Cyclometalated iridium complexes based on monodentate aminophosphanes

Abstract

Monodentate aminophosphanes HNP [NH(4-tolyl)PPh₂] and SiMe₃NP [SiMe₃N(4-tolyl)PPh₂] react with [Ir(μ-Cl)(cod)]₂ affording tetra- or pentacoordinate complexes of formula [IrCl(L)_n(cod)] (L = HNP, n = 1, 2; L = SiMe₃NP, n = 1). The reaction of [IrCl(SiMe₃NP)(cod)] with carbon monoxide smoothly renders [Ir(CO)₃(SiMe₃NP)₂][IrCl₂(CO)₂]. The reaction of HNP or SiMe₃NP with [Ir(CH₃CN)₂(cod)][PF₆] yields the cyclometalated iridium(III)-hydride derivatives [IrH{κ²C,P-NR(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (R = H, SiMe₃) as a result of the intramolecular oxidative addition of the tolyl C²–H bond to iridium. The straighforward formation of [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺ was observed when the reaction was monitored by NMR spectroscopy at 233 K, whereas a more complex reaction sequence was observed in the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺, including the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ and [Ir(cod)(HNP)₂]⁺. The “mixed” complex [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ was obtained upon reaction of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] with SiMe₃NP at 233 K. Finally, the reaction of [Ir(CH₃CN)₂(coe)₂][PF₆] with SiMe₃NP or HNP resulted in the formation of [Ir(CH₃CN)₂(SiMe₃NP)₂][PF₆] and [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)][PF₆], respectively. Both the OC-6-35 and the OC-6-52 isomers of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)]⁺ – featuring facial and meridional dispositions of the phosphorus atoms, respectively – were isolated depending on the reaction solvent. Several compounds described herein catalyse the dehydrogenation of formic acid in DMF, [IrCl(HNP)₂(cod)] being the most active, with TOF_{1 min} of about 2300 h⁻¹ (5 mol% catalyst, 50 mol% sodium formate, DMF, 80 °C).

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Introduction

Ligand design is a longstanding, generally accepted strategy which allows for the preparation of new compounds with tailored properties. In this regard, aminophosphanes have been attracting attention due to their modular synthesis from amines and chlorophosphanes,¹ allowing facile structural modifications. As far as aminophosphanes of general formula NHRPR₂ or NR₂PR₂ are concerned, different coordination modes have been reported, so far (Scheme 1). As a matter of fact, in mononuclear complexes, the expected monodentate κP coordination² has been observed along with the η² coordination,³ whereas the 1κN,2κP coordination⁴ has been reported in dinuclear complexes (Scheme 1-top). Additionally, a very limited number of examples of κ²C,P cyclometalated aminophosphano ligands have been reported, when NR₂ is N-indolyl^5a,b or N-carbazolyl,^5b or in the case that a phenyl group^2f,5c is attached to nitrogen (Scheme 1-bottom). In these cases, a five member metalacycle M–P–N–C–C forms as a result of the CH activation. Interestingly, aminophosphano complexes have found application as drugs,^2d catalysts,^2b,g,3a,b and redox-active multimetallic systems.^4e,h

Scheme 1 Coordination modes of aminophosphanes NHRPR₂ or NR₂PR₂.

Relevant to this paper, we have recently described^2a the synthesis of the iridium(I) complex [IrCl(HNP)₂(tfbb)] [HNP = NH(4-tolyl)PPh₂; tfbb = tetrafluorobenzobarrelene] and its reaction with trimethyl phosphite, reporting that one of the peripheral NH groups is able to establish an intramolecular NH⋯O hydrogen bond that triggers the elimination of methylchloride and the formation of the phosphonato complex [Ir{PO(OMe)₂}(HNP)₂(tfbb)], thus showing that the HNP-Ir scaffold is able to activate small molecules via hydrogen bonding (Scheme 2).^2a

Scheme 2 Reaction of [IrCl(HNP)₂(tfbb)] with trimethyl phosphite.^2a

With this in mind, we envisioned that Ir-HNP derivatives could act as catalysts for the dehydrogenation of formic acid. Actually, a number of homogeneous catalysts based on iridium⁶ have proved to be extremely active, and mechanistic studies have revealed that hydrogen bonds between peripheral NH groups and formic acid play a crucial role in stabilizing both intermediates and transition states involved in the formation of dihydrogen and/or carbon monoxide.⁷ As an example, Fig. 1 shows the transition states TS^A and TS^B proposed for the dehydrogenation of formic acid catalysed by [IrHCl(L_PCP)]^6d and [IrCp*Cl(L_CN)],^6g,i respectively, highlighting the role of NH ancillary groups in the formation of both carbon dioxide (TS^A) or dihydrogen (TS^B).

Fig. 1 Selected iridium catalysts (left) for the formic acid dehydrogenation and related transition states (right).^6d,g,i

On this background, we decided to delve into the preparation of new iridium complexes with the aminophosphano ligand NH(4-tolyl)PPh₂ (HNP) and its silylated analogue SiMe₃N(4-tolyl)PPh₂ (SiMe₃NP). Specifically, herein we describe the synthesis of iridium(I) complexes of formula [IrCl(RNP)_n(cod)] (R = H, SiMe₃; n = 1, 2) as well as cyclometalated iridium(III) derivatives resulting from the intramolecular CH oxidative addition of the C²–H bond of the tolyl group to the iridium centre. In addition, a selection of the prepared complexes has been tested as catalysts for the dehydrogenation of formic acid, guessing that the peripheral NH group(s) might foster the catalytic process.

Results and discussion

Synthesis of SiMe₃N(4-tolyl)PPh₂ (SiMe₃NP)

The synthesis of SiMe₃N(4-tolyl)PPh₂ (SiMe₃NP) was carried out following a route similar to that reported for SiMe₂{N(4-tolyl)PPh₂}₂.⁸ Indeed, the parent aminophosphane NH(4-tolyl)PPh₂ was sequentially treated with n-BuLi and SiMe₃Cl affording SiMe₃NP (Scheme 3). As a consequence of the H/SiMe₃ substitution, a well-shaped ¹H doublet is observed at 0.47 ppm for the SiMe₃ moiety (⁴J_HP = 1.0 Hz) instead of the ¹H signal of the NH hydrogen atom of HNP, and a significant downfield shift of the ³¹P{¹H} NMR signal of SiMe₃NP (δ_P 50.5 ppm) with respect to HNP (δ_P 28.7 ppm) was observed.

Scheme 3 Synthesis of SiMe₃NP.

The crystal structure of SiMe₃NP (Fig. 2) shows an almost planar geometry of the nitrogen atom and an almost perpendicular arrangement of the C16–C17–C18–C19–C20–C21 aromatic ring with respect to the nitrogen plane [Si–N–C16–C17 −116.94(10)°]. Also, bond lengths Si–N [1.7574(10) Å], P–N [1.7167(10) Å] and C16–N [1.4383(14) Å] are similar to those reported for the related aminophosphane SiMe₂{N(4-tolyl)PPh₂}₂ (Si–N 1.751 Å, av.; P–N 1.723 Å, av.; C–N 1.446 Å, av.).⁹ On these grounds, similar to what proposed for SiMe₂{N(4-tolyl)PPh₂}₂, back-donation should exist mainly between nitrogen and phosphorus and should primarily be responsible for the planar geometry of the nitrogen atom in SiMe₃NP (vide infra).

Fig. 2 ORTEP plot of SiMe₃N(4-tolyl)PPh₂ (SiMe₃NP). Hydrogen atoms are omitted for clarity and thermal ellipsoids are at 50% probability. Selected bond lengths (Å) and angles (°) are: P–N 1.7167(10), Si–N 1.7574(10), N–C16 1.4383(14), C16–N–P 122.30(7), C16–N–Si 118.88(7), P–N–Si 117.22(5), Si–N–C16–C17 −116.94(10).

Preparation of iridium(I) compounds

Both HNP and SiMe₃NP react with [Ir(μ-Cl)(cod)]₂ (P : Ir = 1) affording the tetracoordinate iridium complexes [IrCl(L)(cod)] (L = HNP, 1; SiMe₃NP, 2) (Scheme 4).

Scheme 4 Reaction of HNP or SiMe₃NP with [Ir(μ-Cl)(cod)]₂.

The crystal structure of 1 and 2 are given in Fig. 3 showing a distorted square planar geometry at iridium with a cis disposition of the chlorido and aminophosphano ligands. The coordination environments in 1 and 2 are virtually superimposable, nevertheless 1 and 2 differ in terms of intramolecular short contacts. Indeed, a NH⋯Cl hydrogen bond was observed in 1, whereas an anagostic CH⋯Ir interaction¹⁰ was observed in 2 (Fig. 3). As a consequence, a significantly different dihedral angle Cl–Ir–N–P is observed [1, 36.05(13)°; 2, −51.40(6)°].

Fig. 3 ORTEP plot of IrCl(cod)(HNP) (1, top) and IrCl(cod)(SiMe₃NP) (2, bottom). Most hydrogen atoms are omitted for clarity and thermal ellipsoids are at 50% probability. Selected bond lengths (Å) and angles (°) are: 1, Ir–P 2.2850(9), Ir–Cl 2.3488(9), P–N 1.675(3), C20–C21 1.384(5), C25–C24 1.414(5), Ir–Ct2 1.98682(17), Ir–Ct1 2.08157(17), N–H 0.96(3), N–C13 1.417(4), P–Ir–Cl 88.31(3), C13–N–P 132.4(3), C13–N–H 113(3), P–N–H 115(3), Cl–Ir–P–N 36.05(13), N–H⋯Cl hydrogen bond: H⋯Cl 2.48(4), N⋯Cl 3.156(3), N–H 0.960(19), Cl–H–N 127(4); Ct1, centroid of C20 and C21; Ct2, centroid of C24 and C25. 2, N–P 1.6926(12), N–Si 1.7814(12), N–C16 1.4419(18), P–Ir 2.3028(4), Ir–Cl 2.3634(4), C28–C27 1.4260(19), C24–C23 1.394(2), Ct1–Ir 2.08575(16), Ct2–Ir 1.98914(14), P–N–Si 129.23(7), C16–N–P 118.78(9), C16–N–Si 111.98(9), P–Ir–Cl 89.136(12), Ct2–Ir–Ct1 86.812(6), C17–C16–N–Si 83.66(17), N–P–Ir–Cl −51.46(6). CH⋯Ir contact: H1c⋯Ir 2.56(3), C1⋯Ir 3.371(2), C1–H1c 1.076(19), Ir–H1c–C30 131.1(9). Ct1, centroid of C23 and C24; Ct2, centroid of C27 and C28.

The NMR spectra of 1 and 2 suggest that the square planar structure observed in the solid state is preserved in solution. Indeed, for both compounds one ³¹P{¹H} NMR signal is observed (δ_P 43.9, 1; 62.2 ppm, 2) along with two olefinic resonances for the cod ligand (δ_H 2.81 and 5.63, 1; 2.39 and 5.48 ppm, 2; see ESI, Fig. SI5† for selected NMR data with the proposed assignment).

The pentacoordinate derivative [IrCl(HNP)₂(cod)] (3) was obtained when [Ir(μ-Cl)(cod)]₂ was reacted with HNP (Ir : HNP molar ratio 1 : 2, Scheme 4). In contrast, the higher steric hindrance of SiMe₃NP prevents the formation of the pentacoordinate complex [IrCl(SiMe₃NP)₂(cod)], even if SiMe₃NP is added in excess. The ¹H and ¹³C{¹H} NMR spectra of 3 indicate that the compound is fluxional in solution even at 213 K. Indeed, at that temperature one broad ¹H resonance was observed for the four olefinic hydrogen atoms of the cod ligand (δ_H 3.37 ppm) and two ¹H resonances at 2.36 and 1.91 ppm for the endo and exo methylene hydrogen atoms of the cod ligand. By the same token, two ¹³C{¹H} resonances at 69.0 ppm (t, ²J_CP = 7.5 Hz) and at 32.5 ppm (br) were observed for the olefinic and methylene carbon atoms of the cod ligand. In this regard, DFT calculations indicate that 3 may adopt two stable configurations, namely TBPY-5-13 ¹¹ (I,‡Fig. 4) and SPY-5-13 ¹¹ (II, Fig. 4), with similar energies (TBPY-5-13 → SPY-5-13; ΔG = −1.6 kcal mol⁻¹). Thus, reasonably the fast equilibrium TBPY-5-13 ⇄ SPY-5-13 should exchange the olefinic =CH moieties as well as the CH₂ groups, consequently accounting for the averaged spectra observed for 3.

Fig. 4 View of the calculated structures of the TBPY-5-13 and SPY-5-13 configurations of 3. The relative Gibbs free energies are given in parentheses (kcal mol⁻¹). For clarity, only ipso carbon atoms of tolyl and phenyl groups are shown. Blue, iridium; gray, carbon; orange, phosphorus; green, chlorine; violet, nitrogen; white, hydrogen.

The CH⋯Ir interaction observed in 2 (vide supra) is reminiscent of the CH⋯Ir short contact in the related square planar intermediate [IrCl(SiNP)(CO)]¹² resulting from the carbonylation of IrCl(SiNP)(cod) (Scheme 5-top). Taking into account that [IrCl(SiNP)(CO)] smoothly undergoes an intramolecular oxidative addition of the SiCH₂–H bond to iridium, the reaction of 2 with CO was carried out, and, in contrast to our expectations, the formation of 4[IrCl₂(CO)₂] was observed (Scheme 5-bottom). The IR spectrum of a CH₂Cl₂ solution of 4[IrCl₂(CO)₂] shows five CO stretching bands (see ESI, Fig. SI3†) confirming the presence of the anion [IrCl₂(CO)₂]⁻ [2055 (s), 1971 (s) cm⁻¹] and the pentacoordinated cation 4⁺ [2080 (w), 2008 (s); 1998 (s) cm⁻¹].§ Accordingly, the ¹³C{¹H} triplet at 173.5 ppm (²J_CP = 11.1 Hz) and the singlet at 169.1 ppm were assigned to the carbonyl ligands of 4⁺ and [IrCl₂(CO)₂]⁻, respectively, and the ³¹P{¹H} NMR spectrum contains one singlet at 42.9 ppm. Despite the fact that 4[IrCl₂(CO)₂] is thermally stable in the solid state and was obtained as an analytically pure sample, it is thermally unstable in CD₂Cl₂ and slowly evolves to unidentified species, rendering mixtures still containing about 85 mol% (³¹P{¹H} NMR) of 4[IrCl₂(CO)₂] after 15 h at room temperature.

Scheme 5 Carbonylation reaction of [IrCl(SiNP)(cod)] (A, previous work¹²) and [IrCl(SiMe₃NP)(cod)] (2, B, this work).

Cyclometalation reactions

The reactivity of HNP and SiMe₃NP towards cationic iridium(I) complexes bearing labile ligands has been explored. In the following subsections, the reactions of HNP and SiMe₃NP with [Ir(CH₃CN)₂(cod)][PF₆] or [Ir(CH₃CN)₂(coe)₂][PF₆] will be discussed.

Reactivity with [Ir(CH₃CN)₂(cod)][PF₆]. The reaction of HNP or SiMe₃NP with [Ir(CH₃CN)₂(cod)][PF₆] yields the iridium(III) derivatives of formula OC-6-24-[IrH{κ²C,P-NR(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (R = H, 5PF₆, R = SiMe₃, 6PF₆) resulting from the intramolecular oxidative addition of the tolyl C²–H bond to iridium (Scheme 6).

Scheme 6 Formation of 5PF₆ and 6PF₆.

The crystal structure of 6PF₆ was determined by single crystal X-ray diffraction measurements. Its asymmetric unit contains four crystallographically independent pairs of the ions OC-6-24-[IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺ and PF₆⁻ (see ESI, Fig. SI1†). As for the cations, despite the fact that they are chemically equivalent, they differ in the configuration of the metal centre. Actually, the metal centre of 6⁺ is stereogenic and two of the cations of the asymmetric unit exhibit C configuration¹¹ at the metal, whereas the remaining two exhibit A configuration¹¹ (ESI, Fig. SI1†). When comparing the cations of the asymmetric unit, related bond lengths and angles are similar, regardless of the configuration at the metal centre. Therefore, for the sake of brevity, only the structure of one of the cation exhibiting C configuration (Fig. 5) will be discussed. A distorted octahedral geometry at the metal centre is observed with a trans disposition of the hydrido and acetonitrile ligands. The five member Ir1–P1–N1–C113–C114 metallacycle is planar and exhibits a bite angle P1–Ir1–C114 of 81.14(14)° nicely fitting two cis coordination sites at the metal centre.

Fig. 5 ORTEP plot of C_Ir-OC-6-24-[IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺ in 6PF₆. For clarity, most hydrogen atoms are omitted and only the ipso carbon atoms of the phenyl groups are shown. Thermal ellipsoids are at 50% probability. Selected bond lengths (Å) and angles (°) are: C114–Ir1 2.075(4), C113–C114 1.410(6), C113–N1 1.455(6), N1–P1 1.681(4), N1–Si1 1.796(4), N128–Ir1 2.127(4), P1–Ir1 2.2640(14), Ir1–H1m 1.596(10), C120–C121 1.390(7), C124–C125 1.380(6), Ir1–Ct1 2.1909(5), Ir1–Ct2 2.1971(6), C114–Ir1–H1m 80(2), P1–Ir1–H1m 87(2), C114–Ir1–P1 81.14(14), N128–Ir1–H1m 168(2), Ct1–Ir1–Ct2 82.52(2), C113–N1–P1 113.2(3), C113–N1–Si1 121.2(3), P1–N1–Si1 125.5(2).

In view of the bond lengths C113–N1 [1.455(6) Å], N1–P1 [1.681(4) Å], and N1–Si1 [1.796(4) Å] and taking into account the planarity of the nitrogen atom N1 , the metalation of the tolyl substituent in 6⁺ and the consequent coplanarity of the tolyl moiety and the N1–P1–Si–C113 fragment appear not to bring about any substantial difference in the electronic distribution at the nitrogen atom with respect to SiMe₃NP and 2. In this regard, the delocalization indexes¹³ (DI, aka Fuzzy Bond Orders, FBO) were calculated for SiMe₃NP, 2 and 6⁺ (Fig. 6) showing that the nitrogen–phosphorus back-donation is still operative in 6⁺ and the nitrogen–carbon bond basically holds its single bond character, similar to SiMe₃NP and 2, where the aromatic ring lies almost perpendicular to the N–Si–C–P plane.

Fig. 6 Delocalization index (DI) for the Si–N, C–N and P–N bonds in SiMe₃NP, 2 and 6⁺.

The solution structure of 6⁺ is similar to that observed in the solid state. In particular, the ¹H signal of the hydrido ligand is a doublet at −16.43 ppm with a ²J_HP of 11.2 Hz indicating a mutual cis disposition of the hydrido ligand and phosphorus. Also, four non-equivalent olefinic hydrogen atoms are observed for the cod ligand (δ_H 5.54, 5.37, 4.69, 4.09 ppm) and a ³¹P{¹H} singlet is observed at 73.9 ppm downfield-shifted with respect to SiMe₃NP (50.5 ppm) and 2 (62.2 ppm), reasonably as a consequence of the oxidation state +3 of the metal centre. The metalation of the tolyl group brings about a ¹H pattern consisting of two mutually coupled doublets at 7.14 and 6.91 ppm (³J_HH = 8.2 Hz) and one broad singlet at 6.72 ppm assigned to the aromatic hydrogen atoms of the [Ir{κ²C,P-SiMe₃N(4-C₆H₃CH₃)P}] moiety. Also, the ¹³C{¹H} signal of the metalated carbon atom (δ_C 126.8 ppm) is a doublet as a consequence of the carbon–phosphorus coupling (²J_CP = 3.5 Hz). Finally the ¹H singlet at 1.80 ppm indicates the presence of coordinated acetonitrile. The NMR data for 5PF₆ are analogous to those discussed for 6PF₆ suggesting that the solution structure of 5PF₆ should be similar. It is worth a mention that, at variance with 2, the NH ¹H signal of 5⁺ is a broad singlet (Δν_1/2 = 5.1 Hz) at 5.22 ppm, which suggests that the cyclometalation causes a reduced hydrogen-phosphorus coupling constant. Fig. SI6 (ESI†) shows the detailed assignment of ¹H, ¹³C{¹H} and ³¹P{¹H} signals relevant for the solution structure elucidation of 5⁺ and 6⁺.

The formation of 5⁺ and 6⁺ was monitored by NMR spectroscopy. In the case of 6⁺, the straightforward and clean formation of 6⁺ was observed even at 233 K, no intermediate being detected by ¹H and ³¹P{¹H} NMR spectroscopy. On the other hand, when monitoring the formation of 5⁺, a more complex reaction sequence was brought to light (Scheme 7). Actually, upon adding HNP to [Ir(CH₃CN)₂(cod)][PF₆] (1 : 1 molar ratio) at 233 K,¶ part of [Ir(CH₃CN)₂(cod)]⁺ remains unreacted (as seen in the ¹H NMR spectrum), whereas HNP reacts completely. In addition, beside 5⁺, the square planar complex [Ir(cod)(HNP)₂]⁺ (7⁺) and the hydrido derivative [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ (8⁺) were observed (Scheme 7). Fig. 7 shows the ³¹P{¹H} NMR spectrum of the reaction mixture at 233 K along with selected NMR data relevant for the identification of 7⁺ and 8⁺. Upon raising the temperature up to 298 K (in 10 min approximately), 5⁺ is observed in solution as the only product. On these grounds, the reaction of [Ir(CH₃CN)₂(cod)]⁺ with HNP is likely to yield 5⁺ reasonably via the putative intermediate [Ir(cod)(HNP)(CH₃CN)]⁺ (III⁺) which undergoes an intramolecular CH oxidative addition to iridium (vide infra). On the other hand, the formation of [Ir(cod)(HNP)₂]⁺ (7⁺) and [IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ (8⁺) indicates that the putative intermediate [Ir(cod)(HNP)(CH₃CN)]⁺ (III⁺) undergoes a substitution reaction, in which HNP replaces CH₃CN rendering [Ir(cod)(HNP)₂]⁺ (7⁺), which in turn undergoes the intramolecular CH oxidative addition to iridium, yielding 8⁺.

Scheme 7 Reaction sequence for the reaction of [Ir(CH₃CN)₂(cod)]⁺ with HNP.

Fig. 7 (Top) ³¹P{¹H} NMR of the CD₂Cl₂ solution resulting from the reaction of [Ir(CH₃CN)₂(cod)][PF₆] and HNP (1 : 1) at 233 K, with the proposed assignment; (bottom) selected ¹H, ¹³C{¹H} and ³¹P{¹H} NMR data for 7⁺ and 8⁺ (δ are given in ppm, J in Hz).

Admittedly, once formed, 5⁺ might react with HNP rendering 8⁺ and releasing CH₃CN (Scheme 7). Nonetheless, the ¹H–¹H EXSY spectrum of the reaction mixture at 233 K shows exchange peaks between [Ir(cod)(HNP)₂]⁺ (7⁺) and [IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ (8⁺) but no exchange peaks have been observed between 8⁺ and 5⁺, suggesting that the reaction 5⁺ + HNP → 8⁺ + CH₃CN may not be viable at 233 K whereas the equilibrium 7⁺ ⇄ 8⁺ should be operative at that temperature.||

Finally, it is worth a mention that the CH oxidative addition to iridium in 1⁺ is totally regiospecific. In fact, the isomers IV⁺ (OC-6-25) and V⁺ (OC-6-35, Fig. 8) were not detected in the reaction mixture. As a confirmation, IV⁺ was calculated to be less stable than 5⁺ (ΔG_rel = +6.6 kcal mol⁻¹) whereas the structure of V⁺ could not even be located on the potential energy surface (PES), reasonably due to the steric congestion between the PPh₂ moiety and the cod ligand, leading to the dissociation of the CH₃CN ligand and a severe distortion of the resulting square pyramidal structure (ΔG_rel = +20.6 kcal mol⁻¹).

Fig. 8 Isomers of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺.

Interestingly, the cyclometalation of the tolyl group can be reverted by reacting 5⁺ or 6⁺ with chloride (Scheme 8). Actually, the reaction of 5PF₆ or 6PF₆ with tetrabutylammonium chloride (1 : 1) cleanly and immediately affords 1 and 2, respectively.** On the other hand, 5PF₆ and 6PF₆ were also obtained via chloride abstraction upon treating 1 and 2, respectively, with TlPF₆ in CH₃CN (Scheme 8). On these grounds, the C–H oxidative addition to iridium in [IrX(RNP)(cod)]ⁿ⁺ (X = Cl, n = 0; X = CH₃CN, n = 1, R = H, SiMe₃) is subtly controlled by the ancillary ligand X. Actually, it should be hampered by the chlorido ligand and facilitated when the acetonitrile ligand is present, instead. With this in mind, the Gibbs free energy profiles of the cyclometalation reaction for 1 and [Ir(cod)(HNP)(CH₃CN)]⁺ (III⁺) were explored by means of DFT calculations (Fig. 9). Despite the fact that the activation barriers are easily accessible at room temperature for both 1 (cf.TS_1-VI, ΔG_act = +17.5 kcal mol⁻¹) and III⁺ (cf.TS_III⁺-5⁺, ΔG_act = +11.5 kcal mol⁻¹), the relative stability of the hydrido derivative vs. the related square complex are different. As a matter of fact, in agreement with the observed outcome of the reactions given in Scheme 8, the putative hydrido complex [IrHCl{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)] (VI) is less stable than the parent square planar complex [IrCl(HNP)(cod)] (1), whereas [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺ (5⁺) is more stable than [Ir(cod)(HNP)(CH₃CN)]⁺ (III⁺, Fig. 9), which indicates that switching from X = CH₃CN to X = Cl⁻ in [IrX(RNP)(cod)]ⁿ⁺ makes the oxidative addition unfavourable.

Scheme 8 Reversible Formation of 5PF₆ and 6PF₆.

Fig. 9 (Top) Gibbs free energy profile for the formation of [IrHX{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(cod)]ⁿ⁺ (X = CH₃CN, 5⁺; Cl⁻, II) along with the calculated Gibbs free energies (kcal mol⁻¹, M06/def2tzvp//B3PW91-GD3BJ/def2svp, CH₂Cl₂, 298 K, 1 atm). (bottom) View of TS_I⁺-5⁺ and TS_1-II along with selected interatomic distances (Å). For clarity, most hydrogen atoms have been omitted and phenyl groups are shown in a wireframe style. Blue, iridium; gray, carbon; orange, phosphorus; green, chlorine; violet, nitrogen; white, hydrogen.

For the sake of comparison, the energy profile of the reaction 7⁺ → 8⁺ was also calculated (Fig. SI46 ESI†) confirming that this transformation is a fast equilibrium under the explored reaction conditions.

In view of the reaction sequence discussed in Scheme 7, the reaction of 5⁺ with HNP (1 : 1) was carried out aiming at preparing 8⁺. Surprisingly, on a preparative scale an inseparable mixture of products was obtained, and monitoring the reaction at 233 K evidenced the formation of a mixture of products mainly containing 8⁺ and a new thermally unstable product, which was independently prepared at 233 K and identified in situ as [Ir(HNP)₃(cod)]⁺ (9⁺).†† In addition, when the reaction mixture was heated up to room temperature, 8⁺ disappeared and a mixture of products was obtained including, among others, hydrido derivatives containing three HNP ligands. This evidence suggests that the outcome of the reaction of 5⁺ with HNP is spoiled by the reversibility of the CH oxidative addition as well as by the formation of substitution products containing up to three HNP ligands per iridium.

Also, the reaction of 5⁺ with SiMe₃NP was carried out, envisioning that the higher steric hindrance of SiMe₃NP could prevent the formation of unwanted products (Scheme 9). As a matter of fact, this reaction smoothly takes place at 233 K in 6 h rendering the unexpected compound 10⁺ (along with residual amounts of unreacted 5⁺, Fig. 10). Notably, the formation of 10⁺ (instead of VII⁺, Scheme 9) indicates that 5⁺ does not simply exchange the CH₃CN ligand with the incoming SiMe₃NP, but also that, at some point, the CH reductive elimination rendering HNP and the following CH oxidative addition at SiMe₃NP take place. The ¹H and ¹³C{¹H} signals assigned to the tolyl groups (see ESI, Fig. SI7†) confirm the presence of the [Ir{κ²C,P-SiMe₃N(4-C₆H₃CH₃)P}] moiety along with the κP HNP ligand. Also the ²J_HP coupling constants (105.5, 12.4 Hz) of the ¹H signal at −10.18 ppm of the Ir–H moiety and the ²J_PP constant (22.2 Hz) support the structure proposed for 10⁺.

Scheme 9 Reaction of 5⁺ with SiMe₃NP.

Fig. 10 ³¹P{¹H} NMR spectrum of 10⁺ (prepared in situ) with traces of residual 5⁺ (6 mol% approx.).

Finally, for the sake of comparison, the relative stability of 10⁺ and of the expected isomer VII⁺ was calculated by DFT methods showing that 10⁺ is 31.5 kcal mol⁻¹ more stable than VII⁺, reasonably because of the high steric congestion that exists between the trimethylsilyl group and the metalated tolyl group (Fig. 11). In our hands, any attempt to isolate 10⁺ was unsuccessful due to its limited thermal stability.

Fig. 11 Calculated structures of 10⁺ and VII⁺ (B3PW91-GD3BJ/def2svp). For clarity, most hydrogen atoms are omitted and most tolyl and phenyl groups are shown in a wireframe style. Blue, iridium; gray, carbon; orange, phosphorus; yellow, silicon; violet, nitrogen; white, hydrogen. Blue, iridium; gray, carbon; orange, phosphorus; yellow, silicon; violet, nitrogen; white, hydrogen.

Reactivity with [Ir(CH₃CN)₂(coe)₂][PF₆]. The reaction of [Ir(CH₃CN)₂(coe)₂][PF₆] with SiMe₃NP or HNP was carried out, a different outcome being observed in each case (coe = cyclooctene, Scheme 10). Indeed, [Ir(CH₃CN)₂(coe)₂][PF₆] reacts with SiMe₃NP (1 : 2 molar ratio) affording the square planar complex [Ir(CH₃CN)₂(SiMe₃NP)₂]⁺ (11⁺), isolated as the hexafluorophosphate salt, as a result of the substitution of the coe ligands. In agreement with the square planar geometry at the metal centre, one ³¹P{¹H} singlet at 61.9 ppm is observed for the two equivalent phosphorus atoms, along with two ¹H singlets at 1.42 and 0.03 ppm for the equivalent acetonitrile ligands and the trimethyl silyl groups, respectively. On the other hand, the reaction of [Ir(CH₃CN)₂(coe)₂][PF₆] with HNP entails the intramolecular C–H oxidative addition to iridium, yielding the iridium(III) hydrido isomers 12⁺ (OC-6-35) and 13⁺ (OC-6-52) of formula [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)]⁺, isolated as the hexafluorophosphate salts when the reaction was carried out in CH₃CN or CH₂Cl₂, respectively (Scheme 10). More specifically, in CD₂Cl₂, 12⁺ is the kinetic product observed in the first place (³¹P{¹H}NMR), whereas 13⁺ is finally obtained as the thermodynamic product. On the contrary, in CD₃CN, only 12⁺ forms, 13⁺ being not observed (³¹P{¹H} NMR). Accordingly, when a CD₂Cl₂ solution of 12⁺ was allowed to stand at room temperature for 12 h, the clean conversion of 12⁺ into 13⁺ takes place (ESI, Fig. SI45†).

Scheme 10 Reaction of [Ir(CH₃CN)₂(coe)₂]⁺ with HNP or SiMe₃NP.

The solid state structure of 12PF₆ was determined by means of single crystal X-ray diffraction measurements (Fig. 12). The asymmetric unit of 12PF₆ contains two crystallographically independent pairs of the ions OC-6-35-[IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)]⁺ and PF₆⁻ (ESI, Fig. S12†). Similar to 6⁺, 12⁺ contains a stereogenic metal centre, and the cations of the asymmetric unit are chemically equivalent, but exhibit different configurations at the metal centre. When comparing bond lengths and angles, similar values are observed for the two enantiomers. Therefore, for the sake of brevity, only the crystal structure of the enantiomer C is discussed in detail (Fig. 12). An octahedral environment is observed at the metal centre with a facial disposition of the three phosphorus atoms P1, P2 and P3. The remaining coordination sites are occupied by the hydrido ligand H1m (trans to P3), the acetonitrile ligand (trans to P1) and the carbon atom C14 of the metalated tolyl group (trans to P2). Similar to 6⁺, the [Ir{κ²C,P-NH(4-C₆H₃CH₃)P}] moiety of 12⁺ is planar and the bond lengths N1–P1 [1.680(4) Å] and C13–N1 [1.412(5) Å] indicate a nitrogen–phosphorus back-donation with a nitrogen–carbon single bond.

Fig. 12 ORTEP plot of C_Rh-OC-6-35-[IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)][PF₆] (C_Rh-12PF₆). For clarity most hydrogen are omitted and only ipso carbon atoms of the phenyl and tolyl groups are shown. Thermal ellipsoids are at 50% probability. Selected bond lengths (Å) and angles (°) are: N58–Ir1 2.062(3), P1–Ir1 2.2484(10), P2–Ir1 2.3387(10), P3–Ir1 2.4027(9), Ir1–H1m 1.54(4), C14–Ir1 2.103(4), C13–C14 1.407(5), C13–N1 1.412(5), N1–P1 1.680(4), C14–Ir1–P1 81.14(11), P1–Ir1–H1m 83.1(19), C14–Ir1–H1m 84.1(18), P3–Ir1–H1m 172.6(19), P1–Ir1–P2 98.90(4), P1–Ir1–P3 97.96(3), P2–Ir1–P3 105.03(3), N58–Ir1–P1 166.85(9).

Based on the NMR spectra of 12⁺, its solid state structure should be maintained in solution. Specifically, three ³¹P{¹H} signals are observed at 49.8 (P¹), 19.8 (P²) and 11.2 ppm (P³) with ²J_PP coupling constants (16.9, 11.5 Hz) indicating a mutually cis disposition. Accordingly, three ¹H signals are observed for the non-equivalent NH groups (δ_H 6.28, HNP¹; 5.17, HNP²; 4.64 ppm, HNP³). The ¹H signal of the hydrido ligand at −11.83 ppm is observed as a doublet of triplets with ²J_HP constants (150.5, 16.2 Hz) underpinning the presence of two phosphorus atoms at cis positions (P¹ and P²) and one (P³) at the trans position. The ¹H and ¹³C{¹H} signals assigned to the tolyl group are similar to those observed for 6⁺, thus confirming its metalation (see ESI Fig. SI8† for selected ¹H, ¹³C{¹H} and ³¹P{¹H} data and the proposed assignment).

Also, the solution structure of 13⁺ was established by means of multinuclear NMR spectroscopy. Relevant for the solution structure elucidation, the ³¹P{¹H} spectrum contains two signals at 43.4 and 19.7 ppm assigned to the metalated Ir{κ²C,P-NH(4-C₆H₃CH₃)P} moiety and the equivalent HNP ligands, respectively. Accordingly, two ¹H signals are observed for the NH moieties at 5.57 (1H) and 5.18 ppm (2H). The hydrido ligand lies trans to the phosphorus atom of the metalated HNP ligand originating a ¹H doublet of triplets at −9.86 ppm (²J_HP = 133.0, 18.6 Hz). Like for 12⁺, the ¹H and ¹³C{¹H} signals assigned the tolyl groups confirm the presence of one metalated tolyl group (see ESI Fig. SI8†).

As for the formation of 12⁺ and 13⁺, DFT calculations were performed in order to shed light on the underlying reaction sequence. Provided that [Ir(CH₃CN)₂(SiMe₃NP)₂]⁺ (11⁺) forms upon reaction of [Ir(CH₃CN)₂(coe)₂]⁺ with SiMe₃NP, the related complex [Ir(CH₃CN)₂(HNP)₂]⁺ (VIII⁺) is assumed as the starting point for the formation of 12⁺ (Fig. 13A). Two routes leading from VIII⁺ to 12⁺ can be envisioned. On one hand (Fig. 13A, red arrows), the oxidative addition of the CH bond to iridium in III⁺ leads to the iridium(III) hydrido derivatives IX⁺ or X⁺, depending on the regiochemistry of the CH oxidative addition. The barriers for the formation of IX⁺ (TS_VIII⁺-IX⁺, +16.6 kcal mol⁻¹) and X⁺ (TS_VIII⁺-X⁺,‡‡ ΔE_act ≈ +16.7 kcal mol⁻¹) are accessible at room temperature. On their turn, IX⁺ or X⁺ might undergo a substitution reaction exchanging CH₃CN with HNP. Indeed, the CH₃CN ligand trans to carbon atom (IX⁺) or hydrido ligand (X⁺) are expected to be labile and prone to being replaced by the incoming HNP ligand, thus rendering 12⁺. On the other hand (Fig. 13A, blue arrows), VIII⁺ might convert into the square planar intermediate XII⁺via a presumably barrierless, exergonic associative mechanism§§ going through the pentacoordinate derivative XI⁺. Subsequently, XII⁺ should undergo the intramolecular CH oxidative addition and, depending on the implied tolyl group and on the regiochemistry of the CH oxidative addition itself, 12⁺, XIII⁺ or XIV⁺ might form. Notably, both XIII⁺ and XIV⁺ are calculated to be less stable than 12⁺, in agreement with the fact that neither of them was obtained. Also the transition state TS_XII⁺-12⁺ (Fig. 13B) leading from XII⁺ to 12⁺ was calculated to be accessible at room temperature (Fig. 13A, ΔG_act = +12.5 kcal mol⁻¹). With this in mind, both routes are accessible under experimental conditions. Nonetheless, from an energetic point of view, starting from VIII⁺, the route viaIX⁺ and X⁺ (red arrows) goes up to approx. +17 kcal mol⁻¹ (corresponding to the transition states TS_VIII⁺-IX⁺ and TS_VIII⁺-X⁺) and then downhill to 12⁺, whereas the route viaXI⁺ (blue arrows) goes through two exergonic initial steps (VIII⁺ → IX⁺, ΔG = −22.5 kcal mol⁻¹; IX⁺ → X⁺, ΔG = −1.5 kcal mol⁻¹) followed by the CH oxidative addition rendering 12⁺, which is feasible under experimental conditions (ΔG_act = +12.5 kcal mol⁻¹, TS_XII⁺-12⁺vs. XII⁺). As a result, even if the route viaIX⁺ and X⁺ (red arrows) is viable, the reaction sequence going through XII⁺ (blue arrows) is more favourable and should be operative under experimental conditions.

Fig. 13 (A) Reaction sequences for the formation of 12⁺ and 13⁺ with the relative Gibbs free energy (kcal mol^–1, B3PW91-GD3BJ/def2svp) of the proposed intermediates and transition states. (B) Calculated structure for the transition state TS_XII⁺-11⁺ along with selected interatomic distances (Å, B3PW91-GD3BJ/def2svp). For clarity, most hydrogen atoms are omitted and phenyl and selected tolyl groups are shown in a wireframe style. (C) Proposed reaction sequence for the isomerization of 12⁺ to 13⁺ (M06/def2tzvp//B3PW91-GD3BJ/def2svp).

As far as the formation of 13⁺ is concerned, the meridional disposition of the phosphorus and carbon atoms and of the hydrido ligand within the IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂} moiety rules out that 13⁺ might result from the straightforward CH oxidative addition to iridium.¶¶

Thus, the transformation of 12⁺ into 13⁺ should be the result of a genuine isomerization process which alters the disposition of the ligands at the metal centre, and more than likely is triggered by the dissociation of CH₃CN when 12⁺ is dissolved in CH₂Cl₂. On this basis, Fig. 13C depicts a plausible reaction sequence (green arrows) for the conversion of 12⁺ into 13⁺ along with the calculated relative Gibbs free energy of the proposed intermediates. In the first place, the dissociation of CH₃CN from 12⁺ renders the pentacoordinated iridium complex XV⁺ which stepwise isomerise to XVII⁺viaXVI⁺. Finally, 13⁺ forms upon reaction of XVII⁺ with CH₃CN.

Catalytic dehydrogenation of formic acid

The iridium(I) derivatives 1–3 and 11PF₆, and the iridium(III) compounds 5PF₆, 6PF₆, and 13PF₆ were tested as catalysts for formic acid dehydrogenation. Preliminary catalytic tests (1 mol% catalyst, 10 mol% NaHCOO, in DMF at 80 °C) revealed that IrCl(HNP)₂(cod) (3) is by far the most active catalyst (Table 1, entries 1–7) reaching TOF_{1 min} of about 1700 h⁻¹ and complete conversion to H₂ and CO₂ after 18 min, CO not being observed in the resulting gas mixture (IR). In this regard, these preliminary data point out that the presence of two aminophosphano ligands, each one with an available NH group, may be responsible for the good catalytic performance of 3. When using 1 mol% of 3 as a catalyst, increasing the amount of sodium formate up to 50 mol% does not increase the activity of the catalyst (Table 1, entry 8). Notably, when 5 mol% of 3 along with 50 mol% of sodium formate was used a short induction time was clearly observed (Fig. 14) and a TOF_{1 min} of about 2300 h⁻¹ (Table 1, entry 9) was reached, which indicates that its activity is similar to that reported for the iridium complexes [IrCp*(H₂O)(2,2′-bipyridine-4,4′-diol)]²⁺ (ref. 6k) and [IrCp*Cl{1,2-(NH)₂C₆H₁₀}]⁺.^6g Also, at a 5 mol% catalyst loading and 50 mol% of sodium formate, 3 showed a decreasing activity upon consecutively adding formic acid, without any induction period (C–F, Fig. 14). When triethylamine was used as a base instead of sodium formate (Table 1, entries 10 and 11), a lower activity was observed for 3 (TOF_{1 min} 1000 h⁻¹, Fig. 14).

Fig. 14 Conversion vs. time profiles for the formic acid dehydrogenation catalysed by 3 (5% catalyst, 50% base, DMF, 80 °C).

Table 1 Selected experimental data for the dehydrogenation of formic acid catalysed by 1–3, 5PF₆, 6PF₆, 11PF₆ and 13PF₆ ^a

Entry	Catalyst	Cat. mol%	Base	Base mol%	Conv.%^b, time	TOF1 min ^c (h^−1)
a Reaction conditions: 20 mL of DMF, 20 μL of HCOOH (0.529 mmol), NaHCOO or NEt3 (10 or 50 mol%), iridium catalyst (1 or 5 mol%), 353 K. b Based on the pressure of the resulting gas mixture. c TOF1 min is calculated as TON at 1 min divided by 1 min.
1	1	1%	NaHCOO	10%	38%, 40 min	400
2	2	1%	NaHCOO	10%	68%, 40 min	240
3	3	1%	NaHCOO	10%	92%, 18 min	1700
4	5PF6	1%	NaHCOO	10%	37%, 40 min	240
5	6PF6	1%	NaHCOO	10%	30%, 40 min	150
6	11PF6	1%	NaHCOO	10%	94%, 35 min	500
7	13PF6	1%	NaHCOO	10%	95%, 18 min	680
8	3	1%	NaHCOO	50%	97%, 18 min	1640
9	3	5%	NaHCOO	50%	99%, 0.8 min	2330
10	3	1%	NEt3	10%	96%, 25 min	1000
11	3	5%	NEt3	50%	98%, 2 min	900

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Conclusions

Monodentate aminophosphanes HNP [NH(4-tolyl)PPh₂] and SiMe₃NP [SiMe₃N(4-tolyl)PPh₂] react with iridium(I) precursors affording a varied family of iridium(I) and cyclometalated iridium(III) complexes, exhibiting κP or κ²C,P coordination modes. The tetra- or pentacoordinate iridium(I) complexes [IrCl(κP-RNP)_n(cod)] (R = H, n = 1, 2; R = SiMe₃, n = 1) result from bridge splitting at [Ir(μ-Cl)(cod)]₂, the steric hindrance of the incoming aminophosphane being decisive in the outcome of the reaction. On the other hand, octahedral iridium(III) complexes can also be obtained as a result of the intramolecular CH oxidative addition of the C²–H bond of the tolyl substituent of both HNP and SiMe₃NP to iridium(I) rendering the [IrH{κ²C,P-RN(4-C₆H₃CH₃)P}] moiety. The calculated activation barrier of the CH oxidative addition is relatively small and the related CH reductive elimination can be smoothly triggered (Scheme 11). Unexpectedly, complex reaction sequences have been observed as a consequence of the reversibility of the CH oxidative addition and/or competing substitution reactions.

Scheme 11 Reversible CH oxidative addition to iridium.

Both iridium(I) and iridium(III) derivatives catalyze the dehydrogenation of formic acid in DMF, the iridium(I) compound [IrCl(HNP)₂(cod)] being the most active species. A detailed study on the reaction mechanism operative in the dehydrogenation of formic acid catalysed by 3 is under run in order to elucidate the role of the NH groups as well as the influence of the structure of the iridium-aminophosphane platform on the observed catalytic activity.

Experimental

General section

All the operations were carried out using standard Schlenk tube techniques under an atmosphere of pre-purified argon or in a Braun glove-box under argon. Organic solvents were dried by standard procedures and distilled under argon or obtained oxygen- and water-free from a Solvent Purification System (Innovative Technologies). The compounds NH(4-tolyl)PPh₂ (HNP),⁸ [Ir(μ-Cl)(cod)]₂,¹⁴ and [Ir(CH₃CN)₂(cod)][PF₆]¹⁵ were prepared according to the literature. [Ir(CH₃CN)₂(coe)₂][PF₆] was obtained as a spectroscopically pure solid following a procedure similar to that reported by Milstein for [Ir(acetone)₂(coe)₂][PF₆].¹⁶ NMR spectra were recorded with Bruker spectrometers (AV300, AV400, and AV500) and are referred to SiMe₄ (¹H, ¹³C{¹H}) and H₃PO₄ (³¹P{¹H}). The proposed ¹H, ¹³C{¹H}, and ³¹P{¹H} assignment relies on the combined analysis of 1D [¹H, ¹H{³¹P}, ¹³C{¹H}-apt, ³¹P{¹H}] and 2D NMR spectra (¹H–¹H COSY, ¹H–¹H NOESY, ¹H–¹³C HSQC, ¹H–¹³C HMBC, ¹H–³¹P HMBC). For 5PF₆ and 6PF₆, olefinic carbon and hydrogen atoms are labelled as “up” or “down” if they point towards the hydride or if they do not, respectively, and as “trans” or “cis” if the HC=CH moiety lies trans to phosphorus or cis to it, respectively. For 12PF₆ and 13PF₆, labels P¹, P², and P³ are used for non-equivalent phosphorus atoms, and, accordingly, superscript labels “tol-P1/2/3” and “PhP1/2/3” are used for hydrogen and carbon atoms belonging to the tolyl and phenyl groups attached/linked to the phosphorus atom P¹/P²/P³. As for the reaction monitored by NMR spectroscopy at low temperature, the working temperature of 233 K was chosen in order to avoid the precipitation of reactants and/or products. C, H, and N analyses were carried out on a Perkin-Elmer 2400 CHNS/O analyzer. Infrared spectra were recorded on a Thermo Nicolet Avatar 360 FT-IT spectrometer on CH₂Cl₂ solutions using KBr windows (1 mm path).

Synthesis of SiMe₃N(4-tolyl)PPh₂ (SiMe₃NP)

A solution of NH(4-tolyl)PPh₂ (2.43 g, 8.34 mmol, 291.33 g mol⁻¹) in THF (10 mL) and toluene (25 mL) at 198 K was treated with n-BuLi (1.6 M in hexane, 5.2 mL, 8.3 mmol). The mixture was allowed to warm up to room temperature (1 h) and SiMe₃Cl (1.06 mL, 8.35 mmol, 0.856 g mL⁻¹, 108.64 g mol⁻¹) was added. After stirring overnight at room temperature, the suspension was heated at 333 K for 3 h. All the volatiles were removed in vacuo and the residue was extracted with CH₂Cl₂. The liquid phase was evaporated up to 3 mL and added with hexane (10 mL), affording the precipitation of a colourless solid, which was filtered off and washed with diethyl ether, dried in vacuo, and finally identified as SiMe₃N(4-tolyl)PPh₂ (1.61 g, 53% yield). Found: C 73.01, H 6.99, N 3.78. Calcd for C₂₂H₂₆NPSi (363.51 g mol⁻¹): C 72.69, H 7.21, N 3.85. ¹H NMR (C₆D₆, 298 K): δ_H 7.66–7.52 (4H, o-PPh), 7.23–7.14 (6H tot; 4H m-PPh, 2H p-PPh), 6.75 (d, 2H, ³J_HH = 8.8 Hz, C³H^tol), 6.73 (d, 2H, ³J_HH = 8.8 Hz, C²H^tol), 2.07 (s, 3H, CH₃^tol), 0.47 (d, 9H, ³J_HP = 1.0 Hz, SiCH₃). ¹³C{¹H} NMR (C₆D₆, 298 K): δ_C 143.4 (d, ²J_CP = 8.0 Hz, C^{1, tol}), 140.7 (d, ¹J_CP = 17.7 Hz, C^{1, PhP}), 134.7 (C^{4, tol}), 134.2 (d, ²J_CP = 21.1 Hz, C^{2, PhP}), 130.9 (d, ³J_CP = 1.3 Hz, C^{2, tol}), 129.6 (C^{3, tol}), 129.3 (C^{4, PhP} C^{3, tol}), 128.7 (d, ³J_CP = 5.9 Hz, C^{3, PhP}), 21.3 (CH₃^tol), 2.2 (d, ³J_CP = 8.4 Hz, SiCH₃), ³¹P{¹H} NMR (C₆D₆, 298 K): δ_P 50.5 (s).

Synthesis of [IrCl(HNP)(cod)] (1). A toluene solution (10 mL) of [Ir(μ-Cl)(cod)]₂ (315 mg, 0.469 mmol, 671.70 g mol⁻¹) was added with HNP (137 mg, 0.470 mmol, 291.33 g mol⁻¹) affording a deep orange solution. After 1 h stirring at room temperature, the solution was evaporated in vacuo up to approx. 5 mL and added with 20 mL of hexane. The resulting suspension was filtered, the solid washed with hexane (2 × 5 mL), dried in vacuo and finally identified as [IrCl(HNP)(cod)] (241 mg, 82% yield). Found: C 51.95, H 4.75, N 2.15. Calcd for C₂₇H₃₀ClIrNP (627.18 g mol⁻¹): C 51.71, H 4.82, N 2.23. ¹H NMR (C₆D₆, 298 K): δ_H 8.01 (ddd, 4H, ³J_HP = 10.9 Hz, ³J_HH = 8.0 Hz, ³J_HH = 1.6 Hz, o-PPh), 7.72 (d, ²J_HP = 13.1 Hz, 1H, NH), 7.21–7.04 (6H tot; 4H, m-PPh; 2H, p-PPh), 6.73 (d, ³J_HH = 8.7 Hz, 2H, C²H^tol), 6.68 (d, ³J_HH = 8.7 Hz, C³H^tol), 5.63 (m, 2H, C^sp2H^cod), 2.81 (m, 2H, C^sp2H^cod), 2.05 (br, 4H, CH₂^cod,endo), 1.99 (s, 3H, CH₃^tol), 1.63 (m, 2H, CH₂^cod,exo), 1.43 (br, 2H, CH₂^cod,exo). ¹³C NMR (C₆D₆, 298 K): δ_C 140.9 (d, ²J_CP = 11.6 Hz, C^{1, tol}), 134.1 (d, ²J_CP = 12.0 Hz, C^{2, PhP}), 132.6 (C^{4, tol}), 131.5 (d, ¹J_CP = 50.8 Hz, C^{1, PhP}), 131.4 (d, ³J_CP = 2.2 Hz, C^{3, PhP}), 130.2 (C^{2, tol}), 129.1 (C^{4, PhP}), 120.3 (C^{3, tol}), 94.5(d, ³J_CP = 14.9 Hz, C^{sp2, cod}), 54.7 (C^{sp2, cod}), 34.1 (d, ⁴J_CP = 3.3 Hz C^{sp3, cod}), 30.0 (d, ⁴J_CP = 2.1 Hz C^{sp3, cod}), 21.0 (CH₃^tol). ³¹P{¹H} NMR (C₆D₆, 298 K): δ_P 43.9 (s).

Synthesis of [IrCl(SiMe₃NP)(cod)] (2). A dichloromethane solution (15 mL) of [Ir(μ-Cl)(cod)]₂ (479 mg, 0.713 mmol, 671.70 g mol⁻¹) was added with SiMe₃NP (546 mg, 1.50 mmol, 363.51 g mol⁻¹). The resulting orange solution was stirred for 3 h, evaporated up to 3 mL and added with hexane (10 mL), affording an orange solid which was filtered off, washed with hexane (12 mL), dried in vacuo and finally identified as [IrCl(SiMe₃NP)(cod)] (839 mg, 84% yield). Found: C 51.97, H 5.43, N 1.92. Calcd for: C₃₀H₃₈ClIrNPSi (699.36 g mol⁻¹): C 51.52, H 5.48, N 2.00. ¹H NMR (C₆D₆ 298 K): δ_H 7.68 (m, 4H, o-PPh), 6.94–6.87 (6H tot; 4H m-PPh, 2H p-PPh), 6.72 (dd, 2H, ³J_HH = 8.2 Hz, ⁴J_HP = 1.4 Hz, C²H^tol), 6.45 (d, 2H, C³H^tol, ³J_HH = 8.2 Hz), 5.48 (m, 2H, C^sp2H^cod), 2.39 (m, 2H, C^sp2H^cod), 2.05–1.88 (4H tot; 2H, C^sp3H^{cod endo}; 2H, C^sp3H^{cod exo}) 1.81 (s, 3H, CH₃^tol), 1.51 (m, 2H, C^sp3H^{cod exo}), 1.22 (m, 2H, C^sp3H^{cod endo}), 0.70 (s, 9H, SiCH₃). ¹³C{¹H} NMR (C₆D₆, 298 K): δ_C 141.1 (d, ²J_CP = 9.4 Hz, C^{1, tol}), 136.0 (d, ³J_CP = 11.6 Hz, C^{2, PhP}), 135.0 (d, ¹J_CP = 51.8 Hz, C^{1, PhP}), 133.7 (d, ³J_CP = 2.5 Hz, C^{2, tol}), 130.6 (d, ⁴J_CP = 1.9 Hz, C^{4, PhP}), 129.7 (d, ⁴J_CP = 1.9 Hz, C^{3, tol}), 127.8 (d, ³J_CP = 9.8 Hz, C^{3, PhP}), 93.8 (d, ²J_CP = 15.73 Hz, C^{sp2, cod}), 53.6 (C^{sp2, cod}), 34.1 (d, ³J_CP = 3.1 Hz, C^{sp3, cod}), 30.2 (d, ³J_CP = 2.2 Hz, C^{sp3, cod )}, 21.3 (CH₃^tol), 3.9 (d, ²J_CP = 1.9 Hz, SiCH₃), ³¹P{¹H} NMR (C₆D₆, 298 K): δ_P 62.2 (s).

Synthesis of [IrCl(HNP)₂(cod)] (3). A dichloromethane solution (5 mL) of [Ir(μ-Cl)(cod)]₂ (166 mg, 0.247 mmol, 671.70 g mol⁻¹) was added with HNP (304 mg, 1.04 mmol, 291.33 g mol⁻¹). The resulting yellow solution was stirred for 4 h, evaporated up to 2 mL and added with hexane (10 mL), affording a light yellow solid which was filtered off and washed with diethyl ether (15 mL), dried in vacuo and finally identified as [IrCl(HNP)₂(cod)] (301 mg, 66% yield). Found: C 59.99, H 5.42, N 3.15. Calcd for: C₄₆H₄₈ClIrN₂P₂ (918.50 g mol⁻¹): C 60.15, H 5.27, N 3.05. ¹H NMR (CD₂Cl₂, 298 K): δ_H 7.60–7.34 (10H tot; 8H, o-PPh; 2H, HNP), 7.33–7.01 (12H tot; 8H, m-PPh; 4H p-PPh), 6.75 (d, 4H, ³J_HH = 8.2 Hz, C³H^tol), 6.32 (d, 4H, ³J_HH = 8.2 Hz, C²H^tol), 3.43 (br, 4H, C^sp2H^cod), 2.32 (m, 4H, C^sp3H^{cod endo}), 2.15 (s, 3H, CH₃^tol), 1.88 (m 4H, C^sp3H^{cod exo}). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 142.0 (br, C^{1, tol}), 132.7 (br, C^{2, PhP}), 130.3 (br, C^{4, PhP}), 130.0 (C^{4, tol}), 129.7 (C^{3, tol}), 128.4 (C^{3, PhP}), 118.7 (C^{2, tol}), 69.4 (br, C^{sp2, cod}), 33.2 (C^{sp3, cod}), 20.9 (CH₃^tol). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 27.2 (br). ¹H NMR (CD₂Cl₂, 213 K): δ_H 7.77 (t, ²J_HP = 6.6 Hz, 2H, HNP), 7.44 (m, 4H, o-PPh), 7.31–7.16 (12H tot; 4H, o-PPh; 4H m-PPh; 4H, p-PPh), 6.99 (t, 4H, ³J_HH = 7.5 Hz, m-PPh), 6.75 (d, 4H, ³J_HH = 8.1 Hz, C³H^tol), 6.27 (d, 4H, ³J_HH = 8.1 Hz, C²H^tol), 3.37 (br, 4H, C^sp2H^cod), 2.36 (m, 4H, C^sp3H^{cod endo}), 2.12 (s, 3H, CH₃^tol), 1.91 (m 4H, C^sp3H^{cod exo}). ¹³C{¹H} NMR (CD₂Cl₂, 213 K): δ_C 141.1 (t, ²J_CP = 5.8 Hz, C^{1, tol}), 132.3 (t, ²J_CP = 4.7 Hz, C^{2, PhP}),131.1 (d, ¹J_CP = 60.0 Hz, C^{1, PhP}), 130.9 (t, ²J_CP = 4.8 Hz, C^{2, PhP}), 130.3 (d, ¹J_CP = 51.7 Hz, C^{1, PhP}), 130.1 (C^{4, PhP}), 129.7 (C^{4, PhP}), 129.3 (C^{4, tol}), 129.1 (C^{3, tol}), 128.0 (t, ³J_CP = 5.2 Hz, C^{3, PhP}), 127.6 (t, ³J_CP = 4.9 Hz, C^{3, PhP}), 117.8 (C^{2, tol}), 69.0 (t, ²J_CP = 7.5 Hz, C^{sp2, cod}), 32.6 (br, C^{sp3, cod}), 20.5 (CH₃^tol); ³¹P{¹H} NMR (CD₂Cl₂, 213 K): δ_P 27.9 (s).

Synthesis of [Ir(SiMe₃NP)₂(CO)₃][IrCl₂(CO)₂] (4[IrCl₂(CO)₂]). An orange dichloromethane solution (2 mL) of [IrCl(SiMe₃NP)(cod)] (174 mg, 0.249 mmol, 699.36 g mol⁻¹) was stirred under carbon monoxide (1 atm approx.) for 1 h. Hexane (15 mL) was added to the resulting pale yellow-green solution, affording a green oil that was washed with hexane (3 × 5 mL) eventually yielding a green solid, which was dried under vacuum and finally identified as [Ir(SiMe₃NP)₂(CO)₃][IrCl₂(CO)₂] (122 mg, 72% yield). Found: C 44.53, H 4.05, N 2.16. Calcd for: C₄₉H₅₂Cl₂Ir₂N₂O₅P₂Si₂ (1322.41 g mol⁻¹): C 44.50, H 3.96, N 2.12. ¹H NMR (CD₂Cl₂, 298 K): δ_H 7.65 (t, 4H, ³J_HH = 7.5 Hz, p-PPh), 7.58 (8H t, ³J_HH = 7.5 Hz, m-PPh), 7.45 (dd, 8H, ³J_HP = 13.4 Hz, ³J_HH = 7.5 Hz, o-PPh), 7.11 (d, 4H, ³J_HH = 8.0 Hz, C³H^tol), 6.93 (d, 4H, ³J_HH = 8.0 Hz, C²H^tol), 2.41 (s, 6H, CH₃^tol), −0.09 (s, 18H, SiCH₃). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 173.5 (t, ²J_CP = 11.1 Hz, P₂IrC̲O), 169.1 (Cl₂IrC̲O), 142.8 (C^{1, tol}), 138.3 (C^{4, tol}), 134.9 (t, ¹J_CP = 34.0 Hz, C^{1, PhP}), 133.7 (C^{4, PhP}), 133.3 (t, ²J_CP = 6.4 Hz, C^{2, PhP}), 131.11 (C^{3, tol}), 131.08 (d, ³J_CP = 3.0 Hz C^{2, tol}), 130.0 (t, ³J_CP = 5.6 Hz, C^{3, PhP}), 21.7 (CH₃^tol), 3.4 (d, ³J_CP = 0.5 Hz, SiCH₃). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 42.9 (s).

Synthesis of [IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (5PF₆). Method (1): A dichloromethane solution (5 mL) of [Ir(CH₃CN)₂(cod)][PF₆] (251 mg, 0.476 mmol, 527.47 g mol⁻¹) at 223 K was added with a dichloromethane solution (5 mL) of HNP (139 mg, 0.476 mmol, 291.33 g mol⁻¹) at 223 K. The resulting dark red solution was stirred for 2 h at 233 K. Afterwards, the solution was allowed to warm up to room temperature and was finally stirred for 30 min. The resulting orange solution was dried in vacuo and the residue was washed with diethyl ether (3 × 5 mL), dried in vacuo and finally identified as [IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (348 mg, 94% yield).

Method (2): An acetonitrile solution (8 mL) of [IrCl(HNP)(cod)] (227 mg, 0.361 mmol, 627.18 g mol⁻¹) was added with TlPF₆ (126 mg, 0.361 mmol, 349.94 g mol⁻¹). The resulting pale yellow suspension was stirred for 1 h and evaporated in vacuo, affording a colorless solid that was extracted with dichloromethane (3 × 3 mL). The resulting pale yellow liquid phase was evaporated up to 1 mL and added with diethyl ether (5 mL), affording a colourless solid which was filtered off, washed with diethyl ether (3 × 5 mL), dried in vacuo and finally identified as [IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (252 mg, 89% yield). Found: C 44.95, H 4.21, N 3.69. Calcd for C₂₉H₃₃F₆IrN₂P₂ (777.74 g mol⁻¹): C 44.78, H 4.28, N 3.60. ¹H NMR (CD₂Cl₂, 298 K): δ_H 7.79 (dd, 2H, ³J_HP = 12.9 Hz, ³J_HH = 7.3 Hz, 2H o-PPh), 7.71–7.66 (3H tot; 2H, m-PPh; 1H, p-PPh), 7.65–7.55 (5H tot; 2H, o-PPh; 2H, m-PPh; 1H, p-PPh), 6.88 (d, 1H, ³J_HH = 7.8 Hz, C⁶H^tol), 6.83 (d, 1H, ³J_HH = 7.8 Hz, C⁵H^tol), 6.70 (br, 1H, C³H^tol), 5.77 (m, 1H, C^sp2H^{cod, up-trans}), 5.40 (m, 1H, C^sp2H^{cod down-trans}), 5.22 (br, 1H, NH), 4.76 (dd, ³J_HP = 9.5 Hz, ³J_HH = 7.4 Hz 1H, C^sp2H^{cod up-cis}), 4.61 (m, 1H, C^sp2H^{cod down-cis}), 2.68 (m, 6H, C^sp3H^cod), 2.49 (m, 2H, C^sp3H^cod), 2.23 (s, 3H, CH₃^tol), 1.92 (s, 3H, CH₃CN), −16.07 (d, 1H, ²J_HP = 10.6 Hz, IrH). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 151.6 (d, ²J_CP = 16.2 Hz, C^{1, tol}), 135.2 (C^{3, tol}), 133.6 (d, ⁴J_CP = 2.8 Hz, C^{4, PhP}), 133.2 (d, ²J_CP = 12.5 Hz, C^{2, PhP}), 132.8 (d, ⁴J_CP = 2.9 Hz, C^{4, PhP}), 131.21 (d, ²J_CP = 11.2 Hz, C^{2, PhP}), 131.19 (d, ¹J_CP = 60.4 Hz, C^{1, PhP}), 130.4 (d, ¹J_CP = 60.2 Hz, C^{1, PhP}), 130.2 (d, ³J_CP = 11.8 Hz, C^{3, PhP}), 130.13 (d, ²J_CP = 11.2 Hz, C^{3, PhP}), 130.09 (C^{4, tol}), 127.4 (br, C^{5, tol}), 122.2 (CH₃C̲N), 122.0 (d, ²J_CP = 2.9 Hz, C^{2, tol}) 111.5 (d, ³J_CP = 14.5 Hz, C^{6, tol}), 100.6 (d, ²J_CP = 10.6 Hz, C^{sp2, cod up-trans}), 98.8 (d, ²J_CP = 12.7 Hz, C^{sp2, cod down-trans}), 92.93 (br, C^{sp2, cod up cis}), 92.87 (C^{sp2, cod down-cis}), 33.3 (d, ³J_CP = 2.2 Hz, C^{sp3, cod}), 32.4 (br, C^{sp3, cod}), 30.3 (d, ³J_CP = 2.8 Hz, C^{sp3, cod}), 28.8 (d, ³J_CP = 1.3 Hz, C^{sp3, cod}), 21.2 (CH₃^tol), 2.31 (d, ³J_CP = 1.2 Hz, SiCH₃), 3.6 (C̲H₃CN). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 62.5 (s, NP), −144.7 (hept, ¹J_PF = 710.1 Hz, PF₆⁻).

Synthesis of [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (6PF₆). Method (1): A dichloromethane solution (10 mL) of [Ir(CH₃CN)₂(cod)][PF₆] (206 mg, 0.390 mmol, 527.47 g mol⁻¹) was added with SiMe₃NP (142 mg, 0.391 mmol, 363.51 g mol⁻¹). The resulting dark red solution was stirred for 1 h, evaporated up to 3 mL and added with hexane (5 mL), affording a pale red solid which was filtered off, washed with hexane (3 × 5 mL), dried in vacuo and finally identified as [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (289 mg, 87% yield).

Method (2): An acetonitrile solution (5 mL) of [IrCl{SiMe₃N(4-C₆H₄CH₃)PPh₂}(cod)] (157 mg, 0.224 mmol, 699.18 g mol⁻¹) was added with TlPF₆ (78.5 mg, 0.224 mmol, 349.94 g mol⁻¹). The resulting red solution was stirred for 1 h and evaporated in vacuo, affording a pale red solid which was eventually extracted with dichloromethane (2 × 5 mL). The resulting red liquid phase was evaporated up to 2 mL and added with hexane (5 mL), affording a pale red solid which was filtered off, washed with hexane (3 × 5 mL), dried in vacuo and finally identified as [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (154 mg, 81% yield). Found: C 44.99, H 4.95, N 3.22. Calcd for: C₃₂H₄₁F₆IrN₄P₂Si (849.92 g mol⁻¹): C 45.22, H 4.86, N 3.30. ¹H NMR (CD₂Cl₂, 298 K): δ_H 7.89 (m, 2H, o-PPh), 7.73 (td, 2H, ³J_HH = 7.3 Hz, ⁴J_HP = 2.9 Hz m-PPh), 7.69–7.49 (6H tot; 2H, o-PPh; 2H, m-PPh; 2H, p-PPh), 7.14 (d, 1H, ³J_HH = 8.2 Hz, C⁶H^tol), 6.91 (d, 1H, ³J_HH = 8.2 Hz, C⁵H^tol), 6.72 (br, 1H, C³H^tol), 5.54 (m, 1H, C^sp2H^{cod up-trans}), 5.37 (m, 1H, C^sp2H^{cod down-trans}), 4.69 (m, 1H, C^sp2H^{cod down-cis}), 4.09 (t, 1H, ³J_HH = 7.8 Hz, C^sp2H^{cod up-cis}), 2.76 (m, 2H, C^sp3H^cod), 2.50 (m, 4H, C^sp3H^cod), 2.25 (s, 3H, CH₃^tol), 2.12 (m, 2H, C^sp3H^cod), 1.80 (s, 3H, CH₃CN), 0.14 (s, 9H, SiCH₃), −16.43 (d, 1H, ²J_HP = 11.2 Hz, IrH). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 154.8 (d, ²J_CP = 16.2 Hz, C^{1, tol}), 135.7 (C^{3, tol}), 132.8 (d, ¹J_CP = 73.7 Hz, C^{1, PhP}), 132.6 (d, ⁴J_CP = 2.8 Hz, C^{4, PhP}), 132.1 (d, ⁴J_CP = 2.9 Hz, C^{4, PhP}), 131.8 (d, ²J_CP = 11.8 Hz, C^{2, PhP}), 131.2 (d, ¹J_CP = 74.7 Hz, C^{1, PhP}),131.1 (d, ³J_CP = 10.5 Hz, C^{2, PhP}), 130.5 (C^{4, tol}), 129.8 (d, ³J_CP = 11.0 Hz, C^{3, PhP}), 129.3 (d, ²J_CP = 11.9 Hz, C^{3, PhP}), 127.1 (d, ²J_CP = 3.5 Hz, C^{2, tol}), 126.2 (C^{5, tol}), 121.4 (CH₃C̲N), 114.4 (d, ³J_CP = 16.1 Hz, C^{6, tol}), 101.1 (d, ²J_CP = 10.6 Hz, C^{sp2, cod down-trans}), 100.6 (d, ²J_CP = 12.2 Hz, C^{sp2, cod up-trans}), 94.9 (d, ²J_CP = 12.2 Hz, C^{sp2, cod up-trans}), 94.4 (C^{sp2, cod down-cis}), 32.7 (d, ³J_CP = 2.5 Hz, C^{sp3, cod}), 31.5 (d, ³J_CP = 1.2 Hz, C^{sp3, cod}), 29.7 (d, ³J_CP = 2.7 Hz, C^{sp3, cod}), 28.1 (d, ³J_CP = 2.0 Hz, C^{sp3, cod}), 20.7 (CH₃^tol), 2.64 (d, ³J_CP = 1.2 Hz, SiCH₃), 2.63 (C̲H₃CN). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 73.9 (s, NP), −144.7 (hept, ¹J_PF = 710.1 Hz, PF₆⁻).

Formation of [Ir(HNP)₃(cod)]⁺ (9⁺). In a 5 mm NMR tube, a CD₂Cl₂ solution (0.4 mL) of [Ir(CH₃CN)₂(cod)][PF₆] (23.7 mg, 0.0450 mol, 527.47 g mol⁻¹) at 223 K was added to a CD₂Cl₂ solution (0.4 mL) of HNP (39.3 mg, 0.135 mmol, 291.33 g mol⁻¹) at 223 K. Soon after the addition, the NMR tube was introduced into the NMR spectrometer at 233 K, observing the formation of [Ir(HNP)₃(cod)]⁺ (9⁺, 80 mol%) along with unidentified by-products (total 15 mol%). ¹H NMR (CD₂Cl₂, 223 K): δ_H 7.63 (br, 6H, o-PPh), 7.45 (t, 6H, ³J_HH = 6.9 Hz, p-PPh), 7.31 (t, 12H, ³J_HH = 7.5 Hz, m-PPh), 7.15–7.00 (7H tot; 6H. o-PPh; 1H, HNP), 6.82 (d, 6H, ³J_HH = 8.0 Hz, C³H^tol), 6.20 (d, 6H, ³J_HH = 8.0 Hz, C²H^tol), 3.41 (br, 4H, C^sp2H^cod), 2.73 (br, 2H, C^sp3H^cod), 2.45 (m, 4H, C^sp3H^cod), 2.19 (s, 9H, CH₃^tol), 1.55 (m, 2H, C^sp3H^cod). ¹³C{¹H} NMR (CD₂Cl₂, 223 K): δ_C 139.1 (d, ²J_CP = 10.6 Hz, C^{1, tol}), 131.54 (C^{4, tol}), 131.47 (C^{4, PhP}), 131.42 (C^{2, PhP}), 129.8 (C^{3, tol}), 129.0 (C^{3, PhP}), 128.6 (C^{2, PhP}), 118.6 (C^{2, tol}), 74.2 (C^{sp2, cod}), 38.1 (C^{sp3, cod}), 28.7 (C^{sp3, cod}), 20.5 (CH₃^tol). ³¹P{¹H} NMR (CD₂Cl₂, 223 K): δ_P 18.5 (br, 3P, HNP).

Formation of [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(HNP)]⁺ (10⁺). A CD₂Cl₂ solution (0.5 mL) of [IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (28.2 mg, 0.0363 mmol, 777.74 g mol⁻¹) at 223 K was added with a CD₂Cl₂ solution (0.5 mL) of SiMe₃NP (13.2 mg, 0.0363 mmol, 363.51 g mol⁻¹) at 223 K. Soon after, the NMR tube was introduced into a NMR spectrometer at 223 K. The formation of [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(HNP)] (10⁺) was observed over the following 5 h. ¹H NMR (CD₂Cl₂, 223 K): δ_H 8.10 (br, 2H, o-P¹Ph), 7.81–7.24 (17H tot; 4H, o-P²Ph; 4H, m-P²Ph, 2H, p-P²Ph, 4H, m-P¹Ph, 2H, p-P¹Ph; 1H, (d, δ 7.42), ³J_HH = 8.3 Hz, C⁶H^tol-P1), 7.19 (br, 1H, o-P¹Ph), 7.07 (d, 1H, ³J_HH = 8.3 Hz, C⁵H^tol-P1), 6.60 (d, 2H, ³J_HH = 8.1 Hz, C³H^tol-P2), 6.55 (br, 1H, o-P¹Ph), 6.35 (br, 1H, C³H^tol-P1), 5.77 (d, 2H, ³J_HH = 8.1 Hz, C²H^tol-P2), 5.22(br, 1H, C^sp2H^{cod up}), 4.81 (d, 1H, ²J_HP = 17.0, HNP), 4.13 (3H tot; 1H, C^sp2H^{cod up}; 2H, C^sp2H^{cod down}), 2.87 (m, 2H, C^sp3H^cod), 2.52 (m, 3H, C^sp3H^cod), 2.29 (s, 3H, CH₃^tol-P1), 2.03 (s, 3H, CH₃^tol-P2), 1.87 (m, 3H, C^sp3H^cod), 0.25 (br, 9H, SiCH₃), −10.18 (dd, 1H, ²J_HPtrans = 105.5 Hz, ²J_HPcis = 12.4 Hz, IrH). ¹³C{¹H} NMR (CD₂Cl₂, 223 K): δ_C 154.4 (d, ²J_CP = 16.0 Hz, C^{1, tol-P1}), 138.9 (d, ²J_CP = 10.6, C^{1, tol-P2}), 138.45 (d, ¹J_CP = 70.8 Hz, C^{1, PhP1}), 138.41 (d, ¹J_CP = 72.1 Hz, C^{1, PhP1}), 136.0 (C^{3I, tol-P1}), 132.3 (C^{3, PhP1}), 132.1 (C^{3, PhP1}), 130.7 (C^{2, PhP1}), 130.4 (C^{3, PhP2}), 130.24 (C^{2, PhP1}), 130.17 (C^{4, tol-P1}), 129.6 (C^{2, PhP1}), 129.2 (C^{3, tol-P2}), 128.7 (C^{4, tol-P2}), 128.0 (C^{2, PhP2}), 127.0 (C^{3, tol-P1}), 124.4 (dd, ²J_CP = 13.4 Hz, ²J_CP = 10.3 Hz, C^{2I, tol-P1}), 117.9 (d, ²J_CP = 5.0 Hz, C^{2, tol-P2}), 116.5 (d, ²J_CP = 16.0 Hz, C^{2, tol-P1}), 96.2 (d, ²J_CP = 8.6 Hz, C^{sp2, cod up}), 93.0 (C^{sp2, cod down}), 91.9 (C^{sp2, cod down}), 89.5 (C^{sp2, cod up}), 37.4 (C^{sp3, cod}), 31.6 (d, ³J_CP = 34.3 Hz, C^{sp3, cod}), 28.4 (C^{sp3, cod}), 20.6 (CH₃^tol-P1), 20.4 (CH₃^tol-P2), 2.6 (SiCH₃). ³¹P{¹H} NMR (CD₂Cl₂, 223 K): δ_P 66.6 (d, 1P, ²J_PP = 22.2 Hz, SiNP¹), 1.2 (d, 1P, ²J_PP = 22.2 Hz, HNP²).

Synthesis of [Ir(SiMe₃NP)₂(CH₃CN)₂][PF₆] (11PF₆). A dichloromethane suspension (5 mL) of [Ir(CH₃CN)₂(coe)₂][PF₆] (136 mg, 0.212 mmol, 639.68 g mol⁻¹) was added with SiMe₃NP (154 mg, 0.424 mmol, 363.51 g mol⁻¹). The resulting orange solution was stirred for 30 min, evaporated up to approx. 2 mL and added with hexane (5 mL), affording an orange solid which was filtered off, washed with diethyl ether (3 × 5 mL), dried in vacuo and finally identified as [Ir(SiMe₃NP)₂(CH₃CN)₂][PF₆] (149 mg, 61% yield). Found: C 49.95, H 5.19, N 4.78. Calcd for C₄₈H₅₈F₆IrN₄P₃Si₂ (1146.30 g mol⁻¹): C 50.29, H 5.10, N 4.89. ¹H NMR (CD₂Cl₂, 298 K): δ_H 7.55 (m, 8H, o-PPh), 7.47–7.40 (12H tot; 8H, m-PPh; 4H, p-PPh), 7.01 (d, 4H, ³J_HH = 8.4 Hz, C²H^tol), 6.97 (d, 4H, ³J_HH = 8.4 Hz, C³H^tol), 2.30 (s, 6H, CH₃^tol), 1.42 (s, 6H, CH₃CN), 0.03 (s, 18H, Si(CH₃)₃). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 142.7 (C^{1, tol}), 136.6 (C^{4, tol}), 135.0 (t, ¹J_CP = 26.5 Hz, C^{1, PhP}), 133.2 (t, ²J_CP = 6.5 Hz, C^{2, PhP}), 132.9 (C^{3, tol}), 130.8 (C^{4, PhP}), 129.8 (C^{2, tol}), 128.6 (t, ³J_CP = 4.8 Hz, C^{3, PhP}), 115.7 (CH₃C̲N), 21.5 (CH₃^tol), 4.2 (C̲H₃CN), 2.7 (SiCH₃). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 61.8 (s, 2P, NP), −144.7 (hept, ¹J_PF = 710.1 Hz, PF₆⁻).

Synthesis of OC-6-34-[IrH{κ³C,P-HN(4-C₆H₃CH₃)PPh₂}(CH₃CN)(HNP)₂][PF₆] (12PF₆). An acetonitrile solution (5 mL) of [Ir(CH₃CN)₂(coe)₂][PF₆] (108 mg, 0.169 mmol, 639.68 g mol⁻¹) was added with HNP (148 mg, 0.508 mmol, 291.33 g mol⁻¹) yielding a yellow solution which was stirred for 2 h, evaporated up to 2 mL and added with diethyl ether (5 mL). The resulting suspension was filtered and the yellow solid was washed with diethyl ether (3 × 5 mL), dried in vacuo and finally identified as OC-6-34-[IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(CH₃CN)(HNP)₂][PF₆] (125 mg, 59% yield). Found: C 57.01, H 4.88, N 4.35. Calcd for C₅₉H₅₇F₆IrN₄P₄ (1252.21 g mol⁻¹): C 56.59, H 4.59, N 4.47. ¹H NMR (CD₃CN, 298 K): δ_H 7.90 (m, 2H, o-P¹Ph), 7.79 (td, 2H, ³J_HH = 8.0 Hz, ⁴J_HP = 2.2 Hz, m-P¹Ph), 7.71–7.57 (4H tot; 2H, o-P³Ph; 2H, m-P¹Ph;), 7.57–7.49 (4H tot; 2H, m-P³Ph, 2H, p-P¹Ph), 7.49–7.30 (4H tot; 2H, o-P³Ph, 2H, m-P²Ph), 7.32–7.11 (8H tot; 2H, o-P¹Ph; 2H, m-P³Ph; 2H, m-P²Ph; 2H, p-P²Ph), 7.11–6.93 (4H tot; 2H, o-P²Ph; 2H, p-P³Ph), 6.88 (dd, 1H, ³J_HH = 7.8 Hz, ⁴J_HP = 2.3 Hz, C⁶H^tol-P1), 6.83–6.72 (3H tot; 2H, o-P2Ph; 1H, C⁵H^tol-P1), 6.69 (d, 2H, ³J_HH = 8.2 Hz, C³H^tol-P2), 6.66 (d, 2H, ³J_HH = 8.2 Hz, C³H^tol-P3), 6.38 (d, 1H, ⁴J_HP = 5.2 Hz, C³H^tol-P1), 6.28 (br, 1H, HNP¹), 5.89 (d, 2H, ³J_HH = 8.2 Hz, C²H^tol-P2), 5.67 (d, 2H, ³J_HH = 8.2 Hz, C²H^tol-P3), 5.17 (d, 1H, ²J_HP = 19.6 Hz, HNP²), 4.64 (d, 1H, ²J_HP = 18.5 Hz, HNP³), 2.11 (s, 3H, CH₃^tol-P3), 2.10 (s, 3H, CH₃^tol-P1), 2.07 (s, 3H, CH₃^tol-P2), 1.64 (s, 3H, CH₃CN), −11.83 (dt, 1H, ²J_HP = 150.5 Hz, ²J_HP = 16.2 Hz). ¹³C{¹H} NMR (CD₃CN, 298 K): δ 149.8 (dd, ²J_CP = 14.3 Hz, ⁴J_CP = 2.1 Hz, C^{1, tol-P1}), 140.5 (d, ²J_CP = 11.0 Hz, C^{1, tol-P3}), 140.2 (d, ²J_CP = 11.1 Hz, C^{1, tol-P2}), 137.9 (dd, ³J_CP = 2.6 Hz, ⁵J_CP = 0.9 Hz, C^{3, tol-P1}), 133.7 (d, ³J_CP = 2.7 Hz C^{3, PhP1}), 133.6 (d, ²J_CP = 9.9 Hz, C^{2, PhP2}), 133.4 (dd, ²J_CP = 11.2 Hz, ⁴J_CP = 0.9 Hz, C^{2, PhP3}), 133.3 (d, ²J_CP = 10.7 Hz, C^{2, PhP1}), 132.6 (d, ⁴J_CP = 2.6 Hz C^{4, PhP2}), 132.53–131.27 (C^{2, PhP1}, C^{2, PhP3}, C^{2, PhP2}, C^{4, PhP1}, C^{4, PhP3}), 132.1 (d, ²J_CP = 2.0 Hz, C^{2, PhP3}), 131.7 (C^{4, tol-P3}), 131.5 (d, ⁴J_CP = 2.3 Hz, C^{4, PhP3}), 131.3 (C^{4, tol-P2}), 131.0 (d, ³J_CP = 9.3 Hz, C^{3, PhP2}), 130.8 (d, ³J_CP = 11.3 Hz, C^{3, PhP1}), 130.47 (C^{4, PhP2}), 130.44 (C^{3, tol-P2}), 130.34 (C^{4, PhP1}), 130.2(C^{3, tol-P3}), 129.80 (d ³J_CP = 11.5 Hz, C^{3, PhP3}), 129.75 (C^{4, tol-P1}), 129.6 (d, ³J_CP = 10.6 Hz, C^{3, PhP2}), 128.6 (d, ⁵J_CP = 9.8 Hz, C^{4, PhP3}), 127.0 (d, ⁴J_CP = 1.5 Hz, C^{3, tol-P1}), 126.0 (d, ³J_CP = 4.9 Hz, C^{2, tol-P1}), 121.1 (CH₃C̲N), 119.6 (d, ³J_CP = 5.3 Hz, C^{2, tol-P3}), 119.0 (d, ³J_CP = 5.4 Hz, C^{2, tol-P2}), 112.2 (d, ³J_CP = 14.0 Hz, C^{6, tol-P1}), 21.4 (CH₃^tol-P1), 20.8 (CH₃^tol-P3, CH₃^tol-P2), 3.8 (C̲H₃CN). ³¹P{¹H} NMR (CD₃CN, 298 K): δ_P 49.8 (t, 1P, ²J_CP = 11.5 Hz, NP¹), 19.8 (dd, 1P, ²J_CP = 16.0 Hz, ²J_CP = 11.5 Hz, NP²), 11.2 (dd, 1P, ²J_CP = 16.0 Hz, ²J_CP = 11.5 Hz, NP₃), −144.7 (hept, ¹J_PF = 710.1 Hz, PF₆⁻).

Synthesis of OC-6-52-[IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(CH₃CN)(HNP)₂][PF₆] (13PF₆). Method (1): A dichloromethane suspension (5 mL) of [Ir(CH₃CN)₂(coe)₂][PF₆] (116 mg, 0.181 mmol, 639.68 g mol⁻¹) was added with HNP (158 mg, 0.542 mmol, 291.33 g mol⁻¹) giving a yellow solution, which was stirred for 36 h, evaporated up to 2 mL and added with hexane (10 mL). The resulting suspension was filtered and the yellow solid washed with diethyl ether (5 mL) and hexane (5 mL), dried in vacuo and finally identified as OC-6-52-[IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(CH₃CN)(HNP)₂][PF₆] (132 mg, 58% yield). Found: C 57.01, H 4.75, N 4.92. Calcd for C₅₉H₅₇F₆IrN₄P₄ (1252.21 g mol⁻¹): C 56.59, H 4.59, N 4.47. ¹H NMR (CD₂Cl₂, 298 K): δ_H 7.47–7.21 (22H tot; 4H, o-P¹Ph; 4H, o-P^2/3Ph; 4H, m-P¹Ph; 4H, m-P^2/3Ph; 2H, p-P¹Ph; 4H, p-P^2/3Ph), 7.14 (tt, 4H, ³J_HH = 7.6 Hz, ⁴J_HP = 1.7 Hz, m-P^2/3Ph), 6.99–6.86 (5H tot; 4H, o-P^2/3Ph 1H, C⁶H^tol-P1), 6.78 (d, 1H, ³J_HH = 8.0 Hz, C⁵H^tol-P1), 6.66 (d, ³J_HH = 8.0 Hz, 4H, C³H^tol-P2/3), 6.23 (s, 1H, C³H^tol-P1), 5.95 (d, ³J_HH = 8.0 Hz, 4H, C²H^tol-P2/3), 5.57 (d, 1H, ²J_HP = 2.2 Hz, HNP¹), 5.18 (t, ²J_HP = 8.3 Hz, 2H HNP^2/3), 2.08 (s, 6H CH₃^tol-P2/3), 1.95 (s, 3H, CH₃^tol-P1), 1.56 (s, 3H, CH₃CN), −9.86 (dt, 1H, ²J_HP = 133.0 Hz, ²J_HP = 18.6 Hz). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_H 151.8 (C^{1, tol-P1}), 145.0 (C^{3, tol-P1}), 139.8 (t, ³J_CP = 3.1 Hz, C^{1, tol-P2/3}), 139.1 (C^{1, PhP}), 132.5 (dd, ²J_CP = 11.4 Hz, ⁴J_CP = 5.7 Hz, C^{2, PhP2/3}), 131.1 (dd, ²J_CP = 6.4 Hz, ⁴J_CP = 3.5 Hz C^{2, PhP1}), 130.9 (C^{4, tol-P2/3}), 131.2 (C^{4, PhP1}), 129.9 (C^{5, tol-P1}), 129.8 (C^{4, PhP2/3}), 129.6 (C^{2, tol-P2/3}), 129.0 (dd, ⁴J_CP = 5.9 Hz, ⁶J_CP = 4.8 Hz, C^{3, PhP2/3}), 128.5 (dd, ⁴J_CP = 5.8 Hz, ⁶J_CP = 4.6 Hz, C^{3, PhP1}), 126.1 (C^{4, tol-P1}), 120.8 (CH₃C̲N), 118.8 (C^{3, tol-P2/3}), 113.5 (dd, ²J_CP = 10.0 Hz, ²J_CP = 1.6 Hz, C^{2, tol-P1}), 111.7 (d, ³J_CP = 13.4 Hz, C^{6, tol-P1}), 21.3 (CH₃^tol-P1), 20.8 (CH₃^tol-P2/3), 2.5 (C̲H₃CN). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 43.4 (t, 1P, ²J_PP = 16.0 Hz, NP¹), 19.7 (d, 2P, ²J_PP = 16.0 Hz, NP^2/3), −144.7 (hept, ¹J_PF = 710.1 Hz, PF₆⁻).

Method (2): A CD₂Cl₂ solution (0.4 mL) of 12PF₆ (14.8 mg, 0.0118 mmol, 1252.21 g mol⁻¹) was allowed to stand at room temperature for 12 h in a standard 5 mm NMR tube. ¹H and ³¹P{¹H} NMR indicated the clean and complete conversion of OC-6-34-[IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)]⁺ to OC-6-52-[IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)]⁺.

General procedure for the catalytic dehydrogenation of formic acid

The catalytic tests were carried out on a Man on the Moon kit (Series X102, https://www.manonthemoontech.com/x102-gas-evolu-tion.html), employing a reactor with a total volume of 19 mL. A DMF solution (1.7 mL) of formic acid (20 μL, 0.529 mmol) and sodium formate or trimethylamine (10 mol%, 0.0529 mmol; 50 mol%, 0.264 mmol) was introduced into the reactor and placed in a thermostated oil bath at 353 K. After 10 min approx., a solution of the catalyst (1 mol%, 0.00529 mmol; 5 mol%, 0.0264 mmol) in 0.3 mL of DMF was added and the recording of the curve pressure vs. time started (cf.Table 1).

DFT calculations

Molecular structure optimizations and frequencies calculations were carried out with the Gaussian09 program (revision D.01)¹⁷ using the method B3PW91,¹⁸ including the D3 dispersion correction scheme by Grimme with Becke–Johnson damping.¹⁹ The def2-SVP²⁰ basis and pseudo potential were used for all atoms and the “ultrafine” grid was employed in all calculations. Stationary points were characterized by vibrational analysis. The structures were optimized in dichloromethane (298 K, 1 atm) using the PCM method.²¹ In order to improve the accuracy of the calculated energies, in selected cases, single point energy calculations were carried out on the optimized structures of intermediates and transition states using the method M06,²² the def2-TZVP²⁰ basis and pseudo potentials, where appropriate, and the SMD model²³ for the solvent (dichloromethane). Delocalization indexes (DI) were calculated using Multiwfn.²⁴

Crystal structure determination

Single crystals of SiMe₃NP, 1, 2, and 6PF₆ were obtained by slow evaporation of dichloromethane solutions of the compounds; single crystals of 12PF₆ were grown by slow diffusion of diethyl ether into a CH₃CN solution of the compound. X-ray diffraction data were collected at 100(2) K on a SMART APEX (1), APEX DUO (2, 6PF₆) or D8 VENTURE (SiMe₃NP, 12PF₆) Bruker diffractometers with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) using ω-scans (and φ-scans for SiMe₃NP and 12PF₆). Intensities were integrated and corrected for absorption effects with SAINT-PLUS²⁵ and SADABS²⁶ programs, both included in APEX4 package. The structures of 1, 6PF₆, and 12PF₆ were solved by the Patterson method with SHELXS-97 ²⁷ and refined by full matrix least-squares on F² with SHELXL-2014 ²⁸ under WinGX.²⁹

The structure of 2 was solved with the ShelXS-2013 ³⁰ solution program using direct methods and by using Olex2 1.5-dev³¹ as the graphical interface. The model was refined with olex2.refine 1.5-dev³² using full matrix least squares minimisation on F₂. Eventually the crystal structure of 2 was refined using NoSpherA2, an implementation of NOn-SPHERical Atom-form-factors in Olex2.³³ NoSpherA2 implementation of Hirshfeld atom refinement makes use of tailor-made aspherical atomic form factors calculated on-the-fly from a Hirshfeld-partitioned electron density (ED), not from spherical-atom form factors. The ED is calculated from a gaussian basis set single determinant SCF wavefunction – either Hartree–Fock or DFT using selected funtionals – for a fragment of the crystal. This fragment can be embedded in an electrostatic crystal field by employing cluster charges or modelled using implicit solvation models, depending on the software used. The following options were used: software, Orca 5.0; partitioning, NoSpherA2; int. accuracy, normal; method, r2scan; basis set: x2c-tzvp; charge: 0; multiplicity: 1; relativistic: dkh2.

Crystal data and structure refinement for SiMe₃N(4-tolyl)PPh₂ (SiMe₃NP)

C₂₂H₂₆NPSi, 363.50 g mol⁻¹, triclinic, P1̄, a = 8.6970(3) Å, b = 11.2407(4) Å, c = 11.9717(4) Å, α = 117.9100(10)°, β = 97.2470(10)°, γ = 95.1980(10)°, V = 1010.90(6) Å, Z = 2, D_calc = 1.194 g cm³, μ = 0.200 mm⁻¹, F(000) = 388, colourless prism, 0.190 × 0.180 × 0.120 mm, θ_min/θ_max 2.053/28.276°, −11 ≤ h ≤ 11, −14 ≤ k ≤ 13, −15 ≤ l ≤ 15, reflections collected/independent 39 891/4998 [R(int) = 0.0300], T_max/T_min 0.7457/0.7210, data/restraints/parameters 4998/0/230, GooF(F²) 1.051, R₁ = 0.0327 [I > 2σ(I)], wR₂ = 0.0876 (all data), largest diff. peak/hole 0.368/−0.261 e Å⁻³. CCDC deposit number 2178754.†

Crystal data and structure refinement for [IrCl(HNP)(cod)] (1)

C₂₇H₃₀ClIrNP, 627.14 g mol⁻¹, monoclinic, P2₁/c, a = 18.6429(17) Å, b = 10.8746(10) Å, c = 12.3111(11) Å, β = 104.9190(10)°, V = 2411.7(4) ų, Z = 4, D_calc = 1.727 g cm⁻³, μ = 5.728 mm⁻¹, F(000) = 1232, orange prism, 0.250 × 0.180 × 0.160 mm, θ_min/θ_max 2.188/28.755°, −24 ≤ h ≤ 25, −14≤k ≤ 14, −16≤l ≤ 16, reflections collected/independent 33 669/5870 [R(int) = 0.0444], T_max/T_min 0.7458/0.6264, data/restraints/parameters 5870/1/285, GooF(F²) 1.028, R₁ = 0.0270 [I > 2σ(I)], wR₂ = 0.0604 (all data), largest diff. peak/hole 1.495/−0.949 e Å⁻³. CCDC deposit number 2178757.†

Crystal data and structure refinement for [IrCl(SiMe₃NP)(cod)] (2)

C₃₀H₃₈ClIrNPSi, 699.37 g mol⁻¹, triclinic, P1̄, a = 9.4160(10) Å, b = 10.5553(11) Å, c = 14.9182(16) Å, α = 86.0960(10)°, β = 74.7700(10)°, γ = 76.9410(10)°, V = 1393.6(3) ų, Z = 2, D_calc = 1.667 g cm⁻³, μ = 5.007 mm⁻¹, F(000) = 696, orange prism, 0.300 × 0.190 × 0.180 mm, θ_min/θ_max 1.41/28.37°, −11 ≤ h ≤ 12, −14 ≤ k ≤ 14, 0 ≤ l≤19, reflections collected/independent 18 124/6925 [R(int) = 0.0158], T_min/T_max 0.3302/0.2449, data/restraints/parameters 6925/21/658, GooF(F²) 1.064, R₁ = 0.0115 [I > 2σ(I)], wR₂ = 0.0261 (all data), largest diff. peak/hole 1.092/−0.443 e Å⁻³. CCDC deposit number 2178753.†

Crystal data and structure refinement for [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (6PF₆)

C₃₂H₄₁F₆IrN₂P₂Si, 849.90 g mol⁻¹, triclinic, P1̄, a = 15.413(5) Å, b = 16.528(6) Å, c = 28.517(10) Å, α = 106.793(6)°, β = 94.609(7)°, γ = 102.683(8)°, V = 6704(4) ų, Z = 8, D_calc = 1.684 g cm⁻³, μ = 4.173 mm⁻¹, F(000) = 3376, yellow prism, 0.210 × 0.150 × 0.050 mm, θ_min/θ_max 0.755/26.372°, −19 ≤ h ≤ 19, −20 ≤ k ≤ 20, −35 ≤ l ≤ 35, reflections collected/independent 121 535/27 407 [R(int) = 0.0464], T_max/T_min 0.5959/0.4686, data/restraints/parameters 27 407/4/1621, GooF(F²) 1.018, R₁ = 0.0309 [I > 2σ(I)], wR₂ = 0.0695 (all data), largest diff. peak/hole 2.061/−0.848 e Å⁻³. CCDC deposit number 2178755.†

Crystal data and structure refinement for [IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)][PF₆] (12PF₆)

2[C₅₉H₅₇IrN₄P₃][PF₆]·3 CH₃CN·C₄H₁₀O, 2701.61 g mol⁻¹, monoclinic, P2₁/c, a = 24.8378(10) Å, b = 23.4102(8) Å, c = 20.6295(8) Å, β = 91.556(2)°, V = 11 990.8(8) ų, Z = 4, D_calc = 1.497 g cm⁻³, μ = 2.399 mm⁻¹, F(000) = 5472, colourless prism, 0.190 × 0.040 × 0.020 mm³, θ_min/θ_max 1.975/25.681°, index ranges −30 ≤ h ≤ 30, −28 ≤ k ≤ 28, −25 ≤ l ≤ 25, reflections collected/independent 337 915/22 758 [R(int) = 0.0705], T_max/T_min 0.8621/0.7473, data/restraints/parameters 22 758/10/1504, GooF(F²) 1.056, R₁ = 0.0339 [I > 2σ(I)], wR₂ = 0.0860 (all data), largest diff. peak/hole 3.415/−0.872 e Å⁻³. CCDC deposit number 2178756.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the Spanish Ministerio de Ciencia e Innovación MCIN/AEI/10.13039/501100011033, under the project PID2019-103965GB-I00, and the Departamento de Ciencia, Universidad y Sociedad del Conocimiento del Gobierno de Aragón (group E42_20R) is gratefully acknowledged. Authors are grateful for the reviewer's insightful comments on the refinement of the crystal structure of 2 using non-spherical form factors.

References

1. (a) G. Ewart, D. S. Payne, A. L. Porte and A. P. Lane, J. Chem. Soc., 1962, 3984; (b) H. H. Sisler and N. L. Smith, J. Org. Chem., 1961, 26, 611; (c) W. A. Hart and H. H. Sisler, Inorg. Chem., 1964, 3, 617–622.
2. (a) V. Passarelli, J. J. Pérez-Torrente and L. A. Oro, Dalton Trans., 2016, 45, 951–962; (b) J. Jansa, T. Řezníček, L. Dostál, Z. Růžičková, F. Bureš and R. Jambor, Appl. Organomet. Chem., 2016, 30, 1036–1042; (c) S. Pavlik, K. Mereiter, R. Schmid and K. Kirchner, Organometallics, 2003, 22, 1771–1774; (d) A. D. Burrows, M. F. Mahon and M. T. Palmer, J. Chem. Soc., Dalton Trans., 2000, 1669–1677; (e) L. Melounková, M. Syková, R. Jirásko, R. Jambor, R. Havelek, E. Peterová, J. Honzíček and J. Vinklárek, New J. Chem., 2021, 45, 19506–19514; (f) P. Chen, Y. Li, Z. Chen, W. Du and Y. Chen, Angew. Chem., Int. Ed., 2020, 59, 7083–7088; (g) K. G. Gaw, M. B. Smith, J. B. Wright, A. M. Z. Slawin, S. J. Coles, M. B. Hursthouse and G. J. Tizzard, J. Organomet. Chem., 2012, 699, 39–47; (h) M. Aydemir, A. Baysal, N. Gürbüz, I. Özdemir, B. Gümgüm, S. Özkar, N. Çaylak and L. T. Yildirim, Appl. Organomet. Chem., 2010, 24, 17–24; (i) S. Priya, M. S. Balakrishna and J. T. Mague, J. Organomet. Chem., 2003, 679, 116–124.
3. (a) Y. Chen, D. Song, J. Li, X. Hu, X. Bi, T. Jiang and Z. Hou, ChemCatChem, 2018, 10, 159–164; (b) H. Lin, Y. Li, J. Wang, M. Zhang, T. Jiang, J. Li and Y. Chen, Appl. Organomet. Chem., 2021, 35, e6345,  DOI:10.1002/aoc.6345.
4. Selected references are: (a) B. S. Mitchell, W. Kaminsky and A. Velian, Inorg. Chem., 2021, 60, 6135–6139; (b) A. Aloisi, É. Crochet, E. Nicolas, J.-C. Berthet, C. Lescot, P. Thuéry and T. Cantat, Organometallics, 2021, 40, 2064–2069; (c) J. A. Kephart, A. C. Boggiano, W. Kaminsky and A. Velian, Dalton Trans., 2020, 49, 16464–16473; (d) H. Zhang, G. P. Hatzis, C. E. Moore, D. A. Dickie, M. W. Bezpalko, B. M. Foxman and C. M. Thomas, J. Am. Chem. Soc., 2019, 141, 9516–9520; (e) J. A. Kephart, B. S. Mitchell, A. Chirila, K. J. Anderton, D. Rogers, W. Kaminsky and A. Velian, J. Am. Chem. Soc., 2019, 141, 19605–19610; (f) K. M. Gramigna, D. A. Dickie, B. M. Foxman and C. M. Thomas, ACS Catal., 2019, 9, 3153–3164; (g) M. L. Bin Ismail and C.-W. So, Chem. Commun., 2019, 55, 2074–2077; (h) B. A. Barden, G. Culcu, J. P. Krogman, M. W. Bezpalko, G. P. Hatzis, D. A. Dickie, B. M. Foxman and C. M. Thomas, Inorg. Chem., 2019, 58, 821–833; (i) A. J. Ayres, A. J. Wooles, M. Zegke, F. Tuna and S. T. Liddle, Inorg. Chem., 2019, 58, 13077–13089.
5. (a) K. A. Grice, W. Kaminsky and K. I. Goldberg, Inorg. Chim. Acta, 2011, 369, 76–81; (b) D. Wang, M. Li, X. Chen, M. Wang, Y. Liang, Y. Zhao, K. N. Houk and Z. Shi, Angew. Chem., Int. Ed., 2021, 60, 7066–7071; (c) K. G. Gaw, A. M. Z. Slawin and M. B. Smith, Organometallics, 1999, 18, 3255–3257.
6. Selected references are: (a) A. Luque-Gómez, S. García-Abellán, J. Munarriz, V. Polo, V. Passarelli and M. Iglesias, Inorg. Chem., 2021, 60, 15497–15508; (b) N. Onishi, R. Kanega, E. Fujita and Y. Himeda, Adv. Synth. Catal., 2019, 361, 289–296; (c) S.-M. Lu, Z. Wang, J. Wang, J. Li and C. Li, Green Chem., 2018, 20, 1835–1840; (d) S. Cohen, V. Borin, I. Schapiro, S. Musa, S. De-Botton, N. V. Belkova and D. Gelman, ACS Catal., 2017, 7, 8139–8146; (e) S. Siek, D. B. Burks, D. L. Gerlach, G. Liang, J. M. Tesh, C. R. Thompson, F. Qu, J. E. Shankwitz, R. M. Vasquez, N. Chambers, G. J. Szulczewski, D. B. Grotjahn, C. E. Webster and E. T. Papish, Organometallics, 2017, 36, 1091–1106; (f) C. Fink and G. Laurenczy, Dalton Trans., 2017, 46, 1670–1676; (g) J. Li, J. Li, D. Zhang and C. Liu, ACS Catal., 2016, 6, 4746–4754; (h) J. J. A. Celaje, Z. Lu, E. A. Kedzie, N. J. Terrile, J. N. Lo and T. J. Williams, Nat. Commun., 2016, 7, 11308; (i) J. H. Barnard, C. Wang, N. G. Berry and J. Xiao, Chem. Sci., 2013, 4, 1234–1244; (j) J. F. Hull, Y. Himeda, W.-H. Wang, B. Hashiguchi, R. Periana, D. J. Szalda, J. T. Muckerman and E. Fujita, Nat. Chem., 2012, 4, 383–388; (k) Y. Himeda, Green Chem., 2009, 11, 2018–2022.
7. M. Iglesias and L. A. Oro, Eur. J. Inorg. Chem., 2018, 2125–2138.
8. V. Passarelli and F. Benetollo, Inorg. Chem., 2011, 50, 9958–9967.
9. M. Palmese, V. Passarelli and J. J. Pérez-Torrente, Dalton Trans., 2022, 51, 7142–7153.
10. M. Brookhart, M. L. H. Green and G. Parkin, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 6908–6914.
11. Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005, the Red Book, compiled by N. G. Connelly, R. M. Hartshorn, T. Damhus and A. T. Hutton, Published for the International Union of Pure and Applied Chemistry by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 0WF, UK ISBN 0-85404-438-8.
12. V. Passarelli, J. J. Pérez-Torrente and L. A. Oro, Inorg. Chem., 2014, 53, 972–980.
13. E. Matito, J. Poater, M. Solá, M. Duran and P. Salvador, J. Phys. Chem. A, 2005, 109, 9904–9910.
14. J. L. Herde, J. C. Lambert and C. V. Senoff, Inorg. Synth., 1974, 15, 18–20.
15. V. W. Day, W. G. Klemperer and D. J. Main, Inorg. Chem., 1990, 29, 2345–2355.
16. R. Dorta, R. Goikhman and D. Milstein, Organometallics, 2003, 22, 2806–2809.
17. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, C. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2009.
18. J. P. Perdew, in Electronic Structure of Solids ‘91, ed. P. Ziesche and H. Eschrig, Akademie Verlag, Berlin, 1991.
19. S. Grimme, S. Ehrlich and L. Goerigk, J. Comput. Chem., 2011, 32, 1456–1465.
20. F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 7, 3297–3305.
21. J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005, 105, 2999–3093.
22. Y. Zhao and D. G. Truhlar, J. Chem. Phys., 2006, 125, 194101.
23. A. V. Marenich, C. J. Cramer and D. G. Truhlar, J. Phys. Chem. B, 2009, 113, 6378–6396.
24. T. Lu and F. Chen, J. Comput. Chem., 2012, 33, 580–592.
25. SAINT+: Area-Detector Integration Software, version 6.01, Bruker AXS, Madison, WI, 2001.
26. G. M. Sheldrick, SADABS program, University of Göttingen, Göttingen, Germany, 1999.
27. G. M. Sheldrick, SHELXS 97, Program for the Solution of Crystal Structure, University of Göttingen, Göttingen, Germany, 1997.
28. G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3–8.
29. L. J. Farrugia, J. Appl. Crystallogr., 2012, 45, 849–854.
30. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 339–341.
31. O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.
32. L. J. Bourhis, O. V. Dolomanov, R. J. Gildea, J. A. K. Howard and H. Puschmann, Acta Crystallogr., Sect. A: Found. Adv., 2015, 71, 59–71.
33. F. Kleemiss, O. V. Dolomanov, M. Bodensteiner, N. Peyerimhoff, M. Midgley, L. J. Bourhis, A. Genoni, L. A. Malaspina, D. Jayatilaka, J. L. Spencer, F. White, B. Grundkoetter-Stock, S. Steinhauer, D. Lentz, H. Puschmann and S. Grabowsky, Chem. Sci., 2021, 12, 1675–1692.
34. M. J. Church, M. J. Mays, R. N. F. Simpson and F. P. Stefanini, J. Chem. Soc. (A), 1970, 2909–2914.

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Footnotes

† Electronic supplementary information (ESI) available: Crystal structures, IR and NMR spectra, NMR data, coordinates of calculated structures. CCDC 2178753–2178757. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt02081e

‡ Note that all non-observed compounds (either expected or calculated) are labelled with sequential roman numerals, whereas sequential arabic numerals have been used exclusively for observed/isolated compounds, including isomers.

§ For the sake of comparison, the IR spectrum of the related complex [Ir(CO)₃(PPh₃)₂]⁺ shows one weak band at 2074 cm^–1 (A₁ mode) and two strong bands at 2010 and 2018 cm^–1 (split E mode), cf. ref. 34

¶ A CD₂Cl₂ (0.5 mL) solution of [Ir(CH₃CN)₂(cod)][PF₆] (0.031 mmol) at 233 K was added with a CD₂Cl₂ solution (0.5 mL) of HNP (0.031 mmol) and introduced into an NMR spectrometer at 233 K.

|| As far as the equilibrium 7⁺ ⇄ 8⁺ is concerned, ¹H–¹H EXSY crosspeaks have been observed between the following groups of signals (see Fig. 7 for labelling): (a) 2.01 (Me^II8⁺), 2.10 (Me 7⁺), 2.20 (Me^I8⁺), (b) 5.96 (NH^I8⁺), 5.57 (NH^II8⁺), 4.43 (NH 7⁺); (b) –9.90 (IrH 8⁺), 5.87 (C²H^II8⁺), 7.19 (C⁶H^I8⁺); (c) 4.36 (CH^cod7⁺), 5.29 (C^dH 8⁺), 4.01 (C^bH 8⁺); d) 6.14 (C³H^II8⁺), 6.59 (C³H^I8⁺), 6.72 (C³H 7⁺), 6.96 (C⁵H^I8⁺)].

** A CD₂Cl₂ solution (0.5 mL) of 5PF₆ or 6PF₆ (0.021 mmol) was added with [NBu₄]Cl (0.021 mmol). Soon after mixing, the clean formation of 1 and 2, respectively, was observed (³¹P{¹H} NMR spectroscopy).

†† [Ir(HNP)₃(cod)]⁺ (9⁺) was prepared by reaction of HNP and [Ir(cod)(CH₃CN)₂][PF₆] (Ir : HNP = 1 : 3) at 223 K in CD₂Cl₂ as a thermally unstable compound and was spectroscopically identified in situ in the presence of about 20 mol% of unidentified by-products. At 233 K, its ³¹P{¹H} NMR spectrum shows a broad signal at 18.5 ppm (Δν_1/2 = 26 Hz). Accordingly, one ¹H singlet is observed at 2.19 ppm for the methyl moiety of the tolyl groups and one broad ¹H signal at 3.41 ppm is observed for the cod HC=CH moieties. On this ground, [Ir(HNP)₃(cod)]⁺ is likely to undergo a fluxional process exchanging the three HNP ligands as well as averaging the coordinated cod ligand. In this regard, similar to 3, an equilibrium between square pyramidal (SPY-3-12) and trigonal bipyramidal structures (TBPY-5-12) may account for the observed solution behaviour.

‡‡ The transition state TS_VIII⁺-X⁺ could not be located on the PES. So, the reported activation barrier was estimated (ΔE_act ≈ +16.7 kcal·mol^–1) scanning the C–H coordinate of the carbon–hydrogen bond undergoing oxidative addition to the iridium centre.

§§ The dissociative route was also considered and finally discarded since the tricoordinate intermediate [Ir(HNP)₂(CH₃CN)]⁺ was calculated to lie at +14.5 kcal mol^–1vs.VIII⁺.

¶¶ Actually, the meridional disposition of the hydrido ligand, the phosphorus and carbon (tolyl) atoms within the IrH{κ²C,P-HN(4-C₆H₃CH₃)PPh₂} moiety may only result from a parallel arrangement of the reacting C–H bond and the iridium–phosphorus bond, which is incompatible with the topology of the Ir(HNP) fragment. In other words, the tether of the tolyl group to the phosphorus atom restricts the reacting C–H bond to lying perpendicular to the iridium–phosphorus bond (of the aminophosphano ligand undergoing the CH oxidative addition), which may uniquely affords a mutually cis disposition of the hydrido ligand, and the phosphorus and carbon (tolyl) atoms (cf.TS_XII⁺-12⁺, Fig. 13B).

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