Synthesis and reactivity of an iridium complex based on a tridentate aminophosphano ligand

Abstract

The iridium(III) hydride compound [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆) was obtained by reaction of [Ir(SiNP)(cod)][PF₆] with CN^tBu as the result of the intramolecular oxidative addition of the SiCH₂–H bond to iridium(I) [SiNP = Si(CH₃)₂{N(4-tolyl)PPh₂}₂, SiNP–H = CH₂Si(CH₃){N(4-tolyl)PPh₂}₂]. The mechanism of the reaction was investigated by NMR spectroscopy and DFT calculations showing that the pentacoordinated intermediate [Ir(SiNP)(cod)(CN^tBu)][PF₆] (2PF₆) forms in the first place and that further reacts with CN^tBu, affording the square planar intermediate [Ir(SiNP)(CN^tBu)₂][PF₆] (3PF₆) that finally undergoes the intramolecular oxidative addition of the SiCH₂–H bond. The reactivity of 1PF₆ was investigated. On one hand, the reaction of 1PF₆ with N-chlorosuccinimide or N-bromosuccinimide provides the haloderivatives [IrX{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (X = Cl, 4PF₆; Br, 5PF₆), and the reaction of 5PF₆ with AgPF₆ in the presence of acetonitrile affords the solvato species [Ir{κ³C,P,P′-(SiNP–H)}(CH₃CN)(CN^tBu)₂]²⁺ (6²⁺) isolated as the hexafluorophosphate salt. On the other hand, the reaction of 1PF₆ with HBF₄ gives the iridium(III) compound [IrH(CH₂SiF₂CH₃)(HNP)₂(CN^tBu)₂][BF₄] (7BF₄) as the result of the formal addition of hydrogen fluoride to the Si–N bonds of 1⁺ [HNP = HN(4-tolyl)PPh₂]. A similar outcome was observed in the reaction of 1PF₆ with CF₃COOH rendering 7PO₂F₂. In this case the intermediate [IrH{κ²C,P-CH₂SiMeFN(4-tolyl)PPh₂}(HNP)(CN^tBu)₂]⁺ (8⁺) was observed and characterised in situ by NMR spectroscopy. DFT calculations suggests that the reaction goes through the sequential protonation of the nitrogen atom of the Si–N–P moiety followed by the formal addition of fluoride ion to silicon. Also, the crystal structures of SiNP, 1PF₆, 4PF₆ and 7BF₄ have been determined by X-ray diffraction measurements.

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Introduction

Aminophosphanes are easily accessible ligands and both nitrogen and phosphorus substituents can be easily varied thanks to the ample diversity of commercially available precursors. So far, aminophosphano ligands of general formula NHRPR₂ have been used to prepare mononuclear,¹ di- or oligonuclear species² supported by 1κN,2κP aminophosphanes (Fig. 1, top). Alternatively, bidentate aminophosphano ligands have been employed to prepare mononuclear complexes (Fig. 1, bottom).^3–6 In addition, the aminophosphano functionality has been successfully used to decorate ligating functionalities,⁷ in some cases as elusive as the silylene^7a,i and germylene^7a groups. Remarkably, the applications of these complexes are varied and span catalysis, bond activation, metaloenzyme mimics, drugs, and redox-active multimetallic systems, among others.

Fig. 1 Selected amino-phosphane scaffolds and related metal–ligand structures.

Relevant to this paper, in 2001 Woolins reported⁵ the synthesis of SiMe₂{N(2-pyridyl)PPh₂}₂ (SiN^pyP) and of its palladium and platinum κ²P,P′-derivatives and thereafter we reported on the preparation of SiMe₂{N(4-tolyl)PPh₂}₂ (SiNP) and its rhodium^6a and iridium^6b,c complexes. Notably, beside the expected κ²P,P′ coordination of SiNP, we reported two unprecedented examples^6b,c of a κ³C,P,P′ coordination of SiNP as a result of the intramolecular SiCH₂–H oxidative addition to iridium(I), triggered by π-acceptor ligands such as carbon monoxide or trimethyl phosphite on [Ir(SiNP)(cod)]⁺ (Scheme 1).

Scheme 1 Reactivity of [Ir(SiNP)(cod)]⁺ towards CO or P(OMe)₃.

On this background, aiming at further expand the family of metal complexes containing aminophosphano ligands, we decided to assess the capability of tert-butyl isocyanide, isoelectronic with CO, to promote the intramolecular SiCH₂–H oxidative addition to iridium(I) and eventually explore the reactivity of the resulting complex. So, herein we report on the synthesis of a novel iridium(III) complex of formula [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂]⁺ as well as the detailed theoretical and experimental study of the course of its formation. In addition, a reactivity study was carried out on the above mentioned hydrido derivative, including the hydride-halide exchange and the following halide abstraction as well as protonation reactions rendering the unexpected fragmentation of the aminophosphano ligand.

Results and discussion

Synthesis of [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆)

The reaction of [Ir(SiNP)(cod)][PF₆] with tert-butyl isocyanide (1 : 2 molar ratio) yields the hydrido iridium(III) derivative [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆) (room temperature, 24 h) as a result of the intramolecular SiCH₂–H oxidative addition to the metal centre along with the substitution of the cod ligand with two tert-butyl isocyanide ligands (SiNP–H = CH₂SiMe{N(4-tolyl)PPh₂}₂) (Scheme 2).

Scheme 2 Reaction of [Ir(SiNP)(cod)]⁺ with CN^tBu showing the observed intermediates.

The crystal structure of 1PF₆ was determined by single crystal X-ray diffraction measurements, and Fig. 2-top shows the ORTEP plot of the cation [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂]⁺ (1⁺). For the sake of comparison, the crystal structure of SiNP was also determined (Fig. 2-bottom). The metal centre of 1⁺ exhibits an octahedral environment, the metalated κ³C,P,P′-(SiNP–H) ligand occupying three mutually cis coordination sites [C1–Ir–P2 82.79(9)°, P1–Ir–P2 97.00(3)°, C1–Ir–P1 84.83(9)°]. The hydrido ligand lies cis to P1 and trans to P2 [P1–Ir–H 87.6(16)°, P2–Ir–H 169.2(16)°]. The remaining coordination sites are occupied by two mutually cis isocyanide ligands [C47–Ir–C41 86.18(13)°], one trans to P1 [C47–Ir–P1 169.51(10)°] and the other trans to the metallated carbon atom C1 [C41–Ir–C1 175.56(13)°]. Reasonably as a consequence of the metalation, the C1–Si–C2 angle of 1⁺ [C1–Si–C2 124.65(15)°] is wider than the C1–Si–C2 angle of SiNP [C1–Si–C2 111.48(10)°]. Also, the formation of two fused five member metalacycles in 1⁺ should account for the smaller Si–N–P angles of 1⁺ [P2–N2–Si 114.31(15)°, P1–N1–Si 112.48(15)°] when compared with SiNP [P1–N1–Si 121.40(9)°, P2–N2–Si 120.94(9)°]. In addition, it is also remarkable that the nitrogen atoms N1 and N2 of both 1⁺ and SiNP exhibit a planar geometry, i.e. the fragments N1–Si–P1–C15 and N2–Si–P2–C34 are almost planar in both 1⁺ and SiNP suggesting that a p–d(π) backdonation could imply nitrogen and phosphorus and/or silicon. In this connection, it is worth a mention that the tolyl rings attached to N1 and that attached to N2 lie almost perpendicular to the corresponding N1–Si–P1–C15 and N2–Si–P2–C34 planes in 1⁺ as well as SiNP (1⁺, N1 78.5°; N2 85.3°; SiNP, N1 71.5°, N2 72.2°), which rules out the delocalization of the nitrogen lone pair on the aromatic ring as the cause of the above mentioned planarity of the fragments N1–Si–P1–C15 and N2–Si–P2–C34.

Fig. 2 ORTEP plots of [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂]⁺ (1⁺) (top) and of SiNP (bottom). For clarity, most hydrogen are omitted and the tolyl and phenyl rings are represented in a wireframe style for [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂]⁺. Selected bond lengths (Å) and angles (°) are given in ESI.†

As a confirmation, a QTAIM analysis was carried out on both the crystal and the calculated structures of 1⁺ and SiNP showing that the delocalization index DI (aka fuzzy bond orders, FBO) of the bonds at the nitrogen atoms are 1.08–1.11 (N–C), 1.28–1.32 (N–P) and 1.08–1.15 (N–Si) (Fig. 3) pointing at that some p–d(π) backdonation actually should exist mainly between nitrogen and phosphorus and that therefore it should be responsible for the planar geometry of the nitrogen atoms.

Fig. 3 Selected delocalization indexes (DI) for SiNP and 1⁺ (normal type, values for the X-ray structure; italic type, values for the DFT-calculated structure).

The crystal structure of 1⁺ should be preserved in solution. Indeed, its ³¹P{¹H} NMR spectrum shows two doublets at 40.3 and 34.4 ppm with a ²J_PP coupling constant of 20.4 Hz, in agreement with a cis arrangement of the two phosphorus atoms. Also, two ¹H singlets at 1.37 and 1.20 ppm are indicative of two non-equivalent tert-butyl isocyanide ligands. As for the HIrCH₂Si moiety, one ¹³C doublet of doublets at −29.7 ppm (²J_CP = 2.1, 3.7 Hz) and one ¹H multiplet (vide infra) at −10.58 ppm have been observed. In addition, similar to the related trimethyl phosphito derivative [IrH{κ³C,P,P′-(SiNP–H)}{P(OCH₃)₃}₂]⁺,^6b the coupling pattern (see ⁿJ_XY in Fig. 4) of the IrHCH₂ moiety suggests that the conformation observed in the solid state is maintained in solution.‡

Fig. 4 Selected ¹H and ³¹P NMR data for the IrHCH₂ moiety of 1⁺.

The formation of the hydrido iridium(III) derivative 1⁺ was observed to be stepwise (Scheme 2). As a matter of fact, the formation of 1⁺ was monitored by NMR spectroscopy at −80 °C showing that [Ir(SiNP)(cod)(CN^tBu)]⁺ (2⁺) forms in the first place and further reacts with CN^tBu upon raising the temperature, rendering the square planar intermediate [Ir(SiNP)(CN^tBu)₂]⁺ (3⁺, vide infra) which eventually evolves to 1⁺. As a confirmation, [Ir(SiNP)(cod)(CN^tBu)][PF₆] (2PF₆) could be prepared in high yield upon reacting [Ir(SiNP)(cod)][PF₆] with tert-butyl isocyanide (1 : 1 molar ratio) at −80 °C. Furthermore, the reaction of 2PF₆ with CN^tBu cleanly yielded 1PF₆ through 3PF₆. Remarkably, also in this case, 3⁺ formed along with 1⁺, which indicates that the formation of 3⁺ and its conversion into 1⁺ should exhibit similar activation barriers. 2PF₆ was fully characterised in solution by means of multinuclear NMR spectroscopy. A ³¹P{¹H} singlet is observed at 41.4 ppm along with one ¹H singlet at 2.04 ppm for the two methyl moieties of the tolyl groups, suggesting that the two Si-Ntol-PPh₂ arms of 2⁺ are equivalent. On the contrary, two ¹H singlets at 0.56 and −0.21 ppm are observed for the two SiCH₃ groups of 2⁺, which indicates that they are non-equivalent reasonably as a consequence of the coordination of the isocyanide ligand to iridium in [Ir(SiNP)(cod)]⁺ rendering a distorted square pyramidal geometry at the metal centre (vide infra for the DFT calculated structure). As for the cod ligand, broad ¹H signals are observed even at −60 °C for the olefinic (3.46 ppm) and aliphatic hydrogen atoms (1.76 ppm), respectively, suggesting that even at that temperature the putatively non-equivalent olefinic CH moieties as well as the methylene hydrogen atoms are exchanging and their signals are averaged.

As far as the intermediate 3⁺ is concerned, it could be spectroscopically identified§in situ (¹H, ³¹P NMR). Indeed, a ³¹P{¹H} singlet at 53.7 ppm was assigned to its equivalent phosphorus atoms. Accordingly two equivalent tolyl groups as well as two equivalent ^tBu and two equivalent SiMe groups were observed.¶

DFT calculations nicely underpinned the proposed pathway for the formation of 1PF₆. Fig. 5 shows the simplified Gibbs free energy profile for the reaction Ir(SiNP)(cod) + 2 CNtBu → 1⁺ + cod, including the calculated structure of the detected intermediates 2⁺ and 3⁺ as well as the transition state TS_3⁺-1⁺ of the oxidative addition of SiCH₂–H to iridium. The first step is the exoergonic formation of the distorted square pyramidal complex [Ir(SiNP)(cod)(CN^tBu)]⁺ (2⁺). In the following, [Ir(SiNP)(CN^tBu)₂]⁺ (3⁺) is obtained by reaction of 1⁺ with CN^tBu (ΔG_r = −13.3 kcal mol⁻¹). Remarkably 3⁺ exhibits a boat conformation of the six member ring Ir–P–N–Si–N–P which allows one of the SiCH₃ group, namely the flagpole one, to approach the metal centre (CH⋯Ir 2.683 Å, Fig. 5) and eventually add oxidatively to it (TS_3⁺-1⁺, Fig. 5).

Fig. 5 (left) Relative Gibbs free energy profile (kcal mol⁻¹) of the reaction Ir(SiNP)(cod) + 2 CN^tBu → 1⁺ + cod (M06/def2tzvp//B3PW91-GD3BJ/def2svp, 298 K, 1 atm); (right) view of the calculated structures of 2⁺, 3⁺ and TS_3⁺-1⁺ with selected interatomic distances (Å) and angles (°) (for clarity, most hydrogen atoms are omitted and only the ipso carbon atoms of the phenyl and tolyl moieties are shown).

Reactivity of [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆)

In order to assess the applicability of 1PF₆ as a catalyst in the functionalization of multiple carbon–carbon bonds, a preliminary reactivity study was undertaken. We observed that 1PF₆ does not react either with alkynes – phenylacetylene or 1-hexyne – or alkenes – styrene or 1-hexene – even after prolonged reaction times (up to 48 h) and heating (70 °C in THF). Reasonably the stable κ³C,P,P′ coordination of the SiNP–H ligand along with the substitutional inertness of the CN^tBu ligands hampered the reactivity of 1PF₆. Thus, anticipating that the abstraction of a halido ligand could trigger some reactivity at the Ir{κ³C,P,P′-(SiNP–H)} platform, we decided to exchange the hydrido ligand with a halido ligand. Thus, the iridium haloderivatives of formula [IrX{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (X = Cl, 4PF₆; Br, 5PF₆) were prepared by reaction of 1PF₆ with N-chlorosuccinimide or N-bromosuccinimide (Scheme 3).

Scheme 3 Hydride-halide exchange on [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆).

The crystal structure of 4PF₆ was determined by means of X-ray diffraction measurements and it exhibits an octahedral environment of the metal centre with a κ³C,P,P′-(SiNP–H) ligand along with the chlorido ligand and two tert-butyl isocyanide ligands (Fig. 6). The Ir{κ³C,P,P′-(SiNP–H)} moiety of 4⁺ and 1⁺ are virtually superimposable, and by the same token no significant differences are observed between the isocyanide ligands when comparing 1⁺ and 4⁺.

Fig. 6 ORTEP plot of [IrCl{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂]⁺ (4⁺). For clarity, most hydrogen are omitted and the tolyl and phenyl rings are represented in a wireframe style. Selected bond lengths (Å) and angles (°) are given in ESI.†

The solution structure of 4⁺ and 5⁺ should be similar to that of 4⁺ in the solid state. Indeed, the ³¹P{¹H} NMR spectrum shows two doublets at 28.7 and 26.3 ppm (4⁺), and at 25.7 and 25.1 ppm (5⁺) with a coupling constant indicating a mutually cis disposition of the phosphorus atoms (²J_PP = 18.2 Hz, 4⁺; 17.4 Hz, 5⁺). Also, the ¹H NMR spectra contains two singlets at 1.45 and 1.25 (4⁺), and 1.48 and 1.29 (5⁺) ppm for the tert-butyl isocyanide ligands, two multiplets at 1.30 and 1.21 (4⁺), and 1.37 and 1.24 (5⁺) for the IrCH₂ moiety, and one singlet at 0.15 ppm (4⁺) and 0.17 ppm (5⁺) for the SiCH₃ group.

The bromido ligand of 5⁺ was easily abstracted by reaction with AgPF₆ but a clean product, namely [Ir{κ³C,P,P′-(SiNP–H)}(CH₃CN)(CN^tBu)₂][PF₆]₂ (6[PF₆]₂), could be isolated only in the presence of acetonitrile (Scheme 4), whereas intractable mixtures of products were obtained with styrene or phenylacetylene.

Scheme 4 Bromide abstraction from [IrBr{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂]⁺ (5⁺).

The κ³C,P,P′ coordination of SiNP–H is preserved in 6²⁺ as judged by the ³¹P{¹H} doublets observed at 25.1 and 18.0 ppm (²J_PP = 19.4 Hz) and the ¹H doublet of doublets at 1.44 and 1.30 ppm assigned to the IrCH₂ moiety as well as the ¹H singlet at 0.34 ppm for the SiCH₃ group. The ¹H singlets at 2.09, 1.48 and 1.29 ppm confirm the presence of one CH₃CN and two CN^tBu ligands, respectively. Unfortunately no reaction of 6²⁺ with either styrene or phenylacetylene was observed indicating that neither CH₃CN nor CN^tBu ligands in 6²⁺ are labile.

Protonation of [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆)

On another note, the reaction of 1PF₆ with Brønsted acid was explored envisioning that the hydrido moiety could undergo protonation rendering dihydrogen and eventually an accessible coordination vacant. As a matter of fact, 1⁺ does react with Brønsted acids but with an unexpected outcome (Scheme 5). Indeed the reaction of 1⁺ with HBF₄ (1 : 2 molar ratio) renders the hydrido iridium(III) derivative [IrH(CH₂SiF₂CH₃)(HNP)₂(CN^tBu)₂][BF₄] (7BF₄) [HNP = NH(4-tolyl)PPh₂] as a result of the formal addition of two hydrogen fluoride molecules to the κ³C,P,P′-(SiNP–H) ligand along with the counterion exchange (Scheme 5). In addition, formally PF₅ and BF₃ should also result from the reaction, but unfortunately neither they nor any chemically related species could be identified in the course of the reaction.

Scheme 5 Reaction of [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] with HBF₄.

The crystal structure of 7BF₄ was determined by means of single crystal X-ray diffraction measurements (Fig. 7). The metal centre exhibits an octahedral environment in which the newly formed ligands NH(4-tolyl)PPh₂ and CH₂SiF₂CH₃ are mutually cis, rendering an arrangement at the metal centre which is reminiscent of the κ³C,P,P′ coordination of the SiNP–H ligand of 1⁺ [P(1)–Ir–P(2) 98.52(2)°, C(1)–Ir–P(2) 89.12(7)°, C(1)–Ir–P(1) 92.46(7)°]. The hydrido ligand lies cis to C1 and P1 and trans to P2 and the remaining cis coordination sites are occupied by the tert-butyl isocyanide ligands. Remarkably the N1–H1N group is involved in an intramolecular NH⋯F hydrogen bond to F1 [N1–H1N, 0.940(19); H1N⋯F1 2.11(2); N1⋯F1 3.001(3), N1–H1N–F1 158(3)°]. The crystal structure of 7BF₄ should be preserved in solution. Indeed two ³¹P doublets are observed at 11.4 and 3.1 ppm with a coupling constant ²J_PP of 20.5 Hz pointing at a cis disposition of the two HNP ligands. The ¹H NMR spectrum shows two singlets at 1.25 and 1.21 ppm, assigned to the tert-butyl isocyanide ligands, and a doublet of doublets of triplets at −11.86 ppm for the hydrido ligand as a result of the scalar coupling of the IrH hydrogen to the trans phosphorus (²J_HP = 152.9 Hz), the cis phosphorus (²J_HP = 17.4 Hz), and fluorine (⁴J_HF = 2.2 Hz). As for the CH₂SiF₂CH₃ moiety, two ¹⁹F signals at −127.5 and −129.0 ppm (²J_FF = 20.6 Hz), and two ¹H multiplets at 0.01 and −0.42 ppm for the IrCH₂Si hydrogen atoms are observed (cf. Experimental section), which is indicative of two non-equivalent fluorine atoms and two non-equivalent hydrogen atoms, respectively. Also, while the ¹⁹F{¹H} signal at −129.0 ppm is a doublet, the ¹⁹F{¹H} signal at −127.5 ppm is a doublet of doublets due to the above mentioned fluorine–fluorine coupling and to a fluorine–phosphorus coupling (⁴J_FP = 2.7 Hz, vide infra for this assignment).

Fig. 7 ORTEP plot of [IrH(CH₂SiF₂Me)(HNP)₂(CN^tBu)₂]⁺ (7⁺). For clarity, most hydrogen are omitted and the tolyl and phenyl rings are represented in a wireframe style. Selected bond lengths (Å) and angles (°) are given in ESI.†

Remarkably the NH⋯F hydrogen bond observed in the solid state is maintained in solution. For the sake of clarity, the numbering scheme of the crystal structure given in Fig. 7 will be used in the following discussion of the NMR data. While a ¹H doublet (²J_HP = 15.9 ppm) at 4.21 ppm is observed for the N2–H2N moiety, a ¹H doublet of doublets at 5.33 ppm is observed for the N1–H1N group as a consequence of the scalar coupling of hydrogen to the phosphorus atom P1 (²J_HP = 15.9 Hz) and to the fluorine atom F1 (J_HF = 3.9 Hz). As a confirmation of the NH⋯F hydrogen bond and the consequent hampered rotation around the Ir–CH₂Si bond, NOE cross peaks are observed in the ¹H–¹H NOESY spectrum between H1B (−0.42 ppm) and H1N (5.35 ppm) and between H2N (4.21 ppm) and both H1A (0.01 ppm) and H1B (−0.42 ppm). Finally for the sake of confirmation, selected NMR data were calculated by DFT methods (mPW1PW91/def2TZVP) confirming the proposed assignment.||

Aiming at investigating the influence of the acid – more specifically of its conjugated base – on the outcome of the reaction, 1PF₆ was treated with different Brønsted acids, namely HPF₆ in water (54% w/w) and CF₃COOH. Surprisingly no reaction between 1PF₆ and HPF₆ was observed even after 48 h at room temperature. On the other hand, the reaction of 1PF₆ with CF₃COOH is slower than that with HBF₄ and completeness is reached after 4 days and in the presence of a moderate excess of CF₃COOH (1 : 4) at room temperature, rendering 7⁺ and the anion PO₂F₂⁻. Notably when the reaction was monitored by ³¹P{¹H} NMR spectroscopy, the anion PF₆⁻ is quantitatively converted into PO₂F₂⁻ (δ_F = −84.1, δ_P = −19.5, ¹J_PF = 957 Hz) after 24 h. As a confirmation, the reaction of NBu₄PF₆ with CF₃COOH (1 : 4 molar ratio, in CD₂Cl₂) has a similar outcome cleanly affording PO₂F₂⁻. On these grounds, reasonably the formation of PO₂F₂⁻ should not be metal-assisted and might follow a route similar to the chlorination of carboxylic acids with PCl₅ and POCl₃.⁸

When the reaction of 1⁺ with CF₃COOH was monitored by ¹H, ¹⁹F and ³¹P NMR spectroscopy, [IrH{κ²C,P-CH₂SiMeFN(4-tolyl)PPh₂}(HNP)(CN^tBu)₂]⁺ (8⁺) was detected as an intermediate as a result of the formal addition of one hydrogen fluoride molecule to one Si–N bond (Scheme 6). Fig. 8 shows selected areas of the ¹H, ¹⁹F and ³¹P NMR spectra with the proposed assignment.**

Scheme 6 Reaction of [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] with HPF₆ or CF₃COOH.

Fig. 8 Selected areas of the ¹H, ¹⁹F, ¹⁹F{¹H} and ³¹P{¹H} NMR spectra of the mixture resulting from the reaction of 1PF₆ with CF₃COOH in CD₂Cl₂ after 24 h with the proposed assignment. Inset A shows the experimental and simulated ¹⁹F signal at −125.8 ppm of 8⁺. Inset B shows the expanded view of the ¹⁹F{¹H} signal at −127.5 ppm of 7⁺. * unassigned.

In view of the ³¹P{¹H} and ¹H NMR spectra, the formal addition of hydrogen fluoride to 1⁺ affording 8⁺ is regiospecific since four products (I⁺–IV⁺, Fig. 9) might form depending on which nitrogen atom undergoes protonation (N¹ or N²) and on the orientation of the formal addition of the fluoride ion to silicon (syn or anti with respect to the protonated nitrogen atom).

Fig. 9 Sequential addition of HF to 1⁺ showing the possible intermediates and their relative Gibbs free energies (M06/def2tzvp//B3PW91-GD3BJ/def2svp, 298 K, 1 atm).

In order to shed light on the above mentioned regiospecifity, a thorough examination of the calculated structure of 1⁺ suggested that the steric hindrances at each nitrogen atoms are similar. Further, the NBO charges of the nitrogen atoms are virtually identical (−1.171, −1.185 a.u.), suggesting that no preferential attack of H⁺ to one of the two nitrogen atoms should be expected as a consequence of the atomic charges at the reacting sites. On these grounds, the observed selectivity in the formation of 8⁺ should rely on the thermodynamic stability of the intermediate itself. With this in mind, the proton affinities (PA) of 1⁺ were calculated along with the relative Gibbs free energy for the sequential addition of hydrogen fluoride to the two Si–N bonds 1⁺, namely Si–N1 and Si–N2 (Fig. 9). In agreement with the proposed structure of 8⁺, the most stable protonated species 1H²⁺ is that resulting from the protonation of N2, that is the nitrogen atom bonded to the phosphorus trans to the hydrido moiety (PA = 143.2, N2; 139.4 kcal mol⁻¹, N1, Fig. 9). Accordingly, the most stable intermediate I⁺ results from the formal addition of hydrogen fluoride to the bond Si–N2 of 1⁺ with an anti orientation of the attack of fluoride to silicon 1⁺.

Conclusions

tert-Butyl isocyanide triggers the oxidative addition of the SiCH₂–H bond to iridium(I) in [Ir(SiNP)(cod)][PF₆] yielding the iridium(III) hydrido derivative [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆). Reasonably as a consequence of the stable κ³C,P,P′ coordination of the SiNP–H ligand along with the substitutional inertness of the CN^tBu ligands, 1⁺ as well as the related haloderivatives [IrX{κ³C,P,P′-(SiNP–H)}(CNtBu)₂]⁺ (X = Cl, 4⁺; Br, 5⁺) and solvato complex [Ir{κ³C,P,P′-(SiNP–H)}(CH₃CN)(CN^tBu)₂]²⁺ (6²⁺) do not react with unsaturated molecules such as olefins or alkynes.

On the other hand, 1⁺ does react with Brønsted acids such as HBF₄ and CF₃COOH undergoing the unexpected fragmentation of the SiNP backbone. Indeed the formal addition of two molecules of hydrogen fluoride to the Si–N bonds affords the iridium(III) derivative [IrH(CH₂SiCH₃F₂)(HNP)₂(CN^tBu)₂] (7⁺) through the formation of the intermediate [IrH{κ²C,P-CH₂SiMeFN(4-tolyl)PPh₂}(HNP)(CN^tBu)₂]⁺ (8⁺), observed as the result of the regiospecific formal addition of HF to one Si–N bond. Accordingly DFT calculation suggests that a sequential protonation of the Si–N–P moieties takes place followed by the formal addition of fluoride ion to silicon as well as that the observed regiospecificity relies on the thermodynamic stability of the observed intermediate.

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 dinitrogen or 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 SiMe₂{(N(4-tolyl)(PPh₂)}₂ (SiNP)^6a and [Ir(SiNP)(cod)][PF₆]^6b were prepared according to the literature. NMR spectra were recorded with Bruker spectrometers (AV300 and AV400) and are referred to SiMe₄ (¹H, ¹³C) and H₃PO₄ (³¹P), CFCl₃ (¹⁹F). The proposed ¹H, ¹³C, and ³¹P 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). In compounds containing two non-equivalent phosphorus atoms, namely P¹ and P², P¹ indicates the phosphorus atom trans to H (1⁺, 8⁺), Cl (4⁺), Br (5⁺), or CH₃CN (6²⁺) and superscript labels “tol-P1/2” and “PhP1/2” are used for hydrogen and carbon atoms belonging to the tolyl and phenyl groups attached/linked to the phosphorus atom P1/2. C, H, and N analyses were carried out on a PerkinElmer 2400 CHNS/O analyzer.

Synthesis of [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆)

Method 1. A dichloromethane solution (15 mL) of [Ir(SiNP)(cod)][PF₆] (199.80 mg, 0.184 mmol, 1084.15 g mol⁻¹) was added with CN^tBu (53.5 μL, 0.473 mmol, 83.13 g mol⁻¹, 0.735 g mL⁻¹). The yellow resulting solution was stirred for 24 h, partially evaporated up to 1 mL and added with hexane (5 mL), affording a pale yellow solid which was filtered off and washed with tetrahydrofuran/hexane (1 : 1, 5 mL), dried in vacuo and finally identified as [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆, 149 mg, 0.130 mmol, 71% yield).

Method 2. A dichloromethane solution (12 mL) of [Ir(SiNP)(cod)(CN^tBu)][PF₆] (2PF₆, vide infra, 619 mg, 0.530 mmol, 1167.29 g mol⁻¹) was added with CN^tBu (60.0 μL, 0.530 mmol, 83.13 g mol⁻¹, 0.735 g mL⁻¹) at 313 K. The resulting yellow solution was stirred for 14 h, partially evaporated and added with diethyl ether/hexane (1 : 1, 20 mL), affording a pale yellow solid which was filtered off and washed with tetrahydrofuran/hexane (1 : 1, 5 mL), dried in vacuo and finally identified as [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆, 464 mg, 0.406 mmol, 77% yield). Found: C, 52.99; H, 5.07; N, 4.85. Calcd for C₅₀H₅₈F₆IrN₄P₃Si (1142.24): C, 52.57; H, 5.12; N, 4.91. ¹H NMR (CD₂Cl₂ 298 K): δ_H 7.68–7.56 (6H tot; 4H, o-P¹Ph, 2H, m-P¹Ph), 7.55–7.43 (4H tot; 2H, m-P²Ph, 2H, p-P¹Ph), 7.44–7.32 (6H tot; 2H, p-P²Ph, 2H, o-P²Ph, 2H, m-P¹Ph), 7.00 (d, 2H, ³J_HH = 8.3 Hz, C³H^tol-P2), 6.92 (td, 2H, ³J_HH = 7.9 Hz, ⁴J_HP = 2.3 Hz, m-P²Ph), 6.76 (d, 2H, ³J_HH = 8.3 Hz, C²H^tol-P2), 6.74 (d, 2H, ³J_HH = 8.3 Hz, C³H^tol-P1), 6.61 (ddd, 2H, ³J_HP = 11.5 Hz, ³J_HH = 8.1 Hz, ⁵J_HP = 1.1 Hz, o-P²Ph), 6.24 (d, 2H, ³J_HH = 8.3 Hz, C²H^tol-P1), 2.29 (s, 3H, CH₃^tol-P1), 2.15 (s, 3H, CH₃^tol-P2), 1.37 (s, 9H, CH₃^tBu), 1.20 (s, 9H, CH₃^tBu), 0.67 (dddd, 1H, ³J_HP = 15.0 Hz, ²J_HH = 12.6 Hz, ³J_HH = 2.6 Hz, ³J_HP = 1.2 Hz, SiCH^aH^bIr), 0.46 (ddd, 1H, ²J_HH = 12.6 Hz, ³J_HP = 9.0 Hz, ³J_HP = 1.2 Hz, SiCH^aH^bIr), −0.21 (s, 3H, SiCH₃,), −10.58 (ddd, 1H, ²J_{HP trans} = 149.6 Hz, ²J_{HP cis} = 17.9 Hz, ³J_HH = 2.6 Hz, IrH). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 140.2 (dd, ¹J_CP = 45.0 Hz, ³J_CP = 1.9 Hz, C^{1, PhP}), 139.0 (d, ²J_CP = 10.1 Hz, C^{1, tol-P2}), 138.5 (d, ²J_CP = 9.6 Hz, C^{1, tol-P1}), 136.0 (d, ⁵J_CP = 1.6 Hz, C^{4, tol-P1}), 135.5 (C^{4, tol-P2}), 135.4 (d, ²J_CP = 13.8 Hz, C^{2, PhP2}), 134.64 (d, ¹J_CP = 60.2 Hz, C^{1, PhP}), 134.60 (d, ²J_CP = 12.0 Hz, ⁴J_CP = 1.2 Hz C^{2, PhP1}), 133.90 (dd, ¹J_CP = 63.4 Hz, ³J_CP = 4.5 Hz, C^{1, PhP}), 131.8 (d, ²J_CP = 10.9 Hz, C^{2, PhP1}), 131.7 (d, ⁴J_CP = 2.2 Hz, C^{4, PhP2}), 131.5 (d, ⁴J_CP = 2.0 Hz, C^{4, PhP1}), 131.1 (d, ⁴J_CP = 2.4 Hz, C^{4, PhP2}), 130.5 (d, ²J_CP = 10.2 Hz, C^{2, PhP2}), 130.3 (d, ⁴J_CP = 2.0 Hz, C^{4, PhP1}), 129.7 (d, ⁴J_CP = 1.4 Hz, C^{3, tol-P2}), 129.64 (d, ⁴J_CP = 0.7 Hz, C^{3, tol-P1}), 129.61 (d, ⁴J_CP = 1.4 Hz, C^{2, tol-P1}), 129.1 (d, ³J_CP = 1.4 Hz, C^{2, tol-P2}), 128.4 (d, ³J_CP = 10.8 Hz, C^{3, PhP2}), 128.3 (d, ³J_CP = 9.7 Hz, C^{3, PhP1}), 128.0 (d, ³J_CP = 11.1 Hz, C^{3, PhP2}), 127.4 (d, ³J_CP = 10.8 Hz, C^{3, PhP1}), 58.4 (C^tBu2), 58.2 (C^tBu1), 30.0 (CH₃^tBu1), 29.8 (CH₃^tBu2), 20.5 (CH₃^tol-P2), 20.4 (CH₃^tol-P1), −0.9 (t, ³J_CP = 7.6 Hz, CH₃Si), −29.7 (dd, ²J_CP = 3.7, 2.1 Hz, CH₂Si). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 40.3 (d, ²J_PP 20.4 Hz, P^{1, SiNP}), 34.4 (d, ²J_PP 20.4 Hz, P^{2, SiNP}), −144.4 (hept, ¹J_PF = 710.2 Hz, PF₆⁻).

Synthesis of [Ir(SiNP)(cod)(CN^tBu)][PF₆] (2PF₆)

A dichloromethane solution (10 mL) of [Ir(SiNP)(cod)][PF₆] (178 mg, 0.164 mmol, 1084.15 g mol⁻¹) was added with CN^tBu (37.4 μL, 0.331 mmol, 83.13 g mol⁻¹, 0.735 g mL⁻¹) at 253 K. The yellow resulting solution was stirred for 30 minutes, partially evaporated and added with hexane (10 mL) affording a light yellow solid which was filtered off, dried in vacuo and finally identified as [Ir(SiNP)(cod)(CN^tBu)][PF₆] (2PF₆, 176 mg, 0.151 mmol, 92% yield). Found: C, 53.97; H, 5.31; N, 3.65. Calcd for C₅₃H₆₁F₆IrN₃P₃Si (1167.29): C, 54.53; H, 5.27; N, 3.60. ¹H NMR (C₆D₆ 298 K), the labels “up” and “down” are used for the CH or CH₃moieties pointing towards the CN^tBu ligand and apart from it, respectively: δ_H 7.70–7.55 (8H, o-PPh), 7.43–7.24 (12H, m-PPh and p-PPh), 7.19 (d, 2H, ³J_HH = 8.2 Hz, C²H^{tol down}), 6.83 (d, 2H, ³J_HH = 8.2 Hz, C³H^{tol down}), 6.64 (d, 2H, ³J_HH = 8.2 Hz, C³H^{tol up}), 6.24 (d, 2H, ³J_HH = 8.2 Hz, C²H^{tol up}), 3.47 (br, 4H, C^sp2H^cod), 2.03 (s, 6H, CH₃^tol), 1.97 (s, 9H, CH₃^tBu), 1.76 (br, 8H, C^sp3H^cod), 0.56 (s, 3H, SiCH₃^up), −0.21 (s, 3H, SiCH₃^down). ¹³C{¹H} NMR (C₆D₆, 298 K): δ_C 139.6 (C^{4, tol}), 136.7 (C^{1, tol}), 135.6 (C^{2, PhP}), 132.6 (C^{2, PhP}), 132.0–131.9 (C^{2, tol up} and C^{2, tol down}), 131.8 (C^{3, PhP}), 130.5 (C^{4, PhP}), 129.6 (C^{3, tol down}), 129.1 (C^{3, tol up}), 127.6 (C^{3, PhP}), 78.4 (C^{sp2, cod}), 59.9 (C^tBu), 32.6 (C^{sp3, cod}), 30.4 (CH₃^tBu) 20.5 (CH₃^tol), 3.4 (t, ³J_CP = 2.2 Hz, CH₃Si^down), 2.1 (CH₃Si^up). ³¹P{¹H} NMR (C₆D₆, 298 K): δ_P 41.3 (s, P^SiNP), −142.5 (hept, ¹J_PF = 708.8 Hz, PF₆⁻).

Synthesis of [IrCl{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (4PF₆)

A dichloromethane solution (8 mL) of [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆, 87.3 mg, 0.0764 mmol, 1142.24 g mol⁻¹) was added with N-chlorosuccinimide (10.1 mg, 0.0756 mmol, 133.53 g mol⁻¹). The resulting colourless solution was stirred for 32 h, partially evaporated and added with hexane (5 mL), affording a colourless solid which was filtered off and washed with diethyl ether (3 × 5 mL), dried in vacuo and finally identified as [IrCl{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (4PF₆, 81.5 mg, 0.0693 mmol, 92% yield). Found: C, 51.65; H, 4.78; N, 4.70. Calcd for C₅₀H₅₇ClF₆IrN₄P₃Si (1176.68): C, 51.04; H, 4.88; N, 4.76. ¹H NMR (CD₂Cl₂ 298 K): δ_H 7.62–7.47 (10H tot: 4H, o-PPh, 4H, m-P²Ph, 2H, p-P²Ph), 7.45 (m, 2H, m-P¹Ph), 7.43–7.31 (4H tot: 2H o-P¹Ph, 2H p-P²Ph), 7.11 (td, 2H, ³J_HH = 7.9 Hz, ³J_HP = 3.0 Hz, m-P²Ph), 6.92 (d, 2H, ³J_HH = 8.2 Hz, C³H^tol-P2), 6.86 (ddd, 2H, ³J_HP = 12.5 Hz, ³J_HH = 8.2 Hz, ³J_HP = 1.0 Hz, o-P²Ph), 6.81 (d, 2H, ³J_HH = 8.2 Hz, C³H^tol-P1), 6.52 (d, 2H, ³J_HH = 8.2 Hz, C²H^tol-P2), 6.41 (d, 2H, ³J_HH = 8.2 Hz, C²H^tol-P1), 2.24 (s, 3H, CH₃^tol-P1), 2.18 (s, 3H, CH₃^tol-P2), 1.45 (s, 9H, CH₃^tBu2), 1.30 (dd, 1H, ²J_HH = 13.1 Hz, ³J_HP = 6.6 Hz, SiCH₂Ir), 1.25 (s, 9H, CH₃^tBu1), 1.21 (ddd, 1H, ²J_HH = 13.1 Hz, ³J_HP = 3.6 Hz, ³J_HP = 1.3 Hz, SiCH₂Ir), 0.15 (s, 3H, SiCH₃). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 138.3 (d, ¹J_CP = 67.5 Hz, C^{1, PPh}), 137.6 (d, ²J_CP = 9.6 Hz, C^{1, tol-P1}), 137.2 (C^{1, tol-P2}), 136.5 (d, ⁵J_CP = 1.9 Hz, C^{4, tol-P2}), 135.8 (d, ⁵J_CP = 1.5 Hz, C^{4, tol-P1}), 134.3 (d, ²J_CP = 10.0 Hz, C^{2, PhP1}), 134.2 (d, ²J_CP = 10.6 Hz, C^{2, PhP2}), 133.2 (d, ²J_CP = 10.8 Hz, C^{2, PhP2}), 132.4 (d, ⁴J_CP = 2.8 Hz, C^{4, PhP2}), 132.04 (d, ⁴J_CP = 2.9 Hz, C^{4, PhP1}), 131.99 (d, ⁴J_CP = 2.8 Hz, C^{4, PhP2}), 131.4 (d, ⁴J_CP = 2.7 Hz, C^{4, PhP1}), 131.1 (d, ²J_CP = 10.2 Hz, C^{2, PhP1}), 129.8 (d, ⁵J_CP = 1.7 Hz, C^{3, tol-P2}), 129.5 (C^{3, tol-P1}), 129.2 (d, ⁴J_CP = 3.4 Hz, C^{2, tol-P2}), 128.8 (d, ³J_CP = 11.5 Hz, C^{3, PhP1}), 128.6 (dd, ³J_CP = 6.6 Hz, ⁵J_CP = 3.2 Hz, C^{2, tol-P1}), 128.5 (d, ³J_CP = 11.3 Hz, C^{3, PhP1}), 127.9 (d, ³J_CP = 11.7 Hz, C^{3, PhP2}), 127.2 (d, ³J_CP = 11.3 Hz, C^{3, PhP1}), 59.3 (C^tBu2), 59.0 (C^tBu1), 29.8 (CH₃^tBu1), 29.7 (CH₃^tBu2), 20.5 (CH₃^tol-P2), 20.3 (CH₃^tol-P1), −1.2 (t, ³J_CP = 7.0 Hz, CH₃Si), −16.2 (CH₂Si). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 28.6 (d, ²J_PP = 18.2 Hz, SiNP¹), 26.3 (d, ²J_PP = 18.2 Hz, SiNP²), −144.4 (hept, ¹J_PF = 710.2 Hz, PF₆⁻).

Synthesis of [IrBr{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (5PF₆)

A dichloromethane solution (8 mL) of [IrH{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (1PF₆, 175 mg, 0.153 mmol, 1142.24 g mol⁻¹) was added with N-bromosuccinimide (27.3 mg, 0.153 mmol, 177.98 g mol⁻¹). The resulting colourless solution was stirred for 30 min, partially evaporated and added with hexane (5 mL), affording a colorless solid which was filtered off and washed with diethyl ether (3 × 5 mL), dried in vacuo and finally identified as [IrBr{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (5PF₆, 147 mg, 0.120 mmol, 79% yield). Found: C, 49.27; H, 4.72; N, 4.39. Calcd for C₅₀H₅₇BrF₆IrN₄P₃Si (1221.13): C, 49.18; H, 4.70; H, 4.59. ¹H NMR (CD₂Cl₂ 298 K): δ_H 7.66–7.41 (12H tot: 4H o-PPh, 4H m-PPh, 4H p-PPh), 7.37–7.28 (4H tot: 2H o-P¹Ph, 2H m-P¹Ph), 7.15 (td, 2H, ³J_HH = 7.9 Hz, ⁴J_HP = 3.0 Hz, m-P²Ph), 6.94 (ddd, 2H, ³J_HP = 12.3 Hz, ³J_HH = 7.9 Hz, ⁵J_HP = 1.1 Hz, o-P²Ph), 6.91 (d, 2H, ³J_HH = 8.2 Hz, C³H^tol-P2), 6.83 (d, 2H, ³J_HH = 8.2 Hz, C³H^tol-P1), 6.50 (d, 2H, ³J_HH = 8.2 Hz, C²H^tol-P2), 6.43 (d, 2H, ³J_HH = 8.2 Hz, C²H^tol-P1), 2.24 (s, 3H, CH₃^tol-P2), 2.19 (s, 3H, CH₃^tol-P1), 1.44 (s, 9H, CH₃^tBu1), 1.37 (ddd, 1H, ²J_HH = 13.2 Hz, ³J_HP = 3.3 Hz, ³J_HP = 1.3 Hz, SiCH₂Ir), 1.25 (s, 9H, CH₃^tBu2), 1.24 (dd, 1H, ²J_HH = 13.2 Hz, ³J_HP = 8.6 Hz, SiCH₂Ir), 0.17 (s, 3H, SiCH₃). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 137.8 (dd, ¹J_CP = 65.3 Hz, ³J_CP = 2.7 Hz, C^{1, PhP}), 137.7 (d, ²J_CP = 9.1 Hz, C^{1, tol-P1}), 137.1 (d, ²J_CP = 9.0 Hz, C^{1, tol-P2}), 136.4 (d, ⁵J_CP = 2.0 Hz, C^{4, tol-P2}), 135.8 (d, ⁵J_CP = 1.6 Hz, C^{4, tol-P1}), 134.1 (d, ²J_CP = 10.1 Hz, C^{2, PhP2}), 134.0 (d, ²J_CP = 10.0 Hz, C^{2, PhP1}), 133.4 (d, ²J_CP = 10.8 Hz, C^{2, PhP1}), 132.5 (d, ⁴J_CP = 2.7 Hz, C^{4, PhP2}), 132.1 (d, ⁴J_CP = 2.8 Hz, C^{4, PhP1}), 132.0 (d, ⁴J_CP = 2.5 Hz, C^{4, PhP2}), 131.5 (dd, ¹J_CP = 62.7 Hz, ³J_CP = 1.6 Hz, C^{1, PhP}), 131.4 (d, ⁴J_CP = 2.8 Hz, C^{4, PhP1}), 131.3 (d, ³J_CP = 10.1 Hz, C^{2, PhP2}), 130.3 (dd, ¹J_CP = 57.1 Hz, ³J_CP = 1.4 Hz, C^{1, PhP}), 129.7 (d, ⁵J_CP = 1.6 Hz, C^{3, tol-P2}), 129.5 (d, ⁵J_CP = 1.2 Hz, C^{3, tol-P1}), 129.0 (d, ⁴J_CP = 3.5 Hz, C^{2, tol-P2}), 128.8 (d, ²J_CP = 11.3 Hz, C^{3, PhP2}), 128.6 (d, ⁴J_CP = 3.3 Hz, C^{2, tol-P1}), 128.40 (d, ³J_CP = 10.8 Hz, C^{3, PhP1}), 128.37 (dd, ¹J_CP = 66.5 Hz, ³J_CP = 1.2 Hz, C^{1, PhP}), 128.1 (d, ³J_CP = 11.6 Hz, C^{3, PhP2}), 127.2 (d, ³J_CP = 11.2 Hz, C^{3, PhP1}), 59.3 (C^tBu2), 58.9 (C^tBu1), 29.8 (CH₃^tBu1), 29.6 (CH₃^tBu2), 20.4 (CH₃^tol-P2), 20.4 (CH₃^tol-P1), −1.1 (t, ³J_CP = 7.1 Hz, CH₃Si), −18.9 (d, ²J_CP = 3.8 Hz, CH₂Si). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 25.7 (d, ²J_PP = 17.4 Hz, P^{1 SiNP}), 25.1 (d, ²J_PP = 17.4 Hz, P^{2 SiNP}), −144.4 (hept, ¹J_PF = 710.2 Hz, PF₆⁻).

Synthesis of [Ir{κ³C,P,P′-(SiNP–H)}(CH₃CN)(CN^tBu)₂][PF₆]₂ (6[PF₆]₂)

An acetonitrile solution (10 mL) of [IrBr{κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆] (5PF₆, 233 mg, 0.191 mmol, 1221.13 g mol⁻¹) was added with AgPF₆ (53.2 mg, 0.210 mmol, 252.83 g mol⁻¹). The resulting colorless suspension was stirred for 12 h and filtered. The resulting solution was partially evaporated and added with diethyl ether (5 mL), affording a colourless solid which was filtered and washed with diethyl ether (3 × 5 mL), dried in vacuo and finally identified as [Ir(CH₃CN) {κ³C,P,P′-(SiNP–H)}(CN^tBu)₂][PF₆]₂ (6[PF₆]₂, 179 mg, 0.0135 mmol, 71% yield). Found: C, 46.81; H, 4.69; N, 5.32. Calcd for: C₅₂H₆₀F₁₂IrN₅P₄Si (1327.24): C, 47.06; H, 4.56; N, 5.28. ¹H NMR (CD₂Cl₂ 298 K): δ_H 7.73–7.46 (12H tot: 4H m-PPh, 4H p-PPh, 4H o-PPh), 7.46–7.31 (4H tot: 2H o-P²Ph, 2H m-P²Ph), 7.18 (td, 2H, ³J_HH = 7.8 Hz, ³J_HP = 3.2 Hz, m-P¹Ph), 6.95 (m, 2H, m-P²Ph), 6.90 (br d, 4H, ³J_HH = 8.0 Hz, C³H^tol-P1 and C³H^tol-P2), 6.50 (d, 2H, ³J_HH = 8.0 Hz C²H^tol-P1), 6.41 (d, 2H, ³J_HH = 8.0 Hz, C²H^tol-P2), 2.23 (br s, 6H, CH₃^tol-P1 and CH₃^tol-P2), 2.09 (s, 3H, CH₃CN), 1.48 (s, 9H, CH₃^tBu1), 1.44 (dd, 1H, ²J_HH = 13.0 Hz, ³J_HP = 7.5 Hz, SiCH₂Ir), 1.30 (dd, 1H, ²J_HH = 13.0 Hz, ³J_HP = 2.7 Hz, SiCH₂Ir), 1.29 (s, 9H, CH₃^tBu2), 0.34 (s, 3H, SiCH₃). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 137.0 (d, ⁵J_CP = 2.1 Hz, C^{4, tol-P2}), 136.7 (d, ²J_CP = 8.7 Hz, C^{1, tol1} or C^{1, tol2}), 136.5 (d, ⁵J_CP = 1.5 Hz, C^{4, tol-P1}), 135.81 (d, ²J_CP = 8.0 Hz, C^{1, tol1} or C^{1, tol2}), 135.79 (d, ²J_CP = 70.6 Hz, C^{2, PhP}), 133.9 (d, ²J_CP = 10.7 Hz, C^{2, PhP2}), 133.7 (d, ²J_CP = 11.1 Hz, C^{2, PhP1}), 133.2 (d, ⁴J_CP = 2.8 Hz, C^{4, PhP2}), 133.0 (d, ⁴J_CP = 1.9 Hz, C^{4, PhP1}), 132.9 (d, ⁴J_CP = 1.8 Hz, C^{4, PhP2}), 132.5 (d, ⁴J_CP = 2.8 Hz, C^{4, PhP1}), 132.3 (d, ³J_CP = 10.1 Hz, C^{2, PhP2}), 131.4 (d, ³J_CP = 10.2 Hz, C^{2, PhP1}), 130.8 (d, ¹J_CP = 60.7 Hz, C^{1, PhP}), 130.0 (d, ⁴J_CP = 1.5 Hz, C^{3, tol-P2}), 129.9 (d, ⁴J_CP = 1.4 Hz, C^{3, tol-P1}), 129.4 (d, ³J_CP = 11.7 Hz, C^{2, PhP2}), 129.4 (d, ³J_CP = 11.2 Hz, C^{2, PhP1}), 129.1 (d, ³J_CP = 11.7 Hz, C^{3, PhP2}), 128.9 (d, ⁴J_CP = 3.7 Hz, C^{2, tol-P2}), 128.6 (d, ³J_CP = 12.0 Hz, C^{3, PhP1}), 127.8 (d, ⁴J_CP = 3.8 Hz, C^{2, tol-P1}), 127.7 (d, ¹J_CP = 62.0 Hz, C^{1, PhP}), 126.8 (d, ¹J_CP = 70.9 Hz, C^{1, PhP}), 121.9 (d, ³J_CP = 18.6 Hz, NCCH₃), 61.1 (C^tBu2), 60.7 (C^tBu1), 29.4 (CH₃^tBu1), 29.3 (CH₃^tBu2), 20.4 (CH₃^{tol1 or tol2}), 20.3 (CH₃^{tol1 or tol2}), 2.6 (CH₃CN), −1.5 (t, ³J_CP = 6.8 Hz, CH₃Si), −18.7 (t, ²J_CP = 3.6 Hz, CH₂Si). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 25.1 (d, ²J_PP 19.4 Hz, P^{1 SiNP}), 18.0 (d, ²J_PP 19.4 Hz, P^{2 SiNP}), −144.4 (hept, ¹J_PF = 710.2 Hz, PF₆⁻).

Synthesis of [IrH(CH₂SiF₂CH₃)(HNP)₂(CN^tBu)₂][BF₄] (7BF₄)

A dichloromethane solution (5 mL) of [IrH(SiNP–H)(CN^tBu)₂][PF₆] (1PF₆, 116 mg, 0.102 mmol, 1142.24 g mol⁻¹) was added with HBF₄·Et₂O (27.8 μL, 0.203 mmol, 161.93 g mol⁻¹, 1.18 g mL⁻¹) at 193 K. The resulting colourless solution was stirred for 30 min, allowed to warm up at room temperature, partially evaporated and added with hexane (10 mL), affording a colourless solid which was filtered off and washed with diethyl ether/hexane (1 : 10) (12 mL), dried in vacuo and finally identified as [IrH(CH₂SiF₂CH₃)(HNP)₂(CN^tBu)₂][BF₄] (7BF₄, 64.4 mg, 0.0573 mmol, 56% yield). Found: C, 53.29; H, 5.47; N, 5.09. Calcd for C₅₀H₆₀BF₆IrN₄P₂Si (1124.09): C, 53.42; H, 5.38; N, 4.98. ¹H NMR (CD₂Cl₂ 298 K): δ_H 7.74–7.62 (4H tot: 2H p-P²Ph and 2H o-P²Ph), 7.62–7.45 (12H tot: 2H o-P¹Ph, 2H o-P²Ph, 4H m-P¹Ph, 2H m-P²Ph and 2H p-P¹Ph,), 7.40 (td, 2H, ³J_HH = 7.7 Hz, ⁴J_HP = 2.8 Hz, m-P²Ph), 7.31 (m, 2H, o-P¹Ph), 6.77 (d, 2H, ³J_HH = 8.4 Hz, C³H^tol-P1), 6.73 (d, ³J_HH = 8.4 Hz, C³H^tol-P2), 6.11 (d, 2H, ³J_HH = 8.4 Hz, C²H^tol-P1), 5.94 (d, 2H, ³J_HH = 8.4 Hz, C²H^tol-P2), 5.33 (dd, 1H, ²J_HP = 15.9 Hz, ¹J_HF = 3.9 Hz, NH^P1), 4.21 (d, 1H, ²J_HP = 15.9 Hz, NH^P2), 2.16 (s, 3H, CH₃^tol-P1), 2.15 (s, 3H, CH₃^tol-P2), 1.25 (s, 9H, CH₃^tBu1), 1.21 (s, 9H, CH₃^tBu2), 0.33 (t, 3H, ³J_HF = 6.3 Hz, SiCH₃), 0.01 (ddddd, 1H, ²J_HH = 13.4 Hz, ³J_HF = 13.4 Hz, ³J_HP = 10.5 Hz, ³J_HP = 4.7 Hz, ³J_HF = 2.4 Hz, SiCH^aH^bIr), −0.42 (ddddd, 1H, ²J_HH = 13.4 Hz, ³J_HF = 13.4 Hz, ³J_HP = 11.6 Hz, ³J_HP = 3.6 Hz, ³J_HF = 3.5 Hz, SiCH^aH^bIr), −11.86 (ddt, 1H, ²J_{HP trans} = 152.9 Hz, ²J_{HP cis} = 17.4 Hz, ⁴J_HF = 2.2 Hz, IrH). ¹³C{¹H} NMR (CD₂Cl₂, 298 K): δ_C 138.9 (d, ²J_CP = 9.0 Hz, C^{1, tol-P2}), 138.0 (d, ²J_CP = 11.1 Hz, C^{1, tol-P1}), 132.2 (d, ²J_CP = 10.2 Hz, C^{2, PhP1}), 131.9 (d, ⁴J_CP = 2.3 Hz, C^{4, PhP1}), 131.73 (d, ³J_CP = 10.6 Hz, C^{2, PhP2}), 131.67 (d, ³J_CP = 11.0 Hz, C^{2, PhP2}), 131.5 (dd, ³J_CP = 10.0, ⁵J_CP = 2.8 Hz, C^{2, PhP1}), 130.8 (C^{4, tol-P1} and C^{4, tol-P2}), 130.7 (d, ³J_CP = 11.2 Hz, C^{3, PhP1}), 129.1 (d, ³J_CP = 11.2 Hz, C^{3, tol-P1}), 129.0 (d, ⁴J_CP = 7.6 Hz, C^{3, tol-P2}), 128.9 (d, ⁴J_CP = 1.6 Hz, C^{4, PhP2}), 128.6 (d, ³J_CP = 11.2 Hz, C^{3, PhP2}), 118.4 (d, ³J_CP = 5.7 Hz, C^{2, tol-P1}), 117.9 (d, ³J_CP = 5.1 Hz, C^{2, tol-P1}), 58.9 (C^tBu2), 58.8 (C^tBu1), 29.7 (CH₃^tBu1), 29.4 (CH₃^tBu2), 20.0 (CH₃^tol-P1), 20.0 (CH₃^tol-P2), −3.6 (t, ²J_CF = 19.3 Hz, CH₃Si), −34.9 (CH₂Si). ¹⁹F{¹H} NMR (CD₂Cl₂, 298 K): δ_F −127.5 (dd, 1F, ²J_FF = 20.6 Hz, ⁴J_FP = 2.7 Hz, SiF^aF^b), −129.0 (d, 1F, ²J_FF = 20.6 Hz, SiF^aF^b). ³¹P{¹H} NMR (CD₂Cl₂, 298 K): δ_P 11.4 (d, ²J_PP 20.5 Hz, P¹), 3.1 (d, ²J_PP 20.5 Hz, P²).

DFT calculations

Molecular structure optimizations and frequencies calculations were carried out with the Gaussian09 program (revision D.01)⁹ using the method B3PW91,¹⁰ including the D3 dispersion correction scheme by Grimme with Becke–Johnson damping.¹¹ The def2-SVP¹² 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.¹³ In order to improve the accuracy of the calculated energies, single point energy calculations were carried out on the optimized structures of intermediates and transition states using the method M06,¹⁴ the def2-TZVP¹² basis and pseudo potentials, where appropriate, and the SMD model¹⁵ for the solvent (dichloromethane). Finally a correction of +1.89 kcal mol⁻¹ to Gibbs free energy was also applied for the change of the standard state from gas phase (1 atm) to solution (1 M) at 298 K.¹⁶ Delocalization indexes (DI) were calculated using Multiwfn.¹⁷

Crystal structure determination

Single crystals of SiNP and 7BF₄ were obtained by slow evaporation of dicholoromethane solutions of the compounds; single crystals of 1PF₆ and 4PF₆ were grown by slow diffusion of hexane into a THF (4PF₆) or dichloromethane solution (1PF₆) of the compounds. X-ray diffraction data were collected at 100(2) K on a Bruker APEX SMART (1PF₆, 4PF₆, 7BF₄) or APEX DUO (SiNP) diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) using 0.6° ω rotations. Intensities were integrated and corrected for absorption effects with SAINT–PLUS¹⁸ and SADABS¹⁹ programs, both included in APEX2 package. The structures were solved by the Patterson method with SHELXS-97²⁰ and refined by full matrix least-squares on F² with SHELXL-2014²¹ under WinGX.²²

Crystal data for SiNP

C₄₀H₄₀N₂P₂Si, 638.77 g mol⁻¹, monoclinic, P21/c, a = 23.093(3) Å, b = 9.0864(13) Å, c = 17.039(2) Å, β = 107.176(2)°, V = 3415.9(8) ų, Z = 4, reflections collected/independent 65 969/6980 [R(int) = 0.0406], R₁ = 0.0435 [I > 2σ(I)], wR₂ = 0.1380 (all data). CCDC deposit number 2155640.†

Crystal data for 1PF₆

2C₅₀H₅₈F₆IrN₄P₃Si·CH₂Cl₂·C₆H₁₄, 2455.50 g mol⁻¹, triclinic, P1̄, a = 10.9615(7) Å, b = 12.7205(8) Å, c = 20.4705(12) Å, α = 86.1880(10)°, β = 77.4030(10)°, γ = 85.9510(10)°, V = 2774.8(3) ų, Z = 1, reflections collected/independent 34 093/11 291 [R(int) = 0.0280], R₁ = 0.0293 [I > 2σ(I)], wR₂ = 0.0762 (all data). CCDC deposit number 2155643.†

Crystal data for 4PF₆

4C₅₀H₅₇ClF₆IrN₄P₄Si·3C₄H₈O, 4922.88 g mol⁻¹, monoclinic, C2/c, a = 46.824(5) Å, b = 10.9773(11) Å, c = 25.668(3) Å, β = 121.1930(10)°, V = 11 286(2) ų, Z = 2, reflections collected/independent 58 271/12 421 [R(int) = 0.0495], R₁ = 0.0364 [I > 2σ(I)], wR₂ = 0.0815 (all data). CCDC deposit number 2155642.†

Crystal data for 7BF₄

C₅₀H₆₀BF₆IrN₄P₂Si·CH₂Cl₂, 1208.98 g mol⁻¹, monoclinic, P2₁/c, a = 16.3517(10) Å, b = 11.6078(7) Å, c = 28.9040(17) Å, β = 93.8810(10)°, V = 5473.6(6) ų, Z = 4, reflections collected/independent 61 703/11 193 [R(int) = 0.0364], R₁ = 0.0231 [I > 2σ(I)], wR₂ = 0.0525 (all data). CCDC deposit number 2155641.†

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.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Crystal data, NMR spectra, and atomic coordinates of calculated structures. CCDC 2155640–2155643. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt00794k

‡ The hydrido signal was observed as a ¹H{³¹P} doublet due to the scalar coupling of the IrH hydrogen atom to one of the IrCH₂ hydrogen atoms (³J_HH = 2.6 Hz). Accordingly, the ¹H{³¹P} signals at 0.46 and 0.67 ppm, assigned to the IrCH2 moiety, are a doublet (²J_HH = 12.6 Hz) and a doublet of doublets (²J_HH = 12.6, ³J_HH = 2.6 Hz), respectively. As already discussed for the related hydrido derivative [IrH{κ³C,P,P′-(SiNP–H)}{P(OCH₃)₃}₂]⁺,^6b this pattern is the consequence of the dependence of the ³J_HH constant on the H–X–Y–H dihedral angle (cf. M. J. Minch, Concepts Magn. Reson., 1994, 6, 41–56).

§ It is worth a mention that in the course of the related reaction of [Ir(SiNP)(cod)][PF₆] with P(OCH₃)₃ the pentacoordinated derivative [Ir(SiNP)(cod){P(OCH₃)₃}][PF₆], analogous to 2⁺, could not be isolated and was characterised in situ whereas the putative square planar complex [Ir(SiNP){P(OCH₃)₃}₂]⁺, analogous to 3⁺, could not even be observed and was proposed only based on DFT calculations (see ref. 6b).

¶ Selected ¹H NMR data for 3⁺ (CD₂Cl₂, 298 K) are: δ_H 7.14 (d, ³J_HH = 8.1 Hz, 2H, C²H^tol), 6.87 (d, ³J_HH = 8.1 Hz, 2H, C³H^tol), 2.54 (s, 6H, CH₃^tol), 1.41 ppm (s, 18H, CH₃^tBu), 0.86 ppm (s, 6H, SiCH₃).

|| The calculated heteronuclear spin–spin constants J_XY are negligible in all the cases except for F2 and P2 (6.5 Hz, calc.), thus the observed fluorine-phosphorus coupling is the consequence of the conformation adopted by the F–Si–C–Ir–P fragment rather than of the NH⋯F hydrogen bond. On the other hand, when it comes to the NH⋯F hydrogen bond, the calculated heteronuclear spin–spin constants are negligible in all the cases except for H1N and F1 (23.2 Hz, calc.), nicely matching the proposed assignment.

** The ³¹P{¹H} doublets at 43.0 and 11.9 ppm (²J_PP = 19.0 Hz) of 8⁺ are indicative of two mutually cis phosphorus atoms. The ¹⁹F doublet of quartets of doublets at −125.8 ppm results from the scalar coupling of the fluorine atom to the SiCH₃ moiety (³J_HF = 6.4 Hz) and the SiCH₂ non-equivalent hydrogen atoms (³J_HF = 16.4 Hz, 2.0 Hz). Accordingly, the ¹H doublet at 0.22 ppm (³J_HF = 6.4 Hz) was assigned to the SiCH₃ group (δ_C = −1.3 ppm, doublet, ²J_CF = 6.7 Hz) and the ¹H multiplets at 0.20 and −0.24 ppm were assigned to the two non-equivalent SiCH₂ hydrogen atoms. The ¹H signal of the hydrido ligand was observed at −10.8 ppm as a doublet of doublets (²J_{HP, trans} = 146.1 Hz, ²J_{HP, cis} = 26.3 Hz).

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