Synthesis and reactivity of iridium complexes of a macrocyclic PNP pincer ligand

Abstract

Having recently reported on the synthesis and rhodium complexes of the novel macrocyclic pincer ligand PNP-14, which is derived from lutidine and features terminal phosphine donors trans-substituted with a tetradecamethylene linker (Dalton Trans., 2020, 49, 2077–2086 and Dalton Trans., 2020, 49, 16649–16652), we herein describe our findings critically examining the chemistry of iridium homologues. The five-coordinate iridium(I) and iridium(III) complexes [Ir(PNP-14)(η²:η²-cyclooctadiene)][BAr^F₄] and [Ir(PNP-14)(2,2′-biphenyl)][BAr^F₄] are readily prepared and shown to be effective precursors for the generation of iridium(III) dihydride dihydrogen, iridium(I) bis(ethylene), and iridium(I) carbonyl derivatives that highlight important periodic trends by comparison to rhodium counterparts. Reaction of [Ir(PNP-14)H₂(H₂)][BAr^F₄] with 3,3-dimethylbutene induced triple C–H bond activation of the methylene chain, yielding an iridium(III) allyl hydride derivative [Ir(PNP-14*)H][BAr^F₄], whilst catalytic homocoupling of 3,3-dimethylbutyne into Z-tBuC≡CCHCHtBu could be promoted at RT by [Ir(PNP-14)(η²:η²-cyclooctadiene)][BAr^F₄] (TOF_initial = 28 h⁻¹). The mechanism of the latter is proposed to involve formation and direct reaction of a vinylidene derivative with HC≡CtBu outside of the macrocyclic ring and this suggestion is supported experimentally by isolation and crystallographic characterisation of a catalyst deactivation product.

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1. Introduction

Phosphine-based pincers are robust ancillary ligands that continue to find notable applications in organometallic chemistry and homogenous catalysis.^1,2 In particular, rhodium and iridium complexes of these ligands are associated with fundamental and applied breakthroughs in the chemistry of C–H bond activation reactions; exemplified by the characterisation of σ-alkane complexes and development of high-performance alkane dehydrogenation catalysts, respectively.^3,4 These mer-tridentate ligands are evidently well-suited to supporting the formation of highly reactive M(I) derivatives necessary to bring about cleavage of strong C–H bonds,⁵ able to accommodate the changes in geometry associated with M(I)/M(III) redox shuttling, and suitably modular in composition to enable augmentation of metal-based reactivity through considered change of the central donor atom, wingtip substituents, or backbone constituents.² As marked out by lower carbonyl stretching frequencies,⁶ the heavier iridium congeners are particularly notable for a more pronounced tendency for oxidative addition of H₂,^7,8 C(sp³)–H bonds,^3,9 and C(sp²)–H bonds¹⁰ compared to their rhodium counterparts.

Motivated by the potential to exploit additional reaction control though their unique steric profile, use in the construction of interlocked assemblies, and as an extension of our related work with NHC-based variants,^11–13 we have recently become interested in the chemistry of macrocyclic phosphine-based pincers.^14–16 Last year we reported on the synthesis and rhodium complexes of the lutidine-derived macrocyclic pincer PNP-14, where the chiral P-donors are trans-substituted with a tetradecamethylene linker (Chart 1).^14,15 As a novel platform for exploring the organometallic chemistry of Group 9 pincer complexes, we now present our findings critically examining the chemistry of iridium PNP-14 homologues aided by reference to acyclic complexes of 2,6-(R₂PCH₂)₂C₅H₃N (PNP-R; e.g. R = tBu, iPr).

Chart 1 Complexes of the macrocyclic pincer ligand PNP-14.

2. Results and discussion

Mirroring synthetic strategies that we have successfully employed for rhodium homologues,^14,15 the preparation of iridium(I) and iridium(III) complexes of PNP-14 was attempted through substitution reactions of [Ir(COD)₂][BAr^F₄]¹⁷ in 1,2-difluorobenzene (DFB) and [Ir(COD)(biph)Cl]₂¹⁸ in fluorobenzene, respectively, with the latter exploiting Na[BAr^F₄] as a halide abstracting agent (COD = 1,5-cyclooctadiene, Ar^F = 3,5-(CF₃)₂C₆H₃, biph = 2,2′-biphenyl; Scheme 1). These reactions produced five-coordinate cationic derivatives [Ir(PNP-14)(η²:η²-COD)][BAr^F₄] 1 and [Ir(PNP-14)(biph)][BAr^F₄] 2 under mild conditions, which were subsequently isolated as analytically pure crystalline materials in good yield (ca. 80%) and fully characterised (Scheme 1).

Scheme 1 Synthesis and solid-state structures of iridium pincer complexes 1 and 2: thermal ellipsoids at 50% and 30% probability, respectively; anions omitted. Selected bond lengths (Å) and angles (°): 1, Ir1–P2, 2.3743(7); Ir1–P3, 2.3846(7); P2–Ir1–P3, 115.24(2); Ir1–N101, 2.107(2); Ir1–Cnt(C4,C5), 2.083(2); Ir1–Cnt(C8, C9), 2.036(2); N101–Ir1–Cnt(C4, C5), 172.52(8); Cnt(C4, C5)–Ir–Cnt(C8, C9), 84.29(9); 2, Ir1–P2, 2.3253(15); Ir1–P3, 2.2849(15); P2–Ir1–P3, 163.24(5); Ir1–N101, 2.158(5); Ir1–C4, 2.048(6); Ir1–C15, 2.044(6); N101–Ir1–C15, 172.5(2); C4–Ir1–C15, 81.9(2); Ir1⋯C129, 3.152(7); I̲r̲1̲⋯H̲C129, 2.54; Cnt = centroid.

Iridium(I) complex 1 adopts a distorted trigonal bipyramidal metal geometry (18 VE), with the terminal phosphine donors positioned in the equatorial coordination sites conferring a distinctly puckered pincer ligand geometry. Distortion of the PNP ligand towards a fac coordination mode in this manner is associated with a compressed P–Ir–P bite angle of 115.24(2)° in the solid state and a pair of ³¹P resonances at δ 17.0 and 13.4 with no appreciable ²J_PP coupling in DFB solution. While unusual, the formulation of 1 simply appears to be a consequence of COD chelation, although this is contingent upon the flexible lutidine-based backbone and asymmetric steric profile of the phosphine donors.¹⁹ Moreover, given the rhodium(I) homologue is instead characterised as a C₁-symmetric square planar complex, viz. [Rh(PNP-14)(η²-COD)]⁺(1′, 16 VE; δ_³¹P 57.4, 45.9, ²J_PP = 312 Hz),^15,20 the capacity of the heavier metal congener to from stronger metal–ligand bonds is clearly a decisive factor. Bulk purity was established by combustion analysis and the structure of 1 was fully corroborated in solution by NMR spectroscopy and HR ESI-MS.

Coordination of PNP-14 is more conventional in the formally 16 VE square pyramidal iridium(III) complex 2, as evidenced by a P–Ir–P bite angle of 163.24(5)° in the solid state and C₁ symmetry in CD₂Cl₂ solution; with a pair of ³¹P resonances at δ 38.7 and 20.9 exhibiting a characteristically large trans-phosphine ²J_PP coupling constant of 307 Hz.²¹ The crystal structure of 2 is isomorphous to the direct rhodium homologue 2′,¹⁴ with the tetradecamethylene linker skewed to one side of the basal plane away from the biph ligand and contorted in such a way as to enable adoption of a weak γ-agostic interaction (2, I̲r̲1̲⋯H–C̲1̲2̲9̲ = 3.152(7); cf. 3.184(2) Å for 2′). Previously reported five-coordinate complexes of the form [M(pincer)(biph)][BAr^F₄] provide further structural precedent for 2 and the metal-based metrics of the acyclic analogue [Ir(PNP-tBu)(biph)][BAr^F₄] (II) are similar.^6,11,14 Moreover, as II is fluxional in solution as a result of facile biph pseudorotation on the NMR timescale,⁶ retention of C₁ symmetry in solution suggests that buttressing with the methylene strap prevents such dynamics in 2.

Reaction of 2 with dihydrogen (1 atm) in DFB at RT resulted in immediate and full conversion into [Ir(PNP-14)(2-biphenyl)H][BAr^F₄] 3 (δ_³¹P 40.6, 36.5, ²J_PP = 302 Hz; δ_¹H −21.6; Scheme 2). No further reaction was observed after 18 h, but heating at 85 °C for 6 h resulted in complete hydrogenolysis of the biph ligand and formation of dihydride dihydrogen complex [Ir(PNP-14)H₂(H₂)][BAr^F₄] 4 in quantitative spectroscopic yield. Hydrogenolysis also occurs for 2′ and II, but longer reaction times are required under otherwise equivalent conditions (both ca. 2 days).¹⁴ Coordinatively saturated 1 rapidly affords 4 upon reaction with dihydrogen (1 atm) in DFB at RT (<5 min), invoking facile and reversible chelation of COD.

Scheme 2 Synthesis and reactivity of iridium dihydride dihydrogen complex 4. [BAr^F₄]⁻ counter anions omitted for clarity.

Complex 4 was characterised in situ using NMR spectroscopy, with adoption of time-averaged C₂ symmetry, a single ³¹P resonance at δ 42.1, and a broad 4H resonance at δ −9.26 (T₁ = 88.8 ± 0.7 ms, 600 MHz, argon) the most diagnostic features at 298 K. The hydride signal remained broad upon cooling to 253 K but exhibits faster spin–lattice relaxation (δ −9.27, T₁ = 50 ± 1 ms, 600 MHz, argon). The acyclic analogue [Ir(PNP-tBu)H₂(H₂)]BF₄ (IV) is known and formulation as a dihydride dihydrogen complex was corroborated in a similar manner in situ by NMR spectroscopy.⁷ Whilst the data was recorded under difference conditions, the similarly of the hydride signatures is striking (IV, δ_¹H −9.31, T₁ = 24 ms, 400 MHz, 233 K in CD₃OD). In line with the reduced propensity for oxidative addition, the rhodium homologue of 4 is instead observed as a dihydrogen complex, viz. [Rh(PNP-14)(H₂)][BAr^F₄] 4′.¹⁴

Further supporting the assignment of 4, reaction with ethylene (1 atm) generated the corresponding C₁-symmetric dihydride π-complex 5 (δ_³¹P 33.4, 12.4, ²J_PP = 314; δ_¹H −7.89, −17.80) within 5 min at RT (Scheme 2). Subsequent heating at 85 °C for 16 h yielded the bis(ethylene) complex 6 (δ_³¹P 9.0) in quantitative spectroscopic yield, with concomitant formation of ethane. C₂ symmetry and coordination of two molecules of ethylene was established in situ by NMR spectroscopy. The latter is associated with four chemically inequivalent 2H signals at δ 3.23, 2.80, 2.49 and 1.97 and two ¹³C resonances at δ 26.0 and 18.0, and reinforces the disposition of iridium(I) centres to adopt five-coordinate geometries: as seen in 2, but contrasting that observed under the same conditions in the rhodium(I) system, viz. [Rh(PNP-14)(C₂H₄)][BAr^F₄] 6′.¹⁴ Structurally-related bis(ethylene) iridium(I) complexes of CNC- and pybox-based pincer ligands have been crystallographically characterised, exhibiting distorted trigonal bipyramidal metal geometries with the ethylene ligands located in the equatorial sites,²² but are unknown for PNP- and PONOP-based ligands.²³

When 2 was instead treated with an excess of 3,3-dimethylbutene (5 equivalents) the allyl hydride derivative [Ir(PNP-14*)H][BAr^F₄] 7 was produced in quantitative spectroscopic yield after 5 days heating at 100 °C, presumably through intramolecular transfer dehydrogenation of the methylene chain followed by allylic C–H activation (Scheme 2).²⁴ As precedent for this reactivity, examples of cyclometallated rhodium(III) and iridium(III) pincer complexes can be found in the literature.²⁵ Complex 7 was subsequentially isolated in 49% yield and fully characterised, including in the solid state by single crystal X-ray diffraction (Fig. 1). In solution 7 is distinctly C₁-symmetric, with a pair of ³¹P resonances at δ 81.4 and 25.8 with ²J_PP = 313 Hz, allyl ¹³C resonances at δ 81.9, 66.0, and 38.9, and a 1H hydride resonance at δ −8.49 (²J_PH = 19.6, 9.7 Hz). The crystal structure demonstrates that 7 adopts a pseudo-octahedral metal geometry in the solid state, with κ₆-coordination of PNP-14* creating iridacyclopentyl and iridacyclododecyl rings, and the hydride ligand was located from the Fourier difference map. Little distortion of the PNP-core is evident in 7 and the associated metal-based metrics are broadly comparable to those in 2 (e.g. P–Ir–P ca. 163°). There is considerable variance in the allyl Ir–C bond lengths, with the longest contact trans to the hydride ligand (Ir1–C119 = 2.311(4) Å, Ir1–C120, 2.155(3) Å, Ir1–C121, 2.199(3) Å), but all the internal carbon bond angles are >120°. The geometry of the iridacyclododecyl ring is reminiscent of the quadrilateral conformations adopted by 12-membered cycloalkanes.²⁶

Fig. 1 Solid-state structure of 7: the hydride ligand was located from the Fourier difference map; thermal ellipsoids at 30% probability; solvent molecule and anion omitted. Selected bond lengths (Å) and angles (°): Ir1–P2, 2.2694(8); Ir1–P3, 2.3339(9); P2–Ir1–P3, 163.04(3); Ir1–N101, 2.126(2); Ir1–H1, 1.39(3); Ir1–C119, 2.311(4); Ir1–C120, 2.155(3); Ir1–C121, 2.199(3); C119–C120, 1.430(5); C120–C121, 1.409(5); C119–C120–C121, 124.4(3); N101–Ir1–C121, 169.59(11).

The onward reactivity of 6 was harnessed to access the C₂-symmetric Ir(I) carbonyl derivative 8 (δ_³¹P 62.5) by reaction with carbon monoxide, which was isolated in 89% yield (overall from 2; Scheme 2). Complexes of this nature are of interest as the carbonyl ligand is a convenient spectroscopic reporter group for the electronic characteristics of the metal-pincer fragment.^27,28 In this case, the ν(CO) band of 8 (1984 cm⁻¹) is shifted to considerably lower frequency compared to the rhodium(I) homologue 8′ (1997 cm⁻¹) under the same conditions (CH₂Cl₂ solution, Table 1). This is in line with expected periodic trends, which are also apparent from the IR data collected for tBu- and iPr-substituted analogues. These data suggest that PNP-14 is a marginally weaker net donor than PNP-tBu, but equivalent to PNP-iPr.¹⁴

Table 1 Carbonyl stretching frequencies (CH₂Cl₂)

Pincer	ν(CO)/cm^−1	Ref.
a Measured in the solid state (ATR).
[Ir(PNP-14)(CO)][BAr^F4] 8	1984	This work
[Rh(PNP-14)(CO)][BAr^F4] 8′	1997	14
[Ir(PNP-tBu)(CO)][BAr^F4]	1977	6
[Rh(PNP-tBu)(CO)][BAr^F4]	1990	6
[Ir(PNP-iPr)(CO)][BAr^F4]	1986^a	29
[Rh(PNP-iPr)(CO)][BAr^F4]	1998	27

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Of the organometallic chemistry we have discovered so far using PNP-14, the capacity for rhodium complexes to promote the stoichiometric homocoupling of 3,3-dimethylbutyne through the annulus of the macrocyclic ligand stands out (1′ → 9′ in Scheme 3).¹⁵ Given that a structurally related Ir(PCP) system has also been shown to promote stoichiometric terminal alkyne coupling reactions,^30,31 we were very interested to ascertain if similar reactivity could be brought about in iridium complexes of PNP-14. Iridium(I) complex 1 was selected as the most suitable precursor and initial screening studies using a twofold excess of HC≡CtBu in DFB at RT indicated rapid production of the corresponding Z-enyne (δ_¹H 5.53, 5.25; ³J_HH = 11.9 Hz)³² without any observable consumption of 1 (Scheme 3). This stereochemistry contrasts that observed for the rhodium system and, as homocoupling through the macrocycle would be expected to result in an interpenetrated enyne complex, it appears that production of the enyne occurs catalytically outside the ring. Subsequent detailed investigation of this reaction using 100 equivalents HC≡CtBu confirmed that 1 is an effective precatalyst.³³ Under these conditions, the dimerisation of HC≡CtBu into Z-tBuC≡CCHCHtBu proceeds with an initial TOF of 28 h⁻¹. After 6 h, analysis by ¹H NMR spectroscopy indicated complete consumption of HC≡CtBu and exclusive production of Z-tBuC≡CCHCHtBu. From the ³¹P{¹H} NMR spectrum generation of a new organometallic species was apparent (10, δ 25.0, 16.3; ²J_PP = 364 Hz), accounting for 88% of the metal-containing species with 1 making up 10%. Addition of a further 50 equivalents of HC≡CtBu induced complete conversion of 1 into 10 within 24 h but coincided with a halt in homocoupling, which plateaued at 65 TONs.

Scheme 3 Terminal alkyne coupling reactions promoted by 1 and 1′: [M] = M(PNP-14)⁺.

Repeating the homocoupling reaction on a larger scale under similar conditions enabled isolation of 10 from solution in 76% yield, which was subsequently identified as iridium(III) bis(alkenyl) complex [Ir(PNP-14)(η³-E-C(C≡CtBu)CHtBu)(η¹-E-CHCHtBu)][BAr^F₄] (Scheme 4). In the solid state, 10 adopts a very distorted octahedral geometry with the alkenyl ligands in a cis configuration (C4–Ir1–C6 = 102.50(9)°) and coordination of the σ-organyl derived from Z-tBuC≡CCHCHtBu reinforced by π-complexation of the alkyne (Ir1–alkyne = 2.505(2) Å): a binding mode, for which there are no crystallographically characterised Group 9 precedents to our knowledge (CSD 5.41).³⁴ The respective alkenyl Ir–C and C=C bond lengths are not statistically different (Ir1–C4 = 2.043(2) Å, Ir1–C6 = 2.053(2) Å; C4–C5 = 1.330(3) Å, C6–C9 = 1.329(3) Å). The structure of 10 determined by X-ray crystallography was fully corroborated in solution using NMR spectroscopy. For instance, the alkenyl ¹H resonances are located at δ 7.81 (IrCH̲CHtBu; ³J_HH = 15.0 Hz), 5.68 (IrCCH̲tBu), and 4.78 (IrCHCH̲tBu; ³J_HH = 15.2 Hz) in a 1 : 1 : 1 ratio, with the associated ¹³C resonances at δ 143.5 (IrCHC̲HtBu), 142.2 (IrCC̲HtBu), 105.3 (IrC̲CHtBu), and 95.7 (IrC̲HCHtBu); the α-carbons exhibiting coupling to ³¹P (²J_PC = 5–10 Hz). The HR-ESI MS of 10 is also notable for a strong [M]⁺ ion signal at 916.5623 (calcd 916.5628) m/z and bulk purity of was confirmed by combustion analysis.

Scheme 4 Mechanistic features of the terminal alkyne coupling reactions promoted by 1 and 1′: [M] = M(PNP-14)⁺ and [BAr^F₄]⁻ counter anions omitted. Solid-state structure of 10 depicted with thermal ellipsoids at 30% probability for selected atoms; minor disordered component (IrP₂ core), most H atoms, and anion omitted; structural diagram provided in the experimental section. Selected bond lengths (Å) and angles (°): Ir1–P2, 2.3141(5); Ir1–P3, 2.3659(5); P2–Ir1–P3, 163.55(2); Ir1–N101, 2.122(2); Ir–C4, 2.043(2); Ir–C4–C5, 129.59(15); C4–C5, 1.330(3); Ir1–C6, 2.053(2); Ir1–C6–C9, 147.4(2); C6–C9, 1.329(3); Ir1–Cnt(C7, C8), 2.505(2); C6–C7–C8, 161.9(2); C7–C8, 1.216(3); N101–Ir1–C6, 159.35(8); C4–Ir1–Cnt(C7, C8), 153.16(7); C4–Ir1–C6, 102.50(9); Cnt = centroid.

Terminal alkyne homocoupling reactions that produce Z-enyne products are generally understood to proceed via vinylidene intermediates, with 11 implicated in this case (Scheme 4).^35,36 Indeed, the rhodium homologue 11′ is produced initially upon reaction of 1′ with HC≡CtBu.¹⁵ Reaction with the second alkyne equivalent, by net 1,2-addition of the constituent C(sp)–H bond across the vinylidene M=C linkage (concerted or step-wise) followed by reductive elimination, would thereafter confer the enyne product. E-Enyne isomers such as that observed in the rhodium system can also be produced in this manner, although an indirect route involving equilibrium generation of the rhodium(III) alkynyl hydride 12′ is instead invoked in the formation of 9′ from 11′ (Scheme 4). Whilst 11 was not detected during the formation of Z-tBuC≡CCHCHtBu, the generation of 10 provides strong circumstantial evidence for its intermediate presence. No reaction between 1 and independently synthesised Z-tBuC≡CCHCHtBu in DFB was observed, even upon heating at 50 °C for 1 h. The formation of the bis(alkenyl) is, therefore, most reasonably reconciled by irreversible reaction of Z-tBuC≡CCHCHtBu with 11; involving net 1,2-addition of the {C≡C}C(sp²)–H bond across the Ir=C linkage. The addition evidently takes place through the ring in this instance, with the macrocycle preventing subsequent reductive elimination.¹² We therefore attribute the generation of 10 to catalyst deactivation by irreversible product inhibition. The postulated reactivity of the Group 9 vinylidenes derived from 1 and 1′ is clearly nuanced by the nature of the metal and impact of the unique steric constraints imposed by the tetradecamethylene linker. We believe that the more facile Ir(I)/Ir(III) redox couple and propensity of the {Ir(PNP-14)}⁺ fragment to adopt geometries with the pincer ligand in a non-meridional conformation are the decisive factors. Specifically, we propose that the Z-selective homocoupling of HC≡CtBu proceeds catalytically outside the ring via C(sp)–H bond oxidative addition of HC≡CtBu to 11 affording fac-[Ir(PNP-14)(CCHtBu)(C≡CtBu)H]⁺, alkynyl migration yielding an enynyl hydride, and finally release of Z-tBuC≡CCHCHtBu by reductive elimination. In contrast, 1′ mediates the stoichiometric E-selective homocoupling of HC≡CtBu through the ring ultimately via a pathway bypassing the vinylidene intermediate 11′. Further computational analysis would be required to corroborate these suggestions, although accurately modelling the effect of the methylene chain is non-trivial.

3. Conclusions

The organometallic chemistry of iridium complexes of the macrocyclic PNP-14 pincer ligand has been explored. The five-coordinate iridium(I) and iridium(III) complexes [Ir(PNP-14)(η²:η²-COD)][BAr^F₄] 1 and [Ir(PNP-14)(biph)][BAr^F₄] 2 are readily prepared and fully characterised derivatives, with the former notable for a distorted trigonal bipyramidal metal geometry in which the pincer ligand adopts an unusual non-meridional confirmation, and the latter for adoption of a square pyramidal metal geometry stabilised by a weak γ-agostic interaction between the metal and the tetradecamethylene linker. These well-defined complexes have been shown to be effective precursors for the generation of iridium(III) dihydride dihydrogen (4), iridium(I) bis(ethylene) (6), and iridium(I) carbonyl (8) derivatives that highlight important periodic trends by comparison to rhodium counterparts: i.e. the more facile oxidative addition of dihydrogen, propensity to from five-coordinate d⁸ complexes, and greater π-basicity of the heavier metal conger, respectively.

Onward reactivity of the {Ir(PNP-14)}⁺ fragment was also explored with the bulky unsaturated substrates 3,3-dimethylbutene and 3,3-dimethylbutyne. Reaction of 4 with 3,3-dimethylbutene induced triple C–H bond activation of the methylene chain yielding an iridium(III) allyl hydride complex [Ir(PNP-14*)H][BAr^F₄] 7, whilst 1 is an effective pre-catalyst for the homocoupling of 3,3-dimethylbutyne into Z-tBuC≡CCHCHtBu under mild conditions. The latter is particularly remarkable given that reaction of the homologous rhodium precursor 1′ results in the formation of an interpenetrated E-enyne complex (9′). The mechanism of the homocoupling promoted by 1 is proposed to involve formation and direct reaction of the (unobserved) vinylidene derivative [Ir(PNP-14)(CCHtBu)][BAr^F₄] (11) with HC≡CtBu outside of the macrocyclic ring. This suggestion is supported experimentally by isolation and crystallographic characterisation of [Ir(PNP-14)(η³-E-C(C≡CtBu)CHtBu)(η¹-E-CHCHtBu)][BAr^F₄] (10), which results from deactivation of the catalyst by product inhibition.

4. Experimental

4.1 General methods

All manipulations were performed under an atmosphere of argon using Schlenk and glove box techniques unless otherwise stated. Dihydrogen and ethylene were dried by passage through a column of activated 3 Å molecular sieves. Glassware was oven dried at 150 °C overnight and flame-dried under vacuum prior to use. Molecular sieves were activated by heating at 300 °C in vacuo overnight. Fluorobenzene and DFB were pre-dried over Al₂O₃, distilled from calcium hydride and dried twice over 3 Å molecular sieves.³⁷ CD₂Cl₂ was freeze–pump–thaw degassed and dried over 3 Å molecular sieves. C₆D₆ was distilled from sodium and stored over 3 Å molecular sieves. SiMe₄ was distilled from liquid Na/K alloy and stored over a potassium mirror. Other anhydrous solvents and liquid reagents were purchased from Acros Organics or Sigma-Aldrich, freeze–pump–thaw degassed and stored over 3 Å molecular sieves. PNP-14,¹⁴ [Ir(COD)₂][BAr^F₄],¹⁷ [Ir(biph)(COD)Cl]₂,¹⁸ Na[BAr^F₄],³⁸ and [Ir(PNP-tBu)(biph)][BAr^F₄] (II)⁶ were synthesized according to published procedures. All other solid reagents are commercial products and were used as received. NMR spectra were recorded on Bruker spectrometers under argon at 298 K unless otherwise stated. Chemical shifts are quoted in ppm and coupling constants in Hz. Virtual coupling constants are reported as the separation between the first and third lines. NMR spectra in DFB were recorded using an internal capillary of C₆D₆ or acetone-d₆.³⁷ High resolution ESI-MS were recorded on Bruker Maxis Plus instrument. Infrared spectra were recorded on a Jasco FT-IR-4700 using a KBr transmission cell in CH₂Cl₂. Microanalyses were performed at the London Metropolitan University by Stephen Boyer or Elemental Microanalysis Ltd.

4.2 Preparation of [Ir(PNP-14)(η²:η²-COD)][BAr^F₄] (1)

A solution of [Ir(COD)₂][BAr^F₄] (29.3 mg, 23.0 μmol) and PNP-14 (11.0 mg, 23.0 μmol) in DFB (0.5 mL) was mixed for 5 min at RT, the volatiles were removed in vacuo and the resulting orange oil washed with pentane (2 × 2 mL). The analytically pure product was obtained as a yellow crystalline solid by slow diffusion of SiMe₄ (ca. 10 mL) into a DFB solution (0.5 mL) at −30 °C. Yield: 29.0 mg (17.7 μmol, 77%).

¹ H NMR (500 MHz, DFB): δ 8.11–8.16 (m, 8H, Ar^F), 7.50 (br, 4H, Ar^F), 7.33 (t, ³J_HH = 7.7, 1H, py), 7.08 (d, ³J_HH = 7.7, 1H, py), 7.03 (obscured, py), 4.25–4.34 (m, 1H, Ir(CH=CH){axial}), 4.34–4.44 (m, 1H, Ir(CH=CH){axial}), 3.72 (d, ²J_PH = 6.6, 2H, pyCH̲₂), 3.51 (dd, ²J_HH = 18.2, ²J_PH = 6.0, 1H, pyCH̲₂), 3.34 (dd, ²J_HH = 18.2, ²J_PH = 9.7, 1H, pyCH̲₂), 2.54–2.66 (m, 2H, CH₂), 2.15–2.45 (m, 6H, CH₂ + 1 × Ir(CH=CH){equatorial} [δ 2.39]), 1.38–2.04 (m, 9H, CH₂ + 1 × Ir(CH=CH){equatorial} [δ 1.83]), 1.38 (d, ³J_PH = 12.7, 9H, tBu), 0.94–1.32 (m, 19H, CH₂), 0.65 (d, ³J_PH = 12.5, 9H, tBu).

¹³ C{ ¹ H} NMR (126 MHz, DFB): δ 163.7 (dd, J_PC = 5, 4, py), 162.5 (q, ¹J_CB = 50, Ar^F), 162.4 (obscured, py), 138.2 (s, py), 135.1 (s, Ar^F), 129.7 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 124.9 (q, ¹J_FC = 272, Ar^F), 122.0 (d, ³J_PC = 6, py), 120.7 (d, ³J_PC = 8, py), 117.6 (sept, ³J_FC = 4, Ar^F), 64.7 (br, Ir(CH=CH){axial}), 60.9 (d, ²J_PC = 3, Ir(CH=CH){axial}), 60.2 (dd, ²J_PC = 27, 4, Ir(CH=CH){equatorial}), 50.1 (dd, ²J_PC = 29, ²J_PC = 5, Ir(CH=CH){equatorial}), 45.6 (dd, ¹J_PC = 27, ³J_PC = 4, pyC̲H₂) 42.3 (d, ¹J_PC = 22, pyC̲H₂), 35.8 (d, ³J_PC = 10, CH₂), 35.5 (br, CH₂), 35.2 (dd, ¹J_PC = 19, ³J_PC = 4, tBu{C}), 34.1 (d, ¹J_PC = 7, PCH₂), 33.4 (br, tBu{C}), 31.1 (d, ³J_PC = 10, CH₂), 29.8 (d, ³J_PC = 9, CH₂), 29.7 (s, CH₂), 29.5 (s, CH₂), 29.3 (d, ¹J_PC = 17, PCH₂), 29.0 (s, CH₂), 28.8 (s, CH₂), 28.7 (s, CH₂), 28.6 (br, CH₂), 28.3 (s, CH₂), 28.12 (s, CH₂), 28.05 (s, CH₂), 27.8 (d, ²J_PC = 4, tBu{CH₃}), 27.1 (d, ³J_PC = 7, CH₂), 27.0 (s, CH₂), 26.6 (s, CH₂), 26.4 (d, ²J_PC = 5, tBu{CH₃}).

³¹ P{ ¹ H} NMR (162 MHz, DFB): δ 17.0 (s, 1P), 13.4 (s, 1P).

HR ESI-MS (positive ion 4 kV): 778.4221 ([M]⁺, calcd 778.4218) m/z.

Anal. calcd for C₆₉H₇₇BF₂₄IrNP₂ (1641.32 g mol⁻¹): C, 50.49; H, 4.73; N, 0.85. Found: C, 50.40; H, 4.64; N, 0.87.

4.3 NMR scale reaction of 1 with dihydrogen

A solution of 1 (12.2 mg, 7.43 μmol) in DFB (0.5 mL) within a J. Young valve NMR tube was freeze–pump–thaw degassed and placed under an atmosphere of dihydrogen (1 atm). Analysis by NMR spectroscopy indicated quantitative formation of 4 with concomitant formation of COD within 5 min at RT.

4.4 Preparation of [Ir(PNP-14)(biph)][BAr^F₄] (2)

A suspension of PNP-14 (13.3 mg, 27.8 μmol) and [Ir(biph)(COD)Cl]₂ (13.7 mg, 14.0 μmol) in fluorobenzene (0.50 mL) was stirred for 2 days at 50 °C to give a pale-yellow solution. Na[BAr^F₄] (24.7 mg, 27.9 μmol) was added and the suspension stirred for a further 4 h at RT. The volatiles were removed in vacuo and the resulting purple oil was washed with pentane (2 × 1 mL), dried in vacuo and then the product extracted into CH₂Cl₂ (2 mL)_. The analytically pure product was obtained as a purple crystalline solid by recrystallisation from CH₂Cl₂: hexane (1 : 20) at −30 °C. Yield: 36.4 mg (21.6 μmol, 78%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.00 (t, ³J_HH = 7.9, py, 1H), 7.70–7.76 (m, 10H, py + Ar^F), 7.64 (d, ³J_HH = 7.6, 1H, biph), 7.60 (d, ³J_HH = 7.5, 1H, biph), 7.56 (br, 4H, Ar^F), 7.39 (d, ³J_HH = 7.6, 1H, biph), 7.19 (t, ³J_HH = 7.3, 1H, biph), 7.10 (t, ³J_HH = 7.5, 1H, biph), 6.88 (t, ³J_HH = 7.4, 1H, biph), 6.33 (t, ³J_HH = 7.8, 1H, biph), 5.35 (d, ³J_HH = 8.2, 1H, biph), 4.02 (dd, ²J_HH = 19.4, ²J_PH = 9.6, 1H, pyCH̲₂), 3.75–3.89 (m, 2H, 2 × pyCH̲₂), 3.50 (dd, ³J_HH = 17.0, ³J_HH = 9.2, 1H, pyCH̲₂), 3.02–3.15 (m, 1H, PCH₂), 2.78–2.88 (m, 1H, PCH₂), 1.88–1.98 (m, 1H, CH₂), 0.66–1.73 (m, 23H, CH₂), 1.16 (d, ³J_PH = 14.0, 9H, tBu), 0.49 (d, ³J_PH = 16.1, 9H, tBu), 0.22–0.37 (m, 2H, CH₂).

¹³ C{ ¹ H} NMR (126 MHz, CD₂Cl₂): δ 164.6 (app t, J_PC = 4, py), 163.3 (br, py), 162.3 (q, ¹J_CB = 50, Ar^F), 150.6 (d, ³J_PC = 2 biph), 149.6 (s, biph), 145.3 (dd, ²J_PC = 8, 6, biph{IrC}), 139.9 (s, py), 135.4 (s, Ar^F), 135.2 (s, biph), 129.42 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 129.39 (s, biph), 126.3 (s, biph), 125.6 (s, biph), 125.3 (s, biph), 125.1 (q, ¹J_FC = 272, Ar^F), 123.5 (s, biph), 123.2 (d, ³J_PC = 10, py), 123.1 (d, ³J_PC = 9, py), 122.1 (s, biph), 121.3 (s, biph), 121.2 (app t, ²J_PC = 6, biph{IrC}), 118.0 (sept, ³J_FC = 4, Ar^F), 40.9 (d, ¹J_PC = 29, pyC̲H₂), 39.5 (d, ¹J_PC = 26, pyC̲H₂), 35.0 (d, ¹J_PC = 23, tBu{C}), 32.9 (d, ²J_PC = 14, CH₂), 32.8 (obscured, tBu{C}), 30.4 (s, CH₂), 29.6 (s, CH₂), 29.5 (s, CH₂), 29.42 (s, CH₂), 29.35 (s, CH₂), 29.2 (s, CH₂), 29.1 (d, ²J_PC = 3, tBu{CH₃}), 28.1 (s, CH₂), 27.9 (d, ¹J_PC = 28, PCH₂), 27.3 (s, CH₂), 25.8 (br, CH₂), 25.5 (s, tBu{CH₃}), 24.7 (s, CH₂), 24.1 (s, CH₂), 19.7 (dd, ¹J_PC = 22, ³J_PC = 3, PCH₂).

³¹ P{ ¹ H} NMR (162 MHz, CD₂Cl₂): δ 38.7 (d, ²J_PP = 307, 1P), 20.9 (d, ²J_PP = 307, 1P).

HR ESI-MS (positive ion, 4 kV): 822.3912 ([M]⁺, calcd 822.3906) m/z.

Anal. calcd for C₇₃H₇₃BF₂₄IrNP₂ (1685.33 g mol⁻¹): C, 52.03; H, 4.37; N, 0.83; found: C, 51.88; H, 4.28; N, 0.81.

4.5 NMR scale reactions of 2

The following reactions were carried out starting with a solution of 2 (16.9 mg, 10.0 μmol) in DFB (0.5 mL) within a J. Young valve NMR tube and analysed in situ by NMR spectroscopy.

4.5.1 Synthesis of [Ir(PNP-14)(2-biphenyl)H][BAr^F₄] (3). The solution of 2 was freeze–pump–thaw degassed and placed under dihydrogen (1 atm), resulting in quantitative formation of 3 within 5 min at RT. No free biphenyl was observed.

¹ H NMR (400 MHz, DFB, H₂, selected data): δ 7.61 (t, ³J_HH = 7.8, 1H py), 3.66–3.82 (m, 2H, 2 × pyCH̲₂), 3.06 (dd, ²J_HH = 17.3, ²J_PH = 6.4, 1H, pyCH̲₂), 0.79 (d, ³J_PH = 13.4, 9H, tBu), 0.67 (d, ³J_PH = 14.1, 9H, tBu), −21.6 (app t, ²J_PH = 15, 1H, IrH).

³¹ P{partial ¹ H} NMR (162 MHz, DFB, H₂): δ 40.6 (dd, ²J_PP = 302, ²J_PH = 14, 1P), 36.5 (dd, ²J_PP = 302, ²J_PH = 14, 1P).

4.5.2 Synthesis of [Ir(PNP-14)H₂(H₂)][BAr^F₄] (4). The solution of 3 was heated at 85 °C for 6 h, resulting quantitative formation 4 with concomitant formation of biphenyl (δ_¹H 7.42, 7.25, 7.16).

¹ H NMR (500 MHz, DFB, H₂): δ 8.10–8.16 (m, 8H, Ar^F), 7.49 (br, 4H, Ar^F), 7.47 (t, ³J_HH = 7.8, 1H, py), 7.19 (d, ³J_HH = 7.8, 2H, py), 3.96 (dvt, ²J_HH = 17.6, J_PH = 8, 2H, pyCH̲₂), 3.12 (dvt, ²J_HH = 17.6, J_PH = 10, 2H, pyCH̲₂), 2.11–2.23 (m, 2H, PCH₂), 1.12–1.69 (m, 26H, CH₂), 0.91 (vt, J_PH = 16, 18H, tBu), −9.27 (br, fwhm = 24 Hz, 4H, IrH₄).

¹³ C{ ¹ H} NMR (126 MHz, DFB, H₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 162.4 (vt, J_PC = 6, py), 138.9 (s, py), 135.1 (s, Ar^F), 129.7 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 124.9 (q, ¹J_FC = 272, Ar^F), 120.4 (vt, J_PC = 10, py), 117.6 (sept, ³J_FC = 4, Ar^F), 44.4 (vt, J_PC = 28, pyC̲H₂), 30.0 (vt, J_PC = 32, tBu{C}), 29.0 (s, CH₂), 28.9 (vt, J_PC = 8, CH₂), 28.3 (s, CH₂), 28.2 (s, CH₂), 27.5 (s, CH₂), 26.3 (s, CH₂), 25.8 (vt, J_PC = 32, PCH₂), 24.5 (vt, J_PC = 6, tBu{CH₃}).

³¹ P{ ¹ H} NMR (162 MHz, DFB, H₂,): δ 42.1 (s, 2P).

¹ H NMR (600 MHz, DFB, Ar, 298 K, selected data): δ −9.26 (br, fwhm = 29 Hz, T₁ = 88.8 ± 0.7 ms, 4H, IrH).

¹ H NMR (600 MHz, DFB, Ar, 253 K, selected data): δ −9.27 (br, fwhm = 21 Hz, T₁ = 50 ± 1, 4H, IrH).

4.5.3 Synthesis of [Ir(PNP-14)H₂(C₂H₄)][BAr^F₄] (5). The solution of 4 was freeze–pump–thaw degassed and placed under ethylene (1 atm), resulting in quantitative formation of 5 within 5 min at RT.

¹ H NMR (500 MHz, DFB, C₂H₄, selected data): δ 7.43 (t, ³J_HH = 7.8, 1H, py), 3.71–3.84 (m, 2H, 2 × pyCH̲₂), 3.18–3.37 (m, 5H, 1 × pyCH̲₂ + 4 × C₂H₄), 3.08 (dd, ²J_HH = 17.6, ²J_PH = 10.4, 1H, pyCH̲₂), 0.89 (d, ³J_PH = 14.9, 9H, tBu), 0.87 (d, ³J_PH = 13.9, 9H, tBu), −7.89 (dd, ²J_PH = 17.6, ²J_PH = 13.1, 1H, IrH), −17.80 (app t, J_PH = 11, 1H, IrH).

¹³ C{ ¹ H} NMR (126 MHz, DFB, C₂H₄, selected data): δ 48.7 (s, C₂H₄).

³¹ P{ ¹ H} NMR (162 MHz, DFB, C₂H₄): δ 33.4 (d, ²J_PP = 314, 1P), 12.4 (d, ²J_PP = 314, 1P).

4.5.4 Synthesis of [Ir(PNP-14)(C₂H₄)₂][BAr^F₄] (6). The solution of 5 was heated at 85 °C for 16 h, resulting in quantitative formation of 6, with concomitant formation of ethane (δ_¹H 0.70, δ_¹³C 6.1).

¹ H NMR (600 MHz, DFB, C₂H₄): δ 8.11–8.15 (m, 8H, Ar^F₄), 7.50 (br, 4H, Ar^F₄), 7.46 (t, ³J_HH = 7.8, 1H, py), 7.15 (d, ³J_HH = 7.8, 2H, py), 3.63 (dvt, ²J_HH = 16.8, J_PH = 8, 2H, pyCH̲₂), 3.28 (dvt, ²J_HH = 16.8, J_PH = 8, 2H, pyCH̲₂), 3.23 (br, 2H, C₂H₄), 2.80 (br, 2H, C₂H₄), 2.49 (br, 2H, C₂H₄), 1.97 (br, 2H, C₂H₄), 1.03–1.48 (m, 28H, CH₂), 0.79 (br, 18H, tBu).

¹³ C{ ¹ H} NMR (126 MHz, DFB, C₂H₄): δ 164.1 (d, ³J_PC = 3, py), 162.6 (q, ¹J_CB = 50, Ar^F), 139.1 (s, py), 135.1 (s, Ar^F), 129.7 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 124.9 (q, ¹J_FC = 272, Ar^F), 119.8 (vt, J_PC = 8, py), 117.6 (sept, ³J_FC = 4, Ar^F), 42.6 (vt, J_PC = 28, pyC̲H₂), 33.5 (vt, J_PC = 26, tBu{C}), 31.0 (s, CH₂), 29.1 (vt, J_PC = 12, CH₂), 27.8 (s, CH₂), 27.3 (s, CH₂), 27.2 (s, CH₂), 26.3 (s, tBu{CH₃}), 26.0 (s, C₂H₄), 24.6 (s, CH₂), 18.0 (s, C₂H₄), 14.3 (vt, J_PC = 24, PCH₂).

³¹ P{ ¹ H} NMR (162 MHz, DFB, C₂H₄): δ 9.0 (s, 2P).

4.5.5 Synthesis and isolation of [Ir(PNP-14)(CO)][BAr^F₄] (8). The solution of 6 was freeze–pump–thaw degassed and placed under carbon monoxide (1 atm), resulting in an immediate colour change from colourless to bright yellow. The volatiles were removed in vacuo, and the resulting yellow oil washed with SiMe₄ (2 × 0.5 mL) and thoroughly dried in vacuo to give the analytically pure product as a yellow foam. Yield: 13.9 mg (8.90 μmol, 89%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 7.87 (t, ³J_HH = 7.8, 1H, py), 7.70–7.76 (m, 8H, Ar^F), 7.56 (br, 4H, Ar^F), 7.51 (d, ³J_HH = 7.9, 2H, py), 3.94 (dvt, ²J_HH = 17.6, J_PH = 8, 2H, pyCH̲₂), 3.47 (dvt, ²J_HH = 17.6, J_PH = 8, 2H, pyCH̲₂), 2.18–2.27 (m, 4H, PCH₂), 1.94–2.07 (m, 2H, CH₂), 1.76–1.89 (m, 2H, CH₂), 1.64–1.74 (m, 2H, CH₂), 1.54–1.64 (m, 2H, CH₂), 1.23–1.48 (m, 16H, CH₂), 1.14 (vt, J_PH = 16, 18H, tBu)

¹³ C{ ¹ H} NMR (126 MHz, CD₂Cl₂): δ 182.5 (vt, ²J_PC = 18, CO), 165.4 (vt, J_PC = 10, py), 162.3 (q, ¹J_CB = 50, Ar^F), 141.9 (s, py), 135.4 (s, Ar^F), 129.4 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 125.2 (q, ¹J_FC = 272, Ar^F), 122.0 (vt, J_PC = 10, py), 118.0 (sept, ³J_FC = 4, Ar^F), 39.4 (vt, J_PC = 24, pyC̲H₂), 34.7 (vt, J_PC = 30, tBu{C}), 30.3 (vt, J_PC = 10 CH₂), 29.3 (s, CH₂), 29.0 (s, CH₂), 28.9 (s, CH₂), 28.3 (s, CH₂), 27.5 (vt, J_PC = 6, tBu{CH₃}), 26.1 (s, CH₂), 23.4 (vt, J_PC = 30, PCH₂).

³¹ P{ ¹ H} NMR (162 MHz, CD₂Cl₂): δ 64.6 (s, 2P).

³¹ P{ ¹ H} NMR (162 MHz, DFB): δ 62.5 (s, 2P).

IR (CH₂Cl₂): ν(CO) 1984 cm⁻¹.

HR ESI-MS (positive ion, 4 kV): 698.3217 ([M]⁺, calcd 698.3228) m/z.

Anal. calcd for C₆₂H₆₅BF₂₄IrNOP₂ (1561.14 g mol⁻¹): C, 47.70; H, 4.20; N, 0.90; found: C, 47.89; H, 4.13; N, 0.97.

4.6 NMR scale reaction of [Ir(PNP-tBu)(biph)][BAr^F₄] (II) with dihydrogen

A solution of II (16.0 mg, 9.98 μmol) in DFB (0.5 mL) within a J. Young valve NMR tube was freeze–pump–thaw degassed and placed under dihydrogen (1 atm) and then heated at 80 °C for 2 days. Analysis by NMR spectroscopy indicated quantitative formation of [Ir(PNP-tBu)H₂(H₂)][BAr^F₄]. The spectroscopic data are consistent with the literature for the related BF₄⁻ salt.⁷

¹ H NMR (400 MHz, CD₂Cl₂, selected data): δ 8.11–8.17 (m, 8H, Ar^F), 7.50 (s, 4H, Ar^F), 3.52 (vt, J_PH = 8, 4H, pyCH̲₂), 1.11 (vt, J_PH = 14, 36H, tBu), −9.48 (br, 4H, IrH).

³¹ P{ ¹ H} NMR (162 MHz, DFB): δ 64.6 (s, 2P).

4.7 Preparation of [Ir(PNP-14*)H][BAr^F₄] (7)

A solution of 4 (13.6 μmol, generated in situ as described above) in DFB (0.5 mL) within a J. Young valve NMR tube was treated with 3,3-dimethylbutene (8.8 μL, 71.4 μmol) and the solution heated at 100 °C for 5 days. Analysis by NMR spectroscopy indicated quantitative formation of the product. The volatiles were removed in vacuo and the resulting pink/red oil washed with SiMe₄ (2 × 0.5 mL). The analytically pure product was obtained as pale red blocks by the slow diffusion of excess SiMe₄ into a DFB solution at −30 °C. Yield: 10.2 mg (6.66 μmol, 49%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 7.70–7.75 (m, 8H, Ar^F), 7.65 (t, ³J_HH = 7.8, 1H, py), 7.56 (br, 4H, Ar^F), 7.34 (d, ³J_HH = 7.8, 1H, py), 7.31 (d, ³J_HH = 7.8, 1H, py), 4.86 (app q, J_HH = 9, 1H, IrCH), 4.47 (app t, ³J_HH = 7, 1H, IrCH), 3.71–3.81 (m, 2H, 2 × pyCH̲₂), 3.41 (dd, ²J_HH = 17.1, ²J_PH = 9.3, 1H, pyCH̲₂), 3.26 (dd, ²J_HH = 17.4, ²J_PH = 9.2, 1H, pyCH̲₂), 2.35–2.52 (m, 1H, CH₂), 2.15–2.29 (m, 1H, CH₂), 1.98–2.11 (m, 1H, CH₂), 0.94–1.95 (m, 20H, IrCH [δ 1.89] + CH₂), 1.17 (d, ³J_PH = 15.4, 9H, tBu), 1.01 (d, ³J_PH = 13.5, 9H, tBu), −8.49 (dd, ²J_PH = 19.6, 9.7, 1H, IrH).

¹³ C{ ¹ H} NMR (126 MHz, CD₂Cl₂): δ 163.4 (br, py), 162.3 (q, ¹J_CB = 50, Ar^F), 161.9 (br, py), 138.5 (s, py), 135.4 (s, Ar^F), 129.4 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 125.1 (q, ¹J_FC = 272, Ar^F), 121.0 (d, ³J_PC = 9, py), 120.8 (d, ³J_PC = 9, py), 118.0 (sept, ³J_FC = 4, Ar^F), 81.9 (s, IrCH), 66.0 (d, ²J_PC = 5, IrCH), 41.9 (d, ¹J_PC = 30, pyC̲H₂), 39.2 (d, ¹J_PC = 29, pyC̲H₂), 38.9 (s, IrCH), 35.9 (s, CH₂), 35.0 (dd, ¹J_PC = 22, ³J_PC = 5, tBu{C}), 30.4 (dd, ¹J_PC = 22, ³J_PC = 5, tBu{C}), 29.6 (s, CH₂), 27.8 (d, ²J_PC = 14, CH₂), 27.4 (s, CH₂), 27.0 (d, ²J_PC = 3, tBu{CH₃}), 25.7 (d, ²J_PC = 4, tBu{CH₃}), 24.8 (dd, ¹J_PC = 28, ³J_PC = 1, PCH₂), 24.4 (s, CH₂), 24.3 (d, ²J_PC = 5, CH₂), 22.9 (s, CH₂), 22.3 (s, CH₂), 21.6 (s, CH₂), 20.8 (dd, ¹J_PC = 24, ³J_PC = 4, PCH₂).

³¹ P{ ¹ H} NMR (162 MHz, CD₂Cl₂): δ 81.4 (d, ²J_PP = 313, 1P), 25.8 (d, ²J_PP = 313, 1P).

³¹ P{ ¹ H} NMR (162 MHz, DFB): δ 80.9 (d, ²J_PP = 313, 1P), 25.5 (d, ²J_PP = 313, 1P).

HR ESI-MS (positive ion 4 kV): 668.3128 ([M]⁺, calcd 668.3122) m/z.

Anal. calcd for C₆₂H₆₇BF₂₄IrNP₂ (1531.12 g mol⁻¹): C, 47.85; H, 4.15; N, 0.91. Found: C, 47.65; H, 4.03; N, 1.01.

4.8 NMR scale reaction of 7 with dihydrogen

A solution of 7 (7.7 mg, 5.03 μmol) in DFB (0.5 mL) within a J Young valve NMR tube was freeze–pump–thaw degassed, placed under dihydrogen (1 atm) and heated at 80 °C for 16 h. Analysis by NMR spectroscopy indicated quantitative formation of 4.

4.9 Catalytic homocoupling of 3,3-dimethylbutyne promoted by 1

A solution of 1 (8.2 mg, 5.0 μmol) in DFB (400 μL) within a J. Young NMR tube was treated with a solution of 3,3-dimethylbutyne (62 μL, 503 μmol) and the resulting homocoupling reaction producing only Z-tBuC≡CCHCHtBu followed at RT in situ using NMR spectroscopy. The solution was mixed constantly when not in the spectrometer. Analysis after 1 hour indicated 28 TONs, with complete conversion observed within 6 h. At this point, 10 was observed as the major organometallic species (88%), with 1 the minor organometallic species (10%). Further 3,3-dimethylbutyne (31 μL, 252 μmol) was added, resulting in further homocoupling, totalling 65 TONs and complete conversion of 1 to 10 after 24 h. Spectroscopic data for Z-tBuC≡CCHCHtBu is consistent with literature values.³²

Data for Z-tBuC≡CCHCHtBu:¹H NMR (400 MHz, DFB, selected data): δ 5.53 (d, ³J_HH = 11.9, 1H, CH=CH), 5.25 (d, ³J_HH = 11.9, 1H, CH=CH), 1.12 (s, 9H, tBu), 1.11 (s, 9H, tBu).

4.10 Preparation of [Ir(PNP-14)(η³-E-C(C≡CtBu)CHtBu)(η¹-E-CHCHtBu)][BAr^F₄] (10)

A solution of 1 (14.0 mg, 8.53 μmol) in DFB (0.5 mL) within a J Young valve NMR tube was treated with 3,3-dimethylbutyne (210 μL, 1.71 mmol) and stirred for 6 h at RT. Analysis by NMR spectroscopy indicated quantitative formation of the product. The volatiles were removed in vacuo, the resulting yellow oil washed with SiMe₄ (2 × 0.5 mL) and dried in vacuo to afford the analytically product as a yellow solid. Yield: 11.6 mg (6.52 μmol, 76%). Crystals suitable X-ray crystallography were obtained by slow diffusion of excess SiMe₄ into an Et₂O solution. From the SiMe₄ washings Z-tBuC≡CCHCHtBu was obtained as a colourless oil in low yield. Yield: 4.6 mg (28.0 μmol, 2%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 7.83 (t, ³J_HH = 6.7, 1H, py), 7.81 (dd, ³J_HH = 15.0, ³J_PH = 3, 1H, IrCH̲CHtBu), 7.70–7.75 (m, 8H, Ar^F), 7.56 (br, 4H, Ar^F), 7.52 (d, ³J_HH = 7.9, 1H, py), 7.49 (d, ³J_HH = 7.9, 1H, py), 5.68 (t, ⁴J_PC = 2, 1H, IrCCH̲tBu), 4.78 (dt, ³J_HH = 15.2, ⁴J_PH = 2, 1H, IrCHCH̲tBu), 4.28 (ddd, ²J_HH = 16.5, ²J_PH = 10.3, ⁴J_PH = 3.0, 1H, pyCH̲₂), 3.69 (ddd, ²J_HH = 17.4, ²J_PH = 10.1, ⁴J_PH = 1.3, 1H, pyCH̲₂), 3.64 (dd, ²J_HH = 17.4, ²J_PH = 9.9, 1H, pyCH̲₂), 3.22 (dd, ²J_HH = 16.5, ²J_PH = 7.9, 1H, pyCH̲₂), 2.28–2.43 (m, 2H, CH₂), 2.08–2.19 (m, 1H, CH₂), 0.88–1.96 (m, 25H, CH₂), 1.24 (s, 9H, IrCCHt̲B̲u̲), 1.17 (s, 9H, C≡Ct̲B̲u̲),1.02 (d, ³J_PH = 14, 18H, 2 × PtBu), 1.00 (s, 9H, IrCHCHt̲B̲u̲).

¹³ C{ ¹ H} NMR (126 MHz, CD₂Cl₂): δ 164.2 (app t, J_PC = 3, py), 163.3 (app t, J_PC = 3, py), 162.3 (q, ¹J_CB = 50, Ar^F), 143.5 (app t, ³J_PC = 4, IrCHC̲HtBu), 142.2 (br, IrCC̲HtBu), 139.4 (s, py), 135.4 (s, Ar^F), 129.4 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 125.1 (q, ¹J_FC = 272, Ar^F), 121.7 (d, ³J_PC = 8, py), 121.5 (d, ³J_PC = 8, py), 118.0 (sept, ³J_FC = 4, Ar^F), 116.9 (s, C≡C̲tBu), 105.3 (dd, ²J_PC = 8, ²J_PC = 5, IrC̲CHtBu), 95.7 (app t, ²J_PC = 10, IrC̲HCHtBu), 60.1 (s, C̲≡CtBu), 46.0 (d, ¹J_PC = 25, pyC̲H₂), 40.8 (d, ¹J_PC = 30, pyC̲H₂), 37.3 (s, CHCHtBu{C}), 37.1 (s, CCHtBu{C}), 36.6 (dd, ¹J_PC = 21, ³J_PC = 4, PtBu{C}), 34.6 (dd, ¹J_PC = 21, ³J_PC = 6, PtBu{C}), 33.3 (d, ²J_PC = 15, CH₂), 32.2 (s, CH₂), 31.8 (s, C≡CtBu{C}), 31.4 (s, CH₂), 31.28 (s, tBu{CH₃}), 31.26 (s, tBu{CH₃}), 31.1 (s, CH₂), 30.5 (s, CH₂), 30.3 (s, CH₂), 30.2 (s, CH₂), 29.7 (s, CHCHtBu{CH₃}), 28.8 (br, CH₂), 28.20 (d, ²J_PC = 4, CH₂), 28.17 (s, CH₂), 27.1 (d, ²J_PC = 3, 2 × PtBu{CH₃}), 26.6 (s, CH₂), 24.9 (s, CH₂), 21.9 (dd, ¹J_PC = 30, ³J_PC = 2, PCH₂), 19.5 (dd, ¹J_PC = 19, ³J_PC = 3, PCH₂).

³¹ P{ ¹ H} NMR (162 MHz, CD₂Cl₂): δ 25.5 (d, ²J_PP = 364, 1P), 16.8 (d, ²J_PP = 364, 1P).

³¹ P{ ¹ H} NMR (121 MHz, DFB): δ 25.0 (d, ²J_PP = 364, 1P), 16.3 (d, ²J_PP = 364, 1P).

HR ESI-MS (positive ion, 4 kV): 916.5623 ([M]⁺, calcd 916.5628) m/z.

Anal. calcd for C₇₉H₉₅BF₂₄IrNP₂ (1779.57 g mol⁻¹): C, 53.35; H, 5.38; N, 0.79. Found: C, 53.26; H, 5.09; N, 0.77.

4.11 NMR scale reaction of 1 with Z-tBuC≡CCHCHtBu

A solution of 1 (16.1, 9.8 μmol) in DFB (0.5 mL) within a J. Young valve NMR tube was treated with Z-tBuC≡CCHCHtBu (4.6 mg, 28.0 μmol) and then heated at 50 °C for 1 h. No reaction was apparent upon analysis by NMR spectroscopy.

4.12 Crystallography

Data were collected on a Rigaku Oxford Diffraction SuperNova AtlasS2 CCD diffractometer using graphite monochromated Mo Kα (λ = 0.71073 Å) or CuKα (λ = 1.54184 Å) radiation and an Oxford Cryosystems N-HeliX low temperature device [150(2) K]. Data were collected and reduced using CrysAlisPro and refined using SHELXL,³⁹ through the Olex2 interface.⁴⁰ Full details about the collection, solution, and refinement are documented in CIF format, which have been deposited with the Cambridge Crystallographic Data Centre under CCDC 2051203 (1), 2051204 (2), 2051205 (7), 2051206 (10), 2051207 ([Rh(PNP-14)(η²-norbornene)][BAr^F₄]).†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the European Research Council (ERC, grant agreement 637313) and Royal Society (UF100592, UF150675, A.B.C.) for financial support. High-resolution mass-spectrometry data were collected using instruments purchased through support from Advantage West Midlands and the European Regional Development Fund. Crystallographic data were collected using an instrument that received funding from the ERC under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 637313).

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Footnote

† Electronic supplementary information (ESI) available: Catalytic homocoupling of 3,3-dimethylbutyne promoted by 6, synthesis and characterisation of [Rh(PNP-14)(η²-norbornene)][BAr^F₄]; NMR, IR and ESI-MS spectra of new compounds, and selected reactions (PDF). Primary NMR data (MNOVA). CCDC 2051203–2051207. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0dt04303f

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