Attempted syntheses of ZnPhos-ruthenium complexes (ZnPhos = bis(2-diphenylphosphinophenyl)zinc)

Abstract

Efforts to prepare Ru-ZnPhos complexes (ZnPhos = bis(2-diphenylphosphinophenyl)zinc) through in situ reactions of the bis-cyclometallated phosphine RuZn₂ complex [Ru(PPh₃)(C₆H₄PPh₂)₂(ZnMe)₂] (1) with the N-Et and N-ⁱPr substituted N-heterocyclic carbenes IEt₂Me₂ (1,3-diethyl-4,5-dimethylimidazol-2-ylidene) and IⁱPr₂Me₂ (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) gave instead the cycloruthenated/cyclozincated bimetallic species [Ru(IEt₂Me₂′)(C₆H₄PPh₂)(PPh₂(C₆H₄)Zn{IEt₂Me₂})H] (5; IEt₂Me₂′ = cyclometallated IEt₂Me₂) and [Ru(PPh₃)(C₆H₄PPh₂)(PPh(C₆H₄)₂Zn{IⁱPr₂Me₂})H] (7) respectively, both of which feature new Zn-NHC bonds. An alternative approach involving substitution of free ZnPhos into ruthenium monodentate phosphine precursors proved marginally more successful. Heating [Ru(PPh₃)₃(CO)H₂] with excess ZnPhos gave a tetrametallic species 13 comprised of a Ru centre coordinated to ZnPhos and two ZnC₆H₄PPh₂ ligands formed via Zn–C cleavage of two ZnPhos ligands. Substitution into the NHC analogue [Ru(PPh₃)₂(IMe₄)(CO)H₂] (IMe₄ = 1,3,4,5-tetramethylimidazol-2-ylidene) was successful and generated the bridging dihydride complex [Ru(ZnPhos)(IMe₄)(CO)(μ-H)₂] (14).

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Introduction

Interest in pincer phosphine ligands (R₂P-E-PR₂, Fig. 1) stems from their highly tuneable stereoelectronic properties,^1–4 which in combination with their enhanced stability to the types of degradation reactions that affect monodentate phosphine ligands,⁵ can result in favourable catalytic properties.⁶

Fig. 1 Representative examples of P–E–P pincer ligands based on non-metallic E elements.

Changing E from a non-metal to a Lewis acidic metal can alter the properties further by introduction of σ-accepting, Z-type character.⁷ In contrast to the widespread studies of the group 13 elements Al, Ga and In,^8–16 investigations of pincer ligands with E = Zn, Cd and Hg remain sparse.^17–21 Whilst toxicity issues may explain the reluctance to develop Cd and Hg derivatives,^20,21 the relevance of Zn in conjunction with late transition metals²² in reactions such as Pd-catalysed Negishi cross-coupling^23,24 suggests that R₂P-Zn-PR₂ pincers might offer worthwhile properties as ligands.²⁵

We recently reported the serendipitous in situ formation of bis(2-diphenylphosphinophenyl)zinc (abbreviated as ZnPhos by analogy to bis(2-diphenylphosphinophenyl)ether, or DPEphos) complexes of ruthenium upon treating the bis-cyclometallated phosphine RuZn₂ complex [Ru(PPh₃)(C₆H₄PPh₂)₂(ZnMe)₂] (1) with CO and the N-heterocyclic carbene IMe₄ (1,3,4,5-tetramethylimidazol-2-ylidene).²⁶ The resulting products, 2 and 3 respectively (Scheme 1), both reacted with H₂; the former photochemically to give the isolable bridging dihydride species 4, the latter thermally to give terminal hydride 5 that existed in equilibrium with 3. A short time later, Takaya's group²⁷ reported a synthetic route to the free ZnPhos ligand and utilised it in reactions with Ru₃(CO)₁₂ and Pd(PPh₃)₄ to give 2 and [Pd(ZnPhos)(PPh₃)] (6) respectively (Scheme 1).

Scheme 1 Previously reported coordination chemistry of ZnPhos.^26,27

Prompted by the absence of any further reports of metal-ZnPhos complexes in the meantime, we set out to synthesise new Ru derivatives by (i) extending the in situ approach in Scheme 1 to NHCs other than IMe₄ and (ii) attempting to substitute free ZnPhos into labile Ru-PPh₃ precursors. We now show that in situ ZnPhos is limited to just IMe₄, while the substitution chemistry of the ligand illustrates a susceptibility to degradative Zn–C bond cleavage.

Results and discussion

Attempted in situ formation of Ru-ZnPhos complexes from 1 and NHCs

While the exact sequence of steps in Scheme 1 that transforms 1 to 3 is not known, the overall reaction entails elimination of PPh₃ and ZnMe₂ (the latter being trapped as the structurally characterised bis-carbene adduct (IMe₄)₂ZnMe₂) from 1, insertion of Zn between two (2-diphenylphosphino)phenyl groups to generate the ZnPhos ligand and coordination of two molecules of IMe₄ to Ru.

Changing IMe₄ to IEt₂Me₂ (1,3-diethyl-4,5-dimethylimidazol-2-ylidene) did not result in Ru-ZnPhos formation, but gave instead the RuZn bimetallic complex 7 shown in Scheme 2. The structure of 7, which was established by X-ray crystallography (Fig. 2), contained one intact IEt₂Me₂ ligand bound to Zn and a second IEt₂Me₂ ligand cyclometallated onto Ru (Ru1–C45 = 2.174(3) Å; Table 1).^28,29 The resulting hydride that was generated was located and freely refined in the X-ray structure as bridging between Ru and Zn; their separation of 2.5429(7) Å is well within the sum of the covalent radii (2.68 Å).³⁰ The X-ray structure also showed a phosphine ligand (P2) cyclometallated onto Zn (Zn1–C18 = 2.013(3) Å), and a second (P1) cyclometallated mainly onto Ru, but with some interaction to Zn based on comparison of bond metrics (Scheme 3) to those reported for 1 and ZnPh₂.^26,31 The different rings sizes that arise from these interactions was clear from the ³¹P{¹H} NMR spectrum, which showed one high (δ 83) and one low (δ −9) frequency resonance, indicative of the presence of five- and four-membered phosphorus ring chelates, respectively.^32,33

Scheme 2 Formation of the IEt₂Me₂ compound 7.

Fig. 2 Molecular structures of (left) 7 (one of the two molecules present in the structure) and (right) 9. Ellipsoids are represented at 30% probability. In both cases, hydrogen atoms (hydride ligands excepted) are omitted for clarity. Solvent is also omitted for 7 for the same reason and peripheral substituents are depicted as wireframes for visual ease throughout.

Scheme 3 Comparison of M–C_{bridging aryl} distances (Å) in 1,³⁶7–10 and ZnPh₂.³¹

Table 1 Selected bond lengths (Å) and angles (°) for compounds 7, 9, 10 and 11

	7	9	10	11
Ru–Zn	2.5429(4)	2.6286(3)	2.6045(7)	2.5731(3)
Ru–P	Ru1–P1 2.3227(7)	Ru1–P1 2.3141(5)	Ru1–P1 2.3483(11)	Ru1–P1 2.3532(4)
	Ru1–P2 2.2937(7)	Ru1–P2 2.3652(5)	Ru1–P2 2.3745(12)	Ru1–P2 2.3404(4)
		Ru–P3 2.3852(5)	Ru1–P3 2.3244(11)	Ru1–P3 2.3651(4)
Ru–Caryl	2.221(3)	Ru1–C32 2.0852(19)	Ru1–C12 2.121(4)	2.1072(17)
		Ru1–C43 2.1401(18)	Ru1–C30 2.114(5)
Ru–Cother	Ru1–C37 2.070(3)	—	—	—
	Ru1–C45 2.174(3)
Zn–Caryl	Zn1–C18 2.013(3)	Zn1–C54 2.0197(19)	Zn1–C49 2.005(4)	Zn1–C38 1.9956(18)
	Zn1–C35 2.311(3)	Zn1–C48 2.393(2)	Zn1–C12 2.489(4)
Zn–CNHC	2.035(3)	2.036(2)	2.053(4)	2.0321(18)
P–Ru–P	169.76(3)	P1–Ru1–P2 106.929(17)	167.43(4)	P1–Ru1–P2 100.859(16)
		P2–Ru1–P3 102.352(17)		P2–Ru1–P3 100.210(16)
		P3–Ru1–P1 102.221(16)		P3–Ru1–P1 110.112(16)

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As in the case of 3, clean formation of 7 took place only in the presence of a significant excess of carbene. With 2 or 6 equivalents IEt₂Me₂, it formed a minor component alongside other hydride containing species,³⁴ whereas 10 equivalents of carbene gave 7 as the major product. Crude reaction mixtures contained ca. 25% of a second species, which we believe to be an isomer, although we were unable to find any conditions under which the yield of this product was increased.³⁵ Ultimately, 7 was isolated as orange-yellow microcrystalline material in 43% upon low temperature crystallisation from toluene/hexane, followed by manual separation away from colourless crystalline material assumed to be (IEt₂Me₂)₂ZnMe₂, by analogy to what was found during formation of 3 (Scheme 1).³⁶ No efforts were made to characterise or determine the yield of this side product.

Increasing the size of the carbene N-substituent further from N-Et to the N-ⁱPr carbene IⁱPr₂Me₂ (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) gave 9 (Scheme 4), which was isolated in 58% yield as yellow crystals from benzene/hexane.³⁷ The X-ray structure (Fig. 2 and Table 1) confirmed that 9 contained a Zn-NHC ligand, but no Ru-NHC ligand, making it a direct analogue of the previously reported Zn(IMes) derivative 8 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; shown in Scheme 4 for comparison).³⁶ The X-ray structure of 9 showed three different types of phosphine ligands (one intact PPh₃, one cyclometallated onto Ru and the third with interaction from one of the phenyl rings to Ru and from a second ring to Zn) in a fac-geometry (Scheme 4). A hydride ligand bridging the Ru and Zn centres (separated by 2.6286(3) Å, slightly elongated in comparison to 7) was again located and freely refined. The ¹H and ³¹P NMR spectra of 9 were consistent with the X-ray data. Thus, the trans H-Ru-PPh₂(C₆H₄) geometry was confirmed by ¹H{selective-³¹P} measurements, while the ³¹P{¹H} NMR spectrum exhibited low frequency (δ −12, δ −24) resonances for the two cyclometallated phosphines, and a mid-frequency signal (δ 48) for the Ru-PPh₃ group.

Scheme 4 Formation of the IⁱPr₂Me₂ compounds 9–11. The previously reported IMes complex 8 is shown for comparison.³⁶

For reasons that remain unclear, the formation of 9 required use of a much smaller excess of IⁱPr₂Me₂ (only 4 eq.) than the number of carbene equivalents used to generate 3 or 7. Interestingly, even with 10 eq. IⁱPr₂Me₂, we observed no coordination of NHC onto Ru.

Broadness of the ¹H NMR methine signal led us to record spectra at 55 °C. While this yielded sharper spectra, it resulted over longer times in isomerisation of 9 to 10 (Scheme 4), which was formed alongside minor amounts of the bridging dihydride complex 11 (Scheme 4). We attribute formation of the latter to adventitious moisture. The identities of the two compounds were confirmed by X-ray crystallography and NMR spectroscopy following independent syntheses; 11 upon heating 1 with IⁱPr₂Me₂ at 70 °C under H₂, 10 by repeating the same process under argon. The X-ray structure of the latter (Fig. 3 and Table 1) revealed a mer-arrangement of three cyclometallated (two onto Ru, one onto Zn) phosphines, whereas 11 exhibited one intact PPh₃, one cycloruthenated ligand and one cyclozincated phosphine arranged in a fac-geometry. Despite the presence of the two bridging hydrides (which were both located in the X-ray structure; Fig. 3 and Table 1), the Ru⋯Zn distance was reduced to <2.60 Å. In the ¹H NMR spectrum of 11, the two hydrides appeared as highly coupled resonances at δ −7.1 and δ −11.1, which were characterised as being trans to PPh₃ and trans to RuC₆H₄PPh₂ through ¹H{selective-³¹P} NMR experiments.

Fig. 3 Molecular structures of (left) 10 and (right) 11. Ellipsoids are represented at 30% probability. In both cases, hydrogen atoms (hydride ligands excepted) are omitted for clarity. Solvent, in 11, is similarly absent. Peripheral substituents are depicted as wireframes, also for visual ease.

Reactivity of ZnPhos with Ru–H precursors

Substitution reactions of free ZnPhos with Ru-PPh₃ complexes employed precursors used successfully in the synthesis of Ru(DPEphos) and related Ru(P–O–P) products.^38–40 Attempts to incorporate ZnPhos into [Ru(PPh₃)₃HCl] was thwarted by the insolubility of the latter in C₆H₆ and of the ligand in C₆H₅F. Use of CH₂Cl₂, in which [Ru(PPh₃)₃HCl] is fully soluble, resulted only in degradation of ZnPhos to PPh₃.⁴¹ Although THF proved a viable solvent for reactions with both [Ru(PPh₃)₃HCl] and the carbonyl derivative [Ru(PPh₃)₃(CO)HCl], mixtures of products were formed in each case.

No reaction of the dihydride complex [Ru(PPh₃)₃(CO)H₂] with ZnPhos took place in C₆H₆ at room temperature, but heating at 80 °C overnight generated one main species which displayed three broad ³¹P NMR resonances at δ 68, δ 60 and δ −17. Layering with hexane afforded deep-red crystals, which were identified by X-ray crystallography as the unexpected RuZn₃ tetrametallic complex 13 shown in Scheme 5. The X-ray structure (Fig. 4) revealed a 7-coordinate Ru centre coordinated to ZnPhos and CO ligands as well as to the Zn atom (Zn2) of a ZnC₆H₄PPh₂ moiety (Ru1–Zn2 = 2.5129(6) Å). The phosphorus atom was bound to the Zn (Zn3) of the second ZnC₆H₄PPh₂ ligand, which then completed the coordination sphere of Ru being κ²-Zn,P bonded (Ru1–Zn3 = 2.6386(5) Å, Ru1–P4 = 2.4296(9) Å). Selected metrics for the structure are given in Table 2.

Scheme 5 Reactivity of [Ru(PPh₃)₂(L)(CO)H₂] (L = PPh₃, IMe₄) with ZnPhos.

Fig. 4 Plots depicting (left) the structure of 13 and (right) one of the four molecules present in the structure of 14. Ellipsoids are shown at 30% probability. Solvent in 13 and hydrogen atoms in both structures (hydride ligands excepted) have been omitted for clarity. Peripheral substituents are depicted as wireframes, also for visual ease.

Table 2 Selected bond lengths (Å) and angles (°) for compounds 13 and 14

	13	14 ^a
a Data are for one of the four molecules of the compound in the unit cell.
Ru–Zn1	2.6544(5)	2.6437(15)
Ru–Zn2	2.5129(6)	—
Ru–Zn3	2.6386(5)	—
Ru–P1	2.3784(8)	2.329(3)
Ru–P2	2.3561(8)	2.319(2)
Ru–P4	2.4296(9)	—
Ru–Cother	Ru1–C1: 1.881(4)	Ru1–C1: 1.886(12)
		Ru1–C2: 2.137(9)
Zn–P	Zn3–P3: 2.4885(11)	—
Zn–Zn	Zn1–Zn2: 2.7815(7)	—
	Zn2–Zn3: 2.5371(7)
Zn–Caryl	Zn1–C7: 1.987(3)	Zn1–C26: 1.989(7)
	Zn1–C20: 1.961(4)	Zn1–C34: 1.990(7)
	Zn2–C38: 1.980(4)
	Zn3–C56: 1.972(4)
P1–Ru–P2	151.66(3)	167.07(10)
P1–Ru–P4	104.59(3)	—
P2–Ru–P4	103.53(3)	—

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Formation of the two ZnC₆H₄PPh₂ groups arises through Zn–C cleavage of two ZnPhos ligands. In turn, this also generates 2 eq. of –C₆H₄PPh₂, which upon combination with two Ru–H ligands, yields two of the five equivalents of PPh₃ (three lost from [Ru(PPh₃)₃(CO)H₂]) generated overall in the reaction, as established by a ³¹P{inverse-gated ¹H} NMR measurement.

Crystalline 13 failed to redissolve in benzene but did dissolve in THF, yielding a ³¹P{inverse-gated ¹H} NMR spectrum comprised of a 1 : 2 : 1 ratio of three resonances at similar chemical shifts to those in the crude reaction mixture, but now resolved into triplets (albeit still quite broad), all with J_PP splitting of just 6 Hz. Removal of THF gave a red residue that did now redissolve in C₆D₆ to give a spectrum identical to that of the crude material.

The triplet multiplicity of phosphorus P3 (Fig. 4) appears to be at odds with the solid-state structure, as is the size of the P,P-couplings, which are significantly smaller than cis P–Ru–P J_PP values which are typically ca. 20 Hz.^42,43 These findings are suggestive of fluxionality in the system; indeed, cooling to very low temperature (−85 °C) broadened significantly the signal at ca. δ 60; only three resonances were observed at all temperatures. There was also noticeable shift (Δδ) of +0.5 ppm in the lowest frequency resonance upon cooling, as well as an even more pronounced Δδ of −2 ppm in the signal for free PPh₃ in the sample; both observations further support one or more processes of fluxionality/exchange. Higher temperature spectra were compromised by degradation of starting material resonances and an increase in free PPh₃, a process that also took place in solution even at room temperature over time.

In light of the X-ray structure, solid-state ³¹P{¹H} CPMAS NMR spectra of crystalline 13 were measured. These also revealed three resonances at ca. δ 67, δ 55 and δ −14, along with a signal for free PPh₃ (ca. δ −7, proven by addition of PPh₃, which was then used as an internal reference)⁴⁴ and a broad resonance of unknown origin at δ −5. ³¹P spin-diffusion measurements showed correlations between all three resonances, confirming they originated from within the same molecule. This was supported by a ³¹P-detected ¹H T₁ measurement. As protons in the same molecule should show the same T₁ (the result of dipolar coupling), the ³¹P signals associated with each molecule should then show the same ¹H T₁ value; this was what was observed (Table S1;† the small discrepancies are attributed to issues with fittings signals of low intensity).⁴⁵ The different ¹H T₁ associated with the broad −5 ppm signal, together with the absence of cross-peaks to this in the ³¹P spin diffusion experiment, are consistent with this resonance being from a different species.

When the formation of 13 was followed by ³¹P{¹H} NMR spectroscopy, free PPh₃, together with signals attributed to the substitution product [Ru(ZnPhos)(PPh₃)(CO)H₂] (12, Scheme 5), appeared within 1 h. Full consumption of [Ru(PPh₃)₃(CO)H₂] took place over ca. 3–4 h, leaving 13 as the major solution component after ca. 5 h.⁴⁷ Under more concentrated conditions, 13 precipitated directly from solution, leaving a filtrate comprised, in part, of 12. Heating this with ZnPhos gave 13, consistent with 12 being a precursor to the final tetrametallic product (Scheme 5). The exact structure of 12 (it is shown with bridging hydrides in Scheme 5 based on the similarity of the ¹H NMR hydride resonances to those of 4 and 14 (vide infra)) and the mechanism of conversion to 13 remain unknown, although it is tempting to suggest that an intramolecular attack of Ru–H on a Zn–C bond of coordinated ZnPhos is involved.⁴⁶ Attempts to generate just 12via sequentially adding up to 4 equivalents of ZnPhos to [Ru(PPh₃)₃(CO)H₂] gave 12 as part of a mixture together with unreacted [Ru(PPh₃)₃(CO)H₂] or tetrametallic 13.

In contrast to the degradative reaction with [Ru(PPh₃)₃(CO)H₂], ZnPhos (2 eq.) afforded the intact substitution product [Ru(ZnPhos)(IMe₄)(CO)(μ-H)₂] (14, Scheme 5) in 32% isolated yield after heating with [Ru(PPh₃)₂(IMe₄)(CO)H₂] at 40 °C for 24 h. This time could be cut to just 6 h by increasing the temperature to 70 °C, allowing us to exclude elevated temperatures as being responsible for the Zn–C cleavage that yields 13. Based on the proposed intramolecular attack mooted above, it may be that the change from PPh₃ to the strongly donating IMe₄ ligand in 14 disfavours such a process.

The X-ray crystal structure of 14 (Fig. 4) confirmed the presence of bridging hydrides analogous to the arrangement in 4 (Scheme 1). Four molecules were present in the unit cell of 14, with the Ru⋯Zn separation ranging from 2.6422(14)–2.6672(15) Å, considerably shorter than that in 4 (2.8080(9) and 2.8184(9) Å, 2 molecules in the unit cell). As shown in Scheme 1, a contrasting structure with terminal hydrides was found for the bis-IMe₄ derivative 5, which computational studies showed relates to the kinetics of the addition of H₂ to [Ru(ZnPhos)(IMe₄)₂] (3).²⁷ The bridging hydride structure of 14 (and, indeed, 12) may result from the hydride ligands being present in the precursor, rather than formed through addition of H₂.

Conclusions

Two approaches have been used in an effort to synthesise new ZnPhos complexes of ruthenium. Following on from our previous report of the formation of [Ru(ZnPhos)(IMe₄)₂] (3, Scheme 2) from [Ru(PPh₃)(C₆H₄PPh₂)₂(ZnMe)₂] (1) and IMe₄, we have found that the latter appears to be the only NHC able to bring about in situ ZnPhos formation from 1, with both N-ethyl and N-isopropyl substituted carbenes showing behaviour analogous to IMes in giving instead Ru⋯Zn(NHC) containing products. Substitution of free ZnPhos into labile Ru precursors has proven to be slightly more successful, giving [Ru(ZnPhos)(IMe₄)(CO)(μ-H)₂] (14) from the NHC precursor [Ru(PPh₃)₂(IMe₄)(CO)H₂]. However, with the all-phosphine complex [Ru(PPh₃)₃(CO)H₂], Zn–C cleavage takes place to give the tetrametallic species 13, revealing a vulnerability of ZnPhos to degradative processes that may ultimately limit its value for catalysis. Whilst disappointing, we feel that this still represents a valuable observation of the type often overlooked.⁴⁸

Experimental section

All manipulations were carried out at room temperature under argon using standard Schlenk, high vacuum and glovebox techniques using dry and degassed solvents. C₆D₆, C₆D₅CD₃ and THF-d₈ were vacuum transferred from potassium. NMR spectra were recorded on Bruker Avance 400 and 500 MHz NMR spectrometers and referenced as follows: C₆D₆ (δ_H 7.15; δ_C 128.0), C₆D₅CD₃ (δ_H 2.09; δ_C 20.4) and THF-d₈ (δ_H 3.58; δ_C 25.4). ³¹P{¹H} spectra were referenced externally to 85% H₃PO₄ at δ 0. Coupling constants are defined using ^xJ_AB nomenclature only where there is absolute certainty in assignments; vt = virtual triplet. The ³¹P CPMAS and spin diffusion experiments and the ³¹P-detected ¹H T₁ measurement of solid 13 were acquired on a Bruker Avance 500 MHz spectrometer using a 4 mm iProbe CPMAS. The spin diffusion experiment was carried out using a DARR (Dipolar Assisted Rotational Resonance) pulse sequence; the T₁ measurement using an inversion recovery method. IR spectra were recorded on a Bruker ALPHA ATR-IR spectrometer. Elemental analyses were performed by Elemental Microanalysis Ltd, Okehampton, Devon, U.K. Literature routes were used for the synthesis of [Ru(PPh₃)(C₆H₄PPh₂)₂(ZnMe)₂] (1),²⁶ [Ru(PPh₃)₃HCl],⁴⁹ [Ru(PPh₃)₃(CO)HCl],⁵⁰ [Ru(PPh₃)₃(CO)H₂],⁵¹ IⁱPr₂Me₂ and IEt₂Me₂.⁵² ZnPhos was prepared according to the literature,²⁷ albeit starting from 2-(diphenylphosphino)phenylboronate, which was prepared from 1,2-dibromobenzene.^53,54

Synthesis and characterisation of 7

A J. Youngs resealable ampoule was charged with 1 (50 mg, 0.05 mmol), IEt₂Me₂ (72 mg, 0.5 mmol) and benzene (3 mL) and the solution stirred at room temperature overnight.^‡ After filtration, the filtrate was reduced to dryness and the residue recrystallised from toluene/hexane at −37 °C to afford a mixture of orange-yellow crystals of the product and colourless crystals of what are assumed to be (IEt₂Me₂)₂ZnMe₂, which were separated manually. Yield: 21 mg (43%). ¹H NMR: δ_H (500 MHz, C₆D₅CD₃, 233 K): 8.28 (br t, ³J_HH = 7.0 Hz, 2H, Ar), 8.11–8.02 (br m, 4H, Ar), 7.71 (br m, 1H, Ar), 7.53 (vbr s, 1H, Ar), 7.32 (br t, ³J_H,H = 6.9 Hz, 1H, Ar), 7.27 (t, ³J_H,H = 7.4 Hz, 2H, Ar), 6.98–6.86 (m, 5H, Ar), 6.85–6.68 (m, 6H, Ar), 6.57 (br t, ³J_H,H = 8.2 Hz, 1H, Ar),* 4.55 (m, 1H, NCHH), 3.86 (t, ³J_H,H = 9.5 Hz, 1H, RuCH₂CHH), 3.72 (m, 1H, NCHH), 3.16 (m, 1H, NCHH), 3.00 (m, 1H, RuCH₂CHH), 2.58 (vbr m, 1H, RuCHHCH₂), 2.42 (m, 1H, NCHH), 2.16 (m, 1H, NCHH), 1.65–1.59 (overlapping m + s, 4H, NCHH + NC=CCH₃),^† 1.57 (s, 3H, NC=CCH₃), 1.32–1.19 (overlapping m + s, 7H, RuCHHCH₂ + NC=CCH₃),^† 1.07 (t, ³J_HH = 7.1 Hz, 3H, NCH₂CH₃), 0.87 (t, ³J_H,H = 7.1 Hz, 3H, NCH₂CH₃), 0.46 (t, ³J_H,H = 6.9 Hz, 3H, NCH₂CH₃), −6.78 (br dd, ²J_H,P = 17.4 Hz, ²J_H,P = 15.1 Hz, 1H, RuH). *The remaining aromatic resonances are assumed to be obscured by solvent signals. ^†Established by ¹H COSY. ³¹P{¹H} NMR: δ_P (202 MHz, C₆D₅CD₃, 233 K): 83.0 (d, ²J_P,P = 298 Hz), −8.6 (d, ²J_P,P = 298 Hz). Selected ¹³C{¹H} DEPTQ NMR: δ_C (126 MHz, C₆D₅CD₃, 233 K): 192.4 (br t, ²J_C,P = 11 Hz, Ru–C_NHC), 177.2 (s, Zn–C_NHC), 53.3 (s, NCH₂CH₂Ru), 42.5 (s, NCH₂CH₃), 41.7 (s, NCH₂CH₃), 40.7 (s, NCH₂CH₃), 17.6 (s, NCH₂CH₃), 16.9 (s, NCH₂CH₃), 15.5 (s, NCH₂CH₃), 9.6 (s, NC=CCH₃), 9.3 (s, NC=CCH₃), 7.7 (s, NC=CCH₃), 7.6 (s, NC=CCH₃), 3.9 (br s, NCH₂CH₂Ru). Elemental analysis. Found: C, 59.85; H, 6.18; N, 5.90. C₅₄H₆₀N₄P₂ZnRu requires C, 65.28; H, 6.09; N, 5.64. Multiple attempts repeatedly gave low %C values. ^‡Crude reaction mixtures showed ca. 25% of a second hydride containing species, which we propose is an isomer of 7. Selected ¹H NMR: δ_H (500 MHz, C₆D₆, 298 K): −8.49 (dd, ²J_H,P = 51.7 Hz, ²J_H,P = 4.0 Hz, 1H, Ru–H). ³¹P{¹H} NMR: δ_P (202 MHz, C₆D₆, 298 K): −5.8 (d, ²J_P,P = 20 Hz), −15.4 (d, ²J_P,P = 20 Hz).

Synthesis and characterisation of 9

A J. Youngs resealable ampoule was charged with a benzene (3 mL) solution of 1 (50 mg, 0.05 mmol) and IⁱPr₂Me₂ (43 mg, 0.24 mmol) and the mixture stirred overnight at room temperature. Following filtration, the filtrate was reduced to dryness, washed with hexane (3 mL) and recrystallised from benzene and hexane to yield yellow crystals of 9. Yield: 35 mg (58%). ¹H NMR: δ_H (400 MHz, C₆D₆, 298 K): 7.68 (t, J = 7.8 Hz, 2H, Ar), 7.57 (d, J = 6.4 Hz, 1H, Ar), 7.51–7.34 (br, 11H, Ar), 7.28–7.18 (br, 4H, Ar), 7.13–7.02 (br, 4H, Ar),* 7.01–6.78 (br m, 15 H, Ar), 6.78–6.64 (m, 3H, Ar), 6.55 (t, J = 7.2 Hz, 1H, Ar), 6.44 (t, J = 7.2 Hz, 1H, Ar), 4.24 (br s, 2H, CH(CH₃)₂), 1.50 (s, 6H, NC=C(CH₃)), 0.96 (overlapping d, ³J_H,H = 6.8 Hz, 12H, CH(CH₃)₂), −9.31 (br ddd, ²J_H,P = 53.5 Hz, ²J_H,P = 24.5 Hz, ²J_H,P = 2.6 Hz, 1H, Ru–H). *Partially overlapped with resonance for C₆D₅H. ¹H NMR: δ_H (400 MHz, C₆D₅CD₃, 328 K): 7.63 (br t, J = 7.6 Hz, 2H, Ar), 7.44 (br d, J = 6.4 Hz, 1H, Ar), 7.39–7.31 (br, 9H, Ar), 7.28–7.16 (br, 4H, Ar), 6.96–6.64 (br, 17 H, Ar), 6.78–6.64 (m, 3 H, Ar), 6.54 (br m, 1H, Ar), 6.45 (br t, J = 7.2 Hz, 1H, Ar), 6.36 (br t, J = 7.3 Hz, 1H, Ar),* 4.23 (sept, ³J_H,H = 8.8 Hz, 2H, CH(CH₃)₂), 1.65 (s, 6H, NC=C(CH₃)), 1.02–0.98 (overlapping d, ³J_H,H = 6.8 Hz, 12H, CH(CH₃)₂), −9.51 (ddd, ²J_H,P = 52.9 Hz, ²J_H,P = 24.5 Hz, ²J_H,P = 3.1 Hz, 1H, Ru–H). *The remaining aromatic resonances are assumed to be obscured by solvent signals. ³¹P{¹H} NMR: δ_P (162 MHz, C₆D₆, 298 K): 48.2 (t, ²J_P,P = 25 Hz), −11.9 (dd, ²J_P,P = 25 Hz, ²J_P,P = 16 Hz), −23.9 (dd, ²J_P,P = 25 Hz, ²J_P,P = 16 Hz). ¹³C{¹H} NMR: δ_C (101 MHz, C₆D₆, 298 K): 177.8 (ddd, J_C,P = 67 Hz, J_C,P = 12 Hz, J_C,P = 6 Hz, Ar), 176.2 (br dm, J_C,P = 5 Hz, Zn–C_NHC), 170.3 (br dm, J_C,P = 42 Hz, Ar), 168.5 (d, J_C,P = 61 Hz, Ar), 165.4 (dd, J_C,P = 57 Hz, J_C,P = 12 Hz, Ar), 153.4 (dd, J_C,P = 47 Hz, J_C,P = 5 Hz, Ar), 149.0 (d, J_C,P = 35 Hz, Ar), 142.7 (br dm, J_C,P = 29 Hz, Ar), 142.0 (d, J_C,P = 10 Hz, Ar), 140.6 (d, J_C,P = 20 Hz, Ar), 140.5 (d, J_C,P = 21 Hz, Ar), 137.2 (d, J_C,P = 23 Hz, Ar), 137.0 (d, J_C,P = 9 Hz, Ar), 135.8 (d, J_C,P = 25 Hz, Ar), 134.5 (d, J_C,P = 13 Hz, Ar), 134.4 (d, J_C,P = 12 Hz, Ar), 133.8 (d, J_C,P = 10 Hz, Ar), 132.4 (d, J_C,P = 10 Hz, Ar), 128.8 (m, Ar), 128.5 (s, Ar), 127.4 (d, J_C,P = 8 Hz, Ar), 127.2 (d, J_C,P = 8 Hz, Ar), 126.2 (d, J_C,P = 5 Hz, Ar), 125.9 (s, Ar), 125.4 (s, NC=CCH₃), 125.3 (m, Ar), 122.5 (s, Ar), 119.5 (d, J_C,P = 8 Hz, Ar), 53.3 (s, NCH(CH₃)₂), 22.4 (s, NCH(CH₃)₂), 22.1 (s, NCH(CH₃)₂), 9.8 (s, NC=CCH₃). Elemental analysis. Found: C, 69.14; H, 5.72; N, 2.56. C₆₅H₆₃N₂P₃ZnRu requires C, 68.99; H, 5.61; N, 2.48.

Synthesis and characterisation of 10

Using the same masses/volumes as for 9, the contents of the ampoule were heated at 70 °C for 15 h. After filtration/washing as per 9, the residue was crystallised from benzene/hexane to give 30 mg (50%) of 10 as yellow crystals. ¹H NMR: δ_H (400 MHz, C₆D₆, 298 K): 8.44 (t, J = 8.5 Hz, 2H, Ar), 8.14 (dt, J = 19.3 Hz, J = 8.2 Hz, 4H, Ar), 7.78 (dd, J = 7.2 Hz, J = 5.1 Hz, 1H, Ar), 7.59–7.50 (m, 3H, Ar), 6.97 (app quart, J = 7.6 Hz, 3H, Ar), 6.93–6.49 (br m, 19H, Ar), 6.26 (t, J = 8.5 Hz, 2H, Ar),* 4.73 (br sept, ³J_H,H = 6.3 Hz, 1H, CH(CH₃)₂), 2.47 (sept, ³J_H,H = 6.4 Hz, 1H, CH(CH₃)₂), 1.57 (d, ³J_H,H = 6.4 Hz, CHMe₂, 3H), 1.56 (s, NC=CCH₃, 3H), 1.49 (d, ³J_H,H = 6.4 Hz, CHMe₂, 3H), 1.43 (s, NC=CCH₃, 3H), 0.79 (d, ³J_H,H = 6.5 Hz, CHMe₂, 3H), 0.09 (d, ³J_H,H = 6.4 Hz, CHMe₂, 3H), −5.68 (td, ²J_H,P = 12.7 Hz, ²J_H,P = 6.7 Hz, 1H, RuH). *The remaining aromatic signals are obscured by C₆D₅H. ³¹P{¹H} NMR: δ_P (162 MHz, C₆D₆, 298 K): 78.2 (d, ²J_P,P = 288 Hz, ²J_P,P = 22 Hz), −14.9 (dd, ²J_P,P = 288 Hz, ²J_P,P = 26 Hz), −20.0 (dd, ²J_P,P = 26 Hz, ²J_P,P = 22 Hz). ¹³C{¹H} NMR: δ_C (101 MHz, C₆D₆, 298 K): 179.5 (dt, J_C,P = 10 Hz, J_C,P = 6 Hz, Ar), 179.4 (s, Zn–C_NHC), 175.3 (dd, J_C,P = 70 Hz, J_C,P = 3 Hz, Ar), 163.8 (ddd, J_C,P = 53 Hz, J_C,P = 7 Hz, J_C,P = 4 Hz, Ar), 158.0 (dd, J_C,P = 43 Hz, J_C,P = 5 Hz, Ar), 152.0 (dd, J_C,P = 45 Hz, J_C,P = 3 Hz, Ar), 151.0 (d, J_C,P = 54 Hz, Ar), 144.2 (dd, J_C,P = 18 Hz, J_C,P = 5 Hz, Ar), 142.6 (dd, J_C,P = 23 Hz, J_C,P = 3 Hz, Ar), 140.8 (d, J_C,P = 31 Hz, Ar), 139.5 (d, J_C,P = 17 Hz, Ar), 138.4 (d, J_C,P = 22 Hz, Ar), 137.8 (d, J_C,P = 27 Hz, Ar), 137.3 (d, J_C,P = 33 Hz, Ar), 136.7 (d, J_C,P = 5 Hz, Ar), 136.4 (d, J_C,P = 12 Hz, Ar), 135.2 (br d, J_C,P = 5 Hz, Ar), 135.0 (br d, J_C,P = 14 Hz, Ar), 134.2 (d, J_C,P = 9 Hz, Ar), 133.9 (d, J_C,P = 4 Hz, Ar), 132.8 (d, J_C,P = 9 Hz, Ar), 132.7 (d, J_C,P = 10 Hz, Ar), 130.2 (d, J_C,P = 8 Hz, Ar), 129.6 (br s, Ar), 128.8 (br s, Ar), 128.5 (s, Ar), 127.4 (d, J_C,P = 8 Hz, Ar), 127.2 (d, J_C,P = 22 Hz, Ar), 127.0–126.3 (m, Ar), 125.2 (s, NC=CCH₃), 125.0 (br m, Ar), 124.5 (br s, Ar), 123.7 (s, NC=CCH₃), 120.9 (br s, Ar), 120.1 (br s, Ar), 53.8 (s, NCH(CH₃)₂), 51.5 (s, NCH(CH₃)₂), 23.5 (s, NCH(CH₃)₂), 22.8 (s, NCH(CH₃)₂), 22.7 (s, NCH(CH₃)₂), 21.4 (s, NCH(CH₃)₂), 10.0 (s, NC=CCH₃), 9.2 (s, NC=CCH₃). Elemental analysis. Found: C, 65.28; H, 5.07; N, 2.37. C₆₅H₆₃N₂P₃ZnRu requires C, 68.99; H, 5.61; N, 2.48. Multiple attempts repeatedly gave low %C values.

Synthesis and characterisation of 11

Using the same masses/volumes as for 9, the ampoule was freeze–pump–thaw degassed three times, placed under 1 atm H₂ and stirred at room temperature overnight. After filtration/washing as per 9, the residue was crystallised from benzene/hexane to give 26 mg (43%) of yellow crystals of 11. ¹H NMR: δ_H (400 MHz, C₆D₆, 318 K): 8.07 (d, J = 6.7 Hz, 1H, Ar), 7.98 (t, J = 8.1 Hz, 2H, Ar), 7.81 (br t, J = 8.5 Hz, 2H, Ar), 7.42–7.17 (m, 9H, Ar),* 7.09–6.97 (m, 6H, Ar), 6.97–6.75 (m, 13H, Ar), 6.73–6.56 (m, 6H, Ar), 4.37 (sept, ³J_H,H = 6.8 Hz, 2H, CH(CH₃)₂), 1.51 (s, NC=CCH₃, 6H), 1.29–1.12 (br, CHMe₂, 6H), 1.04 (br s, CHMe₂, 6H), −7.10 (dddd, ²J_H,P = 59.1 Hz, ²J_H,P = 20.2 Hz, ²J_H,P = 15.6 Hz, ²J_H,H = 7.3 Hz, 1H, RuH),^† −11.12 (dm, ²J_H,P = 51.7 Hz, ²J_H,P = 11.7 Hz, ²J_H,P = 9.4 Hz, ²J_H,H = 7.3 Hz, 1H, RuH).^† *The remaining aromatic signals are obscured by C₆D₅H. ^†Values determined by ¹H{selective-³¹P} measurements. ³¹P{¹H} NMR: δ_P (202 MHz, C₆D₆, 298 K): 63.7 (dd, ²J_P,P = 20 Hz, ²J_P,P = 16 Hz), 57.6 (t, ²J_P,P = 20 Hz), −19.5 (dd, ²J_P,P = 20 Hz, ²J_P,P = 16 Hz). ¹³C{¹H} NMR: δ_C (101 MHz, C₆D₆, 318 K): 176.9 (br m, Zn–C_NHC), 175.7–174.1 (overlapping m, Ar),* 155.5 (d, J_C,P = 47 Hz, Ar), 154.0 (d, J_C,P = 49 Hz, Ar), 145.3 (dd, J_C,P = 18 Hz, J_C,P = 3 Hz, Ar), 145.0 (d, J_C,P = 18 Hz, Ar), 142.8 (d, J_C,P = 28 Hz, Ar), 141.3 (d, J_C,P = 23 Hz, Ar), 140.4 (d, J_C,P = 30 Hz, Ar), 140.0 (d, J_C,P = 12 Hz, Ar), 135.1 (br, Ar), 133.1 (br, Ar), 132.6 (d, J_C,P = 11 Hz, Ar), 127.5 (br s, Ar), 127.1–126.7 (overlapping m, Ar), 126.3 (s, Ar), 125.0 (d, J_C,P = 4 Hz, Ar), 124.6 (s, NC=CCH₃), 124.3 (d, J_C,P = 5 Hz, Ar), 119.4 (d, J_C,P = 8 Hz, Ar), 51.8 (s, NCH(CH₃)₂), 22.8 (s, NCH(CH₃)₂), 9.2 (s, NC=CCH₃). Elemental analysis. Found: C, 68.88; H, 6.13; N, 2.35. C₆₅H₆₄N₂P₃ZnRu requires C, 68.87; H, 5.78; N, 2.47.

Synthesis and characterisation of 13

[Ru(PPh₃)₃(CO)H₂] (20 mg, 0.02 mmol) and ZnPhos (38 mg, 0.06 mmol) were dissolved in C₆D₆ (0.6 mL) in a J. Youngs resealable NMR tube and the sample heated at 80 °C for 5 h. Standing at room temperature, precipitated 13 as a deep-red microcrystalline solid, although always contaminated with traces of PPh₃. An alternative approach of concentrating the solution and layering with hexane afforded material which always contained a greater amount of PPh₃. Yield: 15 mg (50%). ¹³C{¹H} NMR: δ_C (126 MHz, C₆D₆, 298 K): 215.5 (m, Ru–CO), 174.6 (dt, J_C,P = 33 Hz, J_C,P = 6 Hz, Ar), 173.6 (d, J_C,P = 33 Hz, Ar), 173.2 (d, J_C,P = 55 Hz, Ar), 157.9 (dvt, J_C,P = 31 Hz, J_C,P = 9 Hz, Ar), 146.4 (d, J_C,P = 15 Hz, Ar), 146.0 (d, J_C,P = 15 Hz, Ar), 144.5 (m, Ar), 143.9 (vt, J_C,P = 18 Hz, Ar), 139.2 (s, Ar), 139.1 (s, Ar), 139.0 (s, Ar), 138.8 (s, Ar), 138.4 (d, J_C,P = 32 Hz, Ar), 135.4 (d, J_C,P = 13 Hz, Ar), 135.1 (vt, J_C,P = 6 Hz, Ar), 134.0 (d, J_C,P = 17 Hz, Ar), 133.8 (vt, J_C,P = 5 Hz, Ar), 133.3 (vt, J_C,P = 5 Hz, Ar), 133.0 (d, J_C,P = 10 Hz, Ar), 132.4 (vt, J_C,P = 5 Hz, Ar), 130.7 (d, J_C,P = 5 Hz, Ar), 130.5 (s, Ar), 129.3 (d, J_C,P = 8 Hz, Ar), 128.6 (s, Ar), 128.5 (m, Ar), 127.9 (m, Ar), 127.9 (s, Ar), 127.6 (s, Ar), 127.1 (s, Ar), 125.9 (s, Ar), 125.1 (d, J_C,P = 3 Hz, Ar). ATR-IR (cm⁻¹): 1896 (ν_CO). Elemental analysis was precluded by the contamination with PPh₃ noted above (see Fig. S41†).

Synthesis and characterisation of 14

[Ru(IMe₄)(PPh₃)₂(CO)H₂] (20 mg, 0.026 mmol) and ZnPhos (30 mg, 0.051 mmol) were dissolved in C₆D₆ (0.6 mL) in a J. Youngs resealable NMR tube and the sample heated at 70 °C for 6 h. Upon concentrating and layering with hexane, colourless crystals of 14 were formed. Yield: 7 mg (32%). ¹H NMR: δ_H (400 MHz, C₆D₆, 298 K): 8.60 (d, ³J_H,H = 7.0 Hz, 2H, Ar), 7.97 (m, 5H, Ar), 7.58 (m, 2H, Ar), 7.12 (t, ³J_H,H = 7.6 Hz, 2H, Ar), 7.08–6.94 (m, 9H, Ar), 6.71 (t, J_H,H = 7.2 Hz, 2H, Ar), 6.58 (t, ³J_H,H = 7.4 Hz, 4H, Ar), 3.05 (s, 3H, NCH₃), 1.69 (s, 3H, NCH₃), 1.32 (s, 3H, NC=CCH₃), 0.94 (s, 3H, NC=CCH₃), −6.63 (td, ²J_H,P = 20.9 Hz, ²J_H,H = 5.7 Hz, 1H, RuH), −9.65 (td, ²J_H,P = 16.7 Hz, ²J_H,H = 5.7 Hz, 1H, RuH). ³¹P{¹H} NMR: δ_P (162 MHz, C₆D₆, 298 K): 68.6 (s). ¹³C{¹H} NMR: δ_C (101 MHz, C₆D₆): 207.3 (t, ²J_C,P = 8 Hz, Ru–CO), 189.0 (t, ²J_C,P = 9 Hz, Ru–C_NHC), 172.9 (vt, J = 31 Hz, ZnAr_Cquat), 149.1 (vt, J = 32 Hz, Ar), 140.7 (vt, J = 18 Hz, Ar), 140.4 (vt, J = 18 Hz, Ar), 139.2 (vt, J = 12 Hz, Ar), 139.1 (vt, J = 12 Hz, Ar), 133.0 (m, Ar), 132.5 (vt, J = 5 Hz, Ar), 128.9 (s, Ar), 127.1 (s, Ar), 126.6 (br vt, J = 3 Hz, Ar), 125.0 (m, Ar), 123.5 (s, NCH=CHN), 123.3 (s, NCH=CHN), 36.7 (s, NCH₃), 35.7 (s, NCH₃), 9.8 (s, H₃CC=CCH₃), 9.3 (s, H₃CC=CCH₃). Elemental analysis. Found: C, 61.00; H, 4.78; N, 3.13. C₄₄H₄₂N₂OP₂ZnRu requires C, 62.67; H, 5.02; N, 3.32. Multiple attempts repeatedly gave low %C values.

X-ray crystallography

Data for all structures (Table S2†) were obtained using an Agilent SuperNova instrument and a Cu-Kα source. All experiments were conducted at 150 K and structures were universally solved using SHELXT.⁵⁵ Refinements were conducted using SHELXL⁵⁶via the Olex2 ⁵⁷ interface. Convergences were generally straightforward. Where disorder prevailed, appropriate distance and ADP restraints were included, in these regions, to assist convergence. Only additional, noteworthy, points follow. Two independent molecules of the bimetallic complex plus 2.5 molecules of benzene were noted to constitute the asymmetric unit in the structure of 7. The hydride ligand present in each of the two complexes was located and refined freely. Solvent was manifest as half of an ordered molecule (proximate to a crystallographic inversion centre) and two complete benzene moieties that were treated for 75 : 25 and 57 : 43 disorder, respectively. The hydride was located and refined freely in the structure of 9. In the structure of 10, the hydride was also readily located and it was freely refined, but with a riding U_iso value. Data for this structure were impacted by sample twinning plus decay of the crystal in the beam, both of which were addressed during integration of the raw frames.

The asymmetric unit in 11 contains one molecule of the bimetallic complex and half of a molecule of hexane. The latter lies proximate to a crystallographic inversion centre, which serves to generate the remainder of the solvent. The hydrides were located and refined without restraints. There is evidence for some smearing of the solvent-electron density, but this precluded any sensible modelling efforts that were trialled.

The asymmetric unit in the structure of 13 was seen host to one molecule of benzene in addition to one molecule of the metal complex. In 14, the asymmetric unit was seen to contain four independent molecules which are broadly similar. However, the overlay of these four molecules shows that there are significant conformational differences with respect to the positioning of the pendant phenyl rings. All hydrides were located and refined subject to some comparative distance restraints. These separate similarity restraints were applied to the following four, chemically equivalent, sets of bonds (i) Ru4–H7D, Ru3–H5D, Ru1–H1D, Ru2–H3D (ii) Ru2–H4D, Ru1–H2D, Ru4–H8D, Ru3–H6D (iii) Zn2–H3D, Zn1–H1D, Zn4–H7D, Zn3–H5D and (iv) Zn3–H6D, Zn1–H2D, Zn2–H4D, Zn4–H8D. The requirement for restraints reflects the fact that the crystal was not entirely single in nature, but that it did not readily index as a multi-twinned sample either. As a result, the largest unassigned electron density peak is 1.87 electrons per cubic Å, while the largest hole is 3.51 e Å⁻³.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for 9, 10, 11, 7, 13, and 14 have been deposited at the CCDC under accession numbers 2409552, 2409553, 2409554, 2409555, 2409556 and 2409557† and can be obtained from https://www.ccdc.cam.ac.uk/.

Acknowledgements

The work was supported by an EPSRC Doctoral Training Award to AMW. We thank Dr Robin Stein (Bruker) for solid-state NMR assistance and Dr David Liptrot for use of his ATR-IR spectrometer. We dedicate this paper to Professor Paul Pringle, on the occasion of his upcoming retirement, in recognition of his many contributions to the chemistry of metal-phosphine complexes.

References

1. J. I. van der Vlugt, Angew. Chem., Int. Ed., 2010, 49, 252–255.
2. M. A. Bennett, S. K. Bhargava, N. Mirzadeh and S. H. Privér, Coord. Chem. Rev., 2018, 370, 69–128.
3. G. M. Adams and A. S. Weller, Coord. Chem. Rev., 2018, 355, 150–172.
4. M. T. Whited, Dalton Trans., 2021, 50, 16443–16450.
5. Y. H. Lee and B. Morandi, Coord. Chem. Rev., 2019, 386, 96–118.
6. P. W. N. M. van Leeuwen and P. C. J. Kamer, Catal. Sci. Technol., 2018, 8, 26–113.
7. M. Vogt and R. Langer, Eur. J. Inorg. Chem., 2020, 3885–3898.
8. M. Sircoglou, M. Mercy, N. Saffon, Y. Coppel, G. Bouhadir, L. Maron and D. Bourissou, Angew. Chem., Int. Ed., 2009, 48, 3454–3457.
9. A. Amgoune and D. Bourissou, Chem. Commun., 2011, 47, 859–871.
10. R. C. Cammarota and C. C. Lu, J. Am. Chem. Soc., 2015, 137, 12486–12489.
11. G. Bouhadir and D. Bourissou, Struct. Bonding, 2017, 171, 141–201.
12. M. V. Vollmer, J. Xie and C. C. Lu, J. Am. Chem. Soc., 2017, 139, 6570–6573.
13. J. Takaya and N. Iwasawa, J. Am. Chem. Soc., 2017, 139, 6074–6077.
14. R. C. Cammarota, L. J. Clouston and C. C. Lu, Coord. Chem. Rev., 2017, 334, 100–111.
15. D. You and F. P. Gabbai, Trends Chem., 2019, 1, 485–496.
16. J. Takaya, Chem. Sci., 2021, 12, 1964–1981.
17. P. Steinhoff, R. Steinbock, A. Friedrich, B. G. Schieweck, C. Cremer, K. N. Truong and M. E. Tauchert, Dalton Trans., 2018, 47, 10439–10442.
18. P. Steinhoff, M. Paul, J. P. Schroers and M. E. Tauchert, Dalton Trans., 2019, 48, 1017–1022.
19. J. P. Schroers, M. N. Kliemann, J. M. A. Kollath and M. E. Tauchert, Organometallics, 2021, 40, 3893–3906.
20. M. A. Bennett, M. Contel, D. C. R. Hockless and L. L. Welling, Chem. Commun., 1998, 2401–2402.
21. M. A. Bennett, M. Contel, D. C. R. Hockless, L. L. Welling and A. C. Willis, Inorg. Chem., 2002, 41, 844–855.
22. Y. P. Cai, S. J. Jiang, L. Q. Dong and X. Xu, Dalton Trans., 2022, 51, 3817–3827.
23. M. García-Melchor, B. Fuentes, A. Lledós, J. A. Casares, G. Ujaque and P. Espinet, J. Am. Chem. Soc., 2011, 133, 13519–13526.
24. M. V. Polynski and E. A. Pidko, Catal. Sci. Technol., 2019, 9, 4561–4572.
25. A. M. Messinis, S. L. J. Luckham, P. P. Wells, D. Gianolio, E. K. Gibson, H. M. O'Brien, H. A. Sparkes, S. A. Davis, J. Callison, D. Elorriaga, O. Hernandez-Fajardo and R. B. Bedford, Nat. Catal., 2019, 2, 123–133.
26. F. M. Miloserdov, C. J. Isaac, M. L. Beck, A. L. Burnage, J. C. B. Farmer, S. A. Macgregor, M. F. Mahon and M. K. Whittlesey, Inorg. Chem., 2020, 59, 15606–15619.
27. K. Fukuda, T. Harada, N. Iwasawa and J. Takaya, Dalton Trans., 2022, 51, 7035–7039.
28. S. Burling, M. F. Mahon, B. M. Paine, M. K. Whittlesey and J. M. J. Williams, Organometallics, 2004, 23, 4537–4539.
29. N. Bramananthan, E. Mas-Marzá, F. E. Fernandéz, C. E. Ellul, M. F. Mahon and M. K. Whittlesey, Eur. J. Inorg. Chem., 2012, 2213–2219.
30. B. Cordero, V. Gómez, A. E. Platero-Prats, M. Revés, J. Echeverría, E. Cremades, F. Barragán and S. Alvarez, Dalton Trans., 2008, 2832–2838.
31. P. R. Markies, G. Schat, O. S. Akkerman and F. Bickelhaupt, Organometallics, 1990, 9, 2243–2247.
32. P. E. Garrou, Chem. Rev., 1981, 81, 229–266.
33. F. Mohr, S. H. Privér, S. K. Bhargava and M. A. Bennett, Coord. Chem. Rev., 2006, 250, 1851–1888.
34. The major product formed with 2 equivalents of IEt₂Me₂ appeared to be an analogue of the IMes and IⁱPr₂Me₂ compounds 8 and 9 shown in Scheme 4, based on the similarity of the hydride and ³¹P NMR resonances shown in Fig. S1 and S2† to those in Fig. S10 and S11† (see also ref. 36). No attempts were made to isolate this species for further characterisation.
35. Repeating the reaction at higher temperature (55–80 °C) or in a different solvent (toluene or THF) failed to generate any more of the proposed isomer. Based on the ³¹P NMR resonances (Fig. S2;† see also Experimental section), it contains two cyclometallated phosphine ligands cis to one another. Recrystallisation of the crude mixture of 7 and the proposed isomer from toluene/hexane at low temperature gave just 7.
36. F. M. Miloserdov, A.-F. Pécharman, L. Sotorrios, N. A. Rajabi, J. P. Lowe, S. A. Macgregor, M. F. Mahon and M. K. Whittlesey, Inorg. Chem., 2021, 60, 16256–16265.
37. Again the crystals required manual separation from a colourless solid, assumed to be (IⁱPr₂Me₂)₂ZnMe₂.
38. A. E. W. Ledger, A. Moreno, C. E. Ellul, M. F. Mahon, P. S. Pregosin, M. K. Whittlesey and J. M. J. Williams, Inorg. Chem., 2010, 49, 7244–7256.
39. D. Pingen, T. Lebl, M. Lutz, G. S. Nichol, P. C. J. Kamer and D. Vogt, Organometallics, 2014, 33, 2798–2805.
40. J. Alós, T. Bolaño, M. A. Esteruelas, M. Oliván, E. Oñate and M. Valencia, Inorg. Chem., 2014, 53, 1195–1209.
41. It is unclear whether this degradation is due to the chlorinated solvent itself, or traces of water in the solvent. See also ref. 27.
42. S. B. Duckett, R. J. Mawby and M. G. Partridge, Chem. Commun., 1996, 383–384.
43. H. Samouei, F. M. Miloserdov, E. C. Escudero-Adán and V. V. Grushin, Organometallics, 2014, 33, 7279–7283.
44. L. Bemi, H. C. Clark, J. A. Davies, C. A. Fyfe and R. E. Wasylishen, J. Am. Chem. Soc., 1982, 104, 438–445.
45. Z. Yan and R. Zhang, J. Magn. Reson., 2023, 357, 107597.
46. In the low frequency region of the spectrum, there were small changes in chemical shift (Δδ = 0.1–0.2 ppm) for both ZnPhos and PPh₃ resonances over the course of the reaction. This observation shows some similarities to the behaviour of resonances for 13 and PPh₃ as a function of temperature described in the main text.
47. We have reported intramolecular attack of Ru–H on the C–O bond of DPEphos. M. K. Cybulski, N. A. Beattie, S. A. Macgregor, M. F. Mahon and M. K. Whittlesey, Chem. – Eur. J., 2020, 26, 11141–11145.
48. R. H. Crabtree, Chem. Rev., 2015, 115, 127–150.
49. R. A. Schunn, E. R. Wonchoba and G. Wilkinson, Inorg. Synth., 1971, 13, 131–134.
50. N. Ahmad, J. J. Levison, S. D. Robinson and M. F. Uttley, Inorg. Synth., 1974, 15, 45–48.
51. H. Samouei and V. V. Grushin, Organometallics, 2013, 32, 4440–4443.
52. N. Kuhn and T. Kratz, Synthesis, 1993, 561–562.
53. S. Harder, L. Brandsma, J. A. Kanters, A. Duisenberg and J. H. van Lenthe, J. Organomet. Chem., 1991, 420, 143–154.
54. S. Porcel, G. Bouhadir, N. Saffon, L. Maron and D. Bourissou, Angew. Chem., Int. Ed., 2010, 49, 6186–6189.
55. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Adv., 2015, 71, 3–8.
56. G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3–8.
57. O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2409552–2409557. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt01160d

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