Structural and reactivity investigations using organo-copper(I) and zinc(II) complexes with hydrogen and carbon dioxide

Abstract

Heterogeneous copper zinc catalysts are widely used in syn-gas or carbon dioxide hydrogenation to methanol, but the structures and reactivity at the catalytic active sites are still not fully understood. Well-defined metal complexes and clusters may help reveal the structures of key intermediates, including for catalytic reactions involving gaseous species. Here, a tri-copper(I) complex, [Cu₃{µ:κ³(PPC)-dppm′}₂{µ:κ²(PP)-dppm′}] 1 (dppm′ = [(Ph₂P)₂CH]⁻) is used as the starting point to investigate reactivity with hydrogen and carbon dioxide in solution and at low temperatures (20–60 °C) and gas pressures (1 bar or lower). Complex 1 reacts with hydrogen to afford a copper-hydride cluster, [Cu₅(µ-H)(µ₃-H)₂(µ:κ³(PPC)-dppm′)₂(µ:κ²(PP)-dppm)₂] 2, which contains five Cu^I centres and three hydrido ligands. The reaction of 2 with carbon dioxide provides a tri-copper(I)-bis(formate)(hydrido) complex, [Cu₃(µ₃-H)(κ¹(O)-O₂CH)(µ:κ²(OO)-O₂CH)(µ:κ²(PP)-dppm)₃] 3, in which two molecules of carbon dioxide react with the hydrides to produce formate ligands. Treating 1 with an organozinc(II) complex, [Zn(C₆F₅)₂], affords a hetero-tetrametallic Cu^I₂Zn^II₂ complex, [Cu₂(µ:κ³(PPC)-dppm′)₂{Zn(C₆F₅)₂}₂] 4. Complex 4 features two cationic Cu(I) and two anionic Zn(II) centres, adopting an unusual copper zincate speciation, and reacts with carbon dioxide to form a bis(carboxylate)-Cu^I₂Zn^II₂ complex, [Cu₂(µ:κ⁴(PPOO)-dppm″)₂{Zn(C₆F₅)₂}₂] 5; carbon dioxide inserts into the Zn–C bond and forms a new C–C bond with the methine carbon of the dppm′ ligand. Overall, the structures and reactivity indicate that Cu^I sites readily form hydrides, on exposure to hydrogen; these cuprous hydrides react with CO₂ to produce formate, and cuprous zincates react with CO₂ to afford carboxylates.

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Introduction

The hydrogenation of carbon dioxide (CO₂) to form methanol is important for the production of sustainable liquid transport fuels and energy vectors.¹ To limit the resulting greenhouse gas emissions, the hydrogen should be renewably sourced, e.g. by solar powered water electrolysis.^2–4 Currently, methanol is industrially produced on a 100 Mt per annum scale from synthesis gas via hydrogenation of both CO and CO₂, with the key reactions involving both direct hydrogenations of these carbon oxides and exchange between them by the reverse water–gas shift reaction.^5,6 The syn-gas to methanol reactions operate at high temperatures (250 °C) and pressures (>50 bar) using heterogeneous Cu/ZnO/Al₂O₃ catalysts.⁷ Computational⁸ and isotopic-labelling studies⁹ have revealed that, in syn-gas transformations, most of the methanol produced originates from the hydrogenation of CO₂, with CO playing a key role in the catalytic cycle through reactions with surface-oxygen atoms originating from water production, as well as participating in CO-assisted hydrogenation of surface intermediates.^8,10,11 Direct CO₂ hydrogenation is also feasible using the same Cu/ZnO catalysts and, once again, low temperature reactions are desirable to drive equilibria.¹² In both reactions, there is considerable debate regarding the catalyst active sites, including different hypotheses regarding metal oxidation states, structures, speciation, and mechanisms/functions; there is general consensus that active sites occur at interfaces between copper(0/I) and zinc(0/II) surfaces.^{5,6,10,12–24} There are many approaches to exploring this complex catalytic problem; this work focusses on using well-defined copper and zinc complexes to better understand reactivity with hydrogen and carbon dioxide.

Homogeneous copper or copper–zinc catalysts are not yet known for CO₂ hydrogenation to methanol, but well-defined complexes or clusters may be useful to investigate key hydrogen and carbon dioxide insertion reactions occurring during the catalytic cycle. There are two widely accepted mechanisms for heterogeneous Cu/ZnO catalysed CO₂ hydrogenation to methanol. Firstly, direct CO₂ hydrogenation, which proceeds via formate intermediates ([O₂CH]⁻). Secondly, the reverse water–gas-shift reaction (RWGSR) and CO hydrogenation mechanism, in which CO₂ is converted to CO, followed by CO hydrogenation to methanol via formyl ([OCH]⁻) and formaldehyde (O=CH₂) intermediates.²⁵ Formaldehyde is common to both mechanisms and is hydrogenated to a methoxy intermediate which is protonated to liberate methanol.²⁶

It is hypothesized that most CO₂ hydrogenation occurs via the formate route,^37,38 with the Cu-sites responsible for the adsorption and dissociation of the H₂, while the ZnO inserts and activates the CO₂. The key steps in the formate mechanism are: (1) the reaction/absorption of CO₂ with ZnO possibly by insertion into surface hydride/hydroxide species to activate the carbon dioxide; (2) the reaction of the copper surface with hydrogen to form copper-hydrides or ‘activated’ hydrogen; and (3) the reaction between copper-hydride and the ZnO–carbon dioxide species, occurring at Cu–ZnO interfaces, to form formate intermediates.³⁹ Here, our objective is to isolate well-defined homogeneous metal complexes and clusters that react readily with hydrogen and carbon dioxide. We use these complexes as model compounds informing upon the structures and reactivity of metal hydride species.

There have been several prior investigations into the reactivity of well-defined homogeneous copper or zinc complexes with hydrogen or carbon dioxide.^30,40–45 Often, the copper or zinc hydride complexes were reacted with carbon dioxide to produce formate complexes.^30,40–48 For example, Bertrand and co-workers reported a transiently-formed copper(I) hydride complex [(^EtCAAC)CuH] (^EtCAAC = (C₆H₃-2,6-ⁱPr₂)NC(Me₂)CH₂C(Et)₂C; Fig. 1A) which reacted with CO₂ to yield a copper(I)-formate complex, [(^EtCAAC)Cu(O₂CH)].²⁷ Bertrand, Jazzar and co-workers also showed that a trinuclear Cu⁰₂Cu^I complex, [(^EtCAAC)Cu]₃, reduced CO₂ to CO, possibly paralleling the reactivity on copper surfaces during reductive CO₂ dissociation.⁴⁹ Gray, Sadighi and co-workers reported a hydride-bridged dicopper(I) cationic complex, [{(IDipp)Cu}₂(µ-H)][BF₄] (Fig. 1B), which reacted with CO₂ to provide a formate-bridged dicopper cation, [{(IDipp)Cu}₂(µ-O₂CH)][BF₄].²⁸ Tran, Bullock and co-workers isolated di-copper(I) dihydride bis(NHC) crystals and studied sequential carbon dioxide insertion reactions to give a di-copper(I)formate hydride crystal and, ultimately, a di-copper(I)bis(formate) speciation.⁴⁵ Whilst dimers were maintained in the crystal-to-crystal transformations, the equivalent complexes dissociated to monomers in solution. Appel and co-workers showed that a copper(I)-hydride complex, coordinated by a tris(phosphine) ligand, [{κ³(PPP)-MeC(CH₂PPh₂)₃}CuH] (Fig. 1C), reacted with CO₂ to form a copper(I)-formate product, [{κ³(PPP)-MeC(CH₂PPh₂)₃}Cu(O₂CH)].²⁹ Recently, Buss and co-workers reported a series of cationic tricopper(I) mono-, di- and trihydrido complexes, Cu₃H_x (x = 1–3) coordinated by tris(carbene) ligands, and showed that the CO₂ reactivity depended on the number of hydrido ligands (Fig. 1D).³⁰ The di- and trihydrido complexes, [LCu₃(µ-H)₂]BArF and [LCu₃(µ-H)₃] (L = C₆-1,3,5-{CH₂{NCN(Ar)CH₂CH₂}}₃-2,4,6-Et₃; Ar = C₆H₃-2,6-ⁱPr₂; BArF = B{C₆H₃-3,5-(CF₃)₂}₄), reacted with carbon dioxide to form copper formate products, whereas the monohydrido complex, [LCu₃(µ₃-H)][BArF]₂, was unreactive.

Fig. 1 Selected homogenous molecular copper and zinc complexes investigated for CO₂ hydrogenation in the literature.^27–36 Ar = C₆H₃-2,6-ⁱPr₂; BArF = B{C₆H₃-3,5-(CF₃)₂}₄; R = OMe, ^tBu, NO₂.

There is also precedent for reactions between zinc-hydride complexes and carbon dioxide.^{32,33,50–54} For example, Parkin and co-workers reported a zinc(II)-hydride complex, coordinated by a tris(2-pyridylthio)methyl ligand, [{κ³(NNC)-tptm}ZnH] (Fig. 1E), which reacted with CO₂ to afford a zinc(II)-formate complex, [{κ³(NNC)-tptm}Zn(O₂CH)].³¹ Okuda and co-workers reported CO₂ insertion into the Zn–H bond of [(Me₆tren)ZnH]BArF (Me₆tren = N{(CH₂)₂NMe₂}₃; Fig. 1F) to provide the zinc-formate product, [(Me₆tren)Zn(O₂CH)]BArF.³² The zinc-hydride complex of a related but less sterically hindered ligand, [(pmdta)ZnH][B(C₆F₅)₄] (pmdta = MeN{(CH₂)₂NMe₂}₂, Fig. 1G), produced a dimeric zinc-formate complex, [{(pmdta)Zn(O₂CH)}₂][B(C₆F₅)₄]₂, when treated with CO₂.³³ Our group reported a series of zinc(II)-hydride complexes, coordinated by a κ³(NNO)-di(amino)phenolate ligand, which reacted with CO₂ to provide zinc(II)-formate complexes (Fig. 1I).^34,35 It was shown that the rate of carbon dioxide insertion was equivalent or faster than the rate of carbon dioxide diffusion/solubilisation even at low pressures (in THF). Grapperhaus and co-workers reported an equilibrium between a zinc(II)–methanol complex, [(dmth)Zn(HOMe)] (dmth = diacetyl-2-(4-methyl-3-thiosemicarbazonate)-3-(2-pyridinehydrazonato)) and a zinc(II)-methoxide complex, [(Hdmth)Zn(OMe)], facilitated by ligand protonation (Fig. 1H). The zinc(II)-methoxide complex reversibly inserts CO₂ into the Zn–OMe bond to form a zinc(II)-methylcarbonate complex, [(Hdmth)Zn(O₂COMe)], which critically, facilitates CO₂ reduction; treatment chemically with NaBH₄, or electrochemically using a Pt electrode to generate Pt–H via the hydrogen evolution reaction (HER), produces methanol and generates a zinc(II)-formate complex, [(dmth)Zn(O₂CH)]. Ligand substitution of HCO₂⁻ for MeOH regenerates [(Hdmth)Zn(OMe)], closing the CO₂ reduction cycle.³⁶

The insertion of carbon dioxide into copper or zinc hydride bonds is quite well precedented but most of those hydride complexes were formed by metathesis reactions with alkali hydrides and silanes, or by the protonation of reactive metal alkoxides.^55,56 For example, copper hydride complexes were often prepared by reacting a copper precursor, e.g. CuCl(PR₃) with KHB(OⁱPr)₃, in the presence of strongly donating ligands (i.e. phosphines, carbenes, dithiophosphates or dithiocarbamates).⁵⁷ There is much less precedent for copper hydride formation by reaction of well-defined copper(I) complexes directly with hydrogen despite its importance in many of the catalytic cycles.^29,58–61 Further, the active site is often proposed to involve a copper–zinc interface, yet, there is very little precedent for well-defined heterometallic Cu/Zn complexes reacting with hydrogen or carbon dioxide.^62–64 Thus, we targeted well-defined copper and zinc complexes, including heterometallic complexes, and their reactivity with hydrogen and carbon dioxide. We selected a known copper(I) bis(phosphine) complex as the precursor since the chelating phosphine ligands may stabilise highly reactive intermediates and the ligand enables use of ³¹P NMR spectroscopy to characterise products of insertion reactions.^65–67 In the heterogeneous catalysis, chemisorption of hydrogen is proposed to occur at Cu⁰ active sites and may form Cu(I)–H surface species, akin to multimetallic oxidative addition, and play a key role in carbon dioxide reduction. Here, we targeted the reaction of the Cu(I) precursor with hydrogen to form copper(I) hydride complexes, but by H₂ heterolysis between a Brønsted basic phosphine ligand and the Cu(I) centre to which it is bound; we focus on multi-metallic complexes since surface active sites are proposed to involve several copper atoms/ions.^68,69 We also targeted heterometallic Cu(I)Zn(II) complexes to understand reactions with carbon dioxide, in particular whether CO₂ insertions would occur at zinc or copper sites.

Results

The synthesis and solid-state structure of the tri-copper(I) starting complex, [Cu₃{µ:κ³(PPC)-dppm′}₂{µ:κ²(PP)-dppm′}] (1; dppm′ = [(Ph₂P)₂CH]⁻), was previously described and is formed by reacting mesitylcopper(I) ([CuMes]_z; z = 4, 5) with bis(diphenylphosphino)methane (dppm; (Ph₂P)₂CH₂) (Scheme 1).^65–67 The structure of complex 1 features three Cu^I centres coordinated by three anionic bis(phosphine) ligands, [(Ph₂P)₂CH]⁻ (dppm′), in which the methylene carbon atom of dppm has been deprotonated. Complex 1 shows a solid-state structure with the following features: (1) two of the dppm′ ligands are κ³(PPC)-coordinated to the Cu^I centres and the other dppm′ ligands is κ²(PP)-coordinated, leaving a free anionic CH_methine; (2) two of the Cu centres are 4-coordinate and phosphine-coordinated, whereas one is 2-coordinate and only bound to anionic CH_methine groups. We proposed that these structural features might allow 1 to react readily with hydrogen, carbon dioxide and/or organo-zinc reagents without involving any metal redox chemistry; as such, it affords an opportunity to investigate Cu(I) reactivity which might be relevant to catalyst surface reactivity.

Scheme 1 Reactions of 1 with hydrogen to form [Cu₅(µ-H)(µ₃-H)₂(µ:κ³(PPC)-dppm′)₂(µ:κ²(PP)-dppm)₂] (2) and, subsequently with carbon dioxide and hydrogen to form [Cu₃(µ₃-H)(κ¹(O)-O₂CH)(µ:κ²(OO)-O₂CH)(µ:κ²(PP)-dppm)₃] (3).

First, the reaction of 1 with low pressures of H₂ (0.5 bar) was undertaken at 60 °C, in toluene, over 3 days. The reaction afforded [Cu₅(µ-H)(µ₃-H)₂(µ:κ³(PPC)-dppm′)₂(µ:κ²(PP)-dppm)₂] (2, Scheme 1) as a bright yellow solid, which was isolated in 63% yield. Complex 2 is a molecular cluster comprising 5 Cu^I centres, 2 anionic dppm′ ligands, 2 neutral dppm ligands and 3 hydride ligands. The reaction also affords neutral dppm ligand as a by-product; using in situ NMR spectroscopy to investigate the formation of 2 (from 1) shows only the formation of 2 and free dppm (i.e. no intermediates). The ³¹P{¹H} NMR spectrum for 2, at 298 K in benzene-d₆, shows distinct resonances for each of the 8 phosphines with broadened singlets observed at 0.2, −0.6, −8.5 (2 PR₃), −13.9 and −14.6 ppm, and sharp doublets at −11.7 and −12.7 and ppm (²J_P,P = 27 Hz; Fig. S14). Variable temperature NMR spectroscopy did not show any changes (coalescence/splitting) to the broadened resonances from −80 to 70 °C. The peak broadening prevented use of 2-D NMR techniques (e.g.¹H, ³¹P-HMBC NMR spectroscopy) (Fig. S17 and S20).

The ¹H NMR spectrum of 2, at 298 K in benzene-d₆, shows methine-CH resonances at 3.08 ppm, and two sets of methylene-CH₂ protons at 2.72 and 2.06 ppm, respectively. The methylene-CH₂ resonances were confirmed using ²H-labelling experiments, i.e. after the reaction of 1 with D₂ and identification of D on the methylene unit in the ligand (Fig. S21). The copper-hydride resonances could not be located by ¹H or ²H NMR spectroscopy, at room temperature or through variable temperature NMR experiments (−80 to 70 °C). This lack of a clear copper-hydride signal is proposed to arise from peak broadening due to the quadrupolar Cu (I = 3/2), combined with the likely fluxionality of the hydride ligands in solution (see SI). The FT-IR spectra of 2 and 2-d₃ are identical, indicating Cu–H stretches occur at very low frequencies (Fig. S23).

Crystals suitable for X-ray diffraction experiments were obtained by slow evaporation of a benzene solution of 2 at room temperature (Fig. 2). The solid-state structure shows 5 Cu^I centres and 3 hydrido ligands; the Cu centres form a distorted trigonal bipyramid, in which two apical Cu centres are located above and below an equatorial Cu₃-plane. The neutral dppm ligands are κ²(PP)-coordinated to an apical and equatorial Cu(I) atom on the outer faces of the cluster, respectively. The anionic dppm′ ligands occupy the open faces of the cluster via κ³(PPC)-coordination to the Cu(I) atoms (Fig. 2). The Cu–Cu bond lengths range from 2.683(1)–2.774(1) Å, which exceeds the sum of the covalent radii (2.24–2.64 Å)^70,71 but is significantly shorter than the sum of the van der Waals radii (3.84–4 Å),^72,73 The Cu–Cu bond length, in 2, is also shorter than that observed for complex 1 (2.836(4) Å).⁶⁷ The intermetallic separation is, however, consistent with other neutral copper(I)-hydride cluster complexes (Table S3).^74–76 The orientation of the three Cu(I) centres in the equatorial sites is somewhat similar to bonding motifs observed at the Cu(111) surface; this surface is often invoked as a key interface during catalytic CO₂ hydrogenation.^77,78 Nonetheless, such analogies must be treated carefully since whilst the geometry may be somewhat similar, the Cu–Cu bond lengths in complex 2 are considerably longer than on a Cu(111) surface, where d_Cu–Cu is ∼2.41 Å.⁷⁹ The Cu–P bond lengths range from 2.211(2)–2.318(2) Å, consistent with the average Cu–P bond length reported in 1 (2.317(5) Å).⁶⁷ Two of the hydrido ligands are µ₃-coordinated to an apical Cu(I) and two equatorial Cu(I) atoms, and one of the hydrido ligands is µ₂-coordinated between two equatorial Cu(I) centres. The Cu–H bond lengths range from 1.44(6)–1.94(6) Å, which are similar to bonds reported for other neutral Cu^I-hydride cluster complexes^74,75,80,81 and cationic bridging copper(I)-hydride complexes.^30,61 Given the inherent uncertainty in locating hydrido ligands by X-ray diffraction methods, the number and positions of the hydrido ligands in complex 2 were corroborated computationally by DFT. Three hydrido ligands were found to occupy nearly the exact locations as those determined by XRD, providing strong support for the experimental data (see Computational methods in the SI).

Fig. 2 Illustration of the solid-state structure of 2·4benzene obtained from single crystal X-ray diffraction experiments. Displacement ellipsoids are drawn with 50% probability, and carbon atoms of the P-phenyl groups are drawn wireframe. For clarity, only one of two molecules in the asymmetric unit are depicted, and hydrogen atoms, aside for the bridging hydride ligands and methylene/methine protons, and lattice solvent are omitted.

As mentioned, there is precedent for copper(I) hydride complexes reacting with CO₂ to yield copper(I) formate complexes.^{27–30,45,49} To establish whether similar reactivity would be achieved using complex 2, it was dissolved in toluene and exposed to 1 bar of carbon dioxide, at 60 °C, for 3 days which resulted in a colour change from bright yellow to colourless. The reaction product is [Cu₃(µ₃-H)(κ¹(O)-O₂CH)(µ:κ²(OO)-O₂CH)(µ:κ²(PP)-dppm)₃] (3; Scheme 1) but complete transformation of 2 into 3 did not occur until the solution was treated with H₂ (0.5 bar) and heated at 60 °C for 16 hours (Fig. S24–S26). Complex 3 features a Cu₃-core coordinated to two formate ligands. Labelling experiments, using D₂ and ¹³CO₂, confirmed that the formate ligands are produced by CO₂ insertion into the Cu–H bonds in 2 (the reactions to form 3-d₃ and 3-¹³C are described in the supplementary information). The tricopper complex is also coordinated to 3 neutral dppm ligands, which are κ²(PP)-coordinated to the Cu(I) centres, indicating the complete protonation of the anionic dppm′ ligands in 2. The ³¹P{¹H} NMR spectrum shows a single resonance, at −8.4 ppm, indicating all phosphines are equivalent, in line with the solid-state structure (Fig. S27). The ¹H NMR spectrum of 3 contains a singlet, at 3.41 ppm, assigned to the methylene protons of the dppm ligands. It also contains a sharp singlet, at 9.55 ppm, assigned to the formate protons and a broadened singlet, at 1.57 ppm, assigned to the µ₃-hydrido ligand, the relative integrals for these resonances are 2 : 1, respectively (Fig. 3A and S28). When D₂ was used to afford 3-d₃, the reaction product was characterised using ²H NMR spectroscopy which revealed broadened resonances at 9.42, 3.29 and 1.52 ppm, which are assigned to the formate, methylene and µ₃-D groups, respectively (Fig. 3A and S29). The isotope experiments substantiate the assignment of the bridging hydrido ligand. Further, while the hydrido ligands in 2 were not observed using NMR spectroscopy (see above), the hydrido ligand resonance, in the ²H NMR spectrum of 3-d₃, may indicate that similar bonding occurs in 2-d₃ and that these bridging hydrides undergo CO₂ insertion to form the bridging formate ligands. The ¹H NMR spectrum of 3-¹³C, in which the carbon atom of the formate ligands has been ¹³C-labelled, contains a doublet at 9.55 ppm (¹J_H,C = 188 Hz), confirming its assignment as the formate proton (Fig. S30). Furthermore, the ¹³C{¹H} NMR spectra of both 3 and 3-¹³C contain a singlet, at 168.3 ppm, confirming its assignment as the carbon atom of the formate ligands (Fig. S31 and S32). The FT-IR spectrum of 3 contains high intensity resonances at 1588, 1577 and 1569 cm⁻¹, assigned as the asymmetric ν(OCO) formate ligand stretches, and intense resonances at 1476 and 1426 cm⁻¹, assigned as the symmetric ν(OCO) formate stretching vibrations (Fig. S33). The combined spectroscopic features are consistent with data reported for some other copper(I) κ¹(O)- and κ²(OO)-formate complexes.^27,40,42 In particular, a tri-copper(I) formate complex, [Cu₃(µ₃-H)(µ:κ²(OO)-O₂CH)(dppm)₃][PF₆],⁸² which was formed by reacting [Cu₈H₆(dppm)₅][PF₆] with CO₂. The previously reported copper(I) formate complex was cationic and contained a single formate ligand, bridging two Cu(I) centres. In contrast, complex 3 is neutral and features two bridging formate ligands (Table S4). The formate coordination chemistry is related to a previously reported tri-Cu(I) formate complex, [Cu₃(µ₃-H)(µ:κ²(OO)-OAc)(κ¹(O)-OAc)(dppm)₃] (OAc = acetate), reported by Hayton and co-workers, which was formed by reacting copper(I) acetate with 0.5 equiv. of Ph₂SiH₂, and dppm ligand.⁸⁰ In both of these literature complexes, the Cu(I) clusters also feature a residual hydrido ligand and in both cases its stability was attributed to steric protection imparted by the Cu centres and dppm ligands.

Fig. 3 (A) Stacked ¹H NMR (top; benzene-d₆, 298 K, 600 MHz) and ²H NMR spectra (bottom; toluene, 298 K, 77 MHz) of 3 and 3-d₃, respectively; the red highlighted resonance in the ²H NMR spectrum of 3-d₃ is assigned as the µ₃-D resonance. Where other resonances marked * = benzene-d₆, Δ = PPh₂ and ○ = toluene. (B) Two orientations of the solid-state structure of 3·3benzene. Displacement ellipsoids are drawn with 50% probability, and carbon atoms of the P-phenyl groups are drawn wireframe. For clarity, hydrogen atoms, except for H(1) and H(2), and lattice solvent are omitted. The µ₃-hydrido ligand could not be located in the difference map.

Single crystals of 3·3benzene suitable for X-ray diffraction experiments were obtained from a benzene solution of 3, after hexane diffusion, at room temperature. The solid-state structure shows a trigonal planar Cu₃-core coordinated by 3 neutral dppm ligands and two formate ligands (Fig. 3B). One of the formate ligands adopts κ²-bridging between 2 Cu centres via both its oxygen atoms, whereas the other is κ¹(O)-coordinated to only 1 Cu centre. The µ₃-hydrido ligand could not be located in the difference map, and crystals suitable for neutron diffraction could not be grown. As a result, DFT calculations were consulted to verify the position of the bridging hydrido ligand in 3. The hydrido ligand was located in the centre and slightly off the plane of the Cu₃-ring and coordinated in a µ₃-fashion, in the same position as in the neutral acetate analogue, [Cu₃(µ₃-H)(µ:κ²(OO)-OAc)(κ¹(O)-OAc)(dppm)₃] (OAc = acetate),⁸⁰ corroborating the assignment based on spectroscopic studies and ²H-labelling (vide supra). In the absence of the location for the µ₃-hydrido ligand, each of the Cu centres are best described as basally distorted square pyramidal with the formate ligands occupying the apical site; the τ₅-parameters are 0.06, 0.20 and 0.21 for Cu(1), Cu(2) and Cu(3), respectively.^83,84 Once again, the orientation of the Cu₃ core is reminiscent of a Cu(111) monolayer, but with elongated Cu–Cu bond lengths.⁷⁹ The Cu(1)–Cu(2), Cu(1)–Cu(3) and Cu(2)–Cu(3) bond lengths in 3 are 2.9681(5), 2.9041(5) and 2.8071(5) Å, respectively. The Cu(2)–Cu(3) bond length is similar to the Cu–Cu bond lengths in 1 and 2, however, the others are significantly longer.

The Cu(1)–O(1) bond length, to the κ¹-coordinated formate ligand, is 2.092(2) Å, and the Cu(2)–O(3) and Cu(3)–O(4) bond lengths, to the µ:κ²-coordinated formate ligand, are 2.137(2) and 2.116(2) Å, respectively. The C–O bond lengths are all similar to one another (1.235(5)–1.253(3) Å), indicating charge delocalization across the formate ligands. Consistent with this charge delocalisation, the O–C–O bond angles are 128.3(3)° and 128.4(2)°. The structural parameters in 3 are consistent with reports of other copper formate complexes, e.g. [(κ³(PPP)-triphos)Cu(κ¹(O)-O₂CH)]⁴⁰ and [Cu₂(µ-O₂CH)(µ-meso-dpmppm)₂][BF₄] (see Table S4).⁴² Compared to the cationic tri-copper complex, [Cu₃(µ₃-H)(µ:κ²(OO)-O₂CH)(dppm)₃][PF₆] (Cu–O = 2.192(5) Å; C–O = 1.321(7); O–C–O = 105.2(5)°),⁸² the Cu–O and C–O bond lengths are shorter, and the O–C–O bond angle is less acute. As noted above, the formation of 3 by treating 2 with CO₂ does not reach completion until the reaction is further charged with H₂. Further, in the reaction of 2 with only carbon dioxide, an additional species (to 3) is observed by NMR spectroscopy which suggests there is at least one reaction intermediate which is consumed upon addition of excess H₂. The ¹H NMR spectrum of the crude product (prior to addition of hydrogen) shows signals for 3 and additional peaks for a second species. The intermediate shows a peak at 9.65 ppm, assigned to a formate ligand, and resonances at 4.86 (triplet; ²J_P,H = 10 Hz), 2.98 and 2.47 ppm (2 × broad singlets), with relative integrals of ∼1 : 2 : 2, which are assigned to 1 anionic dppm′ and 2 neutral dppm ligand environments (see Fig. S24 and S25). On the basis of the NMR spectroscopic data, two potential structures are proposed: [Cu₃(µ₃-H)(O₂CH)(dppm′)(dppm)₂] (2A) or [Cu₃(O₂CH)₂(dppm′)(dppm)₂] (2B; Scheme S1). The putative intermediate 2A undergoes a single formate insertion and contains a bridging hydrido ligand, perhaps featuring the same trigonal planar Cu₃-core as 3, with a dppm : dppm′ ratio of 2 : 1 (see Scheme S1 in SI for a discussion of possible reaction mechanisms). All attempts to selectively isolate the proposed intermediates for further characterisation or to elucidate structures using ²H- and ¹³C-labelling experiments and NMR spectroscopy, were unsuccessful. Overall, when balancing the chemical reaction for the formation of 3, 18 H-atoms should be required to cleanly form 3 from 2, but only 9 hydrides are provided by 2, consistent with the requirement for additional H₂ for the complete formation of 3. To understand the reactivity of the formate ligands further, 3 was treated with a common organic reductant, pinacolborane (HBpin). No reaction occurred at room temperature, and upon gentle heating to 60 °C, 3 decomposes to Cu metal and unidentified organic by-products.

Next, the insertion reactions using carbon dioxide were investigated; complex 1 was reacted with CO₂ to understand whether there would be any insertion into its Cu–C bonds, as anticipated from the prominence of organocopper chemistry in carboxylation reactions.^85,86 Exposure of 1 to CO₂ (1 bar), in toluene at room temperature, resulted in a colour change from yellow to nearly colourless with concomitant precipitation of a white solid. Attempts to solubilize the reaction product in benzene-d₆, CD₂Cl₂, THF-d₈ and acetone-d₆ were unsuccessful, suggesting the formation of a copper-carboxylate polymer/aggregate. The solid was analysed by FT-IR spectroscopy which showed high intensity absorptions at 1425 and 1307 cm⁻¹, assigned to asymmetric and symmetric carboxylate stretches and consistent with bridging carboxylate ligands. The solid's elemental analysis was consistent with the molecular formula C₇₅H₆₃Cu₃P₆·3CO₂, i.e. reaction of 1 equiv. CO₂ per dppm′ in 1 (see Fig. S34, SI). Therefore, the product is proposed to be a polymeric copper carboxylate species, formed by CO₂ insertion into Cu–C bonds and by reaction (at C) with the methine carbon atom of the κ²(PP)-coordinated dppm′ ligand (Scheme 1).

Another objective is to understand the reactivity of well-defined heterometallic Cu(I) and Zn(II) complexes, since Cu/Zn(II) interfaces are critical as active sites in heterogeneous carbon dioxide hydrogenation catalysis. Thus, the reaction of tri-copper(I) complex 1 with organo-Zn^II complexes was used to try to isolate mixed metal complexes. The organo-zinc reagent was [Zn(C₆F₅)₂] since we reasoned its relatively high Lewis acidity should enable reaction with the anionic CH_methine group, or even insertion into the Cu–CH_methine bonds. Treating 1 with [Zn(C₆F₅)₂], in toluene, produced an immediate colour change from yellow to colourless, affording [Cu₂(µ:κ³(PPC)-dppm′)₂{Zn(C₆F₅)₂}₂] (4, Scheme 2). Single crystals suitable for X-ray diffraction experiments were obtained by slow evaporation of a benzene solution of the product. The solid-state structure shows that 4 is a tetrametallic Cu₂Zn₂-complex with the Zn(C₆F₅)₂ coordinated to the anionic –CH_methine groups of the dppm′ ligands. There is also κ²(PP)-coordination of two dppm′ ligands to two Cu^I centres in the core of the structure (Fig. 4). Two different isomers were identified: a cis-isomer where the two zinc centres are on the same side (both above/below) of the central Cu₂-plane (cis-4; Fig. 4A), and a trans-isomer, in which the zinc atoms are located above and below the central di-Cu(I) plane (trans-4; Fig. 4B). The Cu(I) centres in both cis-4 and trans-4 both exhibit T-shaped coordination geometry, with P–Cu–P and P–Cu–Cu bond angles of 168.04(2)° and 90.94(1)–96.42(1)° in cis-4, and 171.20(1)° and 88.57(1)–96.05(1)° in trans-4, respectively. There are short Cu⋯F (from ortho-C₆F₅) distances of 2.869 and 2.719 Å in cis-4 and trans-4, respectively. These distances lie well within the sum of the van der Waals radii (3.27 Å) and imply there is some cationic charge on each copper(I) atom.⁷² The Zn centres in each isomer exhibit trigonal planar geometry, with the Σ(CZnC) angles equal to 359.8(2)° in both isomers (theoretical = 360°), implying anionic charge on zinc. Overall, the complex is polarised as a cuprous zincate. The Cu–Cu bond length in cis-4 is 2.6736(4) Å, which is significantly shorter than the Cu–Cu bond lengths in 1 (2.836(4) Å)⁶⁷ and 3 (2.8071(5)–2.9681(5) Å). The Cu–Cu bond lengths are close to those found in 2 (2.682(1)–2.772(1) Å) and in several literature copper(I) cationic clusters, e.g. [Cu₂(µ-dcpm)₂][BF₄]₂ (dcpm = (Cy₂P)₂CH₂; Cy = cyclohexyl; Cu–Cu = 2.691(2) Å)⁸⁷ and [Cu₃(µ₃-H)(µ-dppy)₄][ClO₄]₂ (dppy = py-2-PPh₂; py = pyridyl; Cu–Cu = 2.701(1), 2.749(1), 2.723(1) Å).⁸⁸ The Cu–Cu bond length in trans-4 is 2.8235(4) Å, and may indicate slightly reduced polarisation between the cationic Cu and anionic Zn centres. While the Cu–Cu distances in both cis-4 and trans-4 are longer than the sum of the covalent radii (2.24–2.64 Å),^70,71 they are significantly shorter than the sum of the van der Waals radii (3.84–4 Å).^72,73 The Cu–P bond lengths in 4 (2.2182(3)–2.2283(3) Å) are significantly shorter than those found in 1 (average = 2.317(5) Å),⁶⁷ and 3 (2.2497(7)–2.2910(7) Å). They are, once again, similar to those found in 2 (2.209(2)–2.318(1) Å) and in some literature cationic copper(I) phosphine complexes [Cu(P^tBu₃)₂][PF₆] (Cu–P = 2.2187(6) Å)⁸⁹ and [(Me₃P)₂Pd(µ-2-C₆F₄PPh₂)₂Cu][PF₆] (Cu–P = 2.217(2), 2.214(2) Å).⁹⁰ The Zn–C bond lengths range from 2.008(1)–2.082(1) Å in cis-4, and 2.005(1)–2.097(1) Å in trans-4, which are longer than in neutral Zn-aryl complexes⁹¹ but consistent with prior reports of complexes featuring anionic organo-zincates (see Table S5).⁹² Despite the different orientations of the –Zn(C₆F₅)₂ moieties in the isomers, the Zn⋯Zn distances are similar, at 7.304 and 8.365 Å in cis-4 and trans-4, respectively.

Scheme 2 The reaction of 1 with Zn(C₆F₅)₂ to form heterometallic di-copper(I)–di-Zn(II) complex 4 as cis- and trans-isomers of [Cu₂(µ:κ³(PPC)-dppm′)₂{Zn(C₆F₅)₂}₂] (cis-4 and trans-4). The subsequent reaction of 4 with CO₂ affords heterometallic complex [Cu₂(µ:κ⁴(PPOO)-dppm″)₂{Zn(C₆F₅)₂}₂] (5).

Fig. 4 Solid-state structures of (A) cis-4·2benzene and (B) trans-4·3benzene. Displacement ellipsoids are drawn with 50% probability, and carbon atoms of the P-phenyl groups are drawn wireframe. For clarity, hydrogen atoms and lattice solvent are omitted.

The ³¹P{¹H} NMR spectrum of 4 contains two singlets, at 15.6 and 15.2 ppm (CD₂Cl₂), in an approximate 5 : 2 ratio, respectively. These are tentatively assigned to differing quantities of the cis- and trans-isomers of 4. These signals are shifted from those observed for 1 which are at 0.8 and −18.8 ppm (benzene-d₆; Fig. S10 and S35). The higher chemical shifts for 4 are consistent with the incorporation of Lewis acidic zinc into the complex. The ¹⁹F NMR spectrum contains two sets of ortho-, meta- and para-F resonances, also in an ∼5 : 2 ratio. One set of resonances is located at −116.8, −158.8, and −162.6 ppm, and the second set is located at −117.3, −159.2, and −163.2 ppm; both sets of resonances are shifted relative to [Zn(C₆F₅)₂] (Fig. S36 and S37). The ¹H NMR spectrum of 4 contains resonances at 3.83 and 3.76 ppm which are assigned to the –CH_methine protons, also integrating in a 5 : 2 ratio, respectively (Fig. S38). All attempts to separate the cis- or trans-isomers proved unsuccessful, immediately forming the same 5 : 2 ratio of isomers perhaps suggesting a dynamic equilibration between them. Treating complex 1 with [Zn(Mes)₂], a reagent which should be less Lewis acidic than [Zn(C₆F₅)₂], failed to result in any reaction and both starting species remained unreacted in solution even over pro-longed periods. Therefore, it seems that the higher Zn(II) Lewis acidity of [Zn(C₆F₅)₂] is relevant to its successful coordination by the anionic methine group of dppm′, either by coordination to a free [CH(PPh₂)₂]⁻ or via insertion into a Cu–CH_methine bond.

Next, the reactivity of 4 with CO₂ and H₂ was investigated. Treating 4 with CO₂, at 1 bar in toluene solution and at room temperature, did not result in any reaction. When the solution was heated to 60 °C, there was complete (quantitative) conversion into [Cu₂(µ:κ⁴(PPOO)-dppm″)₂{Zn(C₆F₅)₂}₂] (5; Scheme 2) which was isolated in 72% yield. Complex 5 is the product of CO₂ insertion into the Zn–C_methine bond and results from a new C–C bond forming between the carbon atom of CO₂ and the methine carbon of the ligand. The reaction can be viewed as producing carboxylated dppm′, henceforth referred to as dppm″. The dppm″ ligand is κ²(PP)-coordinated to the two Cu(I) centres, and its carboxylate moiety bridges, via the O-donors, between the Zn(II) and Cu(I) centres (Scheme 2). The ³¹P{¹H} NMR spectrum of 5 shows a singlet, at 11.2 ppm, shifted ∼4 ppm to lower frequency relative to 4 and indicative of reduced cationic charge on the Cu(I) centres (Fig. S41). The ¹H NMR spectrum shows a peak at 5.09 ppm assigned to the HC_methine group (Fig. S42); this resonance is shifted by ∼1.3 ppm to higher frequency relative to 4. The ¹⁹F NMR spectrum shows resonances at −117.3, −158.1 and −163.0 ppm, assigned to the o-, p- and m-(C₆F₅) nuclei, respectively (Fig. S44). The FT-IR spectrum of 5 contains absorptions at 1573 and 1445 cm⁻¹, assigned as the asymmetric and symmetric carboxylate stretches, respectively (Fig. S45).

The ¹³C{¹H} NMR spectrum contains a singlet at 178.6 ppm, assigned to the [RCO₂]⁻ carbon atom and with a chemical shift consistent for a carboxylate group (Fig. S46). Single crystals of 5·2benzene suitable for X-ray diffraction experiments were obtained by slow evaporation of a benzene solution of 5 at room temperature (Fig. 5). The –[Zn(C₆F₅)₂] groups are coordinated in a trans-arrangement, with a Zn⋯Zn distance of 10.852 Å. The coordination geometry of the Cu centres is best described as distorted trigonal pyramidal, with the phosphine- and carboxylate-donors residing in a trigonal-plane, and the Cu–Cu bond occupying the apical coordination site. The Cu–Cu bond length (2.6792(4) Å) is consistent with that found in cis-4 and significantly shorter than that found in trans-4. While the Cu(1′)–P(2) bond length (2.1977(3) Å) is consistent with those found in 4, the Cu(1)–P(1) bond length (2.2779(3) Å) is significantly longer. The C–O bond lengths are relatively similar to one another, with C(2)–O(1) and C(2)–O(2) being 1.267(2) and 1.250(2) Å, respectively, and are consistent with charge delocalisation across the carboxylate ligand. The coordination geometry of the Zn(II) centres is best described as T-shaped, with C(27)–Zn(1)–C(33), C(27)–Zn(1)–O(2) and C(33)–Zn(1)–O(2) bond angles equal to 148.58(6)°, 104.98(5)° and 105.36(5)°, respectively. Given that the coordination geometry of Zn(II) is no longer trigonal planar, as is the case in 4, charge delocalisation seems to favour charge polarisation towards Cu(I) rather than Zn(II) in the precursor complex. Moreover, the Zn–C bond lengths in 5 are 1.970(1) and 1.970(2) Å, which are significantly shorter than in 4 and consistent with the evolution into neutral Zn(aryl)₂ complexes. However, while the Cu–O bond length in 5 (2.0772(9) Å) is consistent with literature copper(I) complexes featuring an anionic O-donor coordinated to Cu (see Table S6),^80,93 the Zn–O bond length (2.020(1) Å) is also similar to that reported for complexes featuring an anionic O-donor coordinated to Zn^II, indicative of charge delocalisation within the carboxylate group (see Table S6).^94–96

Fig. 5 Solid-state structure of 5·2benzene. Displacement ellipsoids are drawn with 50% probability, and carbon atoms of the P-phenyl groups are drawn wireframe. For clarity, hydrogen atoms, except for the methine protons, and lattice solvent are omitted.

In an attempt to isolate a copper/zinc-hydride complex, 4 was treated with H₂ (1 bar) in benzene-d₆. There was no reaction at room temperature but upon gentle heating, 60–80 °C, the complex decomposed, depositing some Cu metal onto the Schlenk flask walls and evolving C₆HF, as identified by ¹⁹F NMR spectroscopy. Finally, complex 5 was treated with H₂ and, in separate reactions, with various metal hydrides, including CaH₂ or HBpin, to determine whether further hydrogenation of the carboxylate ligand was feasible. In all cases, no reaction occurred at room temperature, and heating benzene-d₆ solutions to 60–80 °C resulted in decomposition, with the only assigned product, by ¹⁹F NMR spectroscopy, being C₆HF₅.

Discussion

The tri-copper(I) precursor complex 1 was successfully used to identify and characterise small-molecule clusters formed by reactions with hydrogen, carbon dioxide and Lewis acidic Zn(II) species. Complex 2 is a copper(I)-hydride cluster formed by heterolysis of dihydrogen at the copper(I) sites and the Brønsted basic carbanion backbone of the chelating phosphine ligands. It reacts with carbon dioxide, via a process which is assisted by additional hydrogen, to form a copper(I)-formate complex, 3. Complex 4 is the product of the Cu(I) precursor reaction with zinc(II) precursors to form a heterometallic Cu₂Zn₂-bis(phosphine) complex. It reacts at low pressures (1 bar) of CO₂ to form a heterometallic Cu₂Zn₂ complex featuring bridging carboxylate ligands. In all these reactions, the copper(I) sites readily form Cu-hydrides even when the starting material is exposed to relatively low pressures (concentrations) of hydrogen gas. In contrast, for the heterometallic complexes there was no equivalent Zn–H formation. The copper(I)-hydrides insert carbon dioxide readily to form Cu(I) formate complexes, even using low pressures of carbon dioxide. The heterometallic Cu(I)/Zn(II) complexes react with carbon dioxide to form carboxylates that bridge between the two metals and may facilitate charge delocalisation between them. The complexes suggest that cuprous zincate structures may help to increase reactivity at Zn(II) sites. It is interesting that these alkyl-zincates react with the electrophilic carbon, of carbon dioxide, to form new C–C bonds and carboxylate bridging ligands. There are few examples of low pressure carbon dioxide insertion into alkyl–zinc bonds to effect carboxylation;^97,98 Hou has reported the alkylative carboxylation of allenamides or ynamides using carbon dioxide, dialkyl zinc reagents and a Cu(I) catalyst.^99,100 The carboxylation reactivity using complex 4, which combines Cu(I) and Zn(II) sites, should be investigated for other carboxylations, both stoichiometric and catalytic, using carbon dioxide.¹⁰¹ One challenge that must be overcome in any catalytic chemistry targeting alcohols is the need to further reduce the metal formate and carboxylate species – in this work, preliminary attempts to achieve these reductions have proved unsuccessful. In future, the reactions of cuprous formate (e.g. complexes 3) or copper(I) zinc(II) formates (e.g. complex 5) with higher pressures of hydrogen gas should be undertaken. Recent work has established that cuprous hydride complexes can reduce aldehydes (to alkoxides) and esters.¹⁰² Further reactions should be explored between the cuprous (zinc) formate complexes and stable cuprous hydride complexes, e.g. Strykers copper hydride reagent or even, perhaps, complex 2.^103,104

The objective was to investigate the reactivity of well-defined organo-copper(I) and Zn(II) complexes as possible mimics for the active site surfaces of heterogeneous catalysts. The chemistry observed in this work may be relevant, but caution must be applied since heterogeneous catalytic intermediates likely involve multiple metals and are mobile across the catalyst surface. Nonetheless, the reactions of the well-defined complexes, in this work, indicate that hydrogen can be activated heterolytically at Cu(I) sites in the presence of a suitable ligand environment (i.e. strongly Brønsted basic ligands), and is not limited to Cu(0) sites via oxidative addition, akin to H₂ chemisorption on a heterogenous surface. The other key reagent, carbon dioxide, reacts readily with both cuprous hydrides and organozinc reagents, and forms metal carboxylate ligands that often adopt bridging coordination modes to the metals. These chemistries are consistent with the proposed speciation occurring during the first steps of the formate carbon dioxide hydrogenation mechanism.²⁶ In heterogeneous catalysis, Cu(I) surface species are certainly feasible, particularly during hydrogen activation,¹⁰⁵ and hence the current work may inform upon speciation during hydrogen activation at Cu, either by chemisorption or heterolysis. Although, ZnO surfaces are not usually considered as negatively charged, in heterogeneous catalysis, this work highlights that cuprous zincate interfaces can introduce charge polarisation from Cu(I) to Zn(II); this zincate-induced polarisation may modulate the catalytic activity, for example, enabling carbon dioxide activation via reaction at its electrophilic carbon atom and providing a future route to C–C bond formation during the reductions. The formation of higher alkanol/alkene products would be very valuable and is generally not achieved using only Cu/ZnO catalysts. Future work should continue to prioritise heterometallic clusters featuring hydrides and formates as ligands. Following the success of the complexes described in this work, understanding the onward speciation during later stages of the reduction or coupling chemistry is a future target. The complexes described in this work may also enable reactions with s-block metals, like K, Mg, and Ca, which are well known to form metal zincates;^64,106–108 for example, heteromagnesium complexes showed unusual reactivity with formyl groups and are common promoters of Cu/ZnO heterogeneous catalysts.^109–114

Conclusions

A trimetallic copper(I) complex, [Cu₃{µ:κ³(PPC)-dppm′}₂{µ:κ²(PP)-dppm′}] (1; dppm′ = [(Ph₂P)₂CH]⁻) was used to investigate reactions with hydrogen, carbon dioxide and Lewis acidic organo-zinc precursors. Well-defined copper(I) hydride and formate clusters and a heterometallic Cu(I)Zn(II) carboxylate complex were isolated and characterised. The tri-Cu(I) complex 1 reacted with hydrogen to form a penta-copper-hydride cluster complex, [Cu₅(µ-H)(µ₃-H)₂(µ:κ³(PPC)-dppm′)₂(µ:κ²(PP)-dppm)₂] (2). Complex 2 reacted with carbon dioxide to form a tri-copper(I)-hydrido-bis(formate) complex, [Cu₃(µ₃-H)(κ¹(O)-O₂CH)(µ:κ²(OO)-O₂CH)(µ:κ²(PP)-dppm)₃] (3). A mixed metal copper(I)zinc(II) complex, [Cu₂(µ:κ³(PPC)-dppm′)₂{Zn(C₆F₅)₂}₂] (4), was formed by treating 1 with [Zn(C₆F₅)₂] and features cationic Cu(I) sites and anionic zincate(II) moieties. It reacted with carbon dioxide to form a new cluster Cu^IZn^II-carboxylate complex, [Cu₂(µ:κ⁴(PPOO)-dppm″)₂{Zn(C₆F₅)₂}₂] (5), in which CO₂ inserts into a Zn–C bond to form a new C–C bond and bridging carboxylate species. While neither 3 or 5 reacted with sources of hydride, such as HBpin or CaH₂, to reduce CO₂, future work will investigate other hydride sources, such as silanes, for carbon dioxide hydroelementation.

Overall, the Cu(I) complex reacted with hydrogen to form a molecular hydride species which reduced CO₂ to form a formate complex. In contrast, the heterometallic Cu(I)Zn(II) complex reacted with carbon dioxide to form a metal carboxylate species. In the future, other heterometallic Cu(0/I)Zn(II) complexes should be used to explore the generality of the reactivity. The cuprous zincates may help to moderate intermetallic polarisation and electron transfer processes. These results may even, in the longer term, help inform upon roles for copper and zinc species in CO₂ hydrogenation catalysts. It remains a long-standing goal to discover homogeneous carbon dioxide hydrogenation catalysts able to form higher (C₂ and above) alcohols and alkenes, particularly using earth abundant metals like Cu and Zn.

Author contributions

BEC: conceptualization, data curation, formal analysis, investigation, methodology, validation, visualization, writing – original draft. AP: conceptualization, writing – review & editing. MSPS: conceptualization, funding acquisition, project administration, supervision, writing – review & editing. CKW: conceptualization, funding acquisition, project administration, resources, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

CCDC 2269234–2269239 (1–6) contain the supplementary crystallographic data for this paper.^115a–f

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: full experimental details and procedures, tables of structural and spectroscopic metrics for comparison with similar compounds reported in the literature, characterisation data (¹H, ²H, ¹³C{¹H}, ³¹P{¹H} and ¹⁹F{¹H} NMR, FT-IR, and crystallographic details), and computational methods. See DOI: https://doi.org/10.1039/d5sc05666g.

Acknowledgements

This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) (grant number EP/S018603/1) and the UK Catalysis Hub (EP/R026645/1).

Notes and references

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Footnote

† This publication is dedicated to Professor Martin Cowie, who forged a remarkable career exploring the reactivity of bimetallic complexes of bis(phosphine) ligands, including dppm. Rest in peace Uncle Marty.

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