Reactivity of metal hydrides with CO₂: going beyond formate with a high-valent cationic pentahydride Mo(VI) complex

Abstract

The cationic molybdenum pentahydride complex [MoH₅(depe)₂]⁺ (depe = 1,2-bis(diethylphosphino)ethane) is shown to undergo two consecutive reactions with carbon dioxide. In the initial, room-temperature process, classical insertion of CO₂ into a metal–hydride bond is observed, resulting in the formation of the expected formate complex, [MoH₂(HCOO)(depe)₂]⁺. Further reactivity is triggered at temperature above 100 °C. Complete conversion into two new complexes is indeed observed, resulting from the formal cleavage of a C–O bond of carbon dioxide, [MoH(CO)₂(depe)₂]⁺ and [MoO(HCOO)(depe)₂]⁺. Unprecedented in the absence of ligand assistance, such metal hydride reactivity has been comprehensively studied by a combination of experimental and computational means with the aim of elucidating the underlying mechanism that governs this process.

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Introduction

Carbon dioxide (CO₂) is an appealing primary source of carbon atoms for the development of new sustainable synthetic procedures of both carbon-based commodity chemicals and fuels.^1–6 Although inexpensive, relatively abundant and non-toxic, CO₂ suffers from severe thermodynamic and kinetic limitations that significantly restrict its use as a routine reagent. Transition metal-based complexes have commonly been employed to overcome this inherent kinetic reluctance.^7–9 They have been demonstrated to be highly effective catalysts for the conversion of CO₂ into a range of hydrogenated C₁ products (HCOOH, CH₃OH and CH₄), either through electro-assisted approaches or via traditional catalytic hydrogenation. In the majority of such processes, the formation of a metal formate species through CO₂ insertion into metal hydride bonds represents a fundamental step in catalytic cycles.¹⁰ Consequently, a substantial focus has been directed toward elucidating thermodynamic^11–13 and kinetic^14–18 aspects of this reaction.

Although rare, alternative reactivities of metal hydrides have also been reported (Fig. 1). The group of Parkin described C–O bond cleavage upon exposure of [Cp*Mo(PMe₃)₃H] to a CO₂ atmosphere, resulting in the formation of a carbonyl complex together with trimethyl phosphine oxide.¹⁹ In the same vein, the group of Milstein investigated the reductive cleavage of CO₂ by hydride complexes of group 9 metals (Rh, Ir) supported by PNP pincer ligands.^20,21 This reactivity, triggered by metal–ligand cooperation, again gave access to carbonyl derivatives – the second oxygen atom being released as a water molecule. Interestingly, a diisocyanato/dioxo dititanium complex was obtained from the stepwise, double reductive cleavage of CO₂ by a dihydride dititanium complex, through the concomitant activation of a dinitrogen ligand.²² On-metal C–O bond activation by metal hydride complexes can also be facilitated by the assistance of exogenous main-group Lewis acids (LA).²³ Ultimately, such assistance is capable to promote a cooperative reductive cleavage of CO₂, as recently reported by the group of Camp with a strongly polarized heterobimetallic Ir–Al complex.²⁴ In most of those cases, the transfer of an oxo group to a particularly oxophilic exogenous – or remote – partner thus appears to be an important thermodynamic lever that drives the C–O cleavage reaction.

Fig. 1 Examples of C–O bond cleavages in the CO₂ molecule mediated by transition metal hydride complexes.

Recently, our group reported on the isolation of a cationic molybdenum(VI) pentahydride complex supported with a bis(phosphine) ligand,²⁵ [MoH₅(depe)₂]⁺ (1⁺, depe = 1,2-bis(diethylphosphino)ethane). Related group 6 pentahydride derivatives were previously postulated, observed or isolated by the groups of Wilkinson,²⁶ Semenenko,^27,28 Ito,²⁹ Henderson,³⁰ Cundari/Yousufuddin³¹ and Chirik.³² Their reactivity towards carbon dioxide remains, however, largely uncharted. In this article, we explore the stoichiometric reactivity of pentahydride complex 1⁺ towards CO₂ and its nitrogen- and sulphur-based analogues (N,N′-diisopropylcarbodiimide – DIC – and CS₂). We demonstrate that complex 1⁺ readily loses H₂ in those processes to generate unsaturated derivatives capable of quick reaction with the considered heterocumulenes. In all cases, insertion of the substrates into a metal-hydride bond occurs. Interestingly, further reactivity is observed for CO₂ upon thermal activation. A rare example of unassisted, on-metal C–O bond cleavage is evidenced, with the formation of both Mo(II)–carbonyl and Mo(IV)–oxo cationic complexes. Comprehensive computational investigations have been conducted to assess the overall mechanism of this intriguing process.

Results and discussion

We have recently reported the protonation behaviour of the neutral Mo(IV) tetrahydride complex 2 towards a variety of proton sources, highlighting the importance of the nature of the conjugate base of the acid used to warrant protonation-only processes.²⁵ Based on these previous findings, we selected triethylammonium tetraphenylborate [HNEt₃][BPh₄] – that combines both weakly coordinating counter-anion and conjugate base – to serve as the proton purveyor.

Treating a solution of 2 with 1 equiv. of [HNEt₃][BPh₄] at room temperature resulted in the immediate formation of the expected pentahydride complex [MoH₅(depe)₂][BPh₄], 1·BPh₄ (Scheme 1). A diagnostic signal appears as a quintet at −5.0 ppm in the hydride region of the ¹H NMR spectrum. This complex can be isolated in good yields (80–90%) as a pale-yellow powder.

Scheme 1 Protonation of 2 employing [HNEt₃][BPh₄] as the proton source affords 1·BPh₄.

Single crystals suitable for X-ray diffraction analysis were obtained upon low temperature storage (−40 °C) of THF–pentane solution of the complex. The resulting structure is almost identical to the one we previously reported, should the nature of the counterion be excluded (see ESI†).²⁵

When a solution of 1·BPh₄ in THF was exposed to CO₂ (3 bar) at room temperature, the solution rapidly turned red. The newly formed complex 3·BPh₄ shows a new signal in the hydride region of the ¹H NMR spectrum (in C₆D₄Cl₂), emerging as a pseudo-quintet at −7.94 ppm with an apparent ²J_(P–H) coupling of 42.0 Hz. Another notable, new resonance is observed at 7.77 ppm in the form of a singlet that splits into a doublet (¹J_(C–H) = 209.3 Hz) when ¹³C-labelled CO₂ was used. We assigned this new signal to a formato ligand generated by CO₂ insertion into a Mo–H bond of 1·BPh₄. This NMR data is in good agreement with previously reported formato ligands.^33–37 This was further supported by the appearance of a quintet (³J_(C–P) = 3.5 Hz) at 176.1 ppm in the ¹³C{¹H} NMR spectrum of 3·BPh₄. The ³¹P{¹H} NMR spectrum features two singlets at 43.9 and 78.3 ppm that expanded into triplets (²J_(P–H) = 36.0 and 41.9 Hz, respectively) upon selective ¹H-decoupling centred at 1.5 ppm. Collectively, these results point towards the formulation of 3·BPh₄ as [MoH₂(HCOO-κ²O)(depe)₂][BPh₄] (Scheme 2).³⁸

Scheme 2 Formation of Mo(IV)–formato complex 3·BPh₄ from 1·BPh₄ under CO₂ atmosphere.

This ion pair crystallised from a THF–pentane solution, and the single-crystal X-ray diffraction (sc-XRD) analysis confirmed the κ²O bidentate coordination of the formate anion, with Mo–O bond lengths of ca. 2.26 Å (Fig. 4). If we restrict to the first coordination sphere, the Mo centre is found in a C_2v-symmetric environment. The phosphorus nuclei lie in twos above or under the symmetry plane, in good agreement with the ³¹P{¹H} NMR spectrum showing two resonances. Although the hydrido ligands could not be unambiguously located in the final Fourier difference map, their most likely positions can be estimated from the presence of a large open site located below a mean plane formed by the phosphorus atoms, trans to the formato ligand. It is reasonable to propose that the two hydrides should be staggered relative to the formato ligand, so that d orbital destabilization is minimized in an overall dodecahedral geometry, common for 8-coordination.³⁹

To gain more insight into the reactivity of complex 1⁺ toward CO₂, we carried out Density Functional Theory (DFT) calculations. We initially performed geometry optimizations at B3LYP-D3(BJ)/SDD+f(Mo), 6-31G** (other atoms) level of theory. Further energy refinement through single point calculations was carried out at PCM(THF)-M06-L-D3/def2-TZVPP level of theory considering solvent effects by means of the Polarizable Continuum Model (PCM) method (see ESI†). Optimization of 3⁺ was first performed in order to validate the reliability of our selected computational level of theory. We found an excellent agreement between the X-ray and DFT geometries, with a relative error between experimental and calculated structures consistently below 1.5% (Table S1†). We then turned our attention to the mechanistic pathway followed by the reaction of 1⁺ with CO₂. As illustrated in the reaction profile (Fig. 2), a reductive elimination of H₂ was found to initiate the overall process with a low energy transition state (TS1, ΔG^# = 4.0 kcal mol⁻¹) yielding the σ-complex INT1. This is followed by a barrierless dissociation of H₂ to afford INT1′ (Fig. S1†). Further κO¹ coordination of CO₂ to the metal centre then occurs through TS2 (ΔG^# = 22.0 kcal mol⁻¹). Formation of the resulting INT2 is slightly endergonic from INT1′ (ΔG = 5.5 kcal mol⁻¹). Interestingly, kinetic measurements performed under two different pressures of CO₂ (1.5 vs. 3 bar) showed no influence of the latter (see ESI†). This agrees with the computed energy profile where H₂ release is energetically more demanding than CO₂ addition. Then, a rapid hydride transfer from the Mo centre to the coordinated CO₂ was found to occur (TS3, ΔG^# = 0.9 kcal mol⁻¹ from INT2), granting further stabilization. We also considered that the hydride transfer process could occur via an outer-sphere mechanism, but the corresponding TS was found to be energetically prohibitive at 44.9 kcal mol⁻¹ (Scheme S1†). Interestingly, the resulting intermediate INT3, featuring a formato ligand, is nearly isoenergetic with INT1′, suggesting that CO₂ insertion-deinsertion processes are easily accessible. Rotation around the C–O bond, allowing the expected κ²O coordination of the formato ligand, finally affords complex 3⁺ in a global exergonic process (ΔG: −3.0 kcal mol⁻¹). Overall, this mechanism is in good agreement with the concerted CO₂ insertion pathway, previously reported by Hazari for group 9 and 10 metal hydride complexes.¹⁰

Fig. 2 Computed profile for the reaction of complex 1⁺ with CO₂. Free energy values (ΔG) and distances are given in kcal mol⁻¹ and Å, respectively. All data have been computed at the PCM(THF)-M06-L-D3/def2-TZVPP//B3LYP-D3(BJ)/SDD+f(Mo), 6-31G** (other atoms) level of theory.

An o-dichlorobenzene solution of the 3·BPh₄ complex was heated at 100 °C for 24 hours under an atmosphere of CO₂, resulting in a complex mixture of compounds. Large, unresolved signals were notably observed in the ¹H NMR spectrum, suggesting the formation of a paramagnetic molybdenum species identified as [Mo(depe)₂Cl₂] (see ESI†). Simultaneously, the appearance of new resonances in the aromatic domain is also noted. We tentatively assigned those new signals to the formation of biphenyl (see ESI†), arising from the radical-driven reductive rearrangements of the tetraphenylborate anion.^40,41 Slight chemical shift deviations are, however, noted in comparison with an authentic biphenyl sample, which we attribute to the influence of [Mo(depe)₂Cl₂] paramagnetism. We thus decided to opt for the HB(C₆F₅)₃⁻ anion, that displays increased redox stability. 3·HB(C₆F₅)₃ could easily be prepared by the reaction of 2 with [HP^tBu₃][HB(C₆F₅)₃],²⁵ followed by room temperature reaction with CO₂. Under similar conditions (reaction time increased to 36 h), 3·HB(C₆F₅)₃ indeed granted access to a much cleaner reactivity. More specifically, two new complexes 4·HB(C₆F₅)₃ and 5·HB(C₆F₅)₃ are formed in an approximate 36 : 64 ratio (Scheme 3, top), together with H₂ evolution. The positively charged part of the minor species was identified as [MoH(CO)₂(depe)₂]⁺, 4⁺. This monohydride cationic complex bearing two carbonyl ligands in a trans configuration was independently prepared from the reaction of 1⁺ with CO (Scheme 3, bottom, and Fig. 4). The second complex that formed, 5⁺, features a singlet at 46.0 ppm in the ³¹P{¹H} NMR spectrum. The latter unresolvedly broadens upon selective decoupling of the aliphatic protons, suggesting the absence of hydrido ligands. This is further confirmed by the ¹H NMR spectrum, which is devoid of any new hydride signals. Interestingly, a new proton signal resonating at 7.31 ppm was observed, which correlates with a singlet at 163.8 ppm in the ¹³C{¹H} NMR spectrum. This signal splits upon ¹³C-labeling (¹J_(C–H) = 204.3 Hz), indicating the formation of a new formato derivative. High-resolution mass spectrometry was used to gain further insight into the nature of compound 5⁺. The isotopic pattern of the resulting signal strongly suggests the presence of an additional oxo ligand. Taken together, those results point towards 5⁺ being formulated as [MoO(HCOO)(depe)₂]⁺, a cationic Mo(IV) complex. Despite several attempts, independent synthesis of 5⁺ was unsuccessful: it is, however, interesting to note that ³¹P NMR features are in line with similar cationic d² molybdenum–oxo complexes.⁴² Along these two main products, small amounts of free formate was also noticed, accounting for 8% of the non-gaseous compounds ultimately present in solution.

Scheme 3 CO₂ cleavage induced by thermal activation of the formato complex 3·HB(C₆F₅)₃ (top) and independent synthesis of the monohydrido bis(carbonyl) complex 4·BPh₄ (bottom).

Interested in this unusual process – formally resulting from the cleavage of a CO₂ molecule C–O bond⁴³ – we again decided to investigate the mechanism of this transformation by the use of DFT calculations. We employed the PCM(o-dichlorobenzene)-M06-L-D3/def2-TZVPP//B3LYP-D3(BJ)/SDD+f(Mo), 6-31G** (other atoms) level of theory and corrected the free energies at 373.15 K using GoodVibes package.⁴⁴ Two different pathways have been considered for the thermal evolution from complex 3⁺ to complex 5⁺. As illustrated in Scheme 4, the first pathway (Pathway I) begins with the reductive elimination of H₂, followed by the coordination of CO₂. Then, the breaking of the C–O bond in the CO₂ ligand occurs, followed by the release of CO leading to the final compound 5⁺. The second pathway (Pathway II) involves as a first step a hydride transfer from the formato moiety to Mo centre—formally a de-insertion of CO₂, the reverse of the final elementary step in the mechanism leading from 1⁺ to 3⁺. H₂ is subsequently released, followed by the C–O bond cleavage in the CO₂ ligand. Finally, two routes were envisaged to achieve the formation of the final product 5⁺: either CO₂ can be added followed by the release of CO, or CO can be released first, followed by the addition of CO₂. According to our DFT calculations, Pathway I can be safely ruled out due to several high energy barriers in the process (see the complete reaction profile in Fig. S2†). First, the release of H₂ from 3⁺ is highly endergonic, with the complex INT4-I being 25.3 kcal mol⁻¹ higher in energy than 3⁺. Additionally, subsequent addition of CO₂ requires an overall activation barrier of 45.1 kcal mol⁻¹, which cannot be reached under the reaction conditions. The transition state for the next step (C–O bond cleavage in the CO₂ molecule) also has a prohibitive high energy at 43.9 kcal mol⁻¹. In the reaction profile corresponding to Pathway II (Fig. 3), the first step formally corresponds to the hydride transfer from the formato moiety to the Mo centre, which is the back-side reaction from 3⁺ to INT2, previously described in Fig. 2. The reverse activation barrier of 21.9 kcal mol⁻¹, corrected with GoodVibes software to take the temperature into account, was found accessible at 100 °C. Then, the release of H₂ proceeds very easily through TS4, with a rather low barrier of 1.7 kcal mol⁻¹ from INT2, affording the η²-CO₂–Mo complex INT4.⁴⁵ The subsequent C–O bond cleavage is energetically more costly, with a transition state, TS5, located at 31.3 kcal mol⁻¹ above 3⁺. Finally, DFT calculations suggest that CO is released (TS6) before addition of CO₂ (TS7) to form the final product 5⁺. In this process, CO₂ plays the role of a hydride abstractor, generating a formato moiety, which finally coordinates on Mo in 5⁺. In Pathway II, the rate-determining step (RDS) corresponds to the addition of CO₂ (TS7), with an overall activation barrier of 33.2 kcal mol⁻¹ from 3⁺, which is consistent with a high temperature for the reaction to occur. It is important to note that the released CO (TS6 in Pathway II) is necessary for the thermal evolution of complex 3⁺ into complex 4⁺ (Fig. S3†). Additionally, the CO₂ needed for the final step in Pathway II (TS7) is derived from the reaction profile described in Fig. S3† (thermal evolution from 3⁺ into 4⁺). Consequently, formation of 4⁺ and 5⁺ are intimately connected.

Scheme 4 Plausible reaction pathways for the thermal evolution from 3⁺ to 5⁺.

Fig. 3 Energy profile for the thermal evolution of complex 3⁺ into complex 5⁺ according to Pathway II (Scheme 4). All data have been computed at the PCM(o-dichlorobenzene)-M06-L-D3/def2-TZVPP//B3LYP-D3(BJ)/SDD+f(Mo), 6-31G** (other atoms) level of theory. Free energies (ΔG) have been corrected at 373.15 K. All activation barriers are referred to 3⁺. Distances and energies are given in Å and kcal mol⁻¹, respectively.

As an extension of our investigation on the activation of CO₂, we studied the reactivity of 1·BPh₄ with CS₂ and DIC (Scheme 5). Upon treatment of complex 1·BPh₄ with 1 equiv. of CS₂, an immediate darkening of the THF solution is observed. The formation of complex 6·BPh₄ was characterized by the appearance of a new quintet at −6.09 ppm (²J_(P–H) = 44.0 Hz) and a singlet at 5.76 ppm in the ¹H NMR spectrum, as well as a ³¹P{¹H} resonance at 69.1 ppm. The latter splits into a doublet (²J_(P–H) = 44.0 Hz) upon selective decoupling of the aliphatic protons. Unlike the behaviour observed in the presence of CO₂, these data support a 4e⁻-reduction of CS₂ through double insertion into Mo–H bonds,^46–49 resulting in a monohydride methanedithiolate derivative, [MoH(S₂CH₂)(depe)₂][B(C₆H₅)₄] (6·BPh₄, Scheme 5). Despite several attempts, 6⁺ remained reluctant to crystallization and isolation of an analytically pure complex could not be reached (92% spectroscopic purity, see ESI†). Addition of 1 equiv. of DIC to 1·BPh₄ in THF led to the formation of the expected M–H insertion product 7·BPh₄ (Scheme 5). In the ¹H NMR spectrum, loss of the hydride resonance of 1·BPh₄ was observed along with the formation of two new diagnostic signals: a multi-lined hydride signal at −7.97 ppm and a singlet at 7.95 ppm assigned to the formamidinate methine proton. Typical of the methine carbon,^50–53 a signal at 159.4 ppm is observed in the ¹³C{¹H} NMR spectrum. The ³¹P{¹H} features two triplets at 71.0 and 41.5 ppm. Contrary to 6·BPh₄, complex 7·BPh₄ easily crystallized from a THF–pentane solution, as orange prisms (η = 54%). In the solid-state structure of 7·BPh₄ (Fig. 4), the formamidinate ligand is found in a κ²N coordination, with a N–Mo–N bite angle of 59.0°. As expected, this value and other relevant metrics are very similar to those determined in the molecular structure of 3·BPh₄. Again, the exact location of the hydrido ligands could not be determined, although their most probable positions will similarly be found trans to the formamidinate ligand to fulfil the expected distorted dodecahedral coordination sphere.

Scheme 5 Formation of 6·BPh₄ and 7·BPh₄ from 1·BPh₄.

Fig. 4 From left to right, molecular structures of compounds 3·BPh₄, 4·BPh₄ and 7·BPh₄ in the solid state. The BPh₄⁻ counter ions and the hydrogen atoms (except those on the reduced formato or amidinato ligand or bound to the metal centre) were omitted for clarity. Thermal ellipsoids are shown at the 25% level of probability. Compounds 4·BPh₄ and 7·BPh₄ crystallized with two molecules in the asymmetric unit (Z′ = 2), only one is depicted for clarity.

Conclusions

In summary, we have thoroughly investigated the reactivity of a high-valent cationic pentahydride Mo(VI) complex 1·BPh₄ towards CO₂ and related heteroallenes (CS₂, DIC). The expected insertion of substrate molecules into Mo–H bonds was typically observed, resulting either in two-electron (CO₂, DIC) or four-electron reduction of the substrate (CS₂). Overall, and despite its high-valent character, complex 1·BPh₄ behaves as a potent reductant. Most of those reactivities is triggered by facile hydrogen release, suggesting 1·BPh₄ could be better described as a masked Mo(IV) complex. Beyond this expected insertion chemistry, we have been able to evidence a rare example of C–O bond cleavage mediated by a metal hydride complex, that occurs without assistance of a ligand or another proximal reactive centre. Indeed, upon thermal activation, the formato complex 5⁺ was shown to further evolve quantitatively towards a duet of complexes, namely Mo(II)–carbonyl and Mo(IV)–oxo derivatives. This reaction was particularly scrutinized via a combination of experimental and theoretical tools. The intimate mechanism responsible for the formation of both carbonyl and oxo derivatives has been carefully deciphered and the importance of high temperature reversible CO₂ hydrogenation–dehydrogenation process was evidenced. Ultimately, this reversibility – together with the high thermodynamic drivers associated with Mo–O bond formation and transient CO liberation – allows on-metal C–O bond cleavage to be accessed.

Data availability

The datasets supporting this article have been uploaded as part of the ESI.†

Author contributions

N. Q. and N. D. run the experiments and analysed the data. J. J. C.-T. performed DFT calculations. L. V. recorded single-crystal X-ray diffraction data and solved the structures. K. M. and A. S. obtained funding for the project, managed it and took part in the analysis of the data. N. Q. wrote the first draft of the manuscript. K. M. and J. J. C.-T. analysed the theoretical results and wrote the computational part of the manuscript. The manuscript was then further revised through contributions of all authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

N. Q. and A. S. are indebted to the European Research Council for funding (ERC Starting Grant No. 757501). Mass Spectrometry Service (N. Martins-Froment) of the Institute of Chemistry of Toulouse-UAR 2599 (Université de Toulouse, CNRS, Toulouse, France – https://www.ict.ups-tlse.fr) is acknowledged for technical assistance with ESI-HRMS experiments. J. J. C.-T. and K. M. would like to thank the Carnot ISIFoR institute for its support and funding. The “Direction du Numérique” of the Université de Pau et des Pays de l'Adour and the Mésocentre de Calcul Intensif Aquitain (MCIA) are acknowledged for the support of computational facilities. This work was also granted access to the HPC resources of IDRIS under the allocation 2023-AD010800045R2 made by GENCI. All authors thank their respective institution for providing access to facilities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures, computational details, NMR and FTIR spectra and crystallographic data. CCDC 2365476–2365478 and 2365557. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc04345f

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