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.

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

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

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

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.

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).

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.

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.