Synthesis, characterization and computational study of heterobimetallic CoFe complexes for mimicking hydrogenase

Abstract

A mixture of [CpCo(CO)₂] (Cp = cyclopentadienyl) and [Fe(pdt)(CO)₂(dppe)] (pdt = S(CH₂)₃S; dppe = Ph₂PCH₂CH₂PPh₂) was stirred in refluxing toluene to give a heterobimetallic complex [CpCo(μ-pdt)Fe(CO)(dppe)] (2). The protonation of 2 with HBAr^F₄ (bis-etherate of tetrakis[3,5-bis(trifiuoromethyl)phenyl]boric acid) in CD₂Cl₂ affords the μ-hydrido cation [CpCo(μ-pdt)(μ-H)Fe(CO)(dppe)]⁺ ([2μ-H]⁺) at room temperature. However, the low-temperature protonation reaction of 2 reveals a terminal hydride intermediate [Cp(H)Co(μ-pdt)Fe(CO)(dppe)]⁺ ([2H]⁺), as confirmed by ¹H and ³¹P{¹H} NMR spectroscopy. The detailed process of the protonation of 2, including the relevant intermediate species, the structural and electronic properties of the bimetallic cofactors and the key factors responsible for protonation, has been revealed by a density functional theory (DFT) investigation from both a thermodynamics and kinetics perspective. Three pathways have been explored: path I has protonation at the terminal position on the Co taking place via a transition state TS1_ba–ba (7.3 kcal mol⁻¹) with the terminal hydride as the initial product [2H]⁺_ba–ba (1.2 kcal mol⁻¹), which is the kinetic product, which then proceeds via an isomerization transition state TS_iso (22.2 kcal mol⁻¹) to form the bridging hydride [2μ-H]⁺_ba–ba (−14.4 kcal mol⁻¹); path II has protonation occurring on the Co–Fe bond via a transition state TS2_ba–ba (21.8 kcal mol⁻¹) directly producing the bridging hydride [2μ-H]⁺_ba–ba with good thermodynamic stability; path III is another protonation process which proceeds via a transition state TS3_ba–ap (18.1 kcal mol⁻¹) to form a bridging hydride [2μ-H]⁺_ba–ap (−7.9 kcal mol⁻¹). Calculations indicate that the highest energy species of all the proposed pathways are close and have energies that are not difficult to overcome (lower than 22.2 kcal mol⁻¹), which provides important insight into the process of the hindrance of the spontaneous isomerization of terminal species to produce to bridging species via the consideration of the electronic and steric factors for protonation regiochemistry. Agreement of the calculated and experimental data suggests that the proposed pathways are possible and the whole reaction is determined by the thermodynamic product [2μ-H]⁺_ba–ba at room temperature. The work reported here has implications for the mechanistic interpretation of dinuclear metal-complex assisted protonation, which is helpful for the future design of new non-noble metal complexes for catalyzed hydrogen generation.