Metal formates via oxidation of metal carbonyl hydrides

Selective Reduction of CO2 to a Tantalum Formate Complex and Release of Methyl Formate from the Tantalum(V) Center

A cyclometalated NHC-coordinated hydrido tantalum alkoxide was found to selectively react with CO₂ to afford the genuine tantalum formate (NHC)(HCOO)Ta(OR_F)₃ with OR_F = OC(CF₃)₂CH₃. In the solid state, the presence of a κ²-O,O-formate moiety was established by single-crystal X-ray diffraction and ATR-IR spectroscopy, while NMR experiments and DFT modeling studies suggest that the κ¹-O-coordination mode is preferred in solution. Despite the accessibility of the latter κ¹-O-formate in solution, no over-reduction to a dinuclear methylene diolate was observed. Upon treatment with MeOTf, the κ¹-O-formate was methylated selectively, which led to the formation of a tantalum triflate complex along with methyl formate. This is a rare example in which a value-added oxygen-containing organic product (here HCOOMe) is released from an oxophilic early transition metal (here Ta^V).

Electrocatalytic carbon dioxide reduction by using cationic pentamethylcyclopentadienyl-iridium complexes with unsymmetrically substituted bipyridine ligands

Eight [Ir(bpy)Cp*Cl](+) -type complexes (bpy= bipyridine, Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) containing differently substituted bipyridine ligands were synthesized and characterized. Cyclic voltammetry (CV) of the complexes in Ar-saturated acetonitrile solutions showed that the redox behavior of the complexes could be fine tuned by the electronic properties of the substituted bipyridine ligands. Further CV in CO2 -saturated MeCN/H2 O (9:1, v/v) solutions showed catalytic currents for CO2 reduction. In controlled potential electrolysis experiments (MeCN/MeOH (1:1, v/v), Eapp =-1.80 V vs Ag/AgCl), all of the complexes showed moderate activity in the electrocatalytic reduction of CO2 with good stability over at least 15 hours. This electrocatalytic process was selective toward formic acid, with only traces of dihydrogen or carbon monoxide and occasionally formaldehyde as byproducts. However, the turnover frequencies and current efficiencies were quite low. No direct correlation between the redox potentials of the complexes and their catalytic activity was observed.