Hydrogen Generation by Water Reduction with [Cp2*Ti(OTf)]: Identifying Elemental Mechanistic Steps by Combined In Situ FTIR and In Situ EPR Spectroscopy Supported by DFT Calculations

A detailed mechanism of hydrogen production by reduction of water with decamethyltitanocene triflate [Cp*Ti-2(III)(OTf)] has been derived for the first time, based on a comprehensive in situ spectroscopic study including EPR and ATR-FTIR spectroscopy supported by DFT calculations. It is demonstrated that two H2O molecules coordinate to [Cp*Ti-2(III)(OTf)] subsequently forming [Cp-2*Ti-III(H2O)(OTf)] and [Cp*Ti-III(H2O)(2)(OTf)]. Triflate stabilizes the water ligands by hydrogen bonding. Liberation of hydrogen proceeds only from the diaqua complex [Cp*Ti-III(H2O)(2)(OTf)] and involves, most probably, abstraction and recombination of two H atoms from two molecules of [Cp*Ti-III(H2O)(2)(OTf)] in close vicinity, which is driven by the formation of a strong covalent TiOH bond in the resulting final product [Cp*Ti-2(IV)(OTf)(OH)].