Hydrogen generation by water reduction with [Cp*(2) 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*2 Ti(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 H2 O molecules coordinate to [Cp*2 Ti(III) (OTf)] subsequently forming [Cp2 *Ti(III) (H2 O)(OTf)] and [Cp*Ti(III) (H2 O)2 (OTf)]. Triflate stabilizes the water ligands by hydrogen bonding. Liberation of hydrogen proceeds only from the diaqua complex [Cp*Ti(III) (H2 O)2 (OTf)] and involves, most probably, abstraction and recombination of two H atoms from two molecules of [Cp*Ti(III) (H2 O)2 (OTf)] in close vicinity, which is driven by the formation of a strong covalent TiOH bond in the resulting final product [Cp*2 Ti(IV) (OTf)(OH)].