Synthesis and Characterisation of Heterotrinuclear Transition Metal Complexes for Biomimetic Proton Reduction

Selected structural, spectroscopic, electrochemical and photochemical properties of novel heterotrinuclear [FeFe]-hydrogenase enzyme model compounds carrying a catalytic iron sulfur cluster site directly coupled to different transition metal subunits are reported. Typical metal-to-ligand charge transfer (MLCT) chromophores based on electron donating copper(I), rhenium(I) and tungsten(0) complex fragments were selected to provide both long-wavelength spectral sensitization and photoredox activity. Photoinduced one-electron transfer from the MLCT donor subunit to the diiron acceptor site could be observed with an efficient steady-state accumulation of the corresponding charge-separated state. The reduced iron site of these deeply coloured heterotrinuclear [FeFe]-hydrogenase model systems has been shown to be electrocatalytically active for proton reduction in the presence of trifluoroacetic acid (TFA). However, a successful visible-light powered system for catalytic hydrogen release in homogeneous solution could not be achieved, and the reaction remained stoichiometric with all of the systems investigated.