Pyridine-substituted Fe(III) complexes - on the way to one-component photo-catalytical systems

Linked photosensitizer-catalyst dyads based on earth-abundant metal complexes are currently potential candidates for sustainable proton reduction systems. Fe(III) carbene complexes showing low-lying ligand-to-metal charge transfer (LMCT) states with lifetimes in the 0.2-0.3 ns range are promising candidates for the photosensitizer side. The latter must be linked to a proton reduction catalyst, usually of cationic character, like cobaloxime. We explore the feasibility of this approach with a model compound mimicking the inductive effect of the cobaloxime moiety on the electronic state structure of a Fe(III) photosensitizer. Thus, the homoleptic pyridine-substituted [Fe(ImPPy)2]+ (HImPPy = 1,1'-(5-(pyridin-4-yl)-1,3-phenylene)bis(3-methyl-1H-imidazol-3-ium)) complex was modified by attaching a methyl group at the pyridinyl nitrogen, to model the inductive effect of a positively charged catalytic centre on the Fe(III) photosensitizer. Reduction of ligand electron density leads to stabilization of metal-to-ligand (²MLCT) excited states, as predicted by TD-DFT calculations. Experimental support is provided by a combination of cyclic voltammetry and ultrafast spectroscopies. [Fe(ImPPyMe)2]3+ shows a characteristic lifetime of 80 ± 10 ps for the stabilized ²MLCT state. This constitutes a promising starting point for later use of [Fe(ImPPy)2]+ as a photosensitizer in bimetallic one-component dyads for photoinduced hydrogen production.