Fe-II Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime

The iron carbene complex [Fe-II(btz)(3)](PF6)(2) (where btz = 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a (MLCT)-M-3 state with a 528 ps excited-state lifetime in CH3CN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported Fe-II complex. The low potential of the [Fe(btz)(3)](3+)/[Fe(btz)(3)](2+) redox couple makes the (MLCT)-M-3 state of [Fe-II(btz)(3)](2+) a potent photo-reductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [Fe-III(btz)(3)](3+) form of this complex, these results show that the Fe-II and Fe-III oxidation states of the same Fe(btz)(3) complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.