FeII 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 ](PF 6 ) 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 3 MLCT state with a 528 ps excited-state lifetime in CH 3 CN 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 3 MLCT state of [Fe II (btz) 3 ] 2+ a potent photoreductant 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.