Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds
Narayan Sinha, Björn Pfund, Christina Wegeberg, Alessandro Prescimone, Oliver S. Wenger
Abstract
Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excitedstate properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d 6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (L CNC ), in which a central amido -donor is flanked by two -donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L CNC ) 2 ]-(PF 6 ), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(L CNC ) 2 ](PF 6 ) is long-lived because it does not deactivate as efficiently into lower-lying metalcentered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(L CNC ) 2 ](PF 6 ), as well as structural, electrochemical, and UV-vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.