Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 144, Issue 22, Pages 9859-9873Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jacs.2c02592
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Funding
- Swiss National Science Foundation [200021_178760, 206021_157687]
- NCCR Molecular Systems Engineering [14583224]
- National Research Fund, Luxembourg [14583224]
- Independent Research Fund Denmark [9059-00003B]
- Swiss National Science Foundation (SNF) [206021_157687] Funding Source: Swiss National Science Foundation (SNF)
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Unlike platinum group metals, cobalt(III) complexes with mixed metal-to-ligand and intra-ligand charge transfer properties in the excited state have not been discovered yet. A stable cobalt(III) complex with substantial mixed metal-to-ligand and intra-ligand charge transfer character in the excited state was obtained by using a rigid tridentate chelate ligand.
Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intra-ligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state 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 pi-donor is flanked by two sigma-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L-CNC)(2)](PF6), 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)](PF6) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(L-CNC)(2)](PF6), 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.
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