Electrochemical and structural characterization of a radical cation formed by one-electron oxidation of a cymantrene complex containing an N-heterocyclic carbene ligand

Electrochemical oxidation of the cymantrene derivative MnCp(CO)(2)(IDiip). 1, (Cp = eta(5)-C5H5, IDiip = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene) is a quasi-Nernstian one-electron process in dichloromethane/0.1 M INBu4][PF6]. The E-1/2 potential of the l(0/+) couple, -0.34 V vs. ferrocene, is the most negative yet reported for a mono-substituted cymantrene derivative, reflecting the powerful electron-donating character of the N-heterocyclic carbene (NHC)IDiip ligand. The radical cation 1(+) has been characterized by IR spectroscopy (v(co) = 1958 and 2034 cm(-1)) and by X-ray crystallography as its [PF6](-) salt. Compared to its neutral counterpart MnCp'(CO)(2)(IDiip) (Cp' = eta(5)-C5H4Me), the Mn-C-NHC bond distance in 1(+) is lengthened by 0.031 angstrom and the average M-C(O) bond distances are lengthened by 0.069 angstrom. DFT-based computational results show that the largest contribution to the redox orbital of 1(0/)(+) is found at the metal atom (about 70% manganese-based in 1(+)). Comparison of the E-1/2 potential of 1 with those of other mono-CO-substituted cymantene complexes allows an estimate of the "ligand electronic parameter" of the IDiip ligand as -0.27 V vs. NHE. (C) 2018 Elsevier Ltd. All rights reserved.