Carbodicarbene Ligand Redox Noninnocence in Highly Oxidized Chromium and Cobalt Complexes

Carbodicarbenes (CDCs) possess two lone pairs of electrons on their central carbone C atom (C_carbone). Coordination to a transition metal via a σ bond leaves one pair of electrons with appropriate symmetry for π donation to the metal. However, the high energy of the latter also renders the CDC ligand potentially redox-active. Herein, we explore these alternatives in the redox series [Cr(L)₂] and [Co(L)₂] (n = 2-5), where L is a tridentate ligand comprised of a central CDC and two flanking pyridine donors. To this end, all members of both redox series were synthesized and their electronic structures were investigated by using a combination of ¹H NMR, Evans' NMR, IR, UV-vis, and EPR spectroscopies, SQUID magnetometry, X-ray crystallography, and density functional theory studies. Whereas [Co^II(L)₂]²⁺ is a straightforward low-spin (S = ¹/₂) cobalt(II) complex, the corresponding chromium complex was found to feature an electronic structure that is intermediate between the two limiting resonance forms [Cr^III(L^•-)(L)]²⁺ and [Cr^II(L)₂]²⁺. In the case of the tri-, tetra-, and pentacationic complexes, the qualitatively identical electronic structures [M^III(L)₂]³⁺, [M^III(L^•+)(L)]⁴⁺, and [M^III(L^•+)₂]⁵⁺ were observed for both metals. Thus, the metal ions retain a 3+ oxidation state throughout, and the higher redox states contain oxidized ligands. The majority of the unpaired spin on the cation radical ligands was calculated to be localized in π-symmetry orbitals on the coordinated C_carbone atoms. Analogous behavior was previously reported for the corresponding iron redox series and, as such, redox noninnocence in oxidized CDC and, more broadly, carbone complexes is likely widely accessible.