Valence Tautomerism in Chromium Half-Sandwich Triarylmethylium Dyads

Abstract

Valence tautomerism (VT) may occur if a molecule contains two chemically different redox-active units, which differ only slightly in their intrinsic redox potential. Herein, we present three new half-sandwich complexes [(η6-arene)Cr(CO)2L]+ with a triarylmethylium substituent appended to the π-coordinated arene and different coligands L (L = CO, P(OPh)3, PPh3, 1+–3+) at the chromium atom. Ligand substitution purposefully lowers the half-wave potential for chromium oxidation and thereby the redox potential difference towards tritylium reduction. For the PPh3-substituted complex 3+, cyclic voltammetry measurements indicate that chromium oxidation and tritylium reduction occur at (almost) the same potential. This renders the diamagnetic Cr(0)-C6H4-CAr2+ form 3+, and its paramagnetic diradical Cr(I)+•-C6H4-CAr2• valence tautomer 3+•• energetically nearly degenerate. Temperature-dependent IR spectroscopy indeed shows two pairs of carbonyl bands that are assignable to a Cr(0) and a Cr(I) species, coexisting in a T-dependent equilibrium with almost equal quantities for both at −70 °C. The diradical form with one unpaired spin at the trityl unit engages in a monomer ⇌ dimer equilibrium, which was investigated by means of quantitative EPR spectroscopy. The diradical species 1+••–3+•• were found to be highly reactive, leading to several identified reaction products, which presumably result from hydrogen atom abstraction via the trityl C atom, e.g., from the solvent.