Valence Tautomerism in Chromium Half-Sandwich Triarylmethylium Dyads

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 [(<i>η</i>⁶-arene)Cr(CO)₂L]⁺ with a triarylmethylium substituent appended to the π-coordinated arene and different coligands L (L = CO, P(OPh)₃, PPh₃, <b>1⁺</b>–<b>3⁺</b>) 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 <b>3⁺</b>, 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 <b>3⁺</b>, and its paramagnetic diradical Cr(I)^+•-C₆H₄-CAr₂^• valence tautomer <b>3^+••</b> 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 <i>T</i>-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 <b>1^+••</b>–<b>3^+••</b> 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.