Synthesis and redox reactivity of a phosphine-ligated dichromium paddlewheel

A pentadentate ligand platform, bis[2-(diisopropylphosphino-methylamino)phenyl]ether (1), abbreviated as H₂(PNONP), is introduced that enables the isolation of homodinuclear chromium complexes. In a one-step metalation using Cr(III) and Cr(II) chloride reagents, the bimetallic compounds, [Cr(μ-Cl)(PNONP)]₂ (2) and [Cr(PNONP)]₂ (3), respectively, were synthesized. These complexes have been characterized by X-ray diffraction, NMR spectroscopy, cyclic voltammetry, magnetometry, UV-Vis-NIR spectroscopy, combustion analysis, and computational methods. Complex 3 has a reasonably short Cr-Cr bond length of 2.1342(5) Å. Quantum chemical calculations support a diradical singlet ground-state with a formal triple bond between the chromium centers. By cyclic voltammetry, 3 exhibits two reversible oxidations at E = -470 and -750 mV versus FeCp₂^0/+. The one- and two-electron oxidized analogues, 3⁺ and 3²⁺, were generated in situ via chemical oxidation using ferrocenium. Based on in situ characterization of 3⁺ and 3²⁺, we hypothesize the oxidations are metal-based to yield Cr₂⁵⁺ and Cr₂⁶⁺ cores, respectively.