Synthesis and redox reactivity of a phosphine-ligated dichromium paddlewheel

A pentadentate ligand platform, bis[2-(diisopropylphosphino-methylamino) phenyl] ether (1), abbreviated as H-2(PNONP), is introduced that enables the isolation of homodinuclear chromium complexes. In a onestep metalation using Cr(III) and Cr(II) chloride reagents, the bimetallic compounds, [Cr(mu-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) angstrom. 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-1/2 = -470 and -750 mV versus FeCp20/+. The one-and two-electron oxidized analogues, 3(+) and 3(2+), were generated in situ via chemical oxidation using ferrocenium. Based on in situ characterization of 3(+) and 3(2+), we hypothesize the oxidations are metal-based to yield Cr-2(5+) and Cr-2(6+) cores, respectively. (C) 2014 Elsevier B.V. All rights reserved.