Redox Behavior of Rhodium 9,10-Phenanthrenediimine Complexes

New square-planar rhodium complexes of the redox-active ligand 9,10-phenenthrenediimine (phdi) have been prepared and studied. The complexes [dpp-nacnac(CH3)]Rh(phdi) (2a; [dpp-nacnac(CH3)](-) = CH[C(Me)(N-(Pr2C6H3)-Pr-i)](2-)) and [dpp-nacnac(CF3)]Rh(phdi) (2b; [dpp-nacnac(CF3)](-) = CH[C(CF3)(N-(Pr2C6H3)-Pr-i)](2)(-)) have been prepared from the corresponding [nacnac]Rh(CO)(2) synthons by treatment with Me3NO in the presence of the phdi ligand. Complexes 2a and 2b are diamagnetic, and their absorption spectra are dominated by intense charge-transfer transitions throughout the visible region. Electrochemical studies indicate that both the phdi ligand and the rhodium metal center are redox-active, with the [nacnac](-) ligands serving to modulate the one-electron-oxidation and -reduction redox potentials. In the case of 2a, chemical oxidation and reduction reactions provided access to the one-electron-oxidized cation, [2a](+), and one-electron-reduced anion, [2a](-), the latter of which has been characterized in the solid state by single-crystal X-ray diffraction. Solution electron paramagnetic resonance spectra of [2a](+) and [2a](-) are consistent with S=1/2 spin systems, but surprisingly the low-temperature spectrum of [2a](-) shows a high degree of rhombicity, suggestive of rhodium(II) character in the reduced anion.