Structural and Electronic Characterization of Multi-Electron Reduced Naphthalene (BIAN) Cobaloximes
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Reported here are the syntheses and characterization of cobaloximes that feature a bis(imino) acenaphthene (BIAN) appended ligand. The X-ray crystal structures and spectroscopy (H-1 NMR or EPR) of the complexes within the series [Co(aqdBF(2))(2)(MeCN)(2)] (1), [Co(aqdBF(2))(MeCN)(2)](-) (2) and [Co(aqdBF(2))(2)-( MeCN)2](2-) (3, 3') are reported and the 3-electron reduced complex [Co(aqdBF(2))(2)(MeCN)(2)](3-) (4) has been prepared in situ and characterized by H-1 NMR spectroscopy. The X-ray crystal structures revealed the presence of a 6-coordinate Co-II species (1), a 5-coordinate Co-I species (2), and a 4-coordinate complex (3, 3'). In the case of complex 3, evidence from single crystal EPR spectroscopy (g(parallel to) = 2.017, g(perpendicular to) = 1.987; <10 G linewidths) in conjunction with DFT calculations indicate that the EPR signal originates from a delocalized ligand-based unpaired spin. The frontier orbitals obtained from DFT calculations on 1, 2, 3, & 4 support the electronic assignments that were observed spectroscopically. The cathodic cyclic voltammogram (CV) of the solvato congener in DMF solution, namely [Co(aqdBF(2))(2)(DMF)(2)], exhibits three reversible redox events near -1.0, -1.5 and -2.0 V vs. Fc/Fc(+). Catalytic proton reduction was observed by CV near the third redox peak. Compared with other cobaloximes (E-cat = -1.0 V), the delay of catalytic onset arises from the existence of a series of resonance-stabilized intermediates.