Williams Owen M, Cowley Alan H, Rose Michael J
Department of Chemistry, The University of Texas at Austin, Austin, TX 78712, USA.
Dalton Trans. 2015 Aug 7;44(29):13017-29. doi: 10.1039/c5dt00924c.
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 ((1)H NMR or EPR) of the complexes within the series [Co(aqdBF2)2(MeCN)2] (), Co(aqdBF2)(MeCN)2 () and Co(aqdBF2)2(MeCN)2 (, ) are reported and the 3-electron reduced complex Co(aqdBF2)2(MeCN)2 () has been prepared in situ and characterized by (1)H NMR spectroscopy. The X-ray crystal structures revealed the presence of a 6-coordinate Co(II) species (), a 5-coordinate Co(I) species (), and a 4-coordinate complex (, ). In the case of complex , evidence from single crystal EPR spectroscopy (g‖ = 2.017, g⊥ = 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 , , , & support the electronic assignments that were observed spectroscopically. The cathodic cyclic voltammogram (CV) of the solvato congener in DMF solution, namely [Co(aqdBF2)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 (Ecat = -1.0 V), the delay of catalytic onset arises from the existence of a series of resonance-stabilized intermediates.
