Synthesis and Molecular and Electronic Structures of Reduced Bis(imino)pyridine Cobalt Dinitrogen Complexes: Ligand versus Metal Reduction

Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine cobalt dihalide complexes ((PDI)-P-Ar)CoCl2 and ((iPr) BPDI)CoCl2 ((PDI)-P-Ar = 2,6-(2,6-R-2-C6H3N=CMe)(2)C5H3N (R = Pr-i, Et, Me); (BPDI)-B-iPr = 2,6-(2,6-Pr-i(2)-C6H3N=CPh)(2)C5H3N) in the presence of an N-2 atmosphere furnished the corresponding neutral cobalt dinitrogen complexes ((PDI)-P-Ar)CoN2 and ((BPDI)-B-iPr)CoN2. Magnetic measurements on these compounds establish doublet ground states. Two examples, ((PDI)-P-iPr)CoN2 and ((BPDI)-B-iPr)CoN2, were characterized by X-ray diffraction and exhibit metrical parameters consistent with one-electron chelate reduction and a Co(I) oxidation state. Accordingly, the toluene solution EPR spectrum of ((PDI)-P-iPr)CoN2 at 23 degrees C exhibits an isotropic signal with a g value of 2.003 and hyperfine coupling constant of 8 x 10(-4) cm(-1) to the l = 7/2 Co-59 center, suggesting a principally bis(imino)pyridine-based SOMO. Additional one-electron reduction of ((PDI)-P-iPr)CoN2 was accomplished by treatment with Na[C10H8] in THF and yielded the cobalt dinitrogen anion [((PDI)-P-iPr)CoN2](-), DFT calculations on the series of cationic, neutral, and anionic bis(imino)pyridine cobalt dinitrogen compounds establish Co(I) centers in each case and a chelate-centered reduction in each of the sequential one-electron reduction steps. Frequency calculations successfully reproduce the experimentally determined N N infrared stretching frequencies and validate the computational methods. The electronic structures of the reduced cobalt dinitrogen complexes are evaluated in the broader context of bis(imino)pyridine base metal chemistry and the influence of the metal d electron configuration on the preference for closed-shell versus triplet diradical dianions.