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

Abstract

Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine cobalt dihalide complexes ((Ar)PDI)CoCl(2) and ((iPr)BPDI)CoCl(2) ((Ar)PDI = 2,6-(2,6-R(2)-C(6)H(3)N=CMe)(2)C(5)H(3)N (R = (i)Pr, Et, Me); (iPr)BPDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N=CPh)(2)C(5)H(3)N) in the presence of an N(2) atmosphere furnished the corresponding neutral cobalt dinitrogen complexes ((Ar)PDI)CoN(2) and ((iPr)BPDI)CoN(2). Magnetic measurements on these compounds establish doublet ground states. Two examples, ((iPr)PDI)CoN(2) and ((iPr)BPDI)CoN(2), 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 ((iPr)PDI)CoN(2) 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 I = 7/2 (59)Co center, suggesting a principally bis(imino)pyridine-based SOMO. Additional one-electron reduction of ((iPr)PDI)CoN(2) was accomplished by treatment with Na[C(10)H(8)] in THF and yielded the cobalt dinitrogen anion [((iPr)PDI)CoN(2)](-). 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[triple bond]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.

Metal–Ligand Electron Transfer in 4d and 5d Group 9 Transition Metal Complexes with Pyridine, Diimine Ligands

“…[15a, 16,17,54] This is clearly reflected in the DFT-calculated C imine -N imine and C imine -C pyridine bond lengths and Mayer bond orders. As Pople and Karplus showed, the effect of increased charge on 13 C NMR resonances can also be expressed in terms of the corresponding Mayer bond orders of the involved carbon atoms.…”

Bimetallic Cobalt–Dinitrogen Complexes: Impact of the Supporting Metal on N2 Activation

“…7 Cobalt diketiminate complexes, when subjected to alkali metals, capture N 2 within a Co−NN−Co linkage, representing a formal two-electron reduction of N 2 ( Figure 1A). 5 Of relevance, monocobalt and dicobalt complexes have been discovered to catalyze the silylation of N 2 to two N(SiMe 3 ) 3 molecules in a six-electron redox reaction. 22 The dicobalt system features a metal−metal interaction in the precatalyst.…”

“…Using hemilabile interactions between a catalytic metal center and an ancillary main group ion is a powerful strategy in smallmolecule activation. 11,12 An enlightening example is the ironboratrane system that mediates the reduction of N 2 to NH 3 with seven turnovers at ambient temperature and pressure. 13,14 Recently, the cobalt-boratrane congener ( Figure 1E) was shown to be competent in fixing N 2 to NH 3 with 2.4 turnovers.…”

Synthesis, Electronic Structure, and Ethylene Polymerization Activity of Bis(imino)pyridine Cobalt Alkyl Cations

“…A representation of the solid-state structure is shown in Figure 4. Unlike other structurally characterized bis(imino)pyridine cobalt cations with neutral ligands, [12,22] [ + to account for the experimentally observed ethane. Butane is formed by coupling two cobalt ethyl fragments.…”