Near-Infrared Fourier Transform Raman Spectroscopy of B12 Models. 4. Steric and Electronic Factors Affecting the Co-C Bond in Organocobalt Complexes

Article abstract of DOI:10.1021/ic00128a007

Cobalt(III) σ-alkyls [Co(bpb)R(OH2)] [bpb = 1,2-bis(2-pyridinecarboxamido)benzene; R = CH3, C2H5, i-Pr, n-Bu, Bz, Np, CH2SiMe3] were prepared by reactions of Na[Co(bpb)] with the respective alkyl halides. Oxidation of [Co(bpb)CH3(OH2)] with Ce(IV) gave [Co(bpb)CH3](1+), which shows an isotropic EPR signal at g = 2.0800 with (59)Co hyperfine coupling of ca. 50 G, indicative of Co(IV) character. Treatment of [Co(bpb)CH3](1+) with t-Bupy (4-tert-butylpyridine) led to Co-C bond cleavage and formation of [Co(bpb)(t-Bupy)](1+). The Co-C stretching frequencies for [Co(bpb)CH3(OH2)] and [Co(bpb)(CN)2](1-) have been determined by near-IR FT-Raman spectroscopy to be respectively 515 and 493 cm**-1, the assignment of which has been confirmed by isotopic labeling experiments. Coordination of bases such as PMe3 and t-Bupy to [Co(bpb)CH3(OH2)] decreases the Co-C stretching frequency. Treatment of [Co(bpb)CH3(OH2)] with Ce(IV) results in a downshift of ν(Co-C) by 24 cm**-1, suggesting that the Co-Cbond is weakened on oxidation. The Co-C vibrational modes for the heavier alkyls [Co(bpb)R(OH2)] (R = C2H5, i-Pr, n-Bu, Bz, Np, CH2SiMe3) have been unambiguously determined to be 483, 478, 400, 334, 307, and 270 cm**-1, respectively. The ν(Co-C) modes for methylcobalt(III) porphyrins[Co(TPP)CH3], [Co(OEP)CH3], and [Co(TMP)CH3] were determined to be 504,500, and 459 cm**-1, respectively, indicating that the steric bulk of the porphyrin macrocycle has a profound influence on the axial Co-C bond.