NEAR-INFRARED FOURIER-TRANSFORM RAMAN-SPECTROSCOPY OF B-12 MODELS .4. STERIC AND ELECTRONIC FACTORS AFFECTING THE CO-C BOND IN ORGANOCOBALT COMPLEXES

Cobalt(III) sigma-alkyls [Co(bpb)R(OH2)] [bpb = 1,2-bis(2-pyridinecarboxamido)benzene; R = CH3, C2H5, i-Pr, n-Bu, Bz, Np, CH(2)SiMe(3)] 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](+), which shows an isotropic EPR signal at g = 2.0800 with Co-59 hyperfine coupling of ca. 50 G, indicative of Co(IV) character. Treatment of [Co(bpb)CH3](+) with t-Bupy (4-tert-butylpyridine) led to Co-C bond cleavage and formation of [Co(bpb)(t-Bupy)](+). The Co-C stretching frequencies for [Co(bpb)CH3(OH2)] and [Co(bpb)(CN)(2)](-) 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 PMe(3) 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 nu(Co-C) by 24 cm(-1), suggesting that the Co-C bond 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, CH(2)SiMe(3)) have been unambiguously determined to be 483, 478, 400, 334, 307, and 270 cm(-1), respectively. The nu(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.