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

Cobalt(III) sigma-alkyls [Co(bpb)R(OH2)] [bpb = 1,2-bis(2-pyridinecarb oxamido)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 halide s. Oxidation of [Co(bpb)CH3(OH2)] with Ce(IV) gave [Co(bpb)CH3](+), wh ich shows an isotropic EPR signal at g = 2.0800 with Co-59 hyperfine c oupling 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 cleava ge and formation of [Co(bpb)(t-Bupy)](+). The Co-C stretching frequenc ies 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 exp eriments. Coordination of bases such as PMe(3) and t-Bupy to [Co(bpb)C H3(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 vib rational 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) mo des for methylcobalt(III) porphyrins [Co(TPP)CH3], [Co(OEP)CH3], and [ Co(TMP)CH3]() were determined to be 504, 500, and 459 cm(-1), respecti vely, indicating that the steric bulk of the porphyrin macrocycle has a profound influence on the axial Co-C bond.