Do organocobalt porphyrins have agostic alkyl groups? An investigation of the structure of ethyl cobalt(III) octaethylporphyrin and the nuclear magnetic resonance spectra of C-13-labeled alkyl cobalt(III) porphyrin complexes

The X-ray structure of Co(OEP)Et and the H-1 and C-13 NMR spectroscopy of Co(P)R, where R = alkyl and P = OEP, TTP, OEC and OEiBC, were investigated to determine whether evidence supports the existence of the agostic interaction that had been postulated to explain the anomalous features of the compounds' NMR spectra. Co(OEP)Et, C38H49CoN4, crystallized in the triclinic space group P (1) over bar (Z=2) with unit cell dimensions a = 10.675(1), b = 10.886(1), c = 13.973(1) Angstrom, alpha = 79.539(8), beta = 86.386(7), gamma = 79.298(8)degrees, and V = 1568.2(2) Angstrom(3) at 173(2) K. The structural features of the ordered axial ethyl group were inconsistent with an agostic interaction in the solid state. Synthesis and use of C-13-labeled compounds were necessary to observe the C-13 NMR spectra of the axial alkyl groups and to measure informative coupling constants. The C-13 resonances of carbons bound to cobalt were quite broad and had unusual upheld shifts. Coordination of pyridine as a trans axial ligand narrowed the linewidth of the cw-carbon resonance and caused it to move significantly downfield. The spectroscopic data were inconsistent with an agostic interaction in solution. The anomalous features of the NMR spectra were ascribed to the effects of the quadrupolar Co-59 nucleus and to paramagnetic contact shifts that arise from thermal population of an excited state. (C) 1997 Elsevier Science S.A.