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 inve stigated to determine whether evidence supports the existence of the a gostic interaction that had been postulated to explain the anomalous f eatures of the compounds' NMR spectra. Co(OEP)Et, C38H49CoN4, crystall ized 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 necessa ry to observe the C-13 NMR spectra of the axial alkyl groups and to me asure informative coupling constants. The C-13 resonances of carbons b ound to cobalt were quite broad and had unusual upheld shifts. Coordin ation of pyridine as a trans axial ligand narrowed the linewidth of th e cw-carbon resonance and caused it to move significantly downfield. T he 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 conta ct shifts that arise from thermal population of an excited state. (C) 1997 Elsevier Science S.A.