COMPARISON OF THE STRUCTURES OF THE 5-COORDINATE COBALT(II) PYRIDINE,5-COORDINATE COBALT(III) METHYL, AND 6-COORDINATE COBALT(III) METHYL PYRIDINE COMPLEXES OF OCTAETHYLPORPHYRIN

The molecular structures of the complexes of Co(OEP)CH3 Co(OEP)(CH3)(P y), and Co(OEP)(DMAP) were determined by X-ray diffraction. The comple xes are related to each other and to coenzyme B-12 by a reaction seque nce that involves coordination of pyridine to the five-coordinate alky l-Co(III) porphyrin complex to afford a six-coordinate alkyl-Co(III) p orphyrin pyridine complex followed by homolysis of the alkyl-cobalt bo nd to afford a five-coordinate Co(II) porphyrin pyridine complex. The compound rinato)(4-(N,N'-dimethylamino)pyridine)cobalt(II), Co(OEP)(DM AP); crystallized in the monoclinic space group P2(1)/n (Z = 4) with u nit cell dimensions a = 14.392(6) Angstrom, b = 14.864(6) Angstrom, c = 18.217(5) Angstrom, beta = 95.60(3)degrees, and V = 3878.4(2.6) Angs trom(3). The structure was refined to R = 0.050 on F-o(2) on the basis of 5122 reflections with I>0. The 1.982(2)-Angstrom Co-N-eq distance and the 0.12-Angstrom displacement of the cobalt atom from the porphyr in nitrogen plane are typical of five-coordinate, square-pyramidal com plexes of cobalt(II) porphyrins with nitrogenous bases, but the 2.191( 2)-Angstrom Co-N-ax distance is longer than previously observed for a pyridine axial ligand. The compound (2,3,7,8,12,13,17,18-octaethylporp hyrinato) (methyl)cobalt(III), Co(OEP)CH3, crystallized with two indep endent molecules per asymmetric unit in the triclinic space group <P(1 )over bar> (Z = 4) and had unit cell dimensions a = 13.776(1) Angstrom , b = 14.038(2) Angstrom, c = 16.593(2) Angstrom, alpha = 99.20(1)degr ees, beta = 90.07(1)degrees, gamma = 95.67(1)degrees, and V = 3151.6(6 ) Angstrom(3). The structure refined to R = 0.061 on F-o on the basis of 4699 reflections with I > 2 sigma(I). The agreement of structural d etails between the two independent molecules was excellent, despite so me disordering of ethyl groups. The cobalt atom has square pyramidal g eometry and mean displacement of 0.10 Angstrom from the plane of the f our nitrogen atoms. The average Co-C and Co-N-eq distances are 1.973 a nd 1.966 Angstrom, respectively. The compound taethylporphyrinato)(met hyl)(pyridine)cobalt(III), Co(OEP)-(CH3)(Py), crystallized in the tric linic space group <P(1)over bar> (Z = 2) with unit cell dimensions a = 10.667(2) Angstrom, b = 10.902(2) Angstrom, c = 16.119(3) Angstrom, a lpha = 97.51(3)degrees, beta = 97.67(3)degrees, gamma = 99.96(3)degree s, and V = 1808.5(6) Angstrom(3). The structure was refined to R = 0.0 65 on F-o on the basis of 2320 reflections with F-o > 4 sigma(F-o). Th e cobalt atom has octahedral geometry and is centered in the plane of the porphyrin. The Co-C, Co-N-eq, and Co-N-ax distances are 2.018, 1.9 83, and 2.214 Angstrom, respectively. A strong mutual trans influence of the methyl and pyridine axial ligands is evident from the structure s. Coordination of pyridine to five-coordinate Co(OEP)(CH3) results in a 0.045-Angstrom elongation of the Co-C bond. The Co-N-ax distance in the six-coordinate, formally d(6) cobalt(III) complex Co(OEP)(CH3)(Py ) is between 0.17 and 0.24 A longer than that in other six-coordinate Co(III) porphyrin pyridine X complexes. It is also significantly longe r than the Co-N-ax distance in five-coordinate, d(7) cobalt(II) porphy rin pyridine complexes. The porphyrin core size (Co-N-eq distance) of Co(OEP)(CH3)(Py) is slightly larger than that of Co(OEP)CH3 but is ess entially identical to that of Co(OEP)(DMAP). The centering of the coba lt atom in the porphyrin plane in the six-coordinate complex increases the steric interactions of both the axial methyl and pyridine ligand with the porphyrin. Overall, the data help rationalize the ''base-on'' effect, the acceleration of the homolysis rate of Co-C bonds that acc ompanies ligand binding in B-12 and Bit model compounds.