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)(Py), and Co(OEP)(DMAP) were determined by X-ray diffraction. The complexes are related to each other and to coenzyme B-12 by a reaction sequence that involves coordination of pyridine to the five-coordinate alkyl-Co(III) porphyrin complex to afford a six-coordinate alkyl-Co(III) porphyrin pyridine complex followed by homolysis of the alkyl-cobalt bond to afford a five-coordinate Co(II) porphyrin pyridine complex. The compound (2,3,7,8,12,13,17,18-octaethylporphyrinato)(4-(N,N'-dimethylamino)pyridine)cobalt(II), Co(OEP)(DMAP); crystallized in the monoclinic space group P2(1)/n (Z = 4) with unit 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) Angstrom(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 porphyrin nitrogen plane are typical of five-coordinate, square-pyramidal complexes 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-octaethylporphyrinato) (methyl)cobalt(III), Co(OEP)CH3, crystallized with two independent 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)degrees, 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 details between the two independent molecules was excellent, despite some disordering of ethyl groups. The cobalt atom has square pyramidal geometry and mean displacement of 0.10 Angstrom from the plane of the four nitrogen atoms. The average Co-C and Co-N-eq distances are 1.973 and 1.966 Angstrom, respectively. The compound (2,3,7,8,12,13,17,18-octaethylporphyrinato)(methyl)(pyridine)cobalt(III), Co(OEP)-(CH3)(Py), crystallized in the triclinic 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, alpha = 97.51(3)degrees, beta = 97.67(3)degrees, gamma = 99.96(3)degrees, and V = 1808.5(6) Angstrom(3). The structure was refined to R = 0.065 on F-o on the basis of 2320 reflections with F-o > 4 sigma(F-o). The 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, 1.983, and 2.214 Angstrom, respectively. A strong mutual trans influence of the methyl and pyridine axial ligands is evident from the structures. 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 longer than the Co-N-ax distance in five-coordinate, d(7) cobalt(II) porphyrin 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 essentially identical to that of Co(OEP)(DMAP). The centering of the cobalt 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 accompanies ligand binding in B-12 and Bit model compounds.