CONFORMATIONAL STUDY OF 2,3,5,7,8,12,13,15,17,18-DECAALKYLPORPHYRINS

X-ray crystal structures are reported for two sterically crowded porph yrins: ,13,15,17-hexaethyl-2,8,12,18-tetramethylporphyrin (6, H2DAP) a nd hexaethyl-2,8,12,18-tetramethylporphinato)nickel?? (II) (Ni6, NiDAP ). In the NiDAP structure, steric crowding is relieved by the usual me chanism of the porphyrin adopting a nonplanar conformation. The crysta l structure of H2DAP is unusual, as it shows an essentially planar con formation of the porphyrin macrocycle. In this case, steric crowding o f the meso and pyrrole ethyl groups is relieved by a novel elongation of the macrocycle along the 5,15 axis. A detailed study is made of the solution conformations of H2DAP, NiDAP, and the corresponding zinc(II ) complex (ZnDAP), using optical spectroscopy, variable-temperature pr oton NMR spectroscopy, and molecular mechanics calculations. It is sug gested that the two species observed in low-temperature proton NMR stu dies of H2DAP and ZnDAP correspond to cis and trans conformations of t he methyl groups of the meso ethyls, and not the syn and anti structur es proposed previously (Maruyama; et al. J. Phys. Org. Chem. 1988, 1, 63). Crystal data: H2DAP (6), C36H46N4, M(r) = 534.8, monoclinic, P2(1 )/c, a = 6.458(4) angstrom, b = 16.58(2) angstrom, c = 13.778(7) angst rom, beta = 99.78(4)-degrees, V = 1,454(2) angstrom3, Z = 2, D(x) = 1. 222 Mg.m-3, lambda(Mo Kalpha) = 0.710 73 angstrom, mu = 0.072 mm-1, 13 0 K, R = 0.069 for 1769 reflections with F > 4.0sigma(F); NiDAP (Ni6), C36H44N4Ni, M(r) = 591.5, monoclinic, P2(1)/c, a = 14.350(8) angstrom , b = 16.043(6) angstrom, c = 15.049(6) angstrom, beta = 117.12(3)-deg rees, V = 3,083(2) angstrom3, Z = 4, D(x) = 1.274 Mg.m-3, lambda(Mo Ka lpha) = 0.710 73 angstrom, mu = 0.660 mm-1, 130 K, R = 0.072 for 3717 reflections with F > 4.0sigma(F).