Structural investigation of (1-methylimidazole)-2,3,7,8,12,13,17,18-octaethylporphinatozinc(II)

Zn(C36H44N4)(C4HrN2)), (Zn(OEP)- (1-MeIm)), Mr= 680.26, monoclinic, P2Jn, Z=4, F(000)= 1448, a= 14.099 (3), b= 17.762 (2), c= 14.475 (2)A, fl= 95.48 (1) °, V= 3608.4/~3, T= 294 + 1 K. The calculated and observed densities are 1.25 and 1.26gcm -3. 5700 observed reflections (measured with Mo Ka radiation, ~ = 0.71073/~) were used to refine the structure to a final R 1 = 0.072 and wR = 0.098. The complex crystallizes isomorphously with (Co(OEP)(1-MeIm)) and similarities and dif- ferences in side-chain disorder and core conformations are described. The average Zn-Np bond distance is 2.068 (7)A and the axial Zn-N(Im) distance is 2.106 (4)/~,. The zinc(II) atom is displaced 0.42 (1)/~ from the porphyrin mean plane. The 1-methyl- imidazole ligand plane makes a dihedral angle of 10.5 ° with a Zn-Np vector. This orientation of the 1-methylimidazole ligand corresponds well with that of previously determined imidazole metalloporphyrin structures and further indicates that the preferred orientation of imidazole ligands is essentially insensi- tive to the d" configuration of the metal ion of the metalloporphyrin. Introduction. We have been actively seeking to under- stand the control of and the consequences of axial orientation effects in imidazole-ligated metallo- porphyrin species. We distinguish two possible types of axial ligand orientation effects: (i) those involving the relative orientation of the axial ligands with respect to each other and (ii) the orientation of the axial imidazole(s) with respect to the porphinato core. A definition for the orientation of planar axial ligands was originally suggested by Collins, Countryman & Hoard (1972) as the dihedral angle ~0 between the ligand plane and a coordinate plane defined by a porphinato N atom, the metal atom and the coordinated axial donor atom. A ~0 angle of 0 ° corresponds to an eclipsing of the axial ligand plane with an M--Np bond.t Recent work has