Density-Functional Theory Study of the Stereochemistry of Chloroiron(III) and Chloromanganese(III) Complexes of a Bridled Chiroporphyrin

Abstract

Transition metal complexes of chiroporphyrins, in which two adjacent meso substituents are linked by a strap of eight methylene groups, [M(BCP8)], can exist as either an alpha alpha alpha alpha or alpha beta alpha beta atropisomer depending on the nature of the coordinated metal cation. This remarkable conformational versatility was investigated by density-functional theory calculations for the d(5) chloroiron(III) complex in the low-spin and high-spin states and for the d(4) high-spin chloromanganese(III) complex. The lowest-lying electronic state of all of the conformers of the chloroiron(III) bridled chiroporphyrin is found to be the high-spin state. For the chloroiron(III) complex in the low-spin or the high-spin state and for the high-spin chloromanganese(III) complex, the most stable form is predicted to be the alpha alpha alpha alpha conformer in which the chloride axial ligand is located within the cavity provided by the bridles. The predicted stereochemistries are compared with those similarly obtained (i) for the chloroiron(III) and chloromanganese(III) complexes of the tetramethylchiroporphyrin, which is devoid of straps, and (ii) for the d(10) zinc(II) and low-spin d(8) nickel(II) BCP8 complexes, on the basis of the effects tied to the occupancy of the stereochemically active d(x(2)-y(2))-type antibonding orbital level, to the restraints imposed by the straps, and to the presence of the axial chloride ligand.