Crystallographic and EXAFS studies of conformationally designed nonplanar nickel(II) porphyrins

Abstract

A series of Ni(II) tetraphenylporphyrins with varying [beta] substituents was examined by X-ray crystallography and EXAFS to assess peripheral steric effects on the conformations of the macrocycles. Crystallographic results establish that ethyl, propyl, and cyclohexenyl substitutions lead to saddle conformations of the macrocycle skeletons that minimize steric interactions between the [beta] and meso substituents. We have also taken advantage of the sensitivity of Ni-N distances to the planarity or ruffling of Ni porphyrins to establish that the saddle distortions of three compounds are maintained in solution. The Ni-N distances in solution agree well with the crystallographic values, within the precision of the EXAFS measurements, 0.02 A. The EXAFS results also establish that another compound is very distorted in solution. Although a structure of this compound has not been determined, the results are consistent with the crystallographic saddle structure of the free base analog, H[sub 2]DPP. The different conformations of the cyclopentenyl derivatives thus point to the high sensitivity of the skeletal distortions to steric interactions between [beta] and meso substituents. The present results, previous crystallographic determinations, and ongoing studies unambiguously establish that the skeletons of porphyrins and hydroporphyrins are highly flexible in vitro and in vivo. 31 refs., 3 tabs.