Metal effects on the structure of non-planar porphyrins

Porphyrin planarity, is though to influence chemical and electron-transfer properties of tetrapyrroles in methylreductase and the photosynthetic reaction center. The authors are investigating the effect of differing metals on the conformation of the porphyrin ring for a series of highly non-planar porphyrins. The non-planarity of these octaalkyl-tetraphenylporphyrins results from steric crowding of substituents at the periphery of the planarity of the macrocycle. However, the metals incorporated into the central core effect the planarity of the macrocycle and the metal core geometry. The series of octaethyl-tetraphenylporphyrin (OETPP) metal derivaties (M=N(II), Co(II), Co(III)OAc, Cu(II), Zn(II), Fe(III)Cl) is investigated using UV-visible adsorption spectroscopy, dual-channel resonance Raman spectroscopy, and X-ray crystallography. A comparison is made between the non-planar OETPPs and the planar octaethylporphyrins (OEPs) and tetraphenylporphyrins (TPPs). Molecular mechanics calculations are also used to calculate the molecular conformations for the metal OETPPs and corresponding metal OEPs and TPPs. The calculated conformations and compared with the experimetal stuctures. Generally, the macrocycle planarity of the OETPPs decreases as the metal size increases, although all of the metal derivatives maintain considerable non-plarity. Core sizes for the MOETPP series vary over the entire range of the observed core sizes for all planar metalloporphyrins. Nevertheless, a plot of themore » Raman frequencies versus porphyrin core size indicates that there is little or no core-size dependence of the Raman modes v[sub 2], v[sub 3] and v[sub 4].« less