Conformational analysis of octa- and tetrahalogenated tetraphenylporphyrins and their metal derivatives

A new maximally diagonal force field for molecular modelling of metalloporp hyrins is developed and optimized on the crystal structures of nickel(II) p orphine, nickel(II) mono-tert-butylporphyrin and nickel(II) di-tert-butylpo rphyrin. It is then used to investigate non-planar distortions of octa- and tetrachloro tetraphenylporphyrins (TPP) and their Ni(II) and Tb(III) compl exes. Molecular mechanics (MM) calculations reproduced very well the struct ure of Tb(III) octachloro-TPP (so far the only example of a crystallographi cally characterized chloro TPP metal derivative). Normal-coordinate structu ral decomposition (NSD) analysis was performed on the equilibrium structure s obtained by MM calculations. As expected, sad distortion dominates in oct achloro structures irrespective of the presence or the size of the central metal atom; dom distortion dominates in tetrachloro structures with large T h(III) central atom, while sad, ruf, wav and pro distortions are present in various amounts in other tetrachloro structures (TPP free base and Ni(II) complex) depending on the pattern of peripheral chloro substitution on the pyrrole rings. Other observed regularities are: reduction of the conformati onal flexibility of the porphyrin core upon metallation, and increase of th e dihedral angle between the phenyl groups and the mean LSQ plane of the po rphyrin core, as well as the overall increase in structural regularity upon the increase of the size of the central metal atom. (C) 2001 Elsevier Scie nce B.V. All rights reserved.