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

A new maximally diagonal force field for molecular modelling of metalloporphyrins is developed and optimized on the crystal structures of nickel(II) porphine, nickel(II) mono-tert-butylporphyrin and nickel(II) di-tert-butylporphyrin. It is then used to investigate non-planar distortions of octa- and tetrachloro tetraphenylporphyrins (TPP) and their Ni(II) and Tb(III) complexes. Molecular mechanics (MM) calculations reproduced very well the structure of Tb(III) octachloro-TPP (so far the only example of a crystallographically characterized chloro TPP metal derivative). Normal-coordinate structural decomposition (NSD) analysis was performed on the equilibrium structures obtained by MM calculations. As expected, sad distortion dominates in octachloro structures irrespective of the presence or the size of the central metal atom; dom distortion dominates in tetrachloro structures with large Tb(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 conformational flexibility of the porphyrin core upon metallation, and increase of the dihedral angle between the phenyl groups and the mean LSQ plane of the porphyrin core, as well as the overall increase in structural regularity upon the increase of the size of the central metal atom.