Spectroscopic cis-influences in octahedral tin(IV) meso-tetraphenylporphyrin complexes

Visible absorption and 1H NMR spectra have been measured for a series of octahedral tin(IV) porphyrin complexes Sn(TPP)X2, where TPP is mes-tetraphenylporphyrin and X is Cl, OH, OMe, OAc, NO3, ClO4, Br, I, NCS and OC6H4-p-Me. The tin-proton coupling constants to the β-pyrrole protons decrease from 19.2 Hz (X = ClO4) to 9.9 Hz (X = OME), and for the oxygen-bound ligands, correlate well with the basicity of the ligands. The halides do not fit this relationship, perhaps because of π-bonding effects. The chemical shifts of the β-protons also depend on the nature of X, and vary from 9.35 (X = ClO4) to 9.04 (X = OC6H4-p-Me) ppm. Increasing basicity of X causes red shifts in the visible spectra, as well as a decrease in the molar absorption coefficient ratio (εβ/εα) for the visible absorption bands. Tin-proton coupling constants for the axial ligands OH, OMe and OAc are reported. The complex Sn(TPP)(ClO4)2 exhibits unusual NMR behaviour in CDCl3 solutions, possibly due to self-aggregation.