Bis(aryloxo) derivatives of tin(IV) porphyrins: synthesis, spectroscopy and redox activity

Abstract

Spectroscopic, electrochemical and fluorescence properties of five octahedral tin(IV) porphyrins ([( TpTP ) Sn ( O -m,p- C 6 H 3( CH 3)2)2] (1), [( TpTP ) Sn ( O -p- C 6 H 4( CH 3))2] (2), [( TpTP ) Sn ( O - C 6 H 5)2] (3), [( TpTP ) Sn ( O -p- C 6 H 4 O -o,p- C 6 H 3( NO 2)2)2] (4) and [( TpTP ) Sn ( O -p- C 6 H 4( NO 2))2] (5)) bearing electron donating/electron withdrawing aryloxo subunits as the axial ligands are investigated in detail. These complexes have been synthesized by the reaction of 5,10,15,20-tetra(4-methylphenyl)porphyrinato tin(IV) dihydroxide ([( TpTP ) Sn ( OH )2]) and either 3, 4-dimethylphenol, p-cresol, phenol, 4-(2,4-dinitrophenoxy)phenol or p-nitrophenol and were isolated in good-to-moderate yields. Analysis of the spectral data (FAB mass, IR, UV-vis and 1H NMR) of 1 - 5 suggests that the two trans axial aryloxo ligands are strongly bound in a symmetric manner at the tin center in these complexes. Specifically, the general symmetry observed in the 1H NMR spectra and the ring current effect experienced by the protons present on the axial ligands are revealing in this regard. Each porphyrin ring undergoes two successive, reversible/quasi-reversible, one-electron reductions in CH 2 Cl 2, 0.1 M TBAP. The bound nitroaromatic axial ligands of complexes 4 and 5 could also be reduced under the same set of experimental conditions. The wavelengths of maximum emission, the singlet energies and the Stokes’ shifts observed in the fluorescence spectra of 1 - 5 are close to the corresponding parameters of the reference porphyrin, [( TpTP ) Sn ( OH )2]. On the other hand, fluorescence intensities of complexes 1 - 4 are quenched in comparison with that of [( TpTP ) Sn ( OH )2] in three different solvents. A detailed analysis of the emission and redox potential data indicates that a photoinduced electron transfer from the axial aromatic subunit to singlet state of the basal tin(IV) porphyrin can, in principle, explain the fluorescence quenching observed in these donor-acceptor systems. The spectroscopic and redox features of these tin(IV) porphyrins are compared with those of the analogous phosphorus(V) porphyrins reported by us earlier.