Hydrogen bond controlled formation of trans-dihydroxo porphyrinato platinum(IV) complexes: Synthesis, characterization and catalytic activity in olefin epoxidation

Abstract

A number of trans-dihydroxo porphyrinato platinum(IV) complexes, [(porp)PtIV(OH)2] have been synthesized and characterized from their platinum(II)-porphyrin precursors by spectroscopic methods. The crystal structure of the platinum complex of trans-dihydroxo-meso-tetramesitylporphyrinato platinum(IV) meta-chlorobenzoic acid ([(TMP)PtIV(OH)2](m-CBA)2) has also been determined. The porphyrin periphery is planar and the platinum atom has a slightly distorted octahedral environment. It must be noted that there is a very strong O–H⋯O hydrogen bond between the hydrogen atoms of m-chlorobenzoic acids and the two hydroxo ligands coordinated to the platinum center. This crystal structure has been previously described by us as trans-dihydroxo-meso-tetramesitylporphyrinato platinum(VI) meta-chlorobenzoate ([(TMP)PtVI(OH)2](m-CB)2), but the spectroscopic data has been reinterpreted due to the contradictory +6 oxidation state for platinum. The UV–Vis and NMR spectroscopic data suggest that the porphyrin skeleton is similar to that of normal porphyrins and are also indicative of the +4 oxidation state for platinum. Cyclic voltammetry studies also indicate that PtIITMP can oxidize up to PtIV(TMP+) species via three successive one-electron oxidation steps. The results of DFT studies and NBO analysis confirm that platinum has +4 oxidation state in both [(TMP)Pt(OH)2](m-CBA)2 and [(TMP)Pt(OH)2](m-CB)2 complexes. The catalytic activity of these porphyrin complexes has been investigated for the oxidation of various alkenes in the presence of iodosylbenzene.