‘Click’ silica immobilisation of metallo-porphyrin complexes and their application in epoxidation catalysis

We present the synthesis, via Adler condensation reactions, of mono- and tetrakis-4-(ethynyl-phenyl)porphyrin ligands and the zinc and manganese complexes thereof. The formed complexes were immobilised on silica by reacting the ethynyl groups with azide-functionalised silica in a copper(I) catalysed Huisgens 1,3-dipolar cycloaddition reaction. The synthesised metallo-porphyrin containing materials were thoroughly characterised using various solid-state techniques (NMR, IR, UV–Vis, elemental content analysis). The manganese containing materials were applied as catalysts in the epoxidation of various alkenes (cyclooctene, cyclohexene, styrene) with various oxidants (iodosylbenzene, tert-butylperoxide). The heterogenised homogeneous catalysts show diminished activity and yields compared to the analogous homogeneous catalysts (71% yield cf. 92% for cyclooctene epoxidation, TOF 82 h−1 cf. 230 h−1). Upon recycling, the heterogenised catalysts become gradually less active over five cycles until they are catalytically inactive. The deactivation process is discussed, with spectroscopy suggesting that the catalysts themselves are intact and thus stable to the reaction conditions and recycling, however there is likely some decomplexation, and also both chemical and mechanical decomposition of the silica support resulting in inaccessibility to the catalytic site.