Hydroxylation of cyclohexane catalyzed by iron(III)–iron(III) porphyrin dimers and DABCO with molecular oxygen: evidence for the conformation effect of porphyrin dimers on the catalytic activity

Abstract

A series of p/p type iron(III)–iron(III) porphyrin dimers and corresponding zinc(II)–zinc(II) porphyrin dimers covalently linked by a flexible alkoxy chain (–O(CH2)nO–, n=2–10) have been synthesized and characterized. The hydroxylation of cyclohexane catalyzed by iron(III)–iron(III) porphyrin dimers with molecular oxygen under mild conditions are reported, 1 eq. of bidentate ligand DABCO (1,4-diazobicyclo[2,2,2] octane) is added exactly as axial ligand to the catalytic system. The catalytic activities of these dimers are supposed to be related to the special conformations formed by porphyrin dimers and DABCO. By using 1H-NMR, the conformation equilibrium of corresponding zinc(II)–zinc(II) porphyrin dimers and their binding behavior with 1 eq. DABCO has been discussed in detail. It has been proven that the higher catalytic activities of the catalytic system mainly result from the formation of the ternary sandwich complexes of iron(III)–iron(III) porphyrin dimers with DABCO.