Heterogeneous catalytic properties of unprecedented μ-O-[FeTCPP]2 dimers (H2TCPP = meso-tetra(4-carboxyphenyl)porphyrin): an unusual superhyperfine EPR structure

Abstract

During the past few years, a great deal of effort has been devoted to the anchoring of catalysts into solid coordination networks in order to achieve heterogeneous catalysts. In this sense, an innovative approach consists in using the coordination-network synthons both as structural units and as catalysts. Regarding the latter, metalloporphyrins are suitable candidates for synthons. In fact, a few studies report on coordination compounds based on metalloporphyrins exhibiting these features. On the other hand, highly distorted di-iron oxo dimers containing electron withdrawing groups rank amongst the most effective catalyst models. Thus, the aim of this work was to obtain coordination networks based on iron porphyrins exhibiting those characteristics. In this way, this work reports on the synthesis and characterisation of the μ-O-[FeTCPP]2·16DMF compound (H2TCPP = meso-tetra(4-carboxyphenyl)porphyrin, DMF = N,N-dimethylformamide). This compound is the first example of a μ-oxo dimer with TCPP. The inter-dimer connections give rise to a laminar structure. The structural, spectroscopic and magnetic properties of this compound are consistent with the presence of high-spin FeIII ions, exhibiting a strong antiferromagnetic coupling in the μ-oxo dimer (J = −132 cm−1). An unusual superhyperfine structure has been observed in EPR that is related to the high accessible volume of the compound. The structural features of the dimers and the accessible network are responsible for the excellent behaviour of the compound as a heterogeneous catalyst for different oxidations of alcohols. Therefore, this compound is one of the very few examples of metalloporphyrins where structural units act as catalysts.