Abstract
Heterogeneous catalysts are of great interest in many industrial processes for environmental reasons and, during recent years, a great effort has been devoted to obtain metal-organic frameworks (MOFs) with improved catalytic behaviour. Few supramolecular metal-organic frameworks (SMOFs) are stable under ambient conditions and those with anchored catalysts exhibit favourable properties. However, this paper presents an innovative approach that consists of using metal nodes as both structural synthons and catalysts. Regarding the latter, metalloporphyrins are suitable candidates to play both roles simultaneously. In fact, there are a number of papers that report coordination compounds based on metalloporphyrins exhibiting these features. Thus, the aim of this bioinspired work was to obtain stable SMOFs (at room temperature) based on metallo-porphyrins and explore their catalytic activity. This work reports the environmentally friendly microwave-assisted synthesis and characterization of the compound [H(bipy)]2[(MnTPPS)(H2O)2]·2bipy·14H2O (TPPS = meso-tetra-phenyl-porphine-4,4',4'',4'''-tetra-sulfonic acid and bipy = 4,4'-bi-pyridine). This compound is the first example of an MnTPPS-based SMOF, as far as we are aware, and has been structurally and thermally characterized through single-crystal X-ray diffraction, IR spectroscopy, thermogravimetry and transmission electron microscopy. Additionally, this work explores not only the catalytic activity of this compound but also of the compounds μ-O-[FeTCPP]2·16DMF and [CoTPPS0.5(bipy)(H2O)2]·6H2O. The structural features of these supra-molecular materials, with accessible networks and high thermal stability, are responsible for their excellent behaviour as heterogeneous catalysts for different oxidation, condensation (aldol and Knoevenagel) and one-pot cascade reactions.
Highlights
During recent years, supramolecular materials and metal– organic frameworks (MOFs) have been thoroughly explored in many fields, such as water reuse, photocatalysis, electrochemistry and gas adsorption
The metalloporphyrinic synthons are intended to act as heterogeneous catalysts and structural building units at the same time
Synthetic metalloporphyrin complexes have been largely used for a wide variety of catalytic transformations (Chatterjee et al, 2016; Rayati et al, 2016; Sengupta et al, 2016; Zhou et al, 2016), and in this work we have explored the catalytic activity of the monomeric framework [H(bipy)]2[(MnTPPS)(H2O)2]Á2bipyÁ14H2O, 1, the dimeric framework -O-[FeTCPP]2Á16DMF, 2 (Fidalgo-Marijuan et al, 2015), and the one-dimensional framework [CoTPPS0.5(bipy)(H2O)2]Á6H2O, 3 (Fidalgo-Marijuan et al, 2013)
Summary
Supramolecular materials and metal– organic frameworks (MOFs) have been thoroughly explored in many fields, such as water reuse, photocatalysis, electrochemistry and gas adsorption (de Lange et al, 2015; Dias & Petit, 2015; Li et al, 2016; Li & Hill, 2017; Wang et al, 2016; Gao et al, 2014). In order to achieve heterogeneous catalysis, there are a number of successful approaches such as anchoring the catalyst into the cavities of porous coordination networks (Zhan & Zeng, 2016; Liu et al, 2016), doping the network with the catalyst (Lan et al, 2016) or post functionalizing the network (Andriamitantsoa et al, 2016) In this sense, we have been exploring a new strategy that consists of using porphyrins as structural units in SMOFs and catalytic active centres simultaneously (Fidalgo-Marijuan et al, 2015). Compounds 1, 2 and 3 have been exhaustively characterized by means of single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric analysis and transmission electron microscopy (TEM), after which oxidation, Knoevenagel and aldol condensations, and a one-pot cascade reaction (involving an acetal hydrolysis in the first step and a Knoevenagel condensation in the second) have been successfully tested for these compounds
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