∞3[Cu2(mand)2(hmt)]–MOF: A Synergetic Effect between Cu(II) and Hexamethylenetetramine in the Henry Reaction

Abstract

∞ 3 [Cu 2 (mand) 2 (hmt)]·H 2 O (where mand is totally deprotonated mandelic acid (racemic mixture) and hmt is hexamethylenetetramine) proved to be a stable metal-organic framework (MOF) structure under thermal activation and catalytic conditions, as confirmed by both the in situ PXRD (Powder X-ray diffraction) and ATR-FTIR (Attenuated total reflection-Fourier-transform infrared spectroscopy) haracterization. The non-activated MOF was completely inert as catalyst for the Henry reaction, as the accessibility of the substrates to the channels was completely blocked by H-bonded water to the mand entities and CO 2 adsorbed on the Lewis basic sites of the hmt. Heating at 140 • C removed these molecules. Only an insignificant change in the relative ratios of the XRD facets due to the capillary forces associated to the removal of the guest molecules from the network has been observed. This treatment afforded the accessibility of nitromethane and various aldehydes (4-bromobenzaldehyde, 4-nitrobenzaldehyde, and p-tolualdehyde) to the active catalytic sites, leading to conversions up to 48% and selectivities up to 98% for the desired nitroaldol products. The behavior of the catalyst is solvent-sensitive. Protic solvents completely inhibited the reaction due to the above-mentioned strong H-bonds. Accordingly, very good results were obtained only with aprotic solvents such as acetonitrile and 1,4-dioxane. The synthesized MOF is completely recyclable as demonstrated for five successive cycles.Chemistry 2020, 2 51 resemblance with enzymes, such moieties can be effective in catalysis, influencing the overall catalytic selectivity due to the steric constraints introduced by the morphology of the channels [9].However, to be used as catalysts or adsorbents, MOFs should be firstly evacuated for the adsorbed guest molecules without any collapse of the structure [10]. To achieve such stability, the literature suggested the necessity of either M +III or M +IV nodes with a high polarizing capability [11][12][13] or organic linkers with high pKa values [14]. In the case of M +II -supported MOFs, the stability may be improved by framework interpenetration [15], yet this generates a noticeable decrease in the specific surface area. In terms of catalytic activity, slight framework degradation without total collapse can be beneficial, as this may take place with the generation of defects [16]. Thus, MOFs may afford a very efficient transition from the classical homogeneous catalysis with metal complexes or from more recently organocatalysis to heterogeneous catalysis. The metallic nodes act as a backbone for the overall structure [17] but also regulate the acidity/basicity of the accessible sites [18][19][20].For the specific case of the base catalyzed reactions, to date, several reports highlighted the efficient participation of the MOFs Lewis basic sites for fine organic syntheses such as the Hantzsch coupling reaction [21,22], Friedländer reaction [23], Knoevenagel condensation [24-26], aza-Michael condensation [27], or the Henry reactio...

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