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

Abstract

∞3[Cu2(mand)2(hmt)]·H2O (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 CO2 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.