Design of solvatomorphic structures based on a polyboronated tetraphenyladamantane molecular tecton

Abstract

A series of solvatomorphic structures of tetrakis(4-dihydroxyborylphenyl)adamantane, 1, have been prepared and analyzed by single crystal X-ray diffraction methods. Tetraboronated adamantane represents a valuable scaffold for the formation of various supramolecular structures in the solid state. Through three-component crystallizations of 1 in methanol in the presence of a secondary ingredient (acetone, DMSO, DMF, 1,4-dioxane, urea), seven crystal structures were obtained. In turn, crystallization from a methanol–acetonitrile mixture produced large transparent needles, but the material proved amorphous and solely composed of 1 underlying the pivotal role of solvent molecules in the stabilization of the crystal structure. Six out of the seven obtained structures crystallize in tetragonal space groups of symmetry in accordance with the general molecular symmetry of 1. The most common crystal motif constitutes H-bonded tetramers, where interactions between boronic groups are mediated by solvent molecules. Alternatively, linear discrete H-bonded motifs composed of four molecules of 1 and four molecules of DMF were also observed. According to expectation, the inclusion of 1,4-dioxane favors the formation of 1D linear chains, although the overall H-bonding pattern is quite complex due to the presence of water molecules. Surprisingly, the crystallization of 1 in wet methanol solution resulted in the formation of hemi-ester species containing four B(OMe)(OH) groups and incorporating a water molecule. The same process was observed when crystallization was performed in a methanol–urea mixture. The role of the solvent molecules is additionally discussed in the context of performed DFT calculations. The thermogravimetric analysis shows that solvent evaporation is accompanied by the dehydration of boronic groups presumably leading to the formation of a covalently bonded polymeric material, whose stability is predefined by the initial structure of 1.