Five analogues of a novel group of boron derivatives of aminophosphonic acids-N-benzylamino-(3-boronphenyl)-S-methylphosphonic acid (m-PhS), N-benzylamino-(4-boronphenyl)-S-methylphosphonic acid (p-PhS), N-benzylamino-(2-boronphenyl)-R-methylphosphonic acid (o-PhR), N-benzylamino-(3-boronphenyl)-R-methylphosphonic acid (m-PhR), and N-benzylamino-(4-boronphenyl)-R-methylphosphonic acid (p-PhR)-were studied using Fourier transform infrared (FT IR), Fourier transform Raman (FT RS), and surface-enhanced Raman (SERS) spectroscopies. Analysis of obtained FT IR and FT RS spectra show that all investigated compounds in the solid state exist as dimeric species formed by an H-bonding interaction between -B(OH)(2) moieties of each monomer. In addition, comparison of the wavenumbers, intensities, and broadness of bands from the FT Raman and SERS spectra allowed information to be obtained regarding the adsorption geometry of the investigated compounds immobilized onto an electrochemically roughened silver substrate.
The structures of three ortho-alkoxyphenylboronic acids (2-methoxy-, 2-ethoxy-, 2-isobutoxy-), and three diortho-alkoxyphenylboronic acids (2,6-dimethoxy-, 2,6-diethoxy-and 2-isobutoxy-6-methoxy) were determined by single crystal X-ray diffraction. The study was undertaken with the intention of designing a novel boronic acid having a monomeric structure, which to date has been an unavailable building block for crystal engineering. This motif can be enhanced by involving two hydroxyl groups at the boron atom in intramolecular hydrogen bonds. Although monosubstituted systems form typical dimers in the crystal lattice, disubstituted species reveal a much bigger variety of possible interactions. Among the analyzed compounds, 2,6-dimethoxyphenylboronic acid and 2,6diethoxyphenylboronic acid crystallize in two polymorphic forms each. The unprecedented packing with monomers as the dominant structural motif has been found in the crystal structure of 2-isobutoxy-6-methoxyphenylboronic acid and one of the polymorphs of both 2,6dimethoxyphenylboronic acid and 2,6-diethoxyphenylboronic acid. This description of the molecular packing is also supported by the analysis of fingerprint 2-D plots based on the Hirshfeld surfaces. The variety of possible types of interactions either within a single moiety or between moieties in dimers were additionally analyzed on the basis of the interaction energies, which have been estimated by ab initio calculations at the MP2/6-31+G* and B3LYP/6-311+G** level of theory.
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