Apicophilicity versus Hydrogen Bonding. Intramolecular Coordination and Hydrogen Bonds in <i>N</i>‑[(Hydroxydimethylsilyl)methyl]-<i><i>N,N</i></i>′‑propyleneurea and Its Hydrochloride. DFT and FT-IR Study and QTAIM and NBO Analysis

Conformers of <i>N</i>-[(hydroxydimethylsilyl)­methyl]-<i><i>N,N</i></i>′-propyleneurea (<b>1</b>) and their hydrochlorides (<b>2</b>) with HCl coordinated to different basic sites have been studied experimentally by FT-IR and theoretically using the density functional theory (DFT) method at the B3LYP/6-311+G­(d,p) and M06/6-311+G­(d,p) levels of theory. The structures of silanols <b>1</b> and <b>2</b> are determined by the balance of two competing effects: namely, intramolecular CO→Si coordination and intramolecular CO···H–O or intermolecular X···H–Cl hydrogen bonding. The preferred conformation of silanol <b>1</b> is that with an equatorial hydroxyl group, in apparent contradiction with the apicophilicity rule. In the crystal, silanol <b>1</b> exists as a conformer with a bifurcated bond composed of a weak CO→Si coordinated bond and a substantially more strong CO···H–O hydrogen bond. From the NBO analysis, the energies of the n_O → σ*_Si–X and n_O → σ*_H–O orbital interactions responsible for the formation of the coordination and hydrogen bonds, as well as the lengths of these bonds, change in opposite directions. In solution the equilibrium is shifted toward the conformer having only the hydrogen bond and no coordination bond. Its hydrochloride <b>2</b> exists in the crystal as a conformer with the axial OH group coordinated to HCl, whereas in solution it appears to be in equilibrium with a conformer having the equatorial OH group, in which a four-centered bifurcated bond is formed by two intramolecular components CO→Si and CO···HO and one intermolecular component CO···HCl. The QTAIM analysis showed the O→Si coordination bonds in the studied compounds to fall in the range from partially covalent and weak donor–acceptor to mainly electrostatic in nature and the hydrogen bonds to vary from weak to medium in energy