B[μ-H(CO₃)₂]: An Acentric Boron Hydrogencarbonate with [μ-H(CO₃)₂]³– Complex Anions

The hydrous borocarbonate B[μ-H(CO3)2] was synthesized in a laser-heated diamond anvil cell at moderate pressures (∼20 GPa) and temperatures (∼1500(200) K) by a reaction between B2O3, CO2 and H2O. The crystal structure was obtained from synchrotron single-crystal X-ray diffraction and confirmed by a combination of density functional theory (DFT) calculations and Raman spectroscopy. Second harmonic generation (SHG) measurements corroborated the acentric space group, while DFT calculations provided the complete SHG tensor. In the [μ-H(CO3)2]3- building block two [CO3]2- groups are connected by sharing a hydrogen atom, which is bound in a nearly linear and symmetric O-H-O arrangement. B[μ-H(CO3)2] is a hydrous borocarbonate without further cations, combining interesting chemical and physical properties such as symmetric hydrogen bonds and a significant SHG intensity.