Ba(BO2OH) - A Monoprotonated Monoborate from Hydroflux Showing Intense Second Harmonic Generation

Pure samples of colorless, air-stable Ba(BO2OH) crystals were obtained from Ba(NO3)(2) and H3BO3 under the ultra-alkaline conditions of a KOH hydroflux at about 250 degrees C. The product formation depends on the water-base molar ratio and the molar ratio of the starting materials. B(OH)(3) acts as a proton donor (Bronsted acid) rather than a hydroxide acceptor (Lewis acid). Ba(BO2OH) crystallizes in the non-centrosymmetric orthorhombic space group P2(1)2(1)2(1). Hydrogen bonds connect the almost planar (BO2OH)(2-) anions, which are isostructural to HCO3-, into a syndiotactic chain. IR and Raman spectroscopy confirm the presence of hydroxide groups, which are involved in weak hydrogen bonds. Upon heating in air to about 450 degrees C, Ba(BO2OH) dehydrates to Ba2B2O5. Moreover, the non-centrosymmetric structure of Ba(BO2OH) crystals was verified with power-dependent confocal Second Harmonic Generation (SHG) microscopy indicating large conversion efficiencies in ambient atmosphere.

Automated summary

Pure and stable Ba(BO2OH) crystals were synthesized using Ba(NO3)(2) and H3BO3 under ultra-alkaline conditions. The crystals have a non-centrosymmetric orthorhombic structure, and hydrogen bonding connects the (BO2OH)(2-) anions into chains. Heating the crystals results in dehydration and transformation to Ba2B2O5. The non-centrosymmetric structure of Ba(BO2OH) was confirmed using confocal Second Harmonic Generation microscopy.