Solvation spectra. Part 53.—Infrared and nuclear magnetic resonance studies of the tetrahydroborate anion in various pure solvents and binary aqueous mixtures

Abstract

Borohydride salts have been studied in various solvents by infrared and n.m.r. spectroscopy. Solutions in water containing HOD in low concentration exhibit an O—D stretching frequency profile indicative of relatively strong hydrogen bonding to the anion, comparable with that for chloride ions. The asymmetric B—H stretching band for BH^–₄(v₃) is a broad singlet in water (∼2270 cm^–1) but shifts strongly to lower frequencies as basic aprotic solvents, such as hexamethylphosphoramide, are added. This is again indicative of “hydrogen-bonding” to the anions by water.

The proton resonance for water shifts linearly on adding borohydride salts and from this shift and previously estimated cation shifts, a molal shift for BH^–₄ in water of + 0.08 p.p.m. has been derived. This falls between the shifts induced by chloride and bromide ions. The ¹H resonance for the anion in water (a multiplet from coupling to the two types of magnetic boron nuclei, ¹⁰B and ¹¹B) moves up-field as basic aprotic solvents are added after a small initial down-field shift or plateau. Heating also causes an up-field shift. We conclude that hydrogen-bonding induces a down-field shift which parallels the low-frequency shift in v₃. Possible structures for the hydrogen-bonded complex are considered.

In contrast, addition of t-butyl alcohol or triethylenediamine to aqueous borohydride solutions induces a rapid down-field shift. This is interpreted in terms of clathrate cage formation.