Metal tetrahydroborates and tetrahydroborato metallates, 30 [1]. Alkoxo-substituted alkali metal tetrahydroborates: Studies in solution and structures in the solid state

Article abstract of DOI:10.1515/znb-2006-0906

Reactions of MBH₄ (M = Li, Na, K) with tBuOH, Ph₃COH, PhOH, F₅C₆OH, and 2,4-tBu₂C₆H ₃OH in THF in a 1:1 ratio were followed by ¹¹B NMR spectroscopy. No M[H₂B(OR)₂] species could be detected, but minor amounts of M[H₃BOR] and larger amounts of M[HB(OR) ₃]. In the reaction of LiBH₄ with 2,4-tBu ₂C₆H₃OH also a fair proportion of (RO) ₂BH was generated. The perfluorophenolato borane (F₅C ₆O)₂BH·THF was prepared from the phenol and BH ₃·THF in THF solution. It is unstable to disproportionation. Compound (C₆F₅O)₃B·THF was isolated and its crystal structure determined. Reaction of LiBH₄ with F ₅C₆OH in hexane generated a solid that proved to be Li[H₂B(OC₆F₅)₂]. It is unstable in THF. On the other hand, 2,2′-dihydroxydiphenyl in the presence of secondary amines reacts to give Li[C₁₂H₈O ₂B(NR₂)₂] (3-5). Li[B(O₂C ₁₂H₈)₂], 2, is formed when HN(tBu)Ph is used as a secondary amine. The unstable phthalatoborane H{C₆H ₄[C(O)O]₂}BH·THF (7), is stabilized as its pyridine adduct (phth)BH·py (8). 7 reacts with 3 equivalents of LitBu to give [Li(HBtBu)₃] (11), isolated as its tris(THF) solvate. Analogously, 7 reacts with LiNMePh to produce compound Li[HB(NMePh)₃] (10). Similarly, 7 and NaOtBu (molar ratio 1:3) give access to Na[HB(OtBu) ₃] (9). In attempts to grow single crystals, specimens resulting from a hexane solution showed that partial hydrolysis has occurred to give Na[HB(OtBu)₃]·Na[(tBuO)₂BO]·Na[tBuOB(O)H], which crystallizes as a centrosymmetric dimer. While catecholborane when treated with LitBu in THF and DME gave access to (dme)₂Li[catB(tBu) ₂], 12 (dme)₂, several compounds were observed when Li piperidide was used as nucleophile. Amongst these, the most interesting one was (dme)(THF)Li₂(cat)(catBH), 13 (dme)THF, the crystal structure of which was determined. In all cases where the borate species carried OR groups the O atoms of the RO or PhO group coordinate with the alkali metal cation. DFT calculations for the series of anions H_4-nBX_n^- showed that HBX₃^- is the most stable species for X = F, OH, NH₂. This confirms experimental results.