Metal Tetrahydroborates and Tetrahydroborato Metallates, 30 [1]. Alkoxo-Substituted Alkali Metal Tetrahydroborates: Studies in Solution and Structures in the Solid State

Abstract

Abstract Reactions of MBH4 (M = Li, Na, K) with tBuOH, Ph3COH, PhOH, F5C6OH, and 2,4- tBu2C6H3OH 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-tBu2C6H3OH also a fair proportion of (RO)2BH was generated. The perfluorophenolato borane (F5C6O)2BH·THF was prepared from the phenol and BH3 ·THF in THF solution. It is unstable to disproportionation. Compound (C6F5O)3B·THF was isolated and its crystal structure determined. Reaction of LiBH4 with F5C6OH 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[C12H8O2B(NR2)2] (3 - 5). Li[B(O2C12H8)2], 2, is formed when HN(tBu)Ph is used as a secondary amine. The unstable phthalatoborane H{C6H4[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 H4−nBXn − showed that HBX3 − is the most stable species for X = F, OH, NH2. This confirms experimental results.