Hydridoborates and hydridoborato metallates: Part 26. Preparation and structures of dihydridoborates of lithium and potassium

A series of alkali metal dihydrido borates has been prepared by three different routes. They were characterized by 11B-NMR spectroscopy and, in part, by IR spectroscopy. The following compounds were obtained: lithium dihydrido dimethyl borate and lithium methyl trihydrido borate, 1 and 2, lithium and potassium dihydrido di-tert-butyl borate, 3 and 4, lithium dihydrido dicyclopentyl borate, 5, lithium and potassium dihydrido 9-borata-bicyclo[3.3.0] nonane, 6a,b, potassium dihydrido boratacyclohexane, 7, lithium dihydrido boratacycloheptane, 8, and lithium dihydrido-9-boratafluorene, 9. In the process of the formation of 1 and 7 also Li(H3BMe) 2 and Li2(H3B(CH2)5BH3) are formed, most likely by a ligand redistribution process which is not operative if bulky organyl groups are bound to the boron atom or if the boron atom is part of a ring system. In case of catecholate no H2B(OR)2− anions were detected but for ephedrino or dithiolato ligands the corresponding dihydrido borate complexes were readily detected by 11B-NMR but the latter converted in THF solution into B(S2R’)2 anions. MO calculations show that the ligand redistribution for H2BX2− ions into BH4− and BX4− becomes thermodynamically more favored with increasing electronegativity of the substituent X. Characterization of the new hydrido borate species is usually unambiguous, but 10 shows an anomalous temperature dependent behavior in THF solution which can be attributed to an equilibrium involving Li(2H, 2O), Li(2H, 3O), and Li(3O) structural units. This is supported by the X-ray structure of dimeric 10·THF and monomeric 10·THF·TMEDA. While 9·3THF is monomeric and contains doubly bridging H2B groups all other dihydrido diorganyl borates are dinuclear. The interaction between the alkali metal center and the boron bonded H atoms depends on number and size of the auxiliary ligands. ‘Agostic’ Li⋯HC interactions play a role if β-H atoms are present and when the alkali metal cation is not coordinatively saturated by the auxiliary ligand and the H(B) hydrogens. The most symmetric and so far unique arrangement is found for [6a·2THF]2 where all four H(B) hydrogens form Li⋯H⋯Li bridges. Also lithium bis(dithiocatecholato)borate, 17·2 THF is dimeric. Its Li centers are coordinated by four sulfur and two oxygen atoms. These atoms form a double heterocubane structure with two diametral edges missing.