Coordination Compounds of Alkali Metal Tetrahydroborates with Ethers and Amines

The crystal structure of LiBH 4 (thf) is of the same type as that of LiBH 4 (OEt 2 ) in that it forms double chains containing seven‐coordinate Li atoms and BH 4 groups where one H atom coordinates to three Li atoms. In addition, three H atoms form Li–H–B bridges to three different Li atoms. (NaBH 4 ) 2 (triglyme) shows a sheet structure with eight‐coordinate Na atoms (2 O and 6 H atoms). All H atoms of the BH 4 groups are in bridging positions (1 μ 3 2 and 3 μ 2 1 types) and each triglyme molecule acts as a tris(bidentate) ligand. A more precise structure determination is reported for NaBH 4 (15‐crown‐5), which contains an asymmetric tridentate BH 4 group. The compound crystallizes with half a molecule of pyridine. A tridentate BH 4 group is also present in KBH 4 (18‐crown‐6). In contrast, crystallization of NaBH 4 (18‐crown‐6) from pyridine gives the salt [Na(py) 2 (18‐crown‐6)]BH 4 . The crown ether complex KH 2 BC 5 H 10 (18‐crown‐6) binds through its BH hydrogen atoms to the K atom and there is also a weak K ··· H–C interaction. In contrast to the bidentate BH 4 group in LiBH 4 (py) 3 , the BH 4 group in LiBH 4 (py‐4‐Me) 3 is tridentate. LiBH 4 reacts with py‐2‐Me to produce LiBH 4 (py‐2‐Me) 2 , which is dimeric in the solid state with BH 4 groups bonding to the Li atoms in a μ 3 2 /μ 2 2 manner. The reaction of LiBH 4 with 2‐aminopyridine in THF gives the complex LiBH 4 (thf) 2 (py‐2‐NH 2 ) in which only the pyridine N atom coordinates to the Li atom. NaBH 4 crystallizes from pyridine as NaBH 4 (py) 3 , which has a chain structure in the solid state where the BH 4 groups show one three‐coordinate H atom of the μ 3 2 type and two two‐coordinate H atoms of the μ 2 2 type. The 1:1 dialkylamine complexes of LiBH 4 with HN i Pr 2 and HN i Bu 2 have different structures. The former shows a chain structure with BH 4 groups bridging two Li atoms with bridges of the μ 3 2 and μ 2 2 type, whereas the latter compound forms double strands with one μ 4 3 ‐ and μ 2 3 ‐coordinated H atoms. NaBH 4 (morpholine) 2 forms a three‐dimensional structure with four different Na atoms linked by BH 4 groups in a rather unusual manner ― they exhibit mono‐, bi‐, and tridentate functions. LiBH 4 adds MeHN‐NH 2 in a 2:3 ratio to form the salt [Li 2 (H 2 N‐NHMe) 3 ](BH 4 ) 2 , which contains chains of [Li 2 (H 2 N‐NHMe) 3 ] 2+ cations separated from BH 4 – anions. In contrast, LiBH 4 reacts with PhNH‐NH 2 to give the adduct LiBH 4 (H 2 N‐NHPh) 2 , which associates into chains containing H 2 BH 2 LiH 2 BH 2 Li units. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)