Coordination Compounds of Alkali Metal Tetrahydroborates with Ethers and Amines

The crystal structure of LiBH4(thf) is of the same type as that of LiBH4(OEt2) in that it forms double chains containing seven‐coordinate Li atoms and BH4 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. (NaBH4)2(triglyme) shows a sheet structure with eight‐coordinate Na atoms (2 O and 6 H atoms). All H atoms of the BH4 groups are in bridging positions (1 μ32 and 3 μ21 types) and each triglyme molecule acts as a tris(bidentate) ligand. A more precise structure determination is reported for NaBH4(15‐crown‐5), which contains an asymmetric tridentate BH4 group. The compound crystallizes with half a molecule of pyridine. A tridentate BH4 group is also present in KBH4(18‐crown‐6). In contrast, crystallization of NaBH4(18‐crown‐6) from pyridine gives the salt [Na(py)2(18‐crown‐6)]BH4. The crown ether complex KH2BC5H10(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 BH4 group in LiBH4(py)3, the BH4 group in LiBH4(py‐4‐Me)3 is tridentate. LiBH4 reacts with py‐2‐Me to produce LiBH4(py‐2‐Me)2, which is dimeric in the solid state with BH4 groups bonding to the Li atoms in a μ32/μ22 manner. The reaction of LiBH4 with 2‐aminopyridine in THF gives the complex LiBH4(thf)2(py‐2‐NH2) in which only the pyridine N atom coordinates to the Li atom. NaBH4 crystallizes from pyridine as NaBH4(py)3, which has a chain structure in the solid state where the BH4 groups show one three‐coordinate H atom of the μ32 type and two two‐coordinate H atoms of the μ22 type. The 1:1 dialkylamine complexes of LiBH4 with HNiPr2 and HNiBu2 have different structures. The former shows a chain structure with BH4 groups bridging two Li atoms with bridges of the μ32 and μ22 type, whereas the latter compound forms double strands with one μ43‐ and μ23‐coordinated H atoms. NaBH4(morpholine)2 forms a three‐dimensional structure with four different Na atoms linked by BH4 groups in a rather unusual manner ― they exhibit mono‐, bi‐, and tridentate functions. LiBH4 adds MeHN‐NH2 in a 2:3 ratio to form the salt [Li2(H2N‐NHMe)3](BH4)2, which contains chains of [Li2(H2N‐NHMe)3]2+ cations separated from BH4– anions. In contrast, LiBH4 reacts with PhNH‐NH2 to give the adduct LiBH4(H2N‐NHPh)2, which associates into chains containing H2BH2LiH2BH2Li units. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)