Complexes of lithium tetrahydroaluminate with N,N,N′,N′-tetra-methylethane-1,2-diamine (tmen).: Crystal structures of [{Li(tmen)-(AlH4)}2] and [Li(tmen)2][AlH4] and the use of the 6Li-{1H} nuclear Overhauser effect to study LiAlH4 and LiBH4 in donor solvents

Two crystalline complexes (1:1 and 1:2) of LiAlH2 with N,N,N',N'-tetramethylethane-1,2-diamine (tmen) have been isolated. A crystal structure determination of the 1:1 complex showed that it formed centrosymmetrical dimers in which [AlH4](-) anions and [Li(tmen)](+) cations are linked by mu-hydrogen bridges. Bond lengths and angles within the non-planar eight-membered rings are Al-H 1.55(3), Li-H 1.99(3) Angstrom, H-Li-H 131(1)and H-AI-H 112(1)degrees and the exocyclic Al-H distance was 1.53(4) Angstrom. Cryoscopic data indicated that the dimeric structure with LI-H-Al bridges was preserved in benzene solution and measurements of the Li-6-{H-1} nuclear Overhauser effect (NOE) showed the presence of Li ... H-Al interactions. The 1:2 adduct [Li(tmen)(2)][AlH4] 2, which crystallised with an ionic NaCl structure, was converted into 1 by heating under vacuum at 120 degrees C. The NOE measurements on solutions of 2 in benzene showed the presence of Li ... H-Al interactions even when a large excess of tmen was present. Aluminium-bound hydrogen was shown to be near to lithium in solutions of LiAlH4 in diethyl ether, tetrahydrofuran, mono-and di-glyme [MeO(CH2)(2)O(CH2CH2O)(n)Me, n = 0 or 1]. The NOE measurements on solutions of LiBH4 in Et2O and NMR spectra of partially deuteriated species suggested the formation of contact ion pairs, with Li ... H4B interactions which were fluxional on the NMR time-scale.