Complexes of lithium tetrahydroaluminate with N,N,N′,N′-tetramethylethane-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

Abstract

Two crystalline complexes (1∶1 and 1∶2) of LiAlH₄ 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 µ-hydrogen bridges. Bond lengths and angles within the non-planar eight-membered rings are Al–H 1.55(3), Li–H 1.99(3) Å, H–Li–H 131(1) and H–Al–H 112(1)° and the exocyclic Al–H distance was 1.53(4) Å. Cryoscopic data indicated that the dimeric structure with Li–H–Al bridges was preserved in benzene solution and measurements of the ⁶Li-{¹H} 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 °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)₂O(CH₂CH₂O)_nMe, 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.