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Complexes of lithium tetrahydroaluminate with NNN'N'-tetra-methylethane-12-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 sol

journal contribution
posted on 2023-06-08, 05:21 authored by Mbolatiana M Andrianarison, Anthony G Avent, Miles C Ellerby, Ian B Gorrell, Peter B Hitchcock, J David Smith, David R Stanley
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 oH–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 oH–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, witH4B interactions which were fluxional on the NMR time-scale.

History

Publication status

  • Published

Journal

Journal of the Chemical Society, Dalton Transactions

ISSN

1472-7773

Publisher

RSC Publishing

Page range

249 - 253

ISBN

0300-9246

Department affiliated with

  • Chemistry Publications

Full text available

  • No

Peer reviewed?

  • Yes

Legacy Posted Date

2012-02-06

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