A. Stasch
Feb 20, 2012
Citations
0
Influential Citations
29
Citations
Journal
Angewandte Chemie
Abstract
Lithium hydride is the lightest metal hydride and is considered to be the simplest ionic compound. 2] With a low molecular weight of just under 8 g mol 1 and a relatively high hydrogen content of 12.7% it is of interest for numerous applications including hydrogen storage technologies and uses in organic and inorganic synthesis. LiH crystallizes with a rock salt structure and shows a high lattice energy of approximately 220 kcalmol . This renders the material stable and relatively easy to handle, but also makes it too insoluble and unreactive for many applications. In recent years, a number of well-defined and structurally characterized molecular s-block metal hydride complexes have been forthcoming and show significant differences to the properties of the parent bulk metal hydride, that is, MH for the group 1 metals and MH2 for the group 2 metals. The majority of these achievements concentrate on the molecular compounds of the group 2 metals Be, Mg, and Ca. Some of these hydride complexes have already been employed in hydrometalation reactions and have even been successfully used as a hydrogenation catalyst on alkenes. The majority of hydride complexes involving group 1 metals are mixed metal systems, and can be considered ate complexes, for example LiAlH4 and its derivatives. [2, 10] Some remarkable examples include mixed metal systems such as [[{(Me3Si)2N}AlH2Li]3(LiH)] having a central distorted (LiH)4 cube featuring one hydride moiety solely bound to Li cations. There are also a number of complexes having an interstitial hydride surrounded by seven or eight lithium ions, as in cationic [L6HLi8] + (L = a chelating monoanionic Nligand) and the related neutral complex [L6HLi7]. [12, 13] Active forms of alkali metal hydrides MH (M = Li, Na, K) have been obtained from mixtures of metal alkoxides and alkyl metal compounds in the presence or absence of dihydrogen or from alkyl metal compounds and silanes. 14] Mixed lithium alkoxide/lithium hydride aggregates have been generated thermally or photolytically in solution, and the remarkable ’superaggregate’ [(tBuOLi)16(LiH)17] was, in one instance, obtained in an undetermined yield and structurally characterized. Reaction of the new phosphinoamine DipNHPPh2 (1; Dip = 2,6-iPr2C6H3; Scheme 1), prepared from DipNHLi and ClPPh2, [17] with two equivalents of nBuLi in n-hexane afforded the mixed alkyl phosphinoamido lithium complex [(DipNPPh2)2Li4nBu2] (2). [17] For comparison, the reaction of equimolar amounts of DipNHPPh2 and nBuLi in a hydrocarbon solvent affords the poorly soluble phosphinoamido lithium salt [(DipNPPh2)Li], which is presumably oligoor polymeric in structure. Addition of donor solvents to [(DipNPPh2)Li] afford the soluble complexes [(DipNPPh2)Li(Do)2] [(Do)2 = (THF)2 or (TMEDA)], which have three-coordinate lithium cations showing N,O,O and N,N,N coordination modes, respectively, and no Li···P contacts. A single-crystal X-ray structure analysis of compound 2 revealed a mixed dialkyl bisphosphinoamido lithium complex composition (Figure 1). Compound 2 crystallizes with a full molecule in the asymmetric unit and features a severely distorted Li4 tetrahedron (dashed lines in Figure 1) with interatomic Li···Li distances ranging from 2.407(4) (Li1Li3) to 3.218(4) (Li1-Li2). The P N bond lengths in the phosphinoamide moiety are only marginally shorter than the comparable bond length in the parent phosphinoamine 1 (1.699(5) ). Two n-butyl groups in 2 cap two of the four Li3 faces with Li-C distances ranging from 2.202(3) to 2.270(3) . Both n-butyl groups in 2 are disordered and have been modeled with two positions for the outer three carbon atoms, and only the major butyl components are shown in Figure 1. Two Li cations (Li1, Li2) each coordinate to one N and one P atom from different phosphinoamide ligands and have one contact each to one nBu carbon atom. In addition, each of those two Li cations show Li···H contacts to alkyl groups of one iPr group of the Dip moiety; the shortest Li···H distances being approximately 2.06–2.09 . The other two Li cations (Li3, Li4) bind to one N atom each and to two nBu Scheme 1. Preparation of 2 and 3.