Molecular early main group metal hydrides: synthetic challenge, structures and applications

Abstract

Within the general area of early main group metal chemistry, the controlled synthesis of well-defined metal hydride complexes is a rapidly developing research field. As group 1 and 2 metal complexes are generally highly dynamic and lattice energies for their [MH](∞) and [MH(2)](∞) salts are high, the synthesis of well-defined soluble hydride complexes is an obvious challenge. Access to molecular early main group metal hydrides, however, is rewarding: these hydrocarbon-soluble metal hydrides are highly reactive, have found use in early main group metal catalysis and are potentially also valuable molecular model systems for polar metal hydrides as a hydrogen storage material. The article focusses specifically on alkali and alkaline-earth metal hydride complexes and discusses the synthetic challenge, molecular structures, reactivity and applications.

Methanediide Formation via Hydrogen Elimination in Magnesium versus Aluminium Hydride Complexes of a Sterically Demanding Bis(iminophosphoranyl)methanediide

“…The thermal lability of putative [(HLMgH) n ] and the respective stability of [HLAlH 2 ] 3 may highlight differences in these early main group metal hydride species [36]. MgH complexes are expected to have more ionic M···H interactions compared with more polar covalent AlH systems.…”

Alkali Metal Hydride Complexes: Well-Defined Molecular Species of Saline Hydrides

“…Complex 11 shows very broad resonances at 3.4À4.0 ppm in the 1 H NMR spectrum for the LiH fragments, and broad overlapping peaks in the region from 0.8 to 4.3 ppm in its 7 Li{ 1 H} NMR spectrum for the numerous Li environments. Irreversible LiH elimination from [Li(thf) 4 ][(pz) 12 Li 37 (thf) 2 H 26 ] 11 does occur at elevated temperatures in solution, but appears to be significantly slowed down by the sterically demanding ligands.…”

“…This route is now established [7,76] for a range of electropositive metal complex fragments, because commercially available silanes such as phenyl-and diphenylsilane can be used as hydride sources and nucleophilic alkyl and amide substituents can be conveniently converted. It can be summarised according to Scheme 2: alkyl-or amidometal fragments undergo a metathesis reaction with the silane via a hydridosilicate intermediate (cf.…”

“…The LiÁÁÁH contacts within the central cube are decomposes at 1008C in solution within minutes, but is sufficiently stable at room temperature. The availability of hydrocarbon-soluble 7 allowed reactivity studies to be undertaken and 1 H, 7 Li, and 31 P NMR spectroscopy aided rapid in situ reaction monitoring. [72,73] The lithium hydride complex [(DipNPPh 2 ) 4 Li 8 H 4 ] 7 reacts at room temperature and below in hydrolithiation reactions with benzophenone and dicyclohexylcarbodiimide respectively; see Scheme 1 for a simplified summary.…”

“…Because of the highly electropositive nature of the s-block metals and their electronegativity difference with hydrogen (,1.2-1.3 units), their binary hydrides are classified as ionic or saline, with the exception of the more covalent or polymeric hydride of beryllium. [1][2][3]6,7] The Alkali Metal Hydrides…”