Review
Molecular zinc hydrides

https://doi.org/10.1016/j.ccr.2015.08.010Get rights and content

Highlights

  • This review collates reports on molecular zinc hydrides.

  • Systematic follows ligands used to support zinc hydrides.

  • Synthetic procedures for molecular hydrides are collected.

  • NMR spectroscopic and crystallographic data are discussed.

  • Catalytic applications of zinc hydrides are discussed.

Abstract

An overview of structurally characterized molecular organozinc hydrides is given. The compounds are presented based on the type of supporting ligand. Their synthesis, structure, reactivity, and catalytic applications are summarized in this overview.

Introduction

Zinc dihydride is known for over 60 years [1]. In the last decades, several attempts were made to stabilize zinc hydride with the use of organic ligands, since parent zinc dihydride ZnH2 is an inconvenient reagent due to its low solubility in organic solvents and thermal instability. Organozinc hydrides are compiled here, starting with zinc dihydride and the ongoing attempts to transform this presumably polymeric species into accessible reagents. In this article, molecular zinc hydrides are presented according to the ligand type. The early isolation of molecular zinc hydrides was achieved using anionic nitrogen donor ligands, some of them possessing additional oxygen donors. Later on, molecular zinc hydrides supported by carbon donor ligands were obtained as neutral, anionic, and cationic compounds. In addition to the synthesis and characterization of molecular zinc hydrides, these compounds were used as homogeneous catalysts, as described in the last section. Literature reports up to April 2015 have been included. A review on inorganic hydride compounds of zinc has appeared in 2014 [2]. The focus of the present overview lies on the synthesis and structural characterization of molecular zinc hydrides.

Section snippets

Zinc dihydride

Zinc dihydride, ZnH2, was first reported by Schlesinger et al. in 1947. It was synthesized by the reaction of Me2Zn and LiAlH4 in diethyl ether [1]. ZnH2 was alternatively obtained from ZnCl2 and alkali metal hydrides [3]. It is proposed to be a highly branched, hydride-bridged polymer, which possesses a low thermal stability and a not well defined reactivity pattern [3], [4]. At ambient temperature ZnH2 decomposes to a grey powder over 1–2 days, presumably due to decomposition to elemental

Nitrogen donors for zinc hydrides

Neutral zinc hydride complexes of the composition [LZnH]n were obtained with polydentate mono-anionic nitrogen donors. Complex 1 was prepared from ZnH2 and N,N,N′-trimethylethylenediamine by Bell et al. in 1968 (Scheme 3). In 1980, the crystal structure of the complex 1 as the first structurally authenticated zinc hydride complex was obtained [10], [11]. This complex is dimeric in the solid state and possesses two terminal hydrides with a zinc hydrogen bond length of 161.8(6) pm, as confirmed by

Carbon donor-stabilized zinc hydrides

In the last years, various carbon donor-stabilized zinc hydride complexes, both neutral and ionic, were reported [34], [35], [36], [37], [38], [39], [40], [41], [42], [43].

Catalytic applications of zinc hydrides

In situ prepared zinc hydrides, obtained by the reaction of zinc carboxylates with hydride reducing agents, were used as efficient catalysts for the hydrosilylation of carbonyl substrates [54]. Defined zinc hydrides were also proposed to be catalytically active species in some hydrosilylation reactions [25], [31], [41], [47], [55], [56]. Some of the zinc hydrides described above were utilized as homogeneous catalysts, e.g. for the reduction of carbonyl compounds [54], in the dehydrocoupling of

Conclusion

Zinc dihydride remains one of few binary metal hydrides that have not been fully structurally characterized. Introduction of carefully designed ligands has now allowed the isolation, structural characterization and reactivity studies of several families of molecular zinc hydrides. When compared with the rich chemistry involving zinc alkyl compounds, zinc hydrides offer ample space for further development and most importantly, as homogeneous catalysts and possibly hydrogen storage systems. The

Acknowledgements

We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft through the International Research Training Group “Selectivity in Chemo- and Biocatalysis” (SeleCa) and the Cluster of Excellence at RWTH Aachen EXC 236 “Tailor-Made Fuels from Biomass”.

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