ReviewMolecular zinc hydrides
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”.
References (64)
- et al.
Organomet. J. Chem.
(1980) - et al.
Tetrahedron Lett.
(1977) - et al.
Organomet. J. Chem.
(1980) - et al.
J. Organomet. Chem.
(1977) - et al.
J. Organomet. Chem.
(1978) - et al.
J. Organomet. Chem.
(2011) - et al.
J. Am. Chem. Soc.
(1947) - et al.
Russ. J. Inorg. Chem.
(2014) - et al.
Inorg. Chem.
(1974) - et al.
J. Am. Chem. Soc.
(1951)
J. Chem. Soc. A
Acta Crystallogr. B: Struct. Sci.
J. Chem. Soc. Chem. Commun.
Organometallics
Inorg. Chem.
Eur. J. Inorg. Chem.
J. Am. Chem. Soc.
J. Am. Chem. Soc.
Z. Naturforsch. B: Chem. Sci.
Chem. Commun.
Chem. Commun.
Chem. Commun.
Eur. J. Inorg. Chem.
Organometallics
Angew. Chem. Int.
Chem. Eur. J.
Z. Anorg. Allg. Chem.
Inorg. Chem.
Organometallics
Z. Anorg. Allg. Chem.
Chem. Commun.
Organometallics
Cited by (63)
Synthesis of bis(2-pyridylthio)methyl zinc hydride and catalytic hydrosilylation and hydroboration of CO<inf>2</inf>
2022, Chemical CommunicationsZinc, Cadmium and Mercury
2022, Comprehensive Organometallic Chemistry IV: Volume 1-15Zinc Reagents in Organic Synthesis
2022, Comprehensive Organometallic Chemistry IV: Volume 1-15Zinc hydride cation [ZnH]<sup>+</sup> supported by TMPDA (TMPDA = N,N,N’,N’-tetramethylpropane-1,3-diamine)
2021, PolyhedronCitation Excerpt :The Zn1-Zn1′ distance of 2.4474(13) Å compares with values for zinc-zinc distances in multinuclear cationic zinc hydrides reported in the literature [13,16]. The Zn-H bonds within the Zn2H2 core of 1.74(6) Å (Zn1-H1) and 1.82(4) Å (Zn1-H1′) are in the typical range for zinc µ-hydrido ligands (1.58–1.88 Å) [8] at four-coordinate zinc centers. Compared to the THF adduct 1, the chair conformation shaped by the TMPDA ligand and zinc is less pronounced in 2 whereby it retains the tetrahedral geometry on both zinc centers with pyramidalization of the nitrogen donors.