Two alternative approaches to access mixed hydride-amido zinc complexes: synthetic, structural and solution implications

Abstract

Using bis(amide) Zn(HMDS)₂ (HMDS = 1,1,1,3,3,3-hexamethyldisilazide) as a precursor, this study explores the synthesis of N-heterocyclic carbene stabilized mixed amido-hydride zinc complexes using two alternative hydride sources, namely dimethylamine borane (DMAB) and phenylsilane PhSiH₃. Hydride-rich zinc cluster Zn₄(HMDS)₂H₆·2IPr (1) (IPr = 1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene), which can be envisaged as a co-complex of IPr·ZnH₂ and (HMDS)ZnH, is obtained when DMAB is employed, with the concomitant formation of heteroleptic bis(amido)borane [HB(NMe₂)(HMDS)] and H₂ evolution. NMR studies in d₈-THF show that although the bulky carbene IPr does not bind to the zinc bis(amide), its presence in the reaction media is required in order to stabilise 1. Reactions using the slightly less sterically demanding NHC IXy (IXy = 1,3-bis-(2,6-dimethylphenyl)imidazol-2-ylidene) led to the isolation and structural elucidation of the carbene adduct Zn(HMDS)₂·IXy (2). Contrastingly, mixtures of equimolar amounts of PhSiH₃ and the zinc bis(amide) (60 °C, 3 h, hexane) afforded monomeric heteroleptic hydride (HMDS)ZnH·IPr (3). NMR studies, including DOSY experiments, revealed that while the integrity of 3 is retained in polar d₈-THF solutions, in lower polarity C₆D₆ it displays a much more complex solution behaviour, being in equilibrium with the homoleptic species ZnH₂·IPr, Zn(HMDS)₂ and IPr.