A Systematic Study of Structure and E−H Bond Activation Chemistry by Sterically Encumbered Germylene Complexes

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• 作者：Matthew Usher ; Dr. Andrey V. Protchenko ; Dr. Arnab Rit ; Dr. Jesú ; s Campos ; Dr. Eugene L. Kolychev ; Dr. Ré ; mi Tirfoin and Prof. Dr. Simon Aldridge
• 刊名：Chemistry - A European Journal
• 出版年：2016
• 出版时间：August 8, 2016
• 年：2016
• 卷：22
• 期：33
• 页码：11685-11698
• 全文大小：1653K
• ISSN：1521-3765

文摘

A series of new germylene compounds has been synthesized offering systematic variation in the σ- and π-capabilities of the α-substituent and differing levels of reactivity towards E−H bond activation (E=H, B, C, N, Si, Ge). Chloride metathesis utilizing [(terphenyl)GeCl] proves to be an effective synthetic route to complexes of the type [(terphenyl)Ge(ER_n)] (1–6: ER_n=NHDipp, CH(SiMe₃)₂, P(SiMe₃)₂, Si(SiMe₃)₃ or B(NDippCH)₂; terphenyl=C₆H₃Mes₂-2,6=Ar^Mes or C₆H₃Dipp₂-2,6=Ar^Dipp; Dipp=C₆H₃iPr₂-2,6, Mes=C₆H₂Me₃-2,4,6), while the related complex [{(Me₃Si)₂N}Ge{B(NDippCH)₂}] (8) can be accessed by an amide/boryl exchange route. Metrical parameters have been probed by X-ray crystallography, and are consistent with widening angles at the metal centre as more bulky and/or more electropositive substituents are employed. Thus, the widest germylene units (θ>110°) are found to be associated with strongly σ-donating boryl or silyl ancillary donors. HOMO–LUMO gaps for the new germylene complexes have been appraised by DFT calculations. The aryl(boryl)-germylene system [Ar^MesGe{B(NDippCH)₂}] (6-Mes), which features a wide C-Ge-B angle (110.4(1)°) and (albeit relatively weak) ancillary π-acceptor capabilities, has the smallest HOMO–LUMO gap (119 kJ mol⁻¹). These features result in 6-Mes being remarkably reactive, undergoing facile intramolecular C−H activation involving one of the mesityl ortho-methyl groups. The related aryl(silyl)-germylene system, [Ar^MesGe{Si(SiMe₃)₃}] (5-Mes) has a marginally wider HOMO–LUMO gap (134 kJ mol⁻¹), rendering it less labile towards decomposition, yet reactive enough to oxidatively cleave H₂ and NH₃ to give the corresponding dihydride and (amido)hydride. Mixed aryl/alkyl, aryl/amido and aryl/phosphido complexes are unreactive, but amido/boryl complex 8 is competent for the activation of E−H bonds (E=H, B, Si) to give hydrido, boryl and silyl products. The results of these reactivity studies imply that the use of the very strongly σ-donating boryl or silyl substituents is an effective strategy for rendering metallylene complexes competent for E−H bond activation.