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

A series of new germylene compounds has been synthesized offering systematic variation in the sigma- and pi-capabilities of the alpha-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(ERn)] (1-6: ERn = NHDipp, CH(SiMe3)(2), P(SiMe3)(2), Si(SiMe3)(3) or B(NDippCH)(2); terphenyl=C(6)H(3)Mes(2)-2,6=Ar-Mes or C(6)H(3)Dipp(2)-2,6=Ar-Dipp; Dipp=C(6)H(3)iPr(2)-2,6, Mes=C6H2Me3-2,4,6), while the related complex [{(Me3Si)(2)N}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 (theta>110 degrees) are found to be associated with strongly sigma-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 [(ArGe)-Ge-Mes{B(NDippCH)2}] (6-Mes), which features a wide C-Ge-B angle (110.4(1)degrees) and (albeit relatively weak) ancillary pi-acceptor capabilities, has the smallest HOMO-LUMO gap (119 kJmol(-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, [(ArGe)-Ge-Mes{Si(SiMe3)(3)}] (5-Mes) has a marginally wider HOMO-LUMO gap (134 kJmol(-1)), rendering it less labile towards decomposition, yet reactive enough to oxidatively cleave H-2 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 s-donating boryl or silyl substituents is an effective strategy for rendering metallylene complexes competent for E-H bond activation.