title: A Systematic Study of Structure and E-H Bond Activation Chemistry by Sterically Encumbered Germylene Complexes.
abstract: 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(ERn )] (1-6: ERn =NHDipp, CH(SiMe3 )2 , P(SiMe3 )2 , Si(SiMe3 )3 or B(NDippCH)2 ; terphenyl=C6 H3 Mes2 -2,6=Ar(Mes) or C6 H3 Dipp2 -2,6=Ar(Dipp) ; Dipp=C6 H3 iPr2 -2,6, Mes=C6 H2 Me3 -2,4,6), while the related complex [{(Me3 Si)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 (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(Mes) Ge{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(Mes) Ge{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.
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK.
authors.forename: Matthew
authors.initials: M
authors.lastname: Usher
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK.
authors.forename: Andrey V
authors.initials: AV
authors.lastname: Protchenko
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK.
authors.forename: Arnab
authors.initials: A
authors.lastname: Rit
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK.
authors.forename: Jesús
authors.initials: J
authors.lastname: Campos
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK.
authors.forename: Eugene L
authors.initials: EL
authors.lastname: Kolychev
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK.
authors.forename: Rémi
authors.initials: R
authors.lastname: Tirfoin
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK. simon.aldridge@chem.ox.ac.uk.
authors.forename: Simon
authors.initials: S
authors.lastname: Aldridge
keywords: boryl ligand; germanium; germylene; hydrogen; oxidative addition