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.

Reductive Elimination and Oxidative Addition of Hydrogen at Organostannylium and Organogermylium Cations

“…Under 5 bar H 2 7 could not be detected but besides formation of 8 hydrogenation of the Ge−Ar* bond was also found (See Figures S18, S21 in the SI). Transition metal‐free activation of hydrogen is a topic of major interest . Famous low valent digermyne and distannyne molecules were found to react with hydrogen readily .…”

“…Famous low valent digermyne and distannyne molecules were found to react with hydrogen readily . Diarylgermylene exhibits oxidative addition of hydrogen at 70 °C, a silyl substituted germylene {Ar′Ge[Si(SiMe 3 ) 3 ]} and bis(boryl)stannylene react already at room temperature with hydrogen and digermavinylidene shows addition of hydrogen at 45 °C . The presumed reaction of the low valent germylene cation (Scheme c,d, H 2 addition instead of C 6 H 4 F 2 ) can be compared with the activation of hydrogen by an isoelectronic low valent gallium compound …”

Using N‐Heterocyclic Vinyl Ligands to Access Stable Divinylgermylenes and a Germylium Cation

“…Notably,A ldridgesa rylsilylgermylene Ar*GeSi(SiMe 3 ) 3 has an estimated DE HOMO-LUMO of 32 kcal mol À1 and can readily activate H 2 . [18] Despite the steric bulk about Ge in [(IPr=CH) 2 Ge:] (5a), smooth oxidative addition transpired with I 2 and MeI to give [(IPr = CH) 2 GeI 2 ]( 6)a nd [(IPr = CH) 2 GeMe(I)] (7), respectively (Scheme 4). Of much greater interest was the reaction of 5a with MeOTf,w hich gave an orange-colored product later identified as the three-coordinate germylium salt [(IPr = CH) 2 GeMe]OTf (8;S cheme 4).…”

Diverse Activation Modes in the Hydroboration of Aldehydes and Ketones with Germanium, Tin, and Lead Lewis Pairs

“…Oxidative addition at germylenes is a known reaction and has been studied with many different substrates . The B−Ge−H structural motive is known in the literature from BH 3 adducts with germylene hydrides and products of the oxidative addition of a B−H bond of Me 3 NBH 3 at a germylene . In these structures, the B−Ge distances [2.15(7)–2.064(6) Å] are comparable to the found value of 4 .…”