Small inorganic rings in the 21st century: from fleeting intermediates to novel isolable entities.

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=C6H3Mes2-2,6=ArMes or C6H3Dipp2-2,6=ArDipp; 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 [ArMesGe{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, [ArMesGe{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.

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

A series of germylene, stannylene and plumbylene complexes [η2(N,N)-Me2Si(DippN)2Ge:] (3a), [η2(N,N)-Ph2Si(DippN)2Ge:] (3b), [η2(N,N)-Me2Si(DippN)2Sn:] (4), [η2(N,N)-Me2Si(DippN)2Pb:]2 (5a), and [η2(N,N)-Ph2Si(DippN)2Pb:] (5b) (Dipp = 2,6-iPr2C6H3) bearing bulky bis(amido)silane ligands were readily prepared either by the transamination of M[N(SiMe3)2]2 (M = Sn, Pb) and [Me2Si(DippNH)2] or by the metathesis reaction of bislithium bis(amido)silane [η1(N),η1(N)-R2Si(DippNLi)2] (R = Me, Ph) with the corresponding metal halides GeCl2(dioxane), SnCl2, and PbCl2, respectively. Preliminary atom-transfer chemistry involving [η2(N,N)-Me2Si(DippN)2Ge:] (3a) with oxygen yielded a dimeric oxo-bridged germanium complex [η2(N,N)-Me2Si(DippN)2Ge(μ-O)]2 (6). All complexes were characterized by 1H, 13C, 119Sn NMR, IR, and elemental analysis. X-ray single crystal diffraction analysis revealed that the metal centres in 3b, 4, and 5b are sterically protected to prevent interaction between the metal centre and the nitrogen donors of adjacent molecules while complex 5a shows a dimeric feature with a strong intermolecular Pb⋯N interaction.

Synthesis and structural characterization of two-coordinate low-valent 14-group metal complexes bearing bulky bis(amido)silane ligands.

2,6-Diisopropylphenylamides of Potassium and Calcium: A Primary Amido Ligand in s-Block Metal Chemistry with an Unprecedented Catalytic Reactivity

The synthesis and coordination chemistry of a series of dianionic bis(amido)silyl and bis(amido)disilyl, [NSiN] and [NSiSiN], chelates with N-bound aryl or sterically modified triarylsilyl (SiAr3) groups is reported. In order to provide a consistent comparison of the steric coverage afforded by each ligand construct, various two-coordinate N-heterocyclic germylene complexes featuring each ligand set were prepared and oxidative S-atom transfer chemistry was explored. In the cases where clean oxidation transpired, sulfido-bridged centrosymmetric germanium(IV) dimers of the general form [LGe(μ-S)]2 (L = bis(amidosilyl) ligands) were obtained in lieu of the target monomeric germanethiones with discrete Ge═S double bonds. These results indicate that the reported chelates possess sufficient conformational flexibility to allow for the dimerization of LGe═S units to occur. Notably, the new triarylsilyl groups (4-RC6H4)3Si— (R = tBu and iPr) still offer considerably expanded degrees of steric coverage relative to ...

Expanding the steric coverage offered by bis(amidosilyl) chelates: isolation of low-coordinate N-heterocyclic germylene complexes.

Several of alkaline-earth-metal complexes [(η2:η2:μ(N):μ(N)-Li)+]2[{η2-Me2Si(DippN)2}2Mg]2– (4), [η2(N,N)-Me2Si(DippN)2Ca·3THF] (5), [η2(N,N)-Me2Si(DippN)2Sr·THF] (6), and [η2(N,N)-Me2Si(DippN)2Ba·4THF] (7) of a bulky bis(amido)silane ligand were readily prepared by the metathesis reaction of alkali-metal bis(amido)silane [Me2Si(DippNLi)2] (Dipp = 2,6-i-Pr2C6H3) and alkaline-earth-metal halides MX2 (M = Mg, X = Br; M = Ca, Sr, Ba, X = I). Alternatively, compounds 5–7 were synthesized either by transamination of M[N(SiMe3)2]2·2THF (M = Ca, Sr, Ba) and [Me2Si(DippNH)2] or by transmetalation of Sn[N(SiMe3)2]2, [Me2Si(DippNH)2], and metallic calcium, strontium, and barium in situ. The metathesis reaction of dilithium bis(amido)silane [Me2Si(DippNLi)2] and magnesium bromide in the presence of oxygen afforded, however, an unusual lithium oxo polyhedral complex {[(DippN(Me2Si)2)(μ-O)(Me2Si)]2(μ-Br)2[(μ3-Li)·THF]4(μ4-O)4(μ3-Li)2} (8) with a square-basket-shaped core Li6Br2O4 bearing a bis(aminolato)silane ligand....

Synthesis and structural characterization of alkaline-earth-metal bis(amido)silane and lithium oxobis(aminolato)silane complexes.

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Synthesis and structural characterization of two-coordinate low-valent 14-group metal complexes bearing bulky bis(amido)silane ligands.

Synthesis and structural characterization of alkaline-earth-metal bis(amido)silane and lithium oxobis(aminolato)silane complexes.