S. M. Ibrahim Al-Rafia

Stabilization of the heavy methylene analogues, GeH2 and SnH2, within the coordination sphere of a transition metal.

The heavy group 14 methylene analogues, EH2, (E = Ge and Sn) have been stabilized via efficient methods, thus enabling the chemistry of these novel inorganic hydrides to be explored in depth.

Preparation of Stable Low-Oxidation-State Group 14 Element Amidohydrides and Hydride-Mediated Ring-Expansion Chemistry of N-Heterocyclic Carbenes

Various low oxidation state (+2) group 14 element amidohydride adducts, IPr⋅EH(BH(3))NHDipp (E=Si or Ge; IPr=[(HCNDipp)(2)C:], Dipp=2,6-iPr(2)C(6)H(3)), were synthesized. Thermolysis of the reported adducts was investigated as a potential route to Si- and Ge-based clusters; however, unexpected transmetallation chemistry occurred to yield the carbene-borane adduct, IPr⋅BH(2)NHDipp. When a solution of IPr⋅BH(2)NHDipp in toluene was heated to 100 °C, a rare C-N bond-activation/ring-expansion reaction involving the bound N-heterocyclic carbene donor (IPr) transpired.

Low-coordinate germylene and stannylene heterocycles featuring sterically tunable bis(amido)silyl ligands.

A series of monomeric heterocyclic metallylenes [{(i)Pr(2)Si(NR)(2)}M:] (M = Ge and Sn; R = Dipp = 2,6-(i)Pr(2)C(6)H(3) or SiPh(3)) have been prepared. Preliminary atom-transfer chemistry involving the new low-valent germylenes with the chalcogen sources Me(3)NO and S(8) yielded the corresponding dimeric oxo- and sulfido complexes (e.g., [{(i)Pr(2)Si(NDipp)(2)}Ge(μ-E)](2); E = O and S). Structural analyses of the metallylenes and their oxidized products reveal that incorporation of the umbrella-shaped triarylsilyl groups (SiPh(3)) within the NSiN chelate confers additional steric protection about the group 14 centers relative to a Dipp group. The inclusion of sterically modifiable -SiAr(3) (Ar = aryl) units as part of a bis(amido) ligand array represents a new approach in this field and holds considerable promise with regard to attaining increasingly higher degrees of steric bulk.

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

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 (SiAr(3)) 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-RC(6)H(4))(3)Si- (R = (t)Bu and (i)Pr) still offer considerably expanded degrees of steric coverage relative to the parent congener, -SiPh(3,) and thus the use of substituted triarylsilyl groups within ligand design strategies should be a generally useful concept in advancing low-coordination main group and transition-metal chemistry.

Synthesis and Mössbauer Spectroscopy of Formal Tin(II) Dichloride and Dihydride Species Supported by Lewis Acids and Bases

(119)Sn Mössbauer spectroscopy was performed on a series of formal Sn(II) dichloride and dihydride adducts bound by either carbon- or phosphorus-based electron pair donors. Upon binding electron-withdrawing metal pentacarbonyl units to the tin centers in LB·SnCl2·M(CO)5 (LB = Lewis base; M = Cr or W), a significant decrease in isomer shift (IS) was noted relative to the unbound Sn(II) complexes, LB·SnCl2, consistent with removal of nonbonding s-electron density from tin upon forming Sn-M linkages (M = Cr and W). Interestingly, when the nature of the Lewis base in the series LB·SnCl2·W(CO)5 was altered, very little change in the IS values was noted, implying that the LB-Sn bonds were constructed with tin-based orbitals of large p-character (as supported by prior theoretical studies). In addition, variable temperature Mössbauer measurements were used to determine the mean displacement of the tin atoms in the solid state, a parameter that can be correlated with the degree of covalent bonding involving tin in these species.

Preparation and Structures of Group 12 and 14 Element Halide–Carbene Complexes

The synthesis of a series of N-heterocyclic carbene (NHC) complexes involving zinc, cadmium, and the heavy Group 14 elements germanium, tin, and lead is reported. The direct reaction between the bulky carbene IPr (IPr = (HCNDipp)2C:, Dipp = 2,6-iPr2C6H3) and the Group 14 halide reagents GeCl4 and SnCl4 afforded the 1 : 1 complexes IPr·ECl4 (E = Ge and Sn) in high yield; similarly, ZnI2 interacted with IPr in THF to give the THF-bound complex IPr·ZnI2·THF. CdCl2 underwent divergent chemistry with IPr and the major product isolated was the imidazolium salt [IPrH][IPr·CdCl3], which could be converted into IPr·CdCl2·THF upon treatment with Tl[OTf]. In addition, the stable PbII amide adduct, IPr·PbBr(NHDipp), was prepared. Each of the new carbene–element halide adducts was treated with the hydride sources Li[BH4] and Li[HBEt3] in order to potentially access new element hydride adducts and/or clusters. In most instances scission of the element–carbene bonds transpired, except in the case of IPr·ZnI2·THF, which reacted with two equivalents of Li[BH4] to yield the thermally stable bis(borohydride) zinc complex IPr·Zn(BH4)2.