James T. Goettel

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.

Syntheses and Structures of Xenon Trioxide Alkylnitrile Adducts

The potent oxidizer and highly shock-sensitive binary noble-gas oxide XeO3 interacts with CH3 CN and CH3 CH2 CN to form O3 XeNCCH3 , O3 Xe(NCCH3 )2 , O3 XeNCCH2 CH3 , and O3 Xe(NCCH2 CH3 )2 . Their low-temperature single-crystal X-ray structures show that the xenon atoms are consistently coordinated to three donor atoms, which results in pseudo-octahedral environments around the xenon atoms. The adduct series provides the first examples of a neutral xenon oxide bound to nitrogen bases. Raman frequency shifts and Xe-N bond lengths are consistent with complex formation. Energy-minimized gas-phase geometries and vibrational frequencies were obtained for the model compounds O3 Xe(NCCH3 )n (n=1-3) and O3 Xe(NCCH3 )n ⋅[O3 Xe(NCCH3 )2 ]2 (n=1, 2). Natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM), electron localization function (ELF), and molecular electrostatic potential surface (MEPS) analyses were carried out to further probe the nature of the bonding in these adducts.

Hydrogen-Bonding Interactions in T-2 Toxin Studied Using Solution and Solid-State NMR

The structure of T-2 toxin in the solid-state is limited to X-ray crystallographic studies, which lack sufficient resolution to provide direct evidence for hydrogen-bonding interactions. Furthermore, its solution-structure, despite extensive Nuclear Magnetic Resonance (NMR) studies, has provided little insight into its hydrogen-bonding behavior, thus far. Hydrogen-bonding interactions are often an important part of biological activity. In order to study these interactions, the structure of T-2 toxin was compared in both the solution- and solid-state using NMR Spectroscopy. It was determined that the solution- and solid-state structure differ dramatically, as indicated by differences in their carbon chemical shifts, these observations are further supported by solution proton spectral parameters and exchange behavior. The slow chemical exchange process and cross-relaxation dynamics with water observed between the hydroxyl hydrogen on C-3 and water supports the existence of a preferential hydrogen bonding interaction on the opposite side of the molecule from the epoxide ring, which is known to be essential for trichothecene toxicity. This result implies that these hydrogen-bonding interactions could play an important role in the biological function of T-2 toxin and posits towards a possible interaction for the trichothecene class of toxins and the ribosome. These findings clearly illustrate the importance of utilizing solid-state NMR for the study of biological compounds, and suggest that a more detailed study of this whole class of toxins, namely trichothecenes, should be pursued using this methodology.

Stable Chloro- and Bromoxenate Cage Anions; [X3(XeO3)3]3– and [X4(XeO3)4]4– (X = Cl or Br)

The number of isolable compounds which contain different noble-gas-element bonds is limited for xenon and even more so for krypton. Examples of Xe-Cl bonds are rare, and prior to this work, no Xe-Br bonded compound had been isolated in macroscopic quantities. The syntheses, isolation, and characterization of the first compounds to contain Xe-Br bonds and their chlorine analogues are described in the present work. The reactions of XeO3 with [N(CH3)4]Br and [N(C2H5)4]Br have provided two bromoxenate salts, [N(C2H5)4]3[Br3(XeO3)3] and [N(CH3)4]4[Br4(XeO3)4], in which the cage anions have Xe-Br bond lengths that range from 3.0838(3) to 3.3181(8) Å. The isostructural chloroxenate anions (Xe-Cl bond lengths, 2.9316(2) to 3.101(4) Å) were synthesized by analogy with their bromine analogues. The bromo- and chloroxenate salts are stable in the atmosphere at room temperature and were characterized in the solid state by Raman spectroscopy and low-temperature single-crystal X-ray diffraction, and in the gas phase by quantum-chemical calculations. They are the only known examples of cage anions that contain a noble-gas element. The Xe-Br and Xe-Cl bonds are very weakly covalent and can be viewed as σ-hole interactions, similar to those encountered in halogen bonding. However, the halogen atoms in these cases are valence electron lone pair donors, and the σ*Xe-O orbitals are lone pair acceptors.

Ultraviolet Photolysis Studies on XeO4 in Noble‐Gas and F2 Matrices and the Formation and Characterization of a New XeVIII Oxide, (η2‐O2)XeO3

The photolytic behavior of the thermochemically unstable xenon(VIII) oxide XeO4 was investigated by UV irradiation in noble-gas and F2 matrices. Photolysis of Xe(16) O4 or Xe(18) O4 in noble-gas matrices at 365 nm yielded XeO3 and a new xenon(VIII) oxide, namely, (η(2) -O2 )XeO3 , which, along with XeO4 , was characterized by matrix-isolation IR spectroscopy and quantum-chemical calculations. Calculations of the UV spectrum showed that the photodecomposition is induced by an n→σ* transition, but the nature of the excitation differs when different light sources are used. There is strong evidence for the formation of mobile (1) D excited O atoms in the case of excitation at 365 nm, which led to the formation of (η(2) -O2 )XeO3 by reaction with XeO4 . Matrix-isolation IR spectroscopy in Ne and Ar matrices afforded the natural-abundance xenon isotopic pattern for the ν3 (T2 ) stretching mode of Xe(16) O4 , and (18) O enrichment provided the (16) O/(18) O isotopic shifts of XeO4 and (η(2) -O2 )XeO3 .