Main Group Chemistry

The bis(thioether)silanes Me₂Si(CH₂SR)₂ (abbreviated Bts^R, where R = Me or Prⁱ), the first two members of a new family of bidentate thioether ligands, have been readily prepared and fully characterized. The tin(IV) tetrahalide derivatives (Bts^R)SnX₄ (X = Cl, Br), which comprise the first four metal coordination complexes of these new sulfur-donor ligands, have also been isolated, and the X-ray structure of the tetrabromide derivative (Bts^Me)SnBr₄ confirms the chelating nature of the dithioether.

The organosulfur compounds allicin, methionine and methylcysteine protect against metal-mediated oxidative DNA damage, but few studies have determined the antioxidant behaviour of the oxo-sulfur derivatives of these compounds. Gel electrophoresis experiments were performed to determine the ability of MetSO, MeCysSO, MMTS, MePhSO and Me₂SO₂ to inhibit copper- and iron-mediated DNA damage. Under these conditions, MetSO and MeCysSO significantly inhibit DNA damage, MePhSO and Me₂SO₂ have no effect and MMTS promotes DNA damage. For iron-mediated DNA damage, significantly less antioxidant or pro-oxidant behaviour is observed for these compounds. To determine whether metal coordination is a mechanism for the antioxidant activity of these oxo-sulfur compounds, UV–vis spectroscopy and gel electrophoresis experiments using [Cu(bipy)₂]⁺ or [Fe(EDTA)]²⁻ as the metal source were also performed. Results of these experiments indicate that metal coordination is a significant factor for their antioxidant activity, but another mechanism also contributes to their antioxidant behaviour.

The coordination chemistry of divalent zinc triad metal ions with N-(2-pyridylmethyl)-N-(2-(methylthio)ethyl)amine (L) was investigated by X-ray crystallography and proton NMR. Chloride salts yielded non-homologous M(L)Cl₂ complexes. Hg(L)Cl₂ and Zn(L)Cl₂ were five-coordinate monomers with distorted square pyramidal and trigonal bipyramidal geometries, respectively. Cd(L)Cl₂ formed a six-coordinate polymer with a distorted octahedral geometry. These complexes had similar nearly temperature independent proton NMR spectra suggesting structural differences between them were less extensive in solution due to fluxional processes. Although 1:1 metal to ligand complexes were most thermodynamically stable for the chloride salts at all [M²⁺]/[L], the thermodynamic stability of [M(L)(NCCH₃)_x]²⁺ only exceeded that of [M(L)₂]²⁺ when [M²⁺] ≥ [L] with perchlorate salts. Slow intramolecular and intermolecular exchange conditions for [M(L)₂]²⁺ and slow intermolecular exchange conditions for [M(L)(NCCH₃)_x]²⁺ were found on the proton chemical shift and in some cases J(M¹H) time scales. Strong interactions between divalent zinc triad metal ions and thioether ligand components at physiologically relevant temperatures suggest interactions with methionine should be considered in developing a more complete understanding of the bioactivities of these metal ions.

The use of benzene-1,3-diamidoethanethiol as a covalent surface coating for the prevention of metal leaching was demonstrated with several sulfide minerals including cinnabar (HgS), pyrite (FeS₂), chalcopyrite (CuFeS₂), covellite (CuS), galena (PbS), realgar (As₄S₄) and sphalerite (ZnS). The minerals were coated with sufficient H₂BDT to bind the surface metals in a 1:1 ratio. Leaching at pH 1, 3 and 7 was then conducted on both treated and untreated minerals. ICP and CVAFS (for mercury) analyses revealed that the coated minerals showed a dramatic reduction in metal leaching as compared to uncoated control samples. X-ray photoelectron spectroscopy indicated the formation of covalent bonds between the sulphur of the ligand and the metals from the minerals.

Ab initio and density-functional theory (DFT) modeling have proven to be important tools in the determination of the properties and reactivity of selenium with respect to biological activity. In this review, we address recent applications of quantum chemistry in three areas of interest to selenium chemistry: theoretical ⁷⁷Se chemical shifts, analysis of Se···N,O interactions important to redox chemistry and mechanistic determinations for selenoenzymes and small selenium molecules. High-performance computing and DFT have allowed for large-scale calculations of both the enzyme active site and solution-phase reactivity. The latter development is important for understanding the complex mechanisms of small molecule GPx mimics. Application of solvent-assisted proton exchange to the redox scavenging mechanism of PhSeH by our research group is highlighted.

The first selective palladium-catalyzed phenylselenylation of aryl bromides using a tin-free phenylselenolate source is reported. High selectivity for the desired asymmetric diaryl selenide is achieved using a catalyst derived from palladium and a Josiphos-type ligand with (PhSe)₂/NaBH₄ as the phenylselenylating agent. Aryl bromides are phenylselenylated at 100°C using low catalyst loadings to give the desired asymmetric diaryl selenides in high yield and >95% selectivity. Phenylselenoborane adducts formed by the reduction of diphenyl diselenide with sodium borohydride are more selective phenylselenylating reagents than sodium phenylselenolate.

Phosphine-substituted oligothiophenes are interesting third-order nonlinear materials. Both their linear and nonlinear optical properties can be altered by varying the group attached to the nonbonding pair of electrons on the phosphorus atoms, the number of phosphine groups attached to the oligothiophene and the number of repeat units in the oligothiophene. In this article, we report the syntheses, multinuclear NMR spectroscopic characterizations and X-ray crystal structures for three derivatives of 5,5′-bis(diphenylphosphino)-2,2′-bithiophene, Ph₂(X)P(C₄H₂S)₂P(X)Ph₂ (X = −, 1) in which X is O (2), S (3) and CH₃⁺ · I⁻ (4). The X-ray crystal structures exhibit significantly different π–π stacking, suggesting that these compounds could have significantly different nonlinear optical properties in the solid state.

Using phosphoryl chloride as a starting material, new 1,3,2-diazaphosphorinanes with formula 4-X-C₆H₄NHP(O)[NHCH₂C(Y)₂CH₂NH] (1–6) where [X = F (1), Cl (2), Br (3), I (4), OCH₃ (5); Y = CH₃] and [X = OCH₃, Y = H (6)] were synthesized and characterized by ¹H, ¹³C, ³¹P NMR, IR spectroscopy and elemental analysis. The structures of compounds 1, 2, 5 and 6 have been determined by X-ray crystallography. Compound 1 exists as two symmetrically independent molecules in the crystalline lattice. The endocyclic nitrogen atoms in compounds 1, 1′ and 6 are very close to planar, but in 2 and 5 are distorted from planarity. These structures form two-dimensional polymeric chains through –P=O···H–N– hydrogen bonds. ¹H NMR spectra of compounds 1–5 indicate high value ³J(PNCH) coupling constants about 25.0 Hz which is related to H_equatorial with P–N–C–H torsion angle near to 180° obtained from X-ray crystallography. Ab initio quantum chemical calculations were performed with Gaussian 98 program at HF level of theory to optimize the structures of compounds 1, 2, 5 and 6. The stabilization energy of a molecule highly increases when the level of the basis set increases. Also, the two conformers 1 and 1′ have exactly the same stabilization energies, bond lengths and angles in the gas-phase calculations and they only show differences in the sign of their related torsion angles.

The synthesis, spectroscopic data and crystal structures of two new mononuclear Pd(II) heteroleptic complexes with the crown trithioether 1,4,7-trithiacyclononane (9S3) and the arsine ligand, triphenylarsine (AsPh₃) are presented. Reaction between [Pd(9S3)Cl₂] and AsPh₃ (1:2 stoichiometry) in nitromethane followed by metathesis to the hexafluorophosphate salt results in the formation of two different palladium(II) complexes, the expected bis arsine complex [Pd(9S3)(AsPh₃)₂](PF₆)₂ as well as, surprisingly, the intermediate mono arsine-chloro complex [Pd(9S3)(AsPh₃)(Cl)](PF₆). Both complexes exhibit the anticipated structure consisting of a cis square planar array of two sulfur atoms from the 9S3 and either two As donors (bis AsPh₃ complex) or one As and one Cl donor (mono AsPh₃ complex). The third 9S3 sulfur atom exhibits a long distance interaction with the palladium forming an elongated square pyramidal geometry with a coordination environment around the Pd best described as [S₂As₂ + S₁] or [S₂AsCl + S₁]. Structural, spectroscopic, and reactivity differences are observed between the arsine and related phosphine complexes due to the poorer donor qualities of AsPh₃.

The synthesis and characterization of three new complexes, BiCl₃(mipit)₂, BiCl₃(emit)₂ and BiCl₃(mnpit)₂ are reported where emit = 1-ethyl-3-methyl-2(3H)-imidazolethione; mnpit = 1-methyl-3-(1-propyl)-2(3H)-imidazolethione; mipit = 1-methyl-3-(2-propyl)-2(3H)-imidazolethione. X-ray crystallographic results are reported for the mnpit and mipit complexes whereas the structure of the emit complex is under reinvestigation because of disorder. BiCl₃(mnpit)₂ crystallizes in a triclinic space group (P1) with lattice parameters: a = 9.4223(6) Å, b = 10.6275(6) Å, c = 12.8860(8) Å, α = 108.329(1)°, β = 90.388(1)°, γ = 115.200(1)°, V = 1093(1) ų, Z = 2. BiCl₃(mipit)₂ crystallizes in a triclinic space group (P1) with lattice parameters: a = 10.4347(8) Å, b = 11.0018(9) Å, c = 11.2075(9) Å, α = 95.827(1)°, β = 104.890(1)°, γ = 117.064(1)°, V = 1071(1) ų, Z = 2. Both complexes show the same pattern with two BiL₂Cl₄ (L = ligand) octahedra sharing a common edge through bridging chlorine atoms and the ligands occupying trans positions perpendicular to the plane made by the two bismuth atoms and six chlorine atoms. No strong evidence for a hemidirected lone pair is observed in either of the two complexes.