C. David Garner

Oligomerization in As (III) sulfide solutions: Theoretical constraints and spectroscopic evidence

Bond distances, vibrational frequencies, gas-phase energetics, and proton affinities for various thioarsenite molecules and ions are predicted from molecular orbital theory and used to interpret EXAFS and Raman spectra of dissolved thioarsenites in undersaturated, alkaline 1 M NaHS solutions. From MO predictions, Raman peaks at 325 and 412 cm(-1) are assigned to AsS(SH)(2)(-) and a peak at 382 cm(-1) to AsS2(SH)(2-). At alkaline pH, As-S distances in dissolved thioarsenites are 2.21-2.23 Angstrom and no statistically significant As-As interactions are recorded, consistent with predominance of the monomers, AsS(SH)(2)(-) and AsS2(SH)(2-). Estimated proton affinities suggest that thioarsenites with a negative charge greater than 2 are unstable in water. In seeming contradiction to this spectroscopic evidence, a new analysis of published solubility studies reinforces previous inferences that the trimer, As3S4(SH)(2)(-), is the predominant thioarsenite in systems saturated with As2S3. Previously proposed dimeric species of the form, HxAs2S4x-2, are rejected based on predicted thermodynamic properties. Dimer plus tetramer combinations also are rejected. Estimated free energies for AsS(OH)(SH)(-) and AsS(SH)(2)(-) are presented. We reconcile the spectroscopic and solubility evidence by showing that in undersaturated solutions monomers can become thermodynamically favored over oligomers. This pattern should be looked for in other sulfide systems as well. Sulfidic natural waters are in many cases undersaturated with respect to As2S3 phases, so monomeric thioarsenites could be more important in nature than the trimers that have been characterized in saturated solutions. EXAFS spectra show that amorphous As2S3 resembles orpiment in the first shell around As, but that higher shells are disordered. Disorder may be caused by occasional realgar-like, As-As bonds, consistent with the observation that amorphous As2S3 is slightly S deficient.

Surface coordination chemistry: corrosion inhibition by tetranuclear cluster formation of iron with salicylaldoxime.

A tetranuclear iron cluster is the principal component of the purple coatings produced by treating a mild steel surface with a salicylaldoxime corrosion inhibitor. This was shown by comparison of the spectroscopic data with those of the cluster [{Fe(salH)(HsalH)}4 ], which was obtained from FeCl3 and salicylaldoxime (H2 salH) and has a distorted tetrahedral arrangement of Fe(III) atoms coordinated by terminal (1-) and bridging (2-) salicylaldoximate ligands (the central core of the cluster is depicted).

A phenol–imidazole pro-ligand that can exist as a phenoxyl radical, alone and when complexed to copper(II) and zinc(II)

A new N,O-bidentate, phenol–imidazole pro-ligand 2′-(4′,6′-di-tert-butylhydroxyphenyl)-4,5-diphenyl imidazole (LH) has been designed, synthesised, and characterised. LH possesses no readily oxidisable position (other than the phenol) and involves o- and p-substituents on the phenol ring that prevent radical coupling reactions. LH undergoes a reversible one-electron oxidation to generate the corresponding [LH]˙+ radical cation that possesses phenoxyl radical character. The unusual reversibility of the [LH]/[LH]˙+ redox couple is attributed, at least in part, to a stabilisation of [LH]˙+ by intramolecular O–H⋯N hydrogen bonding. The compounds [CuL2] (1) and [ZnL2] (2) have been synthesised and characterised structurally, spectroscopically, and electrochemically. The crystal structures of 1·4DMF, 1·3MeOH, and 2·2.5MeCN·0.3CH2Cl2 have been determined and each shown to possess an N2O2-coordination sphere, the geometry of which varies with the nature of the metal and the nature of the co-crystallised solvent. 1 and 2 each undergo two, reversible, ligand-based, one-electron oxidations, to form, firstly, [M(L)(L˙)]+ and secondly [M(L˙)2]2+. The [M(L)(L˙)]+ (M = Cu, Zn) cations have been generated by both electrochemical and chemical oxidation and their [ML2][BF4] salts isolated as air-stable, dark green, crystalline solids. The UV/vis, EPR, and magnetic characteristics of these compounds are consistent with each cation involving an MII (M = Cu or Zn) centre bound to a phenoxide (L−) and a phenoxyl radical (L˙). The structural information obtained by a determination of the crystal structures of [CuL2][BF4]·2CH2Cl2 and [ZnL2][BF4]·2CH2Cl2·0.75pentane fully supports this interpretation. For each of these salts, there is a clear indication that the coordinated phenoxyl radical is involved in intramolecular π–π stacking interactions that parallel those in galactose oxidase.

Multinuclearity of aqueous copper and zinc bisulfide complexes: An EXAFS investigation

Copper and zinc bisulfide complexes in NaHS solutions were investigated by Extended X-ray Absorption Fine-Structure spectroscopy (EXAFS). A goal was to investigate the possibility that bisulfide complexes are multinuclear. Copper and zinc K-edge absorption spectra were recorded in fluorescence mode at room temperature and at 80 K. Lowering temperature improved signal to noise ratios, but did not significantly change measured metal-sulfur distances or coordination numbers. In equilibrium with S-absent assemblages in the Cu-S system, dissolved copper consisted of colorless Cu(I) species having Cu-S interatomic distances of 2.30-2.33 A (coord. no. 3 ± 1 ). Second shell Cu atoms were present at 2.72-2.75 A ( coord. no. 1.1). The presence of second shell Cu atoms, which was confirmed by duplicate experiments using different starting materials, demonstrates that the predominant complexes in these solutions are multinuclear. The Cu-Cu distance is shorter than in most known Cu-S cluster complexes and Cu-S minerals, but is in good agreement with known Cu-Cu distances in Cu4(RS)2−6 clusters. In equilibrium with a CuS + S assemblage, shorter Cu-S distances and no evidence of an ordered second shell were observed. In these polysulfide-rich solutions, different Cu complexes apparently predominate. Solutions saturated with sphalerite or amorphous ZnS precipitate yielded Zn-S and Zn-Zn distances similar to those in solid sphalerite, although the Zn-Zn coordination number was lower and second shell S was also present. In the Zn case, but not the Cu case, it is possible that the species being characterized by EXAFS were <200 A particles not in reversible equilibrium with the solution phase. EXAFS is a promising tool for determining molecular structures of complexes of geochemical interest, but at present its application is limited to solutions containing somewhat more than 1 mM of the target element.

Phenoxyl radicals: H-bonded and coordinated to Cu( ii ) and Zn( ii )

Two pro-ligands ((R)LH) comprised of an o,p-di-tert-butyl-substituted phenol covalently bonded to a benzimidazole ((Bz)LH) or a 4,5-di-p-methoxyphenyl substituted imidazole ((PhOMe)LH), have been structurally characterised. Each possesses an intramolecular O-H[dot dot dot]N hydrogen bond between the phenolic O-H group and an imidazole nitrogen atom and (1)H NMR studies show that this bond is retained in solution. Each (R)LH undergoes an electrochemically reversible, one-electron, oxidation to form the [(R)LH] (+) radical cation that is considered to be stabilised by an intramolecular O...H-N hydrogen bond. The (R)LH pro-ligands react with M(BF(4))(2).H(2)O (M = Cu or Zn) in the presence of Et(3)N to form the corresponding [M((R)L)(2)] compound. [Cu((Bz)L)(2)] (), [Cu((PhOMe)L)(2)] (), [Zn((Bz)L)(2)] and [Zn((PhOMe)L)(2)] have been isolated and the structures of .4MeCN, .2MeOH, .2MeCN and .2MeCN determined by X-ray crystallography. In each compound the metal possesses an N(2)O(2)-coordination sphere: in .4MeCN and .2MeOH the {CuN(2)O(2)} centre has a distorted square planar geometry; in .2MeCN and .2MeCN the {ZnN(2)O(2)} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both and , in CH(2)Cl(2)-DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(ii) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)((R)L)((R)L )](+), comprising a M(ii) centre bound to a phenoxide ((R)L) and a phenoxyl radical ((R)L ) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF(4)]. The second oxidation produces [M(II)((R)L )(2)](2+). The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [(R)LH] (+) (R = Bz or PhOMe), or by coordination to a metal, as in [M(II)((R)L)((R)L )](+) (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu(ii) is slightly more effective than Zn(II) in this respect.

14N-coordination to VO2+ in reduced vanadium bromoperoxidase, an electron spin echo study

Vanadium bromoperoxidase from the brown seaweed Ascophyllum nodosum was studied with electron spin echo envelope modulation (ESEEM) spectroscopy. After comparing the Fourier transformed (FT) ESEEM spectra with those of a number of vanadyl model compounds, it could be concluded that nitrogen is present in the equatorial plane of the vanadyl cation of reduced bromoperoxidase (14N frequencies occurred at 3.1, 4.2, 5.3 and 8.1 MHz). Furthermore, the FT‐ESEEM spectra of reduced bromoperoxidase exhibited an intense 1H modulation (13.8 MHz), which was completely replaced by a deuterium modulation at ∼2 MHz when bromoperoxidase was dissolved in D2O, instead of H2O. These latter data confirm earlier EPR experiments on reduced bromoperoxidase [(1988) Biochemistry 27, 1629–1635], showing that the oxo‐vanadium (IV) ion is coupled to exchangeable protons.

The nature of the metalmetal interaction in tetra-μ-carboxylatochromium(II) systems

Abstract The results of an ab initio molecular orbital calculation for Cr 2 (O 2 CH) 4 ,2H 2 O are presented. They indicate that the eight chromium d electrons, corresponding to a d 4 configuration of each metal atom are accommodated in the metalmetal σ, δ, δ * and σ * orbitals to give no net bonding over the CrCr separation of 2.362(1) A. The He(I) photoelectron spectrum for Cr 2 (O 2 CCH 3 ) 4 is described and interpreted with the aid of ΔSCF calculations.

Correlation effects and the nature of the metalmetal bond in DI-chromium and DI-molybdenum complexes

Abstract Calculations, both at the SCF level and including correlation effects have been performed for Cr4+2, Mo4+2, [Cr2(CH38] 4−, and Cr2(O2CH)4 · 2H2O. For Mo4+2 the SCF ground state corresponds to the usual σ2 π4 δ2 quadruple bond description, whilst for Cr4+2 and the dichromium(II) complexes, the ground state for the metalmetal interaction is the σ2δ2σ*2 δ*2 no-bond configuration. Correlation effects, which allow bonding configurations of higher energy to mix into the ground state, are seen to provide a novel interpretation of the metalmetal interactions in these complexes.

Investigations of Amavadin

The development of the understanding of the co-ordination chemistry and the properties of Amavadin, the chemical form in which vanadium is accumulated by the Amanita genus of mushrooms, is reviewed.

4-(2,2-dimethyldioxalan-4-yl)-5-(quinoxalin-2-yl)-1,3-dithiol-2-one, a proligand relating to the cofactor of the oxomolybdoenzymes

Abstract The coupling of 2-iodoquinoxaline to 4-(2,2-dimethyl-1,3-dioxolan-4-yl)-5-(tri-n-butylstannyl)-1,3-dithiol-2-one5 gave 4-(2,2-dimethyl-1,3-dioxolan-4-yl)-5-(quinoxalin-2-yl)-1,3-dithiol-2-one 4.