Tetrahedral Centers Research Articles

On the antiperiplanar lone pair hypothesis and its application to catalysis by glycosidases.

The idea that sp3 lone pairs of electrons on a heteroatom direct the departure of a leaving group from an adjacent tetrahedral carbon centre is shown to be untenable as a general principle, and the permissive evidence in favour of it to be accounted for by the principle of least nuclear motion. The application of the idea to the majority of glycopyranosidases, which work by a double-displacement mechanism, is shown to require implausible contortions of the pyranose ring.

Modification of the thermodynamic properties of the electron-transferring groups in mitochondrial succinate dehydrogenase upon binding of succinate.

The redox properties of the covalently-bound flavin and of the tetrahedral iron-sulfur center S1 of succinate dehydrogenase were studied as a function of the binding of different ligands to the enzyme. The midpoint potential of both flavin and S1 increases by some 200 mV when protein binds succinate to a site having Kdsucc = 0.8-1.0 mM, thus different from the substrate binding site. Succinate binding increases the potential of the oxidized flavin/semiquinone half-cell more than that of the semiquinone/reduced flavin one: this results in higher semiquinone formation with increasing succinate. Malonate and fumarate appear to mimic, in this regard, the effect of succinate. The increase in midpoint potential of S1 upon binding of dicarboxylic acid is related to an increase in hydrophobicity of the cluster environment. The possible molecular basis for the modulation of the flavin potential is discussed together with the significance of this shift on the catalytic behaviour of the protein.

Copper(I) and silver(I) ions in unusual poly donor-atom environments; X-ray crystal and molecular structures of [M{( R)( S)-1,2-(5-Me-thiophene-2-CHz.dbnd;N) 2-cyclohexane} 2](O 3SCF 3), M = Cu I or Ag I

Coordination properties of 0.t’ and Cu’ with ligand systems containing N and S donor atoms as potential coordinating sites continue to be of interest as models for the redox active sites in the ‘blue’ proteins [ 1] . The conformation and rigidity of the carbon skeleton connecting the hetero-atoms, as well as the types and number of hetero-atoms in the model ligand systems, are important with respect to both the number of metal atoms which can be bonded and the ultimate configuration at the metal centres. Recently we have shown that the quadridentate N-donor ligands (R)(S)-1 ,2-(6-R-py-2-CH=N)Z-cyclohexane (R = H or Me), in which the inner two N-donor sites are connected by the (R)(S)-1,2cyclohexanediyl ring, act as di-bidentate ligands binding two Cur or Ag’ atoms (see Fig. 1) [2]. The application of INEPT “N and ro9Ag NMR spectroscopy revealed that the structural features found for the silver(I) complex [Ag,{(R@)-1,2(py-2-CH=N)z-cyclohexane),l(03SCF3)2 in the solid, with characteristic alternating two short and two long Ag-N distances around the distorted tetrahedral silver centres [2], are retained in solution [3] . Attempts to form mononuclear Cu’ or Ag’ complexes with these N4donor ligands, using ‘A Curor Agr-to-ligand ratios were unsuccessful. Interpretation of these results requires more information concerning the influence of the donor sites on the coordination properties of these types of potentially quadridentate ligands. For this purpose we have now investigated the bonding features of the N2S, ligand system (R)(S)-1,2-(5-R-thiophene-

INTERACTIONS OF NUCLEOPHILES WITH QUATERNARY PHOSPHONIUM CENTERS °Crystal and Molecular Structure of Benzyltriphenylphosphonium Iodide and Structural Correlation of Related Systems

Abstract The Crystal and molecular structure of benzyltriphenylphosphonium iodide was determined. This salt crystallises in space group P21/c, with a = 9.692(5), b = 21.58(1), c = 11.211(6) Å; β = 107.2(2)°; Z = 4. The structure was refined to a final R value of 0.052. This structure, together with all structures of the quaternary phosphonium salts R4P+ X−, available through the CCDC file, was subject to the correlation treatment, introduced by Dunitz et al. Exclusive “face” orientation of X− with respect to the tetrahedral phosphonium center was demonstrated. Analysis of the secondary interactions of X− with the organic cation indicated that systems studied yield better models for ylid formation (α-hydrogen abstraction) than models for nucleophilic attack at the PIV atom.

Spectroscopic studies of ascorbate oxidase. Electronic structure of the blue copper sites.

Low-temperature electronic absorption and room temperature circular dichroism (CD) and magnetic circular dichroism (MCD) spectra are reported for ascorbate oxidase. Bands attributable to d-d electronic transitions in the type 1 (blue) coppers (2B2 ground state) have been observed at 5800, 10000, and 12000 cm-1 (1725, 1000, and 835 nm). The three bands are assigned to the transitions 2B2 leads to 2E, 2B2 leads to 2B1, and 2B2 leads to 2A1, respectively, in the slightly flattened tetrahedral blue copper centers. The ligand field theoretical treatment predicts a type 1 copper reduction potential of 417 mV. The MCD spectrum of ascorbate oxidase contains one major band at 14000 cm-1 (710 nm) and a very weak feature at 13 200 cm-1 (550 nm). The intensities of the CD and MCD bands are consistent with two or three type 1 coppers in each enzyme molecule, and the near-infrared spectroscopic results suggest that the electronic structures of the blue coppers are closely similar.

Complexes of substituted benzothiazoles 3. Cobalt (II) complexes of the ‘tripod’ ligand tris(2-benzothiazolymethyl)amine

Tetradentate ‘tripod’ ligands are often capable of forcing transition metal ions to adopt a trigonal bipyramidal stereochemistry by virtue of their own geometrical contraints and the bulk of the pendant groups. The tetradentated ‘tripod’ ligand tris(2-benzothiazolimethyl)amine(NTBT) has been found to form only two trigonal bipyramidal cations, [Co(NTBT)X] + (X = Cl,Br), present in the complexes [Co(NTBT)X]BPh 4·(CH 3CN) 2·yH 2O (X = Cl, y = 035, X = Br, y = 0), [Co(NTBT)X][CoX 3(CH 3CN)]- (MeNO 2) 2·1·5EtOH. Coordinated acetonitrile occurs in two of the mixed stereochemistry systems, but rather than occupying an axial site int the trigonal bipyramidal cation it appears to be coordinated at the tetrahedral centre. Pseudo-octahedral structures exist in the solid state in all the other systems investigated; i.e Co(NTBT)Cl 2·2H 2O, Co(NTBT)Br 2·(CH 3CN) 2, [Co(NTBT)(NCS) 2], [Co(NTBT)(NO 3)]NO 3·H 2O, [CO(NTBT)(H 2O) 2](ClO 4) 2. The comple Co(NTBT)Br 2·(CH 3CN) 2, which is octahedral in the solid state, dissociates in solution in nitromethane to form the carion [Co(NTBT)Br] +. Structural assignments are supported by ligand field and ifrared spectra and magnetism and conductance data.

Ru4+ site occupancy in yttrium iron garnet

Ru4+ site occupancy in YIG has been investigated by means of optical absorption and magnetic circular dichroism (MCD). Octahedral Ru4+ was observed and evidence found of a tetrahedral ruthenium center in a stable valence state. The presence of Zn enhances the octahedral site concentration of Ru4+.

Stereochemical Aspects of Optical Resolution of Octahedral Metal Chelates by Liquid Chromatography

Abstract Discrimination of enantiomers of octahedral metal complexes should be done in a mechanism different from that of organic compounds having a tetrahedral asymmetric center. In this respect, the elucidation of the mechanism of optical resolution by chromatography has been one of the most fascinating subjects in coordination chemistry. Since metal complexes usually exist as ions, they are charged on ion-exchange resin and eluted into enantiomers with the eluent containing suitable chiralions(reso1ving agent). While so many chiral metal complexes can be resolved into enantiomers by way of diastereomeric crystals, comparatively few examples of complete resolution by chromatographic techniques have been reported. This is because there are only two resolving agents, d-tartrate and antimony d-tartrate, which are conveniently used for chromatographic separation(1). Here I will describe the outline of works which were done in my laboratory to clarify the mechanism of chromatographic resolution of metal chel...

Spectroscopic and redox studies of some copper(II) complexes with biomimetic donor atoms: implications for protein copper centres

The redox chemistry and e.s.r. spectra of a number of copper(II) complexes with nitrogen- and sulphur-donor ligands have been examined, including complexes with 2,2′-bi-imidazole, histamine, and cyclic and acyclic saturated amine and thioether ligands. The tetrahedral CuIIS4 centre generated by γ-irradiation of single crystals of a thioacetamide complex of CuI has a low value of |A∥|. Tetrahedral distortion of otherwise tetragonal copper(II) centres increases g∥ and decreases |A∥| for N-, S-, and O-donor ligands, and is a more effective source of reduction of |A∥| than charge effects. The quotient g∥/|A∥| appears to be a convenient empirical index of distortion of the donor set from planar toward tetrahedral, indicating that most type I copper in proteins is tetrahedrally, rather than tetragonally, co-ordinated.

Computer graphics determination and display of stereoisomers in coordination compounds

An interactive computer program has been written to determine and display stereoisomers of coordination compounds, which include one trigonal bipyramid or one octahedral stereocenter, and up to six tetrahedral centers. Recognition of isomers is made via a stereodescriptor calculated by means of a naming algorithm.

Intramolecular migration of an amino-group via a transannular process during the reaction of O-salicyloylglycolamides in alkaline solution: an analogue of the reverse of the Brenner aminoacyl insertion reaction

O-Saticyloylglycolamides undergo cleavage in the pH range 7–13 to yield salicylamide and salicylate anion in the high and low ranges respectively. Scanning the u.v: spectrum during reaction gives evidence for the accumulation and decay of intermediates at both ends of the pH range. Intermediates are not observed to accumulate during alkaline hydrolysis of O-benzoylglycolamides and benzamides are also not products. Amine is transferred intramolecularly in the salicyloyl reaction as judged from the absence of ammonia in the reactant solution. We postulate that the initial reaction involves the attack of the 2-oxyanion on the amido-group to yield a seven membered ring tetrahedral adduct: diacylamide and O-saliyloylisoamide intermediates are formed by transannular migration of oxygen or the amino-group from the tetrahedral centre to the ester carbonyl. The formation of the diacylamide is inhibited at low pH but when formed it cleaves to give salicylamide. The reaction of O-saliyloylglycolamides to produce salicylamide is essentially an analogue of the reverse of the Brenner aminoacyl insertion reaction. The driving force for the extrusion of the acid from the salicyloyl compound is probably the hydrolysis of the ester link. The effective molarity for the intramolecular attack of oxyanion on the amide has an estimated upper limit of ca. 108M.

Properties and maps of stereochemical reaction cycles that involve compounds with four ligands attached to a tetrahedral chiral center

Properties and maps of stereochemical reaction cycles that involve compounds with four ligands attached to a tetrahedral chiral center

Spectroscopic Behavior and Coordination of Nickel(II) in Liquid Mixtures of Zinc and Cesium Chlorides

Optical absorption spectra of dilute solutions of NiCl2 in molten ZnCl2–CsCl mixtures were measured over the complete range of solvent compositions. The temperature extremes investigated were 260°C for melts containing 38 mole % CsCl and 1000°C for 75 mole % CsCl. At temperatures within 100–300°C of the liquidus (depending on melt composition) the spectra were highly composition and temperature dependent and consisted of relatively narrow absorption bands. Analysis of these spectra showed the presence under different conditions of six kinds of nickel(II) centers with well-defined coordination geometries. An equilibrium between octahedral and tetrahedral centers occurred in 0–8 mole % CsCl such that the octahedral form was favored with increasing CsCl content. A different octahedral–tetrahedral pair occurred in 20–50 mole % CsCl with the tetrahedral form favored by increasing CsCl content and temperature. In CsCl-rich melts tetrahedrally coordinated Cl3NiClZnCl33− and NiCl42− were found. In ZnCl2-rich melts at high temperatures the absorptions became broad and only mildly dependent on solvent composition. Suggestions are made regarding the relationships between these coordination effects and structure in the solvent.