Cyanide as a copper and quinone-directed inhibitor of amine oxidases from pea seedlings (<Emphasis Type="Italic">Pisum sativum</Emphasis>) and<Emphasis Type="Italic"> Arthrobacter globiformis</Emphasis>: evidence for both copper coordination and cyanohydrin derivatization of the quinone cofactor

The interactions of cyanide with two copper-containing amine oxidases (CuAOs) from pea seedlings (PSAO) and the soil bacterium Arthrobacter globiformis (AGAO) have been investigated by spectroscopic and kinetic techniques. Previously, we rationalized the effects of azide and cyanide for several CuAOs in terms of copper coordination by these exogenous ligands and their effects on the internal redox equilibrium TPQ_amr-Cu(II)TPQ_sq-Cu(I). The mechanism of cyanide inhibition was proposed to occur through complexation to Cu(I), thereby directly competing with O₂ for reoxidation of TPQ. Although cyanide readily and reversibly reacts with quinones, no direct spectroscopic evidence for cyanohydrin derivatization of TPQ has been previously documented for CuAOs. This work describes the first direct spectroscopic evidence, using both model and enzyme systems, for cyanohydrin derivatization of TPQ. K_d values for Cu(II)-CN^– and Cu(I)-CN^–, as well as the K_i for cyanide inhibition versus substrate amine, are reported for PSAO and AGAO. In spite of cyanohydrin derivatization of the TPQ cofactor in these enzymes, the uncompetitive inhibition of amine oxidation is determined to arise almost exclusively through CN^– complexation of Cu(I).Abbreviations AGAO Arthrobacter globiformis amine oxidase - APAO Arthrobacter P1 amine oxidase - APT attached proton test - BPAO bovine plasma amine oxidase - CuAO quinone-copper containing amine oxidase - LTQ lysyl tyrosylquinone - MAO monoamine oxidase - PKAO porcine kidney amine oxidase - PPAO porcine plasma amine oxidase - PSAO pea seedling amine oxidase - TPQ 2,4,5-trihydroxyphenylalaninequinone - TPQ_amr TPQ aminoresorcinol - TPQ_imq TPQ iminoquinone - TPQ_ox TPQ oxidized - TPQ_sq TPQ semiquinone - WT wild-typeE.M. Shepard and G.A. Juda contributed equally to this workThis revised version was published online in February 2004: Hansenula polymorpha was not italicised at the end of the Introduction, Equation 3 appeared twice, and the resolution of Scheme 3 was insufficient.An erratum to this article can be found at     

An unexpected destabilisation of copper(II) phenoxyl radical species by steric protection

The complexes [CuL(Tp^Ph)] (HL = 5-tertbutylsalicylaldehyde, 5-tertbutyl-3-methylsulfanylsalicylaldehyde or 5-tertbutyl-3-phenylsulfanylsalicylaldehyde; [Tp^Ph]⁻ = tris-{3-phenylpyrazolyl}hydridoborate) have been prepared, and adopt square-pyramidal coordination geometries. Each compound exhibits a ligand-based oxidation in CH₂Cl₂ that is chemically reversible by voltammetry. However, Coulometric determinations showed that the resultant phenoxyl radical products decomposed rapidly at low temperatures in bulk solution. This instability may reflect intramolecular steric repulsions between the phenoxide tertbutyl substituents, and a pyrazolylborate phenyl group. These results contrast with a previously reported analogous compound, bearing a 5-methyl-3-methylsulfanylsalicylaldehydato ligand, which yields a phenoxyl radical oxidation product that is stable for hours under the same conditions.     

Synthesis, structure and spectroscopic properties of 2,3-bis(diphenylphosphino)quinoxaline (dppQx) and its copper(I) complexes

Phosphinoquinoxalines were prepared by treatment of 2,3-dichloroquinoxaline (3) with phosphorus nucleophiles. The Arbuzov reaction of 3 with PPh(O-i-Pr)₂ gave a mixture of diastereomers of 2,3-(PPh(O)(O-i-Pr))₂quinoxaline (6); the crystal structure of rac-6 was determined, but attempts at reduction to yield bis(phenylphosphino)quinoxaline 7 resulted in P-C cleavage and formation of phenylphosphine. The bis(secondary phosphine) 7 could be generated from 3 and LiPHPh(BH₃), but was not isolated in pure form. Copper-catalyzed coupling of PHPh₂ with 3 gave 2,3-bis(diphenylphosphino)quinoxaline (4, dppQx), whose coordination chemistry was investigated, with comparison to data for the analogous 1,2-bis(diphenylphosphino)benzene (dppBz) complexes. Reaction of dppQx with [Cu(NCMe)₄][PF₆] gave [Cu(dppQx)₂][PF₆] (8); CuCl yielded [Cu(dppQx)Cl]₂ (9). Reaction of [Cu(NCMe)₄][PF₆] with one equiv of DPEphos, followed by one equiv of dppQx, gave [Cu(dppQx)(DPEphos)][PF₆] (10). Ligand 4 and copper complexes 8 and 9 were crystallographically characterized. The UV-Vis spectra of dppQx and its copper complexes were red-shifted from those of the dppBz analogs; in contrast to results for the dppBz complexes, those of dppQx were not luminescent in solution.     

Involvement of a quinoprotein (PQQ-containing) alcohol dehydrogenase in the degradation of polypropylene glycols by the bacterium Stenotrophomonas maltophilia

Previous work has shown that when the bacterium Stenotrophomonas maltophilia is grown on polypropylene glycol, different dye-linked polypropylene glycol dehydrogenase (PPG-DH) activities are induced during growth. Here the purification and characterization of the dehydrogenase activity induced in the stationary phase, and present in the periplasmic space, is described. The homogeneous enzyme preparation obtained consists of a homodimeric protein with a molecular mass of about 123 kDa and an isoelectric point of 5.9. The cofactor of the enzyme appeared to be pyrroloquinoline quinone (PQQ), no heme c was present, and holo-enzyme contained two PQQ molecules per enzyme molecule. In these respects, PPG-DH described here is similar to already known quinoprotein alcohol dehydrogenases, but in other respects, it is different. Therefore, it is suggested that PPG-DH could be a new type of quinoprotein alcohol dehydrogenase. Based on its strong preference for polyols, PPG-DH seems well fitted to carry out the first step in the degradation of PPGs, synthetic polymers containing a variety of hydroxyl groups.     

Quenching of bathocuproine disulfonate fluorescence by Cu(I) as a basis for copper quantification

In this paper we report the up to now ignored fluorescence properties of the specific Cu(I)-chelator bathocuproine disulfonate and their application in assays of total copper and Cu(I). The method is based on the linear quenching of the bathocuproine disulfonate emission at 770 nm (lambda(ex)580 nm) by increasing concentrations of Cu(I), at pH 7.5. Copper concentrations as low as 0.1 microM can be determined. Other metal ions (iron, manganese, zinc, cadmium, cobalt, nickel) do not interfere. The procedure for total copper determination in proteins includes HCl treatment to release the copper, neutralization to pH 7.5 in the presence of citrate to stabilize the copper, and reduction of the copper to Cu(I) by ascorbate in the presence of the chelator. This assay gave results coincident with the analysis by atomic absorption spectroscopy in two selected proteins. In addition, conditions are described (omitting HCl treatment and reduction by ascorbate) for direct measurement of Cu(I) in native proteins, as illustrated for the Escherichia coli NADH dehydrogenase-2. Data show that the fluorometric assays described in this paper are simple and convenient procedures for total copper and direct Cu(I) quantification in determined biological samples.     

Structure and luminescence of copper(I) cyanide-amine and -sulfide networks

Copper(I) cyanide reacts with various liquid amines and sulfides (L) under solvent-less conditions to form (CuCN)L_n, n = 0.5, 0.57, 0.75, 0.8, 1, 1.25, 1.5, 2. New X-ray structures are reported for L = Py (Py = pyridine, n = 0.57), 2-MePy (n = 1), 3-EtPy (n = 1.5), 2-ClPy (n = 1), 3-ClPy (n = 2), 3-MeOPy (n = 2), 4-^tBuPy (n = 1.5), piperidine (n = 1.25), N-methylmorpholine (n = 1), N,N-dimethylcyclohexylamine (n = 1), 1-methylimidazole (n = 3), Me₂S (n = 1), and tetrahydrothiophene (n = 1). The amine structures (except for the monomeric 1-methylimidazole complex) reveal 1D CuCN chains decorated with 0-2 L per metal atom. Chain structures observed include zigzag, helical and figure-8 helical. The CuCN-sulfide structures show sulfur-bridging of CuCN chains. In some cases (CuCN)L_?1.5 species are transformed to (CuCN)L under vacuum. Thermal analysis shows facile release of ligand, yielding CuCN. Most of the (CuCN)L_n products are photoluminescent, emitting in the visible region. In some cases, coordination of very similar amines results in remarkably different emission spectra.     

The amyloidogenic region of the human prion protein contains a high affinity (Met)2(His)2 Cu(I) binding site

The prion protein (PrP^c) is a cuproprotein implicated in a number of human neurodegenerative diseases. Although many physiological functions have been ascribed to PrP, its potential to act as a neuronal antioxidant, based in part on its copper binding ability, is controversial and unresolved. A number of studies have shown that copper bound to PrP^c is not redox silent, and recent data shows that the Cu(II) sites at histidines 96 and 111 display reversible electrochemistry. Reversible electrochemistry implies redox cycling whilst the metal remains bound and with the absence of permanent oxidation or reduction of the protein. Despite this indirect evidence of Cu(I) binding to PrP, the nature of the Cu(I) binding site/s is unclear, although previous extended X-ray absorption fine structure (EXAFS) data has implicated methionines in the Cu(I) binding site. Using spectroscopic techniques we find that the PrP region encompassing histidines 96 and 111 can bind a Cu(I) ion in a site comprising His 96, His 111, Met 109 and Met 112. The four-coordinate (His)₂(Met)₂ Cu(I) site has a K_d = 10⁻¹⁵–10⁻¹² M indicative of high affinity. Mutation of histidine residues reduces the Cu(I) affinity. Although alluding to the fact the PrP could act in a direct superoxide dismutase-like fashion, the Cu(I)–PrP(91–124) site and affinity is comparable to that observed for bacterial periplasmic Cu(I) transporters.     

EPR spectroscopic study of a dinuclear copper(II) complex of tolfenamic acid

The copper(II) complex with tolfenamic acid [Cu(tolf)₂(H₂O)]₂ was studied by X-band and K-band EPR spectroscopies in the temperature range from 90 to 300 K. The Cu²⁺ ions in dinuclear complex show a strong antiferromagnetic exchange interaction with |J| = 292 cm⁻¹. The EPR spectra, which were observed for [Cu(tolf)₂(H₂O)]₂, are typical powder spectra of the copper pairs. The spectra exhibit the hyperfine structure in low temperature range. The values of the spin-Hamiltonian parameters were determined on the basis of the best fit for the simulated spectra at both K-band (0.75 cm⁻¹) at T = 298 K and X-band (0.3 cm⁻¹) at T = 93 K as compared with the experimentally observed spectra. These values show that the local environment around the copper species is distorted tetragonal pyramid. This EPR evidence is consistent with the crystallographic data.     

Molecular structure and catechol oxidase activity of a new copper(I) complex with sterically crowded monodentate N-donor ligand

The attempted alkylation of 1,3-bis(2′-pyridylimino)isoindoline (indH) by the use of n-BuLi and subsequent alkyl halides led to quaternization of the pyridine nitrogens and the zwitterionic monodentate N-ligand (Me₂ind)I was formed. By the use of the ligand the copper(I) complex [Cu^I(Me₂ind)I₂] was prepared and its structure determined. It was found to be good catalyst for the oxidation of 3,5-di-tert-butylcatechol (DTBCH₂) to 3,5-di-tert-butyl-1,2-benzoquinone (DTBQ) and H₂O₂ by dioxygen. Detailed kinetic studies revealed first-order dependence on the catalyst and dioxygen concentration and saturation type behavior with respect to the substrate.     

Homoleptic copper(II) and copper(I) complexes of 5,6-dihydro-5,6-epoxy-1,10-phenanthroline. Six-coordinate copper(I) in solution

Reaction of 5,6-dihydro-5,6-epoxy-1,10-phenanthroline (L) with Cu(ClO₄)₂·6H₂O in methanol in 3:1 M ratio at room temperature yields light green [CuL₃](ClO₄)₂·H₂O (1). The X-ray crystal structure of the hemi acetonitrile solvate [CuL₃](ClO₄)₂·0.5CH₃CN has been determined which shows Jahn-Teller distortion in the CuN₆ core present in the cation [CuL₃]²⁺. Complex 1 gives an axial EPR spectrum in acetonitrile-toluene glass with g_|| = 2.262 (A_|| = 169 × 10⁻⁴ cm⁻¹) and g_⊥ = 2.069. The Cu(II/I) potential in 1 in CH₂Cl₂ at a glassy carbon electrode is 0.32 V versus NHE. This potential does not change with the addition of extra L in the medium implicating generation of a six-coordinate copper(I) species [CuL₃]⁺ in solution. B3LYP/LanL2DZ calculations show that the six Cu-N bond distances in [CuL₃]⁺ are 2.33, 2.25, 2.32, 2.25, 2.28 and 2.25 Å while the ideal Cu(I)-N bond length in a symmetric Cu(I)N₆ moiety is estimated as 2.25 Å. Reaction of L with Cu(CH₃CN)₄ClO₄ in dehydrated methanol at room temperature even in 4:1 M proportion yields [CuL₂]ClO₄ (2). Its ¹H NMR spectrum indicates that the metal in [CuL₂]⁺ is tetrahedral. The Cu(II/I) potential in 2 is found to be 0.68 V versus NHE in CH₂Cl₂ at a glassy carbon electrode. In presence of excess L, 2 yields the cyclic voltammogram of 1. From ¹H NMR titration, the free energy of binding of L to [CuL₂]⁺ to produce [CuL₃]⁺ in CD₂Cl₂ at 298 K is estimated as −11.7 (±0.2) kJ mol⁻¹.     

Studies on galactose oxidase active site model complexes: effects of ring substituents on Cu(II)-phenoxyl radical formation

Copper(II) complexes of new N₃O- and N₂O₂-donor tripodal ligands bearing one or two o-substituted phenol moieties have been synthesized as models for the galactose oxidase active site. The complexes of 2-[N-(1-methyl-2′-imidazolylmethyl)-N-(6″-methyl-2″-pyridylmethyl)-aminomethyl)]-4-methyl-6-methylthiophenol (MeSL), [Cu(MeSL)Cl], and N-(6-methyl-2-pyridylmethyl)-N,N-bis(2′-hydroxy-3′,5′-di-tert-butylbenzyl)amine (t-buL2mepy), [Cu(t-buL2mepy)(H₂O)], have been revealed by X-ray structural analysis to have a square-pyramidal structure with one and two phenolate oxygens in the basal plane, respectively. [Cu(MeSL)Cl] was converted into a Cu(II)-o-methylthiophenoxyl radical species by electrochemical or Ce(IV) oxidation. An o-methoxyphenoxyl radical in a similar complex was considerably more stable than the 2,4-di(tert-butyl)phenoxyl radical. While t-buL2mepy reacted with Cu(ClO₄)₂ to give [Cu(t-buL2mepy)(H₂O)] without disproportionation, an N2O2-donor ligand containing an o-methoxyphenol, a 2,4-di(tert-butyl)phenol, and an N-methylimidazole moiety gave a phenoxyl radical complex exhibiting the characteristic absorption peak at 478 nm as a reddish powder by the reaction with Cu(ClO₄)₂ as a result of spontaneous disproportionation. It exhibited a quasi-reversible redox wave at E_1/2=0.34 V (vs. Ag/AgCl) in CH₃CN, which is lower than the potentials of the copper complexes of various N₃O-donor ligands, and oxidized ethanol to acetaldehyde with a low turnover number.     

Structural variations and spectroscopic properties of copper(I) complexes with bis(schiff base) ligands

Six copper(I) complexes {[Cu₂(L1)(PPh₃)₂I₂] · 2CH₂Cl₂}_n (1), {[Cu₂(L2)(PPh₃)₂]BF₄}_n (2), [Cu₂(L3)(PPh₃)₄I₂] · 2CH₂Cl₂ (3), [Cu₂(L4)(PPh₃)₄I₂] (4), [Cu₂(L5)(PPh₃)₂I₂] (5) and [Cu₂(L6)(PPh₃)₂I₂] (6) have been prepared by reactions of bis(schiff base) ligands: pyridine-4-carbaldehyde azine (L1), 1,2-bis(4′-pyridylmethyleneamino)ethane (L2), pyridine-3-carbaldehyde azine (L3), 1,2-bis(3′-pyridylmethyleneamino)ethane (L4), pyridine-2-carbaldehyde azine (L5), 1,2-bis(2′-pyridylmethyleneamino)ethane (L6) with PPh₃ and copper(I) salt, respectively. Ligand L1 or L2 links (PPh₃)₂Cu₂(μ-I)₂ units to form an infinite coordination polymer chain. Ligand 3 or 4 acts as a monodentate ligand to coordinate two copper(I) atoms yielding a dimer. Ligand 5 or 6 chelates two copper(I) atoms using pyridyl nitrogen and imine nitrogen to form a dimer. Complexes 1-4 exhibit photoluminescence in the solid state at room temperature. The emission has been attributed to be intraligand π-π* transition mixed with MLCT characters.     

Crystal structure and catalytic activity of a copper(II) complex based on a tetradentate bis-benzimidazole diamide ligand

A dimeric copper(II) complex bridged via a new tetra dentate bis benzimidazole diamide ligand [N,N′-bis(benzimidazolyl-2-yl)(methyl)pentane diamide](GBGA) with the composition [Cu₂(GBGA)₂(NO₃)₂](NO₃)₂ has been isolated and characterized. The X-ray structure of the above complex reveals that the unit cell consists of two centrosymmetric, crystallographically independent molecules, but differing in the coordination mode of ion. In one case ion is symmetric bidentate while in the other case it is monodentate. The coordination around Cu(II) is either a trigonally distorted octahedron (where the N2–O2 equatorial plane is formed by two benzimidazole N atoms and two carbonyl O atoms) or a distorted square pyramidal. The copper(II) complex carries out the selective oxidation of cinnamyl alcohol (allylic), geraniol (aliphatic-allylic) and 3-pyridyl carbinol (hetero aryl alcohol) to their respective aldehydes in the presence of tertiary butyl hydroperoxide as an alternative source of oxygen. The catalytic efficiency has been found to be much higher for the analogous copper(II) complex formed with the corresponding N-octylated ligand (O-GBGA). The percentage yield of the products viz geranial, cinnamyl aldehyde and 3-pyridyl carbinal varies between 34% and 57%. While the respective turnovers are 13-, 19- and 32-fold with respect to the copper(II) catalyst. A higher turnover in the case of 3-pyridyl carbinol is due to the transformation of the parent Cu(II) catalyst (having a N2–O2 type equatorial plane) to a more active Cu(II) species which have been shown to have a 4N donor equatorial plane as identified by low temperature EPR spectroscopy. Such a switch from a carbonyl O donor to an amine N donor of the peptidic link in the ligand may be important for the redox functioning of copper(II) bound to small peptides.     

Site-specific interactions of copper(II) ions with heparin revealed with complementary (SRCD, NMR, FTIR and EPR) spectroscopic techniques

The interactions between Cu(II) ions and heparin were investigated using several complementary spectroscopic techniques. NMR indicated an initial binding phase involving specific coordination to four points in the structure that recur in slightly different environments throughout the heparin chain; the carboxylic acid group and the ring oxygen of iduronate-2-O-sulfate, the glycosidic oxygen between this residue and the adjacent (towards the reducing end) glucosamine and the 6-O-sulfate group. In contrast, the later binding phase showed little structural specificity. One- and two-dimensional correlated FTIR revealed that complex out of phase (asynchronous) conformational changes also occurred during the titration of Cu(II) ions into heparin, involving the CO and N-H stretches. EPR demonstrated that the environments of the Cu(II) ions in the initial binding phase were tetragonal (with slightly varied geometry), while the later non-specific phases exhibited conventional coordination. Visible spectroscopy confirmed a shift of the absorbance maximum. Titration of Cu(II) ions into a solution of heparin indicated (both by analysis of FTIR and EPR spectra) that the initial binding phase was complete by 15-20 Cu(II) ions per chain; thereafter the ions bound in the non-specific mode. Hetero-correlation spectroscopy (FTIR-CD) improved resolution and assisted assignment of the broad CD features from the FTIR spectra and indicated both in-phase and more complex out of phase (synchronous and asynchronous, respectively) changes in interactions within the heparin molecule during the titration of Cu(II) ions.     

Investigation of the solution behavior of tri-n-butylphosphine copper(I) and silver(I) acetates using P{H} NMR spectroscopy

³¹P{¹H} NMR spectra of metal-organic [(ⁿBu₃P)_mMO₂CMe] (M = Cu, Ag; m = 1, 1.5, 2, 2.5, 3, 3.5, 4) have been studied in the temperature range of 308-178 K. Exchange parameters were determined for the appropriate silver(I) complexes. Possible ligand exchange mechanisms based on dissociative and associative processes are discussed.     

Redox properties of an engineered purple Cu(A) azurin

Purple Cu(A) centers are a class of binuclear, mixed-valence copper complexes found in cytochrome c oxidase and nitrous oxide reductase. An engineered Cu(A) protein was formed by replacing a portion of the amino acid sequence that contains three of the ligands to the native type I copper center of Pseudomonas aeruginosa azurin with the corresponding portion of sequence from the Cu(A) center of cytochrome c oxidase from Paracoccus denitrificans [Proc. Natl. Acad. Sci. USA 93 (1996) 461]. Oxidation-reduction midpoint potential (E(m)) values of the Cu(A) azurin of +399+/-10 and +380+/-2mV, respectively, were determined by cyclic voltammetry and spectrochemical titration. An n value of one was obtained, indicating that the redox reaction is cycling between the mixed valence and the fully reduced states. Whereas the E(m) value of native azurin is pH dependent, the E(m) value of Cu(A) azurin is not, as expected for the Cu(A) center. Similarities and differences in the redox properties are discussed in terms of the known crystal structures of Cu(A) centers in cytochrome c oxidase and Cu(A) azurin.     

Synthesis, spectroscopic and X-ray characterization of a copper(II) complex with the Schiff base derived from pyridoxal and aminoguanidine: NMR spectral studies of the ligand

A copper(II) complex with the pyridoxal-aminoguanidine (PL-AG) Schiff base adduct, as an organic compound of the very potent biological activity and promising pharmacological importance in the treatment of diabetic complications, has been prepared and characterized. The X-ray structural analysis of the [CuCl2(PL-AG)] complex showed that it has a distorted pseudo-square-pyramidal (4+1) structure with the tridentate ONN Schiff base in the equatorial plane, with the Cu-O(1), Cu-N(1) and Cu-N(3) bond lengths of 1.917(2)A, 1.930(2)A and 1.984(2)A, respectively. The bond length of the equatorial Cu-Cl(1) is 2.279(1)A, while that of the apical Cu-Cl(2) is 2.792(1)A. Pyridoxal fragment is coordinated in its zwitterionic form. In addition to the X-ray structural analysis, the complex was characterized by IR spectrometric, conductometric and magnetic techniques, and the ligand itself by IR, 1H and 13C NMR spectra.     

Structure-dependent spectroscopic and redox properties of copper(I) complexes with bidentate iminopyridine ligands

A series of iminopyridine ligands; cyclopropylpyridin-2-ylmethyleneamine (A), cyclopentylpyridin-2-ylmethyleneamine (B), cyclohexylpyridin-2-ylmethyleneamine (C), and cycloheptylpyridin-2-ylmethyleneamine, (D) and their copper(I) complexes, [Cu(L)₂]⁺ (1a-1d) and [Cu(L)(PPh₃)₂]⁺ (2a-2d) have been synthesized and characterized by CHN analyses, ¹H NMR and IR and UV-Vis spectroscopy. Structures of 1a, 1b, 1c and 2a were determined by X-ray crystallography. The coordination polyhedron about the Cu^I center in the complexes is best described as a distorted tetrahedron. The dihedral angles between the least-squares planes of the chelate ligands show considerable variation from 86.1° in 1a to 68.3° in 1b, indicating the importance of packing forces in the crystalline environment. The UV-Vis spectra of the complexes are characterized by first metal to ligand charge transfer bands increasing in wavelength with increasing size of the ring substituents in the ligands, except for the cyclopropyl compounds (1a and 2a), in good agreement with the variation of the dihedral angles between the ligand planes. Cyclic voltammetry of the complexes indicates a quasireversible redox behavior for the complexes. The bulkier ligands (PPh₃) inhibit the geometric distortion within the oxidized form and the redox potentials of complexes 2a-2d are shifted to more positive values, therefore.     

Copper(I) and silver(I) tertiary phosphines complexes: Synthesis, X-ray structures and spectroscopic characterization

Six new complexes, [Cu₄I₄(PPh₂Cy)₄]·2H₂O (1), [CuI(PPhCy₂)₂] (2), [CuCl(PPhCy₂)₂] (3), and [CuBr(PPh₃)₃]·CH₃CN (4), [Ag(PPhCy₂)₂(NO₃)] (5), [Ag(PCy₃)(NO₃)]₂ (6) [where Ph = phenyl, Cy = cyclohexyl], have been synthesized and structurally characterized by X-ray diffraction, IR absorption spectra and NMR spectroscopic studies (except complex 4). The X-ray diffraction analysis of complex (1), pseudo polymorph of complex [Cu₄I₄(PPh₂Cy)₄], reveals a stella quadrangula structure. The four corners of the cube are occupied by copper(I) atoms and four I atoms are present at the alternative corners of the cube, further more the copper(I) atoms are coordinated to a monodentate tertiary phosphine. Complexes (2) and (3) are isostructural with trigonal planar geometry around the copper(I) atom. The crystal structure of complex (4) is a pseudo polymorph of complex [CuBr(PPh₃)₃] and the geometrical environment around the copper(I) centre is distorted tetrahedral. In the Ag^I complexes (5) and (6), the central metal atoms have pseudo tetrahedral and trigonal planar geometry, respectively. Spectroscopic and microanalysis results are consistent with the single crystal X-ray diffraction studies.     

Oxazolone copper(I) complexes inspired by the methanobactin active site

Two oxazolone-derived potential ligands with enethioether substituents have been synthesized that differ by the terminal thioether moiety (S-Et in L¹, S-C₆H₄(OMe)-2 in L²). Both L¹ and L² behave as bidentate {NS} chelate ligands to form stable complexes with copper(I) triflate that crystallize as dimeric complexes [L₂Cu₂(OTf)₂] (4 and 5) featuring a central {Cu₂S₂} diamond core with distinctly different Cu-S bonds. L¹ as well as 4 and 5 have been characterized by single crystal X-ray diffraction. NMR spectroscopy including ¹H and ¹⁹F DOSY experiments reveals that 4 and 5 dissociate into monomeric species [LCu(OTf)] (4′ and 5′) in CDCl₃ solutions. 4′ and 5′ retain the {NS} binding motif of the oxazolone-derived ligands, but are in slow equilibrium with their {OS} isomers 4″ and 5″ that result from E/Z isomerization of the exocyclic enethioether double bond.