A pyridine bridged dicarbene ligand and its silver(I) and palladium(II) complexes: synthesis, structures, and catalytic applications

The potentially tridentate ligand 2,6-bis[(3-methylimidazolium-1-yl)methyl]pyridine dibromide reacts readily with silver(I) oxide in dichloromethane or dimethylsulfoxide to give a dinuclear silver(I)-carbene complex that was isolated as the tetrafluoroborate salt. Single crystal X-ray crystallography shows that each silver(I) ion is bridged by two ligands bonding through the carbene donors. Treatment of the silver(I) complex with suitable palladium(II) precursors gave the complexes PdCl[(CNC)]BF₄ and [PdMe(CNC)]BF₄ (CNC=2,6-bis[(3-methylimidazolin-2-yliden-1-yl)methyl]pyridine), in which the pyridyl and both carbene moieties are coordinated to a single palladium(II). The palladium(II) complexes have been fully characterised, including X-ray crystallography, and exhibit good activities in the Heck coupling reaction of 4-bromoacetophenone and n-butyl acrylate.     

Coordination network solids based on Cu(II) coordination compounds with 1,4-bis-(1,2,4-triazol-1-yl)-butane as a flexible alkyl spacer ligand: Synthesis, characterization and X-ray structures

The coordination chemistry of three selected copper(II) salts with the flexible ligand 1,4-bis(1,2,4-triazol-1-yl)butane (abbreviated as btb) is described. This ligand acts as a bidentate ligand, bridging copper(II) ions, thereby generating polymers in 2D and 3D network solids.     

Hydrogen binding in coordination compounds of 3(5)-(4-methoxyphenyl)pyrazole

The solid state structures of 3(5)-(4-methoxyphenyl)pyrazole and its coordination compounds with a series of two valent transition metals have been investigated. Since pyrazoles provide not only a nitrogen donor site for the coordination to metal ions, but also an additional N–H function, they are ideal ligands for the formation of hydrogen bound coordination polymers or for the implementation of secondary interactions with other ligands bound to the same central ion, resulting in a rigid ligand environment at the central metal. We chose cobalt, nickel, palladium, copper and zinc as twofold positively charged Lewis acids preferring coordination numbers of four and six to prove the capability of pyrazole to undergo intramolecular hydrogen bonds. In the four-coordinate mode, either tetrahedral (Zn²⁺) or square planar coordination geometries (Pd²⁺) are possible, providing different geometric restrictions for hydrogen bonding.     

Electrospray mass spectrometric investigation of the relative ligand properties of EPh3 ligands (E=P, As, Sb or Bi) towards Ag and Cu

Electrospray mass spectrometry (ESMS) has been used to investigate the relative ligand properties of the triphenylpnictogen ligands EPh₃ (E=P, As, Sb and Bi) towards silver(I) and copper(I) ions. It is found that the preferred species formed increase in coordination number from two for PPh₃ in [Ag(PPh₃)₂]⁺ to four for SbPh₃ in [Ag(SbPh₃)₄]⁺, consistent with the decreasing donor ligand ability and increasing metal –E bond length in the series PPh₃–AsPh₃–SbPh₃. With BiPh_3, the spectra were complex, suggesting considerable decomposition. These studies also suggest that silver(I) and copper(I) ions will have widespread utility in the characterisation of tertiary stibine ligands, as has been described previously for phosphines and arsines. These studies demonstrate the power of the ESMS technique in determining the donor properties of a related series of ligands, and this information is of significance in coordination chemistry.     

113Cd-NMR investigation of a cadmium-substituted copper, zinc-containing superoxide dismutase from yeast

113Cd nuclear magnetic resonance spectroscopy has been used to investigate the metal binding sites of cadmium-substituted copper, zinc-containing superoxide dismutase from baker's yeast. NMR signals were obtained for 113Cd(II) at the Cu site as well as for 113Cd(II) at the Zn site. The two subunits in the dimeric enzyme were found to have identical coordination properties towards 113Cd(II) at the Zn site when no copper is coordinated at the Cu site, and when Cu(I) or Cd(II) is coordinated, were found to be very small indicating that 113Cd(II) must be bound to the same number and type of ligands in both cases. Furthermore, the spectra show that the rate of exchange of protein-bound 113Cd(II) and free 113Cd2+ is slow on the NMR time scale also at the Cu site. The present study suggests an explanation for the discrepancy in the literature regarding 113Cd-NMR investigations of bovine superoxide dismutase.     

Preparation, crystal structure and properties of a novel microporous Cu coordination polymer with 6-quinolinecarboxylate

A microporous coordination polymer formulated as {[Cu(L)₂] · (DMF)₂}_n (1) has been prepared by the direct reaction of copper nitrate with 6-quinolinecarboxylic acid (HL) in DMF. X-ray single crystal diffraction of 1 reveals that the [Cu₂(COO)₄] secondary building units are interconnected by the bridging L ligands to generate a layered framework with the terminal L ligands as lateral pendants at both sides. Furthermore, the unusual inserted integration of the coordination layers, regulated and fixed by interlayer aromatic stacking interactions between the terminal ligands, leads to the formation of a novel 3-D microporous crystalline lattice with different 1-D channels along three directions. The gas adsorption and magnetic character of this crystalline material have also been investigated.     

Synthesis and physic-chemical properties of a copper(II) complex with 2-(2-pyridyl)iminotetrahydro-1,3-thiazine hydrochloride-water (1/2) (PyTzHCl.2H2O). Crystal structure of PyTz and [[CuCl(PyTz)]2(mu-Cl)2

The synthesis of 2-(2-pyridyl)iminotetrahydro-1,3-thiazine (PyTz) has been carried out, as well as the determination of its X-ray crystal structure, together with the coordination behaviour and equilibra study of PyTzHCl.2H2O with copper(II) in aqueous solution at 298 K and 0.1 M ionic strength in NaClO4. The formation constants are determined and discussed in terms of the characteristics of the ligand. The compound Di-mu-chloro-bis[chloro[2-(2-pyrydil-kappaN)amino-5,6-dihydro-4H-1,3-thiazine-kappaN]copper] has been isolated and its crystal and molecular structure determined by X-ray analysis. The structure consists of dimeric molecules [Cu2Cl4L2], in which copper ions are bridged by two chloro ligands. The geometry about each copper approximates to a distorted square pyramid with the bridging ligands occupying apical and equatorial sites of each copper ion, while the PyTz ligand and the remaining chloride ion are located in an equatorial plane. The compound was also characterized through elemental analysis, magnetic susceptibility, electron paramagnetic resonance, and electronic and infrared spectroscopies.     

Copper (II) complexes with Ac-HXH-NHMe (X=Gly, Ala and Aib) peptide motifs: influence of increasing CH(3) groups at C(alpha) of residue X on the coordination in solution

The interaction of Cu(II) ion with small peptides has been an interesting subject to clarify the role of copper in detail. As various Cu(II)-oligopeptide complexes can also be good models for the active centers of metalloenzymes, complexes of tripeptide and tetrapeptides are frequently investigated instead of the complexes of large peptides. The histidine side-chains of various metalloproteins frequently take part in the copper(II) coordination. Accordingly, we studied the coordination of Cu(II) to the N and C terminal protected tripeptide ligands L(A) (Ac-HisGlyHis-NHMe), L(B) (Ac-HisAlaHis-NHMe) and L(C) (Ac-HisAibHis-NHMe) in aqueous solution potentiometrially in order to determine the effect of C(alpha) methyl groups at middle residue acid on the ligation of the backbone NH and also on histidine's N(im) of coordination. Species distribution curves indicates that in acidic pH, all three peptides behave as bidentate ligands and a macrochelate forms on the metal coordination with the two histidine imidazolyl N. This coordination remains unaffected with the +I effect of increasing CH(3) groups at C(alpha) of middle residue. In the pH range 4-8, the tridentate coordination from the peptide is seen in ligand L(A) and L(B) while it is absent in L(C) due to +I effect of two C(alpha) methyl groups at middle residue as they makes N-terminal NH deprotonation difficult in this pH range and it takes place along with C terminal NH and only 4N coordinated species formed at higher pH. These 4N (N(im), N(-), N(-), N(im)) coordinated species are formed by all the three ligands at higher pH values.     

Cobalt, nickel, copper and cadmium coordination polymers containing the bis(1,2,4-triazolyl)methane ligand

Co(II), Ni(II), Cu(II) and Cd(II) coordination polymers containing the flexible ditopic bis(1,2,4-triazol-1-yl)methane ligand (Btm) have been prepared by reaction of equimolar quantities of the corresponding cobalt, nickel, copper and cadmium salts in EtOH solution. Structure solution and refinement of polycrystalline materials were performed by powder diffraction technique (XRPD), using conventional laboratory data. The results show that architecturally different coordination polymers were obtained depending on the counter-ion employed. The XRPD results show also that non-covalent interactions are driving forces for the occurrence of different structures.     

Design and DNA-binding of metallo-supramolecular cylinders conjugated to peptides

Di-nuclear metallo-supramolecular “cylinders”, based on bis-pyridylimine ligands, are end-functionalised with short peptides. The design and synthesis of one tetra-cationic triple-stranded (iron(II)) and three di-cationic double-stranded (copper(I) or silver(I)) cylinder-peptide conjugates are described. DNA-binding experiments, using circular and linear dichroism spectroscopies, confirm the binding and indicate that the iron(II) complexes cause DNA to bend or coil. Artificial nuclease activity by the copper(I) complexes is demonstrated by gel electrophoresis studies.     

Inhibition of the catecholase activity of biomimetic dinuclear copper complexes by kojic acid

The inhibition of the catechol oxidase activity exhibited by three dinuclear copper(II) complexes, derived from different diaminotetrabenzimidazole ligands, by kojic acid [5-hydroxy-2-(hydroxymethyl)-γ-pyrone] has been studied. The catalytic mechanism of the catecholase reaction proceeds in two steps and for both of these inhibition by kojic acid is of competitive type. The inhibitor binds strongly to the dicopper(II) complex in the first step and to the dicopper-dioxygen adduct in the second step, preventing in both cases the binding of the catechol substrate. Binding studies of kojic acid to the dinuclear copper(II) complexes and a series of mononuclear analogs, carried out spectrophotometrically and by NMR, enable us to propose that the inhibitor acts as a bridging ligand between the metal centers in the dicopper(II) catalysts. Received: 23 August 1999 / Accepted: 20 January 2000     

Copper(II) compounds of some rigid dinucleating bis-amine-bis-imidazole ligands

The Mannich condensation of formaldehyde with some amines and imidazoles rendered three bis-(amine-imidazole) ligands in a simple one pot reaction. Although these ligands are formally dinucleating, the copper(II) ions are held far apart, as shown by the Xray structures of five copper(II) compounds obtained with these ligands. Consequentially, these copper(II) compounds behave magnetically mononuclear.     

Synthesis and structural analysis of copper(II) cysteine complexes

This report describes the synthesis and structural analysis of stable copper(II) cysteine complexes. Pale pink copper(II) cysteine complexes were synthesized in mole ratios of 1:2, 1:4, and 1:6 of copper(II):cysteine in ethanol. Infrared spectroscopy and X-ray absorption spectroscopy confirmed that copper(II) binding occurred via the thiol ligand of cysteine. XANES analysis showed that the oxidation state of copper remained as copper(II) and the local atomic geometry was similar in all of the cysteine complexes. The EXAFS data indicate that the copper(II) cysteine complexes are forming ring type structures with sulfur ligands from the cysteines acting as bridging ligands. X-ray diffraction revealed that the copper(II) cysteine complexes formed monoclinic cells with maximum crystallinity found in the 1:4 copper(II):cysteine complex.     

Structural preferences and thermal stability of complexes with the exo-bidentate ligand 1,2-bis(2-methylimidazol-1-ylmethyl)benzene

We describe a series of new coordination polymers of Cd(II), Co(II) and Ag(I) with 1,2-bis(2-methylimidazol-1-ylmethyl)benzene. All complexes were characterized by single crystal X-ray diffraction which reveals polymeric bridging of metal centers by the ligand in all cases. The cadmium center in complex 1 has a slightly irregular octahedral geometry involving two Cl⁻ ions and four N atoms from individual ligands, resulting in the formation of undulated (4,4) layers. In complex 2 the cobalt(II) ion is coordinated by two Cl⁻ ions and two N atoms from separate ligands. This yields a slightly irregular tetrahedral coordination environment around the metal center and the formation of a 1D zigzag-chain structure. Each of the three Ag(I) complexes (3-5) forms an infinite 1D chain. These three complexes are similar both in conformation and packing mode despite modification of the counterions. The size of the counterion appears to affect the thermal stabilities of the resulting networks.     

Syntheses, structures and vibrational spectroscopy of some 1:1 and 1:2 adducts of silver(I) oxyanion salts with 2,2′-bis(pyridine) chelates

Syntheses and room-temperature single-crystal X-ray structure determinations are recorded for a number of adducts of 1:1 stoichiometry of silver(I) oxyanion salts (perchlorate, nitrate, trifluoroacetate (‘tfa’) (increasing basicity)) with 2,2′-bis(pyridine) ligands (2,2′-bipyridyl, ‘bpy’; 2,2′-biquinolyl, ‘bq’; 2,2′-dipyridylketone, ‘dpk’; 2,9-dimethylphenanthroline, ‘dmp’). The adducts take two forms: (a) neutral mononuclear molecules, in which the 2,2′-bis(pyridine) ligand behaves as a chelate, with the silver coordination number dependent on the denticity of the anion; these are Agtfa:bpy (1:1) and AgClO₄:bq (1:1) (and various (ionic) acetonitrile or pyridine solvates AgClO₄:bq/dmp:MeCN/py (1:1:1), in which the solvent molecules are coordinated); and (b) one-dimensional polymers. The latter are diverse: in AgClO₄:bpy, dpk (1:1), the anion is discrete, the polymer made up of an array of two-coordinate silver atoms linked by bpy ligands twisted about their central connecting element. In AgNO₃:bpy, bq (1:1), the bpy ligands are chelating with the oxyanions bridging, cf. previously reported AgNO₃:dpk (1:1), in which the nitrate chelates the metal, with the dpk bridging, chelating N,O to one silver, while the other nitrogen bridges to the next. With Agtfa, a novel binuclear adduct has been isolated in conjunction with the hydrated ligand, Agtfa:dpk:(dpk · H₂O) (1:1:2). The far-IR spectra of several of these complexes show bands that can be assigned to the ν(AgN) modes, the positions of these bands correlating well with the relative Ag-N bond lengths.Syntheses and single-crystal X-ray structural characterizations are also reported for various adducts of silver(I) perchlorate, nitrate and trifluoromethanesulfonate with bpy, bq, ‘phen’ (= 1,10-phenanthroline), and ‘dmp’, of stoichiometry AgX:L (1:2). In each case the complex is ionic [AgL₂]X; the silver atom is four-coordinate, but diverse and remarkable variations in stereochemistries associated with changes in the interligand N-Ag-N angles, presumably influenced by the different packing arrangements, are observed.     

Complexes of copper and gold with phosphorus-sulfur bidentate ligands

Copper(II) salts react with the bidentate hybrid ligands 1-thiophenyl-2-diphenylphosphinoethane (PSPh) and 1-thioethyl-2-diphehylphosphinoethane (PSEt) to yield copper(I) and/of copper(II) complexes of composition [Cu(PSR)₂]Y andCu- (OPSR)₂] Y₂(Y=C10₄, BF₄; OPSR=phosphinooxide of the PSR ligand). A mixed-valence species cu″?(OPSEt)₂Y₂·Cu′(PSR)₂Y has been isolated. The interaction of the PSR ligands with silver(I) and gold(III) salts has been also studied and gold(I) derivatives of the type [AuX(PSR)] have been isolated.     

One-dimensional zinc(II) polymers with 4,6-dimethyl-1,2,3-triazolo[4,5-d]pyrimidin-5,7-dionato built by bipyridyl-based ligands acting as spacers both in the first and in the second sphere

Three solids formed by the reaction of the hexaaquazinc(II) salt of 4,6-dimethyl-1,2,3-triazolo[4,5-d]pyrimidin-5,7-diona (Hdmax) and the potentially bridging ligands 1,2-bis(4-pyridyl)ethane (bpe), 1,3-bis(4-pyridyl)propane (bpp) and trans-1,2-bis(4-pyridyl)ethene (bpethe) are described. The crystal structure of those containing bpe and bpethe has been solved by X-ray diffraction turning out to be one-dimensional polymers with zigzag (bpe) or linear (bpethe) disposition of the metal atoms. The triazolopyrimidinato anion is in both cases monodentately coordinated through the N atom furthest from the pyrimidine ring whereas the bipyridyl ligands bridge through their N atoms consecutive metal atoms in the polymer. For the bpethe compound, an additional non-coordinated molecule of the spacer per Zn atom is present, accepting H-bonds from coordinated water molecules and linking in this way the one-dimensional chains into a two-dimensional superstructure. Therefore, bpethe acts as spacer both in the first and second coordination spheres.     

Sugar hydrazone-metal complexes: transition- and non-transition metal complexes of monosaccharide S-alkylhydrazonecarbodithioates and dehydro-L-ascorbic acid bis(S-alkylhydrazonecarbodithioates)

Copper(II), nickel(II) and palladium(II) complexes with aldehydo-D-arabinose-, L-arabinose-, D-galactose-, D-glucose- and D-mannose- S-methyl- and S-benzylhydrazonecarbodithioates were synthesized and characterized by elemental analyses, infrared and UV-Vis. In these complexes the aldehydo sugar hydrazone acts as a mononegative NS bidentate ligand. The reaction of Cu(II) chloride, however, proceeded with reduction, and copper(I) complexes were isolated. The hydrazone molecule in these Cu(I) complexes acts as neutral NS bidentate ligand. Dehydro-L-ascorbic acid bis(S-methylhydrazinecarbodithioate) and bis(hydrazinecarbothioamide), as well as their corresponding Cu(II), Ni(II), zinc(II) and Pd(II) complexes were prepared and characterized. Electrospray (ES) and field desorption (FD) mass spectra suggest that the Cu(II), Ni(II), and Pd(II) complexes are monomeric (square planar), whereas the Zn(II) are dimeric and pentacoordinate.     

Copper(I) and copper(II) complexes of biologically relevant tridentate ligands

The preparation of a series of tridentate ligands of formulae X(CH₂C₇H₅N₂)₂ (X = NH, S, O, S₂) is described. The ligands contain two benzimidazole moieties and one of NH, S, O, or S₂ as the donor groups. Cu(I) and Cu(II) complexes of these ligands are prepared and characterized. Spectroscopic and X-ray data imply that the geometric constraints of these ligands impose a distorted coordination geometry at copper. The implications and relevance of this chemistry to copper proteins is discussed.     

Ligand dependence in the copper-catalyzed oxidation of hydroquinones

Transition metal-mediated oxidation of hydroquinones is an important physiologic reaction, and copper(II) effectively catalyzes the reaction in phosphate-buffered saline (PBS). Studies reported herein in phosphate buffer alone demonstrate that copper(II) is an ineffective catalyst in the absence of coordinating ligands, but that 1,10-phenanthroline and histamine facilitate the copper(II)-mediated oxidation of hydroquinone and its 2,5- and 2,6-di-tert-butyl analogs to the corresponding benzoquinones. The high concentration of chloride in PBS is the key element that allows copper(II) to work in this system. Although the bis-bathocuproine disulfonate complex of Cu(II), (BC)₂Cu(II), is a strong stoichiometric oxidant, stoichiometric amounts of copper(II) in the presence of ligands other than BC oxidize hydroquinones very slowly under anaerobic conditions. Thus, the rapid copper(II)-catalyzed reaction operating aerobically does not involve a simple ping-pong reduction of copper(II) to copper(I) by hydroquinone and reoxidation of copper(I) by O₂.