Willem L. Driessen

Homogeneous bio-inspired copper-catalyzed oxidation reactions

The controlled oxidation of C–H bonds is one of the most challenging and difficult reactions in organic chemistry. Generally, it requires either stoichiometric amounts of toxic heavy metal salts or very expensive catalysts containing transition metals such as palladium, rhodium or ruthenium. The scientific community used to focus their investigations towards these relatively rare and costly elements while neglecting to look at how Nature performs these types of reactions. Biological systems only employ abundantly available metals like iron, zinc and copper. This review summarizes the background and the state of the art of enzymatic and biomimetic oxidation catalysts involving copper as the active metal center. Recent developments have shown the first very promising results in this incipient field.

New polydentate and polynucleating N-donor ligands from amines and 2,4,6-trichloro-1,3,5-triazine

An efficient synthesis of a novel class of multidentate polynucleating ligands has been developed based on high-yielding chloride substitutions of 2,4,6-trichloro-1,3,5-triazine by primary and secondary amines.

Unidentate versus symmetrically and unsymmetrically bidentate nitrate co-ordination in pyrazole-containing chelates. The crystal and molecular structures of (nitrato-O)[tris(3,5-dimethylpyrazol-1-ylmethyl)amine]copper(II) nitrate, (nitrato-O,O′)[tris(3,5-dimethylpyrazol-1-ylmethyl)amine]nickel(II) nitrate, and (nitrato-O)(nitrato-O,O′)[tris(3,5-dimethylpyrazol-1-ylmethyl)amine]cadmium(II)

The crystal structures of three compounds ML(NO3)2 are described, where M = Cu, Ni, or Cd and L is the tripodal quadridentate ligand tris(3,5-dimethylpyrazol-1-ylmethyl)amine. The structure of the copper compound can best be described as tetragonally distorted trigonal bipyramidal. As in the other compounds, the ligand L utilizes each of its four potential donor sites. One nitrate ion is unidentate, yielding a CuN4O chromophore. In the nickel compound a nitrate ion is symmetrically bidentate, yielding a distorted octahedral cis-NiN4O2 chromophore. In the cadmium compound one of the nitrate ions is unsymmetrically bidentate, the other symmetrically bidentate, yielding a CdN4O3 chromophore. The structure is best described as distorted bicapped octahedral. The nitrate co-ordination modes were investigated using several spectroscopic techniques. Criteria to differentiate between the unidentate, unsymmetrically bidentate, and symmetrically bidentate modes are presented. In this type of compound discrimination between the co-ordination modes solely on the basis of spectroscopic behaviour appears not to be possible. Slight changes in the i.r. spectra of CuL(NO3)2, and of the analogous cobalt and zinc compounds, upon applying pressure can be understood in terms of changes in the nitrate co-ordination.

An unprecedented 1D ladder coordination polymer based on a pentanuclear copper(ii) 2,4,6-tris(dipyridin-2-ylamino)-1,3,5-triazine building blockElectronic supplementary information (ESI) available: additional characterization data for 1 (µeff vs. T and B curves), TGA, XRD. See http://www.rsc.org/suppdata/cc/b2/b203394a/

Copper(II) nitrate reacts with the rigid polydentate triple-connecting dpyatriz ligand in acetonitrile to an unprecedented infinite molecular ladder in which five-coordinated copper pseudo-dimer are bridged by nitrate anions and the coordination polymer chains are linked by hexacoordinated copper ions leading to the formation of large guest cavities.

Preferred uptake of Cu(II) and Cd(II) by novel pyrazole-functionalized chelating polymers

Abstract The synthesis and characterization of two novel pyrazole-functionalized chelating polymers are reported. Functionalization of p-aminomethyl substituted poly(styrene-co-divinylbenzene) was effected by treatment with 3,5-dimethyl-1-hydroxymethylpyrazole (product: LDp) and 1-hydroxymethylpyrazole (product: LPz), respectively. For LDp a chelating group with one dimethylpyrazole per amine nitrogen resulted; in the case of LPz both amine hydrogen atoms were substituted by pyrazole groups. The capacities for Cu2+ and for Cd2+ and their distribution coefficients appeared to be substantially larger than those for Ca2+, Co2+, Ni2+ and Zn2+ for both chelating polymers in the pH range 3.5–5.5. These results were confirmed by competitive experiments between pairs of metal ions. Regeneration (stripping) of Cu2+-loaded LPz with strong mineral acids resulted in the decomposition of the chelating ligand. In contrast, the capacity of LDp was unchanged after several cycles of consecutive loading (with Cu2+) and stripping. Sulphuric acid (0.5 M) was the most suitable stripping reagent.

Selective extraction of metal ions by azathiacrown ether-modified polar polymers

Abstract The metal-ion uptake behaviour of two chelating ion exchange resins has been investigated. A 16-membered azathiacrown ether, 7-aza-1,4,10,13-tetrathiacyclohexadecane ([16]aneS 4 N), has been immobilised on poly(glycidyl methacrylate- co -ethylene glycol dimethacrylate), p(GMA-O) and on its corresponding sulfur analogue, p(GMA-S). A lariat ether moiety on the polymer is formed after ring-opening of the oxirane or thiirane ring, respectively. The hydrophilic resin p(GMA-O)-[16]aneS 4 N shows a very high capacity and selectivity towards Ag + ions in the presence of Cu 2+ , Zn 2+ and Cd 2+ ions. The Ag + ion uptake of the ring-opened thiirane backbone appeared to be equal to the Ag + ion uptake of the ligand-functionalised polymer and therefore the thiirane-based resin p(GMA-S)-[16]aneS 4 N has been tested for metal-ion mixtures without silver. With this thiirane-based resin Cu 2+ ions have been extracted selectively from mixtures of divalent metal ions. The regeneration characteristics of the loaded polymers have been tested by utilising a range of potential stripping agents. The p(GMA-O)-[16]aneS 4 N resin, loaded with Ag + or Cu 2+ ions, was successfully regenerated with an aqueous thiourea solution, while the p(GMA-S)-[16]aneS 4 N resin, loaded with Cu 2+ , could be regenerated best with an aqueous solution of ethylenediamine in combination with ammonium nitrate.

New didentate bispyrazole ligands forming uncommon eight-ring chelates with divalent copper, zinc and cobalt

Abstract The novel ligands 1,3-bis(pyrazol-1′-yl)propane (bpp) and 1,3-bis(3′,5′-dimethylpyrazol-1′-yl)propane (bdpp) have been prepared by reacting 2 equiv. of the corresponding potassium pyrazolate with 1 equiv. of 1,3-dibromopropane. Compound 1,3-bis(5′-methylpyrazol-1′-yl)propane (bmpp) has been prepared from bpp by lithiation at the five position and the subsequent addition of iodomethane. These new ligands form M(II) coordination compounds with the general formula [M(ligand)X2] where X=Cl or NO3, and M=Cu, Zn, or Co. The single-crystal X-ray structures of [Cu(bmpp)(NO3)2], [Cu(bdpp)Cl2], [Zn(bdpp)Cl2], and [Zn(bpp)Cl2] show the ligands to act as didentate chelates forming an uncommon eight-atom chelate ring with the metal ion. In [Cu(bmpp)(NO3)2] each copper ion is distorted octahedrally surrounded, whereas the metal ions in the chloride complexes have distorted tetrahedral geometries. The spectral properties are in accord with the structural data.

Synthesis, characterization and crystal structures of nickel complexes with dissymmetric tetradentate ligands containing a mixed-donor sphere

Abstract Four new nickel(II) complexes of dissymmetric tetradentate ligands, containing mixed-ligand donor sets of NSNS or NSNO, have been synthesized. These complexes were prepared by facile template reactions of the appropriate aldehyde and amine in the presence of [Ni(H 2 O) 6 ](BF 4 ) 2 , resulting directly in the desired nickel compounds. The nickel compounds were characterized by analytical, spectroscopic and electrochemical methods. The structures of [Ni(pyzs)]BF 4 , [Ni(pyrs)]BF 4 and [Ni(pyzo)]BF 4 (see Scheme 1 ) have been determined by single-crystal X-ray crystallography, showing the geometry of the nickel ion to be square-planar. Vis–NIR spectra show that the phenolate-containing complexes [Ni(pyzo)]BF 4 and [Ni(pyro)]BF 4 (see Scheme 1 ) are essentially square-planar in nitromethane, but tetragonal octahedral in methanol, whereas the thiophenolate-containing compounds [Ni(pyzs)]BF 4 and [Ni(pyrs)]BF 4 remain square-planar in both solvents. Titration of the thiophenolate-containing complexes with 1-methylimidazole results in diamagnetic five-coordinated complexes. Electrochemistry shows quasi-reversible reductions to Ni(I) to occur for [Ni(pyzo)]BF 4 , [Ni(pyrs)]BF 4 and [Ni(pyro)]BF 4 .

A kinetic and spectroscopic study on the copper catalyzed oxidative coupling polymerization of 2,6-dimethylphenol. X-ray structure of the catalyst precursor tetrakis(N-methylimidazole)bis(nitrato)copper(II)

The complex of copper(II) nitrate with N-methylimidazole (Nmiz) ligand has been studied as a catalyst for the oxidative coupling of 2,6-dimethylphenol by means of kinetic and spectroscopic measurements. The order of the reaction in copper is fractional and depends on the N/Cu ratio and the base/Cu ratio, indicating that there are at least two possible rate-determining steps, i.e. the formation of a dinuclear copper species and the phenol oxidation. EPR spectroscopy performed on frozen solutions with varying ligand to copper ratios shows that all Cu(II) is converted into the precursor complex at a ratio of 4 to 1, whereas in kinetic experiments, maximum activity and selectivity are reached only at a ratio of at least 30 to 1. Base is needed as a co-catalyst, and the maximum reaction rate is reached at a base to copper ratio of 1.8 to 1. The solid-state X-ray structure of the catalyst precursor complex has been determined to be [Cu(Nmiz)(4)(NO3)(2)], monoclinic, space group P2(1)/n, a=8.452(1)Angstrom, b=10.376(2)Angstrom, c=12.821(2)Angstrom, beta=94.88(2)degrees, Z=1, R=0.049 for 3525 reflections. This structure consists of an axially elongated octahedral CuN4O2 chromophore, which is in agreement with frozen-solution EPR spectra. Investigations under conditions where water and dioxygen were carefully excluded, have shown that for the phenol oxidation step the presence of dioxygen is not required. However, the reaction does require a trace of water (or hydroxide) to form the reactive intermediate. A modified reaction mechanism for the oxidative coupling is presented with special attention to the first steps of the reaction and the equilibrium species present in solution. The role of dioxygen appears to be only to reoxidize the formed Cu(I) species and to regenerate base.

Ligand-templated four-metal chains dimerize into a unique [CuII8] cluster.

Within the context of what has been termed coordination supramolecular chemistry, remarkable progress has been made in the design and preparation of multidentate ligands capable of participating in complicated molecular structures upon complexation, in predetermined manners, with transition metals.[1] This approach is starting to take hold in the area of molecular magnetism. Thus, in recent years, a few important reports have appeared describing the formation of new magnetic transition metal clusters through the template action of specifically designed multinucleating ligands.[2] We have reported the synthesis of an O-pentadentate donor possessing a phenol and two -diketone units (H3L), which was conceived to direct the formation of chains