Zn2+-catalyzed methanolysis of phosphate triesters: a process for catalytic degradation of the organophosphorus pesticides paraoxon and fenitrothion

The methanolyses of two neutral phosphorus triesters, paraoxon (1) and fenitrothion (3), were investigated as a function of added Zn(OTf)(2) or Zn(ClO(4))(2) in methanol at 25 degrees C either alone or in the presence of equimolar concentrations of the ligands phenanthroline (4), 2,9-dimethylphenanthroline (5), and 1,5,9-triazacyclododecane (6). The catalysis requires the presence of methoxide, and when studied as a function of added NaOCH(3), the rate constants (k(obs)) for methanolysis of Zn(2+) alone or in the presence of equimolar 4 or 5 maximize at different [(-)OCH(3)]/[Zn(2+)](total) ratios of 0.3, 0.5, and 1.0, respectively. Plots of k(obs) vs [Zn(2+)](total) either alone or in the presence of equimolar ligands 4 and 5 at the [(-)OCH(3)]/[Zn(2+)](total) ratios corresponding to the rate maxima are curved and show a nonlinear dependence on [Zn(2+)](total). In the cases of 4 and 5, this is explained as resulting from formation of a nonactive dimer, formulated as a bis-mu-methoxide-bridged form (L:Zn(2+)((-)OCH(3))(2)Zn(2+):L) in equilibrium with an active monomeric form (L:Zn(2+)((-)OCH(3))). In the case of the Zn(2+):6 system, no dimeric forms are present as can be judged by the strict linearity of the plots of k(obs) vs [Zn(2+)](total) in the presence of equimolar 6 and (-)OCH(3). Analysis of the potentiometric titration curves for Zn(2+) alone and in the presence of the ligands allows calculation of the speciation of the various Zn(2+) forms and shows that the binding to ligands 4 and 6 is very strong, while the binding to ligand 5 is weaker. Overall the best catalytic system is provided by equimolar Zn(2+), 5, and (-)OCH(3), which exhibits excellent turnover of the methanolysis of paraoxon when the substrate is in excess. At a concentration of 2 mM in each of these components, which sets the pH of the solution at 9.5, the acceleration of the methanolysis of paraoxon and fenitrothion relative to the methoxide reaction is 1.8 x 10(6)-fold and 13 x 10(6)-fold, respectively. A mechanism for the catalyzed reactions is proposed which involves a dual role for the metal ion as a Lewis acid and source of nucleophilic Zn(2+)-bound (-)OCH(3).     

Catalytic transesterification of dialkyl phosphates by a bioinspired dicopper(II) macrocyclic complex

For a number of phosphoryltransfer enzymes, including the exonuclease subunit of DNA polymerase I, a mechanism involving two-metal ions and double Lewis-acid activation of the substrate, combined with leaving group stabilization, has been proposed. Inspired by the active site structure of this enzyme, we have designed as a synthetic phosphoryl transfer catalyst the dicopper(II) macrocyclic complex LCu(2). Crystal structures of complexes [(L)Cu(2)(mu-NO(3))(NO(3))](NO(3))(2) (1), [(L)Cu(2)(mu-CO(3))(CH(3)OH)](BF(4))(2) (2), and [(L)Cu(2)(mu-O(2)P(OCH(3))(2))(NO(3))](NO(3))(2) (3) illustrate various possibilities for the interaction of oxoanions with the dicopper(II) site. 1 efficiently promotes the transesterification of dimethyl phosphate (DMP) in CD(3)OD, k(cat) = 2 x 10(-)(4) s(-)(1) at 55 degrees C. 1 is the only available catalyst for the smooth transesterification of highly inert simple dialkyl phosphates. From photometric titrations and the pH dependence of reactivity, we conclude that a complex [(L)Cu(2)(DMP)(OCH(3))](2+) is the reactive species. Steric bulk at the -OR substituents of phosphodiester substrates O(2)P(OR)(2)(-) drastically reduces the reactivity of 1. This is explained with -OR leaving group stabilization by Cu coordination, an interaction which is sensitive to steric crowding at the alpha-C-atom of substituent R. A proposed reaction mechanism related to that of the exonuclease unit of DNA polymerase I is supported by DFT calculations on reaction intermediates. The complex [(L)Cu(3)(mu(3)-OH)(mu-CH(3)O)(2)(CH(3)CN)(2)](ClO(4))(3) (4) incorporates a [Cu(OH)(OCH(3))(2)(CH(3)CN)(2)](-) complex anion, which might be considered as an analogue of the [PO(2)(OCH(3))(2)(OCD(3))](2)(-) transition state (or intermediate) of DMP transesterification catalyzed by LCu(2).     

Billion-fold acceleration of the methanolysis of paraoxon promoted by La(OTf)3 in methanol

The methanolysis of the insecticide paraoxon (2) was investigated in methanol solution containing varying [La(OTf)(3)] (OTf = (-)OS(O)(2)CF(3)) as a function of at 25 degrees C. Plots of the pseudo-first-order rate constants (k(obs)) for methanolysis as a function of [La(OTf)(3)](total) were obtained under buffered conditions from 5.15 to 10.97, and the slopes of the linear parts of these were used to determine the second-order rate constants (k(2)(obs)) for the La(3+)-catalyzed methanolysis of 2. Detailed analysis of the potentiometric titration data of La(OTf)(3) in methanol through fits to a multicomponent equilibrium mixture of dimers of general stoichiometry La(3+)(2)((-)OCH3)n, where n assumes values of 1-5, gives the equilibrium distribution of each as a function of. These data, when fit to a second expression describing k(2)(obs) in terms of a linear combination of individual rate constants k(2)(2:1), k(2)(2:2).k(2)(2:)n for the dimers, allow one to describe the overall catalytic profile in terms of the individual contributions. The most catalytically important species are the three dimers La(3+)(2)((-)OCH3)1, La(3+)(2)((-)OCH3)2, and La(3+)(2)((-)OCH3)3. The catalysis of the methanolysis of 2 is spectacular: a 2 x 10(-3) M solution of [La(3+)](total), at neutral, affords a 10(9)-fold acceleration relative to the base reaction (t(1/2) approximately 20 s at 8.2) with excellent turnover. A mechanism of the catalyzed reaction involving the La(3+)(2)((-)OCH3)2 species is proposed.     

La3+-catalyzed methanolysis of N-aryl-beta-lactams and nitrocefin

The kinetics of the La(3+)-catalyzed methanolysis of N-phenyl-beta-lactam (2) and N-p-nitrophenyl-beta-lactam (3) as well as that of nitrocefin (1) were studied at 25 degrees C under buffered conditions. In the case of 2 and 3, the observed second-order rate constants (k(2)(obs)) for catalysis plateau at pH 7.5-7.8, reaching values of 1 x 10(-)(2) and 35 x 10(-)(2) M(-)(1) s(-)(1) respectively. Potentiometric titrations of solutions of 2 x 10(-)(3) M La(OTf)(3) were analyzed in terms of a dimer model (La(3+)(2)((-)OCH(3))(n)()), where the number of methoxides varies from 1 to 5. The species responsible for catalysis in the pH range investigated contain 1-3 methoxides, the one having the highest catalytic activity being La(3+)(2)((-)OCH(3))(2), which comprises 80% of the total La(3+) forms present at its pH maximum of 8.9. The catalysis afforded by the La(3+) dimers at a neutral pH is impressive relative to the methoxide reactions: at pH 8.4 a 1 mM solution of catalyst (generated from 2 mM La(OTf)(3)) accelerated the methanolysis of 2 by approximately 2 x 10(7)-fold and 3 by approximately 5 x 10(5)-fold. As a function of metal ion concentration, the La(3+)-catalyzed methanolysis of 1 proceeds by pathways involving first one bound metal ion and then a second La(3+) leading to a plateau in the k(obs) vs [La(3+)](total) plots at all pH values. The k(max)(obs) pseudo-first-order rate constants at the plateaus, representing the spontaneous methanolysis of La(3+)(2)(1(-)) forms, has a linear dependence on [(-)OCH(3)] (slope = 0.84 +/- 0.05 if all pH values are used and 1.02 +/- 0.03 if all but the two highest pH values are used). The speciation of bound 1 at a La(3+) concentrations corresponding to that of the onset of the kinetic plateau region was approximated through potentiometric titration of the nonreactive 3,5-dinitrobenzoic acid in the presence of 2 equiv of La(OTf)(3). A total speciation diagram for all bound forms of La(3+)(2)(1(-))((-)OCH(3))(n)(), where n = 0-5, was constructed and used to determine their kinetic contributions to the overall pH vs k(max)(obs) plot under kinetic conditions. Two kinetically equivalent mechanisms were analyzed: methoxide attack on La(3+)(2)(1(-))((-)OCH(3))(n)(), n = 0-2; unimolecular decomposition of the forms La(3+)(2)(1(-))((-)OCH(3))(n)(), n = 1-3.     

Influence of the [2.1.1]-(2,6)-pyridinophane macrocycle ring size constraint on the structure and reactivity of copper complexes

The macrocycle [2.1.1]-(2,6)-pyridinophane (L) binds to CuCl to give a monomeric molecule with tridentate binding of the ligand but in a distorted tetrahedral "3 + 1" geometry, where one nitrogen forms a longer (by 0.12 A) bond to Cu. In dichloromethane solvent this pyridine donor undergoes facile site exchange with a second pyridine in the macrocycle, to give time-averaged mirror symmetry. Both experimental and density functional theory studies of the product of chloride abstraction, using NaBAr(F)(4) in CH(2)Cl(2), show that the Cu(+) binds in a trigonal pyramidal, not planar, arrangement in LCu(+). This illustrates the ability of macrocyclic ligand constraint to impose an electronically unfavorable geometry on 3-coordinate Cu(I). LCuBAr(F)(4) and a triflate analogue LCu(I)(OTf) readily react with oxygen in dichloromethane to produce, in the latter case, a hydroxo-bridged dimer [LCu(II)(micro-OH)](2)(OTf)(2), of the intact (unoxidized) ligand L. Since the analogous LCuCl does not react as fast with O(2) in CH(2)Cl(2), outer-sphere electron transfer is concluded to be ineffective for oxidation of cuprous ion here.     

Pyrophosphate-mediated magnetic interactions in Cu(II) coordination complexes

The reaction in water of Cu(NO(3))(2)·2.5H(2)O with 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), or 1,10-phenanthroline-5-amine (phenam), and sodium pyrophosphate (Na(4)P(2)O(7)), at various pHs, afforded three new copper(II)-pyrophosphate complexes, namely, {[Cu(bipy)(cis-H(2)P(2)O(7))](2)}·3H(2)O (1a), {[Cu(phen)(H(2)O)](4)(HP(2)O(7))(2)}(ClO(4))(2)·4H(2)O (2), and {[Cu(2)(phenam)(2)(P(2)O(7))](2)·25H(2)O}(n) (3). A solvent free crystalline phase of 1a was also isolated with formula {[Cu(bipy)(trans-H(2)P(2)O(7))](2)} (1b), which can be regarded as a pseudo-polymorph of 1a. Single crystal X-ray analyses revealed these compounds to have uncommon molecular architectures, with 3 being an unprecedented pyrophosphate-containing two-dimensional (2D) polymer. Compounds 1a/1b and 2 are discrete di- and tetra-nuclear complexes, respectively. The cationic {[Cu(phen)(H(2)O)](4)(HP(2)O(7))(2)}(2+) unit in 2 presents a unique quasi-flat structure, held together by solely in-plane pyrophosphate bridging modes (short O(eq)-P-O(eq) and long O(eq)-P-O-P-O(eq) pathways), a coordination arrangement also not previously reported. A different tetranuclear copper(II)-pyrophosphate arrangement is found in 3, with two classically bridged dimers (O(eq)-P-O(eq) pathway) joined together by auxiliary equatorial-axial μ-O pyrophosphate bridges. Here, the bidimensionality is reached through bridging phenam ligands, which provide further inter-"tetramer" metal-metal connections [(N,N')(eq)-(N')(ax) pathway], leading to the formation of an expanded covalent network based on the [Cu(2)(phenam)(2)(P(2)O(7))](2) moiety. Variable-temperature magnetic susceptibility measurements on polycrystalline samples of 2 and 3 revealed net antiferromagnetic coupling between metal centers with J(2a) = -7.9(2) cm(-1), J(2b) = -46.9(3) cm(-1), J(2c) = 0 cm(-1) in 2 (H = -J(2a)[S(Cu(1))·S(Cu(2)) + S(Cu(1a))·S(Cu(2a))] - J(2b)[S(Cu(1))·S(Cu(2a)) + S(Cu(1a))·S(Cu(2))] - J(2c)S(Cu(2))·S(Cu(2a))), and J(3a) = -87.9(2) cm(-1), J(3b) = -5(1) cm(-1) and J(3c) = +5(3) cm(-1) in 3 (H = -J(3a)[S(Cu(1))·S(Cu(2)) + S(Cu(1a))·S(Cu(2a))] - J(3b)[S(Cu(1))·S(Cu(2a)) + S(Cu(1a))·S(Cu(2))] - J(3c)S(Cu(2))·S(Cu(2a))). For 1a, a net ferromagnetic coupling is observed with J(1a) = +0.86(1) cm(-1) (H = -J S(A)·S(B) + S(A)·D· S(B) + βH (g(A)S(A) + g(B)S(B)). This is the first example of ferromagnetic coupling in pyrophosphate-complexes reported to date. A structure-function correlation study focusing on magnetic exchange across the observed diverse pyrophosphate-bridges is described with density functional theory (DFT) calculations included to support the stated observations.     

Disproportionation of O-methylhydroxylamine catalyzed by aquapentacyanoferrate(II)

The aquapentacyanoferrate(II) ion, [Fe(II)(CN)(5)H(2)O](3-), catalyzes the disproportionation reaction of O-methylhydroxylamine, NH(2)OCH(3), with stoichiometry 3NH(2)OCH(3) → NH(3) + N(2) + 3CH(3)OH. Kinetic and spectroscopic evidence support an initial N coordination of NH(2)OCH(3) to [Fe(II)(CN)(5)H(2)O](3-) followed by a homolytic scission leading to radicals [Fe(II)(CN)(5)(?)NH(2)](3-) (a precursor of Fe(III) centers and bound NH(3)) and free methoxyl, CH(3)O(?), thus establishing a radical path leading to N-methoxyamino ((?)NHOCH(3)) and 1,2-dimethoxyhydrazine, (NHOCH(3))(2). The latter species is moderately stable and proposed to be the precursor of N(2) and most of the generated CH(3)OH. Intermediate [Fe(III)(CN)(5)L](2-) complexes (L = NH(3), H(2)O) form dinuclear cyano-bridged mixed-valent species, affording a catalytic substitution of the L ligands promoted by [Fe(II)(CN)(5)L](3-). Free or bound NH(2)OCH(3) may act as reductants of [Fe(III)(CN)(5)L](2-), thus regenerating active sites. At increasing concentrations of NH(2)OCH(3) a coordinated diazene species emerges, [Fe(II)(CN)(5)N(2)H(2)](3-), which is consumed by the oxidizing CH(3)O(?), giving N(2) and CH(3)OH. Another side reaction forms [Fe(II)(CN)(5)N(O)CH(3)](3-), an intermediate containing the nitrosomethane ligand, which is further oxidized to the nitroprusside ion, [Fe(II)(CN)(5)NO](2-). The latter is a final oxidation product with a significant conversion of the initial [Fe(II)(CN)(5)H(2)O](3-) complex. The side reaction partially blocks the Fe(II)-aqua active site, though complete inhibition is not achieved because the radical path evolves faster than the formation rates of the Fe(II)-NO(+) bonds.     

Comparison of protective effects against reactive oxygen species of mononuclear and dinuclear Cu(II) complexes with N-substituted benzothiazolesulfonamides

Copper(II) complexes of N-benzothiazolesulfonamides (HL1=N-2-(4-methylphenylsulfamoyl)-6-nitro-benzothiazole, HL2=N-2-(phenylsulfamoyl)-6-chloro-benzothiazole, and HL3=N-2-(4-methylphenylsulfamoyl)-6-chloro-benzothiazole) with ammonia have been synthesized and characterized. The crystal structures of the [Cu(L1)2(NH3)2].2MeOH, [Cu(L2)2(NH3)2], and [Cu(L3)2(NH3)2] compounds have been determined. Compounds and present a distorted square planar geometry. In both compounds the metal ion is coordinated by two benzothiazole N atoms from two sulfonamidate anions and two NH3 molecules. Complex is distorted square-pyramidal. The Cu(II) ion is linked to the benzothiazole N and sulfonamidate O atoms of one of the ligands, the benzothiazole N of another sulfonamidate anion, and two ammonia N atoms. We have tested the superoxide dismutase (SOD)-like activity of the compounds and compared it with that of two dinuclear compounds [Cu2(L4)2(OCH3)2(NH3)2] and [Cu2(L4)2(OCH3)2(dmso)2] (HL4=N-2-(phenylsulfamoyl)-4-methyl-benzothiazole). In vitro indirect assays show that the dimeric complexes are better SOD mimics than the monomeric ones. We have also assayed the protective action provided by the compounds against reactive oxygen species over Deltasod1 mutant of Saccharomyces cerevisiae. In contrast to the in vitro results, the mononuclear compounds were more protective to SOD-deficient S. cerevisiae strains than the dinuclear complexes.     

Solvent deuterium kinetic isotope effects for the methanolyses of neutral C=O, P=O and P=S esters catalyzed by a triazacyclododecane : Zn(2+)-methoxide complex

The methanolyses of several organophosphate/phosphonate/phosphorothioate esters (O,O-diethyl O-(4-nitrophenyl) phosphate, paraoxon, ; O,O-diethyl S-(3,5-dichlorophenyl) phosphorothioate, ; O-ethyl O-(2-nitro-4-chlorophenyl) methylphosphonate, ; O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate, fenitrothion, ; O-ethyl S-(3,5-dichlorophenyl) methylphosphonothioate ) and a carboxylate ester (p-nitrophenyl acetate, ) catalyzed by methoxide and the Zn(2+)((-)OCH(3)) complex of 1,5,9-triazacyclododecane ( : Zn(2+)((-)OCH(3))) were studied in methanol and d(1)-methanol at 25 degrees C. In the case of the methoxide reactions inverse skie's were observed for the series with values ranging from 2 to 1.1, except for where the k(D)/k(H) = 0.90 +/- 0.02. The inverse k(D)/k(H) values are consistent with a direct nucleophilic methoxide attack involving desolvation of the nucleophile with varying extents of resolvation of the TS. With the : Zn(2+)((-)OCH(3)) complex all the skie values are k(D)/k(H) = 1.0 +/- 0.1 except for where the value is 0.79 +/- 0.06. Arguments are presented that the fractionation factors associated with complex : Zn(2+)((-)OCH(3)) are indistinguishable from unity. The skie's for all the complex-catalyzed methanolyses are interpreted as being consistent with an intramolecular nucleophilic attack of the Zn(2+)-coordinated methoxide within a pre-equilibrium metal : substrate complex.     

Bis(mu-oxo)dicopper(III) complexes of a homologous series of simple peralkylated 1,2-diamines: steric modulation of structure, stability, and reactivity

We have synthesized and characterized bis(mu-oxo)dicopper(III) dimers 1b-4b (Os) based on a core family of peralkylated trans-(1R,2R)-cyclohexanediamine (CD) ligands, self-assembled from the corresponding [LCu(MeCN)]CF3SO3 species 1a-4a and O2 at 193 K in aprotic media; additional Os based on peralkylated ethylenediamine and tridentate polyazacyclononane ligands were synthesized analogously for comparative purposes (5b-7b and 8b-9b, respectively). Trigonal-planar [LCu(MeCN)]1+ species are proposed as the active O precursors. The 3-coordinate Cu(I) complexes [(L(TE))Cu(MeCN)]CF3SO3 (4a) and [(L(TB))Cu(MeCN)]CF3SO3 (10a) were structurally characterized; the apparent O2-inertness of 10a correlates with the steric demands of its four benzyl substituents. The rate of O formation, a multistep process that likely proceeds via associative formation of a 1:1 [LCu(O2)]1+ intermediate, exhibits significant dependence upon ligand sterics and solvent: oxygenation of 4a-the slowest-reacting O precursor of the CD series-is first-order with respect to [4a] and proceeds at least 300 times faster in tetrahydrofuran than in CH2Cl2. The EPR, UV-vis, and resonance Raman spectra of 1b-9b are all characteristic of the diamagnetic bis(mu-oxo)dicopper(III) core. The intense ligand-to-metal charge transfer absorption maxima of CD-based Os are red-shifted proportionally with increasing peripheral ligand bulk, an effect ascribed to a slight distortion of the [Cu2O2] rhomb. The well-ordered crystal structure of [(L(ME))2Cu2(mu-O)2](CF3SO3)2.4CH2Cl2 ([3b. 4CH2Cl2]) features the most metrically compact [Cu2O2]2+ core among structurally characterized Os (av Cu-O 1.802(7) A; Cu...Cu 2.744(1) A) and exemplifies the minimal square-planar ligation environment necessary for stabilization of Cu(III). The reported Os are mild oxidants with moderate reactivity toward coordinating substrates, readily oxidizing thiols, certain activated alkoxides, and electron-rich phenols in a net 2e-, 2H+ process. In the absence of substrates, 1b-9b undergo thermally induced autolysis with concomitant degradation of the polyamine ligands. Ligand product distribution and primary kinetic isotope effects (kobsH/kobsD approximately 8, 1b/d24-1b, 293 K) support a unimolecular mechanism involving rate-determining C-H bond cleavage at accessible ligand N-alkyl substituents. Decomposition half-lives span almost 3 orders of magnitude at 293 K, ranging from approximately 2 s for 4b to almost 30 min for d(24)-1b, the most thermally robust dicationic O yet reported. Dealkylation is highly selective where ligand rigidity constrains accessibility; in 3b, the ethyl groups are attacked preferentially. The observed relative thermal stabilities and dealkylation selectivities of 1b-9b are correlated with NC(alpha)-H bond dissociation energies, statistical factors, ligand backbone rigidity, and ligand denticity/axial donor strength. Among the peralkylated amines surveyed, bidentate ligands with oxidatively robust NC(alpha)-H bonds provide optimal stabilization for Os. Fortuitously, the least sterically demanding N-alkyl substituent (methyl) gives rise to the most thermally stable and most physically accessible O core, retaining the potential for exogenous substrate reactivity.     

New magnetic copper(II) coordination polymers with the polynitrile ligand (C[C(CN)2]3)2- and N-donor co-ligands

Reactions between CuCl2 and K2tcpd (tcpd2- = [C10N6]2- = (C[C(CN)2]3)2-) in the presence of neutral co-ligands (bpym = 2,2'-bipyrimidine, and tn = 1,3-diaminopropane) in aqueous solution yield the new compounds [Cu2(bpym)(tcpd)2(H2O)4] x 2H2O (1), [Cu(tn)(tcpd)] (2), and [Cu(tn)2(tcpd)] x H2O (3), which are characterized by X-ray crystallography and magnetic measurements. Compound 1 displays a one-dimensional structure in which the bpym ligand, acting with a bis-chelating coordination mode, leads to [Cu2(bpym)]4+ dinuclear units which are connected by two mu2-tcpd2- bridging ligands. Compound 2 consists of a three-dimensional structure generated by [Cu(tn)]2+ units connected by a mu4-tcpd2- ligand. The structure of 3 is made up of centrosymmetric planar [Cu(tn)]2+ units connected by a mu2-tcpd2- ligand leading to infinite zigzag chains. In compounds 1 and 3, the bridging coordination mode of the tcpd2- unit involves only two nitrogen atoms of one C(CN)2 wing, while in 2, this ligand acts via four nitrogen atoms of two C(CN)2 wings. Despite this difference, the structural features of the tcpd2- units in 1-3 are essentially similar. Magnetic measurements for compound 1 exhibit a maximum in the chi(m) vs T plot (at approximately 150 K) which is characteristic of strong antiferromagnetic exchange interactions between the Cu(II) metal ions dominated by the magnetic exchange through the bis-chelating bpym. The fit of the magnetic data to a dimer model gives J and g values of -90.0 cm(-1) and 2.12, respectively. For compounds 2 and 3 the thermal variations of the magnetic susceptibility show weak antiferromagnetic interactions between the Cu(II) metal ions that can be well reproduced with an antiferromagnetic regular S = 1/2 chain model that gives J values of -0.07(2) and -0.18(1) cm(-1) with g values of 2.12(1) and 2.13(1) for compounds 2 and 3, respectively (the Hamiltonian is written in all the cases as H = -2JS(a)S(b)).     

Dicopper(II) complexes of H-BPMP-type ligands: pH-induced changes of redox, spectroscopic ((19)F NMR studies of fluorinated complexes), structural properties, and catecholase activities

Substitution of the methyl group from the H-BPMP (HL(CH)3) ligand (2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol) by electron withdrawing (F or CF(3)) or electron donating (OCH(3)) groups afforded a series of dinucleating ligand (HL(OCH)3, HL(F), HL(CF)3), allowing one to understand the changes in the properties of the corresponding dicopper complexes. Dinuclear Cu(II) complexes have been synthesized and characterized by spectroscopic (UV-vis, EPR, (1)H NMR) as well as electrochemical techniques and, in some cases, by single-crystal X-ray diffraction: [Cu(2)(L(OCH)3)(muOH)][(ClO(4))(2)].C(4)H(8)O, [Cu(2)(L(F))(muOH)][(ClO(4))(2)], [Cu(2)(L(F))(H(2)O)(2)][(ClO(4))(3)].C(3)D(6)O, and [Cu(2)(L(CF)3)(H(2)O)(2)][(ClO(4))(3)].4H(2)O. Significant differences are observed for the Cu-Cu distance in the two mu-hydroxo complexes (2.980 A (R = OCH(3)) and 2.967 A (R = F)) compared to the two bis aqua complexes (4.084 A (R = F) and 4.222 A (R = CF(3))). The mu-hydroxo and bis aqua complexes are reversibly interconverted upon acid/base titration. In basic medium, new species are reversibly formed and identified as the bis hydroxo complexes except for the complex from HL(CF)3 which is irreversibly transformed near pH = 10. pH-driven interconversions have been studied by UV-vis, EPR, and (1)H NMR, and the corresponding pK are determinated. In addition, with the fluorinated complexes, the changes in the coordination sphere around the copper centers and in their redox states are evidenced by the fluorine chemical shift changes ((19)F NMR). For all the complexes described here, investigations of the catechol oxidase activities (oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone) are of interest in modeling the catecholase enzyme active site and in understanding aspects of structure/reactivity. These studies show the pH-dependence for the catalytic abilities of the complexes, related with changes in the coordination sphere of the metal centers: only the mu-hydroxo complexes from HL(CH)3, HL(F), and HL(OCH)3 exhibit a catecholase activity. Modification on R-substituent induces a drastic effect on the catecholase activity: the presence of an electron donating group on the ligand increases this activity; the reverse effect is observed with an electron withdrawing group.     

Synthesis of new Cu(II)-chelating ligand amphiphiles and their esterolytic properties in cationic micelles

Four new tetradentate 2,6-disubstituted pyridine and tridentate 2-substituted pyridine ligands were synthesized. Two of these compounds possessed a metal ion binding subunit in the form of a 2,6-disubstituted-4-N,N'-dimethylamine pyridine moiety. Cu(2+)-complexes of these ligands incorporated in cetyltrimethylammonium bromide (CTABr) micelles speeded the cleavage of p-nitrophenyldiphenyl phosphate and p-nitrophenyl hexanoate at pH 7.6. On the basis of a kinetic version of Job plot analysis, a 1:1 ligand/Cu(2+) stoichiometry was found to be the most active species. In CTABr micelles, the pK(a) values for the Cu(2+)-coordinated hydroxyl or pendant -CH(2)OH in these ligands were between 7.8 and 7.9. The metallomicellar systems displayed catalytic (turnover) behavior in the presence of excess substrates.     

Aggregation of [Cu(II)4] building blocks into [Cu(II)8] clusters or a [Cu(II)4]infinity chain through subtle chemical control

Coordination complexes of the ligand H3L [1,3-bis(3-oxo-3-phenylpropionyl)-2-hydroxy-5-methylbenzene] with Cu(II) are reported. Clusters showing various nuclearities or modes of supramolecular organization have been prepared by slightly changing the reaction conditions and have been crystallographically characterized. The reaction of H3L with one equivalent of Cu(OAc)2 in DMF yields the dinuclear complex [Cu2(HL)2(dmf)2] (1). Reaction in MeOH of H3L with an increased amount of metal, in the form of Cu(NO3)2, and excess strong base (nBu4NOH) affords the cluster [Cu8(L)2(OMe)8(NO3)2] (2). Complex 2 is a dimer of two linear [Cu4] arrays bridged by methoxide ligands, where the polynucleating ligand is fully deprotonated. The [Cu4]2 clusters are linked to each other by NO3- bridges to form one-dimensional coordination polymers. The link between [Cu8] units and their relative spatial positioning can be modified by changing the anion of the Cu(II) salt, as demonstrated by the synthesis of the cluster polymers [Cu8(L)2(OMe)8Cl2] (3) and [Cu8(L)(OMe)7.86Br2.14] (4), where only NO3- has been replaced by Cl- or Br-, respectively. Similarly, when ClO4- is used, compound [Cu8(L)2(OMe)8(ClO4)2(MeOH)4] (5) can be isolated. It contains independent [Cu8] units. A slight change in the stoichiometry of the reaction leading to 2 affords the related complex catena-[Cu4(L)(OMe)3(NO3)2(H2O)0.36] (6). This polymer contains essentially the same [Cu4] moiety as 2, albeit organized in a completely different arrangement. Each [Cu4] unit in 6 is linked by OMe- ligands to two such equivalent groups to form an infinite chain. Magnetic susceptibility measurements reveal weak antiferromagnetic exchange between Cu(II) centers in 1 (J = -0.73 cm(-1)) and strong antiferromagnetic coupling within [Cu4] chains in 2, 5, and 6 (most negative J values of -113.8 and -177.3 cm(-1) for 2 and 6, respectively).     

Dodecanuclear [Cu(II)6Gd(III)6] nanoclusters as magnetic refrigerants

A novel dodecanuclear complex, [{(HL)(L)(DMF)Cu(II)Gd(III)(DMF)(H(2)O)}(6)]·6DMF (1; DMF = N,N-dimethylformamide), has been obtained using the ligand resulting from the condensation of 3-formylsalicylic acid with hydroxylamine (H(3)L). The exchange interaction between the phenoxo-bridged Cu(II) and Gd(III) ions is weak ferromagnetic (J = +1.01 cm(-1)). The combination of a high-spin ground state with small anisotropy leads to a significant magnetocaloric effect [-ΔS(m)(0-7 T) = 23.5 J K g(-1) K(-1) at ～2 K].     

Oxamato-bridged trinuclear Ni(II)Cu(II)Ni(II) complexes with irregular spin state structures and a binuclear Ni(II)Cu(II) complex with an unusual supramolecular structure: crystal structure and magnetic properties

Four oxamato-bridged heterotrinuclear Ni(II)Cu(II)Ni(II) complexes of formula ([Ni(bispictn)](2)Cu(pba))(ClO(4))(2).2.5H(2)O (1), ([Ni(bispictn)](2)Cu(pbaOH))(ClO(4))(2).H(2)O (2), ([Ni(cth)](2)Cu(pba))(ClO(4))(2) (3), and ([Ni(cth)](2)Cu(opba))(ClO(4))(2).H(2)O (4) and a binuclear Ni(II)Cu(II) complex of formula [Cu(opba)Ni(cth)].CH(3)OH (5) have been synthesized and characterized by means of elemental analysis, IR, ESR, and electronic spectra, where pba = 1,3-propylenebis(oxamato), pbaOH = 2-hydroxyl-1,3-propylenebis(oxamato), opba = o-phenylenebis(oxamato), bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, and cth = rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. The crystal structures of 1, 3, and 5 have been determined. The structures of complexes 1 and 3 consist of trinuclear cations and perchlorate anions, and that of 5 consists of neutral binuclear molecules which are connected by hydrogen bonds and pi-pi interactions to produce a unique supramolecular "double" sheet. In the three complexes, the copper atom in a square-planar or axially elongated octahedral environment and the nickel atom in a distorted octahedral environment are bridged by the oxamato groups, with Cu.Ni separations between 5.29 and 5.33 A. The magnetic properties of all five complexes have been investigated. The chi(M)T versus T plots for 1-4 exhibit the minimum characteristic of antiferromagnetically coupled NiCuNi species with an irregular spin state structure and a spin-quartet ground state. The chi(M)T versus T plot for 5 is typical of an antiferromagnetically coupled NiCu pair with a spin-doublet ground state. The Ni(II)-Cu(II) isotropic interaction parameters for the five complexes were evaluated and are between 102 and 108 cm(-)(1) (H = -JS(Cu).S(Ni)).     

A Cu(II)-mediated C-H oxygenation of sterically hindered tripyridine ligands to form triangular Cu(II)3 complexes

Two sterically hindered tris-pyridyl methane ligands, tris(6-methyl-2-pyridyl)methane (L1) and bis(6-methyl-2-pyridyl)pyridylmethane (L2), are newly synthesized. Under aerobic conditions, Ln (n = 1 or 2) reacts with CuX2 (X = Cl or Br), oxygenated at the methine position to LnOH or LnOMe. The former alcoholate ligand creates trinuclear Cu(II) complexes [Cu3(X)(LnO)3](PF6)2 [(X, n) = (Br, 1) 1, (C1, 1) 2, (Br, 2) 3, or (C1, 2) 4] in which the alkoxide oxygen atoms bridge copper centers. The crystal structures of 1-4 are presented along with their magnetic susceptibility data. The weak antiferromagnetic coupling between the Cu(II) centers in this trinuclear arrangement is due to weak interaction of the magnetic orbitals (dz2) which are oriented along three alternate sides in a hexagon of the Cu3O3 core in 1-4. Under anaerobic conditions, L1 reacts with CuBr2 to form a square pyramidal complex [CuL1Br2] (9) with the ligand facially capping. [Cu(Br)2(L1OMe)] (10) was obtained after the suspension of 9 in MeOH was stirred under air for 48 h. In the presence of cyclohexene, 9 is converted to [Cu(Br)(L1)]m (m = 1 or 2) 5 quantitatively to give trans- 1,2-dibromocyclohexane, indicating that Br2 is generated during the reaction. The FAB MS spectrum of [18O]-1 prepared by the reaction of L1 with CuBr2 under 18O2 shows that the ligand of [18O]-1 is L1(18O-.) L1(18OH), L1OCD3, and bis(6-methyl-2-pyridyl) ketone were obtained from reaction of L1 with CuBr2 in CD3OD under 18O2. These results indicate that the origins of the O atom in L1OH and L1OMe are O2 and MeOH, respectively. On the basis of these results, a mechanism of the oxygenation of L1 in the present system will be proposed.     

Coordination behavior toward copper(II) and zinc(II) ions of three ligands joining 3-hydroxy-2-pyridinone and polyaza fragments

The synthesis and characterization of new polydentate ligand 2-(N),2'-(N')-bis[2-(3-hydroxy-2-oxo-2H-pyridin-1-yl)acetamido]-1(N'),2(N),2'(N')-trimethyl-2,2'-diaminodiethylamine (L3) is reported. The coordination properties of L3 and of two analogous macrocyclic ligands (L1 and L2) toward Cu(II) and Zn(II) metal ions are reported. All three ligands show the 3-hydroxy-2(1H)-pyridinone (HPO) groups attached as sidearms to a polyaza fragment, which is a macrocyclic framework in the case of L1 and L2 while it is an open chain in the case of L3. The role of the polyaza fragments in preorganizing the two sidearms was investigated. The basicity of L3 and the binding properties of L1-L3 were determined by means of potentiometric measurements in aqueous solution (298.1 +/- 0.1 K, I = 0.15 mol dm(-3)). UV-vis spectra as well 1H and 13C NMR experiments were used to understand the role of the HPO and of the polyaza fragments in the stabilization of the cations. While L1 forms stable mono- and dinuclear complexes, L2 and L3 can form only mononuclear species with each of the metal ions investigated. In the main mononuclear species of L2 and L3, the two HPO moieties stabilize the M(II) in a square planar geometry due to the two oxygen atoms of each HPO. The coordination sphere of the metal is completed by adding a secondary ligand such as water molecules in the case of Cu(II) systems or OH- in the Zn(II) systems. These results are confirmed by the crystal structures of the [CuH(-1)L2]+ and [CuH(-1)L3]+ species reported herein. Two conformations of L1 can be hypothesized in the formation of the dinuclear species, as suggested by NMR experiments on the [ZnH(-2)L1] species, which shows two conformers slowly interchanging on the NMR time scale, one of which was found to be more insoluble.     

Tri- and tetranuclear copper(II) complexes consisting of mononuclear Cu(II) chiral building blocks with a sugar-derived Schiff's base ligand

A new sugar-derived Schiff's base ligand N-(3-tert-butyl-2-hydroxybenzylidene)-4,6-O-ethylidene-beta-D-glucopyranosylamine (H3L1) has been developed which afforded the coordinatively labile, alcoholophilic trinuclear Cu(II) complex [Cu3(L1)2(CH3OH)(H2O)] (1). Complex 1 has been further used in the synthesis of a series of alcohol-bound complexes with a common formula of [Cu3(L1)2(ROH)2] (R = Me (2), Et (3), nPr (4), nBu (5), nOct (6)). X-ray structural analyses of complexes 2-6 revealed the collinearity of trinuclear copper(II) centers with Cu-Cu-Cu angles in the range of 166-172 degrees . The terminal and central coppers are bound with NO3 and O4 atoms, respectively, and exhibit square-planar geometry. The trinuclear structures of 2-6 can be viewed as the two {Cu(L1)}- fragments capture a copper(II) ion in the central position, which is further stabilized by a hydrogen-bonding interaction between the alcohol ligands and the sugar C-3 alkoxo group. Complex 2 exhibits a strong antiferromagnetic interaction between the Cu(II) ions (J = -238 cm(-1)). Diffusion of methanol into a solution of complex 1 in a chloroform/THF mixed solvent afforded the linear trinuclear complex [Cu(3)(L1)2(CH3OH)2(THF)2] (7). The basic structure of 7 is identical to complex 2; however, THF binding about the terminal coppers (Cu-O(THF) = 2.394(7) and 2.466(7) A) has introduced the square-pyramidal geometry, indicating that the planar trinuclear complexes 2-6 are coordinatively unsaturated and the terminal metal sites are responsible for further ligations. In the venture of proton-transfer reactions, a successful proton transfer onto the saccharide C-3 alkoxo group has been achieved using 4,6-O-ethylidene-d-glucopyranose, resulting in the self-assembled tetranuclear complex, [Cu4(HL1)4] (8), consisting of the mononuclear Cu(II) chiral building blocks, {Cu(HL1)}.     

Electronic tuning of beta-diketiminate ligands with fluorinated substituents: effects on the O2-reactivity of mononuclear Cu(I) complexes

Copper(i) complexes with the beta-diketiminate ligands HC{C(R)N(Dipp)}{C(R')N(Dipp)}(-) (Dipp = C(6)H(3)(i)Pr(2-)2,6; L(1), R = CF(3), R' = CH(3); L(2), R = R' = CF(3)) have been isolated and fully characterized. On the basis of X-ray structural comparisons with the previously reported complex LCu(CH(3)CN) (L = HC{C(CH(3))N(Dipp)}(2)(-)), the ligand environments at the copper centers in the analogous nitrile adducts with L(1) and L(2) impose similar steric demands. L(1)Cu(CH(3)CN) reacts instantaneously at low temperature with O(2) to form a thermally-unstable intermediate with an isotope-sensitive vibration at 977 cm(-1) (928 cm(-1) with (18)O(2)), in accord with the peroxo O-O stretch associated with side-on coordination for LCu(O(2)). However, L(2)Cu(CH(3)CN) is unreactive toward O(2) even at room temperature. Evaluation of the redox potentials of the nitrile adducts and the CO stretching frequencies of the carbon monoxide adducts revealed an incremental adjustment of the electronic environment at the copper center that correlated with the extent of ligand fluorination. Furthermore, theoretical calculations (DFT, CASPT2) predicted that an increasing extent of Cu(ii)-superoxo character and end-on coordination of the O(2) moiety in the Cu/O(2) product (L(2) > L(1) > L) are accompanied by increases in the free energy for the oxygenation reaction, with L(2) unable to support a Cu/O(2) intermediate. Calculations also predict the 1 : 1 Cu/O(2) adducts to be unreactive with respect to hydrogen atom abstraction from hydrocarbon substrates on the basis of their stability towards both reduction and protonation.