Negative-charge driven fragmentations for evidencing zwitterionic forms from doubly charged coppered peptides

In aqueous solution, amino acids (AA) and peptides are known to exist as zwitterions over a large pH range. However, in the gas phase, i.e. in electrospray (ESI), the zwitterionic form becomes unfavorable owing to the absence of stabilizing effects from intermolecular solvation. Nevertheless, during mass spectrometry experiments, the presence of a metallic cation can reinforce the zwitterionic character of the molecule and thus influence its fragmentation under low energy collision-induced dissociation (CID) conditions. The [M + Cu(II)](2+) complexes of six pentapeptides (YGGFL, YGGFL(NH(2)), YGGFK, YGGFQ, KYGGF and QYGGF) were analyzed by collision to highlight the presence of zwitterions. The experiments were performed on a 3D-ion trap equipped with an orthogonal ESI source. For each peptides studied, negative-charge driven fragmentations on globally positively charged ions were observed. These fragmentation mechanisms, generally observed in the negative mode, suggest the competitive deprotonation of the C-terminal carboxylic acid or of the tyrosine side-chain residue for each peptide studied and thus a zwitterionic form to preserve the charge balance. Moreover, the specific loss of (CH(3)--C(6)H(4)--O)(*) characterizes YGGFK compared to YGGFQ and the specific loss of styrene characterizes KYGGF compared to QYGGF. These results allow the differentiation of the two couples of isobaric pentapeptides. An unusual loss of NH(4) (+), which occurred from the N-terminus, was also observed for YGGFL, YGGFL(NH(2)), YGGFK and YGGFQ. Finally, the reduction of Cu(II) to Cu(I), concomitant with the (CH(3)--C(6)H(4)--O)(*) release, was pointed out for YGGFK.     

Improvement of durability of an organic photocatalyst in p-xylene oxygenation by addition of a Cu(II) complex

The catalytic durability of an organic photocatalyst, 9-mesityl-10-methyl acridinium ion (Acr(+)-Mes), has been dramatically improved by the addition of [{tris(2-pyridylmethyl)amine}Cu(II)](ClO(4))(2) ([(tmpa)Cu(II)](2+)) in the photocatalytic oxygenation of p-xylene by molecular oxygen in acetonitrile. Such an improvement is not observed by the addition of Cu(ClO(4))(2) in the absence of organic ligands. The addition of [(tmpa)Cu](2+) in the reaction solution resulted in more than an 11 times higher turnover number (TON) compared with the TON obtained without [(tmpa)Cu(II)](2+). In the photocatalytic oxygenation, a stoichiometric amount of H(2)O(2) formation was observed in the absence of [(tmpa)Cu(II)](2+), however, much less H(2)O(2) formation was observed in the presence of [(tmpa)Cu(II)](2+). The photocatalytic mechanism was investigated by laser flash photolysis measurements in order to detect intermediates. The reaction of O(2)˙(-) with [(tmpa)Cu(II)](2+) monitored by UV-vis spectroscopy in propionitrile at 203 K suggested formation of [{(tmpa)Cu(II)}(2)O(2)](2+), a transformation which is crucial for the overall 4-electron reduction of molecular O(2) to water, and a key in the observed improvement in the catalytic durability of Acr(+)-Mes.     

New heterometallic coordination polymers self-assembled from copper(II) nitrate and (diamine)Pt(II)(pyridinecarboxylates)(2)

One-dimensional (1-D), two-dimensional (2-D), and three-dimensional (3-D) coordination polymers were prepared by self-assembly of binary metal complex systems, copper(II) nitrate and (en)Pt(II)(nic)(2) or (dmpda)Pt(II)(isonic)(2) (en = ethylenediamine, dmpda = 2,2'-dimethyl-1,3-propanediamine, nic = nicotinate, and isonic = isonicotinate), in aqueous solutions. Equimolar reactions of copper(II) nitrate with (dmpda)Pt(II)(isonic)(2) and (en)Pt(II)(nic)(2) resulted in 1-D ([(dmpda)Pt(isonic)(2)Cu(OH(2))(3)](NO(3))(2))(n)() (1) and 2-D ([(en)Pt(nic)(2)Cu(OH(2))](NO(3))(2))(n) (2), respectively, but the reaction of (en)Pt(II)(nic)(2) with excess copper(II) nitrate gave 3-D ([((en)Pt(nic)(2))(3)Cu(5)(OH)(2)(OH(2))(6)](NO(3))(8))(n) (3). The local structure of crystal 1 has a mononuclear copper unit, 2 has a dinuclear copper unit with a Cu-Cu distance of 2.659(5) A, and 3 has a pentanuclear copper unit. The methyl groups of the dmpda ligand are located in the space between two isonicotinate ligands of 1, which is presumed to be an important factor to determine the final structure of the crystal formed by self-assembly. Magnetic behaviors of crystals 1-3 examined in the temperature range of 4-300 K appear to be governed by the local structures around the copper(II) ions and do not indicate any significant long-range magnetic exchange interactions along the polymeric chain.     

Copper(II)-mediated oxidative transformation of vic-dioxime to furoxan: evidence for a copper(II)-dinitrosoalkene intermediate

The mononuclear copper(II) complex [Cu(H(2)L(1))(2)(H(2)O)](ClO(4))(2) (1) (where H(2)L(1) = 1,10-phenanthroline-5,6-dioxime) reacts with copper(II) perchlorate in acetonitrile at ambient conditions in the presence of triethylamine to afford a copper(II) complex, [Cu(L(3))(2)(H(2)O)](ClO(4))(2) (2a), of 1,10-phenanthroline furoxan. A similar complex [Cu(L(3))(2)Cl](ClO(4)) (2) is isolated from the reaction of H(2)L(1) with copper(II) chloride, triethylamine, and sodium perchlorate in acetonitrile. The two-electron oxidation of the vic-dioxime to furoxan is confirmed from the X-ray single crystal structure of 2. An intermediate species, showing an absorption band at 608 nm, is observed at -20 °C during the conversion of 1 to 2a. A similar blue intermediate is formed during the reaction of [Cu(HDMG)(2)] (H(2)DMG = dimethylglyoxime) with ceric ammonium nitrate, but H(2)DMG treated with ceric ammonium nitrate does not form any intermediate. This suggests the involvement of a copper(II) complex in the intermediate step. The intermediate species is also observed during the two-electron oxidation of other vic-dioximes. On the basis of the spectroscopic evidence and the nature of the final products, the intermediate is proposed to be a mononuclear copper(II) complex ligated by a vic-dioxime and a dinitrosoalkene. The dinitrosoalkene is generated upon two-electron oxidation of the dioxime. The transient blue color of the dioxime-copper(II)-dinitrosoalkene complex may be attributed to the ligand-to-ligand charge transfer transition. The intermediate species slowly decays to the corresponding two-electron oxidized form of vic-dioxime, i.e. furoxan and [Cu(CH(3)CN)(4)](ClO(4)). The formation of two isomeric furoxans derived from the reaction of an asymmetric vic-dioxime, hexane-2,3-dioxime, and copper(II) perchlorate supports the involvement of a dinitrosoalkene species in the intermediate step. In addition, the oxidation of 2,9-dimethyl-1,10-phenanthroline-5,6-dioxime (H(2)L(2)) to the corresponding furoxan and subsequent formation of a copper(I) complex [Cu(L(4))(2)](ClO(4)) (3) (where L(4) = 2,9-dimethyl-1,10-phenanthroline furoxan) are discussed.     

Copper(II)-binding ability of stereoisomeric cis- and trans-2-aminocyclohexanecarboxylic acid-L-phenylalanine dipeptides. A combined CW/pulsed EPR and DFT study

With the aim of an improved understanding of the metal-complexation properties of alicyclic β-amino acid stereoisomers, and their peptides, the complex equilibria and modes of coordination with copper(II) of L-phenylalanine (F) derivatives of cis/trans-2-aminocyclohexanecarboxylic acid (c/tACHC), i.e. the dipeptides F-c/tACHC and c/tACHC-F, were investigated by a combination of CW and pulsed EPR methods. For the interpretation of the experimental data, DFT quantum-chemical calculations were carried out. Simulation of a pH-dependent series of room-temperature CW-EPR spectra revealed the presence of EPR-active complexes ([Cu(aqua)](2+), [CuL](+), [CuLH(-1)], [CuLH(-2)](-), and [CuL(2)H(-1)](-)), and an EPR-inactive species ([Cu(2)L(2)H(-3)](-)) in aqueous solutions for all studied cases. [CuLH](2+) was included in the equilibrium model for the c/tACHC-F-copper(II) systems, and [CuL(2)], together with two coordination isomers of [CuL(2)H(-1)](-), were also identified in the F-tACHC-copper(II) system. Comparison of the complexation properties of the diastereomeric ligand pair F-(1S,2R)-ACHC and F-(1R,2S)-ACHC did not reveal significant differences. Considerably lower formation constants were obtained for the trans than for the cis isomers for both the F-c/tACHC and the c/tACHC-F pairs in the case of [CuLH(-1)] involving tridentate coordination by the amino, the deprotonated peptide, and the carboxylate groups. A detailed structural analysis by pulsed EPR methods and DFT calculations indicated that there was no significant destabilization for the complexes of the trans isomers. The lower stability of their complexes was explained by the limitation that only the conformer with donor groups in equatorial-equatorial ring positions can bind to copper(II), whereas both equatorial-axial conformers of the cis isomers are capable of binding. From a consideration of the proton couplings obtained with X-band (1)H HYSCORE, (2)H exchange experiments, and DFT, the thermodynamically most stable cyclohexane ring conformer was assigned for all four [CuLH(-1)] complexes. For the F-cACHC case, the conformer did not match the most stable conformer of the free ligand.     

Differentiation of isomeric 2-aryldimethyltetrahydro-5-quinolinones by electron ionization and electrospray ionization mass spectrometry

A series of isomeric 2-aryl-6,6-dimethyltetrahydro-5-quinolinones (set I) and 2-aryl-7,7-dimethyltetrahydro-5-quinolinones (set II) were studied under positive ion electron ionization (EI) and electrospray ionization (ESI) techniques. Under EI conditions, the molecular ions were found to be less stable in set I isomers, and they resulted in abundant fragment ions, i.e., [M-CH(3)](+), [M-CO](+.), [M-HCO](+), [M-(CH(3),CO)](+), and [M-(CH(3),CH(2)O)](+), when compared with set II isomers. In addition, the set I isomers showed specific fragment ions corresponding to [M-OH](+) and [M-OCH(3)](+). The retro-Diels-Alder (RDA) product ion was always higher in set II isomers. The ESI mass spectra produced [M + H](+) ions, and their decomposition showed favorable loss of CH(3) radical, CH(4) and C(2)H(6) molecules in set I isomers. The set II isomers, however, showed predominant RDA product ions, and specific loss of H(2)O. The selectivity in EI and ESI was attributed to the instability of set I isomers by the presence of a gem-dimethyl group at the α-position, and it was supported by the data from model compounds without a gem-dimethyl group. Density functional theory (DFT) calculations successfully corroborated the fragmentation pathways for diagnostic ions. This study revealed the effect of a gem-dimethyl group located at the α-position to the carbonyl having aromatic/unsaturated carbon on the other side of the carbonyl group.     

Supramolecular isomerism within three-dimensional 3-connected nets: unusual synthesis and characterization of trimorphic copper(I) 3,5-dimethyl-1,2,4-triazolate

Solvothermal treatments of acetonitrile, aqueous ammonia and copper(II) salts frequently yielded alpha-[Cu(mtz)](1)(Hmtz = 3,5-dimethyl-1,2,4-triazole) under different reaction conditions. However, two new supramolecular isomers of 1, namely beta-[Cu(mtz)](2) and gamma-[Cu(mtz)](3) were successfully isolated upon introducing 4,4'-bipyridine or nitrate as an additive. 1-3 represent the first example of true supramolecular isomerism within 3D 3-connected nets. Besides the 4.8.10 net for the tetragonal phase 1 (P4(2)/n, a= 13.470(2), c= 6.142(2)A), two novel 3-connected nets, 8(2)10-a and 6.10(2), were observed in the new tetragonal phase 2 (I42d, a= 14.0247(5), c= 5.2884(4)A) and orthorhombic phase 3 (Iba2, a= 8.0423(7), b= 11.5310(10), c= 17.3505(16)A), respectively. The three isomers also displayed distinct physical properties related to their different supramolecular structures.     

Chiral monomeric and homochiral dimeric copper(II) complexes of a new chiral ligand, N-(1,2-bis(2-pyridyl)ethyl)pyridine-2-carboxamide: an example of molecular self-recognition

Reaction of Cu(ClO(4))(2) x 6H(2)O with a racemic mixture of the novel chiral ligand N-(1,2-bis(2-pyridyl)ethyl)pyridine-2-carboxamide (PEAH) affords only the homochiral dimeric copper(II) complexes [Cu(2)((R)()PEA)(2)](ClO(4))(2) and [Cu(2)((S)()PEA)(2)](ClO(4))(2) in a 1:1 ratio. The phenomenon of molecular self-recognition is also observed when a racemic mixture of the monomeric copper(II) complex [Cu((R(S))()PEA)(Cl)(H(2)O)] is converted into the homochiral dimeric species [Cu(2)((R(S))()PEA)(2)](ClO(4))(2) via reaction with Ag(+) ion. This is the first report of direct conversion of a racemic mixture of a chiral monomeric copper(II) complex to a mixture of the homochiral dimers.     

Structural characterization of fatty acids cationized with copper by electrospray ionization mass spectrometry under low-energy collision-induced dissociation

Fatty acids have for many years been characterized by mass spectrometry using electron ionization after chemical derivatization. When fatty acids are ionized using desorption/ionization methods such as electrospray ionization or fast atom bombardment, structural information is usually obtained through high-energy collision-induced dissociation (CID) using sector instruments. It has been shown that copper displays very interesting properties in the gas phase during CID. In this study, the reactivity of saturated and unsaturated fatty acid-copper [M-H+Cu(II)]+ complex and the role of the copper ion in promoting fragmentations were investigated under low-energy collisional activation conditions. The decomposition of these species in an ion trap instrument led to diagnostic ion series that reflect C--C bond cleavage, which involves Cu(II) reduction followed by the release of an alkyl radical. It was demonstrated that in this way the localization of one or two homoconjugated double bonds is possible using low-energy CID. Moreover, the distinction of cis and trans isomers is possible through characteristic product ions related to a specific loss of CO2. When these experiments are repeated using a triple-quadrupole instrument with argon as collision gas, a different behavior is observed as in this case, in addition to the product ion distributions observed in the ion trap, other distributions are observed that reflect the influence of the different kinetic shifts and the occurrence of consecutive decompositions. Different examples are presented with various saturated and unsaturated fatty acid chains. Mechanisms are proposed in order to rationalize the experimental observations.     

Identification of a copper(I) intermediate in the conversion of 1-aminocyclopropane carboxylic acid (ACC) into ethylene by Cu(II)-ACC complexes and hydrogen peroxide

Several Cu(II) complexes with ACC (=1-aminocyclopropane carboxylic acid) or AIB (=aminoisobutyric acid) were prepared using 2,2'-bipyridine, 1,10-phenanthroline, and 2-picolylamine ligands: [Cu(2,2'-bipyridine)(ACC)(H2O)](ClO4) (1a), [Cu(1,10-phenanthroline)(ACC)](ClO4) (2a), [Cu(2-picolylamine)(ACC)](ClO4) (3a), and [Cu(2,2'-bipyridine)(AIB)(H2O)](ClO4) (1b). All of the complexes were characterized by X-ray diffraction analysis. The Cu(II)-ACC complexes are able to convert the bound ACC moiety into ethylene in the presence of hydrogen peroxide, in an "ACC-oxidase-like" activity. A few equivalents of base are necessary to deprotonate H2O2 for optimum activity. The presence of dioxygen lowers the yield of ACC conversion into ethylene by the copper(II) complexes. During the course of the reaction of Cu(II)-ACC complexes with H2O2, brown species (EPR silent and lambda max approximately 435 nm) were detected and characterized as being the Cu(I)-ACC complexes that are obtained upon reduction of the corresponding Cu(II) complexes by the deprotonated form of hydrogen peroxide. The geometry of the Cu(I) species was optimized by DFT calculations that reveal a change from square-planar to tetrahedral geometry upon reduction of the copper ion, in accordance with the observed nonreversibility of the redox process. In situ prepared Cu(I)-ACC complexes were also reacted with hydrogen peroxide, and a high level of ethylene formation was obtained. We propose Cu(I)-OOH as a possible active species for the conversion of ACC into ethylene, the structure of which was examined by DFT calculation.     

Synthesis, structural and magnetic properties of a series of copper(II) complexes containing a monocarboxylated perchlorotriphenylmethyl radical as a coordinating open-shell ligand

A series of complexes of copper(II)-containing a perchlorotriphenylmethyl radical functionalized with a carboxylic group as a new ligand is reported. The compounds [Cu(PTMMC)(2)(L)(3)](PTMMC = (tetradecachloro-4-carboxytriphenyl)methyl radical; L =(1) H(2)O, (2) pyrimidine and ethanol or (3) pyridine), [Cu(2)(PTMMC)(2)(MeCOO)(2)(H(2)O)(2)](4) and [Cu(HPTMMC)(2)(L)(3)](HPTMMC =alpha-H-(tetradecachlorotriphenyl)methane-4-carboxylic acid; L = pyridine)(5) were structurally characterized. In complexes 1, 2, 3, and 5, the copper(II) ion is coordinated to two PTMMC (or HPTMMC) units in a slightly distorted square planar surrounding, while 4 shows a paddle-wheel copper(II) dimer structure, where each Cu metal ion has four O atoms of different carboxylate groups, two of them belonging to two PTMMC radicals. The copper(II)-radical exchange couplings are antiferromagnetic for complexes 1, 2 and 3. A linear three-spin model was applied to complexes 1, 2 and 3 to give J/k(B)=-24.9, -15.0 and -20.7 K, respectively. Magnetic properties of 4 show that it is one of the scarce examples of a spin-frustrated system composed of organic radicals and metal ions. In this case, experimental data were fitted to a magnetic model based on a symmetrical butterfly arrangement to give a copper(II)-copper(II) exchange coupling of J/k(B)=-350.0 K and a copper(II)-radical exchange coupling of J/k(B)=-21.3 K, similar to that observed for the copper(II)-radical interactions in complexes, and.     

Reversible shuttle action upon dehydration and rehydration processes in cationic coordinatively-bonded (4,4) square-grid nets threaded by supramolecular bonded anions, {[Cu(II)(4,4'-bpy)2(H2O)][Cu(II)(2-pySO3)3](NO3)}·H2O

{[Cu(II)(4,4'-bpy)(2)(H(2)O)][Cu(II)(2-pySO(3))(3)](NO(3))}·H(2)O, obtained serendipitously by the reaction of the constituents in water, consists of parallel coordinatively bonded cationic (4,4) corrugated square-grids polymer of {[Cu(II)(4,4'-bpy)(2)(H(2)O)](2+)}(n) threaded by π-π and H-bonded supramolecular chains of [Cu(II)(2-pySO(3))(3)](-) through the open squares. A single-crystal to single-crystal transformation takes place upon removal of the noncoordinated water by controlled heating. The resulting structure exhibits a rearrangement of the coordination of the copper atoms in the grids, where the Cu-H(2)O bond is elongated from 2.250(3) to 2.628(3) ? while the Cu-NO(3) is shortened from 3.122(3) to 2.796(1) ?. This process is reversible as demonstrated by the single crystal structure after rehydration with corresponding bond distances of 2.224(3) and 3.152(3) ?. Such a cooperative effect may be associated with the Jahn-Teller distortion of the copper(II) ion accompanying the shuttle action of the hydrogen-bonded water and nitrate moiety.     

Helical-chain copper(II) complexes and a cyclic tetranuclear copper(II) complex with single syn-anti carboxylate bridges and ferromagnetic exchange interactions

Tridentate Schiff-base carboxylate-containing ligands, derived from the condensation of 2-imidazolecarboxaldehyde with the amino acids beta-alanine (H2L1) and 2-aminobenzoic acid (H2L5) and the condensation of 2-pyridinecarboxaldehyde with beta-alanine (HL2), D,L-3-aminobutyric acid (HL3), and 4-aminobutyric acid (HL4), react with copper(II) perchlorate to give rise to the helical-chain complexes [[Cu(mu-HL1)(H2O)](ClO4)]n (1), [[Cu(mu-L2)(H2O)](ClO4).2H2O]n (2), and [[Cu(mu-L3)(H2O)](ClO4).2H2O]n (3), the tetranuclear complex [[Cu(mu-L4)(H2O)](ClO4)]4 (4), and the mononuclear complex [Cu(HL5)(H2O)](ClO4).1/2H2O (5). The reaction of copper(II) chloride with H2L1 leads not to a syn-anti carboxylate-bridged compound but to the chloride-bridged dinuclear complex [Cu(HL1)(mu-Cl)]2 (6). The structures of these complexes have been solved by X-ray crystallography. In complexes 1-4, roughly square-pyramidal copper(II) ions are sequentially bridged by syn-anti carboxylate groups. Copper(II) ions exhibit CuN2O3 coordination environments with the three donor atoms of the ligand and one oxygen atom belonging to the carboxylate group of an adjacent molecule occupying the basal positions and an oxygen atom (from a water molecule in the case of compounds 1-3 and from a perchlorate anion in 4) coordinated in the apical position. Therefore, carboxylate groups are mutually cis oriented and each syn-anti carboxylate group bridges two copper(II) ions in basal-basal positions with Cu...Cu distances ranging from 4.541 A for 4 to 5.186 A for 2. In complex 5, the water molecule occupies an equatorial position in the distorted octahedral environment of the copper(II) ion and the Cu-O carboxylate distances in axial positions are very large (>2.78 A). Therefore, this complex can be considered as mononuclear. Complex 6 exhibits a dinuclear parallel planar structure with Ci symmetry. Copper(II) ions display a square-pyramidal coordination geometry (tau = 0.06) for the N2OCl2 donor set, where the basal coordination sites are occupied by one of the bridging chlorine atoms and the three donor atoms of the tridentate ligand and the apical site is occupied by the remaining bridging chlorine atom. Magnetic susceptibility measurements indicate that complexes 1-4 exhibit weak ferromagnetic interactions whereas a weak antiferromagnetic coupling has been established for 6. The magnetic behavior can be satisfactorily explained on the basis of the structural data for these and related complexes.     

Copper complexes of nitrogen-anchored tripodal N-heterocyclic carbene ligands

Incorporation of a nitrogen functionality into a tripodal N-heterocyclic carbene ligand system affords the first N-anchored tetradentate tris-carbene ligands TIMEN(R) (R = Me (5a), t-Bu (5b), Bz (5c)). Treatment of the methyl derivatized [H(3)TIMEN(Me)](PF(6))(3) imidazolium salt (H(3)5a) with silver oxide yields the silver complex [(TIMEN(Me))(2)Ag(3)](PF(6))(3) (9), which, in a ligand transfer reaction, reacts with copper(I) bromide to give the trinuclear copper(I) complex [(TIMEN(Me))(2)Cu(3)](PF(6))(3) (10). Deprotonation of the tert-butyl and benzyl derivatives [H(3)TIMEN(t-Bu)](PF(6))(3) and [H(3)TIMEN(Bz)](PF(6))(3) yields the free tris-carbenes TIMEN(t-Bu) (5b) and TIMEN(Bz) (5c), which react readily with copper(I) salts to give mononuclear complexes [(TIMEN(t-Bu))Cu](PF(6)) (11b) and [(TIMEN(Bz))Cu]Br (11c). The solid-state structures of 10, 11b, and 11c were determined by single-crystal X-ray diffraction. While the TIMEN(Me) ligand yields trinuclear complex 10, with both T-shaped three-coordinate and linear two-coordinate copper(I) centers, the TIMEN(t-Bu) and TIMEN(Bz) ligands induce mononuclear complexes 11b and 11c, rendering the cuprous ion in a trigonal planar ligand environment of three carbenoid carbon centers and an additional, weak axial nitrogen interaction. Complexes 11b and 11c exhibit reversible one-electron redox events at half-wave potentials of 110 and -100 mV vs Fc/Fc(+), respectively, indicating sufficient electronic and structural flexibility of both TIMEN(R) ligands (R = t-Bu, Bz) to stabilize copper(I) and copper(II) oxidation states. Accordingly, a copper(II) NHC complex, [(TIMEN(Bz))Cu](OTf)(2) (12), was synthesized. Paramagnetic complex 12 was characterized by elemental analysis, EPR spectroscopy, and SQUID magnetization measurements.     

Resonance raman investigation of equatorial ligand donor effects on the Cu(2)O(2)(2+) core in end-on and side-on mu-peroxo-dicopper(II) and bis-mu-oxo-dicopper(III) complexes

The effect of endogenous donor strength on Cu(2)O(2) bonds was studied by electronically perturbing [[(R-TMPA)Cu(II)]](2)(O(2))](2+) and [[(R-MePY2)Cu](2)(O(2))](2+) (R = H, MeO, Me(2)N), which form the end-on mu-1,2 bound peroxide and an equilibrium mixture of side-on peroxo-dicopper(II) and bis-mu-oxo-dicopper(III) isomers, respectively. For [[(R-TMPA)Cu(II)](2)(O(2))](2+), nu(O-O) shifts from 827 to 822 to 812 cm(-1) and nu(Cu)(-)(O(sym)) shifts from 561 to 557 to 551 cm(-1), respectively, as R- varies from H to MeO to Me(2)N. Thus, increasing the N-donor strength to the copper decreases peroxide pi(sigma) donation to the copper, weakening the Cu-O and O-O bonds. A decrease in nu(Cu-O) of the bis-mu-oxo-dicopper(III) complex was also observed with increasing N-donor strength for the R-MePY2 ligand system. However, no change was observed for nu(O-O) of the side-on peroxo. This is attributed to a reduced charge donation from the peroxide pi(sigma) orbital with increased N-donor strength, which increases the negative charge on the peroxide and adversely affects the back-bonding from the Cu to the peroxide sigma orbital. However, an increase in the bis-mu-oxo-dicopper(III) isomer relative to side-on peroxo-dicopper(II) species is observed for R-MePY2 with R = H < MeO < Me(2)N. This effect is attributed to the thermodynamic stabilization of the bis-mu-oxo-dicopper(III) isomer relative to the side-on peroxo-dicopper(II) isomer by strong donor ligands. Thus, the side-on peroxo-dicopper(II)/bis-mu-oxo-dicopper(III) equilibrium can be controlled by electronic as well as steric effects.     

Selective DNA strand scission with binuclear copper complexes: implications for an active Cu2-O2 species

A homologous series of binuclear copper(II) complexes [Cu(II)(2)(Nn)(Y)(2)](2+) (1-3) (n = 3-5 and Y = (ClO(4))(-) or (NO(3))(-)) were studied to investigate the intermediate(s) responsible for selective DNA strand scission in the presence of MPA/O(2) (MPA = 3-mercaptopropanoic acid). While the N3 complex does not react, the N4 and N5 analogues show comparable activity with strand scission occurring at a single-strand/double-strand junction. Identical reactivity is also observed in the alternate presence of H(2)O(2). Spectroscopic and reactivity studies with [Cu(II)(2)(N4)(Y)(2)](2+) (2) and H(2)O(2) are consistent with DNA oxidation mediated by formation of a side-on peroxodicopper(II) (Cu(2)-O(2)) complex.     

Electrospray ionization mass spectrometric study of Cu(I) and Cu(II) bipyridine complexes employed in atom transfer radical polymerization

We report an electrospray ionization mass spectrometric study of Cu(I) and Cu(II) bipyridine complexes employed in atom transfer radical polymerization. Mass spectra of Cu(I)Br complexed with 2 equiv. of 4,4'-di(5-nonyl)-2,2'-bipyridine (dNbpy) in toluene, methyl acrylate or styrene showed the presence of [Cu(I)(dNbpy)(2)](+) cation and [Cu(I)Br(2)](-) anion. For the Cu(II)Br(2)/2dNbpy system, [Cu(II)(dNbpy)(2)Br](+), [Cu(II)(dNbpy)Br](+), [Cu(I)Br(2)](-), [Cu(II)Br(3)](-) and [Cu(II)(dNbpy)Br(3)](-) species were observed. In addition, for mixed Cu(I)Br/2dNbpy and Cu(II)Br(2)/2dNbpy systems, the negative ion mode showed only the presence of [Cu(I)Br(2)](-) anions, which are potentially formed through halogen exchange between [Cu(II)Br(3)](-) and [Cu(I)(dNbpy)(2)](+). Copyright 2000 John Wiley & Sons, Ltd.     

Rational assembly of soluble copper(II) phosphonates: synthesis, structure and magnetism of molecular tetranuclear copper(II) phosphonates

The reactions of the dinuclear copper complexes [Cu(2)(L)(OAc)] [H(3)L = N,N'-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) or [Cu(2)(L')(OAc)] (H(3)L' = N,N'-(2-hydroxypropane-1,3-diyl)bis(4,5-dimethylsalicylaldimine)] with various phosphonic acids, RPO(3)H(2) (R = t-Bu, Ph, c-C(5)H(9), c-C(6)H(11) or 2,4,6-i-Pr(3)-C(6)H(2)), leads to the replacement of the acetate bridge affording tetranuclear copper(II) phosphonates, [Cu(4)(L)(2)(t-BuPO(3))](CH(3)OH)(2)(C(6)H(6)) (1), [Cu(4)(L)(2)(PhPO(3))(H(2)O)(2)(NMe(2)CHO)](H(2)O)(2) (2), [Cu(4)(L')(2)(C(5)H(9)PO(3))](CH(3)OH)(2) (3), [Cu(4)(L')(2)(C(6)H(11)PO(3)](MeOH)(4)(H(2)O)(2) (4) and [Cu(4)(L')(2)(C(30)H(46)P(2)O(5))](PhCH(3)) (5). The molecular structures of 1-4 reveal that a [RPO(3)](2-) ligand is involved in holding the four copper atoms together by a 4.211 coordination mode. In 5, an in situ formed [(RPO(2))(2)O](4-) ligand bridges two pairs of the dinuclear subunits. Magnetic studies on these complexes reveal that the phosphonate ligand is an effective conduit for magnetic interaction among the four copper centers present; a predominantly antiferromagnetic interaction is observed at low temperatures.     

Synthesis, structure, magnetism and nuclease activity of tetranuclear copper(II) phosphonates containing ancillary 2,2'-bipyridine or 1,10-phenanthroline ligands

The reaction of cyclohexylphosphonic acid (C(6)H(11)PO(3)H(2)), anhydrous CuCl(2) and 2,2'-bipyridine (bpy) in the presence of triethylamine followed by a metathesis reaction with KNO(3) afforded [Cu(4)(mu-Cl)(2)(mu(3)-C(6)H(11)PO(3))(2)(bpy)(4)](NO(3))(2) (1). In an analogous reaction involving Cu(OAc)(2).H(2)O, the complex [Cu(4)(mu-CH(3)COO)(2)(mu(3)-C(6)H(11)PO(3))(2)(2,2'-bpy)(4)](CH(3)COO)(2) (2) has been isolated. The three-component reaction involving Cu(NO(3))(2).3H(2)O, cyclohexylphosphonic acid and 2,2'-bipyridine in the presence of triethylamine afforded the tetranuclear assembly [Cu(4)(mu-OH)(mu(3)-C(6)H(11)PO(3))(2)(2,2'-bpy)(4) (H(2)O)(2)](NO(3))(3) (3). Replacing 2,2'-bipyridine with 1,10-phenanthroline (phen) in the above reaction resulted in [Cu(4)(mu-OH)(mu(3)-C(6)H(11)PO(3))(2)(phen)(4)(H(2)O)(2)](NO(3))(3) (4). In all the copper(II) phosphonates (1-4) the two phosphonate ions bridge the four copper(II) ions in a capping coordination action. Each phosphonate ion bridges four copper(II) ions in a mu(4), eta(3) coordination mode or 4.211 of the Harris notation. Variable-temperature magnetic studies on reveal that all four complexes exhibit moderately strong intramolecular antiferromagnetic coupling. The DNA cleavage activity of complexes 1-4 is also described. Compounds 1 and 3 were able to completely convert the supercoiled pBR322 DNA form I to nick form II without any co-oxidant. In contrast, 50% conversion occurred with and 40% with 4. In the presence of magnesium monoperoxyphthalate all four compounds achieved rapid conversion of form I to form II.