One-electron Reduction Waves Research Articles

Preparation and electrochemical properties of novel cyclic dinuclear acetylacetonato ruthenium complexes doubly bridged with sulfur and/or disulfur

A novel cyclic dinuclear acetylacetonato ruthenium complex doubly bridged with sulfur and/or disulfur at the γ-position of acetylacetonato ligand has been obtained by two different synthetic methods. The molecular structure of the dinuclear complex has been determined by single crystal X-ray diffraction study. Other two cyclic dinuclear β-diketonato ruthenium complexes were also prepared in good yields by the reaction of single bridged dinuclear complexes as starting materials with disulfur dichloride. The cyclic voltammograms of all the dinuclear complexes exhibit two one-electron reduction and oxidation waves in acetonitrile (AN) and dichloromethane (DM). The comproportionation constants ( K c) for mixed-valence state of both Ru II/Ru III and Ru III/Ru IV were evaluated in both solvents at 25 or −30 °C. The values of both K c (Ru II/Ru III) and log 10 K c (Ru III/Ru IV) for double bridged complex are large compared to those of corresponding single bridged complexes. This fact was rationally explained by the double bridging effect caused by the spread of electronic communication and also demonstrated the usefulness of the double bridged dinuclear complexes.

Structural, Electronic, and Theoretical Description of a Series of Cobalt Clathrochelate Complexes in the Co(III), Co(II) and Co(I) Oxidation States

Encaged hexacoordinated metal complexes have long been a fascinating family of complexes, as they confer the same ligand environment to metal ions in different oxidation states. We recently reported that cobalt clathrochelate complexes behave as hydrogen-producing catalysts at quite modest overpotential ( Pantani et al. Angew. Chem., Int. Ed. 2008 , 47 , 9948 ). The electrochemical properties evidenced two quasireversible one-electron reduction waves starting from the cobalt(III) derivative, indicating that there are no dramatic changes in the coordination sphere. The intriguing question is the mechanistic pathways for this observed reactivity. In this work, we compare our observed electrochemical and spectroscopic data (UV-visible and EPR spectroscopies) with our theoretical findings based on DFT, TD-DFT, and CASSCF calculations. The properties of the Co(III) and Co(II) species can be explained as low-spin complexes. In contrast, the doubly reduced species, the "Co(I)" form, is a high-spin complex and its electronic description involves partial reduction of the ligand cage. This point is of major importance to understand the catalytic activity.

Synthesis, characterization, crystal structure and antimicrobial activities of new transN, N-substituted macrocyclic dioxocyclam and their copper(II) and nickel(II) complexes

New trans-disubstituted macrocyclic ligands, 1,8-[ N,N-bis(3-formyl-12-hydroxy-5-methyl)benzyl]-5,12-dioxo-1,4,8,11-tetraazacyclotetradecane (L 1), 1,8-[ N,N-bis(3-formyl-12-hydroxy-5-bromo)benzyl]-5,12-dioxo-1,4,8,11-tetraazacyclotetradecane (L 2), N, N-bis[1,8-dibenzoyl]-5,12-dioxo-1,4,8,11-tetraazacyclotetradecane (L 3), N, N-bis[1,8-(2-nitrobenzoyl)]-5,12-dioxo-1,4,8,11-tetraazacyclotetradecane (L 4), and N, N-bis[1,8-(4-nitrobenzoyl)]-5,12-dioxo-1,4,8,11-tetraazacyclotetradecane (L 5) were synthesized. The ligands were characterized by elemental analysis, FT IR, 1H NMR and mass spectrometry studies. The crystal structure of L 1 is also reported. The copper(II) and nickel(II) complexes of these ligands were prepared and characterized by elemental analysis, FT IR, UV–Vis and mass spectral studies. The cyclic voltammogram of the complexes of ligand L 1–3 show one-electron quasi-reversible reduction wave in the region −0.65 to −1.13 V, whereas that of L 4 and L 5 show two quasi-reversible reduction peaks. Nickel(II) complexes show one electron quasi-reversible oxidation wave at a positive potential in the range +0.95 to +1.06 V. The ESR spectra of the mononuclear copper(II) complexes show four lines, characteristic of square-planar geometry with nuclear hyperfine spin 3/2. All copper(II) complexes show a normal room temperature magnetic moment value μ eff 1.70–1.73 BM which is close to the spin only value of 1.73 BM. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the copper(II) complexes as catalysts and hydrolysis of 4-nitrophenylphosphate using the copper(II) and nickel(II) complexes as catalysts were carried out. The ligands and their complexes were also screened for antimicrobial activity against Gram-positive, Gram-negative bacteria and human pathogenic fungi.

Electrochemistry and Electrogenerated Chemiluminescence of Dithienylbenzothiadiazole Derivative. Differential Reactivity of Donor and Acceptor Groups and Simulations of Radical Cation−Anion and Dication−Radical Anion Annihilations

We report here the electrochemistry and electrogenerated chemiluminescence (ECL) of a red-emitting dithienylbenzothiadiazole-based molecular fluorophore (4,7-bis(4-(4-sec-butoxyphenyl)-5-(3,5-di(1-naphthyl)phenyl)thiophen-2-yl)-2,1,3-benzothiadiazole, 1b). 1b contains two substituted thiophene groups as strong electron donors at the ends connected directly to a strong electron acceptor, 2,1,3-benzothiadiazole, in the center. Each thiophene moiety is substituted in position 2 by 3,5-di(1-naphthyl)phenyl and in position 3 by 4-sec-butoxyphenyl. Cyclic voltammetry of 1b, with scan rate ranging from 0.05 to 0.75 V/s, shows a single one-electron reduction wave (E°(red) = -1.18 V vs SCE) and two nernstian one-electron oxidation waves (E°(1,ox) = 1.01 V, E°(2,ox) = 1.24 V vs SCE). Reduction of the unsubstituted 2,1,3-benzothiadiazole center shows nernstian behavior with E°(red) = -1.56 V vs SCE. By comparison to a digital simulation, the heterogeneous electron-transfer rate constant for reduction, k(r)° = 1.5 × 10(-3) cm/s, is significantly smaller than those for the oxidations, k(o)° > 0.1 cm/s, possibly indicating that the two substituted end groups have a blocking effect on the reduction of the benzothiadiazole center. The ECL spectrum, produced by electron-transfer annihilation of the reduced and oxidized forms, consists of a single peak with maximum emission at about 635 nm, consistent with the fluorescence of the parent molecule. Relative ECL intensities with respect to 9,10-diphenylanthracene are 330% and 470% for the radical anion-cation and radical anion-dication annihilation, respectively. Radical anion (A(-•))-cation (A(+•)) annihilation produced by potential steps shows symmetric ECL transients during anodic and cathodic pulses, while for anion (A(-•))-dication (A(2+•)) annihilation, transient ECL shows asymmetry in which the anodic pulse is narrower than the cathodic pulse. Digital simulation of the transient ECL experiments showed that the origin of the observed asymmetry is asymmetry in the amount of generated charges rather than instability of the electrogenerated species.

Electrochemistry and Electrogenerated Chemiluminescence of a Novel Donor−Acceptor FPhSPFN Red Fluorophore

We report here the electrogenerated chemiluminescence (ECL) of a red fluorophore diphenylaminospirobifluorenylfumaronitrile (FPhSPFN). FPhSPFN contains two diphenylamino groups as strong electron donors at the ends linked through a nonplanar bulky spirobifluorene linker to a common fumaronitrile group as a strong electron acceptor. The cyclic voltammetry of this compound shows two one-electron transfer reduction waves and one single overall two-electron oxidation wave consisting of closely spaced waves with a peak separation of 63 mV. Chronoamperometry with an ultramicroelectrode (UME) confirmed that each reduction wave involves a 1e transfer and that the oxidation wave contains an overall 2e transfer. The first reduction wave is Nernstian, Eo1,red = −1.09 V versus SCE, and the second reduction wave, Eo2,red = −1.50 V versus SCE, produces a film on the electrode. By comparison to a digital simulation, the oxidation is assigned to two reversible, closely spaced, one-electron processes for oxidation with Eo...

Optical and electrochemical properties of cyclometalated Rh(III) complexes based on 4,6-diphenylpyrimidine with ethylenediamine, 2,2′-bipyridyl, and 1,10-phenanthroline

The [Rh(Hdp)2(N∧N)]ClO4 complexes (Hdp− is the monodeprotonated form of 4,6-diphenylpyrimidine and (N∧N) is ethylenediamine, 2,2′-bipyridyl, and 1,10-phenanthroline) are synthesized and characterized by 1H and 13C NMR, IR, electronic absorption, and emission spectroscopy, as well as by cyclic voltammetry. The magnetic equivalence of two cyclometalated 4,6-diphenylpyrimidine ligands in the composition of complexes points to the cis position of metalated phenyl rings in the inner sphere. Quasi-reversible one-electron reduction waves are attributed to the ligand-centered electron transfer to the π* antibonding orbital of heterocyclic ligands, while irreversible oxidation waves are associated with electron detachment from the Rh-C σ bonding orbital of the {Rh(Hdp)2} metal-complex fragment. The characteristic long-wave-length absorption bands and the vibrationally structured phosphorescence bands of complexes are assigned to the spin-allowed and spin-forbidden charge-transfer optical transitions between the σRh-C and πHdp* orbitals localized on the {Rh(Hdp)2} fragment of the complex.

Synthesis, characterization, and antimicrobial activities of nickel(II) and copper(II) Schiff-base complexes

Ni(II) and Cu(II) metal complexes of simple unsymmetrical Schiff-base ligands derived from salicylaldehyde/5-methylsalicylaldehyde and ethylenediamine or diaminomaleonitrile (DMN) were synthesized. The ligands and their complexes were characterized by elemental analysis, 1H NMR, FT IR, and mass spectroscopy. The electronic spectra of the complexes show d–d transitions in the region at 450–600 nm. Electrochemical studies of the complexes reveal that all mononuclear complexes show a one-electron quasi-reversible reduction wave in the cathodic region. ESR spectra of the mononuclear copper(II) complexes show four lines, characteristic of square-planar geometry, with nuclear hyperfine spin 3/2. The copper(II) complexes show a normal room temperature magnetic moment value μ eff = 1.70–1.74 BM which is close to the spin only value of 1.73 BM. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the copper(II) complexes as catalysts were also carried out. The in vitro antimicrobial activity of the investigated compounds was tested against human pathogenic bacterias such as Staphylococcus aureus, Bacillus subtilis, Klebsiella pneumonia, Pseudomonas aeruginosa and Escherichia coli. The antifungal activity was tested against Candida albicans. Generally, the metal complexes have higher antimicrobial activity than the free ligands.

Synthesis, X-ray structures and properties of the first tris-dioximate cobalt clathrochelates with nonequivalent chelate ribbed fragments

One-pot macrocyclization and reduction of the kinetically inert nonmacrocyclic cobalt(III) bis-α-benzyldioximate and dimethylglyoximate with BF 3·O(C 2H 5) 2 and metallic silver followed by cycloaddition of the corresponding α-dioxime to the generated in situ macrocyclic cobalt(II) bis-dioximates afforded the cobalt(II) clathrochelates with nonequivalent α-dioximate fragments. The complexes obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV–Vis, 1H, 13C{ 1H} and 19F NMR spectroscopies, magnetochemistry, EPR, and X-ray crystallography. The coordination polyhedra of an encapsulated in a three-dimensional macrobicyclic ligand cavity cobalt(II) ion have a distorted trigonal prismatic geometry. The displacement of a caged metal ion from the centers of these polyhedra is caused mainly by the Jahn–Teller effect. Magnetochemical data for cobalt(II) clathrochelates obtained characterize them as the low-spin complexes in the temperature range of 2–400 K. The cyclic voltammograms of the synthesized clathrochelates contain the one-electron oxidation and reduction waves. The increase of the electron-donating properties of the ribbed substituents causes the negative shift of these waves. A comparative analysis of the reduction and oxidation potentials allowed to assign these processes to the cobalt-centered reduction and oxidation. The “electrochemical gap” values for clathrochelates studied are very small and characteristic of the complexes with the redox processes localized on the molecular orbitals which are close in energy.

A comparative study of electrochemical reduction of isatin and its synthesized Schiff bases at HMDE

The electrochemical behaviour of a series of Schiff base i.e. 3-[5-phenylpyrazol-3-ylimino]indol-2-ones (IIa-e) synthesized by the reaction of various 5-substituted isatins with 3-amino-5-phenyl-pyrazole has been investigated and compared with corresponding isatin in dimethylformamide in 0·1 M LiCl using cyclic voltammetry at Hanging Mercury Drop Electrode. All synthesized Schiff bases exhibit a single irreversible two-electron reduction wave in contrast with the two discrete one-electron transfer reduction waves observed for isatin in this medium. Observation of a well-developed single reduction wave can be attributed to the higher basicity of the nitrogen species of the imine bond of Schiff bases, making proton abstraction as well as second electron transfer both rapid. The compounds are subjected to constant potential preparative electrolysis. The products are identified as secondary amines by spectroscopic methods. A mechanism for the electro-reduction process has been proposed. Kinetic parameters have also been calculated.

Synthesis, spectral, magnetic, electrochemical and kinetic studies of copper(II), nickel(II) and zinc(II) complexes derived from a phenol-based unsymmetrical “end-off” ligand

A new unsymmetrical end-off, aminomethylated N-methylpiperazine and aminomethylated diethanolamine armed binucleating ligand, 2-[bis(2-hydroxyethyl)aminomethyl]-6-[(4-methylpiperazin-1-yl)methyl]-4-formylphenol (HL), was synthesized by following sequential aromatic Mannich reactions. Mononuclear and binuclear Cu(II), Ni(II) and Zn(II) complexes were synthesized and characterized by elemental and spectral analysis. The EPR spectrum of the mononuclear copper complex shows four hyperfine splittings and the binuclear complex shows a broad signal due to anti-ferromagnetic interaction. The room temperature magnetic moment of the mono and binuclear copper complexes are 1.72 and 2.68 BM, respectively. Variable temperature magnetic moment study of the binuclear copper(II) complex shows weak antiferromagnetic coupling (−2J value, 21 cm−1). The mononuclear Ni(II) complex is square planar and diamagnetic. The six-coordinate binuclear Ni(II) complex shows a magnetic moment of 3.06 BM. Electrochemical studies of the complexes reveal that all mononuclear complexes show a single irreversible one-electron reduction wave and the binuclear complexes show two irreversible one-electron reduction waves in the cathodic region. Catecholase activity of copper(II) complexes using pyrocatechol as a model substrate and the hydrolysis of 4-nitrophenylphosphate using copper(II), nickel(II) and zinc(II) complexes as catalysts showed that binuclear complexes have higher rate constants than corresponding mononuclear complexes.

Electrochemical and spectroscopic detection of self-association of octa-alkyl phthalocyaninato cadmium compounds into dimeric species

The solution phase behaviour of non-peripherally substituted octa-hexyl cadmium phthalocyanine 3 and peripherally substituted octa-2-ethylhexyl cadmium phthalocyanine has been investigated in fresh solutions of CH(2)Cl(2), CHCl(3)-d(1) and THF/THF-d(8) using (1)H NMR spectrometry, UV-Vis spectroscopy, cyclic voltammetry, square wave voltammetry and linear sweep voltammetry. The compounds show an unexpected propensity to form dimeric species in CH(2)Cl(2) and CHCl(3)-d(1), and, in the case of , also to a lesser extent in THF/THF-d(8). This phenomenon is not observed for their metal-free analogues or . The electrochemical results provide particularly strong evidence for the dimeric structures. In particular both the first one-electron oxidation and one-electron reduction waves for 3 and 4, unlike those of and , are split. This is consistent with sequential oxidation/reduction of the two Pc ligands within a dimer. The dimeric species are likely to be the immediate precursors of the recently discovered bis-cadmium tris-phthalocyanine triple-decker sandwich complexes and formed from and over a period of time. The electrochemical data for compounds also show that (i) relative to the metal-free phthalocyanines, the cadmium phthalocyanines exhibit smaller formal reduction potentials for all but one of the observed electron transfer processes and (ii) the electron transfer processes associated with the peripherally substituted compounds, 2 and 4, are observed at more positive potentials than those for the corresponding non-peripherally substituted analogues 1 and 3.

An Electrochemical Study of Antineoplastic Gallium, Iron and Ruthenium Complexes with Redox Noninnocent α-N-Heterocyclic Chalcogensemicarbazones

The electrochemical properties of a series of alpha-N-heterocyclic chalcogensemicarbazones (HL), namely, thiosemicarbazones, selenosemicarbazones, and semicarbazones, and their gallium(III), iron(III), and ruthenium(III) complexes with the general formula [ML(2)][Y] (M = Ga, Fe or Ru; Y = PF(6)(-), NO(3)(-), or FeCl(4)(-)) were studied by cyclic voltammetry. The novel compounds were characterized by elemental analysis, a number of spectroscopic methods (NMR, UV-vis, IR), mass spectrometry and by X-ray crystallography. All complexes show several, mostly reversible, redox waves attributable to the reduction of the noninnocent chalcogensemicarbazone ligands at lower potentials (<-0.4 V vs NHE) than the metal-centered iron or ruthenium redox waves (>0 V vs NHE) in organic electrolyte solutions. The cyclic voltammograms of the gallium complexes display at least two consecutive reversible one-electron reduction waves. These reductions are shifted by approximately 0.6 V to lower potentials in the corresponding iron and ruthenium complexes. The electrochemical, chemical, and spectroscopic data indicate that the ligand-centered reduction takes place at the CH(3)CN double bond. Quantum chemical calculations on the geometric and electronic structures of 2-acetylpyridine (4)N,(4)N-dimethylthiosemicarbazone (HL(B)), the corresponding metal complexes [Ga(L(B))(2)](+) and [Fe(II)(L(B))(2)], and the one-electron reduction product for each of these species support the assignment of the reduction site and elucidate the observed order of the ligand-centered redox potentials, E(1/2)([Fe(II)(L)(2)]) < E(1/2)(HL) < E(1/2)([Ga(L)(2)](+)). The influence of water on the redox potentials of the complexes is reported and the physiological relevance of the electrochemical data for cytotoxicity as well as for ribonucleotide reductase inhibitory capacity are discussed.

Disulfido iron–manganese carbonyl cluster complexes: Synthesis, structure, bonding and properties of the radical CpFeMn 2(CO) 7(μ 3-S 2) 2

Two new compounds CpFeMn 2(CO) 7(μ 3-S 2) 2 ( 2) and Cp 3Fe 3Mn(CO) 4(μ 3-S 2) 2(μ 3-S) ( 3) were obtained by the treatment of [CpFeMn(CO) 5(μ 3-S 2)] 2 ( 1) with CO at room temperature in the presence of room light. Compound 2 contains two triply bridging disulfido ligands on opposite sides of an open FeMn 2 triangular cluster. EPR and temperature-dependent magnetic susceptibility measurements show that it is paramagnetic with one unpaired electron per formula equivalent. The electronic structure of 2 was established by DFT and Fenske-Hall (FH) molecular orbital calculations which show that the unpaired electron occupies a low lying antibonding orbital that is located principally on the iron atom. The cyclic voltammogram of 2 exhibits one reversible one-electron oxidation wave at +0.34 V and one irreversible one-electron reduction wave at −0.66 V vs. Ag/AgCl. Compound 3 contains three iron atoms and one manganese atom with two triply bridging disulfido ligands and one triply bridging sulfido ligand and has no unpaired electrons. The molecular structures of compounds 2 and 3 were established by single crystal X-ray diffraction analyses.

Mixed-Valent Metals Bridged by a Radical Ligand: Fact or Fiction Based on Structure-Oxidation State Correlations

Electron-rich Ru(acac)2 (acac- = 2,4-pentanedionato) binds to the pi electron-deficient bis-chelate ligands L, L = 2,2'-azobispyridine (abpy) or azobis(5-chloropyrimidine) (abcp), with considerable transfer of negative charge. The compounds studied, (abpy)Ru(acac)2 (1), meso-(mu-abpy)[Ru(acac)2]2 (2), rac-(mu-abpy)[Ru(acac)2]2 (3), and (mu-abcp)[Ru(acac)2]2 (4), were calculated by DFT to assess the degree of this metal-to-ligand electron shift. The calculated and experimental structures of 2 and 3 both yield about 1.35 A for the length of the central N-N bond which suggests a monoanion character of the bridging ligand. The NBO analysis confirms this interpretation, and TD-DFT calculations reproduce the observed intense long-wavelength absorptions. While mononuclear 1 is calculated with a lower net ruthenium-to-abpy charge shift as illustrated by the computed 1.30 A for d(N-N), compound 4 with the stronger pi accepting abcp bridge is calculated with a slightly lengthened N-N distance relative to that of 2. The formulation of the dinuclear systems with monoanionic bridging ligands implies an obviously valence-averaged Ru(III)Ru(II) mixed-valent state for the neutral molecules. Mixed valency in conjunction with an anion radical bridging ligand had been discussed before in the discussion of MLCT excited states of symmetrically dinuclear coordination compounds. Whereas 1 still exhibits a conventional electrochemical and spectroelectrochemical behavior with metal centered oxidation and two ligand-based one-electron reduction waves, the two one-electron oxidation and two one-electron reduction processes for each of the dinuclear compounds Ru2.5(L*-)Ru2.5 reveal more unusual features via EPR and UV-vis-NIR spectroelectrochemistry. In spite of intense near-infrared absorptions, the EPR results show that the first reduction leads to Ru(II)(L*-)Ru(II) species, with an increased metal contribution for system 4*-. The second reduction to Ru(II)(L2-)Ru(II) causes the disappearance of the NIR band. One-electron oxidation of the Ru2.5(L*-)Ru2.5 species produces a metal-centered spin for which the alternatives RuIII(L0)Ru(II) or Ru(III)(L*-)Ru(III) can be formulated. The absence of NIR bands as common for mixed-valent species with intervalence charge transfer (IVCT) absorption favors the second alternative. The second one-electron oxidation is likely to produce a dication with Ru(III)(L0)Ru(III) formulation. The usefulness and limitations of the increasingly popular structure/oxidation state correlations for complexes with noninnocent ligands is being discussed.

Comparative voltammetric behaviour of isatin and some of its Schiff bases at a solid electrode

The electrochemical behaviour of isatin (1) and its Schiff base, 3-arylimino-2H-indol-2-ones (2a-g), has been investigated and compared using cyclic (CV) and differential pulse voltammetry (DPV) techniques at glassy carbon electrode (GCE) in different solvent systems. CV of isatin (1) solution in DMF in 0.1 M LiCl showed two irreversible reduction step due to successive transfer of two electrons leading to formation of dioxindole whereas Schiff bases in this system gave a single two-electron transfer irreversible reduction peak at all scan rates studied. This is due to high basicity of nitrogen atom of imine bond, proton abstraction and second electron transfer are both rapid. The product was secondary amine in case of Schiff bases. However, in acidic BR buffer, isatin (1) is preprotonated and showed a single irreversible one-electron reduction wave due to formation of its dimer, isatide. Controlled potential electrolysis was carried out to find the number of electrons transferred and products were isolated and identified by spectroscopy. Kinetic parameters i.e. charge transfer coefficient (α na ), forward rate constant (K f,h ), diffusion constant (D 0 ½) have also been calculated.

Synthesis of new oxamide-based ligand and its coordination behavior towards copper(II) ion: Spectral and electrochemical studies

A new ligand N, N′-bis{3-(2-formyl-4-methyl-phenol)-6-iminopropyl}oxamide ( L) and its mono- and binuclear copper(II) complexes have been synthesized and characterized. The ligand shows absorption maxima at 249 and 360 with a weak transition at 455 nm. The ligand was found to be fluorescent and shows an emission maximum at 516 nm on excitation at 360 nm. The electronic spectra of the mono- and binuclear Cu(II) complexes exhibited a d–d transition in the region 520–560 nm characteristic of square planar geometry around Cu(II) ion. The ESR spectrum of the mononuclear complex showed four lines with nuclear hyperfine splitting. The binuclear complex showed a broad ESR spectrum with g = 2.10 due to antiferromagnetic interaction between the two Cu(II) ions. The room-temperature magnetic moment values ( μ eff) for the mono- and binuclear Cu(II) complexes are found to be 1.70 μ B and 1.45 μ B, respectively. The electrochemical studies of the mononuclear Cu(II) complex showed a single irreversible one-electron wave at −0.70 V ( E pc) and the binuclear Cu(II) complex showed two irreversible one-electron reduction waves at −0.75 V ( E pc 1 ) and −1.27 V ( E pc 2 ) in the cathodic region.

6,6″-Azobis(2,2′-bipyridine) and Its Dinuclear Ruthenium Complex: A Comparative Study with Positional Isomers

Abstract The azobis(bipyridine) ligand 6,6″-azobis(2,2′-bipyridine) (6-azobpy) was prepared as a new member of the family of azopolypyridine derivatives. This compound undergoes reversible trans/cis photoisomerization. Reaction of 6-azobpy and [Ru(bpy)2Cl2] afforded a dinuclear complex [{Ru(bpy)2}2(μ-6-azobpy)]4+. An analogue with deuterated bpy fragments was also prepared to help clarify the 1H NMR signals in the part of the 6-azobpy ligand. The analysis of 1H NMR spectra clearly identified the structure of the complex, in which the Ru2+ ions are chelated by the bpy parts of the bridging 6-azobpy ligand. Electronic absorption and electrochemical properties are characterized by the presence of a low-lying π* level of the bridging ligand, manifested by a metal-to-ligand charge-transfer band extending down to 800 nm and by two consecutive one-electron reduction waves at relatively less negative potentials in cyclic voltammetry. These properties were compared with those of previously reported positional isomers with the help of density functional theory (DFT), time-dependent DFT, and ZINDO calculations. The analysis of the redox couple of Ru3+/2+ showed that the redox potentials differ slightly between the diastereomers and that there is an intermetallic electronic interaction.

Synthesis, spectral, magnetic, electrochemical and kinetic studies of copper(II), nickel(II) and zinc(II) acetate complexes derived from phenol based ‘end-off’ ligands: Effect of p-substituents

A new class of symmetric, end-off, N-methyl piperazine armed binucleating ligands 2,6-bis(4-methyl piperazin-1-yl-methyl)-4-acetyl phenol (HL 1) and 2,6-bis[(4-methyl piperazin-1-yl-methyl)]-(4-methylcarboxy) phenol (HL 2) were synthesized by the Mannich reaction. Their mononuclear and binuclear Cu(II), Ni(II) and Zn(II) complexes have been synthesized. These complexes were characterized by elemental analysis, infra-red and electronic spectral analysis. In the electronic spectra, the lower electron withdrawing nature of the C(O)CH 3 p-substituent (HL 1) compared with the C(O)OCH 3 p-substituent (HL 2) of the phenolic ring causes a red shift in the LMCT-charge transfer band. The mononuclear Cu(II) complexes 1 and 7 have a magnetic moment value close to the spin only value with four hyperfine EPR signals. The binuclear Cu(II) complexes 4 and 10 illustrate an antiferromagnetic interaction ( μ eff 1.56 and 1.55 BM) at 298 K with a broad EPR signal. A variable temperature magnetic moment study of the binuclear copper(II) complexes shows that the extent of antiferromagnetic coupling increases in the order: CHO [K. Shanmuga Bharathi, A. Kalilur Rahiman, K. Rajesh, S. Sreedaran, P.G. Aravindan, D. Velmurugan, V. Narayanan, Polyhedron 25 (2006) 2859] < C(O)CH 3 < C(O)OCH 3 (−2 J values 134 [Shanmuga Bharathi et al., mentioned above], 149 and 158 cm −1, respectively). The mononuclear Ni(II) complexes 2 and 8 are square planar and diamagnetic. The six coordinated binuclear Ni(II) complexes 5 and 11 show a magnetic moment value of 2.96 and 2.95 BM, respectively. Electrochemical studies of the complexes reveal that all the mononuclear complexes show a single irreversible one-electron transfer reduction wave and the binuclear complexes show two irreversible one-electron transfer reduction waves in the cathodic region. There is an anodic shift in the reduction of the metal centres when the electron withdrawing nature of the p-substituent of the phenolic ring increases. The catecholase activity of the mono and binuclear copper(II) complexes, using pyrocatechol as a model substrate, and the hydrolysis of 4-nitrophenyl phosphate using the mono and binuclear copper(II), nickel(II) and zinc(II) complexes as catalysts showed that the binuclear complexes have higher rate constant values than those of the corresponding mononuclear complexes. A comparison of the spectral, electrochemical and magnetic behaviour of the complexes derived from the ligands is discussed on the basis of the substituent at the para position of the phenolic ring.

Crystal Structure and Electrochemical Behavior of a Novel Polyoxometalate [Ni(bpy)3]2[W4V2O19] with Lindqvist‐Type Structure.

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Crystal structure and electrochemical behavior of a novel polyoxometalate [Ni(bpy)3]2[W4V2O19] with Lindqvist-type structure

A novel polyoxometalate [Ni(bpy)3]2[W4V2O19] (Ni2V2W4) has been synthesized by the hydrothermal method and the structure determined by X-ray crystallography. Ni2V2W4 crystallizes in a trigonal system with space group R -3c (a = 15.8984 (5) A, b = 15.8984 (5) A, c = 43.855 (3) A). In the structure of the compound, terminal and bridging oxygen atoms are bond to the metal centers by σ or π bonds. The W6+ and V5+ ions in isopolyanion [W4V2O19]4- statistically distribute over the six metal centers. Using cathodic adsorptive voltammetric method with a carbon paste electrode, the redox property and the electron transferring process were studied. The results show that electrochemical behavior about W(VI) and V(V) atoms give one-electron, three-electron and two-electron reduction waves. Three successive oxidation waves are observed too. The compound was also characterized by thermal gravimetric analysis and IR spectra. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)