Irreversible Two-electron Reduction Research Articles

Synthesis, substitution kinetics, DNA/BSA binding and cytotoxicity of tridentate N^E^N (E = NH, O, S) pyrazolyl palladium(II) complexes.

The pincer complexes, [Pd(L1)Cl]BF4 (PdL1), [Pd(L2)Cl]BF4 (PdL2), [Pd(L3)Cl]BF4 (PdL3), [Pd(L4)Cl]BF4 (PdL4) were prepared by reacting the corresponding ligands, 2,6-bis[(1H-pyrazol-1-yl)methyl]pyridine (L1), bis[2-(1H-pyrazol-1-yl)ethyl]amine (L2), bis[2-(1H-pyrazol-1-yl)ethyl]ether (L3), and bis[2-(1H-prazol-1-yl)ethyl]sulphide (L4) with [PdCl2(NCMe)]2 in the presence NaBF4. The solid-state structures of complexesPdL1-PdL4 confirmed a tridentate coordination mode, with one chloro ligand completing the coordination sphere to afford square-planar complexes. Chemical behaviour of the complexes in solution confirms their stability in both aqueous and DMSO stock media. The electrochemical properties of the compounds showed irreversible two-electron reduction process. Kinetic reactivity of Pd complexes with the biological nucleophiles viz, thiourea (Tu), L-methionine (L-Met) and guanosine 5'-diphosphate disodium salt (5'-GMP) followed the order: PdL2 < PdL3 < PdL4, and PdL2 < PdL1. The kinetic reactivity is subject to the electronic effects of the spectator ligand(s), and the trend was supported by the DFT computed results. The palladium complexes PdL1-PdL4 bind to calf thymus (CT-DNA) via intercalation mode. In addition, the bovine serum albumin (BSA) showed good binding affinity to the complexes. The mode of quenching mechanism of the intrinsic fluorescence of CT-DNA and BSA by the complexes was found to be static. The order of interactions of the complexes with DNA and BSA was in tandem with the rate of substitution kinetics. The complexes, however, displayed relatively low cytotoxicity (IC50 > 100µM) when tested against the human cervical adenocarcinoma (HeLa) cell line and the transformed human lung fibroblast cell line (MRC-5 SV2).

Dependence of Quinone Electrochemistry on Solvent Polarity and pH

We studied the polarographic reduction of quinones in aqueous/organic mixtures with dielectric constants (ε) from 78.4 down to 47. Added organic co-solvents were either protic or aprotic. The increase in polarographic half-wave potential, E 1/2, with declining ε was successfully fit to the Born equation down to ε ≈ 55. Cu2+ and ubiquinone0 were reduced in a single two-electron step that was more ε-sensitive when the added organic co-solvent was aprotic. Naphtho- and anthraquinone were reduced in two successive one-electron steps that were influenced identically by protic or aprotic organic co-solvents. The product of the first reduction wave was neutral semiquinone (·QH), which surprisingly, had to be protonated to ·QH2 + before reduction in the second wave. Except for r H+, radii derived from fitting our E 1/2 vs ε results to the Born equation were too small; in other words, non-electrostatic effects destabilized the oxidized species, greatly enhancing the Born electrostatic increase in E 1/2 with declining ε. Additionally, for ε < 55, we observed deviation from the Born equation, which may be due to changes in solvent structure and dynamics, and solvent-solute interactions. Finally, we studied quinones incorporated into phosphatidyl choline sonicated bilayer vesicles: Ubiquinone0 had two distinct irreversible two-electron reduction waves, one due to a population bound at the membrane surface, and another (whose E 1/2 was negatively shifted by 150 mV) due to a population localized in the membrane interior. Ubiquinone10 had a single irreversible two-electron reduction wave that was 250 mV more negative than the UQ0 membrane-interior population.

Electrochemical Study of the Redox Properties of Levulinic Acid in Organic and Aqueous-Organic Solutions

The redox properties of levulinic acid (LA) were studied by electrochemical methods in acetonitrile, a mixture of acetonitrile with water, ethanol, acetone, and dimethyl sulfoxide. It was shown that the irreversible two-electron reduction of LA under the studied conditions leads to the formation of γ-valerolactone. This reduction potential depends on the solvent and shifted anodically by following order: dimethyl sulfoxide

Synthesis, characterization, interactions with DNA and bovine serum albumin (BSA), and antibacterial activity of cyclometalated iridium(III) complexes containing dithiocarbamate derivatives

Three mononuclear cyclometalated iridium(III) complexes having dithiocarbamate ligands, [IrIII(2-C6H4py)2(L)] (where 2-C6H4py = 2-phenylpyridine; and L1H = 4-MePipzcdtH, L2H = MorphcdtH, and L3H = 4-BzPipercdtH for 1, 2, and 3, respectively), were synthesized from [Ir(2-C6H4py)2Cl]2·1/4CH2Cl2 by displacing the two bridging chlorides with one dithiocarbamate ligand. The complexes were characterized using physicochemical and spectroscopic tools along with structural analysis of [Ir(2-C6H4py)2(L2)] (2) by single crystal X-ray diffraction. Structural analysis of 2 showed a distorted octahedron in which the nitrogen donor of one 2-phenylpyridine and the carbon donor of another 2-phenylpyridine are in axial positions, trans to one another. Electrochemical analysis by cyclic voltammetry showed the irreversible two-electron equivalent reduction voltammograms of 1, 2, and 3 attributable to Ir(III) to Ir(I). Electronic characterizations of these complexes are consistent with significant delocalization of the sulfur electron density onto the empty metal d-orbital. The intercalative interaction of the complexes with calf thymus DNA was evaluated using absorption, fluorescence quenching, and viscosity measurements. The binding affinities of these complexes with bovine serum albumin were estimated in terms of quenching constants using the Stern–Volmer equation. Study of antibacterial activities of the complexes by agar disk diffusion against some species of pathogenic bacteria was also performed.

Molecular Gallium Arsenide Phosphide Clusters Prepared from AsP3, P4, and [{GaC(SiMe3)3}4

Treatment of AsP(3) with 0.75 equivalents of [{GaC(SiMe(3))(3)}(4)] resulted in selective insertion of three equivalents of {GaC(SiMe(3))(3)} into the three As--P bonds to give [As{GaC(SiMe(3))(3)}(3)P(3)] (1-As) with an intact cyclo-P(3) ring. This yellow compound has been characterized by NMR spectroscopy, combustion analysis, single-crystal X-ray diffraction, UV/Vis spectroscopy, Raman spectroscopy, and cyclic voltammetry (THF, 0.2 M [TBA][B(C(6)F(5))(4)]; TBA=tetrabutyl ammonium). Computational models of 1-As and the isomeric [P{GaC(SiMe(3))(3)}(3)AsP(2)] (1-P) have been investigated as well, revealing several interesting electronic features of these cage molecules. Following from the cyclic voltammetry studies of 1-As that highlight an irreversible two-electron reduction at -2.2 V versus Fc/Fc(+), treatment with one equivalent of [Mg(C(14)H(10))(thf)(3)] resulted in two-electron reduction to provide [As{GaC(SiMe(3))(3)}(3)P(3)Mg(thf)(3)] (2), in which the Mg(2+) ion has inserted into one of the P--P bonds of the cyclo-P(3) ring. It was also found that treatment of AsP(3) or P(4) with one equivalent of [{GaC(SiMe(3))(3)}(4)] resulted in formation of the quadruple insertion products [As{GaC(SiMe(3))(3)}(4)P(3)] (3) and [P{GaC(SiMe(3))(3)}(4)P(3)] (4), respectively.

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.

Reactivity of Electrochemically Generated Rhenium (II) Tricarbonyl α-Diimine Complexes: A Reinvestigation of the Oxidation of Luminescent Re(CO)3(α-Diimine)Cl and Related Compounds

The oxidative electrochemistry of luminescent rhenium (I) complexes of the type Re(CO) 3(LL)Cl, 1, and Re(CO) 3(LL)Br, 2, where LL is an alpha-diimine, was re-examined in acetonitrile. These compounds undergo metal-based one-electron oxidations, the products of which undergo rapid chemical reaction. Cyclic voltammetry results imply that the electrogenerated rhenium (II) species 1 ( + ) and 2 ( + ) disproportionate, yielding [Re(CO) 3(LL)(CH 3CN)] (+), 7, and additional products. Double potential step chronocoulometry experiments confirm that 1 ( + ) and 2 ( + ) react via second-order processes and, furthermore, indicate that the rate of disproportionation is influenced by the basicity and steric requirements of the alpha-diimine ligands. The simultaneous generation of rhenium (I) and (III) carbonyl products was detected upon the bulk oxidation of 1 using infrared spectroelectrochemistry. The rhenium (III) products are assigned as [Re(CO) 3(LL)Cl 2] (+), 5; an inner-sphere electron-transfer mechanism of the disproportionation is proposed on the basis of the apparent chloride transfer. Chemically irreversible two-electron reduction of 5 yields 1 and Cl (-). No direct spectroscopic evidence was obtained for the generation of rhenium (III) tricarbonyl bromide disproportionation products, [Re(CO) 3(LL)Br 2] (+), 6; this is attributed to their relatively rapid decomposition to 7 and dibromine. In addition, the 17-electron radical cations, 7 ( + ), were successfully characterized using infrared spectroelectrochemistry.

Electrocatalytic reduction of dioxygen at glassy carbon electrodes modified with polypyrrole/anthraquinonedisulphonate composite film in various pH solutions

Functionalized polypyrrole (PPy) film with anthraquinonedisulphonate (AQDS) incorporated as dopant was prepared by anodic polymerization of pyrrole (Py) at a glassy carbon electrode from aqueous solution. The electrochemical behavior of AQDS in PPy matrix and the electrocatalytic reduction of dioxygen on the resulting composite film were investigated in various pH solutions. The formal potential of AQDS and the reduction potential of dioxygen both exhibit pH dependence. In all pH solutions employed, the electrocatalytic reduction of dioxygen at the PPy/AQDS composite film establishes a pathway of irreversible two-electron reduction to form hydrogen peroxide. The pH 6.0 buffer solution is a more suitable medium for the reduction of dioxygen, where the PPy/AQDS composite film showed a more efficient electrocatalytic performance. It was found that AQDS is an effective mediator for the reduction of dioxygen and the reduced AQ is responsible for the enhanced catalytic activity. The catalytic current is under mixed kinetic-diffusion control. The number of electrons transferred and kinetic parameters of dioxygen reduction were determined using cyclic voltammetry, rotating disk voltammetry and Tafel polarization technique.

Tetranuclear iron(iii) complexes of an octadentate pyridine-carboxylate ligand and their catalytic activity in alkane oxidation by hydrogen peroxide

Reaction of the octadentate ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (LH) with Fe(ClO4)3 leads to the formation of the tetranuclear complexes [Fe4(mu-O)2(LH)2(ClCH2-CO2)4](ClO4)4 (1), [{Fe2(mu-O)L(R-CO2)}2](ClO4)4 (2 R = C6H5-, 3 R = CH3-, 4, R = ClCH2-). The crystal structures of complexes 1 and 2 reveal that they consist of two Fe(III)2(mu-O)(mu-RCO2)2 cores that are linked via the two LH/L ligands to give a "dimer of dimers" structure. Complex assumes a helical shape, with protonated carboxylic acid moieties of the two ligands forming a hydrogen-bonded pair at the center of the cation. In complexes 2, 3 and 4, central carboxylates of the two ligands bridge the iron ions in each of the two Fe2O units, with an interdimer iron-iron separation of approximately 10 A and an intradimer separation of approximately 3.1 A. The second carboxylate bridge within the Fe2O units is defined by exogenous benzoate (2), acetate (3) or chloroacetate (4) ligands. The aqua complex [{Fe2(mu-O)L(H2O)2}2](ClO4)6 (5) is proposed to have a similar structure, but with the exogenous bridging carboxylates replaced by two terminal water ligands. These complexes exhibit electronic and Mössbauer spectral features that are similar to those of (mu-oxo)diiron(III) proteins as well as other related (mu-oxo)bis(mu-carboxylato)diiron(III) complexes. This similarity shows that these properties are not significantly affected by the nature of the bridging exogenous carboxylate, and that the octadentate framework ligand is essential in stabilizing the "dimer of dimers" structure. This structural feature remains in highly diluted solution (10(-5) M) as evidenced by electrospray ionization mass-spectroscopy (ES MS). Cyclic voltammetric studies of complexes 2 and 5 showed two irreversible two-electron reductions, indicating that the two Fe2O units of the tetranuclear complexes behave as distinct redox entities. Complexes 2, 3 and, especially, the aqua complex 5 are active alkane oxidation catalysts. Catalytic reactions carried out with alkane substrate molecules and hydrogen peroxide predominantly gave alcohols. High stereospecificity in the oxidation of cis-1,2-dimethylcyclohexane supports the metal-based molecular mechanism of O-insertion into C-H bonds postulated for non-heme iron enzymes such as methane monooxygenase.

Synthesis and Electronic Characterization of Bipyridine Dithiolate Rhodium(III) Complexes

Five new mixed diimine 1,1'-dithiolate or dithiocarbamate ligand complexes of the form [Rh(bpy)2(SS)][PF6]n, where bpy = 2,2'-bipyridine and SS = various substituted dialkyldithiocarbamates or 1,1'-dithiolates, were synthesized from cis-[Rh(bpy)2(OTf)2][OTf]. The triflate ligands are easily displaced by other ligands and allow these syntheses to proceed in high yields (80-90% overall) under relatively mild reaction conditions and to give high purity products. Electrochemistry shows irreversible two-electron reduction of Rh(III) to Rh(I) and a concomitant loss of one bipyridine ligand; this is followed by reversible one-electron reduction of the remaining 2,2'-bipyridine ligand. The electronic characterizations of these complexes are consistent with significant delocalization of the sulfur electron density onto the empty metal d orbitals. The 1,1'-dithiolate ligands induce larger red shifts in the absorption and emission spectra than the dithiocarbamates as the 1,1'-dithiolates have a more extensive conjugation system.

Synthesis and physicochemical study of NiII complexes with tetradentate acyclic and macrocyclic N2S2 ligands as thiosalen analogs

N,N’-Polymethylenebis(thiosalicylidene)iminate and macrocyclic dithiadiazadibenzocycloalkadiene complexes of nickel(II) were synthesized and their electrochemical and spectroscopic properties were studied. Dithiadiazadibenzocycloalkadiene complexes containing two DMSO molecules coordinated to Ni2+ and two outer-sphere ClO4 − anions were synthesized by the reaction of the corresponding macrocyclic ligands with Ni(ClO4)2·6H2O. The structure of 3,6-dithia-10,14-diazadibenzo[a,g]cyclopentadeca-9,14-dienylnickel(II)[bis(dimethyl sulfoxide) bis-perchlorate] was established by X-ray diffraction. The UV-Vis spectroscopic data are consistent with octahedral structures of diiminobis(sulfide) complexes, a square-planar structure of the thiosalen complex, and distorted tetrahedral structures of other diiminodithiolate complexes. The reaction of S-tert-butylthiosalicylaldehyde with hydrazine hydrate afforded di(ortho-tert-butylthiobenzal)azine. The reaction of the latter with anhydrous NiCl2 produced a colored complex with the simplest molecular formula Ni(C16H12N2S2) in 15% yield. Semiempirical PM3(tm) calculations and the results of UV-Vis, ESR, and 1H NMR spectroscopy demonstrate that this complex has most probably a dimeric structure, in which two Ni centers adopt a nearly square-planar configuration. The complexes are clearly divided into two types according to their electrochemical behavior in DMF solutions. The type 1 is characterized by reversibility of the first reduction steps. The type 2 is characterized by irreversible two-electron reduction as the first step accompanied by deposition of Ni metal on the electrode surface. Rapid electrochemically initiated alkylation occurs in the presence of various alkylating agents (BunI, BunBr, (DmgH)2CoCH3) in a solution of complex 1 in DMF.

Reaction of Mixed-Ligand Iron–Sulfur Cluster [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2)] (Cp* = C5Me5) with Methyl Iodide. Synthesis, Structure, and Redox Behavior of [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2Me)

Abstract The reaction of the mixed-ligand iron–sulfur cluster [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2)] ([3]) (Cp* = C5Me5) with iodomethane gave a methylated cluster, [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2Me)]I ([4]I), in 60% yield. The anion exchange of [4]I with KPF6 gave [4]PF6 as a pure sample. A similar treatment of [3] using iodoethane afforded an ethylated cluster, [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2Et)]I ([5]I), and oxidized parent cluster [3]I. The reaction of [3] with 2-iodopropane did not give an alkylated cluster. An unreacted cluster [3] was recovered with a small amount of oxidized cluster [3]I. The molecular structure of [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2Me)] ([4]), which was obtained by the reduction of [4]PF6 with cobaltocene, was determined by an X-ray diffraction study. Crystallographic data are: Monoclinic, P21/a, a = 21.707(2) Å, b = 21.382(2) Å, c = 11.433(1) Å, β = 98.98(1)°, V = 5241.5(9) Å3, Z = 4, and R = 0.067 for 7412 reflections with |Fo| &amp;gt; 3σ(Fo). The methyl group transferred from MeI is located on the singly ligating sulfur atom in the disulfide ligand of the parent cluster [3]. A cyclic voltammogram of [4]PF6 in 0.1 mol dm−3 tetrabutylammonium tetrafluoroborate–CH3CN showed five reversible one-electron redox waves at E1/2 = +1.42 V, +0.92 V, +0.41 V, −0.38 V, and −1.22 V vs SCE, corresponding to 4+/3+, 3+/2+, 2+/1+, 1+/0, and 0/1− charged cluster couples, respectively. An irreversible two-electron reduction wave was also observed at Epc = −1.97 V vs SCE.

Coordination complexes of molybdenum with 3,6-di-tert-butylcatechol. Addition products of DMSO, pyridine N-oxide, and triphenylarsine oxide to the putative [MoVIO(3,6-DBCat)2] monomer and self-assembly of the chiral [(MoVIO(3,6-DBCat)2)4] square.

Molybdenum complexes of 3,6-di-tert-butylcatechol have been prepared from the reaction between [Mo(CO)(6)] and 3,6-di-tert-butyl-1,2-benzoquinone. A putative "[MoO(3,6-DBCat)(2)]" monomer is assumed to form initially by reaction with trace quantities of oxygen. Condensation of the reaction mixture leads to the formation of oligomeric products, including the [(MoO(3,6-DBCat)(2))(4)] chiral square isolated by chromatographic separation. Molybdenum centers at the corner of the square are bridged by oxo ligands centered along edges. Four-fold and inversion crystallographic symmetry gives tetramers as either LambdaLambdaLambdaLambda or DeltaDeltaDeltaDelta isomers, and the crystal structure consists of parallel columns of squares with the same chirality. Addition of O-Subst (O-Subst = dmso, pyridine N-oxide, triphenylarsine oxide) ligands to [MoO(3,6-DBCat)(2)] occurs selectively to give cis-[MoO(O-Subst)(3,6-DBCat)(2)] products. All three addition complexes are fluxional in solution. The temperature-dependent stereodymanic behavior of [MoO(dmso)(3,6-DBCat)(2)] has been shown to occur via a trigonal prismatic intermediate (Bailar twist) that conserves the cis disposition of oxo and dmso ligands. Electrochemical and chemical reduction reactions have been investigated for [MoO(dmso)(3,6-DBCat)(2)] with interest in displacement of SMe(2) with formation of cis-[MoO(2)(3,6-DBCat)(2)](2-). Cyclic voltammetry shows an irreversible two-electron reduction for the complex at -0.852 V (vs Fc/Fc(+)). Chemical reduction using CoCp(2) was observed to give a product with an electronic spectrum that is generally associated with cis-[MoO(2)(Cat)(2)](2-) complexes. Structural characterization revealed that the product was [CoCp(2)][MoO(3,6-DBCat)(2)], possibly formed as the product of dmso displacement upon one-electron reduction of [MoO(dmso)(3,6-DBCat)(2)].

Specificity in template syntheses of hexaaza-macrobicyclic cages: [Pt(Me5-tricosatrieneN6)]4+ and [Pt(Me5-tricosaneN6)]4+.

The racemic C3 hexadentate cage complex, [Pt(Me5-tricosatrieneN6)]Cl4 (1,5,9,13,20-pentamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosa- 3,14,18-triene)platinum(IV) tetrachloride), was synthesised stereospecifically and regiospecifically from a reaction of the bis-triamine template [Pt(tamc)2]Cl4 (bis[1,1,1-tris(aminomethyl)ethane]- platinum(IV) tetrachloride) with formaldehyde and then propanal, in acetonitrile under basic conditions. Largely, one racemic diastereoisomer was obtained in a surprisingly high yield (approximately 50%), even though the molecule has seven chiral centres. The origins of the stereoselective synthesis are addressed. The crystal structure of [Pt(Me5-tricosatrieneN6)]-(ZnCl4)1.5Cl.2H2O showed that all three imines were attached to one tame fragment with a chiral amine site ([symbol: see text] SSS, delta RRR) and a chiral methine carbon site ([symbol: see text] RRR, delta SSS) on each ligand strand. The PtN6(4+) moiety had a slightly distorted octahedral configuration with the two types of Pt-N bonds related to the imine and the amine donors, 2.050(7) and 2.072(6) A, respectively. Treatment with sodium borohydride (15 s, 20 degrees C) at pH approximately 12.5 reduced the imine groups, but not the Pt(IV) ion, producing a C3 saturated ligand complex [Pt(Me5-tricosaneN6)]Cl4 ((1,5,9,13,20- pentamethyl-3,7,11,15,18,22- hexaazabicyclo[7.7.7]tricosane)platinum(IV)tetrachloride). X-ray crystallographic analysis showed that the average Pt-N bond distance in the cation increased upon imine reduction to 2.10 (av) A. The cyclic voltammograms of the two cage complexes displayed irreversible two-electron reduction waves in aqueous media and a approximately 0.3 V shift to more positive potentials compared to that of the smaller cavity sar (3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) analogue. After reduction, net dissociation of one strand of the cage was also evident, to give unstable square planar Pt(II) macrocyclic products.

Electrochemistry of Carbidopentaruthenium C60 Complexes and Related Clusters

The electrochemical behavior of the face-coordinated C 6 0 -carbidopentaruthenium cluster complexes Ru 5 C(CO) 1 1 (PPh 3 )(μ 3 -η 2 ,η 2 ,η 2 -C 6 0 ) (1), Ru 5 C(CO) 1 0 (μ-η 1 ,η 1 -dppf)(μ 3 -η 2 ,η 2 ,η 2 -C 6 0 ) (2) (dppf = 1,1'-bis(diphenylphosphino)ferrocene), and PtRu 5 C(CO) 1 1 (η 2 -dppe)μ 3 -η 2 ,η 2 ,η 2 -C 6 0 ) (3) (dppe = 1,2-bis(diphenylphosphino)ethane) has been examined by cyclic voltammetry, rotating disk electrode voltammetry, and differential pulse voltammetry methods. The behavior of compounds Ru 5 C(CO) 1 5 (4), Ru 5 C(CO) 1 4 (PPh 3 ) (5), Ru 5 C(CO) 1 3 (μ-η 1 ,η 1 -dppf) (6), PtRu 5 C(CO) 1 6 (7), and PtRu 5 C(CO) 1 4 (η 2 -dppe) (8) was studied also for comparison. For both 1 and 3, the voltammetric scans show an initial irreversible two-electron reduction feature, followed by three quasi-reversible, one-electron reductions of the C 6 0 ligand. In contrast, similar scans for 2 indicate an initial quasi-reversible, one-electron reduction of the C 6 0 ligand that is dynamically coupled with a second one-electron, irreversible reduction. Stepwise reduction of the C 6 0 ligand proceeds at more negative potentials. Interpretation of the electrochemical behavior of compounds 1-8 has been enhanced by studying their chemical reduction with cobaltocene. In all cases uptake of two electrons results in irreversible loss of a CO ligand from the cluster, and the resulting dianionic complexes have been characterized by their IR (ν C O ) spectra.

Mixed-metal cluster chemistry. 19. Crystallographic, spectroscopic, electrochemical, spectroelectrochemical, and theoretical studies of systematically varied tetrahedral group 6-iridium clusters.

A systematically varied series of tetrahedral clusters involving ligand and core metal variation has been examined using crystallography, Raman spectroscopy, cyclic voltammetry, UV-vis-NIR and IR spectroelectrochemistry, and approximate density functional theory, to assess cluster rearrangement to accommodate steric crowding, the utility of metal-metal stretching vibrations in mixed-metal cluster characterization, and the possibility of tuning cluster electronic structure by systematic modification of composition, and to identify cluster species resultant upon electrochemical oxidation or reduction. The 60-electron tetrahedral clusters MIr(3)(CO)(11-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (5), C(5)HMe(4) (6), C(5)Me(5) (7); M = W, Cp = C(5)H(4)Me, x = 1 (13), x = 2 (14)] and M(2)Ir(2)(CO)(10-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (8), C(5)HMe(4) (9), C(5)Me(5) (10); M = W, Cp = C(5)H(4)Me, x = 1 (15), x = 2 (16)] have been prepared. Structural studies of 7, 10, and 13 have been undertaken; these clusters are among the most sterically encumbered, compensating by core bond lengthening and unsymmetrical carbonyl dispositions (semi-bridging, semi-face-capping). Raman spectra for 5, 8, WIr(3)(CO)(11)(eta(5)-C(5)H(4)Me) (11), and W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(4)Me)(2) (12), together with the spectrum of Ir(4)(CO)(12), have been obtained, the first Raman spectra for mixed-metal clusters. Minimal mode-mixing permits correlation between A(1) frequencies and cluster core bond strength, frequencies for the A(1) breathing mode decreasing on progressive group 6 metal incorporation, and consistent with the trend in metal-metal distances [Ir-Ir < M-Ir < M-M]. Cyclic voltammetric scans for 5-15, MoIr(3)(CO)(11)(eta(5)-C(5)H(5)) (1), and Mo(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (3) have been collected. The [MIr(3)] clusters show irreversible one-electron reduction at potentials which become negative on cyclopentadienyl alkyl introduction, replacement of molybdenum by tungsten, and replacement of carbonyl by phosphine. These clusters show two irreversible one-electron oxidation processes, the easier of which tracks with the above structural modifications; a third irreversible oxidation process is accessible for the bis-phosphine cluster 14. The [M(2)Ir(2)] clusters show irreversible two-electron reduction processes; the tungsten-containing clusters and phosphine-containing clusters are again more difficult to reduce than their molybdenum-containing or carbonyl-containing analogues. These clusters show two one-electron oxidation processes, the easier of which is reversible/quasi-reversible, and the more difficult of which is irreversible; the former occur at potentials which increase on cyclopentadienyl alkyl removal, replacement of tungsten by molybdenum, and replacement of phosphine by carbonyl. The reversible one-electron oxidation of 12 has been probed by UV-vis-NIR and IR spectroelectrochemistry. The former reveals that 12(+) has a low-energy band at 8000 cm(-1), a spectrally transparent region for 12, and the latter reveals that 12(+) exists in solution with an all-terminal carbonyl geometry, in contrast to 12 for which an isomer with bridging carbonyls is apparent in solution. Approximate density functional calculations (including ZORA scalar relativistic corrections) have been undertaken on the various charge states of W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (4). The calculations suggest that two-electron reduction is accompanied by W-W cleavage, whereas one-electron oxidation proceeds with retention of the tetrahedral core geometry. The calculations also suggest that the low-energy NIR band of 12(+) arises from a sigma(W-W) --> sigma*(W-W) transition.

Mixed-metal cluster chemistry: Part 20. Syntheses, crystal structures and electrochemical studies of W 2Ir 2(μ-L)(CO) 8(η 5-C 5H 4Me) 2 (L=dppe, dppf)

The 60-electron tetrahedral clusters W 2Ir 2(μ-L)(CO) 8(η 5-C 5H 4Me) 2 [L=dppe ( 2), dppf ( 3)] have been prepared from reaction between W 2Ir 2(CO) 10(η 5-C 5H 4Me) 2 ( 1) and the corresponding diphosphine in 52 and 66% yields, respectively. A structural study of 2 reveals that three edges of a WIr 2 face are spanned by bridging carbonyls, that the iridium-ligated diphosphine coordinates diaxially and that the tungsten-bound methylcyclopentadienyls coordinate axially and apically with respect to the plane of bridging carbonyls. A structural study of 3 reveals that the dppf ligand bridges an IrIr bond which is also spanned by a bridging carbonyl; tungsten-ligated methylcyclopentadienyl ligands and terminal carbonyls result in electronic asymmetry (17e and 19e iridium atoms) in the electron-precise cluster. Both clusters show two reversible one-electron oxidation processes and an irreversible two-electron reduction; the dppf-containing cluster 3 has a further, irreversible, one-electron oxidation process. UV–vis-NIR spectroelectrochemical studies of the 2→ 2 +→ 2 2+ progression reveal the appearance of a low-energy transition on oxidation to 2 + which persists on further oxidation to 2 2+.

The Redox Chemistry of 4-Benzoyl-N-methylpyridinium Cations in Acetonitrile with and without Proton Donors: The Role of Hydrogen Bonding

In anhydrous CH3CN, 4-benzoyl-N-methylpyridinium cations undergo two reversible, well-separated (ΔE1/2 ∼ 0.6 V) one-electron reductions in analogy to quinones and viologens. If the solvent contains weak protic acids, such as water or alcohols, the first cyclic voltammetric wave remains unaffected while the second wave is shifted closer to the first. Both voltammetric and spectroelectrochemical evidence suggest that the positive shift of the second wave is due to hydrogen bonding between the two-electron reduced form of the ketone and the proton donors. While the one-electron reduction product is stable both in the presence and in the absence of the weak-acid proton donors, the two-electron reduction wave is reversible only in the time scale of cyclic voltammetry. Interestingly, at longer times, the hydrogen bonded adduct reacts further giving nonquaternized 4-benzoylpyridine and 4-(α-hydroxybenzyl)pyridine as the two main terminal products. In the presence of stronger acids, such as acetic acid, the second wave merges quickly with the first, producing an irreversible two-electron reduction wave. The only terminal product in this case is the quaternized 4-(α-hydroxybenzyl)-N-methylpyridinium cation. Experimental evidence points toward a common mechanism for the formation of the nonquaternized products in the presence of weaker acids and the quaternized product in the presence of CH3CO2H.

Synthesis, Structure, and Physicochemical Characterizations of a New Cationic Ruthenium(I)−Ruthenium(I) Tetracarbonyl Bipyridine Dimer Precursor for the Electrochemical Synthesis of an Organometallic Ruthenium(0) Polymer

The synthesis and structure of the Ru(I) dimer [Ru(bpy)(CO)2(CH3CN)]2(PF6)2 (1; bpy = 2,2‘-bipyridine) are described. The electrochemical properties of this complex have been examined. The irreversible two-electron reduction of 1 gives [Ru(bpy)(CO)2]n polymer film on the electrode surface. The physicochemical properties of this metal−metal bond based polymer equal those of the polymers formed from the reduction of trans(CH3CN)-[Ru(bpy)(CO)2(CH3CN)2]2+ or trans(Cl)-[Ru(bpy)(CO)2Cl2] monomer complexes previously described. The Ru(0) polymer can be oxidized in a stepwise manner, leading first to the Ru(I) dimer 1 and then to the Ru(II) monomer 2: cis(CH3CN)-[Ru(bpy)(CO)2(CH3CN)2]2+. On the other hand, the dimer 1 exhibits an irreversible two-electron oxidation, leading to the same Ru(II) monomer (2). Carbon electrodes modified with the polymer resulting from 1 catalyze the electrochemical reduction of CO2 into CO with a 98% electrical yield at a low potential in hydro-organic or pure aqueous electrolytes.

Synthesis, Characterisation and Electrochemical Behaviour of Rhodium(III) Complexes Containing 1,2-Naphthoquinone-2-oxime and Formation of Imine Complexes through N−O Bond Cleavage

The new rhodium(III) complexes [Rh(η2-nqo)L2Cl2] (1a−1d) and [Rh(η2-nqo)2LCl] (2b−2d) [1a, L = PPh3; 1b,2b, L = pyridine (py); 1c,2c, L = 4-phenylpyridine (ppy); 1d,2d, L = 4-acetylpyridine (apy)] have been prepared by treatment of the reaction mixture of RhCl3·3H2O and 1,2-naphthoquinone-2-oxime (nqo) in ethanol by P or N donor ligands. Cyclic voltammetric studies show that the new complexes display an irreversible metal-localised two-electron reduction from RhIII to RhI, accompanied by the loss of the chloride ligands. The 1,2-naphthoquinone-2-imine (nqi) complexes [Rh(η2-nqo)(η2-nqi)Cl2]·L (3b−3d) (3b, L = py; 3c, L = ppy; 3d, L = apy), [Rh(η2-nqo)(η2-nqi)Cl2] (4) and [Rh(η2-nqo)2(nqi)Cl] (5) were obtained by deoxygenation of the oxime group in which N−O bond cleavage is observed. The molecular structures of 1a, 2b, 4 and 5 were established by single crystal X-ray analyses.