Irreversible Reduction Wave Research Articles

Coordinative versatility of 2,3-bis(2-pyridyl)-5,8-dimethoxyquinoxaline ( L) to different metal ions: syntheses, crystal structures and properties of [Cu(I) L] 2 2+ and [M L] 2+ (M=Cu(II), Ni(II), Zn(II) and Co(II))

The complexes of Cu(I), Cu(II), Ni(II), Zn(II) and Co(II) with a new polypyridyl ligand, 2,3-bis(2-pyridyl)-5,8-dimethoxyquinoxaline ( L), have been synthesized and characterized. The crystal structures of these complexes have been elucidated by X-ray diffraction analyses and three types of coordination modes for L were found to exist in them. In the dinuclear complex [Cu(I) L(CH 3CN)] 2·(ClO 4) 2 ( 1), L acts as a tridentate ligand with two Cu(I) centers bridged by two L ligands to form a box-like dimeric structure, in which each Cu(I) ion is penta-coordinated with three nitrogen atoms and a methoxyl oxygen atom of two L ligands, and an acetonitrile. In [Cu(II) L(NO 3) 2]·CH 3CN 2, the Cu(II) center is coordinated to the two nitrogen atoms of the two pyridine rings of L which acts as a bidentate ligand. The structures of [Ni(II) L(NO 3)(H 2O) 2]·2CH 3CN·NO 3 ( 3), [Zn(II) L(NO 3) 2 (H 2O)]·2CH 3CN ( 4) and [Co(II) LCl 2(H 2O)] ( 5) are similar to each other in which L acts as a tridentate ligand by using its half side, and the metal centers are coordinated to a methoxyl oxygen atom and two bipyridine nitrogen atoms of L in the same side. The formation of infinite quasi-one-dimensional chains ( 1, 4 and 5) or a quasi-two-dimensional sheet ( 2) assisted by the intra- or intermolecular face-to-face aryl stacking interactions and hydrogen bonds may have stabilized the crystals of these complexes. Luminescence studies showed that 1 exhibits broad, structureless emissions at 420 nm in the solid state and at 450 nm in frozen alcohol frozen glasses at 77 K. Cyclic voltammetric studies of 1 show the presence of an irreversible metal-centered reduction wave at approximately −0.973 V versus Fc +/0 and a quasi-reversible ligand-centered reduction couple at approximately −1.996 V versus Fc +/0. The solution behaviors of these complexes have been further studied by UV–Vis and ESR techniques.

Specific labelling of sulfhydryl-containing biomolecules with redox-active N-(ferrocenyl)iodoacetamide

The synthesis, characterisation, X-ray crystal structure and electrochemical properties of a new redox-active labelling reagent, N-(ferrocenyl)iodoacetamide (Fc-IAA), are reported. This compound exhibits a reversible ferrocenium/ferrocene couple at ca. +0.345 V vs. SCE at a sweep rate of 100 mV s−1 in CH3CN at 298 K, and two irreversible reduction waves at ca. −1.324 and −2.048 V, attributable to the reduction of the iodoacetamide group. Since the iodoacetamide moiety can react specifically with the sulfhydryl group, Fc-IAA has been coupled to various biomolecules including a sulfhydryl-modified oligonucleotide, cysteine, glutathione and sulfhydryl-modified bovine serum albumin. The electrochemical properties of the bioconjugates have also been investigated.

Electrochemistry of Vanadium Oxides and Oxyhalides in Chloroaluminate Room Temperature Ionic Liquids: Formation of a New Ionic Liquid

The electrochemistry of several vanadium oxides and oxyhalides has been examined in room temperature ionic liquids made by mixing 1-ethyl-3-methylimidazolium chloride (EMIC) and aluminum chloride. By varying the mole ratio of the two components, Lewis basic, neutral, and acidic ionic liquids were formed, while the addition of NaCl to an acidic ionic liquid resulted in a Lewis buffered, neutral solvent. Because of the ionic nature of these liquids, most oxides have very low solubility: and are insoluble, while and are slightly soluble with solubility limits of less than 5 mM. The solubilities of the salts and and the oxyhalides and are significantly higher. In acidic ionic liquids, and all exhibit three irreversible reduction waves between 1.7 and 0.5 V vs. a reference electrode consisting of an aluminum wire in a 0.6 mole fraction ionic liquid. and gave similar reduction peaks, suggesting that the oxides may have reacted with the ionic liquid to form oxychlorides. A new ionic liquid was formed by using as the Lewis acid instead of In acetonitrile underwent a one-electron reversible reduction to © 2002 The Electrochemical Society. All rights reserved.

Synthesis and structure of a highly fluorinated 7-bora-7 H-benzo[ de]anthracene

1,8-Dilithionaphthalene·tmeda reacts with bis(pentafluorophenyl)boronfluoride·Et 2O in diethylether to afford 7-pentafluorophenyl-8,9,10,11-tetrafluoro-7-bora-7 H-benzo[ de]anthracene ( 1) in moderate yield. Compound 1, which features a boron atom incorporated in a fused tetracyclic system with 16 π-electrons, has been characterized by multinuclear NMR spectroscopy, and its structure determined by single crystal X-ray crystallography. Cyclic voltammetry studies in THF show an irreversible reduction wave at −1.73 V versus Fc +/Fc.

Catalytic and electrochemical properties of new manganese(III) compounds of 2-(2′-hydroxyphenyl)-oxazoline (Hphox or HClphox). Molecular structures of [Mn(Clphox) 2(MeOH) 2](ClO 4) and [Mn(phox) 2(MeOH) 2][Mn(phox) 2(ClO 4) 2](H 2O) 2

New manganese(III) complexes of Hphox (2-(2′-hydroxyphenyl)-oxazoline) and HClphox (2-(5′-chloro-2′-hydroxyphenyl)-oxazoline) have been synthesised. The X-ray structures of [Mn(phox) 2(MeOH) 2][Mn(phox) 2(ClO 4) 2](H 2O) 2 and [Mn(Clphox) 2(MeOH) 2](ClO 4) show the manganese(III) ions to be octahedrally coordinated with methanol or perchlorate at the axial coordination sites. The cyclic voltammograms of the complexes, with the exception of [Mn(phox) 2(acac)] (Hacac=2,4-pentanedione), show an irreversible reduction wave of manganese(III) to manganese(II). After addition of an excess of 1-methylimidazole (1-Meim), the reduction process shifts towards lower potentials and becomes (quasi-) reversible, indicating that the presence of 1-Meim affects the catalytic efficiency of the complexes. The complexes catalyse the epoxidation of styrene by dihydrogen peroxide. The cumulative turnover numbers towards styrene oxide obtained after 15 min. vary from 16 for [Mn(Clphox) 2(MeOH) 2](ClO 4) to 26 for [Mn(phox) 2(acac)]. Ligand degradation appears to be the limiting factor for obtaining higher turnover numbers.

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, characterization and reactivity of linear-type S-bridged trinuclear complexes containing vanadium(III) ion. Crystal structures of [VIII{M(aet)3}2](ClO4)3 (M = RhIII, IrIII; aet = 2-aminoethanethiolate)†

Reactions of fac(S)-[M(aet)3] (M = RhIII, IrIII; aet = 2-aminoethanethiolate) with an excess of VCl3 gave linear-type S-bridged trinuclear complexes [VIII{M(aet)3}2]3+ (M = RhIII 1, IrIII 2). The ΔΛ isomers (1a(ClO4)3 and 2a(ClO4)3) were selectively crystallized and their structures were determined by X-ray diffraction. Both the complex cations consist of two terminal fac(S)-[M(aet)3] units and a central vanadium atom, which is situated in an octahedral environment with a VIIIS6 chromophore. The Ir complex 2a was also formed by using VIVOCl2 instead of VIIICl3 in a redox reaction, whereas the Rh complex 1a was not formed by a similar method. The optically active isomer (ΛΛ-1b) of the Rh complex was selectively obtained by using Λ-fac(S)-[Rh(aet)3], and characterized by IR, UV-VIS absorption and CD spectra. All the obtained complexes exhibit a unique strong absorption band at ca. 18 × 103 cm−1. Stability and reactivity relating to V–S bond cleavage for the complexes were investigated using UV-VIS absorption spectroscopic methods. These studies show first order decomposition or metal exchange. While 2a has weaker V–S bonds than 1a (from the X-ray analyses), the former is considerably more stable in water than the latter. Cyclic voltammograms of 1a and 2a in water show only irreversible oxidation and reduction waves including VIV/III and VIII/II processes. The V(III) ions in the S-bridged trinuclear structures exhibit a lower magnetic moment (2.59 μB for 1a and 2.27 μB for 2a) at room temperature than the spin-only value for a d2 electronic configuration.

Ligand-Centered Oxidation in a Diiron s-Indacene Complex

The reaction of 1,3,5,7 tetra-tert-butyl-s-indacene (Ic‘) with [Fe2(CO)9] gives the bimetallic complex [{Fe(CO)3}2Ic‘], 1, where the two Fe(CO)3 fragments are coordinated to the same face of the Ic‘ ligand. The cyclic voltammogram of 1 displays two quasi-reversible molecular oxidation waves and an irreversible molecular reduction wave. 1 can be chemically oxidized to its monocation with silver tetrafluoroborate. Comparison of the crystal structures of 1 and [{Fe(CO)3}2Ic‘]+[BF4]-·CH2Cl2, 2, shows little change in bond lengths on oxidation. Although, at first glance, 2 may appear to be an Fe0/FeI mixed-valence compound, Mossbauer and EPR spectroscopy suggest that that the ionized electron originates from a largely indacene-based orbital. The structural and spectroscopic features of 1 and 2 have been rationalized by density functional calculations and compared to those of related systems.

Synthesis and characterization of photosensitive, dinuclear palladium(I) complexes with 1,1′-bis(diphenylphosphino)ferrocene (dppf), [Pd2(dppf)2(RNC)2]2+ (R=xylyl and mesityl)

Reactions of the dinuclear palladium(I) complex, [Pd 2 (RNC) 6 ](PF 6 ) 2 (R=2,6-xylyl (Xyl), 2,4,6-mesityl (Mes)), with 1,1′-bis(diphenylphosphino)ferrocene (dppf) gave dipalladium(I) complexes with dppf ligands, [Pd 2 (dppf) 2 (RNC) 2 ](PF 6 ) 2 ( 1 , R=Xyl, 66%; 2 , R=Mes, 18%), which were characterized by elemental analysis, 1 H- and 31 P-NMR spectroscopy, IR and UV–vis absorption spectroscopic analyses, and cyclic voltammetry. The structure of 1 was characterized by X-ray crystallography. The cation of compound 1 is composed of two Pd(I) atoms joined by a PdPd σ-bond (2.602(1) A), and each palladium ion has a square planar structure ligated by a terminal isocyanide, two P atoms of dppf, and the neighboring Pd atom. The dppf ligands chelate to the metal with an average PPdP bite angle of 99.19° and an average Pd⋯Fe distance of 4.236 A. The cyclopentadienyl rings of dppf ligands are in staggered form. The 1 H- and 31 P-NMR and the electronic absorption spectra of 1 and 2 indicated that the metalmetal bonded structure as observed in the crystal of 1 was retained in the solution. Complexes 1 and 2 were extremely photosensitive, and underwent a homolytic cleavage even under a room light. The reaction was monitored by the electronic absorption spectral changes and might generate a cation radical, [Pd(dppf)(RNC)] + . The cyclic voltammograms of 1 and 2 in acetonitrile solution showed two successive quasi -reversible oxidation waves at E 1/2 =0.60, 0.72 V (vs. Ag/AgPF 6 ) ( 1 ) and 0.62, 0.73 V ( 2 ) and an irreversible reduction wave at E 1/2 =−1.23 V ( 1 ) and −1.22 V ( 2 ). The former oxidation waves can be assigned to Fe(II)/Fe(III) processes of the two ferrocenyl groups and demonstrated that a charge-transfer communication between the Fe centers occurred through the PdPd single bond.

Rhodium Derivatives of Lacunary Heteropolytungstates Illustrate Metalloporphyrin Analogies. Reductive Dimerization to Rh2-Linked Keggin Moieties

The heteropolyanion [PW11O39RhCl]5- (1-Cl) was synthesized by hydrothermal reaction of [PW11O39]7- and RhCl3 at 150 °C for 20 h. The tetramethylammonium salt of 1-Cl was characterized by elemental analysis, 31P and 183W NMR, and solution molecular weight determination by ultracentrifugation (found 2744 ± 63, calcd 2816). The cyclic voltammogram of 1-Cl shows an irreversible reduction wave at −0.45 V vs Ag/AgCl, and controlled potential reduction at −0.5 V generates the dimeric metal−metal bonded species [(PW11O39Rh)2]10- (2). The composition and structure of 2 have been confirmed by elemental analysis of the cesium salt, analytical ultracentrifugation (ionic weight 5821 ± 186, calcd 5561), P and W NMR, and a limited structural analysis of the potassium salt, which revealed a Rh−Rh bond length of 2.52(2) Å (K10[(PW11O39Rh)2]·xH2O, triclinic, P1̄; a = 12.703(6), b = 17.868(8), and c = 19.131(9) Å; α = 96.56(2), β = 97.12(2), and γ = 91.318(33)°; Z = 2; V = 4278(3) Å3). Aqueous solutions of 2 show an absorption band at 720 nm attributed to the π*→σ* transition of the metal−metal bond. Photolysis (λ > 670 nm) of acetone solutions of 2 with PhCH2Br yields [PW11O39RhBr]5- (1-Br) and PhCH2CH2Ph. Oxidation of 2 by air, Br2, and hypochlorite yields [PW11O39Rh(H2O)]4- (1-aq), 1-Br, and 1-Cl, respectively. Photochemical, electrochemical, and ligand substitution routes to other [PW11O39RhL]n- species, L = I-, CN-, CH3COO-, pyridine, or S-bonded dimethyl sulfoxide, are described. Each of these complexes has a characteristic P NMR resonance, and mixtures are separable by chromatography on Sephadex.

Reductive Coupling of Carbonyl Ligands on a Cubane-Type Tetrairon Cluster [Cp4Fe4(CO)4] or [(MeC5H4)4Fe4(CO)4] To Give an Acetylene-Coordinated Cluster [Cp4Fe4(HC⋮CH)2] or [(MeC5H4)4Fe4(HC⋮CH)2

Treatment of [Cp4Fe4(CO)4] with LiAlH4 in tetrahydrofuran (THF) resulted in the reductive coupling of carbonyl ligands to produce a nonsubstituted acetylene-coordinated cluster [Cp4Fe4(HC⋮CH)2]. X-ray structure analysis revealed that the cluster consists of a puckered rhombus of four iron atoms. The coordination mode of the acetylene ligands was determined to be μ4-η2:η2:η1:η1-HC⋮CH. The cyclic voltammogram of the acetylene-coordinated cluster exhibits three reversible or quasireversible one-electron oxidation waves and one irreversible one-electron reduction wave, corresponding to +3/+2, +2/+1, +1/0, and 0/−1 charged couples, respectively. Similarly, [(MeC5H4)4Fe4(CO)4] reacts with LiAlH4 in THF to give [(MeC5H4)4Fe4(HC⋮CH)2].

Effects of chlorine substituents upon the formation, reactivity and electrochemical properties of platinum(II) and platinum(IV) metallacycles

The reactions of [Pt 2Me 4( μ-SMe 2) 2] ( 1) with chlorinated ligands Me 2NCH 2CH 2NCHAr (Ar=C 6Cl 5 ( 2a); 2,3,6-C 6H 2Cl 3 ( 2b); 2,3,5-C 6H 2Cl 3 ( 2c); 2,4-C 6H 3Cl 2 ( 2d); 3,5-C 6H 3Cl 2 ( 2e) and 3-C 6H 4Cl ( 2f)) yield either cyclometallated [C,N,N′] platinum(IV) complexes [PtMe 2Cl(Me 2NCH 2CH 2NCHR–C, N, N′)], arising from C–Cl bond activation, or cyclometallated [C,N,N′] platinum(II) complexes [PtMe(Me 2NCH 2CH 2NCHR– C, N, N′)], arising from C–H bond activation, followed by methane elimination. These processes occur at room temperature except for the formation of compound [PtMe{Me 2NCH 2CH 2NCH(3,5-C 6H 2Cl 2)}] ( 4e) which is produced in refluxing toluene, since at room temperature cyclometallation of ligand 2e is not achieved. Compound [PtMe 2{Me 2NCH 2CH 2NCH(3,5-C 6H 3Cl 2)– N, N′)}] ( 3e), arising from coordination of the ligand to the platinum center, is obtained at room temperature. The reactions of 1 with ligands PhCH 2NCHAr (Ar=2,3,6-C 6H 2Cl 3 ( 2g) and 2,3,5-C 6H 2Cl 3 ( 2h)) produce cyclometallated [C,N] platinum(IV) complexes. The reactivities of the platinum complexes towards phosphines and methyl iodide have been studied. All complexes have been characterized by NMR spectroscopy and the X-ray crystal structure of [PtMe 2Cl{Me 2NCH 2CH 2NCH(3,5-C 6H 2Cl 2)– C, N, N′}] ( 4c) has been determined. The electrochemical properties of the compounds based on cyclic voltammetry have also been studied. While the first reduction step is nearly reversible for cyclometallated platinum(II) compounds, coordination complex 3e and cyclometallated platinum(IV) compounds exhibit an irreversible reduction wave. In all cases oxidation occurs in an irreversible manner. The processes involved and the influence of the chlorine substituents are discussed.

Preparation, Stereochemistry, and Reactivity of Linear-Type S-Bridged MCrIIIM (M = RhIII, IrIII) Trinuclear Complexes with 2-Aminoethanethiolate (aet). Crystal Structures of ΔΛ-[Cr{M(aet)3}2]3+

Abstract S-Bridged trinuclear complexes, [CrIII{M(aet)3}2]3+, (M = RhIII (1), IrIII (2); aet = NH2CH2CH2S−) were newly prepared by the reaction of fac(S)-[M(aet)3] with chromium(III) nitrate. They were separated and optically resolved into the ΔΔ, ΛΛ, and ΔΛ isomers. Of these isomers for 1 and 2, the crystal structures of the ΔΛ isomers (1a and 2a) were determined by an X-ray diffraction method. Each of 1a and 2a consists of two fac(S)-[M(aet)3] subunits, a central Cr atom, three nitrates, and three water molecules, [Cr{M(aet)3}2](NO3)3·3H2O, in which three metals are aligned so as to be exactly linear. The central Cr atom is situated in an octahedral environment with the CrIIIS6 chromophore, which is formed by the coordination of two terminal fac(S)-[M(aet)3] units. The other isomers for 1 and 2 were characterized by the absorption and CD spectra, the molar conductivity, and the magnetic susceptibility. Each cyclic voltammogram of ΔΛ-[Cr{M(aet)3}2]3+ in water showed a reversible redox couple (M(IV)/M(III)) and an irreversible reduction wave (Cr(III)/Cr(II)). The stabilities of the S-bridged MCrIIIM complexes are also discussed in relation to the Cr–S bond strengths.

Fulvalenyl Mono- and Diiron Complexes: Photolytic and Electron-Transfer-Induced Substitution of the Benzene Ligands by Phosphines and CO in the Diiron Fulvalenyl Dibenzene Complexes [Fe2Fv(C6H6)2]2+/0. Generation of the Average-Valence Species [Fe2FvL6]3+(L = Phosphine)

The bi-sandwich complex [Fe2Fv(C6H6)2]2+(PF6-)2 (12+; Fv = μ2-η5:η5-fulvalenyl, unless noted otherwise) synthesized from biferrocene, was photolyzed with visible light in acetonitrile in the presence of 1,2-bis(diphenylphosphino)ethane (dppe) or bis(diphenylphosphino)methane (dppm) at −15 °C to give [Fe2Fv(dppe)2(NCMe)2]2+(PF6-)2 (2a2+) or [Fe2Fv(dppm)2(NCMe)2]2+(PF6-)2 (2b2+). The complexes 2a2+ and 2b2+ reacted in refluxing 1,2-dichloroethane with CO to give [Fe2Fv(dppe)2(CO)2]2+(PF6-)2 (3a2+) and [Fe2Fv(dppm)2(CO)2]2+(PF6-)2 (3b2+), and 2a2+ reacted similarly with PMe3 to give [Fe2Fv(dppe)2(PMe3)]2+(PF6-)2 (42+). The direduced 38-electron (38e) complex 1 reacted at −15 °C with 1 atm of CO to give [Fe2(μ2-η4:η4-Fv)(CO)6] (7) and with PMe3 to give [Fe2Fv(PMe3)4] (9). When Na+PF6- was present in stoichiometric amounts in THF, these reactions followed a different course and Na+PF6- induced electron transfer (disproportionation) by irreversibly dislocating ion pairs: the reaction of 1 with 1 atm of CO gave [Fe(η5-Fv)(η6-C6H6), Na+PF6-] (5), and that with PMe3 gave [Fe2Fv(PMe3)6]2+(PF6-)2 (82+) and the known complex [Fe(PMe3)4] (10). The cyclic voltammograms (CV) of 2a2+ and 2b2+ contain irreversible oxidation and reduction waves, but the CVs of 3a2+ and 3b2+ showed two close reversible monoelectronic reduction waves (no oxidation wave). The CVs of the hexaphosphine complexes indicated partially or fully irreversible reduction waves, respectively, but two reversible waves at +0.71 and +0.95 V for 42+ and +0.70 and +1.08 V for 82+ (vs SCE, Pt, DMF, 0.1 M n-Bu4NBF4 −30 °C). The bielectronic reduction of 2a2+ and the bielectronic oxidation of 42+ and 82+ using redox reagents led to decomposition, but the monoelectronic oxidation of 42+ and 82+ using (p-Br-C6H4)3N+SbCl6- in CH2Cl2 gave the stable mixed-valence trications 43+ and 83+, for which the Mössbauer spectra showed delocalized average valency on the Mössbauer time scale. These studies have opened the route to a variety of mono- and diiron fulvalenyl organometallic compounds, confirming the great importance of the presence of Na+PF6-. This salt can change reaction pathways and, in particular, induce electron-transfer reactions, underlining the extraordinary electronic flexibility of the fulvalenyl ligand and its ability to transfer the electron flow between two metal centers.

Synthesis, Structure, Reactivity, and Electrochemical Study of a (2,2‘-Biphosphinine)(η5-pentamethylcyclo- pentadienyl)chlororuthenium(II) Complex

[RuCp*(η4-C6H10)Cl] (Cp* = C5Me5; 1) reacts with the tmbp ligand (2; tmbp = 4,4‘,5,5‘-tetramethyl-2,2‘-biphosphinine) in THF to afford the [RuCp*(tmbp)Cl] complex 3, which has also been characterized by a single-crystal X-ray diffraction study. Complex 3 crystallizes with one THF molecule. The environment about the Ru atom corresponds to that of a classical three-legged piano-stool structure. Reaction of 3 with LiBr and KCN in CH2Cl2/MeOH afforded [RuCp*(tmbp)Br] (4) and [RuCp*(tmbp)CN] (5), respectively. 3 also reacts in CH2Cl2, in the presence of NH4PF6, with various monodentate ligands to produce a series of stable cationic complexes of the type [RuCp*(tmbp) (L)]+[PF6]- (L = acetonitrile (6), pyridine (7), trimethyl phosphite (8), triphenylphosphine (9), 2-bromo-4,5-dimethylphosphinine (10), tert-butyl isocyanide (11), cis-cyclooctene (12), norbornene (13)). All complexes were obtained in good yields and have been characterized by a combination of elemental analyses and spectroscopic methods (IR and 31P, 1H, and 13C NMR). The redox chemistry of 3 has been investigated by cyclic voltammetry in MeCN. Complex 3 is reversibly oxidized in [RuCp*III(tmbp)Cl] at +0.49 V (vs SCE). The first irreversible monoelectronic reduction wave, which occurs at −1.82 V vs SCE, indicates the formation of the [RuICp*(tmbp)] complex with the loss of Cl-. The second reversible reduction wave at −2.24 V was assigned to the formation of the anionic [Ru0Cp*(tmbp)]- complex, which is stable within the time scale of the cyclic voltammetry.

Electrochemical behavior of sodium 12-tungstodicobaltoate in aqueous and mixed solvent electrolytes

The electrochemical behavior of sodium 12-tungestodicobaltoate, Co 2W 12O 42 8− (CoTA), on glassy carbon electrodes (GCE) in aqueous and mixed solvent electrolytes containing 0.5 M H 2SO 4 has been studied. Three of tungeston's (W) reversible redox waves were identified between 0.0 and −0.6 V vs. Ag AgCl . The formal potentials of these three waves are electrolyte dependent. A reversible redox wave of Co 2+ Co 3+ was recorded between 0.75 and 0.90V, which was independent of those W's three waves. The formation of an insoluble film on the electrode surface was observed after scanning the potential between 0.0 and −0.80 V. An irreversible reduction wave was recorded between −0.6 and −0.8 V in aqueous and in mixed solvent acidic electrolyte solutions of CoTA corresponding to the deposition of this insoluble film. The electrocatalytic activities of these deposited films for H + reduction were also investigated. Some electrochemical and physicochemical data are reported.

Reduction of Acetylated Tetraphenylethylenes: Electrochemical Behavior and Stability of the Related Reduced Anions

The synthesis and characterization of 1-(4-acetylphenyl)-1,2,2-triphenylethylene (2), 1,1-bis(4-acetylphenyl)-2,2-diphenylethylene (3a), trans-1,2-bis(4-acetylphenyl)-1,2-diphenylethylene (3b), cis-1,2-bis(4-acetylphenyl)-1,2-diphenylethylene (3c), 1,1,2-tris(4-acetylphenyl)-2-phenylethylene (4), and 1,1,2,2-tetrakis(4-acetylphenyl)ethylene (5) are described. The molecular orbital energies, charge densities, and electronic spectra of the neutral and reduced species of 2 and 3a−3c were calculated using INDO/1 parameters for structures that had been optimized geometrically with AM1 parameters. The calculated electronic spectra for the neutral compounds compare favorably with the experimentally observed absorption spectra. Cyclic voltammetry of these compounds in deoxygenated THF containing 0.3 M [n-Bu4N]PF6 showed that 2 and 3a have irreversible one-electron first reduction waves, whereas 3b, 3c, 4, and 5 display quasi-reversible two-electron first reduction waves. Chemical reduction of 2−4 with Na/Hg in THF results in highly colored solutions, the absorbance spectra of which have been compared to the calculated electronic spectra for the radical anions and dianions of these compounds. Reduction of 2 results in dimerization to a pinacol product, whereas those of 3b and 4 result only in formation of the tetraarylethane product. 3a gives both pinacol and tetraarylethane upon reduction.

Zur Redoxchemie und Struktur von Oxovanadium(IV,V)-KompIexen mit substituierten Dibenzotetraaza[14]annulen Liganden / Redox Chemistry and Structure of Oxovanadium(IV.V) Complexes Containing Substituted Dibenzotetraaza[14]annulene Ligands

The oxovanadium(IV) complexes LVO and L′VO (L = dianion of 5,14-dihydro-6,8,15.17- tetramethyldibenzo[b.i][1,4.8,11]tetraazacyclotetradecine. H2L, and L′ = dianion of 5,14-dihydro-6,15-dimethyl-8,17-diphenyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine, H2L′) are redox active as indicated by cyclovoltammetric measurements and confirmed by preparative methods: Reversible oxidation to the corresponding oxovanadium (V) cations is observed at ca. 0.35 V vs. SCE in CH2Cl2, and these cations are prepared in high yield by oxidation of the oxovanadium(IV) precursors with [(C5H5)2Fe]SbF6 (reversible by addition of cobaltocene). Irreversible reduction waves are observed for both complexes in the range -1 .9 to 2.1 V vs. SCE in CH2Cl2. Preparative reduction of LVO with Na/K alloy affords very unstable anionic oxovanadium(IV) complexes containing a deprotonated or reduced ligand, while with Li[B(C2H5)3H] in THF the Lewis acid/Lewis base adduct LVOxB(C2H3)3 is isolated. Spectroscopic data including ESR [vanadium(IV)] and 51V NMR [vanadium(V)] are given and discussed. The complexes LVO and [LVO]SbF6 have been characterized by single crystal Xray structure determinations.

Synthesis, characterization and electrochemistry of phenylimido-rhenium(V) complex of 1,4,7-triazacyclononane (TACN)

The complex [Re(NPh)(OH)(PPh 3)(tacn)] 2+ ( 1) was prepared by reacting [Re(NPh)Cl 3(PPh 3) 2] with tacn (tacn = 1,4,7-triazacyclononane) in CH 2Cl 2 and has been characterized by X-ray crystal analysis. The ReNPh distance is 1.706(10) Å and the ReNPh angle is 170.2(9)°. In acetonitrile, the cyclic voltammogram of 1 shows an irreversible reduction wave at −0.77 V assignable to the reduction of rhenium(V) to rhenium(IV). Complex 1 catalyses alkene oxidation by PhIO.

Syntheses and electrochemistry of imidochromium(V) compounds. Crystal structure of [PPh4][Cr(NBut)Cl4(OH2)

Reactions of [Cr(NBut)Cl3(dme)](dme = 1,2-dimethoxyethane) or [PPh4][Cr(NBut)Cl4] with TI(C5H5) and NaL′{L′=(η-C5H5)Co[PO(OEt)2]3} gave respective paramagnetic (µeffca. 1.7 µB) half-sandwich imidochromium(V) compounds [CrL(NBut)Cl2](L =η-C5H51 or L′2). The structure of [PPh4][Cr-(NBut)Cl4(OH2)] has been established by X-ray crystallography. The [Cr(NBut)Cl4(OH2)]– anion has an octahedral geometry with Cr–N (imido) 1.619(3)A. Treatment of [Cr(NBut)(L)Cl][H2L =N,N′-bis(salicylidene)ethane-1,2-diamine or its 3,5-But2 derivative] with AgBF4 afforded cationic imidochromium(V) Schiff-base compounds [Cr(NBut)L]BF4. Reactions of the imidochromium(V) compounds with PPh3 at reflux or on irradiation with UV light gave Ph3PNBut and CrIII. Cyclic voltammetry of the imidochromium(V) compounds showed in most cases irreversible oxidation and reduction waves, which are attributed to oxidation and reduction of CrV.