One-electron Oxidation Waves Research Articles

Spectroscopic, structural and computational studies of thiophenolato bridged dirhenium(III,III) complexes

Thiophenol (HLH), 4-methylthiophenol (HLMe) and 4-chlorothiophenol (HLCl) react with dirhenium(III,II) complex [Re2(µ-O2CCH3)Cl4(µ-dppm)2] (1) (dppm = Ph2PCH2PPh2) in refluxing dry toluene to afford the diamagnetic Re2 6+ complexes of the type [Re2(μ-LR)2Cl4(μ-dppm)2] (2(LR)) [LR = C6H5S–, p-MeC6H4S– and p-ClC6H4S–]. These are the first examples of thiophenolato bridged dirhenium complexes with a Re2 6+ core containing dppm ligand. The spectral properties of the complexes are reported. In the electrochemical experiment, the type 2(LR) complexes show one quasi-reversible one-electron oxidation and one-electron reduction waves. The identity of 2(LMe) has been established by single-crystal X-ray structure determination (Re–Re distance = 2.6178(5) Å) and is shown to have an edge-shared bi-octahedral structure. DFT analysis shows that the highest occupied molecular orbitals are mainly composed of metal δ-based orbitals whereas the lowest unoccupied molecular orbitals are metal π*-based orbitals and the electronic ground state for the 2(LR) complexes is σ2π2δ*2δ2. Time-dependent DFT (TD-DFT) analysis shows that the sharp peak at 455 nm in the experimental UV-vis spectrum is mainly due to the [Cl(π)]→[Re2(π*)] transition.

Synthesis, Structures, and Photoluminescent Properties of Tricyanidonitridorhenium(V) Complexes with Bipyridine-Type Ligands.

Tricyanidonitridorhenium(V) complexes with 2,2'-bipyridine (bpy) derivatives in which the 4 and 4' positions were substituted by X, [ReN(CN)3(X2bpy)]- (X = NMe2, NH2, OMe, Me, Cl, and Br), were newly synthesized and characterized. The structures of the new complexes were determined by single-crystal X-ray analysis. UV-vis spectra of the complexes in dimethyl sulfoxide (DMSO) showed that the peak maximum wavelengths of rhenium-to-π* bpy-type-ligand charge transfer were in the range of 474-542 nm. Cyclic voltammograms in n-(C4H9)4NPF6-DMSO showed one-electron oxidation and reduction waves corresponding to the Re(VI/V) and X2bpy0/- processes, respectively. The new complexes and [ReN(CN)3bpy]- showed photoluminescence in the crystalline phase at 295 and 80 K and in DMSO at 295 K. The origin of the emission in DMSO was attributed to the triplet nature of the rhenium-to-π* bpy-type-ligand charge-transfer transition. Density functional theory calculations showed that the highest occupied and lowest unoccupied molecular orbitals were primarily localized on the dxy orbital of the rhenium and π* orbitals of the bpy-type ligand, respectively.

Effect of Distortions on the Geometric and Electronic Structures of One-Electron Oxidized Vanadium(IV), Copper(II), and Cobalt(II)/(III) Salen Complexes.

The ligands N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,2-ethanediamine and N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,3-propanediamine were chelated to V(IV)═O (1, 2), Cu(II) (3, 4), Co(II) (5), and Co(III) (6). The X-ray crystal structures of 1-6 were solved. The vanadium center in 1-2 resides in square pyramidal geometry, with an axially bound oxo ligand, whereas the metal ion displays a tetrahedrally distorted square planar geometry in 3-5. The extent of distortion is correlated to the length of the diamine spacer: The longer the linker, the larger the tetrahedral distortions. Complex 6 is octahedral with a bidentate acetate molecule that completes the coordination sphere. All the complexes were characterized by UV-vis and EPR spectroscopies, as well as DFT calculations and electrochemistry. Complexes 1-6 exhibit a reversible one-electron oxidation wave in the range -0.11-0.26 V vs Fc+/Fc. The cations 1+ and 2+ were structurally characterized, showing an octahedral V(V) ion with one oxo and one water molecule coordinated in axial positions. Their vis-NIR spectra are dominated by a band at 727 and 815 nm, respectively, which is assigned to a phenolate-to-vanadium(V) charge transfer (CT) transition. The crystal structures of 3+ and 4+ are congruent with Cu(II)-radical species, wherein the metal center remains four-coordinated. Both feature a Class II (Robin-Day classification scale) IVCT transition at around 1200 nm (ε > 1 mM cm-1), indicative of partial localization of the radical. The structure of 5+ displays a square pyramidal cobalt ion, where the fifth (axial) coordination is occupied by a water molecule. It displays a NIR feature at 1244 nm and is described as intermediate between high spin Co(III) and Co(II) radical. In the presence of acetate the dimer [(5)2(μ-OAc)]+ forms, which was structurally characterized and shows a blue shift and lowering in intensity of the NIR absorption band in comparison to 5+. Complex 6+ is a genuine Co(III) radical complex, wherein the phenoxyl moiety is localized on one side of the molecule.

A Ru-bda Complex with a Dangling Carboxylate Group: Synthesis and Electrochemical Properties.

Ruthenium complexes containing the tetradentate 2,2'-bipyridine-6,6'-dicarboxylato (bda2-) equatorial ligand and ortho-subsituted pyridines in the axial position have been prepared and characterized using spectroscopic, crystallographic and electrochemical techniques. Complexes [Ru(Hbda)(DMSO)(pyC)] (1) and [Ru(bda)(DMSO)(pyA)] (2) (where pyC is 2-pyridinecarboxylate, pyA is pyridine-2-ylmethanol and DMSO is dimethyl sulfoxide) have been isolated in moderate to high yields. The solid state structures of (1-H)- and 2 reveal the strong chelate effect of the axial pyridine ligand that coordinates in a bidentate fashion leaving the bda2- equatorial ligand coordinating in a tridentate mode. In solution, compound 2 shows a dynamic equilibrium between different coordination modes of the bda2- and pyA ligands. This phenomenon does not occur for 1 because the carboxylate binds stronger than the labile alcohol in 2. Cyclic voltammetry analysis of 1 reveals a complex behavior with a pH-independent wave at E1/2 = 1.12 V that is tentatively associated with the two-electron RuIV/II couple. In sharp contrast, complex 2 shows a pH-dependent one-electron wave at E1/2 = 0.83 V (pH 1), assigned to the proton-coupled electron transfer process of the RuIII/II couple and a pH-independent wave at E1/2 = 1.06 V assigned to the RuIV/III couple. Compound 2 is used to prepare complex [Ru(bda)(pic)(pyA)] (4). This complex is air sensitive and converts to complex [Ru(bda)(pic)(pyE)] (5) (where pyE is methyl 2-pyridine carboxylate) in the presence of methanol. This oxidation also occurs by applying a positive potential to an aqueous solution of 4, producing the derivative [Ru(bda)(pic)(pyC)] (3). Cyclic voltammetry of 3 shows two pH-independent one-electron oxidation waves at E1/2 = 0.64 V and E1/2 = 1.0 V, corresponding to the RuIII/II and RuIV/III couples, respectively. In addition, a water oxidation catalytic wave appears at Eonset ≈ 1.4 V. Foot-of-the-wave analysis of this catalytic wave based on a water nucleophilic attack accounts for a TOFmax = 0.63-0.74 s-1.

Highly conjugated donor-acceptor dyad based on monotetrathiafulvalene covalently attached to a magnesium norphthalocyanine unit

A novel highly conjugated donor–acceptor dyad composed of a tetrathiafulvalene (TTF) moiety covalently attached to a magnesium norphthalocyanine (NPc) unit (1) has been synthesized and full characterized. Ultraviolet–visible spectroscopy and electron paramagnetic resonance data reveal that 1 forms an electron transfer complex with 2,3,5,6-tetrafluoro-7,7,8,8-tetra-cyanoquinodimethane (F4TCNQ). Detailed electrochemical investigations show one irreversible one-electron oxidation wave, two quasi-reversible one-electron oxidation waves, and two irreversible one-electron reduction waves, indicating that 1 is a good π-electron donor. Compound 1 shows clear intramolecular charge transfer (ICT) interactions from the tetrathiafulvalene fragments to the norphthalocyanine core. This phenomenon can be explained by density functional theory (DFT).

Cluster core growth upon the decarbonylation of cyclopentadienyl-iron-dicarbonyl ferrocenyltelluride CpFe(CO)2TeFc: Fe1Te1 to Fe3Te3

Formal substitution of aryl group on to the ferrocenyl in [CpFe(CO)(μ-TeR)]2 (R = Ar, Fc) complex dramatically changes its Fe2Te2 core structure and chemical properties. Introduction of electron-rich and bulky ferrocenyl moiety instead of Ph in [CpFe(CO)TePh]2 provide flattening of Fe2Te2 core in [CpFe(CO)(μ-TeFc)]2 (1), further photochemical decarbonylation resulting unusual [(CpFe)3(μ-TeFc)3(μ-CO)(CO)] cluster (2). The electrochemical oxidation of 1 shows two reversible one-electron oxidation waves attributed to oxidation of Fe2Te2 core. Chemical oxidation of 1 is accompanied by isomerization of its core and results in dicationic salt cis-[CpFe(CO)(μ-TeFc)]2 (PF6)2 (3).This study also provides an illustrative example of the increasing nuclearity in Fe1 → Fe2 → Fe3 row, upon the stepwise electron-compensating decarbonylation of iron-carbonyl complexes and (apart from this) insight into the distribution of toluene molecules inclusion inside the channels of crystals of compound 2.

Bis(tetraphenylarsonium) hexafluoridotechnetate(IV) dihydrate: preparation, structure and spectroscopic analysis.

Reports of quadrivalent transition-metal fluoride salts containing bulky organic cations are limited. In this context, we prepared the bis(tetraphenylarsonium) hexafluoridotechnetate(IV) dihydrate salt, (C24H20As)2[TcF6]·2H2O, by a cation metathesis reaction of (NH4)2[TcF6] in water. This is the first report of an arsonium salt of the hexafluoridotechnetate(IV) dianion. (AsPh4)2[TcF6]·2H2O crystallizes in the triclinic space group P-1. The [TcF6]2- anion adopts a slightly distorted octahedral geometry with an average Tc-F bond length of 1.933 Å. The cyclic voltammogram of (AsPh4)2[TcF6]·2H2O in CH3CN shows a one-electron reversible oxidation wave at 1.496 V.

A near-infrared benzoquinone-coupled BODIPY: Synthesis, spectroscopic and electrochemical properties

A near-infrared absorbing boron-dipyrromethene (BODIPY) chromophore coupled with two benzoquinone moieties at its 3,5-positions, 3, was prepared via Knoevenagel condensation of 1,3,5,7-tetramethyl-8-(4[Formula: see text]-benzonitrile) BODIPY 1 with 3,5-di-tert-butyl-4-hydroxybenzaldehyde to afford 1,7-dimethyl-3,5-di-(4[Formula: see text]-hydroxy-3[Formula: see text],5[Formula: see text]-di-tert-butyl styryl)-8-(4[Formula: see text]-benzonitrile) BODIPY 2, followed by oxidization with Ag2O in good yield (91%). The UV-vis-NIR absorption spectrum of 3 exhibits two major bands at 795 and 895 nm in the near-IR region, while 2shows maximum absorbance at 661 nm and strong fluorescence at 692 nm ([Formula: see text] 0.59). The cyclic voltammetry of 3 consists of two pairs of reversible one-electron reductions at -0.61 V and -0.88 V and two pairs of one-electron oxidation waves at 0.26 V and 0.54 V. Compared with the redox potentials of 2([Formula: see text] V and [Formula: see text] 0.25 V), the first reduction of 3 is anodically shifted for 710 mV, whereas the first oxidation potential is close. Theoretical calculation reveals that conjugation with the benzoquinone moieties on the BODIPY chromophore significantly lowers the LUMO energy level and the HOMO–LUMO energy gap, resulting in a dramatic bathochromic shift of the S0–S1 transition of 3 compared with that of 2. X-ray crystallographic analysis of 3 reveals that the whole molecule adopts a V-type twisted conformation along the delocalized [Formula: see text]-conjugated pathway.

Synthesis and redox chemistry of Pd(ii) complexes of a pincer verdazyl ligand.

A series of palladium(ii) complexes containing a redox-active, tridentate verdazyl ligand of general formula (verdazyl)PdL (L = Cl, CH3CN) are synthesized. The tetrazine core of tridentate verdazyl ligand 5 is flanked by two pyridyl groups, creating a geometry in which the ancillary ligand L is bound trans to the verdazyl ring in the square planar metal complexes. Pd(ii) complexes were isolated with the verdazyl ligand in either its neutral radical charge state (6: L = CH3CN, 12: L = Cl) or its closed-shell monoanionic charge state (10: L = CH3CN, 9: L = Cl). The charge state of the ligand was determined using X-ray crystallography and NMR, EPR, and IR spectroscopy. The cyclic voltammograms of radical complexes 6 and 12 each contain a reversible one-electron reduction wave and an irreversible one-electron oxidation wave. The complexes can be chemically interconverted between radical ligand (6, 12) and reduced, closed-shell anion (9, 10) using decamethylferrocene as the reductant and a mixture of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and fluoroboric acid as the oxidant.

A novel Nb2S4 complex with a dithiophosphinate ligand: Synthesis, structure and redox properties

A new diisobutyldithiophosphinate complex, [Nb2S4(S2PiBu2)4] (1), was obtained by a ligand substitution reaction using (Bu4N)4[Nb2S4(NCS)8] and NaS2PiBu2·3H2O. Its crystal structure has been determined. The NbNb distance is 2.9034(7) Å. The cyclic voltammogram (CV) of 1 in dichloromethane exhibits two successive chemically reversible one-electron oxidation waves at +0.55 and +1.10 V (versus Ag/AgCl) attributed to the NbIV/NbV couple. This is the first {Nb2S4} cluster that shows two reversible oxidations. The paramagnetic mixed-valence Nb(IV/V) product of the one-electron oxidation, [Nb2S4(S2PiBu2)4]+ (1+), was generated in situ. The EPR spectrum of 1+ in acetonitrile reveals a 19-line signal arising from the hyperfine interaction between the unpaired electron and two equivalent 93Nb nuclei with I = 9/2, indicating complete delocalization of the unpaired electron over both metal centers. DFT calculations have been performed for the complexes [Nb2S4(S2PMe2)4], [Nb2S4(S2P(OMe)2)4], [Nb2S4(S2CNMe2)4] and [Nb2S4(S2COMe)4] and their single and double oxidized species in order to explain the peculiarities of their redox behavior. The results show that the HOMOs responsible for oxidation have a predominantly metallic character in all cases, but they are significantly more high-lying for the dithiophosphinate and dithiocarbamate complexes, which explains their ability to be oxidized.

Enhancing the Visible-Light Absorption and Excited-State Properties of Cu(I) MLCT Excited States.

A computationally inspired Cu(I) metal-to-ligand charge transfer (MLCT) chromophore, [Cu(sbmpep)2]+ (sbmpep = 2,9-di(sec-butyl)-3,8-dimethyl-4,7-di(phenylethynyl)-1,10-phenanthroline), was synthesized in seven total steps, prepared from either dichloro- or dibromophenanthroline precursors. Complete synthesis, structural characterization, and electrochemistry, in addition to static and dynamic photophysical properties of [Cu(sbmpep)2]+, are reported on all relevant time scales. UV-Vis absorption spectroscopy revealed significant increases in oscillator strength along with a concomitant bathochromic shift in the MLCT absorption bands with respect to structurally related model complexes (ε = 16 500 M-1 cm-1 at 491 nm). Strong red photoluminescence (Φ = 2.7%, λmax = 687 nm) was observed from [Cu(sbmpep)2]+, which featured an average excited-state lifetime of 1.4 μs in deaerated dichloromethane. Cyclic and differential pulse voltammetry revealed ∼300 mV positive shifts in the measured one-electron reversible reduction and oxidation waves in relation to a Cu(I) model complex possessing identical structural elements without the π-conjugated 4,7-substituents. The excited-state redox potential of [Cu(sbmpep)2]+ was estimated to be -1.36 V, a notably powerful reductant for driving photoredox chemistry. The combination of conventional and ultrafast transient absorption and luminescence spectroscopy successfully map the excited-state dynamics of [Cu(sbmpep)2]+ from initial photoexcitation to the formation of the lowest-energy MLCT excited state and ultimately its relaxation to the ground state. This newly conceived molecule appears poised for photosensitization reactions involving energy and electron-transfer processes relevant to photochemical upconversion, photoredox catalysis, and solar fuels photochemistry.

Formation of Doubly Nitrosyl-Bridged Diruthenium Complex Bearing Tridentate Ethylbis(2-pyridylethyl)amine via Conversion of Nitrate

Abstract A doubly nitrosyl-bridged dinuclear ruthenium complex, {Ru2(µ-NO)2}, bearing tridentate ethylbis(2-pyridylethyl)amine (ebpea) and acetonitrile as supporting and co-existing ligands, respectively, [{Ru(µ-NO)(ebpea)(NCMe)}2]2+ ([1]2+) has been synthesized by a reaction of tris(acetonitrile)ruthenium(II) complex with sodium nitrate in ethanol. The ebpea ligand coordinated with two pyridyl- and one amine-nitrogen atoms in a meridional mode. The dinuclear ruthenium complex containing two nitrosyl ligands bridging between two ruthenium centers as a bending mode, showed two stepwise one-electron oxidation waves at 0.03 and 0.57 V vs. Ag|0.01 M AgNO3 in MeCN and a strong characteristic NO stretching vibrational mode ν(NO) at 1336 cm−1, indicating the electronic structure of the {RuNO}-moieties was an octahedral {RuNO}8-type. The dinuclear ruthenium complex reacted with acid to give mononuclear ruthenium complexes with evolution of dinitrogen oxide.

Role of Mediator and Effects of Temperature on ortho-C-N Bond Fusion Reactions of Aniline Using Ruthenium Templates: Isolation and Characterization of New Ruthenium Complexes of the in-Situ-Generated Ligands.

In this work, ortho-C-N bond fusion reactions of aniline are followed by the use of two different ruthenium mediators. Reaction of aniline with [RuIII(terpy)Cl3] (terpy = 2,2':6',2″-terpyridine) resulted in a trans bis-aniline ruthenium(II) complex [1]+ which upon oxidation with H2O2 produced compound [2]+ of a bidentate ligand, N-phenyl-1,2-benzoquinonediimine, due to an oxidative ortho-C-N bond fusion reaction. Complex [1]+ and aniline (neat) at 185 °C produced a bis-chelated ruthenium complex (3). A previously reported complex [RuII(N-phenyl-1,2-benzoquinonediimine)(aniline)2(Cl)2] (5) undergoes similar oxidation by air at 185 °C to produce complex [3]. A separate chemical reaction between aniline and strongly oxidizing tetra-n-propylammonium perruthenate [(n-pr)4N]+[RuO4]- in air produced a ruthenium complex [4] of a N4-tetraamidophenylmacrocycle ligand via multiple ortho-C-N bond fusion reaction. Notably, the yield of this product is low (5%) at 100 °C but increases to 25% in refluxing aniline. All these complexes are characterized fully by their physicochemical characterizations and X-ray structure determination. From their structural parameters and other spectroscopic studies, complex [2]+ is assigned as [RuII(terpy)(N-phenyl-1,2-benzoquinonediimine)(Cl)]+ whereas complex [4] is described as a ruthenium(VI) complex comprised of a reduced deprotonated N-phenyl-1,2-diamidobenzene and N4-tetraamidophenylmacrocyclic ligand. Complex [2]+ exhibits one reversible oxidation at 1.32 V and one reversible reduction at -0.75 V vs Ag/AgCl reference electrode. EPR of the electrogenerated complexes has revealed that the oxidized complex is a ruthenium(III) complex with an axial EPR spectrum at gav= 2.06. The reduced complex [2], on the other hand, shows a single-line EPR signal at gav= 1.998. In contrast, complex [4] shows two successive one-electron oxidation waves at 0.5 and 0.8 V and an irreversible reduction wave at -0.9 V. EPR studies of the oxidized complexes [4]+ and [4]2+ reveal that oxidations are ligand centered. DFT calculations were employed to elucidate the electronic structures as well as the redox processes associated with the above complexes. Aerial ortho-C-N bond fusion reactions of aniline using two different mediators, viz. [RuIII(terpy)Cl3] and [(n-pr)4N]+[RuO4]-, have been followed. It is found that in the case of oxidizable Ru(III) mediator complex, C-N bond fusion is limited only to dimerization reaction whereas the high-valent Ru(VII) salt mediates multiple C-N bond fusion reactions leading to the formation of a novel tetradentate N4-tetraamidophenylmacrocyclic ligand. Valence ambiguity in the complexes of the resultant redox-active ligands is scrutinized.

Preparation, structure and electrochemistry of heterometallic Ru(II) and Fe(II) complexes

Four ruthenium(II) complexes containing ferrocene-based ligands, Ru(acac)2(dppf) (1), Ru(acac)2(η2-O2CFc) (2), Ru(PPh3)2(η2-O2CFc)2 (3) and Ru(dppf)(η2-O2CFc)2 (4) have been prepared by the reaction of Ru(acac)2(CH3CN)2 with 1,1-bis(diphenylphosphino)ferrocene or Ru(PPh3)3Cl2 with ferrocenecarboxylic acid in the presence of sodium bicarbonate. In these complexes, the Ru center is chelated by the dppf ligand or bound with the ferrocenecarboxylate moiety in a η2 fashion. All the complexes were characterized by microanalyses, 1H and 31P{1H} NMR, ESI-MS, IR, and UV–Vis spectroscopy. The solid state structure of complex 1 was confirmed by X-ray crystallography. Cyclic voltammetry measurements in dichloromethane for all complexes display reversible or quasi-reversible Ru and Fe couples with significant coupling. Two one-electron oxidation waves were observed in the cyclic voltammetry of 3 and 4 due to the stepwise oxidation of [Fe2II,II] into [Fe2II,III]+ and [Fe2II,III]+ into [Fe2III,III]2+.

Synthesis and Characterization of Copper Complexes with CuICuI, Cu1.5Cu1.5m and CuIICuII Core Structures Supported by a Flexible Dipyridylamide Ligand.

A series of copper complexes supported by a simple dipyridylamide ligand (H2pcp) were isolated and characterized. Treatment of H2pcp with NaH and copper(I) salts led to the formation of [Cu2(2pcp)2] (1a) and {Na[(Cu2(2pcp)2)2]PF6}n (1b). The X-ray crystal structures of both complexes feature CuICuI cores with close Cu···Cu interactions. Electrochemical studies of 1a showed a reversible one-electron oxidation wave in CH2Cl2. On the basis of the work on 1a, we began studying the mixed-valence copper species supported by this ligand. The reaction of H2pcp with Cu(OAc)2 and CuCl in different stoichiometries yielded [Cu2(2pcp)2Cl] (2) and [Cu3(2pcp)2Cl2] (3). X-ray crystallography and spectroscopic characterization suggested delocalized Cu1.5Cu1.5 core structures of both compounds. These results further inspired us to explore the coordination properties of H2pcp toward CuII ions. The complexes [HNEt3][Cu2(2pcp)3(ClO4)](ClO4) (4a), [Cu2(2pcp)3(NO3)] (4b), and [Cu2(2pcp)3(H2O)]BF4 (4c) featuring dinuclear CuIICuII cores were prepared and characterized by X-ray crystallography and spectroscopic methods. Structural analysis of these complexes implied that the accommodation of CuICuI, Cu1.5Cu1.5, and CuIICuII is attributed to the structural flexibility of the ligand H2pcp. Complexes 1a, 2, 3, and 4a were examined by X-ray photoelectron spectroscopy, which confirmed the oxidation state assignments. Computational studies were also performed to provide insight into the electronic structures of these complexes.

A Chiral Hemiporphyrazine Derivative: Synthesis and Chiroptical Properties.

The synthesis of an optically active hemiporphyrazine with chiral binaphthyl substituents (1) is reported, providing the first example of the incorporation of an intrinsically chiral moiety into the macrocyclic core of a hemiporphyrazine analogue. A negative circular dichroism (CD) signal is observed in the 325-450 nm region of the CD spectrum of (S,S)-1, while mainly positive bands are observed in the 220-325 nm region. Mirror symmetry is observed across the entire wavelength range of the CD spectra of (R,R)-1 and (S,S)-1. An irreversible one-electron oxidation wave with an onset potential at 1.07 V is observed by cyclic voltammetry, along with a reversible one-electron reduction wave at -0.85 V. Density functional calculations reproduce the experimentally observed data and trends, and provide further insight into the nature of the electronic transitions.

Intramolecular Electron Transfers Thwart Bistability in a Pentanuclear Iron Complex.

With the intention to investigate the redox properties of polynuclear complexes as previously reported for the pentamanganese complex [{Mn(II)(μ-bpp)3}2Mn(III)Mn(II)2(μ3-O)](3+) (2(3+)), we focused on the analogous pentairon complex that was previously isolated as all-ferrous. As Masaoka and co-workers recently published, aerobic synthesis leads to the [{Fe(II)(μ-bpp)3}2Fe(III)Fe(II)2(μ3-O)](3+) complex (1(3+)). This species exhibits in acetonitrile solution four reversible one-electron oxidation waves. Accordingly, the three oxidized species 1(4+), 1(5+), and 1(6+) with a 3Fe(II)2Fe(III), 2Fe(II)3Fe(III), and 1Fe(II)4Fe(III) composition, respectively, were generated by bulk electrolysis and isolated. Mössbauer spectroscopy allowed us to determine the spin states of all the iron ions and to unambiguously locate the sites of the successive oxidations. They all occur in the μ3-oxo core except for the 1(4+) to 1(5+) process that presents a striking electronic rearrangement, with both metals in axial position being oxidized while the core is reduced to the [Fe(III)Fe(II)2(μ3-O)](5+) oxidation level. This strongly differs from the redox behavior of the Mn5 system. The origin of this electronic switch is discussed.

Synthesis, Structure and Electrochemistry of Tetranuclear Oxygen-Centered Copper(II) Clusters with Acetylacetone and Benz-pyrazole Hydrolyzed Derivatives as Ligand

Two copper(II) clusters Cu(4)OCl(6)(pyrazole)4, 1, and Cu(4)OBr(6)(Br-pyrazole)4, 2, have been synthesized by reacting acetylacetone and benzohydrazide (1:1 ratio) with CuX(2) (X = Cl for 1 and X= Br for 2) in methanol solutions. The structures of both clusters have been established by X-ray crystallography. The clusters contain four Cu, one O, six μ(2)-X atoms, and four pyrazole ligands. The pyrazoles was prepared in situ by the reaction of acetylacetone with benzohydrazide in methanol under reflux. In 2, the methine hydrogens of the pyrazole ligands have been replaced by bromine atoms. The four copper atoms encapsulate the central O atom in a tetrahedral arrangement. All copper atoms are five-coordinate and have similar coordination environments with slightly distorted trigonal bipyramidal geometry. The cyclic voltammogram of the clusters 1 and 2 show a one-electron quasi-reversible reduction wave in the region 0.485 to 0.731 V, and a one-electron quasi-reversible oxidation wave in the region 0.767 to 0.898 V. In 1, one irreversible oxidative response is observed on the positive of side of the voltammogram at 1.512 V and this can be assigned to Cu(II) to Cu(III) oxidation.

Synthesis and electron-donating properties of novel norphthalocyanines containing thiacrown ether-linked tetrathiafulvalene moieties

A novel magnesium-base and metal-free norphthalocyanine (6 and 7) containing peripheral thiacrown ether-linked tetrathiafulvalene moieties have been synthesized and fully characterized. Electrochemical investigations showed two quasi-reversible one-electron oxidation waves, one irreversible one-electron oxidation wave and two irreversible one-electron reduction waves, indicating that these triads are good π-electron donors. Triad 6 reacted with 2,3,5,6-tetrafluoro-7,7,8,8-tetra-cyanoquinodimethane (F4TCNQ) to form a charge transfer complex that exhibited absorption bands in the vicinity of 750 and 960nm. Compound 6 showed evident intramolecular charge transfer interactions in ground states and these may be explained on the basis of the density functional theory.

Iron Complexes of Square Planar Tetradentate Polypyridyl-Type Ligands as Catalysts for Water Oxidation.

The tetradentate ligand, 2-(pyrid-2'-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline (ppq) embodies a quaterpyridine backbone but with the quinoline C8 providing an additional sp(2) center separating the two bipyridine-like subunits. Thus, the four pyridine rings of ppq present a neutral, square planar host that is well suited to first-row transition metals. When reacted with FeCl3, a μ-oxo-bridged dimer is formed having a water bound to an axial metal site. A similar metal-binding environment is presented by a bis-phenanthroline amine (dpa) which forms a 1:1 complex with FeCl3. Both structures are verified by X-ray analysis. While the Fe(III)(dpa) complex shows two reversible one-electron oxidation waves, the Fe(III)(ppq) complex shows a clear two-electron oxidation associated with the process H2O-Fe(III)Fe(III) → H2O-Fe(IV)Fe(IV) → O═Fe(V)Fe(III). Subsequent disproportionation to an Fe═O species is suggested. When the Fe(III)(ppq) complex is exposed to a large excess of the sacrificial electron-acceptor ceric ammonium nitrate at pH 1, copious amounts of oxygen are evolved immediately with a turnover frequency (TOF) = 7920 h(-1). Under the same conditions the mononuclear Fe(III)(dpa) complex also evolves oxygen with TOF = 842 h(-1).