Reversible One-electron Oxidation Research Articles

Asymmetrical Diruthenium Complex Bridged by a Redox-Active Ligand

The asymmetrical dinuclear complex [(acac)(2)Ru1(μ-abpy)Ru2(Cym)Cl]PF(6) ([2]PF(6)), with acac(-) = acetylacetonato = 2,4-pentanedionato, abpy = 2,2'-azobis(pyridine), and Cym = p-cymene = 1-isopropyl-4-methylbenzene, has been obtained from the mononuclear precursors [Ru(acac)(2)(abpy)] and [Ru(Cym)Cl(2)](2). X-ray crystal structure analysis suggests the oxidation state formulation [(acac)(2)Ru1(III)(μ-abpy(•-))Ru2(II)(Cym)Cl](+) for 2(+), with antiferromagnetic coupling between one Ru(III) center and the radical-anion bridging ligand (abpy(•-)), based on the N-N distance of 1.352(3) Å. As appropriate references, the newly synthesized mononuclear [(abpy)Ru(II)(Cym)Cl]PF(6) ([1]PF(6)) with an unreduced N═N double bond at d(NN) = 1.269(4) Å and the symmetrical dinuclear [(acac)(2)Ru(2.5)(μ-abpy(•-))Ru(2.5)(acac)(2)] with d(NN) = 1.372(4) Å (rac isomer) support the above assignment for 2(+) as an asymmetrical mixed-valent configuration bridged by a radical ligand. Reversible one-electron oxidation leads to a dication, 2(2+), with largely metal-centered spin (EPR: g(1) = 2.207, g(2) = 2.155, and g(3) = 1.929), and a weak intervalence charge-transfer absorption at 1700 nm, as observed by spectroelectrochemistry. These results support a description of 2(2+) as [(acac)(2)Ru1(III)(μ-abpy(0))Ru2(II)(Cym)Cl](2+). Density functional theory (DFT) calculations suggest that the first reduction of [2]PF(6) also involves the bridging ligand, leading to [(acac)(2)Ru1(III)(μ-abpy(2-))Ru2(II)(Cym)Cl] (2). Experimentally, the first reduction of 2(+) is not fully reversible, with evidence for the loss of chloride to form [(acac)(2)Ru1(μ-abpy)Ru2(Cym)](+) (2a(+); g(1) = 2.454, g(2) = 2.032, and g(3) = 1.947). Further reduction produces [(acac)(2)Ru1(II)(μ-abpy(2-))Ru2(II)(Cym)] (2a), which forms [(acac)(2)Ru1(II)(μ-abpy(2-))Ru2(I)(Cym)](-)/[(acac)(2)Ru(II)(μ-abpy(•-))Ru(0)(Cym)](-) (2a(-)) in yet another one-electron step (g(1) = 2.052, g(2) = 2.008, and g(3) = 1.936). The major electronic transitions for each redox state have been assigned by time-dependent DFT calculations.

Synthesis and structural characterisation of (aryl-BIAN)copper(i) complexes and their application as catalysts for the cycloaddition of azides and alkynes

A series of Ar-BIAN-based copper(I) complexes (where Ar-BIAN = bis(aryl)acenaphthenequinonediimine) were synthesised and characterised by (1)H and (13)C NMR spectroscopies, FT-IR spectroscopy, MALDI-TOF-MS spectrometry, cyclic voltammetry and single crystal X-ray diffraction. The bis-chelated complexes of general formula [Cu(Ar-BIAN)(2)]BF(4) (where Ar = C(6)H(5) (1), 4-iPrC(6)H(4) (3), 2-iPrC(6)H(4) (4)) were prepared by reaction of [Cu(NCMe)(4)]BF(4) with two equivalents of the corresponding Ar-BIAN ligands, in dichloromethane, while the mono-chelated complexes of the type [Cu(Ar-BIAN)L(2)]BF(4) (where Ar = 2,6-iPr(2)C(6)H(3), L = PhCN (6); Ar = 4-iPrC(6)H(4), L = PPh(3) (7)) were readily accessible by treatment of [Cu(NCR)(4)]BF(4) (R = Me, Ph) with one equivalent of the corresponding Ar-BIAN ligands in the absence or presence of two equivalents of PPh(3), in the same solvent. The structures of complexes 3, 4, 6 and 7 were obtained by single crystal X-ray diffraction, showing distorted tetrahedral geometries around the copper centres in all cases. The electrochemical studies of these complexes and of the already reported [Cu(2,4,6-Me(3)C(6)H(2)-BIAN)(2)]BF(4) (2) and [Cu(2,6-iPr(2)C(6)H(3)-BIAN)(NCMe)(2)] (5), demonstrated that the bis-chelated complexes 1-4 undergo a reversible one-electron reduction or oxidation processes on copper, while the mono-chelated complexes 5-7 show a partially reversible oxidation and an irreversible reduction feature. Both kinds of (Ar-BIAN)copper(I) complexes are active catalysts for the copper(I)-catalysed azide-alkyne cycloaddition reaction (CuAAC). Complex 7, bearing PPh(3) ligands, exhibits the highest catalytic activity, which is comparable with that of the typical CuSO(4)-sodium ascorbate catalyst system.

Bis(acetonitrile)bis(acetylacetonato)ruthenium(iii) mediated chemical transformations of coordinated 2-methylthioanilide

Chemical reaction of [Ru(III)(acac)(2)(CH(3)CN)(2)]ClO(4) (1) with 2-methylthioaniline, HL(1) in ethanol under basic conditions yielded three new complexes Ru(II)(acac)(2)(L(1b)) (1b), (L(1b) = 4-imino-3-(methylsulfanyl)cyclohexa-2,5-dien-1-one), Ru(III)(acac)(2)(L(1c)) (1c), (HL(1c) = N-(2-methylthiophenyl)formamide) and (acac)(2)Ru(II)(μ-L(1d))Ru(II)(acac)(2) (1d), (L(1d) = 1,4-bis(2-methylthioaniline)-1,4-diazabutadiene) via the intermediate Ru(III)(acac)(2)(L(1a)) (1a, L(1a) = (L(1))(-) = 2-methylthioanilide). The reaction proceeded through temperature induced valence tautomerisation between the Ru(III) center and its 2-methylthioanilide counterpart in 1a with concomitant reduction of ruthenium from +III to +II oxidation state and oxidation of ligand L(1a), resulting in aromatic ring hydroxylation, N-formylation and C-C bond formation reactions. All the complexes have been characterised by their single-crystal X-ray structure determination and various spectroscopic and electrochemical techniques. The identity of complex 1a has been confirmed by X-ray crystal structure determination of complex 2, a phenyl analogue of complex 1a. The complexes (1a-d) showed intense charge transfer (MLCT/LMCT) transition in the long wavelength region. The paramagnetic compounds, 1a and 1c, along with the diamagnetic compound 1b showed two one-electron responses in the ranges, -0.4 to -1.0 V and 0.3 to 1.1 V. The diamagnetic complex 1d displayed two successive one-electron reversible reductions (-1.31 and -1.55 V) and two one-electron reversible oxidation processes (-0.036 and 0.51 V). The redox processes are characterized by EPR spectroscopy and spectroelectrochemistry. The compound 1c has been found to exhibit solvatochromism and concentration dependent aggregation in solution.

Heterometallic platinum(ii) compounds with β-aminoethylferrocenes: synthesis, electrochemical behaviour and anticancer activity

A new family of heterometallic compounds 3-6 containing ferrocenyl and platinum(II) centers has been synthesized by reaction of 1-β-aminoethylferrocene (1) and 1,1'-bis(β-aminoethyl)ferrocene (2) with Pt(II) precursors. Using K(2)[PtCl(4)] as the Pt(II) source, the cis-square-planar neutral compounds [Fe{η(5)-C(5)H(4)(CH(2))(2)NH(2)}(2)PtCl(2)] (3) and [{Fe(η(5)-C(5)H(4)(CH(2))(2)NH(2))(η(5)-C(5)H(5))}(2)PtCl(2)] (5) were obtained. Reaction of cis-[PtCl(2)(dmso)(2)] with 1 and 2 resulted in the displacement of dmso and chloride ligands from the platinum coordination sphere, affording the cationic and neutral compounds [Fe{η(5)-C(5)H(4)(CH(2))(2)NH(2)}(2)Pt(dmso)Cl]Cl (4) and [Fe(η(5)-C(5)H(4)(CH(2))(2)NH(2))(η(5)-C(5)H(5))Pt(dmso)Cl(2)] (6). Compounds 3-6 were thoroughly characterized using multinuclear ((1)H, (13)C, (195)Pt) NMR, IR spectroscopy, ESI mass spectrometry and elemental analysis. Single-crystal X-ray analysis of heterometallic 6 confirmed the cis geometry of the molecule and revealed that the platinum atom is held in a perfect square-planar geometry. The electrochemical behaviour of the heterometallic compounds 3-6, which has been examined by cyclic (CV) and square wave (SWV) voltammetries in dichloromethane and dmso solution, is characterized by the reversible one-electron oxidation of the ferrocene moieties. The results of the biological activity studies revealed that the organometallic complex 5 is active against all cell lines with GI(50) values in the range 1.7-2.3 μM. When compared to the standard anticancer drug cisplatin, heterotrimetallic 5, possessing two aminoethylferrocenyl units coordinated to the Pt(II) center, showed a greater activity profile in the colon cancer cell line. Cell cycle studies revealed that the new mixed compound exhibits a mechanism of action different to cisplatin.

Redox switching of the antiferromagnetic coupling in permethylated dicopper(ii) paracyclophanes

A unique magnetic electroswitching behavior has been observed in an oxamato-based permethylated dicopper(II) paracyclophane; upon reversible one-electron oxidation of the double tetramethyl-substituted p-phenylenediamidate bridging skeleton, the spin alignment of the two Cu(II) ions (S(Cu) = ½) changes from antiparallel (OFF) to parallel (ON) in the resulting dicopper(II) π-radical cation species.

Protein Conformational Changes of the Oxidative Stress Sensor, SoxR, upon Redox Changes of the [2Fe–2S] Cluster Probed with Ultraviolet Resonance Raman Spectroscopy

The [2Fe-2S] transcription factor, SoxR, a member of the MerR family, functions as a bacterial sensor of oxidative stress in Escherichia coli. SoxR is activated by reversible one-electron oxidation of the [2Fe-2S] cluster and enhances the production of various antioxidant proteins through the SoxRS regulon. Ultraviolet resonance Raman (UVRR) spectroscopic analysis of SoxR revealed conformational changes upon reduction of the [2Fe-2S] cluster in the absence and presence of promoter oligonucleotide. UVRR spectra reflected the environmental or structural changes of Trp following reduction. Notably, the environment around Trp91 contacting the [2Fe-2S] cluster was altered to become more hydrophilic, whereas that around Trp98 exhibited a small change to become more hydrophobic. In addition, changes in cation-π interactions between the [2Fe-2S] cluster and Trp91 were suggested. On the other hand, the environment around Tyr was barely affected by the [2Fe-2S] reduction. Binding of the promoter oligonucleotide triggered changes in Tyr located in the DNA-binding domain, but not Trp. Furthermore, conformational changes induced upon reduction of DNA-bound SoxR were not significantly different from those of DNA-free SoxR.

Persistent Hydrogen-Bonded and Non-Hydrogen-Bonded Phenoxyl Radicals

The production of stable phenoxyl radicals is undoubtedly a synthetic chemical challenge. Yet it is a useful way to gain information on the properties of the biological tyrosyl radicals. Recently, several persistent phenoxyl radicals have been reported, but only limited synthetic variations could be achieved. Herein, we show that the amide-o-substituted phenoxyl radical (i.e. with a salicylamide backbone) can be synthesised in a stable manner, thereby permitting easy synthetic modifications to be made through the amide bond. To study the effect of H-bonding on the properties of the phenolate/phenoxyl radical redox couple, simple H-bonded and non-H-bonded o,p-tBu-protected salicylamidate compounds have been prepared. Their redox properties were examined by cyclic voltammetry and showed a fully reversible one-electron oxidation process to the corresponding phenoxyl radical species. Remarkably, the redox potential appears to be correlated, at least partially, with H-bond strength, as relatively large differences (ca. 300 mV) in the redox potential between H-bonded and non-H-bonded phenolate salts are observed. The corresponding phenoxyl radicals produced electrochemically are persistent at room temperature for at least an hour; their UV/Vis and EPR characterisation is consistent with that of phenoxyl radicals, which makes them excellent models of biological tyrosyl radicals. The analyses of the experimental data coupled with theoretical calculations indicate that both the deviation from planarity of the amide function and intramolecular H-bonding influence the oxidation potential of the phenolate. The latter H-bonding effect appears to be predominantly exerted on the phenolate and not (or only a little) on the phenoxyl radical. Thus, in these systems the H-bonding energy involved in the phenoxyl radical appears to be relatively small.

Experimental and Digital Simulation Studies of the Electrochemical Oxidation of the Metal Anion [CpW(CO)2(IMes)]−and the 17-Electron Metal Radical CpW(CO)2(IMes)•. Kinetics and Thermodynamics of Capture and Release of MeCN by a Metal Radical and a Metal Cation

The electrochemistry of [CpW(CO)2(IMes)]−[K(18-crown-6)]+ (Cp = η5-C5H5, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), CpW(CO)2(IMes)•, and [CpW(CO)2(IMes)(MeCN)]+PF6– were studied by cyclic voltammetry in MeCN (0.2 M nBu4N+PF6–). The tungsten anion [CpW(CO)2(IMes)]− showed a fully reversible one-electron oxidation with E1/2 = −1.65 V vs Cp2Fe+/0. Oxidation of the resultant 17-electron radical CpW(CO)2(IMes)•was irreversible at all scan rates, affording the 18-electron solvent adduct [CpW(CO)2(IMes)(MeCN)]+. Reduction of [CpW(CO)2(IMes)(MeCN)]+ proceeded with loss of coordinated MeCN and was followed by a second reduction to generate [CpW(CO)2(IMes)]−. Simultaneous digital simulation of voltammograms of [CpW(CO)2(IMes)]− and CpW(CO)2(IMes)• over a range of scan rates and concentrations gave estimates for E° of −0.49 V for the [CpW(CO)2(IMes)]+/• couple and −1.92 V for the [CpW(CO)2(IMes)(MeCN)]+/• couple. The neutral 19-electron radical complex CpW(CO)2(IMes)(MeCN)• is a stronger reducing agent (by 1.42 V) than the unsolvated 17-electron radical CpW(CO)2(IMes)•. Binding of MeCN by the unsolvated 16-electron cation [CpW(CO)2(IMes)]+ is exergonic with Keq = 1.2 × 1011 M–1 for [CpW(CO)2(IMes)]+ + MeCN ⇌ [CpW(CO)2(IMes)(MeCN)]+. Expulsion of the MeCN ligand from the 19-electron complex CpW(CO)2(IMes)(MeCN)• is also exergonic, with Keq = 1.2 × 1013 M for CpW(CO)2(IMes)(MeCN)• ⇌ CpW(CO)2(IMes)• + MeCN. Experimental and simulated cyclic voltammograms also account for the chemical reduction of the metal cation [CpW(CO)2(IMes)(MeCN)]+ by the metal anion [CpW(CO)2(IMes)]−, which ultimately produces two equivalents of the metal-centered radical CpW(CO)2(IMes)•.

Electron delocalization in vinyl ruthenium substituted cyclophanes: Assessment of the through-space and the through-bond pathways

Pseudo-para[2.2]paracyclophane- and [2.1]orthocyclophane-bridged diruthenium complexes 2 and 3 with two interlinked electroactive styryl ruthenium moieties have been prepared and investigated. Both complexes undergo two reversible consecutive one-electron oxidation processes which are separated by 270 or 105 mV. Stepwise electrolysis of the neutral complexes to first the mixed-valent radical cations and then the dioxidized dications under IR monitoring reveal incremental shifts of the charge-sensitive Ru(CO) bands and allow for an assignment of their radical cations as moderately or very weakly coupled mixed-valent systems of class II according to Robin and Day. Ground-state delocalization in the mixed-valent forms of these complexes as based on the CO band shifts is considerably larger for the “closed” paracyclophane as for the “half-open” orthocyclophane. Experimental findings are backed by the calculated IR band patterns and spin density distributions for radical cations of slightly simplified model complexes 2 Me + and 3 Me + with the P i Pr 3 ligands replaced by PMe 3. Radical cations 2 + and 3 + feature a characteristic NIR band that is neither present in their neutral or fully oxidized forms nor in the radical cation of the monoruthenium [2.2]paracyclophane complex 1 with just one vinyl ruthenium moiety. These bands are thus assigned as intervalence charge-transfer (IVCT) transitions. Our results indicate that, for the radical cations, electronic coupling “through-space” via the stacked styrene decks is significantly more efficient than the “through-bond” pathway.

Spectroscopic Evidence for Redox Isomerism in the 1,4-Diethynylbenzene-Bridged Heterobimetallic Cation [{Fe(dppe)Cp*}(μ-C≡CC6H4C≡C){Mo(dppe)(η-C7H7)}]PF6

Reaction of [FeCl(dppe)Cp*] with [Mo(C≡CC6H4-4-C≡CH)(dppe)(η-C7H7)], 1, and NaBPh4 in methanol gives the alkynylvinylidene complex [{Fe(dppe)Cp*}{μ-C≡CC6H4(H)C═C}{Mo(dppe)(η-C7H7)}]BPh4, [2A]BPh4, which is deprotonated to form the heterobimetallic, 1,4-diethynylbenzene-bridged complex [{Fe(dppe)Cp*}(μ-C≡CC6H4C≡C){Mo(dppe)(η-C7H7)}], 3. The alkynylvinylidene compound [2A]BPh4 exists as the major component of an equilibrium mixture with [Fe(dppe)Cp*}{μ-C═C(H)C6H4C≡C}{Mo(dppe)(η-C7H7)}]BPh4, [2B]BPh4, and is formed as a consequence of proton migration between the Cβ carbons of the vinylidene and alkynyl ends of the bridging ligand. Cyclic voltammetric investigations reveal that 3 undergoes two reversible one-electron oxidations to cationic [3]+ and [3]2+, which have been isolated as the [PF6]− salts after chemical oxidation. Computational (DFT) studies on [3]n+ indicate that while the HOMO of neutral 3 is rather heavily localized on the Mo center, in [3]+ the frontier orbitals are more evenly distributed over both metals, with the concentration of spin density being sensitive to the relative disposition of the metal end-cap fragments about the diethynylbenzene ligand. Experimental investigations on [3]PF6 by IR and EPR spectroscopy provide evidence for the coexistence of redox isomers [3A]+ and [3B]+, in which spin density is localized at the molybdenum or iron center, respectively. The solution IR spectrum of [3]PF6 exhibits an unusual four-band pattern in the ν(C≡C) region, consistent with the observation of two isomeric forms of [3]PF6, which are “valence trapped” on the short time scale of IR spectroscopy. In the frozen solution EPR spectrum, at 120 K, the spectroscopic signatures of both paramagnetic end-caps Mo(dppe)(η-C7H7) and Fe(dppe)Cp* are observed. The properties of [3]n+ are discussed with reference to the parent homobimetallics [{MLx}2(μ-C≡CC6H4C≡C)]n+ [MLx = Fe(dppe)Cp*, [4]n+, and Mo(dppe)(η-C7H7), [5]n+, n = 0, 1, 2].

Synthesis, characterization, crystal structure and antitumor activity of organotin(IV) compounds bearing ferrocenecarboxylic acid

Four new organotin(IV) complexes [(Bu3Sn)(FcCOO)]n (1), [(μ-Bu2Sn)2(μ-Bu2SnFcCOO)2(μ3-O)2(μ-OCH3)2]2 (2), [Ph3Sn(FcCOO)(H2O)](phen) (3) and [{Ph3Sn(FcCOO)}2(4,4′-bipy)] (4) [Fc=(η5-C5H5)Fe(η5-C5H4)] have been synthesized and characterized by elemental analyses, IR, (1H and 13C) NMR spectra and X-ray single-crystal diffraction analyses. The structure analyses show that all tin atoms in complexes 1–4 are five-coordinated with trigonal bipyramid geometry. Complexes 1–4 and FcCOOH undergo reversible one-electron oxidations in methanol solution. The antitumor activities of complexes 1–4 have also been tested. Complexes 1 and 2 exhibit medium activity towards P388 cell lines and Hela cell lines. Complexes 3 and 4 exhibit medium activity towards P388 cell lines but strong activity towards Hela cell lines. This may result from complexes 3 and 4 including the neutral molecules 1,10-phenanthroline and 4,4′-bipy.

Molecular Scaffolding with Tetrathiafulvalene—Design and Synthesis of New Molecules for Molecular Electronics

Since the field of molecular electronics originated in the 1970s, the sulfur-containing heterocycle tetrathiafulvalene (TTF) has played a leading role. The reason for this is its ability to undergo two reversible one-electron oxidation steps at potentials that can be finely tuned by either peripheral substitution or by insertion of a π-conjugated spacer between the two dithiole rings. This review summarizes our own efforts in developing the synthetic TTF chemistry for obtaining a variety of molecules with potential applications in molecular electronics devices, such as redox-controlled molecular wires incorporating vertically disposed extended TTFs (cruciform molecules), conjugated macrocycles (dehydroannulenes) with fused TTF units, TTF-perylene diimide donor–acceptor dyads with broken conjugation, and redox-controlled dihydroazulene photoswitches. The synthetic strategies are to a large extent based on acetylenic scaffolding with acetylenic dithiafulvene and tetrathiafulvalene building blocks, employing different metal-catalyzed coupling reactions.

Synthesis and spectroelectrochemical investigation of two tetraarylporphyrins

A free-base tetraarylporphyrin was synthesized by the [2+2] macrocyclization of a dipyrromethane derivative with 3,4,5-tris(dodecyloxy)benzaldehyde in 61% yield. The free-base porphyrin was metallated with zinc acetate in 94% conversion. The free-base and metallated porphyrins show typical intense Soret bands at 426 and 425 nm, respectively, along with the expected number and intensity of Q-bands. Both porphyrins are also fluorescent and display small Stokes shifts between 13 and 18 nm. Cyclic voltammetry established that each porphyrin underwent two reversible one-electron oxidations at 492 and 725 mV (vs Fc/Fc+(ferrocene reference)) for the free-base porphyrin and at 329 and 589 mV (vs Fc/Fc+) for the Zn-metalloporphyrin. Electron-transfer rates were also determined to fall between 1.2 × 10−3and 2.4 × 10−3cm s–1. In addition, spectroelectrochemistry and density functional theory calculations of the oxidized products were carried out to confirm the macrocyclic ring oxidations.

Electronic Coupling in Diiron Complexes with the Butadienediyl-Bridged Chain

The butadienediyl-bridged complexes [eta5-Cs Me5]Fe(CO)(PPh3)]2 (micro-CH=CH-CH=CH) have been prepared and characterized by cyclic voltammetry, NMR, and X-ray crystallography. Compound 3 diastereomers showed two reversible one-electron oxidations at -0.053 approximately -0.056 V and -0.02 approximately 0.03 V, respectively. The comproportionation constant, Kc, was calculated from these measurements. Kc values are very high, 1.0 approximately 3.0 x 10(9). These values are consistent with the Robin-Day Mixed-Valence Class III.

Stilbene analogues incorporating the Co(η 4-C 4Ph 4)(η 5-C 5H 4-) end group

A number of organometallic stilbenes of the general type [Co(η 4-C 4Ph 4)(η 5-C 5H 4CH CHR] are reported where R is C 6H 4X-4 (X = H, OMe, Br, NO 2), 1-naphthyl, 9-anthryl, 1-pyrenyl, (η 5-C 5H 4)Co(η 4-C 4Ph 4), and (η 5-C 5H 4)Fe(η 5-C 5H 4Y) {Y = CHO, CH C(CN) 2 and CH CHC 5H 4-η 5)Co(η 4-C 4Ph 4)}. They were prepared by Wittig or Horner–Wadsworth–Emmons reactions which yield both E and Z or only E products respectively. The isomers were separated and all compounds characterised by standard spectroscopic techniques as well as by X-ray diffraction methods in many cases. The electrochemistry of the stilbene analogues in dichloromethane solution is also reported. In most, the (η 5-C 5H 4)Co(η 4-C 4Ph 4) functional group undergoes a reversible one-electron oxidation. For those molecules that also include (η 5-C 5H 4)Fe(η 5-C 5H 4Y), this is preceded by the reversible oxidation of the ferrocenyl group. Spectroscopic and structural data suggests that for most compounds there is little electronic interaction between Co(η 4-C 4Ph 4)(η 5-C 5H 4) and the R end groups which are effectively independent of one another. The only exceptions to this are Z and E-[Co(η 4-C 4Ph 4)(η 5-C 5H 4CH CHC 6H 4NO 2-4], and [Co(η 4-C 4Ph 4)(η 5-C 5H 4CH CHC 5H 4-η 5)Fe{η 5-C 5H 4CH C(CN) 2}] where the electronic spectra are respectively consistent with a significant Co(η 4-C 4Ph 4)(η 5-C 5H 4)/NO 2 donor/acceptor interaction and a less significant Co(η 4-C 4Ph 4)(η 5-C 5H 4)/C(CN) 2 one. However, OTTLE studies show that in the electronic spectra of [Co(η 4-C 4Ph 4)(η 5-C 5H 4CH CHR] + there are low energy absorption bands (950–1800 nm) which are attributed to R → Co(η 4-C 4Ph 4)(η 5-C 5H 4) + or, when R is a ferrocenyl-base group, Co(η 4-C 4Ph 4)(η 5-C 5H 4) → (η 5-C 5H 4)Fe(η 5-C 5H 4Y) + charge transfer transitions. The ferrocenyl compounds undergo cis/trans isomerisation on the OTTLE experiment timescale.

Redox-active nickel and cobalt tris(pyrazolyl)borate dithiocarbamate complexes: air-stable Co(ii) dithiocarbamates

A series of new cobalt(II) and nickel(II) tris(3,5-diphenylpyrazolyl)borate (Tp(Ph2)) dithiocarbamate complexes [Tp(Ph2)M(dtc)] (M = Co, dtc = S₂CNEt₂ 1, S₂CNBz₂ 2 and S₂CN(CH₂)₄ 3; M = Ni, dtc = S₂CNEt₂ 4, S₂CNBz₂ 5 and S₂CN(CH₂)₄ 6) have been prepared by the reaction of [Tp(Ph2)MBr] with Nadtc in CH₂Cl₂. IR spectroscopy indicates that the Tp(Ph2) ligand is κ³ coordinated while the dithiocarbamate ligand is κ² coordinated. ¹H NMR and UV-Vis spectroscopy are consistent with high spin, five-coordinate metal centres. X-ray crystallographic studies of 1, 3 and 6 confirm the κ³ coordination of the Tp(Ph2) ligand and reveal an intermediate five-coordinate geometry with an asymmetrically coordinated dithiocarbamate ligand. Electrochemical studies of 1-6 reveal a metal centred reversible one-electron oxidation to M(III). Attempted oxidation of [Tp(Ph2)Co(dtc)] with [FeCpCp(COMe)]BF₄ yields [Co(dtc)₃], Hpz(Ph2) and a further product which may be [Tp(Ph2)CoBp(Ph2)]. DFT calculations indicate that the low redox potentials in these complexes result from a strongly antibonding M-S σ* HOMO.

Synthesis, spectroscopy and electronic structure of the vinylidene and alkynyl complexes [W(CCHR)(dppe)(η-C7H7)]+and [W(CCR)(dppe)(η-C7H7)]n+(n = 0 or 1)

The first examples of vinylidene complexes of the cycloheptatrienyl tungsten system [W(C=CHR)(dppe)(η-C₇H₇)](+) (dppe = Ph₂PCH₂CH₂PPh₂; R = H, 3; Ph, 4; C₆H₄-4-Me, 5) have been synthesised by reaction of [WBr(dppe)(η-C₇H₇)], 1, with terminal alkynes HC≡CR; a one-pot synthesis of 1 from [WBr(CO)₂(η-C₇H₇)] facilitates its use as a precursor. The X-ray structure of 4[PF₆] reveals that the vinylidene ligand substituents lie in the pseudo mirror plane of the W(dppe)(η-C₇H₇) auxiliary (vertical orientation) with the phenyl group located syn to the cycloheptatrienyl ring. Variable temperature ¹H NMR investigations on [W(C=CH₂)(dppe)(η-C₇H₇)][PF₆], 3, estimate the energy barrier to rotation about the W=C(α) bond as 62.5 ± 2 kJ mol⁻¹; approximately 10 kJ mol⁻¹ greater than for the molybdenum analogue. Deprotonation of 4 and 5 with KOBu(t) yields the alkynyls [W(C≡CR)(dppe)(η-C₇H₇)] (R = Ph, 6; C₆H₄-4-Me, 7) which undergo a reversible one-electron oxidation at a glassy carbon electrode in CH₂Cl₂ with E(½) values approximately 0.12 V negative of Mo analogues. The 17-electron radicals [6](+) and [7](+) have been investigated by spectroelectrochemical IR, UV-visible and EPR methods. The electronic structures of representative vinylidene (3) and alkynyl (6) complexes have been investigated at the B3LYP/Def2-SVP level. In both cases, electronic structure is characterised by a frontier orbital with significant metal d(z²)character and this dominates the structural and spectroscopic properties of the system.

Phenolate and phenoxyl radical complexes of Cu(ii) and Co(iii), bearing a new redox active N,O-phenol-pyrazole ligand

The synthesis and characterisation of the new N,O-phenol-pyrazole pro-ligand, (pz)LH, comprising a pyrazole covalently linked to an o,p-di-tert-butyl-substituted phenol, are herein reported. In CH(2)Cl(2) at room temperature, the cyclic voltammogram (CV) of (pz)LH exhibits a quasi-reversible one-electron oxidation process (at E(1/2) = 0.66 V vs. Fc(+)/Fc) attributed to the formation of the phenoxyl radical cation [(pz)LH]˙(+). (pz)LH reacts with M(II)(BF(4))(2) (M = Cu, Co) in a 2:1 ratio to afford the bis-Cu(pz)L(2) (1) and tris-Co(pz)L(3) (2) complexes respectively. The X-ray structure of 1 reveals a Cu(II) ion in a square-planar trans-Cu(II)-N(2)O(2) coordination environment whereas that of 2 consists of a Co(III) ion with an octahedral mer-N(3)O(3) coordination sphere; formed by the chelation of two (in 1) or three (in 2) N,O-bidentate phenolate ligands respectively. Both structures are preserved in CH(2)Cl(2) solution, as revealed by their NMR (for 2) and EPR (for 1) data. The CVs of 1 and 2 consist of two (at E(1/2): 0.43 and 0.58 V vs. Fc(+)/Fc) and three (E(1/2) = 0.12, 0.54 and 0.89 V vs. Fc(+)/Fc) reversible one-electron oxidation processes, respectively. The one-electron electrochemical oxidation of 1 and 2 produces the oxidised species, 1(+) and 2(+), which are stable for several hours at room temperature under inert atmosphere in CH(2)Cl(2). The UV/vis and EPR data obtained for 1(+) and 2(+) are unambiguously consistent with the latter being formulated as Cu(II)- and Co(III)-phenoxyl radical complexes, as [Cu(II)((pz)L˙)((pz)L)](+) and [Co(III)((pz)L˙)((pz)L)(2)](+) respectively.

Synthesis, Structural Characterization, and Photophysical, Electrochemical, Intercomponent Energy-Transfer, and Anion-Sensing Studies of Imidazole 4,5-Bis(benzimidazole)-Bridged OsIIOsIIand RuIIOsIIBipyridine Complexes

Homo- and heterobimetallic complexes of composition [(bpy)(2)M(II)(H(2)Imbzim)M'(II)(bpy)(2)](ClO(4))(3)·nH(2)O, where M(II) = M'(II) = Os (1), M(II) = Ru and M'(II) = Os (2), H(3)Imbzim = 4,5-bis(benzimidazole-2-yl)imidazole, and bpy = 2,2'-bipyridine, have been synthesized and characterized using standard analytical and spectroscopic techniques. Both of the complexes crystallized in monoclinic form with the space group P2(1)/m for 1 and P2(1)/n for 2. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated. The complexes display very intense, ligand-centered absorption bands in the UV region and moderately intense metal-to-ligand charge-transfer (MLCT) bands in the visible region. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations. The strong fluorescence of free H(3)Imbzim is completely quenched in the metal complexes by energy transfer to the metal-based units, which exhibit their characteristic MLCT phosphorescence. The luminescence data of the heterometallic complex 2 show that electronic energy transfer takes place from the ruthenium center to the osmium-based component. The anion binding properties of the complexes have been studied in solutions using absorption, emission, and (1)H NMR spectral measurements. The metalloreceptors act as sensors for F(-) and AcO(-) ions. Sensing studies indicate the presence of two successive anion-induced deprotonation steps, leading to the formation of [(bpy)(2)M(HImbzim)M'(bpy)(2)](2+) and [(bpy)(2)M(Imbzim)M'(bpy)(2)](+) species. Double deprotonation is also observed in the presence of hydroxide. The binding affinities of different anions toward the receptors have been evaluated. Cyclic voltammetry measurements carried out in acetonitrile have provided evidence in favor of anion-dependent electrochemical responses of the bimetallic metalloreceptors with F(-) and AcO(-) ions.

Synthesis and properties of 5-ferrocenyl-1 H-pyrazole-3-carbaldehydes

New ferrocene derivatives – ethyl esters of 1-aryl-5-ferrocenyl-1 H-pyrazole-3-carboxylic acids were synthesized. The corresponding aldehydes were obtained from acid esters in two steps. The reductive amination reaction of 5-ferrocenyl-1-phenyl-1 H-pyrazole-3-carbaldehyde was studied. Several of these compounds were investigated by cyclic voltammetry. All of them exhibited a reversible one-electron oxidation–reduction wave owing to the ferrocene–ferricinium redox couple with a positive shift (0.51–0.69 V) compared with that of ferrocene (0.46 V). The X-ray crystal structure of the ethyl ether 1-(3-chloro-2-fluorophenyl)-5-ferrocenyl-1 H-pyrazole-3-carboxylic acid is also presented.