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[Pt{(p-BrC6F4)NCH═C(Cl)NEt2}Cl(py)] (1Cl) is the product of the hydrogen peroxide oxidation of the PtII anticancer agent [Pt{(p-BrC6F4)NCH2CH2NEt2}Cl(py)] (1). Insights into electron delocalization and bonding in [Pt{(p-BrC6F4)NCH═C(Cl)NEt2}Cl(py)]+ (1Cl+) obtained by electrochemical oxidation of 1Cl have been gained by spectroscopic and computational studies. The 1Cl/1Cl+ process is chemically and electrochemically reversible on the short time scale of voltammetry in dichloromethane (0.10 M [Bu4N][PF6]). Substantial stability is retained on longer time scales enabling a high yield of 1Cl+ to be generated by bulk electrolysis. In situ IR and visible spectroelectrochemical studies on the oxidation of 1Cl to 1Cl+ and the reduction of 1Cl+ back to 1Cl confirm the long-term chemical reversibility. DFT calculations indicate only a minor contribution to the electron density (13%) resides on the Pt metal center in 1Cl+, indicating that the 1Cl/1Cl+ oxidation process is extensively ligand-based. Published X-ray crystallographic data show that 1Cl is present in only one structural form, while NMR data on the dissolved crystals revealed the presence of two closely related structural forms in an almost equimolar ratio. Solution-phase EPR spectra of 1Cl+ are consistent with two closely related structural forms in a ratio of about 90:10. The average g value for the frozen solution spectra (2.0567 for the major species) is significantly greater than the 2.0023 expected for a free radical. Crystal field analysis of the EPR spectra leads to an estimate of the 5d(xz) character of around 10% in 1Cl+. Analysis of X-ray absorption fine structure derived from 1Cl+ also supports the presence of a delocalized singly occupied metal molecular orbital with a spin density of approximately 17% on Pt. Accordingly, the considerably larger electron density distribution on the ligand framework (diminished PtIII character) is proposed to contribute to the increased stability of 1Cl+ compared to that of 1+.

Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport ...

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Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport processes and high stability over many redox cycles. Consequently, knowledge of reversible potentials and Li

Low-valent iron centers are critical intermediates in chemical and bio-chemical processes. Herein, we show the first example of a low-valent FeI center stabilized in a high-valent polyoxometalate framework. Electrochemical studies show that the FeIII -functionalized molecular vanadium(V) oxide (DMA)[FeIII ClVV12 O32 Cl]3- (DMA=dimethylammonium) features two well-defined, reversible, iron-based electrochemical reductions which cleanly yield the FeI species (DMA)[FeI ClVV12 O32 Cl]5- . Experimental and theoretical studies including electron paramagnetic resonance spectroscopy and density functional theory computations verify the formation of the FeI species. The study presents the first example for the seemingly paradoxical embedding of low-valent metal species in high-valent metal oxide anions and opens new avenues for reductive electron transfer catalysis by polyoxometalates.

Niedrigvalente Eisenzentren sind wichtige Zwischenstufen in chemischen und biochemischen Prozessen. Hier wird das erste Beispiel eines niedrigvalenten Eisen(I)-Zentrums in einem hochvalenten Polyoxometallat-Anion beschrieben. Elektrochemische Studien zeigen, dass das Eisen(III)-funktionalisierte molekulare Vanadium(V)-oxid (DMA)[FeIIIClVV12O32Cl]3− (DMA=Dimethylammonium) durch zwei wohldefinierte, reversible Eisen-basierte Reduktionen in die FeI-Spezies (DMA)[FeIClVV12O32Cl]5− ubergeht. Experimentelle und theoretische Studien, einschlieslich Elektronenspinresonanz-Spektroskopie und Dichtefunktionalrechnungen, bestatigen die Bildung der Eisen(I)-Spezies. Die Studie beschreibt das erste Beispiel des auf den ersten Blick paradoxen Einbettens niedrigvalenter Metallionen in hochvalente Oxometallat-Anionen und eroffnet neue Moglichkeiten fur reduktive Elektronentransferprozesse durch Polyoxometallate.

The structure of the Keggin-type β-[PW12O40]3– (PW12) polyoxometalate, with n-Bu4N+ as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW12 isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH3CN (0.1 M [n-Bu4N][PF6]) reveals that the reversible potential for the β-PW12 isomer always remains ca. 100 mV more positive than that of the α-PW12 isomer on addition of the acid CF3SO3H. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0–5 mM mimic experimental data exceptionally well. These simulation–experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W(VI)/W(V) processes converge into a single two-electron process if sufficient acid is present. 183W ...

The bulk oxidative electrolysis of a 2 mM solution of trans-[Pt-II{(p-HC6F4)NCH2CH2NEt2)Cl(py)] in highly non coordinating dichloromethane (0.05 M [Bu4N][B(C6F5)]) media leads to the formation of about 14% of the Pt-III species trans-[pt(III){(p-HC6F4)NCH2CH2NEt2}Cl(py)](+). The EPR spectrum of this electro-synthesized formally Pt-III species shows Pt-hyperfine coupling with g(x) similar to g(y) > g(z) similar to g(e), and is broadly consistent with the simple crystal field theory prediction for 5d(7) Pt-III in an elongated tetragonal environment where the unpaired electron is in a 5d(z(2)) orbital. The crystal field calculations lead to an estimate of the 5d(z(2)) character of around 37% and indicate partial delocalization of the unpaired electron onto the orbitals of the surrounding ligands. Transient cyclic voltammetric and steady-state microelectrode studies in the same media as used for bulk electrolysis exhibit a chemically reversible one electron oxidation process under their shorter time scale conditions. Analysis of X-ray diffraction data obtained from a single crystal of trans-[pt(II){(p-HC6F4)NCH2CH2NEt2}Cl(py)] shows the square planar geometry of the ligands around the Pt metal center and the 'W' arrangement of the ethyl groups on the ligand is explained in terms of agostic interactions.

This chapter examines the general aspects of the application of electron paramagnetic resonance (EPR) and its related techniques such as electron nuclear double resonance and spin-echo spectroscopy to studies of copper proteins, and discusses their advantages and limitations. The absorption of microwave power by the sample at resonance changes the effective coupling of the cavity to the waveguide and hence the amount of microwave power reflected from the cavity. In the earliest EPR spectrometers, this change was observed directly by measuring the current through the crystal detector with a meter or an oscilloscope. The chapter discusses the development of the appropriate spin Hamiltonians and their application in the interpretation of the EPR spectra of copper proteins. In the rapid-freezing technique, reactants from two or three syringes are driven into a mixing tube by pressurized gas or a motor-driven cam system.

Characterization of the anticancer active compound trans-[PtII{(p-BrC6F4)NCH2CH2NEt2}Cl(py)] is described along with identification of electrochemical conditions that favor formation of a monomeric one-electron-oxidized PtIII derivative. The square-planar organoamidoplatinum(II) compound was synthesized through a carbon dioxide elimination reaction. Structural characterization by using single-crystal X-Ray diffraction reveals a trans configuration with respect to donor atoms of like charges. As PtIII intermediates have been implicated in the reactions of platinum anticancer agents, electrochemical conditions favoring the formation of one-electron-oxidized species were sought. Transient cyclic voltammetry at fast scan rates or steady-state rotating disc and microelectrode techniques in a range of molecular solvents and an ionic liquid confirm the existence of a well-defined, chemically and electrochemically reversible one-electron oxidation process that, under suitable conditions, generates a PtIII complex, which is proposed to be monomeric [PtIII{(p-BrC6F4)NCH2CH2NEt2}Cl(py)]+. Electron paramagnetic resonance spectra obtained from highly non-coordinating dichloromethane/([Bu4N][B(C6F5)4]) solutions, frozen to liquid nitrogen temperature immediately after bulk electrolysis in a glove box, support the PtIII assignment rather than formation of a PtII cation radical. However, the voltammetric behavior is highly dependent on the timescale of the experiments, temperature, concentration of trans-[PtII{(p-BrC6F4)NCH2CH2NEt2}- Cl(py)], and the solvent/electrolyte. In the low-polarity solvent CH2Cl2 containing the very weakly coordinating electrolyte [Bu4N][B(C6F5)4], a well-defined reversible one-electron oxidation process is observed on relatively long timescales, which is consistent with the stabilization of the cationic platinum(III) complex in non-coordinating media. Bulk electrolysis of low concentrations of [Pt{(p-BrC6F4)NCH2CH2NEt2}Cl(py)] favors the formation of monomeric [PtIII{(p-BrC6F4)NCH2CH2NEt2}Cl(py)]+. Simulations allow the reversible potential of the PtII/PtIII process and the diffusion coefficient of [PtIII{(p-BrC6F4)- NCH2CH2NEt2}Cl(py)]+ to be calculated. Reversible electrochemical behavior, giving rise to monomeric platinum(III) derivatives, is rare in the field of platinum chemistry.

A closo-type 11-vertex osmaborane [1-(η(6)-pcym)-1-OsB10H10] (pcym = para-cymene) has been synthesized and characterized by single-crystal X-ray diffraction and elemental analysis, as well as by (11)B and (1)H NMR, UV-visible, and mass spectrometry. The redox chemistry has been probed by dc and Fourier transformed ac voltammetry and bulk reductive electrolysis in CH3CN (0.10 M (n-Bu)4NPF6) and by voltammetry in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (Pyrr1,4-NTf2), which allows the oxidative chemistry of the osmaborane to be studied. A single-crystal X-ray diffraction analysis has shown that [1-(η(6)-pcym)-1-OsB10H10] is isostructural with other metallaborane compounds of this type. In CH3CN (0.10 M (n-Bu)4NPF6), [1-(η(6)-pcym)-1-OsB10H10] undergoes two well-resolved one-electron reduction processes with reversible potentials separated by ca. 0.63-0.64 V. Analysis based on a comparison of experimental and simulated ac voltammetric data shows that the heterogeneous electron transfer rate constant (k(0)) for the first reduction process is larger than that for the second step at GC, Pt, and Au electrodes. k(0) values for both processes are also larger at GC than metal electrodes and depend on the electrode pretreatment, implying that reductions involve specific interaction with the electrode surface. EPR spectra derived from the product formed by one-electron reduction of [1-(η(6)-pcym)-1-OsB10H10] in CH3CN (0.10 M (n-Bu)4NPF6) and electron orbital data derived from the DFT calculations are used to establish that the formal oxidation state of the metal center of the original unreduced compound is Os(II). On this basis it is concluded that the metal atom in [1-(η(6)-pcym)-1-OsB10H10] and related metallaboranes makes a 3-orbital 2-electron contribution to the borane cluster. Oxidation of [1-(η(6)-pcym)-1-OsB10H10] coupled to fast chemical transformation was observed at 1.6 V vs ferrocene(0/+) in Pyrr1,4-NTf2. A reaction scheme for the oxidation involving formation of [1-(η(6)-pcym)-1-OsB10H10](+), which rearranges to an unknown electroactive derivative, is proposed, and simulations of the voltammograms are provided.

Data derived from a voltammetric and spectroscopic study of the VV/IV couple associated with the initial reduction of the Wells–Dawson-type mono vanadium-substituted polyoxometalates, 1- and 4-[S2VVW17O62]5− in CH3CN as a function of CF3SO3H acid concentration have been obtained. 51V NMR (VV component) and EPR (VIV component) spectra were measured in CH3CN in the presence and absence of an acid. These data showed a small fraction of the 1-isomer in the 4-[S2VVW17O62]5− sample and that protonation could occur at both redox levels for both isomers. On the basis of the mechanism postulated from the voltammetric and spectroscopic data, simulations of cyclic voltammograms were undertaken for the reduction of the isomerically pure 1-[S2VVW17O62]5− isomer over a wide acid concentration range, and the results were compared with experimental data. Cyclic voltammograms of the VV/IV couple derived from the reduction of 1- and 4-[X2VVW17O62]7− (X = P, As) were also obtained in CH3CN and the results were compared with those for 1- and 4-[S2VVW17O62]5−. Reversible potentials for the VV/IV couple are dependent on the anion charge of the polyoxometalate. Analysis of cyclic voltammograms obtained for 1- and 4-[S2VVW17O62]5− in acetonitrile, acetone, dimethyl sulfoxide, dimethyl formamide and nitromethane showed that these VV/IV reversible potentials are also dependent on the acceptor numbers and the polarity index (ENT) values of the organic solvents.

The structure of the Keggin-type β-[PW

Detailed electrochemical studies have been undertaken on molecular aggregation of the organic semiconductor 7,14-bis((triisopropylsilyl)-ethynyl)dibenzo[b,def]chrysene (TIPS-DBC), which is used as an electron donor material in organic solar cells. Intermolecular association of neutral TIPS-DBC molecules was established by using (1)H NMR spectroscopy as well as by the pronounced dependence of the color of TIPS-DBC solutions on concentration. Diffusion limited current data provided by near steady-state voltammetry also reveal aggregation. Furthermore, variation of concentration produces large changes in shapes of transient DC and Fourier transformed AC (FTAC) voltammograms for oxidation of TIPS-DBC in dichloromethane. Subtle effects of molecular aggregation on the reduction of TIPS-DBC are also revealed by the highly sensitive FTAC voltammetric method. Simulations of FTAC voltammetric data provide estimates of the kinetic and thermodynamic parameters associated with oxidation and reduction of TIPS-DBC. Significantly, aggregation of TIPS-DBC facilitates both one-electron oxidation and reduction by shifting the reversible potentials to less and more positive values, respectively. EPR spectroscopy is used to establish the identity of one-electron oxidized and reduced forms of TIPS-DBC. Implications of molecular aggregation on the HOMO energy level in solution are considered with respect to efficiency of organic photovoltaic devices utilizing TIPS-DBC as an electron donor material.

Two vanadium(V)-substituted tungsto-polyoxometalate isomers, 1- and 4-[S2VW17O62](5-), were prepared as their tetra-alkyl ammonium salts from a W(VI)-H2SO4-V(V) reaction mixture in aqueous CH3CN solution. X-ray crystallographic structural analysis revealed that both isomers have a Wells-Dawson-type structure with a higher occupancy of vanadium at polar sites and belt sites for 1- and 4-[S2VW17O62](5-), respectively. The isomers were also characterized by elemental analysis, infrared, Raman, UV-vis, and (51)V NMR spectroscopies as well as voltammetry, and the data obtained were compared with that derived from [S2W18O62](4-). Significantly, the reversible potentials for the vanadium(V/IV) couple for both 1- and 4-[S2VW17O62](5-) in CH3CN (0.1 M n-Bu4NPF6) are considerably more positive than the tungstate reduction process exhibited by the [S2W18O62](4-) framework, implying that the presence of vanadium should be useful in catalytic reactions. The one-electron-reduced [S2V(IV)W17O62](6-) forms of both isomers were prepared in solution by controlled potential bulk electrolysis and characterized by voltammetry and EPR spectroscopy.

The tetrabutylammonium complex with a 2:5 stoichiometry, (n-Bu(4)N)(2)(TCNQ)(5), has been prepared and structurally characterized by X-ray crystallography. Diagnostic bands in the Raman spectrum and signature features in the electrochemistry confirm that the TCNQ moieties are partially charged in the solid state. EPR, magnetic susceptibility, and electrical conductivity measurements are all consistent with (n-Bu(4)N)(2)(TCNQ)(5) behaving as a quasi-one-dimensional organic semiconductor.

The electron paramagnetic resonance (EPR) spectra of the trigonal prismatic complexes Tc(abt)3, Tc(bdt)3, Re(abt)3 and Re(bdt)3 (abt, O-aminobenzenethiol; bdt, benzene-1,2-dithiol) in dilute frozen solution are interpreted in terms of an axially symmetric spin Hamiltonian, with g values close to two, principal hyperfine couplings of |A Tc| ~ 5–12 × 10−4 cm−1, nuclear quadrupole couplings of |P Tc| ~ 0.3–0.35 × 10−4 cm−1 and the unusual values 5 × 10−4 cm−1 ~ |A Re

Amyloid-beta peptide (Abeta) toxicity is thought to be responsible for the neurodegeneration associated with Alzheimer's disease. While the mechanism(s) that modulate this toxicity are still widely debated, it has previously been demonstrated that modifications to the three histidine residues (6, 13, and 14) of Abeta are able to modulate the toxicity. Therefore to further elucidate the potential role of the histidine (H) residues in Abeta toxicity, we synthesized Abeta peptides with single alanine substitutions for each of the three histidine residues and ascertained how these substitutions affect peptide aggregation, metal binding, redox chemistry, and cell membrane interactions, factors which have previously been shown to modulate Abeta toxicity. Abeta{42} H13A and Abeta{42} H6A modified peptides were able to induce significant cell toxicity in primary cortical cell cultures at levels similar to the wild-type peptide. However, Abeta{42} H14A did not induce any measurable toxicity in the same cultures. This lack of toxicity correlated with the inability of the Abeta{42} H14A to bind to cell membranes. The interaction of Abeta with cell membranes has previously been shown to be dependent on electrostatic interactions between Abeta and the negatively charged head group of phosphatidylserine. Our data suggests that it is the imidazole sidechain of histidine 14 that modulates this interaction and strategies inhibiting this interaction may have therapeutic potential for Alzheimer's disease.

The oxidative electrochemistry of [CpFe(CO)(2)](2), 1 (Cp = [eta(5)-C(5)H(5)](-)), was examined in detail in ionic liquids (ILs) composed of ions of widely varying Lewis acid-base properties. Cyclic voltammetric responses were strongly dependent on the nucleophilic properties of the IL anion, but all observations are consistent with the initial formation of 1(+) followed by attack from the IL anion. In [NTf(2)](-)-based ILs ([NTf(2)](-) = bis(trifluoromethylsulfonyl)amide), the process shows nearly ideal chemical reversibility as the reaction between 1(+) and [NTf(2)](-) is very slow. This is highly significant, as 1(+) is known to be highly susceptible to nucleophilic attack and its stability indicates a remarkable lack of coordinating ability of these ILs. In 1-methyl-3-butylimidazolium hexafluorophosphate, [bmim][PF(6)], the oxidation of 1 is still largely reversible, but there is more pronounced evidence of [PF(6)](-) coordination. In contrast, 1 exhibits an irreversible two-electron oxidation process in a dicyanamide-based IL. This overall oxidation process is thought to proceed via an ECE mechanism, details of which are presented. Rate constants were estimated by fitting the experimental data to digital simulations of the proposed mechanism. The use of [NTf(2)](-)-based ILs as a supporting electrolyte in CH(2)Cl(2) was examined by using this solvent/electrolyte as a medium in which to perform bulk electrolyses of 1 and 1*, the permethylated analogue [Cp*Fe(CO)(2)](2) (Cp* = [eta(5)-C(5)(CH(3))(5)](-)). These cleanly yielded the corresponding binuclear radical-cation species, 1(+) and 1*(+), which were subsequently characterized by electron paramagnetic resonance (EPR) spectroscopy. In addition to the above oxidation studies, the reduction of 1 was studied in each of the ILs; differences in cathodic peak potentials are attributed, in part, to ion-pairing effects. This study illustrates the wide range of electrochemical environments available with ILs and demonstrates their utility for the investigation of the redox properties of metal carbonyls and other organometallic compounds.

A family of spin crossover cobalt(II) complexes of the type [Co(terpyRX)2](Y)2·nH2O (X = 4, 8, 12 and Y = BF4−, ClO4−, PF6−, BPh4−) has been synthesized, whereby the alkyl chain length, RX, and counteranion, Y, have been systematically varied. The structural (single crystal X-ray diffraction) and electronic (magnetic susceptibility, electron paramagnetic resonance (EPR)) properties have been investigated within this family of compounds. Single crystal X-ray diffraction analysis of [Co(terpyR8)2](ClO4)2, [Co(terpyR8)2](BF4)2·H2O, and [Co(terpyR4)2](PF6)2·3H2O, at 123 K, revealed compressed octahedral low spin Co(II) environments and showed varying extents of disorder in the alkyl tail portions of the terpyRX ligands. The magnetic and EPR studies were focused on the BF4− family and, for polycrystalline solid samples, revealed that the spin transition onset temperature (from low to high spin) decreased as the alkyl chain lengthened. EPR studies of polycrystalline powder samples confirmed these results, showing signals only due to the low spin state at the temperatures seen in magnetic measurements. Further to this, simultaneous simulation of the EPR spectra of frozen solutions of [Co(terpyR8)2](BF4)2·H2O, recorded at S-, X-, and Q-band frequencies, allowed accurate determination of the g and A values of the low spin ground state. The temperature dependence of the polycrystalline powder EPR spectra of this and the R4 and R12 complexes is explained in terms of Jahn−Teller effects using the warped Mexican hat potential energy surface model perturbed by the low symmetry of the ligands. While well recognized in Cu(II) systems, this is one of the few times this approach has been used for Co(II).

A copper(II) complex containing the ligand 4′-octoxy-2,2′:6′,2″-terpyridine (terpyR8), [Cu(terpyR8) 2 ](ClO 4 ) 2 ( 1 ) has been synthesized and characterized by X-ray crystallography and X-band EPR spectroscopy. Structural analysis at 123 K revealed a compressed octahedral geometry, whereby the axial Cu–N bond lengths are significantly shorter than the equatorial ones. The mononuclear units in 1 are close packed such that there is no included solvent. This close packed nature is highlighted through a series of π–π interactions between mononuclear units and an array of hydrogen-bonding interactions between mononuclear units, the long C 8 alkyl chains and ClO 4 - anions. Detailed EPR studies were carried out on both polycrystalline powder and frozen solution samples of 1 , over a temperature range from 2.5 K up to 200–260 K. The polycrystalline powder exhibits a spectrum characteristic of a d x 2 -y 2 d x 2 - y 2 ground configuration at temperatures below ∼10 K. Above ∼100 K, the spectrum had the appearance of originating from a d z 2 d z 2 ground configuration, but it is best interpreted in terms of fluxional disorder with a d x 2 -y 2 d x 2 - y 2 ground state. The spectra of the frozen solutions are consistent with the d x 2 -y 2 d x 2 - y 2 configuration (elongated tetragonal) at all temperatures, although the spectra show complex behaviour below ∼30 K, due to microwave power saturation effects, and above ∼120 K due to motional averaging effects. Apparent contradictions between the structural and EPR work of the kind seen here on the solid sample have recently been discussed elsewhere for related bis-tridentate Cu(II) complexes and are resolved here such that complex 1 has a Jahn–Teller elongated configuration masked by a structural disorder.

Continuous-wave and pulsed electron paramagnetic resonance have been applied to the study of the Cu(II) site of the copper-resistance protein PcoC from Escherichia coli and certain variant forms. Electron spin echo envelope modulation (ESEEM) experiments confirm the presence of two histidine ligands, His1 and His92, at the Cu(II) site of wild-type PcoC, consistent with the available X-ray crystallographic data for the homolog CopC (67% sequence identity) from Pseudomonas syringae pv. tomato. The variants H1F and H92F each lack one of the histidine residues close to the Cu(II) site. The ESEEM data suggest that the surviving histidine residue remains as a ligand. The nA variant features an extra alanine residue at the N terminus, which demotes the His1 ligand to position 2. At least one of the two histidine residues is bound at the Cu(II) site in this form. Simulation of the (14)N superhyperfine structure in the continuous-wave spectra confirms the presence of at least three nitrogen-based ligands at the Cu(II) sites of the wild-type, H92F and nA forms, while the H1F variant has two nitrogen ligands. The spectra of wild-type form can be fitted adequately with a 3N or a 4N model. The former is consistent with the crystal structure of the CopC homolog, where His1 acts as a bidentate ligand. The latter raises the possibility of an additional unidentified nitrogen ligand. The markedly different spectra of the H1F and nA forms compared with the wild-type and H92F proteins further highlight the integral role of the N-terminal histidine residue in the high-affinity Cu(II) site of PcoC.

The synthesis, isolation, and structural characterization of the fully oxidized sulfite-based polyoxotungstate cluster (Pr4N)4{α-[W18O54(SO3)2]}·2CH3CN and the one-electron reduced form (Pr4N)5{α-[W18O54(SO3)2]}·2CH3CN has been achieved. α-[W18O54(SO3)2]5- was obtained as a Pr4N+ salt by reducing the “Trojan Horse” [W18O56(SO3)2(H2O)2]8- cluster via a template orientation transformation. Acetonitrile solutions of pure α-[W18O54(SO3)2]5- also were prepared electrochemically by one-electron bulk reductive electrolysis of α-[W18O54(SO3)2]4-. Cyclic voltammetry of α-[W18O54(SO3)2]4- and α-[W18O54(SO3)2]5- in CH3CN (0.1 M Hx4NClO4) produces evidence for an extensive series of reversible one-electron redox processes, that are associated with the tungsten−oxo framework of the polyoxometalate cluster. Hydrodynamic voltammograms in CH3CN exhibit the expected sign and magnitude of the steady-state limiting current values for the α-[W18O54(SO3)2]4-/5-/6- series and confirm the existence of a stable one-electron redu...

Alzheimer's disease (AD) is the major cause of dementia. Amyloid beta peptide (Abeta), generated by proteolytic cleavage of the amyloid precursor protein (APP), is central to AD pathogenesis. APP can function as a metalloprotein and modulate copper (Cu) transport, presumably via its extracellular Cu-binding domain (CuBD). Cu binding to the CuBD reduces Abeta levels, suggesting that a Cu mimetic may have therapeutic potential. We describe here the atomic structures of apo CuBD from three crystal forms and found they have identical Cu-binding sites despite the different crystal lattices. The structure of Cu(2+)-bound CuBD reveals that the metal ligands are His147, His151, Tyr168 and two water molecules, which are arranged in a square pyramidal geometry. The site resembles a Type 2 non-blue Cu center and is supported by electron paramagnetic resonance and extended X-ray absorption fine structure studies. A previous study suggested that Met170 might be a ligand but we suggest that this residue plays a critical role as an electron donor in CuBDs ability to reduce Cu ions. The structure of Cu(+)-bound CuBD is almost identical to the Cu(2+)-bound structure except for the loss of one of the water ligands. The geometry of the site is unfavorable for Cu(+), thus providing a mechanism by which CuBD could readily transfer Cu ions to other proteins.

The bulk oxidative electrolysis of a 2mM solution of trans-[Pt

Data derived from a voltammetric and spectroscopic study of the V(V/IV) couple associated with the initial reduction of the Wells-Dawson-type mono vanadium-substituted polyoxometalates, 1- and 4-[S2V(V)W17O62](5-) in CH3CN as a function of CF3SO3H acid concentration have been obtained. (51)V NMR (V(V) component) and EPR (V(IV) component) spectra were measured in CH3CN in the presence and absence of an acid. These data showed a small fraction of the 1-isomer in the 4-[S2V(V)W17O62](5-) sample and that protonation could occur at both redox levels for both isomers. On the basis of the mechanism postulated from the voltammetric and spectroscopic data, simulations of cyclic voltammograms were undertaken for the reduction of the isomerically pure 1-[S2V(V)W17O62](5-) isomer over a wide acid concentration range, and the results were compared with experimental data. Cyclic voltammograms of the V(V/IV) couple derived from the reduction of 1- and 4-[X2V(V)W17O62](7-) (X = P, As) were also obtained in CH3CN and the results were compared with those for 1- and 4-[S2V(V)W17O62](5-). Reversible potentials for the V(V/IV) couple are dependent on the anion charge of the polyoxometalate. Analysis of cyclic voltammograms obtained for 1- and 4-[S2V(V)W17O62](5-) in acetonitrile, acetone, dimethyl sulfoxide, dimethyl formamide and nitromethane showed that these V(V/IV) reversible potentials are also dependent on the acceptor numbers and the polarity index (E(T)(N)) values of the organic solvents.

Australian and Canadian calibration coefficients for air kerma and absorbed dose to water for60Co gamma radiation have been compared using transfer standard ionization chambers of types NE 2561 and NE 2611A. Whilst the primary standards of air kerma are similar, both being thick-walled graphite cavity chambers but employing different methods to evaluate the Awall correction, the primary standards of absorbed dose to water are quite different. The Australian standard is based on measurements made with a graphite calorimeter, whereas the Canadian standard uses a sealed water calorimeter. The comparison result, expressed as a ratio of calibration coefficientsR=N ARPANSA /N NRC , is 1.0006 with a combined standard uncertainty of 0.35% for the air kerma standards and 1.0052 with a combined standard uncertainty of 0.47% for the absorbed dose to water standards. This demonstrates the agreement of the Australian and Canadian radiation dosimetry standards. The results are also consistent with independent comparisons of each laboratory with the BIPM reference standards. A ‘trilateral’ analysis confirms the present determination of the relationship between the standards, within the 0.09% random component of the combined standard uncertainty for the three comparisons.

The synthesis and structural determination of the tripodal triethanolamine (teaH3) based octanuclear cluster [FeIII8O3(O2CCH2CH3)6(tea)(teaH)3(F)3]·0.5MeOH·0.5H2O (1) is reported. The octanuclear core in 1 has only been observed once before in the analogous teaH3 based [Fe8O3(O2CPh)9(tea)(teaH)3]·MeCN cluster and, as observed in that case, magnetic susceptibility measurements on 1 indicate a ground spin state of S = 0. Reaction of the trinuclear complex [FeIII3O(O2CCMe3)6(H2O)3](O2CCMe3) with the tripodal ligands 1-[N,N-bis(2-hydroxyethyl)-amino]-2-propanol (bheapH3) and triethanolamine (teaH3) gives [FeIII7O3(O2CCMe3)9(bheapH)3(H2O)3] (2) and [FeIII7O3(O2CCMe3)9(teaH)3(H2O)3] (3), respectively. The cores in 2 and 3 are dome-like in topology, an architecture unseen in any heptanuclear clusters reported to date. Magnetisation and susceptibility measurements show that clusters 2 and 3 exhibit ground spin states of S = 5/2. Low temperature EPR measurements confirm these ground spin states and yield zero-field splitting parameters of D = 0.28 cm−1 and E/D of 0.21 (for 3).

Detailed electrochemical studies in dichloromethane (0.1 M Bu4NPF6) on the oxidation of the half-lantern [Pt2(kappa2As,C-C6H3-5-Me-2-AsPh2)2(mu-kappaAs,kappaC-C6H3-5-Me-2-AsPh2)2] (1) and full-lantern [Pt2(mu-kappaAs,kappaC-C6H3-5-Me-2-AsPh2)4] (2) complexes reveal the presence of an exceptionally stable dinuclear Pt cation 2+. Thus, oxidation of 1 occurs on the voltammetric time scale via a ladder-square scheme to give 2+, whereas 2 is directly converted to 2+. Electrochemically informed chemical synthesis enabled the isolation of solid [2+][BF4-] to be achieved. Single-crystal X-ray structural analysis showed that 2+ also has a lantern structure but with a shorter separation between the Pt centers [2.7069(3) A (2+), 2.8955(4) A (2)]. EPR spectra of 2+ provide unequivocal evidence for axial symmetry of the complex and are noteworthy because of an exceptionally large, nearly isotropic hyperfine coupling constant of about 0.1 cm(-1). Spectroscopic data support the conclusion that the unpaired electron in the 2+ cation is distributed equally between the two Pt nuclei and imply that oxidation of 2 to 2+ leads to the establishment of the metal-to-metal hemibond. Results of extended Huckel molecular orbital and density functional calculations on 2 and 2+ lead to the conclusions that s, p, dz2 mixing of orbitals contributes to the large EPR Pt hyperfine coupling and also that the structural adjustments that occur upon removal of an electron from 2 are driven by the metal-metal bonding character present in 2+.

Electron spin transient nutation (ESTN) experiments show that the spin multiplicity of the ground state of C-60(3-) in frozen solution is a doublet with S = 1/2. In purified samples, there is no evidence for excited states or other species with higher multiplicity. In the anions Of C120On- (n = 2, 3, 4), where the CW EPR experiments have shown that a mixture of species is present, ESTN experiments confirm that a doublet with S = 1/2 is associated with the 3- anion and triplets with S = 1 are associated with the 2- and 4- anions. A weak nutation peak attributable to m(s) = -1/2 1/2 transitions within a quartet state may arise from association of anions with spins of 1/2 and 1 in solute aggregates.

The VV-substituted polyoxotungstate isomers 1- and 4-[S2VW17O62]5- are prepared as Bu4N+ salts from a Na2WO4-H2SO4/NH4 VO3-NaOH reaction mixture in aqueous MeCN.

Electrochemical reduction of cis-stilbene occurs by two well-resolved one-electron reduction steps in acetonitrile with (n-Bu)4NPF6 as the supporting electrolyte and in N-butyl-N-methylpyrrolidinium (Pyrr1,4(+)) and (trimethylamine)(dimethylethylamine)-dihydroborate bis(trifluoromethylsulfonyl)amide (NTf2(-)) ionic liquids (ILs). Mechanistic details of the electroreduction have been probed by dc and Fourier transformed ac voltammetry, simulation of the voltammetry, bulk electrolysis, and EPR spectroscopy. The first one-electron reduction induces fast cis to trans isomerization in CH3CN and ILs, most likely occurring via disproportionation of cis-stilbene radical anions and fast transformation of the cis-dianion to the trans-configuration. The second reduction process is chemically irreversible in CH3CN due to protonation of the dianion but chemically reversible in highly aprotic ILs under high cis-stilbene concentration conditions. Increase of the (n-Bu)4NPF6 supporting electrolyte concentration (0.01-1.0 M) in CH3CN induces substantial positive shifts in the potentials for reduction of cis-stilbene, consistent with strong ion pairing of the anion radical and dianion with (n-Bu)4N(+). However, protection by ion pairing against protonation of the stilbene dianions or electrochemically induced cis-trans-stilbene isomerization is not achieved. Differences in electrode kinetics and reversible potentials for cis-stilbene(0/•-) and trans-stilbene(0/•-) processes are less pronounced in the Pyrr1,4-NTf2 ionic liquid than in the molecular solvent acetonitrile.

Spin Hamiltonian parameters for the neutral trigonal-prismatic Tc(abt)(3) and Re(abt)(3) chelates (abt = o-aminobenzenethiol) are calculated using relativistic density functional theory at the all-electron level. The small magnitude of the calculated g shifts and metal hyperfine interactions is in excellent agreement with previous experimental predictions based upon a ligand-centered ground-state magnetic orbital. The theoretical (14)N ligand hyperfine and quadrupole couplings also reproduce the nuclear frequencies measured by electron spin-echo envelope modulation spectroscopy. The nuclear quadrupole interaction of (187)Re is predicted to be 2 orders of magnitude larger than that of (99)Tc, in agreement with empirical simulation of the continuous-wave electron paramagnetic resonance spectrum. The spectrum of Tc(abt)(3) at high solute concentrations contains a central resonance not predicted for the isolated complex by theoretical calculations. The absence of this resonance at low solute concentrations provides evidence of intermolecular interactions in these systems.

The kinetics of luminescence decay in electron-irradiated sapphire single crystals have been investigated using time-resolved luminescence spectroscopy. The data, observed over timescales from tens of nanoseconds to tens of milliseconds, characteristically feature a rapid decay of intensity punctuated by discrete plateau regions. Simple theoretical models, invoking such theories as first- or second-order mechanisms, rate laws or power-law dependences, are unable to explain these features. A theoretical model comprising bimolecular electron - hole recombination, together with unimolecular electron detrapping from two discrete traps, qualitatively accounts for these features.

The reaction of AsPh4[TcNX4] (X=Cl or Br) with ‘naked’ F− in MeCN solution has been shown by e.p.r. spectroscopy to result in the facile substitution of the halo ligands by fluoride. Addition of six equivalents of NBu4F·3H2O has provided, for the first time, direct evidence for the formation of the six-coordinate [TcNF5]2− in solution. An e.p.r. study of ligand exchange of Cs2[TcNCl5] (4 × 10−2 − 5 × 10−4 mol dm−3) in 28.6 mol dm−3 aqueous HF has shown that the distribution of the five [TcNFnCl4−n]− (n = ;0–4) species is concentration dependent, with the formation of [TcNF4]− being favoured by dilution.

Three highly aprotic bis(trifluoromethylsulfonyl)amide (NTf2(-)) based ionic liquids (ILs) containing the cations trihexyl(tetradecyl)phosphonium (P6,6,6,14(+)), N-butyl-N-methylpyrrolidinium (Pyrr4,1(+)), and (trimethylamine)(dimethylethylammine)dihydroborate ((N111)(N112)BH2(+)) have been examined as media for room temperature voltammetric detection of highly basic stilbene dianions electrochemically generated by the reduction of trans-stilbene (t-Stb) and its derivatives (4-methoxy-, 2-methoxy-, 4,4'-dimethyl-, and 4-chloromethyl-). Transient and steady-state data in the ILs were compared with results obtained in the molecular solvent acetonitrile. In all media examined, the t-Stb(0/•-) process is chemically and electrochemically reversible with a heterogeneous charge transfer rate constant in CH3CN of 1.5 cm s(-1), as determined by Fourier transformed AC voltammetry. However, further reduction to the dianion was always irreversible in this molecular but weakly acidic solvent. On the other hand, a substantial level of chemical reversibility for the reduction of t-Stb(•-) to t-Stb(2-) on the time scale of cyclic voltammetry is achieved when the concentration of trans-stilbene, [t-Stb], appreciably exceeds the concentration of adventitious water or other proton sources. In particular, these conditions are met when [t-Stb] ≥ 0.1 M in thoroughly dehydrated and purified ILs, while in the presence of CH3CN, t-Stb(2-) still suffers fast irreversible protonation under these stilbene concentration conditions. The E0/•-(0) values (vs Fc(0/+)) for substituted trans-stilbenes in acetonitrile and (N111)(N112)BH2-NTf2 do not differ substantially, nor do the E0/•-(0) and E•-/2-(0) differences or other aspects of the voltammetric behavior.

Analysis of the EPR spectra of Tc complexes in frozen solution has given information about the behaviour of these complexes not readily obtainable by other means. In particular, spectral simulations have enabled the determination of the quadrupole interaction parameter,Q, and thus to the evaluation of subtle electronic effects.

The charge-transfer material TTF-SV(IV)Mo(11)O(40) (TTF = tetrathiafulvalene) was prepared by a spontaneous redox reaction between TTF and the vanadium-substituted polyoxometalate (n-Bu(4)N)(3)[SV(V)Mo(11)O(40)] in both solution and solid state phases. Single crystal X-ray diffraction gave the stoichiometry TTF(4)[SVMo(11)O(40)]·2H(2)O·2CH(2)Cl(2), with the single V atom positionally disordered with eight Mo atoms over the whole α-Keggin polyanion [SVMo(11)O(40)](4-). Raman spectra support the 1+ charge assigned to the oxidized TTF deduced from bond lengths, and elemental and voltammetric analysis also are consistent with this formulation. Scanning electron microscopy images showed a rod-type morphology for the new charge-transfer material. The conductivity of the solid at room temperature is in the semiconducting range. The TTF and (n-Bu(4)N)(3)[SV(V)Mo(11)O(40)] solids also undergo a rapid interfacial reaction, as is the case with TTF and TCNQ (TCNQ = tetracyanoquinodimethane) solids. EPR spectra at temperatures down to 2.6 K confirm the presence of two paramagnetic species, V(IV) and the oxidized TTF radical. Spectral evidence shows that the TTF-SV(IV)Mo(11)O(40) materials prepared from either solution or solid state reactions are equivalent. The newly isolated TTF-SV(IV)Mo(11)O(40) material represents a new class of TTF-polyoxometalate compound having dual electrical and magnetic functionality derived from both the cationic and anionic components.

The reaction of [Fe(II)(BF(4))(2)]·6H(2)O with the nitroxide radical, 4,4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition metal complex [Fe(II)(L(•))(2)](BF(4))(2) (1) which has been investigated using temperature dependent susceptibility, Mössbauer spectroscopy, electrochemistry, density functional theory (DFT) calculations, and X-ray structure analysis. Single crystal X-ray diffraction analysis and Mössbauer measurements reveal an octahedral low spin Fe(2+) environment where the pyridyl donors from L(•) coordinate equatorially while the oxygen containing the radical from L(•) coordinates axially forming a linear O(•)··Fe(II)··O(•) arrangement. Magnetic susceptibility measurements show a strong radical-radical intramolecular antiferromagnetic interaction mediated by the diamagnetic Fe(2+) center. This is supported by DFT calculations which show a mutual spatial overlap of 0.24 and a spin density population analysis which highlights the antiparallel spin alignment between the two ligands. Similarly the monocationic complex [Fe(III)(L(-))(2)](BPh(4))·0.5H(2)O (2) has been fully characterized with Fe-ligand and N-O bond length changes in the X-ray structure analysis, magnetic measurements revealing a Curie-like S = 1/2 ground state, electron paramagnetic resonance (EPR) spectra, DFT calculations, and electrochemistry measurements all consistent with assignment of Fe in the (III) state and both ligands in the L(-) form. 2 is formed by a rare, reductively induced oxidation of the Fe center, and all physical data are self-consistent. The electrochemical studies were undertaken for both 1 and 2, thus allowing common Fe-ligand redox intermediates to be identified and the results interpreted in terms of square reaction schemes.

Solid-state electrochemistry of a tetracyanoquinodimethane (TCNQ)-modified electrode in contact with a tetrapropylammonium cation (Pr(4)N(+)) electrolyte showed two electron-transfer steps to give Pr(4)N(TCNQ)(2) (1) and Pr(4)N(TCNQ) (2) rather than the traditional one-electron step to directly give Pr(4)N(TCNQ). Two thermodynamically stable Pr(4)N(+)-TCNQ stoichiometries, 1 and 2, were synthesized and characterized. The degree of charge transfer (ρ) calculated from the crystal structure is -0.5 for the TCNQ moieties in 1 and -1.0 for those in 2. Raman spectra for Pr(4)N(TCNQ)(2) show only one resonance for the extracyclic C=C stretching at 1423 cm(-1), which lies approximately midway between that of TCNQ at 1454 cm(-1) and TCNQ(-) at 1380 cm(-1). Both the magnetic susceptibility and EPR spectra are temperature-dependent, with a magnetic moment close to that for one unpaired electron per (TCNQ)(2) unit in 1, whereas 2 is almost diamagnetic. Pressed discs of both complexes show conductivity (1-2×10(-5) S cm(-1)) in the semiconductor range. For 1, the position of zero current for the steady-state voltammograms implies 50% of TCNQ(-) and 50% TCNQ(0) is present in solution, thereby supporting a dissociation of (TCNQ)(2)(-) in solution, but is indicative of only TCNQ(-) being present for 2.

We report the preparations and electron spin resonance (ESR) studies of a series of Cs + and Rb + crown ether (15-crown-5 and 18-crown-6) salts of [TcNCl 4 ] - and [MoOCl 4 ] - , and a new refinement of the X-ray structure of [Cs(18-crown-6)] [TcNCl 4 ] involving a twinned P4/n scattering model. This refinement addresses some unsatisfactory aspects of the original disordered model. The structure consists of an ‘infinite sandwich’ Cs + cation/crown ether configuration and polymeric [TcNCL 4 ] - anions, with Tc≡N distances of 1.561(36) Å, and anti-parallel chains of –––Tc≡N–––Tc≡N –––with Tc···Tc separations of 4.275(4) Å. The structure determination of [Rb(15-crown-5) 2 ][TcNCl 4 (OH 2 )] is also described and shows a lattice containing 2:1 ‘sandwich’-type crown ether/alkali-metal cations and isolated anions in the disordered lattice. The presence of Tc≡N infrared stretching frequencies at ca . 1074 cm -1 indicates the absence of strong electron donors coordinated to Tc in the sixth position (i. e. isolated anions), whereas the polymeric chain structure of [Cs(18-crown-6)] [TcNCL 4 ] is characterized by a Tc≡N stretching frequency of 1041 cm -1 . Three product compounds, the ‘infinite sandwich’ [Cs(18-crown6)][TcNCl 4 ], [Cs(18-crown-6)] [TcNCL 4 (OH 2 )] with two Tc≡N stretching frequencies at 1055 and 1046.5 cm -1 , and [Cs(18-crown-6) 2 ][TcNCl 4 containing 2:1 crown ether/Cs + ‘sand­wich’ cations and isolated [TcNCl 4 ] - anions, could be obtained from reaction of Cs + /18-crown-6 and [TcNCL 4 ] - depending on reaction conditions and solvent. The ESR spectra of [Cs(18-crown-6)] [TcNCl 4 ] over the temperature range 130-290 K indicate the presence of exchange interactions along the linear ––– Tc≡N––– Tc≡N––– chains. Spectra for other crown ether salts are consistent with isolated [TcNCL 4 ] - or [TcNCL 4 (OH 2 )] - anions, with dipolar broadening effects apparent.

UV-Vis and ESR spectroscopy have been used to study the behaviour of Cs 2 [TcNCl 5 ] and the hydrolysis product [{TcN(OH)(OH 2 )} 2 (μ-O) 2 ] ( 1 ) in concentrated and aqueous solutions of a variety of sulfur and phosphorus oxo acids ranging from the very weakly coordinating CF 3 SO 3 H to H 2 SO 4 and the coordinating acids H 3 PO 4 and H 4 P 2 O 7 . The paramagnetic monomeric Tc VI N 3− (d 1 ) forms were readily identified by ESR spectroscopy and the diamagnetic dimeric μ-oxo (Tc 2 N 2 O 4+ ) and di(μ-oxo) (Tc 2 N 2 O 2 2+ ) forms by their UV-Vis spectra. For solutions of Cs 2 [TcNCl 5 ] the chemical form of Tc VI N and the rate of the conversion of monomer→μ-oxo dimer→di(μ- oxo) dimer is dependent on the presence of Cl − and the concentration and complexing ability of the acid, with the monomeric form being favoured by the acids of greater complexing power. For solutions of 1 the reverse order of conversion was observed. In 1 M CF 3 SO 3 H, MeSO 3 H or toluene- p -sulfonic acid, 1 is converted to the di(μ-oxo) aquanitrido cation [{TcN(OH 2 ) 3 } 2 (μ-O) 2 ] 2+ while in 7.5 M CF 3 SO 3 H it is proposed that the predominant form is the μ-oxo aqua dimer [{TcN(OH 2 ) 4 } 2 (μ-O)] 4+ . In H 2 SO 4 and the phosphorus oxo acids the aqua cations are coordinated by the acid anions. The monomeric aqua cation, [TcN(OH 2 ) 5 ] 3+ , does not appear to be a viable species, as is also the case for MoO 3+ (aq), and spontaneously dimerises in 7.5 M CF 3 SO 3 H. In anhydrous CF 3 SO 3 H or H 2 SO 4 monomeric Tc VI N with CF 3 SO 3 − or HSO 4 − coordination is observed. A characteristic feature of the UV-Vis spectra of monomeric Tc VI N is a strong absorption at 266–300 nm assigned to πN→Tc charge transfer. The μ-oxo dimers show an intense visible absorption at 470–510 nm.

Point defects have been produced by electron irradiation in single-crystal \ensuremath{\alpha}-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ from two different sources. Time-resolved luminescence spectroscopy has been used to study emission spectra and atomic-displacement thresholds for the defects. An emission band at 3.8 eV is present in only one of the crystals while a band at 4.2 eV is observed in both crystals. The former is probably due to ${\mathit{F}}^{+}$-center emission and the latter possibly arises from an F-center transition. Emission kinetics are consistent with detrapping of electrons from two shallow traps.

UV-Vis and ESR spectroscopy have been used to study the behaviour of Cs 2 [TcNCl 5 ] and the hydrolysis product [{TcN(OH)(OH 2 )} 2 (μ-O) 2 ] ( 1 ) in concentrated and aqueous solutions of a variety of sulfur and phosphorus oxo acids ranging from the very weakly coordinating CF 3 SO 3 H to H 2 SO 4 and the coordinating acids H 3 PO 4 and H 4 P 2 O 7 . The paramagnetic monomeric Tc VI N 3− (d 1 ) forms were readily identified by ESR spectroscopy and the diamagnetic dimeric μ-oxo (Tc 2 N 2 O 4+ ) and di(μ-oxo) (Tc 2 N 2 O 2 2+ ) forms by their UV-Vis spectra. For solutions of Cs 2 [TcNCl 5 ] the chemical form of Tc VI N and the rate of the conversion of monomer→μ-oxo dimer→di(μ- oxo) dimer is dependent on the presence of Cl − and the concentration and complexing ability of the acid, with the monomeric form being favoured by the acids of greater complexing power. For solutions of 1 the reverse order of conversion was observed. In 1 M CF 3 SO 3 H, MeSO 3 H or toluene- p -sulfonic acid, 1 is converted to the di(μ-oxo) aquanitrido cation [{TcN(OH 2 ) 3 } 2 (μ-O) 2 ] 2+ while in 7.5 M CF 3 SO 3 H it is proposed that the predominant form is the μ-oxo aqua dimer [{TcN(OH 2 ) 4 } 2 (μ-O)] 4+ . In H 2 SO 4 and the phosphorus oxo acids the aqua cations are coordinated by the acid anions. The monomeric aqua cation, [TcN(OH 2 ) 5 ] 3+ , does not appear to be a viable species, as is also the case for MoO 3+ (aq), and spontaneously dimerises in 7.5 M CF 3 SO 3 H. In anhydrous CF 3 SO 3 H or H 2 SO 4 monomeric Tc VI N with CF 3 SO 3 − or HSO 4 − coordination is observed. A characteristic feature of the UV-Vis spectra of monomeric Tc VI N is a strong absorption at 266–300 nm assigned to πN→Tc charge transfer. The μ-oxo dimers show an intense visible absorption at 470–510 nm.

The di-µ-oxo technetium(VI) complexes [{TcN(S2CNEt2)}2(µ-O)2]1, [{TcN(S2CNC4H8)}2(µ-O)2]2, [AsPh4]2[{TcN(CN)2}2(µ-O)2]4 and [AsPh4]2[{TcN(edt)}2(µ-O)2]5(H2edt = ethane-1,2-dithiol) have been prepared either by reaction of [{TcN(OH2)3}2(µ-O)2]2+ or Cs2[TcNCl5] in Na4P2O7 solution with the appropriate ligand. The ESR spectra of solutions of 1 and 2 in SOCl2 and of 4 in MeCN with added AsPh4Cl·HCl showed that cleavage of the dimer occurs to give the nitridotechnetium(VI) monomers [TcNCl2(S2CNR2)](R = Et or R2= C4H8) or [TcNCl2(CN)2]–. Reaction of [TcVINCl2(S2CNEt2)] with K2[SCOCOS] gave the mixed-ligand complex [AsPh4][TcVN(S2CNEt2)(SCOCOS)]3. Single-crystal X-ray structures (Cu-Kα radiation) were determined for 1, 2 and 3. Complexes 1 and 2 are dimeric and are best described as two edge-sharing square pyramids, with TcN and Tc–Tc distances of 1.623(4), 1.624(4), 2.543(1), and 1.65(2), 1.59(2) and, 2.542(2)A for 1 and 2 respectively. Complex 3 is monomeric with the technetium atom having a square pyramidal geometry, and a TcN distance of 1.54(2)A.