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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 ...

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 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

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...

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.

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.

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.

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.

Electron paramagnetic resonance spectroscopy (EPR) has the potential to give much detail on the structure of the paramagnetic transition ion coordination sites, principally of Cu2+, in a number of proteins associated with central nervous system diseases. Since these sites have been implicated in misfolding/mis-oligomerisation events associated with neurotoxic molecular species and/or the catalysis of damaging redox reactions in neurodegeneration, an understanding of their structure is important to the development of therapeutic agents. Nevertheless EPR, by its nature an in vitro technique, has its limitations in the study of such complex biochemical systems involving self-associating proteins that are sensitive to their chemical environment. These limitations are at the instrumental and theoretical level, which must be understood and the EPR data interpreted in the light of other biophysical and biochemical studies if useful conclusions are to be drawn.

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...

The synthesis, isolation and structural characterization of the sulfite polyoxomolybdate clusters alpha-(D(3h))(C(20)H(44)N)(4){alpha-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN and beta-(D(3d))(C(20)H(44)N)(4){beta-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN is presented. Voltammetric studies in acetonitrile (0.1 M Hx(4)NClO(4), Hx(4)N=tetra-n-hexylammonium) reveal the presence of an extensive series of six one-electron reduction processes for both isomers. Under conditions of bulk electrolysis, the initial [Mo(18)O(54)(SO(3))(2)](4-/5-) and [Mo(18)O(54)(SO(3))(2)](5-/6-) processes produce stable [Mo(18)O(54)(SO(3))(2)](5-) and [Mo(18)O(54)(SO(3))(2)](6-) species, respectively, and the same reduced species may be produced by photochemical reduction. Spectroelectrochemical data imply that retention of structural form results upon reduction, so that both alpha and beta isomers are available at each of the 4-, 5-, and 6-redox levels. However, the alpha isomer is the thermodynamically favored species in both the one- and two-electron-reduced states, with beta--alpha isomerization being detected in both cases on long time scales (days). EPR spectra also imply that increasing localization of the unpaired electron occurs over the alpha- and beta-[Mo(18)O(54)(SO(3))(2)](5-) frameworks as the temperature approaches 2 K where the EPR spectra show orthorhombic symmetry with different g and hyperfine values for the alpha and beta isomers. Theoretical studies support the observation that it is easier to reduce the alpha cluster than the beta form and also provide insight into the driving force for beta--alpha isomerization in the reduced state. Data are compared with that obtained for the well studied alpha-[Mo(18)O(54)(SO(4))(2))](4-) sulfate cluster.

Structures of the n-tetrabutylammonium salts of [SVM11O40](3-) (M = Mo, W) have been determined by X-ray crystallography and exhibit 3D networks with the V atom disordered over several sites. The cyclic voltammetric behavior of SVM11 in neutral and acidified acetonitrile solutions also has been investigated with respect to the V(V)/V(IV) couple. Results have been interpreted in conjunction with data provided by (51)V NMR spectroscopy on the oxidized V(V) form and by EPR spectroscopy on the reduced V(IV) form. Based on mechanistic details inferred from these studies, simulations of the cyclic voltammograms have been undertaken and results compared with experimental data in acidic media (two protonated forms) in order to provide estimates of equilibrium and kinetic parameters. For the V(V)/V(IV) couple in the series [XVM11O40](n-) (X = Si,Ge,P,As,S; M = Mo,W), the reversible potentials in neutral acetonitrile linearly depend on the total charge of the vanadium-substituted polyoxometalates, similar to the dependence previously reported for the non-substituted parent Keggin polyoxometalates [XM12O40](m-).

The one electron electrochemical reduction of vanadium(V) substituted-Keggin type polyoxometalate anions [XV(V)M(11)O(40)](4-) (XV(V)M(11)) where X = P, As; M = Mo or W has been investigated in CH(3)CN as a function of acid concentration. EPR studies confirm that the product is the highly basic and hence readily protonated V(IV), (XV(IV)M(11)), species. In the absence of acid or in the presence of a large concentration excess, the V(V/IV) redox couple gives one well defined chemically and electrochemically reversible process under conditions of cyclic voltammetry. In contrast, three well resolved V(V/IV) processes are observed in the presence of a moderate concentration of acid in the case of XV(V)Mo(11) but only two with XV(V)W(11). NMR and EPR spectra have been obtained as a function of acid concentration when the polyoxometalate anions are in the V(V) and V(IV) oxidation levels respectively. All voltammetric and spectroscopic data indicate that protonation reactions are coupled to the V(V) and V(IV) redox chemistry but that the reduced V(IV) state is much more basic than the V(V) one and that (XV(IV)Mo(11)) is more basic than (XV(IV)W(11)). Digital simulations of voltammograms for reduction of XV(V)Mo(11) and XV(V)W(11) have been undertaken in CH(3)CN as a function of acid concentration in order to define the thermodynamics and kinetics associated with the V(V/IV) process and the equilibrium constants that accompany the coupled acid-base chemistry. EPR spectra allow an estimation of the relative concentrations of protonated species present in frozen glasses derived from one-electron bulk electrolysis and also allow inferences to be drawn on their nature. This study provides a far more detailed analysis of the coupling of proton and electron transfer than previously available.

Naphthalene diimides 1–4 having different N,N-disubstitution undergo single electron reduction processes either chemically or electrochemically to yield the corresponding radical anion in high yield. This study concentrates on 1, bearing pentyl side chains connected through the diimide nitrogens, and compares the results obtained against those bearing isopropyl, propargyl, and phenylalanyl side chains. Compound 1 exhibits mirror image absorption and fluorescence in the near-UV region in CH2Cl2 and dimethylformamide that is typical of monomeric N,N-dialkyl-substituted naphthalene diimides. In toluene, excimer-like emission is observed, which suggests ground-state complexes involving 1 are formed. X-Ray crystallography has been used to characterize 1 in the solid state. Cyclic voltammetry enables the reversible potentials for [NDI]0/– and [NDI]−/2– type processes to be measured. Bulk one-electron reduction of 1–4 is characterized by dramatic changes in the absorption and emission spectra. Additionally, highly structured EPR (electron paramagnetic resonance) signals from dimethylformamide solutions of the radical anions of 1–3 have been obtained and are consistent with coupling between the unpaired electron and the naphthalene diimide nitrogens and hydrogens and the NCH hydrogens of the appropriate side chains. The overall structure of the EPR spectrum is substituent-dependent. These changes in spectroscopic output upon an electronic input may be described as a simple ‘on/off’ switching mechanism with which to apply a ‘bottom-up’ approach to molecular device manufacture.

The complexes [TcVNCl2(EPh3)2](E = P or As) are oxidized to [TcVINCl4]– in thionyl chloride solution at room temperature. ESR spectroscopy has established the presence of technetium(VI) intermediates, [TcNCl3(EPh3)]: E = P, g∥= 2.0316, g⊥= 2.0026, A∥= 0.0267 cm–1, A⊥= 0.0119 cm–1, Q= 0.000 35 cm–1, ax=ay= 0.0017 cm–1, ax= 0.00185 cm–1; E = As, g∥= 2.0246, g⊥= 1.9986, A∥= 0.0271 cm–1, A⊥= 0.0125 cm–1, Q= 0.0003 cm–1, ax=ay= 0.00195 cm–1 and az= 0.0021 cm–1. The crystal structure of [TcNCl2(AsPh3)2]1 has been determined and is compared with those of ReVN and TcVN phosphine complexes. Crystals of 1 are monoclinic, space group I2/a with a= 15.823(4), b= 9.638(2), c= 22.490(7)A, β= 101.91 (2)° and Z= 4. Molecules of 1 possess C2 symmetry with NTc–Cl and NTc–As angles of 110.51 (3) and 99.22(1)°, respectively. The TcN bond length is 1.601(5)A.

The e.s.r. spectra of frozen aqueous-dimethylformamide (dmf) solutions containing binary mixtures of cobalt(II) tetrasulphophthalocyanine (tspc), nickel(II)tspc, copper(II)tspc and zinc tspc have been used to discern various solute–solute interactions. Addition of zinc tspc or nickel(II) tspc to an aqueous solution containing copper(II) tspc has a profound influence on the ability of dmf to bring about the disaggeregation of the copper(II)tspc. The presence of zinc tspc results in a stabilization of the polymeric form of copper(II) tspc while nickel(II) tspc brings about a breakdown of the aggregated form of copper(II)tspc by addition of dmf. There is unambiguous evidence for the formation of a molecular species containing both nickel(II) tspc and cobalt(II) tspc. The e.s.r. spectra of an aqueous–dmf solution containing copper(II) tspc and cobalt(II) tspc provide evidence for the formation of a species in which dissimilar ion coupling occurs between copper(II) and cobalt(II). Spectra due to cobalt(II) tspc were interpreted by an axial model which did not require the inclusion of quadrupole effects.

E.s.r. measurements on manganese 4,4′,4″,4‴-tetrasulphophthalocyanine, [MnII(tspc)], confirm earlier reports that in the solid state this complex is obtained as a manganese(II) chelate. Measurements on aqueous solutions containing [MnII(tspc)] show that disaggregation of the chelate occurs as a result of small additions (5–10% v/v) of polar organic solvents such as NN-dimethylformamide. In these circumstances the e.s.r. spectrum has been assigned to a mixture of low-spin forms of the manganese(II) chelate. The solute–solute interactions which occur as a result of the addition of [NiII(tspc)] or [ZnII(tspc)] to aqueous solutions of [MnII(tspc)] are characterized by the appearance of e.s.r. signals at 77 K attributable to a high-spin form of the manganese(II) chelate. The presence of ligands such as cyanide, pyridine, or imidazole, which are capable of axial interactions with [MnII(tspc)] promotes autoxidation of the manganese chelate in aqueous solution. Measurements at room temperature and 133 K provide evidence for a change in the spin state of the metal ion in the chelates as a result of changes in the pH of the solutions.

Electron-spin-resonance measurements at \ensuremath{\sim}9 and 4 GHz have identified a center with tetragonal symmetry and S=(1/2), I=(1/2) [${g}_{?}$=1.9998(1), ${g}_{\ensuremath{\perp}}$=2.0029(1), ${A}_{?}$=0.002 03(1) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, ${A}_{\ensuremath{\perp}}$ =0.001 34(1) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$] in crystals of highly colored CaO:Mg. The resonances are produced by illumination at \ensuremath{\lambda}l750 nm and Tl200 K and are associated with the thermoluminescence peak (${\ensuremath{\lambda}}_{\mathrm{max}=690}$ nm) at 180 K. The center involves three lattice sites along 〈100〉 with ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ at an anion-vacancy site next to the cation-vacancy site of a ${\mathrm{P}}^{\mathrm{\ensuremath{-}}}$ center. The center appears to arise as a result of crystal-growth conditions and is not specifically associated with the Mg dopant.

E.s.r. spectra of manganese(II) complexes of dimethyl (dmp), diethyl (dep) and diphenyl (dϕp) phosphates and their derivatives show variations arising from subtle changes in both alkyl or aryl groups and associated solvent molecules. The X-band e.s.r. spectra can be divided into two groups. Spectra due to complexes of the type Mn[O2P(OR)2]2, where R = Me or Et, range from 0 to 400 mT indicating that the crystal field splittings of the ground state of manganese(II) are comparable to the microwave absorption energy (i.e. either ∣2D∣ or ∣4D∣≈hν depending on the absolute sign of D). The second group of spectra is associated with the following complexes: Mn[O2P(OR)2]2 L2, where R = Me, Et, Ph and L = pyridine or R = Ph and L = dmf; Mn[O2P(OPh)2]2, Mn[O2PPh2]2 and Mn[O2PPh2]2(dmf)2. In this case, crystal field splittings are smaller than the microwave absorption energy and, from computer simulations of the spectra, estimates of the crystal field parameter, D, are obtained which lie in the range 0.012 to 0.0255 cm–1. The rhombic symmetry component, E, was found to be almost zero.

Electron spin resonance measurements involving aqueous solutions of copper(II) tetraphenylporphyrintetrasulphonate have been carried out both at room temperature and 77 K to provide evidence for the process of aggregation. The addition of dimethylformamide or dimethyl sulphoxide to the aqueous solution containing the copper(II) chelate results in a breaking up of the polymer units into a distribution of species which include dimeric and monomeric species. The dimeric species are formed only to a minor extent. A similar study of solutions containing the copper(II) chelate of tetracarboxyphenylporphyrin showed that breakdown to monomeric species only occurred in solutions containing a high percentage of dimethylformamide or dimethyl sulphoxide, and that here, too, specific dimeric species did not occur in any great amount. The magnetic parameters have been determined for the monomeric chelates.

The e.s.r. spectra of [AsPh4][TcNCl4], Cs2[TcNCl5], [AsPh4][TcNBr4], and Cs2[TcNBr5] have been studied in non-aqueous and concentrated aqueous acid solutions. None of the spectra shows evidence for the co-ordination of a fifth halide ligand in the trans position, even under circumstances such as a 2 000-fold excess of halide ion, which would be expected to favour the formation of the pentahalogenonitridotechnetate ion. The predominant species in solution is the tetrahalogenonitrido-technetate ion, where the trans position may be vacant or occupied by a solvent molecule in the case of the non-aqeuous solvents and by a water molecule in the case of HCl and HBr solutions. This conclusion may be contrasted with the behaviour of a number of tetra- and penta-halogeno-oxometal complexes, where the equilibrium [MOX4]n–+ X–⇌[MOX5](n+ 1)– is clearly established.

E.s.r. spectra of manganese(II) complexes of dimethyl (dmp), diethyl (dep) and diphenyl (dϕp) phosphates and their derivatives show variations arising from subtle changes in both alkyl or aryl groups and associated solvent molecules. The X-band e.s.r. spectra can be divided into two groups. Spectra due to complexes of the type Mn[O2P(OR)2]2, where R = Me or Et, range from 0 to 400 mT indicating that the crystal field splittings of the ground state of manganese(II) are comparable to the microwave absorption energy (i.e. either ∣2D∣ or ∣4D∣≈hν depending on the absolute sign of D). The second group of spectra is associated with the following complexes: Mn[O2P(OR)2]2 L2, where R = Me, Et, Ph and L = pyridine or R = Ph and L = dmf; Mn[O2P(OPh)2]2, Mn[O2PPh2]2 and Mn[O2PPh2]2(dmf)2. In this case, crystal field splittings are smaller than the microwave absorption energy and, from computer simulations of the spectra, estimates of the crystal field parameter, D, are obtained which lie in the range 0.012 to 0.0255 cm–1. The rhombic symmetry component, E, was found to be almost zero.

Electron spin resonance measurements involving aqueous solutions of copper(II) tetraphenylporphyrintetrasulphonate have been carried out both at room temperature and 77 K to provide evidence for the process of aggregation. The addition of dimethylformamide or dimethyl sulphoxide to the aqueous solution containing the copper(II) chelate results in a breaking up of the polymer units into a distribution of species which include dimeric and monomeric species. The dimeric species are formed only to a minor extent. A similar study of solutions containing the copper(II) chelate of tetracarboxyphenylporphyrin showed that breakdown to monomeric species only occurred in solutions containing a high percentage of dimethylformamide or dimethyl sulphoxide, and that here, too, specific dimeric species did not occur in any great amount. The magnetic parameters have been determined for the monomeric chelates.

The e.s.r. spectrum of the title complex has been studied in non-aqueous solution in the liquid and frozen glass phases. The spectrum is characteristic of a low-spin 4d5 technetium(II) ion in an axially symmetric environment with g∥=(+) 1.928, g⊥=(+) 2.045, A‖= 0.0236, and A⊥= 0.0095 cm–1. The small quadrupole interaction observed is solvent dependent. A simple crystal field model, in which the unpaired electron is located in the t2g orbital triplet, is able to explain most features of the e.s.r. spectrum. A consideration of the electronic parameters derived from the g and A values leads to the conclusion that the results are best explained by a large tetragonal distortion from octahedral symmetry with strong π bonding between technetium and the ligands.

E.s.r. spectroscopy has been used to identify the products formed by reduction of the low-spin manganese(II) chelates of tetrasodium 3,10,17,24-tetrasulphonatophthalocyanine Na4(tspc) on addition of various reducing agents. Addition of hydrazine to an aqueous solution containing [MnII(tspc)]4– leads to the formation of low-spin [Mn0(tspc)]6– which is also formed on addition of sodium sulphide or dithionite to an aqueous solution of the manganese(II) chelate containing 10% v/v of pyridine. The addition of sodium tetrahydroborate to an aqueous solution of [MnII(tspc)]4– is thought to lead to diamagnetic [MnI(tspc)]5–. When pyridine or imidazole are present in an aqueous solution of [MnII(tspc)]4–, the addition of Na[BH4] leads to a free-radical species, which is thought to involve the phthalocyanine ring system. The magnetic parameters associated with the e.s.r. spectrum due to [Mn0(tspc)]6– have been determined by computer simulation of the experimental spectrum. The values of these magnetic parameters, together with the absence of extra hyperfine interactions due to certain axial ligands, lead to the proposal of a 3dxy ground-state configuration for the low-spin manganese(0) chelate.

Potentiometric pH-titration and electron spin resonance studies have shown the existence of mono-meric, dimeric and dinuclear vanadyl chelates of certain polyaminocarboxylic acids. The ligands studied include ethylenediaminetetraacetic (edta), ethyleneglycolbis(β- aminoethyl ether)-N,N?-tetra-acetic (egta), diethylenetriaminepentaacetic (dtpa) and triethylenetetraaminehexaace- tic (ttha) acids. All the ligands form 1 : 1 chelates. The addition of ethylene glycol to the aqueous solution in each case provides circumstances for the study of the monomeric chelates, though it is not equally effective in all cases. The 1 : 1 vanadyl egta chelate above pH 5.5 gave rise to ΔMS = � 2 and ΔMS = � 1 transitions from which structural and magnetic parameters have been determined and interpreted in terms of the formation of a dimeric form of a hydrolytic product of the 1 : 1 chelate. Firm evidence for the formation of dinuclear chelates occurs only in the vanadyl ttha system. Here the room tem- perature e.s.r. spectrum shows the presence of weak exchange coupling between the vanadyl ions in the dinuclear chelate.

E.s.r. spectroscopy has been used to show that the addition of NN-dimethylformamide or dimethyl sulphoxide to aqueous solutions of cobalt(II) tetraphenylporphyrintetrasulphonate. [CoII(tpps)], results in a disaggregation of the metalloporphyrin and provides conditions for its oxygenation by molecular oxygen. Larger additions of ethanol lead to formation of a monomeric six-co-ordinate cobalt(II) porphyrin which does not undergo a reaction with molecular Oxygen to any great extent. On long storage in air, [CoII(tpps)] in the solid state forms a complex with molecular oxygen. The magnetic parameters associated with the e.s.r. spectra of the monomeric forms of the metalloporphyrin and its complex with molecular oxygen have been determined. The oxygenated complex is believed to involve a Co–O–O bond angle of 105°. intermediate between the two models proposed previously for oxyhaemoglobin.

Optical spectroscopy, magnetic susceptibility, and e.s.r. have been used to study the behaviour of Cs2[TcNCl5] in aqueous HCl solutions. In 5.6 mol dm–3 HCl the predominant species is the [TcNCl4]– anion (λmax 398 nm, Iµ 440 m2 mol–1) possibly with H2O weakly bound trans to the N atom. Addition of 1 volume of H2O to Cs2[TcNCl5] followed by 2 volumes of 5 mol dm–3 HCl gives a deep blue species (λmax. 558 nm, Iµ 393 m2 mol–1), which is converted to [TcNCl4]– on standing. Cs2[TcNCl5] dissolved in 0.5 mol dm–3 HCl gives a pink solution (λmax. 540 nm, Iµ 215 m2 mol–1) which is stable over several days. The magnetic susceptibility and e.s.r. measurements indicate that the blue and pink species are diamagnetic. Possible structures of the diamagnetic species are discussed by analogy to the behaviour of [MOX5]n– in solutions of varying acidity.