Horácio M. Novais

(n-Bu4N)2[Fe(dcbdt)2]2. Synthesis, crystal structure and magnetic characterisation

Abstract The ( n -Bu 4 N) 2 [Fe(dcbdt) 2 ] 2 complex, where 4,5-dicyanobenzene-1,2-dithiolate (dcbdt), was prepared and characterised by X-ray diffraction, Mossbauer spectroscopy and magnetisation measurements. The crystal structure as determined by single crystal X-ray diffraction is triclinic, space group P 1 with cell parameters a =9.5660(1) A, b =13.0200(2) A, c =16.0850(2) A, α =67.554°(1), β =86.797°(1), γ =70.523°(1), V =1738.77(4) A 3 and it consists in strongly dimerised [Fe(dcbdt) 2 ] − units, which are stacked side by side as chains along a . The magnetic susceptibility shows that the [Fe(dcbdt) 2 ] − units have a low spin S =1/2 configuration and, besides a strong intradimer antiferromagnetic interaction, present also a significant interdimer antiferromagnetic interaction mediated by short side contacts.

Structure and physical properties of (n-Bu4N)2 [Au(dcbdt)2]5

[(n-Bu) 4 N] 2 [Au(dcbdt) 2 ] 5 was obtained as brown thin plate shaped crystals by electrochemical oxidation of (n-Bu) 4 N [Au(dcbdt) 2 ], a Au complex with 4,5-dicyanobenzene-1,2-dithiolate (dcbdt) recently prepared. Cyclic voltammetry and optical spectroscopy studies indicate a mixed valence state. The crystal structure is triclinic and presents slightly pentamerised segregated stacks of the complex along (1,2,0) Single crystal electrical conductivity (σ RT ∼10 S/cm, Ea=13.5 meV) and thermopower measurements (S RT =6μ V/K) suggest a degenerate semiconducting behaviour. The static susceptibility measurements give an almost temperature independent paramagnetic value of 3 5×10 4 emu/mol. These and EPR results are discussed in relation to the crystal structure.

Conductors based on metal-bisdicyanobenzodithiolate complexes☆

Abstract The physical properties of the partially oxidised complexes [( n -Bu) 4 N] 2 [M(dcbdt) 2 ] 5 with M=Au and Ni (dcbdt=4,5-dicyanobenzene-1,2-dithiolate) are compared and related to the different electronic bandfilling and their slightly different structure. Both compounds present a triclinic structure, space group P- 1, with pentamerised segregated stacks of the complex along [-2 1 0]. The lower electrical conductivity with a larger activation energy, as well as the larger magnitude of thermopower and paramagnetic susceptibility, of the Ni compound are a consequence of both its different electronic bandfilling and more pronounced modulation of the stacking.

Charge transfer salts based on M(dcbdt)2 complexes (M=Au and Ni)

BEDT-TTF [Au(dcbdt) 2 ] and TMTSF [Co(dcbdt) 2 ] (dcbdt = 4,5-dicyanobenzene-1,2-dithiolate) were obtained as small single crystals by electrocrystallisation. BEDT-TTF [Au(dcbdt) 2 ] is triclinic, space group P1, and its structure consists in alternated donor acceptor stacks, responsible for electrical insulating properties. TMTSF [Co(dcbdt) 2 ] presents semiconducting properties with a relatively high electrical conductivity (σ RT ∼10-150 S/cm, E a =78 meV) and its thermopower (S RT ∼ 9 μV/K) has a temperature dependence suggesting a two band conduction mechanism.

EPR spectroscopic study of the radical oxidation of hydroxypurines in aqueous solution: acid–base properties of the derived radicals

EPR spectroscopy has been employed to study the reactions of SO4˙– and OH˙ radicals with xanthine, 8-methylxanthine, hypoxanthine and 2-hydroxypurine.Radicals obtained on the reaction of SO4˙– with these hydroxypurines resulted, depending on the pH, from one-electron oxidation of either the parent compound or its anion, followed by deprotonation. For xanthine and 8-methylxanthine the respective radical anion and radical dianion were observed, for hypoxanthine the species detected correspond to the neutral radical and radical anion, whereas for 2-hydroxypurine only its radical anion was observed.The pKa values of the radical anion of xanthine and 8-methylxanthine were estimated as 12.0 ± 0.1 and 12.1 ± 0.5, respectively, whereas the pKa value of the hypoxanthine neutral radical was 7.7 ± 0.1.In the reaction with OH˙, only the radical anion of xanthine was detected.

Intramolecular electron exchange with solvent participation: electron spin resonance study of the radical-anion of benzene-1,3-dicarbaldehyde in propan-2-ol–water mixtures

The short-lived radical anion of benzene-1,3-dicarbaldehyde was generated by in situ photolysis of the parent compound in pure ethanol, propan-2-ol, and propan-2-ol–water mixtures at pH >7 and identified by e.s.r. In one of the three detected rotamers (Z,E) an alternating linewidth effect was observed at temperatures between 252 and 315 K. This phenomenon was studied in terms of a two-jump model involving two asymmetrically solvated radical anions. Rates of interconversion between these two in propan-2-ol were found to be in the range 0.5–1.25 × 109 s–1. Variation of the water content in mixtures of propan-2-ol and water mainly affects the coupling consants. The 3-formyl-α-hydroxybenzyl radical (the conjugate acid of the radical anion) has also been generated by photolysis of benzene-1,3-dicarbaldehyde in pure propan-2-ol at pH 4–7 and has been studied by e.s.r.

Electron spin resonance study of the reaction of SO4˙– with azolinones

The reactions of photolytically generated sulfate radical-anion, SO4˙–, with several nitrogen-containing heterocycles of the azolinone type in aqueous solutions were studied by ESR spectroscopy. SO4˙– reacts by formal hydrogen abstraction from an NH group to yield neutral radicals in acidic and neutral solutions and the respective radical-anions at basic pH. The pKa-values of radicals derived from 2,3-dihydroimidazol-2(1H)-one, 2-oxo-2,3-dihydro-1H imidazole-4-carboxylic acid and 2,3-dihydroindazol-3(1H)-one were determined to be in the range 8.5–10.4. In the cases of 2,3-dihydroimidazol-2(1H)-one and 2,3-dihydropyrazol-3(1H)-one secondary radicals were detected at basic pH. These radicals resulted from the disproportionation of the primary radicals followed by addition of OH– and oxidation of the OH-adduct.

Electron spin resonance study of radicals derived from cyclic hydrazides

Transient radicals derived from cyclic hydrazides (QH2), generated by in situ photolysis of flowing aqueous solutions in the presence of either added oxidants or reductants, have been studied by e.s.r. The radicals obtained through one-electron oxidation at basic pH values have a semiquinone-type structure (Q–˙) of the same type as certain intermediates in the chemiluminescence of luminol and other cyclic hydrazides. At acidic pH values, the conjugate acids (QH˙) of the semiquinone radicals derived from cyclic hydrazides of both phthalic and naphthalene-2,3-dicarboxylic acids were identified as the corresponding o-formyl carboxylate radicals, produced through a mechanism involving loss of dinitrogen. Reduced radicals (QH2–˙ and QH2–˙) generated in aqueous solution in the presence of propan-2-ol were detected only in the case of cyclic maleohydrazide. Reactions of hydrazides with the OH˙ radical were also investigated; an OH-adduct to the CC bond of the protonated maleohydrazide was identified in the pH range 2–3.

Structure of radicals derived from hydroxypyrimidines in aqueous solution

EPR spectra of radicals derived from the three isomeric trihydroxypyrimidines and from 2-hydroxypyrimidine were obtained either in reduction or oxidation conditions at various pH values. Additionally, three other pyrimidine derivatives, 2-thiobarbituric acid, 2-amino-4,6-dihydroxypyrimidine and 2-methyl-4,5,6-trihydroxypyrimidine, were studied in the same conditions. Reduced radicals were produced by reaction of the pyrimidines with the hydrated electron, the hydrogen atom and with the CO2˙− radical. In these conditions, the electron adducts as well as CO2˙− adducts were identified in most cases. Oxidized radicals were obtained by reaction of the substrates with OH˙, O˙−, SO4˙−, and Br2˙− radicals. The radicals detected and studied were either of the OH-adduct type or radicals derived from its dehydration. The radicals were investigated by using in situ radiolysis and photolysis EPR techniques.