Joseph Crayston

Redox properties of iron-sulphur-nitrosyl complexes: Exploratory electrochemistry of mononuclear, dinuclear and tetranuclear complexes

Abstract Cyclic voltammetry shows two reversible, one-electron reductions for the neutral complexes [Fe2(SR)2(NO)4] (R = alkyl), to provide successively [Fe2(SR)2(NO)4]− and [Fe2(SR)2(NO)4]2−. The paramagnetic complexes [Fe2(SR)2(NO)4]−, which can also be prepared by chemical reduction of neutral [Fe2(SR)2(NO)4], have been characterized by ESR spectroscopy: the complexes [Fe2(SR)2(NO)4]2−, which are isoelectronic with the substitution-labile cobalt complexes [Co2(SR)2(NO)4], undergo ready reaction with phosphorus ligands. The di-anion [Fe2S2(NO)4]2− also exhibits two reversible, one-electron reductions, but for [Fe2(S2O3)2(NO)4]2− only the first reduction is reversible. The tetranuclear complex [Fe4S3(NO)7]− undergoes three reversible, one-electron reductions to yield [Fe4S3(NO)7]x−(x = 2, 3 and 4). For all these complexes, the oxidation reactions are complex and irreversible. The neutral paramagnetic complexes [Fe(NO)(S2CNR2)2] (R = alkyl) all show a reversible one-electron reduction: the diamagnetic intermediates, which can also be prepared by chemical reduction, readily lose NO.

Chemical, electrochemical and morphological characterization of poly -{trans -[RuCl2 (vpy )4]} films deposited on Nd-Fe-B magnets and on inert materials

The characterization of poly-{trans-[RuCl2(vpy)4]}films, deposited on Nd-Fe-B magnets and on Pt, Au and Glassy Carbon (GC) electrodes, produced in the present work were carried out by several in situ and extra situ techniques, aiming by the understanding of film structure improve its use as corrosion protection coating and/or electrocatalyst. Cyclic Voltammetry (CV) was used to estimate the surface coverage, film stability and the surface redox potential of films produced at different conditions. By Chronocoulometry films diffusion coefficient (Dct) were evaluated. Film morphology was studied by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Chemical composition analyses were carried out by Energy Dispersive Spectroscopy (EDS), X-ray Photonelectron Spectroscopy (XPS) and Raman Spectroscopy. The results indicated that the film structure is similar to the Ru complex in solution. Films Dct is limited by the electron transfer rate between Ru metallic centers. Films on Nd-Fe-B were composed by Ru3+ whereas on Pt by Ru2+.

A SERRS spectroscopic investigation of nickel(II) porphyrin complexes adsorbed at electrochemical interfaces

Surface enhanced resonance Raman (SERRS) spectra of two water-insoluble metalloporphyrins, nickel(II) octaethylporphyrin (NiOEP) and nickel(II) tetraphenylporphyrin (NiTPP), are reported at a range of excitation energies and applied potentials. By comparison of the RR and SERRS spectra, it was determined that NiOEP adsorbs onto a silver electrode surface in an edge-on orientation, most probably with the meso-carbon atoms closest to the electrode surface. SERRS spectra were independent of applied potential, indicating that the porphyrin layer is stable throughout the potential range employed and that enhancement principally occurs through the electromagnetic mechanism. By contrast, the Raman spectra of NiTPP at the electrode surface suggest little affinity of NiTPP for the silver surface and possibly arise from RR scattering, rather than SERRS.

Synthesis and crystal structure of cis-[NiL(en)](ClO4)2·en·H2O (L=3-hydroxyethyl-1,3,5,8,12-penta-azacyclotetradecane) and solution studies of the planar-cis-folding equilibrium

Abstract The reaction of formaldehyde and ethanolamine with the nickel(II) complex of 1,9-diamino-3,7-diazanonane (2,3,2-tet) in the presence of triethylamine gives the planar nickel(II) complex of the penta-azamacrocycle-3-hydroxyethyl-1,3,5,8,12-penta-azacyclotetradecane (L) which can be readily isolated as the perchlorate salt. The reaction of [NiL](ClO4)2 with 1,2-diaminoethane (en) gives the folded cis-complex [NiL(en)](ClO4)2·en·H2O containing a water and a 1,2-diaminoethane of crystallisation. The nickel(II) centre in the crystal structure is distorted octahedral with Ni–N bond distances to the N(1), N(2), N(3) and N(4) atoms of the macrocycle in the range 2.099(7)–2.129(7) A which are typical of octahedral nickel(II) complexes with other such azamacrocycles ligands. The cis orientated Ni–N bond distances to the two N atoms of the 1,2-diaminoethane ligand are however somewhat longer at 2.168(7) and 2.161(7) A. The hydroxyethyl group is axial with the ligand having the trans V stereochemistry of the chiral sec-NH centres. The six-membered chelate rings have a chair conformation and the five-membered chelate rings are gauche. The planar-trans-octahedral diaqua equilibrium; [NiL]2++2H2O↔[NiL(OH2)2]2+ is represented by K1 (25°C)=0.167, ΔH°=−55±3 kJ mol−1 and ΔS°=−198±30 J K−1 mol−1. The planar-cis-folded equilibrium; [NiL]2++en↔cis-[NiL(en)]2+ is governed by K2 (25°C)=417 dm3 mol−1, ΔH°=−38.5±2 kJ mol−1 and ΔS°=−79±5 J K−l mol−l. The mechanism of the planar-folded octahedral equilibrium is discussed.