Lesley J. Yellowlees

Electrochemistry of Sulfur and Polysulfides in Ionic Liquids

The electrochemistry of elemental sulfur (S(8)) and the polysulfides Na(2)S(4) and Na(2)S(6) has been studied for the first time in nonchloroaluminate ionic liquids. The cyclic voltammetry of S(8) in the ionic liquids is different to the behavior reported in some organic solvents, with two reductions and one oxidation peak observed. Supported by in situ UV-vis spectro-electrochemical experiments, the main reduction products of S(8) in [C(4)mim][DCA] ([C(4)mim] = 1-butyl-3-methylimidazolium; DCA = dicyanamide) have been identified as S(6)(2-) and S(4)(2-), and plausible pathways for the formation of these species are proposed. Dissociation and/or disproportionation of the polyanions S(6)(2-) and S(4)(2-) appears to be slow in the ionic liquid, with only small amounts of the blue radical species S(3)(•-) formed in the solutions at r.t., in contrast with that observed in most molecular solvents.

Synthesis, structure and properties of [Pt(2,2 '-bipyridyl-5,5 '-dicarboxylic acid)(3,4-toluenedithiolate)]: tuning molecular properties for application in dye-sensitised solar cells

The platinum diimine dithiolate complex, [Pt(2,2′-bipyridyl-5,5′-dicarboxylicacid)(3,4-toluenedithiolate)] ([Pt(5,5′-dcbpy)(tdt)]) and its tetrabutylammonium salt [TBA]2[Pt(5,5′-dcbpy)(tdt)] have been prepared, spectroscopically and electrochemically characterised and attached on to TiO2 substrate to be used as solar cell sensitisers. A single-crystal X-ray structure was obtained for [TBA]2[Pt(5,5′-dcbpy)(tdt)]·EtOH·EtOAc. The effect of the position of the two carboxylic acid substituents on the electrochemistry of the 5,5′-disubstituted complexes is discussed in comparison with the previously reported [Pt(4,4′-dcbpy)(tdt)]. Electrochemical studies show no major change in the HOMO after movement of the carboxylic acid groups, consistent with assignment of the HOMO as largely dithiolate based. Movement of the carboxylic acid groups makes the diimine electronic character and hence the LUMO of the complexes different. Electrochemical studies show a change to lower energy of the LUMO represented by changes in reduction potential of the compound on moving the carboxylic acid substituents from the 4,4′ to the 5,5′ positions. Both [Pt(5,5′-dcbpy)(tdt)] and [TBA]2[Pt(5,5′-dcbpy)(tdt)] have been used as solar cell sensitisers, with the di-TBA salt giving lower dye loading but superior photovoltaic performance. The consequences of tuning the complex through the position of the carboxylic acid groups are discussed.

Coordination and release of NO by ruthenium–dimethylsulfoxide complexes—implications for antimetastases activity

Abstract Several Ru(II) and Ru(III)–dimethylsulfoxide (DMSO) complexes, that are not cytotoxic in vitro, are endowed with anticancer and, in particular, antimetastatic activity against animal tumor models. One possibility for explaining the activity of such compounds against disseminated tumors is that they interfere with NO metabolism in vivo. Thus, we investigated the reactivity of ruthenium–chloride–DMSO complexes towards NO with the aim of producing well-characterized models to be used as reference compounds in subsequent biomimetic studies. In this contribution, we report on the synthesis, spectroscopic, structural and electrochemical characterization of anionic (e.g. [(DMSO)2H][trans-RuCl4(DMSO-O)(NO)] (1)), neutral (e.g. mer,cis-RuCl3(DMSO-O)2(NO) (2)) and new cationic (e.g. [cis,fac-RuCl2(DMSO-O)3(NO)][BF4] (9)) Ru–DMSO nitrosyls, derived from both Ru(II) and Ru(III)–chloride–DMSO precursors. Coordination of the strong π-acceptor NO favors coordination of DMSO ligands through oxygen (DMSO-O) to avoid competition for π electrons. The reactivity of some Ru–DMSO–NO complexes towards heterocyclic N-ligands, leading to compounds such as [(Im)2H][trans-RuCl4(Im)(NO)] (Im=imidazole, 6) and [cis,mer-RuCl2(py)3(NO)][BF4] (py=pyridine, 10), is also described. The spectroscopic and X-ray structural features for all these complexes are consistent with the {Ru(NO)}6 formulation, that is a diamagnetic Ru(II) nucleus bound to NO+. Electrochemical measurements on the Ru–NO complexes showed that they are all redox active in DMF solutions and the site of reduction is the NO+ moiety. With the exception of 10, the reduced complexes are not stable and rapidly release the NO radical.

Ligand-ligand inter-valence charge-transfer absorption in reduced ruthenium(II) bipyridine complexes

An electronic absorption band at 4000 cm−1 in incompletely reduced Ru(II)-bipyridine and Ru(II)-bipyride-pyridine complexes is characteristic of co-existing bpyo and bpy−1 ligands and assigned to bpy−/bpy0 inter-valence charger transfer.

Nanoscale Growth of Molecular Oxides: Assembly of a {V6} Double Cubane Between Two Lacunary {P2W15} Polyoxometalates

POM-in-POM: A Wells–Dawson polyoxometalate sandwich compound with a double cubane core consisting of six vanadium atoms has been synthesized (see structure). Cluster formation was followed by mass spectrometry and the reduction of the double cubane was studied by a novel technique combining mass spectrometry and spectroelectrochemistry.

The electronic absorption spectrum and structure of the emitting state of the tris(2,2′-bipyridyl)ruthenium(II) complex ion

The directly determined absorption spectrum (250–650 nm) of the optically excited species *[Ru(bipy)3]2+(bipy = 2,2′-bipyridyl) is presented and assigned. The promoted (metal to ligand charge transfer) electron is localised on one of the three bipy ligands, as in the formulation [RuIII(bipy)2(bipy–)]2+, while the known emission photoselection data imply that the promoted electron resides on the same ligand throughout.

Electrochemical control of bridging ligand conformation in a binuclear complex—A possible basis for a molecular switch

Reaction of the binucleating bridging ligands 3,3′,4,4′-tetrahydroxybiphenyl (H4L1) or 3,3″,4,4″-tetrahydroxy-p-terphenyl (H4L2) with 2 equivalents of [Ru(bipy)2Cl2]·2H2O (bipy = 2,2′-bipyridine) results in the formation of binuclear complexes [{Ru(bipy)2}(µ-L){Ru(bipy)2}]2+1(L = L1) and 2(L = L2) in which the ligand L4– has been oxidized to the semiquinone (sq) form L2–. In each complex the co-ordinated catecholate fragment (cat) may be oxidised reversibly to the semiquinone and quinone (q) redox states, giving the five-membered redox series cat–cat, cat–sq, sq–sq, sq–q and q–q for the bridging ligands. In the sq–sq state the bridging ligands are necessarily planar due to the presence of double bonds between the aromatic rings; in the cat–cat and q–q states there is formally no double-bond character between the aromatic rings and they are free to adopt a twisted conformation. Spectroelectrochemical measurements confirm that in the cat–cat and q–q states, 1 and 2 behave approximately like their mononuclear counterparts [Ru(bipy)2(cat)] and [Ru(bipy)2(bq)]2+[H2cat = catechol (benzene-1,2-diol); bq =o-benzoquinone]; for 1, the results also show that in the mixed-valence sq–q and sq–cat oxidation states the two halves of the ligand are electronically equivalent (i.e. valence delocalised), which is best explained by the bridging ligand retaining the planar conformation of the sq–sq state. The change from planar to twisted therefore occurs at the extremes of the redox series, on formation of the cat–cat and q–q oxidation states. This control of bridging ligand conformation with oxidation state may form the basis for a molecular switch.

Electrochemical and spectroelectrochemical studies on platinum complexes containing 2,2′-bipyridine

The electrochemical and spectroelectrochemical (UV/VIS/NIR and EPR) properties of a series of complexes of general formula [Pt(bipy)L2]n+(L = Cl– or CN–, n= 0; L = NH3, py, PMe3 or L2= en, n= 2; bipy = 2,2′-bipyridine, py = pyridine, en = ethylenediamine) have been investigated. The complexes undergo a reversible one-electron reduction process at potentials of ca.–1 V vs. Ag–AgCl. In each case UV/VIS/NIR spectra of the one-electron reduction products were consistent with co-ordinated bipyridyl anion-radical species rather than d9 metal centres. Electron paramagnetic resonance spectra of the chemically or electrochemically generated 17-electron species, in conjunction with the results of extended-Huckel molecular-orbital calculations, show a significant admixture of metal 5dyz and/or 6pz orbitals in the singly-occupied molecular orbital. The complex [Pt(bipy)(PMe3)2][BF4]2 has been prepared.

Sterically-induced low temperatur polyhedral rearrangements of carbaplatinaboranes: Synthesis and crystal structures of 1-Ph-3,3-(PMe2Ph)2-3,1,2-PtC2B9H10, 1-Ph-3,3-(PMe2Ph)2-3,1,11-PtC2B9H10, 11-Ph-3,3-(PMe2Ph)2-3,1,11-PtC2B9H10 and 1,11-Ph2-3,3-(PMe2Ph)2-3,1,11-PtC2B9H9

Abstract Reaction of cis-Pt(PMe2Ph)2Cl2 with Tl2[7-Ph-7,8-nido-C2B9H10] affords 1-Ph-3,3-(PMe2Ph)2-3,1,2-PtC2B9H10, mild thermolysis (55°C) of which yields 1-Ph-3,3-(PMe2Ph)2-3,1,11-PtC2B9H10 and 11-Ph-3,3-(PMe2Ph)2-3,1,11-PtC2B9H10. Both of the latter compounds are produced by the microwave irradiation of a mixture of cis-Pt(PMe2Ph)2Cl2 and [HNMe3][7-Ph-7,8-nido-C2B9H11]. When cis-Pt(PMe2Ph)2Cl2 is allowed to react with Tl2[7,8-Ph2-7,8-nido-C2B9H9] at room temperature the only isolable species is 1,11-Ph2-3,3-(PMe2Ph)2-3,1,11-PtC2B9H9. The generation of rearranged products with 3,1,11-PtC2B9 architectures is inconsistent with a diamond-square-diamond mechanism for the isomerisation of icosahedral heteroboranes.

Electrochemistry and spectroelectrochemistry of ruthenium(II)-bipyridine building blocks. Different behaviour of the 2,3- and 2,5-bis(2-pyridyl)pyrazine bridging ligands

We report the results of an investigation, using electrochemical and spectroelectrochemical techniques, into redox properties of uncoordinated free bis-chelating 2,3- and 2,5-bis(2-pyridyl)pyrazine ligands (2,3- and 2,5-dpp) and of the complexes of the [Ru(2,3-dpp)n(bpy)3−n]2+ and [Ru(2,5-dpp)n(bpy)3−n]2+ families (bpy=2,2′-bipyridine), which are used as building blocks for obtaining polynuclear complexes. For comparison purposes, the electrochemical behaviour of the [Ru(2,3-dpp)(DCE-bpy)2]2+complex, where DCE-bpy is 5,5′-dicarboxyethyl-2,2′-bipyridine, has also been investigated. Correlations of the E1/2 values observed for the compounds examined (genetic diagrams) have allowed us to assign all the ligand-based reduction processes as well as to discuss electronic interactions. The localisation of the first three reduction processes for each complex has also been established on the basis of the spectroelectrochemical results. Theoretical calculations (AM1 semiempirical and ab-initio level) carried out for the 2,5-dpp and 2,3-dpp ligands show that, in the uncoordinated state, the former ligand does not exhibit any substantial conformation arrangement, whereas the latter has a stable conformation for a large (56°) dihedral angle between the pyridyl and pyrazine rings. The changes in conformation upon mono- and bis-coordination of 2,3-dpp can account for its peculiar electrochemical behaviour consisting in a change of the number of redox processes with varying coordination state.