Bárbara Loeb

ELECTRONIC EFFECTS OF DONOR AND ACCEPTOR SUBSTITUENTS ON DIPYRIDO(3,2-a:2′,3′-c)PHENAZINE (dppz)

Abstract The chelate ligands 11-R-dipyrido[3,2-a:2′,3′-c]phenazine, dppz-R (R = NH2, CH3, H, COOH, NO2) and the Re(dppz-R)(CO)3Cl (R = NH2, COOH, NO2) complexes were synthesized and characterized by conventional techniques. The influence of the donor and acceptor properties of the R substituents on the ligand properties were studied by spectroscopic techniques such as 1H-NMR and UV-Vis. Theoretical calculations were also achieved, mainly to interpret and understand the experimental spectra.

Synthesis of a new polypyridinic highly conjugated ligand with electron-acceptor properties

Abstract A new acceptor polypyridinic ligand functionalized with a quinone fragment is reported. The ligand, dipyrido[3,2- a :2′,3′- c ]-benzo[3,4]-phenazine-11,16-quinone, Nqphen, was synthesized by condensation of 1,10-phenanthroline-5,6-dione and 2,3-diamino-1,4-naphthoquinone. The syntheses of two rhenium complexes with this ligand are also reported.

Synthesis, characterization and crystal structure of [Ru(tppz)(4,4′-(CH3)2bpy)Cl](PF6), (tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine)

Abstract The synthesis and characterization of the complex [Ru(tppz)(4,4′-(CH 3 ) 2 bpy)Cl] + , where tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, is reported. The ion was obtained in a one pot synthesis by reduction of Ru(4,4′-(CH 3 ) 2 bpy)Cl 4 with triethylamine in the presence of tppz, and isolated as the hexafluorophosphate salt. The structure of [Ru(tppz)(4,4′-(CH 3 ) 2 bpy)Cl](PF 6 ), 1 , was established by X-ray diffraction. The ruthenium atom is in a highly distorted octahedral environment with the 4,4′-(CH 3 ) 2 bpy nitrogens, the pyrazine central nitrogen of tppz, and chlorine defining the best mean equatorial plane. The axial positions are occupied by the nitrogens of the coordinated pyridyl rings of tppz. The Ru—N(tppz) bond to the central ring is short [1.962(9) A], while the Ru—N(4,4′(CH 3 ) 2 bpy) bond trans to it is lengthened towards a single bond distance [2.096(10) A]. This enhances the distortion in coordination of the octahedron, mainly produced by the bite angle of tppz. The distortions in 1 are evident in the 1 H-NMR spectra by the shifts of characteristic resonances upfield and downfield relative to the ligand.

Formation of carnosine-Cu(II) complexes prevents and reverts the inhibitory action of copper in P2X4 and P2X7 receptors

To further analyze the action of copper on brain synaptic mechanisms, the brain dipeptide carnosine (β‐alanyl‐l‐histidine) was tested in Xenopus laevis oocytes expressing the rat P2X4 or P2X7 receptors. Ten micromolar copper halved the currents evoked by ATP in both receptors; co‐application of carnosine plus copper prevented the metal induced‐inhibition with a median effective concentration of 12.1 ± 3.9 and 12.0 ± 5.5 µm for P2X4 and P2X7, respectively. Zinc potentiated only the P2X4 ATP‐evoked currents; carnosine had no effect over this metal. The relative potency and selectivity of classical metal chelators to prevent the copper inhibition was compared between carnosine and penicillamine (PA), bathophenanthroline (BPh) or l‐histidine (His). Their rank order of potency in P2X4 and P2X7 receptors was carnosine = PA = His > BPh > Glycine (Gly) and carnosine = BPh = His > PA > Gly, respectively. The potency to prevent the zinc‐induced potentiation in the P2X4 receptor was BPh > PA > His; carnosine, Gly and β‐alanine were inactive. Whereas 1–100 µm carnosine or His alone did not modify the ATP‐evoked currents, 10–100 µm PA augmented and 100 µm BPh decreased the ATP‐evoked currents. Carnosine was able to revert the copper‐induced inhibition restoring the maximal ATP gated current in a concentration‐dependent manner. Electronic spectroscopy confirm the formation of carnosine‐Cu(II) complexes, mechanism that can account for the prevention and reversal of the copper inhibition, revealing its potential in copper intoxication treatment.

Influence of L-type ligands on the relative stability and interconversion of cis–trans-[Ru(phen)2L2]n+ type complexes. A theoretical study

Abstract Polipyridine complexes of type [Ru(N-N) 2 L 2 ] 2+ are useful for the conversion of light into usable energy for which the trans geometry would be preferable. In this paper, the effect of distortion on trans structure stability was studied theoretically. A formal separation of the contributions was carried out, and it explained why the hydrogen repulsions are an important factor. A study of the main aspects related to the cis – trans isomers and also their mutual interconversion was analyzed. Calculations were carried out at the PM3, ZINDO and ab initio level. Energy differences between trans and cis isomers after full geometry optimizations for different L-ligands show a dependence on the L-ligand Jorgensen field strength parameter f . A driving coordinate was performed in order to determine the kinetic preferences for isomer formation. The analysis showed that the entrance of the second phenanthroline is favoured for the cis geometry in regard to the trans geometry. The activation barrier is also dependent on the L-ligand strength. The thermal trans – cis interconversion seems to be very improbable adiabatically (considering only one potential energy surface).

The 2-Mercaptobenzothiazole and Copper(II) Reaction

Abstract A systematic study of different synthetic routes for the reaction of 2-mercaptobenzothiazole with Cu(II) was pursued. The products were carefully analyzed in regard to the oxidation state of copper and its stoichiometry. Conclusions were drawn in relation to the nature of the products obtained and the conditions of the synthesis.

Experimental evidence of the disproportionation equilibrium in copper mixed-valence complexes

Abstract Experimental evidence for a dismutation equilibrium in the mixed-valence (MV) [M n + –M ( n +1)+ ] system type has been elusive and its existence can be established only when the oxidation–reduction processes involved are reversible. Previous research in the field of binuclear Cu(II)Cu(II), Cu(I)Cu(I) and the related MV Cu(II)Cu(I) complexes allowed us to obtain electrochemical evidence for the disproportionation equilibrium in some of these systems. In this communication we report and discuss experiments with [(RCOO) 2 Cu(II)Cu(I)(OOCR) 2 ] − (R=CH 3 , Ph) type MV complexes that give direct non-electrochemical experimental evidence for the presence in solution of the disproportion equilibrium: 2[( RCOO ) 2 Cu ( II ) Cu ( I )( OOCR ) 2 ] − ⇔( RCOO ) 4 Cu 2 ( II )+[( RCOO ) 4 Cu ( I ) 2 ] 2 − It was possible to isolate the different components of the disproportionation equilibrium by varying temperature and solvent conditions. To our knowledge, this is the first non-electrochemical experimental evidence of this equilibrium for copper MV complexes in solution. Furthermore, as is discussed in the text, these results may be the basis for giving an alternative point of view to that given in some studies already reported in the literature, which relate to solvent and temperature effects on the intensity and energy of the intervalence transfer bands and also to changes in the EPR spectra of MV species.

A violet mixed-valence copper-mercaptoethylamine complex generated electrochemically or by reversible interaction with oxygen

Abstract The synthesis, characterization and the reversible interaction with oxygen of a Cu(I)-mercaptoethylamine complex (I) in aqueous solution and room temperature is reported. As a result of the interaction with oxygen a violet species II is formed which has a similar absorption spectrum (λmax = 500 nm) to a violet species III obtained electrochemically from a solution of the Cu-mercarptoethylamine complex in a nitrogen atmosphere. In the absence of dissolved oxygen both violet species are stable but in presence of air they decompose into a greenish-white precipitate which does not contain the mercaptoethylamine ligand in its structure. A mechanism for the complex I-oxygen interaction is proposed which involves a superoxo mixed-valence species as intermediate. The electronic assignation of the 500 nm absorption band is also discussed. The remarkable special characteristics, i.e. (i) the simplicity of the ligand containing a soft donor atom; (ii) the fact that the reversible uptake of oxygen occurs at room temperature and (iii) the spectroscopical similarity of species II and III described above, allow new insights into the interaction of metal complexes with oxygen. These results suggest that a mixed-valence species may be associated with the O2 binding or oxygen activation processes in copper proteins. Also, considering the results recently published by M. Saraste et al., the relevance of the mercaptide bridged mixed-valence species II and III as models for the CuA site in cytochrome oxidase is also discussed.

New cyclometalated Ir(III) complexes with bulky ligands with potential applications in LEC devices: experimental and theoretical studies of their photophysical properties

In the present work we report the synthesis and the electrochemical, photoluminescence and electroluminescence properties of two new Ir(III) cyclometalated complexes denoted as [Ir(F2ppy)2L1](PF6) and [Ir(F2ppy)2L2](PF6), where F2ppy is 2-(2,4-difluorophenyl)pyridine, L1 is 4,4′-diphenylethyl-2,2′-bipyridine and L2 is 4,4′-bis[2-[4-(1,1-dimethylethoxy)phenyl]ethyl]-2,2′-bipyridine. The photoluminescence spectra in solution for both the complexes are characterized by wavelength emission maxima at around 510 nm and higher quantum yields. In the solid state, the emission spectrum of the complex with L2 is characterized by higher emission intensity than the [Ir(F2ppy)2L1](PF6) complex. This behavior is explained as due to the effect of the more bulky structure of the L2 ligand, which prevents, in a more efficient way compared to the complex with L1, the auto-quenching processes in the solid packing. DFT calculations were performed to understand the photophysical behavior of the complexes, and an excellent agreement between experimental and theoretical data was observed. For the complex with L2, the electronic density of the HOMO is located in the chain and phenyl fragment of this ligand. This behavior is quite different from that expected for a typical Ir(III) cyclometalated complex, where the electron density of the HOMO is located on the metal (t2g orbitals)/phenyl fragment of the cyclometalating ligand. In spite of these differences, both complexes are good emitters, and in both cases the emission comes from a single T1 emitter state with the contribution of the MLCT and LLCT. In order to get a first approximation of the behavior of these complexes in LEC (light emitting electrochemical cells) applications, the electroluminescence spectra with an applied bias of 12 V were obtained. Both the complexes show yellow-green emissions at around 550 nm, with (0.41, 0.48) and (0.45, 0.48) CIE coordinates for [Ir(F2ppy2)2L1](PF6) and [Ir(F2ppy2)2L2](PF6), respectively.

SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES OF A SERIES OF SUBSTITUTED POLYPYRIDINE Co(II)/Co(III) COUPLES AND THEIR POTENTIALITY AS MEDIATORS FOR SOLAR CELLS.

ABSTRACT This paper reports the synthesis, characterization and spectroscopic and electrochemical study of a series of polypyridine Co(II) and Co(III) complexes. The effect on the redox potential of the presence of donor or acceptor substituents on the ligand, as well as the effect of an increased ligand aromaticity, was analyzed. Specifically, complexes of [Co(L-L) 3 ] n+ type were prepared, with n = 2,3, and L-L = 1,10-phenanthroline (phen), 5-amino-1,10’-phenanthroline (phen-NH 2 ), 3,4,7,8-tatramethyl phenantroline (phen-4Me), pyrazino[2,3- f ][1,10]phenanthroline (ppl), 2-methylpyrazino[2,3- f ][1,10]phenanthroline (ppl-Me), dipyrido[3,2- a :2’,3’- c ]phenazine (dppz), 7-methyldipyrido[3,2- a :2’,3’- c ]phenazine (dppz-Me) and 7-aminodipyrido[3,2- a :2’,3’- c ]phenazine (dppz-NH 2 ). The results of this study open the possibility to have a battery of potential mediators for photoelectrochemical solar cells. Ultimately, the election of a determined couple as mediator will depend mainly on the electronic properties of the specific dye in a given cell.