16 research outputs found
Directed Design of a AuI Complex with a Reduced Mesoionic Carbene Radical Ligand: Insights from 1,2,3âTriazolylidene Selenium Adducts and Extensive Electrochemical Investigations
Carbene-based radicals are important for both fundamental and applied chemical research. Herein, extensive electrochemical investigations of nine different 1,2,3-triazolylidene selenium adducts are reported. It is found that the half-wave potentials of the first reduction of the selones correlate with their calculated LUMO levels and the LUMO levels of the corresponding triazolylidene-based mesoionic carbenes (MICs). Furthermore, unexpected quasi-reversibility of the reduction of two triazoline selones, exhibiting comparable reduction potentials, was discovered. Through UV/Vis/NIR and EPR spectroelectrochemical investigations supported by DFT calculations, the radical anion was unambiguously assigned to be triazoline centered. This electrochemical behavior was transferred to a triazolylidene-type MIC-gold phenyl complex resulting in a MIC-radical coordinated Au-I species. Apart from UV-Vis-NIR and EPR spectroelectrochemical investigations of the reduction, the reduced gold-coordinated MIC radical complex was also formed in situ in the bulk through chemical reduction. This is the first report of a monodentate triazolylidene-based MIC ligand that can be reduced to its anion radical in a metal complex. The results presented here provide design principles for stabilizing radicals based on MICs
Spin Crossover and Fluorine-Specific Interactions in Metal Complexes of Terpyridines with Polyfluorocarbon Tails
In coordination chemistry and materials science, terpyridine ligands are of great interest, due to their ability to form stable complexes with a broad range of transition metal ions. We report three terpyridine ligands containing different perfluorocarbon (PFC) tails on the backbone and the corresponding FeII and CoII complexes. The CoII complexes display spin crossover close to ambient temperature, and the nature of this spin transition is influenced by the length of the PFC tail on the ligand backbone. The electrochemical properties of the metal complexes were investigated with cyclic voltammetry revealing one oxidation and several reduction processes. The fluorine-specific interactions were investigated by EPR measurements. Analysis of the EPR spectra of the complexes as microcrystalline powders and in solution reveals exchange-narrowed spectra without resolved hyperfine splittings arising from the 59Co nucleus; this suggests complex aggregation in solution mediated by interactions of the PFC tails. Interestingly, addition of perfluoro-octanol in different ratios to the acetonitrile solution of the sample resulted in the disruption of the F... F interactions of the tails. To the best of our knowledge, this is the first investigation of fluorine-specific interactions in metal complexes through EPR spectroscopy, as exemplified by exchange narrowing
A mesoionic carbene complex of manganese in five oxidation states
Reaction between a carbazole-based mesoionic carbene ligand and
manganese(II) iodide results in the formation of a rare air-stable
manganese(IV) complex after aerobic workup. Cyclic voltammetry
reveals the complex to be stable in five oxidation states. The
electronic structure of all five oxidation states is elucidated chemically, spectroscopically (NMR, high-frequency EPR, UV-Vis, MCD),
magnetically, and computationally (DFT, CASSCF)
Mesoscopic models for DNA stretching under force: new results and comparison to experiments
Single molecule experiments on B-DNA stretching have revealed one or two
structural transitions, when increasing the external force. They are
characterized by a sudden increase of DNA contour length and a decrease of the
bending rigidity. It has been proposed that the first transition, at forces of
60--80 pN, is a transition from B to S-DNA, viewed as a stretched duplex DNA,
while the second one, at stronger forces, is a strand peeling resulting in
single stranded DNAs (ssDNA), similar to thermal denaturation. But due to
experimental conditions these two transitions can overlap, for instance for
poly(dA-dT). We derive analytical formula using a coupled discrete worm like
chain-Ising model. Our model takes into account bending rigidity, discreteness
of the chain, linear and non-linear (for ssDNA) bond stretching. In the limit
of zero force, this model simplifies into a coupled model already developed by
us for studying thermal DNA melting, establishing a connexion with previous
fitting parameter values for denaturation profiles. We find that: (i) ssDNA is
fitted, using an analytical formula, over a nanoNewton range with only three
free parameters, the contour length, the bending modulus and the monomer size;
(ii) a surprisingly good fit on this force range is possible only by choosing a
monomer size of 0.2 nm, almost 4 times smaller than the ssDNA nucleobase
length; (iii) mesoscopic models are not able to fit B to ssDNA (or S to ss)
transitions; (iv) an analytical formula for fitting B to S transitions is
derived in the strong force approximation and for long DNAs, which is in
excellent agreement with exact transfer matrix calculations; (v) this formula
fits perfectly well poly(dG-dC) and -DNA force-extension curves with
consistent parameter values; (vi) a coherent picture, where S to ssDNA
transitions are much more sensitive to base-pair sequence than the B to S one,
emerges.Comment: 14 pages, 9 figure
Remarkable Enhancement of Catalytic Activity of Cu-Complexes in the Electrochemical Hydrogen Evolution Reaction (HER) by Using Triply-Fused Porphyrin
Developing efficient molecular catalysts for the electrocatalytic hydrogen evolution reaction (HER) is a highly important goal in contemporary science. We report here on a bimetallic triply fused copper porphyrin complex (1) comprising two monomeric porphyrin units linked through ÎČâÎČ, mesoâmeso, ÎČâČâÎČâČ triple covalent linkages, that exhibits remarkable enhancement of catalytic activity for the electrochemical HER in comparison to the analogous monomeric copper porphyrin complex (2). Spectroscopic characterization, in association with magnetic measurements, clearly establish the ground state structures of both the bimetallic and monometallic complexes as containing two and one copper (II) centers, respectively. The fused metalloporphyrin complex is found to undergo electrochemical reduction at a lower negative applied potential compared to the metalloporphyrin monomer, as evident from the significant anodic shift (~800mV) in the potential of the first reduction process. Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirm that the catalytic activity of the fused metalloporphyrin occurs at a significantly lower onset potential, (overpotential decreased by ~320 mV), compared to the non-fused monomer. Controlled potential electrolysis combined with the kinetic analysis of catalysts 1 and 2 confirm the production of hydrogen, with 96% and 71% faradaic efficiencies and turnover numbers of 102 and 18, respectively. Kinetic investigations further reveal an observed rate constant of around 107 (s-1), implying high efficiency of the bimetallic catalyst towards hydrogen evolution reaction. Mechanistic insights are presented by using a combination of UV-vis-NIR and EPR spectroscopy and electrochemistry. Our results thus firmly establish the triply fused porphyrin ligands as candidates for generating highly efficient molecular electrocatalysts in combination with transition metal centers