1,721,114 research outputs found
Combining organometallic and Werner-type coordination sites in highly preorganized heterobimetallic systems
No full textA first example of a novel class of preorganized bimetallic complexes is reported, in which both an organometallic CpMn(CO)2 fragment and a classical Werner-type coordination site are arranged in close proximity by means of a bridging pyrazolate. The synthetic route starts from N-protected bis(chloromethyl)pyrazole and involves sequential attachment of the manganese part and the chelating N-donor compartment, bis(pyridylmethyl)amine, at the different sides of the ligand backbone. Pyrazole binding to the manganese(I) is achieved by photoinduced substitution of CO, and the adjacent N4-coordination pocket is suited to accommodate a second metal ion. The heterodinuclear MnZn complex 4 is characterized crystallographically and its redox chemistry investigated by spectroelectrochemical methods.publishe
Linker permethylation as a means to foster valence tautomerism and thwart dimerization in ferrocenyl-triarylmethylium cations
No full textpublishe
Synthesis and electrochemical behavior of the ferrocenyl units assembled on imidoalane and carbaalane clusters (vol 358, pg 2349, 2005)
No full textRedox Isomeric Ferrocenyl Styrylruthenium Radical Cations with Diphenyl-Substituted β-Ketoenolato Ligands
No full textRedox Site Confinement in Highly Unsymmetric Dimanganese Complexes
No full textA set of highly preorganized pyrazolate-bridged dimanganese complexes L(Mn)MnX have been prepared and structurally characterized. They can be described as hybrid organometallic/Werner-type systems that consist of a low-spin CpMn(I)(CO)2 subunit (Mn1) and a proximate tripodal tetradentate {N4} binding pocket accommodating a high-spin Mn(II) ion (Mn2), with Mn...Mn distances of approximately 4.3 A and different coligands bound to Mn2. Density functional theory (DFT) calculations (both the hybrid B3LYP and the pure BP86 functionals and the all-electron basis sets 6-311G and 6-311G*) confirm that the valence alpha and beta Kohn-Sham molecular orbitals (MOs) of these mixed-valent Mn(I)Mn(II) compounds have predominant Mn(3d) character and an almost perfectly localized nature: all five unpaired electrons are essentially localized at the Werner-type Mn2, whereas Mn1 possesses an effective closed-shell structure with the MOs of highest energy centered there. One-electron oxidation occurs in a clean process at approximately E(1/2) = -0.6 V (versus ferrocene/ferrocinium), giving the low-spin/high-spin Mn(II)Mn(II) species. UV/vis and IR spectroelectrochemistry as well as a detailed theoretical analysis reveal that the redox process takes place with strict site control at the organometallic subunit, while it does not significantly influence the spin and charge distribution on the Werner-type site. Positions and shifts of the nu(C[triple bond]O) absorptions are largely reproduced by the DFT calculations. These systems thus represent an exceptional example of the effect the unsymmetry of a dinucleating ligand scaffold has on the spin and charge distribution in homobimetallic complexes and might offer interesting prospects for the study of the cooperative effects of bimetallic arrays.publishe
Oxidized Styrylruthenium–Ferrocene Conjugates: From Valence Localization to Valence Tautomerism
No full textWe report on ferrocenyl-styrylruthenium conjugates Fc-C6H4-CH=CH-Ru(CO)(PiPr(3))(2)(L) in which the electron density at the alkenylruthenium site is modified by the variation of the coligand L [L = Cl, acetylacetonate (acac), hexafluoroacetylacetonate (hfac), or dipivaloylmethane (dpvm); Fc = ferrocenyl]. Crystallographic studies on three derivatives provide snapshots of the conformational degrees of freedom for rotation around the vinyl-phenylene and phenylene-ferrocenyl linkages. All four complexes undergo two consecutive, reversible one-electron oxidations, the potentials of which depend on the ligand L. On the basis of IR spectroelectrochemistry results, the first oxidations of the less electron-rich chlorido and hfac complexes are biased strongly towards the ferrocenyl site. However, the radical cation of the acac complex exists as two equilibrating valence tautomers (VTs) Fc(+)-C6H4-CH=CH-{Ru-acac} reversible arrow Fc-[C6H4-CH=CH-{Ru-acac}](+) [{Ru-acac} = Ru(CO)(PiPr(3))(2)(acac)], in which the positive charge is either localized at the ferrocenyl site or delocalized over the styrylruthenium moiety. Variable-temperature electron paramagnetic resonance (EPR) and Mossbauer spectroscopy reveal that the ferrocenium valence tautomer dominates at low temperature. A marked solvent dependence on the position and relative intensities of the separate Ru(CO) bands in the IR spectra reveals environmental effects on the relative stabilities of the two VTs, whereas the strong negative solvatochromism of the prominent near-IR band of the radical cation of the acac complex argues for a sizable (intervalence) charge-transfer component of the underlying excitation.Deutsche Forschungsgemeinschaft (DFG) [WI1262/13-1
Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl-Substituted Tritylium Dyes
No full textAbstract Four monoferrocenyl tritylium derivatives with donor‐substituted (OMe, NMe 2 ) aryl rings are reported, along with their spectroscopic and electrochemical properties. All the complexes show a one‐electron reduction and a quasi‐reversible ferrocene oxidation at a very positive potential. Small quadrupole splittings, Δ E Q , in Mößbauer spectra agree with highly electron‐deficient ferrocenes. Comparison of the experimental half‐wave potentials for ferrocene oxidation, E 1/2 (Fc/Fc + ), with those estimated from established correlations of E 1/2 (Fc/Fc + ) with Δ E Q indicates that the E 1/2 values of the anisyl‐substituted congeners FcOMe + and FcMeOMe + are affected by Coulombic repulsion between the positive charges at the Fe ion and the neighboring methylium site. Electronic spectra are recorded and interpreted with the aid of quantum chemical calculations. UV/Vis spectroelectrochemical measurements as well as chemical reduction provide insight into the redox‐induced color changes upon ferrocene oxidation or upon reduction to the neutral trityl radicals. The neutral radicals reversibly form EPR‐silent dimers. This process is studied by temperature‐dependent EPR spectroscopy, and thermodynamic data for their dimerization are determined. Experimental and quantum chemical data suggest that the dimers assume classical hexaarylethane structures as opposed to normal or “offset” Jacobson–Nauta‐type structures.Deutsche Forschungsgemeinschaft https://doi.org/10.13039/50110000165
Electronic Coupling in a Highly Preorganized Bimetallic Complex Comprising Pyrazolate-Bridged CpMn(CO)2 Moieties
No full textBy means of a multistep synthetic procedure a dimanganese complex has been prepared, in which a N,N′-bridging pyrazolate ligand spans two CpMn(CO)2 subunits in a highly preorganized chelate arrangement. The Xray crystallographic analyses of the MnIMnI complex K+1− and of its non-chelate precursor complex elucidate details of the molecular structure, in particular an unusual pyrazolate binding mode in the solid state and intertwining of the CO ligands in the crowded bimetallic array 1−. The MnIMnI compound (1−), the mixed-valent MnIMnII (1), and the oxidized MnIIMnII form (1+) have been characterized by various analytical and spectrosopic methods, such as electrochemistry, variable-temperature EPR spectroscopy, IR spectroelectrochemistry, and UV/Vis/NIR spectroelectrochemistry as well as by DFT and TD-DFT calculations. Strong electronic coupling in the mixed-valent complex is observed, but time- (and temperature-) dependent valence detrapping occurs, thus placing 1 in class II according to the Robin and Day assignment, close to the class II/III transition. From variable-temperature EPR spectroscopy a rough estimate of the activation energy and rate for thermal electron transfer can be deduced, with Eth≠=13.6 kJ mol−1 and kth=2.6×1010 s−1 at 298 K. Unexpectedly, no intervalence CT transition for 1 is detected in solution, but one appears in the optical spectrum of solid 1. The conclusions drawn from experiments are fully supported by DFT calculations that were carried out for all three forms of the dimanganese complex. A broken symmetry treatment for mixed-valent 1 reveals almost perfect localization of both spin and charge on one Mn center. According to TD-DFT the first excited states of 1 give rise to the IT processes in the NIR-energy region, as observed in the solid-state spectrum. The HOMOs are located at the Mn ions and are favorably arranged for π interactions with the bridging pyrazolate.publishe
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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