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Rainer F Winter

Publications and source records attributed to Rainer F Winter.

8 recordsLinked to original sources
PubMed2005

Redox site confinement in highly unsymmetric dimanganese complexes.

A 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.

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PubMed2004

Electronic interactions in oligoferrocenes with cationic, neutral and anionic four-coordinate boron bridges.

Dinuclear and trinuclear ferrocene complexes {[Fc2BMe2]Li, [Fc-BMe2-fc-BMe2-Fc]Li2, Fc2B(pyind), [Fc2B(bipy)]PF6, [Fc-B(bipy)-fc-B(bipy)-Fc](PF6)2} bearing anionic, uncharged, and cationic four-coordinate boron bridges have been synthesized (Fc: ferrocenyl; fc: 1,1'-ferrocenylene; pyind: 5-fluoro-2-(2'-pyridyl)indolyl; bipy: 2,2'-bipyridyl). The molecular structures of [Fc2BMe2]Li(12-crown-4)2, [Fc-BMe2-fc-BMe2-Fc](Li(12-crown-4)2)2, Fc2B(pyind), and [Fc2B(bipy)]PF6 were determined by X-ray crystallography. The anionic aggregates [Fc2BMe2]- and [Fc-BMe2-fc-BMe2-Fc]2- are very sensitive to air and moisture whereas bromide salts of their cationic counterparts [Fc(2)B(bipy)]+ and [Fc-B(bipy)-fc-B(bipy)-Fc]2+ may be dissolved in water without decomposition. Cyclic voltammograms of the diferrocene species show two well-resolved one-electron transitions separated by 0.21 V ([Fc2BMe2]Li; Eo' = -0.43 V, -0.64 V; vs. FcH/FcH+), 0.18 V (Fc2B(pyind); Eo' = -0.03 V, -0.21 V), and 0.16 V ([Fc2B(bipy)]PF6; Eo' = +0.23 V, +0.07 V), which indicates electronic interactions between the two ferrocenyl substituents. Two redox waves with an intensity ratio of 1:2 are observed in the cyclic voltammograms of the trinuclear derivatives [Fc-BMe2-fc-BMe2-Fc]Li2 and [Fc-B(bipy)-fc-B(bipy)-Fc](PF6)2. In the case of the BMe(2)-bridged species, the electrochemically unique central ferrocenylene unit is oxidized at a much more cathodic potential value (Eo' = -1.21 V) than the two terminal ferrocenyl substituents (Eo' = -0.51 V). The opposite is true in the case of the B(bipy)-bridged trimer where oxidation of the terminal ferrocenyl groups (Eo' = +0.03 V) precedes oxidation of the internal iron atom (Eo' = +0.26 V). The Fe(II)/Fe(III) redox potentials of the mono- and dianionic species differ to a much larger extent from the redox potential of parent ferrocene (Eo' = 0 V) than the Eo' values of the corresponding mono- and dicationic derivatives. Apart from electrostatic interactions, the electrochemical properties of BMe2- and B(bipy)-bridged oligoferrocenes are determined by the pronounced positive inductive effect of triorganoborate substituents together with positive sigma/pi* hyperconjugation on the one hand and ferrocene-to-B(bipy) charge transfer on the other.

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PubMed2004

Pyrrolyl substituted allenylidene complexes of ruthenium.

Pyrrolyl and indolyl substituted allenylidene complexes of ruthenium have been prepared from the trapping of cationic trans-[Cl(dppm)(2)Ru=C=C=C=CH(2)](+) with various pyrroles or N-methylindole. The reaction is rationalized as involving regioselective attack of the organometallic electrophile on the electron-rich heterocycle followed by proton migration to the terminal =CH(2) entity of the intermediate butenynyl substituted sigma-complex. Pyrrolyl substituted allenylidene complexes have spectroscopic and electrochemical properties intermediate between those of amino and aryl substituted congeners and can thus be regarded as vinylogous aminoallenylidene complexes. We present spectroscopic evidence that the pyrrole pi-system is efficiently incorporated into the metallabutatriene chromophore including resonance Raman spectroscopy. According to our results, the respective frontier orbitals are delocalized across the entire ClRuC(3)(pyrrolyl) entity which defies any classification of the individual redox events as metal or ligand centered redox processes. This issue has been specifically addressed by spectroelectrochemistry. The structure of the 1-methylindole-3-yl complex has been determined by X-ray crystallography. Bond parameters along the ruthenium-allenylidene chain are intermediate between those of amino and aryl substituted congeners and support our conclusions drawn from the spectroscopic results. While still electron rich, pyrrolyl substituted allenylidene complexes are easily deprotonated to their conjugate bases, which are substituted butenynyl complexes. This has been exemplified with the tetrahydroindole derived complex 3f.

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PubMed2003

Electronic coupling in a highly preorganized bimetallic complex comprising pyrazolate-bridged CpMn(CO)2 moieties.

By 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 Mn(I)Mn(I) 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 Mn(I)Mn(I) compound (1(-)), the mixed-valent Mn(I)Mn(II) (1), and the oxidized Mn(II)Mn(II) 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 E(th) ( not equal )=13.6 kJ mol(-1) and k(th)=2.6 x 10(10) 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 pi interactions with the bridging pyrazolate.

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PubMed2002

Computational studies on 3-aza-Cope rearrangements: protonation-induced switch of mechanism in the reaction of vinylpropargylamine.

The 3-aza-Cope rearrangements of 3-azoniahexa-1,5-diene (1), 3-azoniahex-1-ene-5-yne (3), and 3-azahex-1-ene-5-yne (5) were investigated up to the coupled-cluster level, CCSD(T), by using a valence triple-zeta basis set. Activation barriers and geometrical parameters of the transition states are provided. Conformational studies were performed for all reactants and products of the reactions. Solvent effects were estimated from self-consistent reaction field calculations. In contrast to the other two species, the Cope rearrangement of 5 was found to proceed by a stepwise mechanism.

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