Ivo Krummenacher

University of Wuerzburg, Würzburg, Bavaria, Germany

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Publications (56)313.77 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: An improved synthetic route to homoleptic complex [Pt(CAAC(Me) )2 ] (CAAC=cyclic (alkyl)(amino)carbenes) and convenient routes to new heteroleptic complexes of the form [Pt(CAAC(Me) )(PR3 )] are presented. Although the homoleptic complex was found to be inert to many reagents, oxidative addition and metal-only Lewis pair (MOLP) formation was observed from one of the heteroleptic complexes. The spectroscopic, structural, and electrochemical properties of the zero-valent complexes were explored in concert with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. The homoleptic [Pt(CAAC)2 ] and heteroleptic [Pt(CAAC)(PR3 )] complexes were found to be similar in their spectroscopic and structural properties, but their electrochemical behavior and reactivity differ greatly. The unusually strong color of the CAAC-containing Pt(0) complexes was investigated by TD-DFT calculations and attributed to excitations into the LUMOs of the complexes, which are predominantly composed of bonding π interactions between Pt and the CAAC carbon atoms. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Chemistry - A European Journal 08/2015; DOI:10.1002/chem.201502048 · 5.70 Impact Factor
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    ABSTRACT: By a ring expansion reaction of boroles with nitrones a straightforward route to eight-membered O,N,B-containing ring systems was developed, which present a new structural motif in heterocyclic chemistry. Formally, these compounds can be considered as BN-analogues of eight-membered ring ethers containing three double bonds. The X-ray crystallographic data of a series of these oxazaborocine derivatives show that they adopt a boat conformation in the solid state.
    Chemical Communications 08/2015; DOI:10.1039/C5CC05368D · 6.83 Impact Factor
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    ABSTRACT: Taking advantage of an improved synthesis of [Ti(η(6) -C6 H6 )2 ], we report here the first examples of ansa-bridged bis(benzene) titanium complexes. Deprotonation of [Ti(η(6) -C6 H6 )2 ] with nBuLi in the presence of N,N,N',N'',N''-pentamethyldiethylenetriamine (pmdta) leads to the corresponding 1,1'-dilithio salt [Ti(η(6) -C6 H5 Li)2 ]⋅pmdta that enables the preparation of the first one- and two-atom-bridged complexes by simple salt metathesis. The ansa complexes were fully characterized (NMR spectroscopy, UV/Vis spectroscopy, elemental analysis, and X-ray crystallography) and further studied electrochemically and computationally. Moreover, [Ti(η(6) -C6 H6 )2 ] is found to react with the Lewis base 1,3-dimethylimidazole-2-ylidene (IMe) to give the bent sandwich complex [Ti(η(6) -C6 H6 )2 (IMe)]. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Chemistry - A European Journal 06/2015; 21(31). DOI:10.1002/chem.201500737 · 5.70 Impact Factor
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    ABSTRACT: The preparation of [3]thoro- and [3]uranocenophanes, the first structurally authenticated ansa-bridged complexes of actinocenes, is reported. Following a flytrap route, 1,2-bis(cyclooctatetraenyldimethylsilyl)methane was synthesized, reduced to its tetraanion, and subsequently converted into bridged uranocene and thorocene complexes by salt metathesis with the corresponding actinide tetrachlorides. In addition, their electronic structures have been investigated by experimental (UV/Vis spectroscopy, cyclic voltammetry) and theoretical (DFT) methods. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Chemistry - A European Journal 05/2015; 21(26). DOI:10.1002/chem.201501545 · 5.70 Impact Factor
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    ABSTRACT: Reduction of 2-(BMes2)pyrene (B1) and 2,7-bis(BMes2)pyrene (B2) gives rise to anions with extensive delocalization over the pyrenylene bridge and between the boron centers at the 2- and 2,7-positions, the typically unconjugated sites in the pyrene framework. One-electron reduction of B2 gives a radical anion, with a centrosymmetric semi-quinoidal structure, while two-electron reduction produces a quinoidal singlet dianion with biradicaloid character and a relatively large S0-T1 gap. These results have been confirmed by cyclic voltammetry, X-ray crystallography, DFT/CASSCF calculations, and NMR, EPR, and UV-vis-NIR-absorption spectroscopies.
    Journal of the American Chemical Society 05/2015; 137(21). DOI:10.1021/jacs.5b03805 · 11.44 Impact Factor
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    ABSTRACT: A star-like compound consisting of a polychlorinated triphenylmethyl radical (PCTM) core linked to three triarylamines (TAA) and a symmetric and an asymmetric hexaarylbenzene (HAB) both substituted with three PCTMs and three TAAs were synthesised. In the star-like compound a strong communication between the redox centres was observed by electron paramagnetic resonance spectroscopy and UV/Vis/NIR absorption measurements, whereas in the HABs only a weak interaction could be detected. The temporal evolution of the excited states was monitored by ultrafast transient absorption measurements. Within the first picosecond stabilisation of the charge transfer state was observed induced by solvent rotation. Anisotropic transient absorption measurements revealed that within the lifetime of the excited state (τ = 1-4 ps) energy transfer does not occur in the HABs whereas in the star-like system ultrafast and possibly coherent energy redistribution is observed. Taken this information together we made the identity between energy transfer and electron transfer in the specific systems apparent.
    Physical Chemistry Chemical Physics 04/2015; 17(17). DOI:10.1039/c4cp05929h · 4.20 Impact Factor
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    ABSTRACT: Wir stellen hier die Ergebnisse unserer Untersuchungen bezüglich des Effekts der ortho-Substitution von Arylaziden auf die Ringerweiterung von Borolen, fünfgliedrigen, ungesättigten Borheterocyclen, vor. Die Studien führten zu der Isolierung der ersten 1,2-Azaborinin-substituierten Azofarbstoffe, bei denen es sich um leuchtend gelbe Feststoffe handelt. In Lösung stellte sich eines der Derivate, (E)-2-Mesityl-1-(mesityldiazenyl)-3,4,5,6-tetraphenyl-1,2-azaborinin, als nicht stabil heraus, sondern geht eine Jacobsen-Reaktion zu einem Indazol und einem 1-Hydro-1,2-azaborinin ein. Um den Mechanismus der unerwarteten Bildung der Azo-Azaborinine nachvollziehen zu können und um Rückschlüsse auf die Rolle der ortho-Substitution auf den Reaktionsverlauf zu ziehen, wurden Dichtefunktionalrechnungen durchgeführt.
    Angewandte Chemie 03/2015; 127(21). DOI:10.1002/ange.201500970
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    ABSTRACT: Herein, we present the results of our investigations on the effect of ortho substitution of aryl azides on the ring-expansion reaction of boroles, five-membered unsaturated boron heterocycles. These studies led to the isolation of the first 1,2-azaborinine-substituted azo dyes, which are bright yellow solids. One of the derivatives, (E)-2-mesityl-1-(mesityldiazenyl)-3,4,5,6-tetraphenyl-1,2-azaborinine, was found to be unstable in solution and to transform through a Jacobsen-like reaction into an indazole and 1-hydro-1,2-azaborinine. DFT calculations were performed to shed light on possible mechanisms to rationalize the unexpected azo-azaborinine formation and to draw conclusions about the role played by the ortho substituents in the reaction. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Angewandte Chemie International Edition 03/2015; 54(21). DOI:10.1002/anie.201500970 · 11.26 Impact Factor
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    ABSTRACT: Die Reaktion des Diboracumulens (CAAC)2B2 (CAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-yliden) mit einem Überschuss an tert-Butylisonitril führte zur Koordination des Isonitrils an das Bor. Die Verbindung kann formal mit einem freien Elektronenpaar am Bor dargestellt werden, doch sind diese Elektronen weit über das konjugierte π-Netzwerk der π-aziden CAAC- und Isonitrilliganden delokalisiert. Das Erhitzen auf 110 °C führte unter Bildung des ersten Dicyandiborens zum Verlust der organischen Substituenten beider Isonitrilliganden. Während alle bislang bekannten Diborene starke Reduktionsmittel sind, zeigt dieses Diboren im Cyclovoltammogramm Reduktionswellen.
    Angewandte Chemie 02/2015; 127(15). DOI:10.1002/ange.201412006
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    ABSTRACT: The reaction of the bisboracumulene (CAAC)2 B2 (CAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) with excess tert-butylisocyanide resulted in complexation of the isocyanide at boron. Though this compound might be formally drawn with a lone pair on boron, these electrons are highly delocalized throughout a conjugated π-network consisting of the π-acidic CAAC and isocyanide ligands. Heating this compound to 110 °C liberated the organic periphery of both isocyanide ligands, yielding the first example of a dicyanodiborene. Cyclic voltammetry conducted on this diborene indicated the presence of reduction waves, making this compound unique among diborenes, which are otherwise highly reducing. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Angewandte Chemie International Edition 02/2015; 54(15). DOI:10.1002/anie.201412006 · 11.26 Impact Factor
  • Marius Schäfer · Ivo Krummenacher · Holger Braunschweig · Maik Finze
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    ABSTRACT: The 12-vertex closo-boron clusters [1-FcC≡C-closo-B12H11]2– (1), [12-FcC≡C-closo-1-CB11H11]– (2), [1-H2N-12-FcC≡C-closo-1-CB11H10]– (3), and [9-FcC≡C-closo-1, 2-C2B10H11] (5) were synthesized from the corresponding iodinated derivatives and Me3SiC≡CMgBr by a Pd-catalyzed Kumada-type cross-coupling reaction. Anion 3 was converted into the inner salt [1-Me3N-12-FcC≡C-closo-1-CB11H10] (4). The nido-derivative [5-FcC≡C-nido-7, 8-C2B9H11]– (6) was obtained from 5 via a deboronation reaction. The tetraethylammonium salts of the anions 2, 3, and 6 as well as compound 5 were characterized by single-crystal X-ray diffraction. The redox properties of the ferrocenylalkynyl-substituted derivatives 1–6 were studied by cyclic voltammetry. An increasing negative charge of the cluster fragment results in a shift of the redox potential of the ferrocene moiety to more negative values. The experimental studies are supported by density functional calculations (DFT).
    Zeitschrift für anorganische Chemie 01/2015; 641(3-4). DOI:10.1002/zaac.201400524 · 1.25 Impact Factor
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    ABSTRACT: The synthesis of the first tin-bridged bis(benzene) vanadium and trovacene sandwich compounds and the investigation of their oxidative addition and insertion behavior are reported. The vanadoarenophanes and the corre-sponding platinum insertion products were fully characterized including electrochemical and EPR measurements. Controllable ring-opening polymerization of the heteroleptic tin-bridged [1]trovacenophane using Karstedt's catalyst yields a high molecular weight polymer (up to Mn = 89,200 g mol-1) composed of d5-vanadium metal centers in the main chain, making it a rare example of a spin-carrying macromolecule. Magnetic susceptibility measurements (SQUID) confirm the paramagnetic scaffold with repeating S = ½ centers in the main chain and suggest antiferromagnetic interactions between adjacent spin sites (Weiss constant Θ = -2.9 K).
    Journal of the American Chemical Society 01/2015; 137(4). DOI:10.1021/ja510884h · 11.44 Impact Factor
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    ABSTRACT: Trotz der grundlegenden Bedeutung von Radikalanionen-Radikalkationen-Paaren in Ein-Elektronen-Transfer-Reaktionen (SET) sind derartige Spezies noch immer sehr selten und meistens nur von kurzlebiger Natur. Die elektronenreiche B-B-Doppelbindung, welche Diborene zu ungewöhnlich starken neutralen Reduktionsmitteln macht, inspirierte uns dazu, ein Bor-zentriertes Radikalanionen-Radikalkationen-Paar durch SET von einem Diboren auf ein Borol zu realisieren. Da sich die Reduktionspotentiale aller bisher bekannten Diborene (E1/2=−1.05/−1.55 V) für die Reduktion von MesBC4Ph4 (E1/2=−1.69 V) jedoch als nicht ausreichend erwiesen, wurde zuerst ein geeignetes Diboren entwickelt. Mit einem Reduktionspotential von E1/2=−1.95 V zählt IiPr⋅ (iPr)BB(iPr)⋅IiPr zu den stärksten bislang bekannten neutralen Reduktionsmitteln überhaupt. Dessen SET-Reaktion mit MesBC4Ph4 verläuft hochselektiv unter Bildung eines Radikalanionen-Radikalkationen-Paares.
    Angewandte Chemie 01/2015; 127(1). DOI:10.1002/ange.201409513
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    ABSTRACT: Despite the fundamental importance of radical-anion radical-cation pairs in single-electron transfer (SET) reactions, such species are still very rare and transient in nature. Since diborenes have highly electron-rich BB double bonds, which makes them strong neutral reductants, we envisaged a possible realization of a boron-centered radical-anion radical-cation pair by SET from a diborene to a borole species, which are known to form stable radical anions upon one-electron reduction. However, since the reduction potentials of all know diborenes (E1/2 =-1.05/-1.55 V) were not sufficiently negative to reduce MesBC4 Ph4 (E1/2 =-1.69 V), a suitable diborene, IiPr⋅(iPr)BB(iPr)⋅IiPr, was tailor-made to comply with these requirements. With a halfwave potential of E1/2 =-1.95 V, this diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC4 Ph4 by SET to afford a stable boron-centered radical-anion radical-cation pair.
    Angewandte Chemie International Edition in English 01/2015; 54(1). DOI:10.1002/anie.201409513 · 13.45 Impact Factor
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    ABSTRACT: In this contribution, we have evaluated the (anti)aromatic character of thirty-four different borole compounds in their neutral and reduced states based on two aromaticity indices, namely nucleus-independent chemical shift (NICS) and multicenter indices (MCI), calculated at the PBE0/6-31+G(d,p) level of theory. Both indices corroborate the notion that neutral borole compounds are antiaromatic and become increasingly aromatic upon addition of electrons. Effects of the ring substituents on the degree of (anti)aromaticity are discussed together with differences in the two theoretical methods, which are on the one hand based on magnetic (NICS) and on the other hand based on electronic criteria (MCI).
    Dalton Transactions 12/2014; 44(15). DOI:10.1039/c4dt03445g · 4.20 Impact Factor
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    ABSTRACT: Die Komplexierung von zwei Äquivalenten eines cyclischen (Alkyl)(amino)carbens (CAAC) an Tetrabromdiboran führte unter nachfolgender Reduktion mit vier Äquivalenten Natriumnaphthalid zur Bildung des CAAC-stabilisierten Diboracumulens (CAAC)2B2. Gegenüber dem bereits bekannten Komplex, der eine von cyclischen Di(amino)carbenen stabilisierte BB-Bindung aufweist, führt das Vermögen des CAAC-Liganden, π-Elektronendichte aufzunehmen (B2→CAAC), zu signifikanten Unterschieden, wie etwa einer längeren BB- und kürzeren BC-Bindungen. Die Analyse der Grenzorbitale lässt auf eine Verteilung der Valenzelektronen entlang der gesamten linearen C-B-B-C Einheit in (CAAC)2B2 schließen, was durch natürliche Populationsanalyse und Cyclovoltammetrie weiter bestätigt wird.
    Angewandte Chemie 08/2014; 126(34). DOI:10.1002/ange.201403888
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    ABSTRACT: The complexation of two equivalents of a cyclic (alkyl)(amino)carbene (CAAC) to tetrabromodiborane, followed by reduction with four equivalents of sodium naphthalide, led to the formation of the CAAC-stabilized linear diboracumulene (CAAC)2 B2 . The capacity of the CAAC ligand to facilitate B2 →CAAC donation of π-electron density resulted in important differences between this species and a previously reported complex featuring a BB triple bond stabilized by cyclic di(amino)carbenes, including a longer BB bond and shorter BC bonds. Frontier orbital analysis indicated sharing of valence electrons across the entire linear C-B-B-C unit in (CAAC)2 B2 , which is supported by natural population analysis and cyclic voltammetry.
    Angewandte Chemie International Edition in English 08/2014; 53(34). DOI:10.1002/anie.201403888 · 13.45 Impact Factor
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    ABSTRACT: Utilizing a cyclic (alkyl)(amino)carbene (CAAC) as a ligand, neutral CAAC-stabilized radicals containing a boryl functionality could be prepared by reduction of the corresponding haloborane adducts. The radical species with a duryl substituent was fully characterized by single-crystal X-ray structural analysis, EPR spectroscopy, and DFT calculations. Compared to known neutral boryl radicals, the isolated radical species showed larger spin density on the boron atom. Furthermore, the compound that was isolated is extraordinarily stable to high temperatures under inert conditions, both in solution and in the solid state. Electrochemical investigations of the radical suggest the possibility to generate a stable formal boryl anion species.
    Angewandte Chemie International Edition in English 07/2014; 53(28). DOI:10.1002/anie.201403514 · 13.45 Impact Factor
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    ABSTRACT: Unter Verwendung eines cyclischen (Alkyl)(amino)carben(CAAC)-Liganden konnten neutrale, CAAC-stabilisierte Radikale mit einer Borylfunktion durch Reduktion der entsprechenden Halogenboranaddukte erhalten werden. Die Duryl-substituierte Radikalspezies wurde vollständig mithilfe von Röntgenstrukturanalyse, EPR-Spektroskopie und DFT-Rechnungen charakterisiert. Die hier isolierten Spezies weisen eine größere Spindichte auf dem Borzentrum als bekannte Borylradikale auf. Weiterhin erwies sich die Titelverbindung unter inerten Bedingungen in Lösung wie auch im Feststoff als überaus temperaturstabil. Elektrochemische Untersuchungen lassen auf die Erzeugung eines stabilen formalen Borylanions schließen.
    Angewandte Chemie 07/2014; 126(28). DOI:10.1002/ange.201403514
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    ABSTRACT: Die Herstellung eines Phosphan-stabilisierten Diborens, Et3P⋅(Mes)BB(Mes)⋅PEt3 (4), durch Reduktion von Et3P⋅B2Me s2Br2 mit KC8 in Benzol ermöglichte erstmals eine detaillierte Untersuchung der elektronischen Eigenschaften der BB-Bindung im Vergleich mit dem kürzlich beschriebenen NHC-stabilisierten Diboren IMe⋅(Dur)BB(Dur)⋅IMe (1). Ein entscheidendes Merkmal dieser Spezies ist die Gegenwart ungewöhnlich elektronenreicher Borzentren. Cyclovoltammetrie, UV/Vis-Spektroskopie und DFT-Rechnungen offenbarten eine starke Abhängigkeit der Reduktionspotentiale und Absorptionseigenschaften der BB-Bindung von der Lewis-Base. Demnach bedingt die stärkere σ-Donorfähigkeit und höhere Elektronegativität des NHC-Liganden ein energetisch höherliegendes HOMO, was 1 zu einem stärkeren neutralen Reduktionsmittel macht als 4 (1: E1/2=−1.55 V; 4: −1.05 V), sowie einen kleineren HOMO-LUMO-Abstand in 1, verbunden mit einer signifikanten Rotverschiebung der niederenergetischsten Absorptionsbande im Vergleich zu 4. Wegen ihrer stark negativen Reduktionspotentiale können die Diborene 1 und 4 relativ einfach zu sehr seltenen Bor-zentrierten Radikalkationen oxidiert werden (5 und 6).
    Angewandte Chemie 05/2014; 126(22). DOI:10.1002/ange.201311110

Publication Stats

353 Citations
313.77 Total Impact Points

Institutions

  • 2012–2015
    • University of Wuerzburg
      • Institute of Inorganic Chemistry
      Würzburg, Bavaria, Germany
  • 2013
    • University of British Columbia - Vancouver
      • Department of Chemistry
      Vancouver, British Columbia, Canada
  • 2011–2012
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2006–2009
    • Karlsruhe Institute of Technology
      • Institute of Inorganic Chemistry
      Karlsruhe, Baden-Wuerttemberg, Germany