Now showing 1 - 10 of 15
  • Publication
    The Potential of N-Heterocyclic Carbene Complexes as Components for Electronically Active Materials
    (Swiss Chemical Society, 2010-03) ; ;
    The application of N-heterocyclic carbene complexes as active sites in materials other than catalysis has been remarkably scarce. Inspired by the — often misleading — ‘carbene’ label, which implies a substantial degree of M=C π bonding, we have been interested in evaluating the potential of N-heterocylclic carbene complexes as building blocks for constructing electronically active materials. Electron mobility via the metal-carbon bond has been investigated in monometallic imidazol-2-ylidene complexes and subsequently expanded to polymetallic systems. In particular, ditopic benzobisimidazolium-derived ligands have been explored for the fabrication of bimetallic molecular switches and main-chain conjugated organometallic polymers. Electrochemical analyses have allowed for quantifying the degree of electronic coupling between the metal sites and for identifying the key parameters that govern the intermetallic communication.
    Scopus© Citations 35  688
  • Publication
    Pyridine-derived N-heterocyclic carbenes: An experimental and theoretical evaluation of the bonding in and reactivity of selected normal and abnormal complexes of Ni(II) and Pd(II)
    We report a thorough investigation of a series of isomeric complexes with the general formula trans-(pyridylidene)M(PPh3)2Cl (M = Pd, Ni). For the first time, a systematic comparison of normal, abnormal and remote bonding modes is presented. X-ray structural and 13C NMR data indicate the importance of carbenoid mesomeric contributions in their compound class. The catalytic performance of the palladium complexes, trans-(pyridylidene)Pd(PPh3)2Cl, as precursors in Suzuki-Miyaura-type cross-coupling suggests a correlation of remote bonding with catalyst robustness and thus effectivity. When metal precursors M(PPh3)4 are reacted with 2,4-dichloro pyridinium salts, preferential formation of remote carbene complexes occurs and indications are that electronic parameters rather than steric influences are responsible for the observed selectivity. Calculations at the BP86/TZ2P level of theory support interpretation of the results.
      296Scopus© Citations 69
  • Publication
    Beyond catalysis: N-heterocyclic carbene complexes as components for medicinal, luminescent, and functional materials applications
    (RSC Publishing, 2010) ;
    This tutorial review compiles the advances that have been achieved in using transition metal complexes containing N-heterocyclic carbene ligands as components for materials. Applications of metal carbene complexes in fields different from catalysis are remarkably scarce. During the last few years, promising results have been accomplished in particular by utilizing such complexes as antimicrobial and cytotoxic agents, as photoactive sites in luminescent materials, for self-assembly into liquid crystalline materials and metallasupramolecular structures, and as synthons for molecular switches and conducting polymeric materials. These initial achievements clearly underline the great potential of N-heterocyclic carbene complexes in various fields of materials science.
      2437Scopus© Citations 648
  • Publication
    Palladium carbene complexes for selective alkene di- and oligomerization
    A series of palladium complexes were synthesized that comprise three sterically different C,N-bidentate coordinating NHC-pyridine ligands (NHC = N-heterocyclic carbene). In one set, the pyridine and the carbene are linked by a flexible CH2 group (a), in the other two sets, the two ligand units are directly linked and feature a shielding mesityl substituent on the carbene and either an unsubstituted pyridine (b) or a xylyl-substituted pyridine unit (c). Investigation of the reactivity of cationic complexes [Pd(C^N)Me(NCMe)]+, 6, analogues to Brookhart’s α-diimine system, towards alkenes showed a strong correlation between the catalytic activity and selectivity and the ligand setting. While 6a was inactive in ethylene conversion, 6b afforded low-molecular weight olefins (oligomerization), and 6c produced exclusively butene (dimerization). With styrene as substrate, exclusive dimerization occurred with all three complexes. Steric and electronic factors were identified that govern the disparate activity and selectivity, and that allow for efficient tailoring of the catalytic performance.
      549Scopus© Citations 57
  • Publication
    Rhodium carbene complexes as versatile catalyst precursors for Si–H bond activation
    (Wiley-VCH, 2012-01-09) ;
    Rhodium(III) complexes comprising monoanionic C,C,C-tridentate dicarbene ligands activate Si–H bonds and catalyse the hydrolysis of hydrosilanes to form silanols and siloxanes with concomitant release of H2. In dry MeNO2, selective formation of siloxanes takes place, while changing conditions to wet THF produces silanols exclusively. Silylethers are formed when ROH is used as substrate, thus providing a mild route towards the protection of alcohols with H2 as the only by-product. With alkynes, comparably fast hydrosilylation takes place, while carbonyl groups are unaffected. Further expansion of the Si–H bond activation to dihydrosilanes afforded silicones and polysilylethers. Mechanistic investigations using deuterated silane revealed deuterium incorporation into the abnormal carbene ligand and hence suggests a ligand-assisted mechanism involving heterolytic Si–H bond cleavage.
    Scopus© Citations 49  643
  • Publication
    [Ru(bpy)3]2+ analogues containing a N-heterocyclic carbene ligand
    A synthetic procedure is described that provides access to [Ru(bpy)3]2+ analogues in which one bpy ligand is replaced by a C,N-bidentate coordinating carbene-benzimidazole ligand (bpy = 2,2’-bipyridine). These new complexes were prepared by first installing the chelating carbene ligand onto a Ru(cymene) platform and subsequent ligand substitution using bpy or terpy (terpy = 2:2’,6’:2’’-terpyridine). The carbene ligand significantly affects the optical properties of the complex and lowers the ruthenium(II) oxidation potential substantially. Such modifications may be advantageous for the development of new classes of photosensitizer materials.
    Scopus© Citations 52  959
  • Publication
    Expanding the family of mesoionic complexes: donor properties and catalytic impact of palladated isoxazolylidenes
    (RSC Publishing, 2010) ;
    Abnormal isoxazolylidene complexes, a new subclass of mesoionic complexes containing an isoxazolium-derived carbene type ligand, have been synthesised via oxidative addition and compared to structurally related mesoionic complexes by using 31P NMR spectroscopy as a convenient probe for their donor ability and in catalytic cross-coupling reactions.
    Scopus© Citations 75  519
  • Publication
    Rhodium-mediated activation of an alkane-type C–H bond
    Abnormal C4-bonding of N-heterocyclic carbenes effectively modulates the electron density at rhodium and allows for the selective cleavage of an unactivated C(sp3)–H bond, whereas no such intramolecular C–H bond breaking is observed when the carbene binds normally through the C2 carbon.
    Scopus© Citations 33  444
  • Publication
    Main-chain organometallic polymers comprising redox-active iron(II) centers connected by ditopic N-heterocyclic carbenes
    Main-chain organometallic polymers were synthesized from bimetallic iron(II)complexes containing a ditopic N-heterocyclic carbene (NHC) ligand [(cp)(CO)LFe(NHC~NHC)Fe (cp)(CO)L]X2 (where NHC~NHC represents a bridging dicarbene ligand, L = I– or CO). Addition of a diimine ligand such as pyrazine or 4,4’-bipyridine interconnected these bimetallic complexes and gave the corresponding co-polymers containing iron centers that are alternately linked by a dicarbene and a diimine ligand. Diimine coordination was depending on the wingtip groups at the carbene ligands and was accomplished either by photolytic activation of a carbonyl ligand from the cationic [Fe(cp)(NHC)(CO)2]+ precursor (alkyl wingtips) or by AgBF4-mediated halide abstraction from the neutral complex [FeI(cp)(NHC)(CO)] (mesityl wingtips). Remarkably, the polymeric materials were substantially more stable than the related bimetallic model complexes. Electrochemical analyses indicated metal-metal interactions in the pyrazine-containing polymers, whereas in 4,4’-bipyridine-linked systems the metal centers were electronically decoupled.
    Scopus© Citations 74  812