Now showing 1 - 4 of 4
  • 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.
      552Scopus© Citations 57
  • Publication
    Platinum(II) and platinum(IV) complexes stabilized by abnormal/mesoionic C4-bound dicarbenes
    Platinum(II) complexes comprising abnormal diimidazolylidene ligands were synthesized from cis-PtMe2(DMSO)(2) using microwave-assisted double C-H bond activation. NMR analysis revealed an unusual solvolysis process, induced by coordinating solvents such as DMSO and MeCN, which has not been observed in related normal dicarbene complexes. NMR and IR spectroscopy and crystallographic analysis of the mono-substituted DMSO complex indicate a sulfur-bonding of the DMSO ligand to the platinum(II) center. Analysis of the DMSO exchange kinetics provided for the first time a quantitative measure of the trans effect of abnormal carbene ligands. The kinetic exchange rate in these bidentate abnormal dicarbene complexes is 0.050(+/- 2) s(-1) and thus similar to analogous platinum(II) complexes containing phenylpyridine, yet significantly slower than that induced by pyridylidene pyridine. Reaction of the dicarbene platinum(II) complexes with PhICl2, Br-2 and I-2 afforded the corresponding platinum(IV) complexes. Linkage isomerism of the Pt-IV-bound DMSO was observed when the bromination reaction was performed in DMSO solution. Moreover, solvolysis was less pronounced in the platinum(IV) complexes than in the corresponding platinum(II) analogues.
      547Scopus© Citations 33
  • Publication
    Catalytic Hydrogenation Using Abnormal N-Heterocyclic Carbene Palladium Complexes: Catalytic Scope and Mechanistic Insights
    Palladium complexes containing abnormally bound C4-bound dicarbene ligands have been exploited for catalytic alkene hydrogenation. Comparison to normally C2-bound homologues indicates that the carbene bonding mode critically influences the catalytic activity. Good catalytic performance in the hydrogenation of cis-disubstituted olefins and non-isomerizable terminal olefins under mild conditions (RT, 0.1 MPa H2) only occurs when the carbene is abnormally bound to the palladium center. Detailed mechanistic investigations using dynamic light scattering in connection with time-dependent analysis of conversions, and also performance of substoichiometric catalytic experiments provide evidence that the catalysis is heterogeneous and that the abnormally bound carbene ligand has the role of an activator.
      382Scopus© Citations 22
  • Publication
    Synthesis of a sterically modulated pyridine-NHC palladium complex and its reactivity towards ethylene
    A new cationic pyridine-carbene palladium complex has been prepared that features a C,N-bidentate coordinating ligand with a shielded pyridine and a sterically less protected carbene moiety; evaluation of this complex in ethylene polymerization revealed competitive reductive elimination processes and provides guidelines for further catalyst design.
      345Scopus© Citations 13