Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines) with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands and silver and palladium complexes

Dalton Trans. 2007 Jul 7:(25):2696-706. doi: 10.1039/b703656f. Epub 2007 Apr 30.

Abstract

The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the nature of the alkylidene arm. For a methylene arm, simple dications 5a,b.2CI were obtained directly. The higher homologue counterparts were conveniently prepared by general multistep routes following a five-step sequence for ethylene dications 6a,b.2Br or a six-step sequence for propylene dications 7a,b.2Br in > or = 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare palladium complexes (17-20) with N-heterocyclic carbenes via transmetallation from well-defined silver compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the palladium species two [Pd(allyl)(bis-oxazoline)]+ (25-26) complexes with the same ligand bridge were synthesized. [PdX2bisL] complexes appeared in solution as mixtures of species, mononuclear with cis- or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)]2 and micro-bis(carbene)(AgX)2 complexes in 1 : 1 or in 0.5 : 1 ratio leads to binuclear compounds [Pd2Cl2(allyl)2(micro-bis-carbene)] (19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]+ (20a,20b) compounds, respectively. The preparation of analogous [Pd(allyl)(bis-oxazoline)]+ complexes showed the formation of one of the four possible isomers. [Pd(allyl)(bis-oxazoline)]PF6 complexes were inactive as catalytic precursors in the allylic substitution reaction.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alkenes / chemical synthesis
  • Alkenes / chemistry*
  • Benzene Derivatives / chemical synthesis
  • Benzene Derivatives / chemistry*
  • Heterocyclic Compounds / chemical synthesis
  • Heterocyclic Compounds / chemistry*
  • Hydrocarbons / chemistry
  • Isomerism
  • Ligands
  • Methane / analogs & derivatives*
  • Methane / chemistry
  • Organometallic Compounds / chemical synthesis
  • Organometallic Compounds / chemistry*
  • Oxazoles / chemical synthesis
  • Oxazoles / chemistry*
  • Palladium / chemistry*
  • Silver / chemistry*

Substances

  • Alkenes
  • Benzene Derivatives
  • Heterocyclic Compounds
  • Hydrocarbons
  • Ligands
  • Organometallic Compounds
  • Oxazoles
  • carbene
  • Silver
  • Palladium
  • Methane