Furan- and thiophene-functionalised bis-carbene ligands: synthesis, silver(I) complexes, and catalysis

New furan- and thiophene-functionalised nucleophilic heterocyclic carbene (NHC) complexes of Ag(I) were prepared via the reaction of novel furan- and thiophene-functionalised bis-imidazolium salts with Ag2O. Samples of both the N-methyl substituted furan- and thiophene-functionalised Ag(l) complexes suitable for single crystal X-ray studies were obtained following anion metathesis to the tetrafluoroborate salts. The structural characterisations revealed dinuclear [Ag-2(MeCEC)(2)](BF4)(2) (E = O, S) formulations with discrete twenty-membered dimetallacycles present in both instances; however, the overall molecular conformation varies considerably, notably in the orientations of the two bridging furan or thiophene heterocycles to the silver coordination plane. The functionalised bis-imidazolium salts were tested as in situ additives in a Pd(O)-catalysed aryl amination coupling reaction, with the best observed activities around 20% of those seen with 1,3-bis(2,6-di-iso-propylphenyl)imidazolium chloride under identical conditions. The bulkier N-'Bu and N-mesityl substituted salts were found to be more active than the N-methyl substituted analogues. (c) 2005 Elsevier B.V. All rights reserved.