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(I) complexes suitable for single crystal X-ray studies were obtained following anion metathesis to the tetrafluoroborate salts. The structural characterisations revealed dinuclear [Ag2(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(0)-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-tBu and N-mesityl substituted salts were found to be more active than the N-methyl substituted analogues.