Synthesis and reaction chemistry of mixed ligand methylpalladium-carbene complexes

A variety of methylpalladium carbene complexes of the type [Pd(Me)(carbene)(chelate)] [where carbene=1,3-di-methylimidazol-2-ylidene (dmiy), chelate=acetylacetonate (acac) 1, trifluoroacetylacetonate (tfac) 2, hexafluoroacetylacetonate (hfac) 3; carbene=1,3,4,5-tetramethylimidazol-2-ylidene (tmiy), chelate=acac, 6] have been synthesised and fully characterised. The bis-carbene complexes trans-[Pd(Me)(carbene)2Cl] [carbene=dmiy, 7 and tmiy, 8] have also been isolated. The unusual cationic complex {[Pd(Me)(dmiy)(cod)]+BF4 -} 4, in which there are three distinct Pd-carbon bonds, including cis alkyl/alkene coordination has been prepared and aspects of its chemistry investigated. The complex which slowly decomposes even at -20°C does not react via the expected migratory insertion of the methyl group with the coordinated cod but decomposes via a pathway involving the methyl group and the carbene ligand. Crystal structures of the bis-carbene complex 7 and the chloro-bridged dimer [Pd(Me)(tmiy)Cl]2 5 reveal square planar coordination. The carbene C3N2 arrays are closely planar, with the ligand planes inclined at significant angles to the coordination planes of the respective complexes. For complex 7 the interplanar dihedral angle is 70.4(3)° and for 5 the angle is 65.07(8)°. Examples of the methylpalladium monocarbene and dicarbene complexes were tested in the Heck reaction and were found to be extremely active with several having turnover frequencies (TOF's) of >20 000 and total turnover numbers (TON's) of >100 000.