The counterion influence on the CH-activation of methane by palladium(II) biscarbene complexes - Structures, reactivity and DFT calculations

Novel bridged palladium(II) biscarbene complexes with different counterions are reported: 1,1′-dimethyl-3,3′-methylene-4- diimidazolin-2,2′-diylidene palladium(II) bischloride (1) and bis(trifluoroacetate) (2) have been synthesized in good yields. Both complexes are active in the catalytic conversion of methane to methanol and show comparable activities to previously published NHC-catalysts. The results of the single-crystal X-ray structure determination of 1,1′-dimethyl-3,3′-methylene-4- diimidazolin-2,2′-diylidene palladium(II) bischloride (1) and 1,1′-dimethyl-3,3′-methylene-4-diimidazolin-2,2′- diylidene palladium (II) bis(trifluoro-acetate) 2 confirmed the structural similarity to the known corresponding palladium bromide and iodide complexes. Since free 1,1′-dimethyl-R-3,3′- methylene-4-diimidazolin-2,2′-diylidene are only available in low yields these compounds have been synthesized via the bromide complex, exchanging the counterion by AgCF₃COO or by exchanging the counterion of the imidazolium salt by NH₄PF₆.