Palladium(II) complexes containing mono-, bi- and tridentate carbene ligands. Synthesis, characterisation and application as catalysts in CC coupling reactions

Palladium complexes of functionalised heterocyclic carbene complexes have been synthesised. Treatment of imidazolium salts with Ag2O yields AgI(carbene)2 complexes, which act as carbene transfer agents when reacted with palladium salts. In this manner, [Pd(Me)(1-(2-ethylpyridyl)-3-methylimidazolin-2-ylidene)Cl]2 (4a) and Pd(Me)(1-benzyl-3-methylimidazolin-2-ylidene)2Cl (4c) have been prepared from PdMeCl(cod) (cod=1,5-cyclooctadiene) and the appropriate silver complex. Similarly, the reaction of a Ag(carbene)2 complex with PdCl2(MeCN)2 gives Pd(1-benzyl-3-methylimidazolin-2-ylidene)2Cl2 (4b). The tridentate carbene complex [Pd(Me)(1,3-di(2-picolyl)imidazolin-2-ylidene)]BF4 (6a) is synthesised via the in situ reaction of the imidazolium salt with Ag2O, followed by PdMeCl(cod) and AgBF4, whilst [PdCl{1,3-bis(diisopropyl-2-ethylamino)imidazolin-2-ylidene}]BF4 (6c) is synthesised in an identical manner from PdCl2(MeCN)2. The chelated complexes [1,1′-dimethyl-3,3′-(1,2-xylylene)diimidazolin-2,2′-diylidene]Pd(II) dibromide (5a), [1,1′-dimethyl-3,3′-(1,3-xylylene)diimidazolin-2,2′-diylidene]Pd(II) dibromide (5b) and Pd(imidazoliophane)Br2 (5c) have been synthesised via the reaction of the appropriate imidazolium salt with Pd(OAc)2. X-ray crystal structures of the imidazolium salt, 1,3-di(2-picolyl)imidazolium chloride (1f) and the complex [Pd(Me)(tetramethylimidazolin-2-ylidene)2Cl] (2) are reported. Complex 2 shows square planar coordination with the two carbene ligands trans to each other. The carbene ligands are inclined at 65.3° to the coordination plane. Several complexes proved to be highly stable and efficient catalysts for intermolecular Heck and Suzuki coupling reactions, giving turnover numbers of up to 980 000 (Heck) and 177 500 (Suzuki).