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 b een synthesised. Treatment of imidazolium salts with Ag2O yields Ag-I(carbe ne)(2) complexes, which act as carbene transfer agents when reacted with pa lladium salts. In this manner, [Pd(Me)(1-(2-ethylpyridyl)-3-methylimidazoli n-2-ylidene)Cl](2)(4a) and Pd(Me)(1-benzyl-3-methylimidazolin-2-ylidene)(2) Cl (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 (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 imidazol ium salt with Ag2O, followed by PdMeCl(cod) andAgBF(4), whilst [PdCl{1,3-bi s(diisopropyl-2-ethylamino)imidazolin-2-ylidene (6c) is synthesised in an i dentical 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)Br-2 (5c) have been synthesised via the reaction of the appropriate imidazolium salt with Pd(OAc)(2). X-ray crystal structu res of the imidazolium salt, 1,3-di(2-picolyl)imidazolium chloride (1f) and the complex [Pd(Me)(tetramethylimidazolin-2-ylidene)(2)Cl] (2) are reporte d. Complex 2 shows square planar coordination with the two carbene ligands trans to each other. The carbene ligands are inclined at 65.30 degrees to t he coordination plane. Several complexes proved to be highly stable and eff icient catalysts for intermolecular Heck and Suzuki coupling reactions, giv ing turnover numbers of up to 980 000 (Heck) and 177 500 (Suzuki). (C) 2001 Elsevier Science B.V. All rights reserved.