Functionalised and chelate heterocyclic carbene complexes of palladium; synthesis and structural studies

Abstract

A series of picolyl-functionalised, [(C^N)PdXY], 1a, 2a, 3a and 4, pyridyl-functionalised, [(C–N)PdXY], 5a, 6, 7 and 8, methoxymethyl-functionalised, [(C^O)Pd(μ-X)X]₂, 10, and trans-[(C^O)₂PdX₂], 11, and diethylcarbamoylmethyl-functionalised, trans-[(C^amide)₂PdX₂], 12, N-heterocyclic carbene (NHC) complexes of palladium(II), (C^N) = (3-R¹)[1-(α-picolyl)imidazol-2-ylidene, (C–N) = (3-R¹)[1-(2-pyridyl)imidazol-2-ylidene, R¹ = Bu^t, mesityl, 2,6-Prⁱ₂C₆H₃, (C^O) = (3-R¹)(1-methoxymethyl)imidazol-2-ylidene, R¹ = 2,6-Prⁱ₂C₆H₃, (C^amide) = (3-R¹)(1-diethylcarbamoylmethyl)imidazol-2-ylidene, R¹ = 2,6-Prⁱ₂C₆H₃, Y = Me, X = Br or Cl, have been synthesised by interaction of (1,5-cod)PdXY or (1,5-cod)PdX₂ with the corresponding silver NHC complexes, or (in certain cases) with the free functionalised NHC ligands. Derivatives of 1a, 2a, 3a and 5a were prepared by substitution of the halide X by weakly coordinating anions. The majority of the complexes described here were characterised by spectroscopic and diffraction methods and show that the (C–N) and (C^N) ligands act as chelates, while the methoxymethyl- and diethylcarbamoylmethyl-functions in 10, 11 and 12, respectively, are dangling. Derivatives of 5a in which the pyridine ring is substituted with electron withdrawing or bulky substituents show only minor variation of the pyridine–palladium distances.