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

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(mu-X)X](2), 10, and trans-[(C<^>O)(2)PdX2], 11, and diethylcarbamoylmethyl-functionalised, trans-[(C<^>amide)(2)PdX2], 12, N-heterocyclic carbene (NHC) complexes of palladium(II), (C<^>N) = (3-R-1)-[1-(alpha-picolyl) imidazol-2-ylidene, (C-N) = (3-R-1)[1-(2-pyridyl)imidazol-2-ylidene, R-1 = Bu-t, mesityl, 2,6-(Pr2C6H3)-C-i, (C<^>O) = (3-R-1)(1-methoxymethyl)imidazol-2-ylidene, R-1 = 2,6-(Pr2C6H3)-C-i, (C<^>amide) = (3-R-1)(1-diethylcarbamoylmethyl)imidazol-2-ylidene, R-1 = 2,6-(Pr2C6H3)-C-i, 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.