Synthesis and structure of dichloropalladium(II) complexes of heteroleptic N,S- and N,Se-donor ligands based on the 2-organochalcogenomethylpyridine motif, and Mizoroki-Heck catalysis mediated by complexes of N,S-donor ligands

Ligands containing the 2-organochalcogenomethylpyridine motif with substituents in the 4- or 6- position of the pyridyl ring, R-4, R-6-pyCH(2)ER(1) [R-4 = R-6 = H, ER1 = SMe (1), SeMe (2), SPh (6), SePh (7); R-4 = Me, R-6 = H, ER1 = SMe (3), SPh (8), SePh (9); R-4 = H, R-6 = Me, ER1 = SMe (4), SPh (10), SePh (11); R-4 = H, R-6 = Ph, ER1 = SMe (5), SPh (12), SePh (13)] are obtained on the reaction of R-4, R-6-pyMe with LiBun followed by (REER1)-E-1. On reaction with PdCl2(NCMe)(2), the ligands with a 6-phenyl substituent form cyclopalladated species PdCl{6-(o-C6H4) pyCH(2)ER(1)-C,N,E} (5a, 12a, 13a) with the structure of 13a (ER1 = SePh) confirmed by X-ray crystallography; other ligands form complexes of stoichiometry PdCl2(R-4, R(6-)pyCH(2)ER(1)). Complexes with R-6 = H are monomeric with N, E-bidentate configurations, confirmed by structural analysis for 3a (R-4 = Me, ER1 = SMe), 7a (R-4 = H, ER1 = SePh) and 9a (R-4 = Me, ER1 = SePh). Two of the 6-methyl substituted complexes examined by X-ray crystallography are oligomeric with trans-PdCl2(N,E) motifs and bridging ligands, trimeric [PdCl2(mu-6-MepyCH(2)SPh-N,S)](3) (10a) and dimeric [PdCl2 (mu-6-MepyCH(2)SePh-N,Se)](2) (11a). This behaviour is attributed to avoidance of the Me center dot center dot center dot Cl interaction that would occur in the cis-bidentate configuration if the pyridyl plane had the same orientation with respect to the coordination plane as observed for 3a, 7a and 9a [dihedral angles 8.0(2)-16.8(2)degrees]. When examined as precatalysts for the Mizoroki-Heck reaction of n-butyl acrylate with aryl halides in N,N-dimethylacetamide at 120 degrees C, the complexes exhibit the anticipated trends in yield (ArI > ArBr > ArCl, higher yield for electron withdrawing substituents in 4-RC6H4Br and 4-RC6H4Cl). The most active precatalysts are PdCl2(R-4-pyCH(2)SMe-N,S) (R = H (1a), Me (3a)); complexes of the selenium containing ligands exhibit very low activity. For closely related ligands, the changes SMe to SPh, 6-H to 6-Me, and 6-H to 6-Ph lead to lower activity, consistent with involvement of both the pyridyl and chalcogen donors in reactions involving aryl bromides. The precatalyst PdCl2(pyCH(2)SMe-N,S) (1a) exhibits higher activity for the reaction of aryl chlorides in (Bu4NCl)-N-n at 120 degrees C as a solvent under non-aqueous ionic liquid (NAIL) conditions. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.