Cyclopalladation to form planar tridentate [N–C–N]– intramolecular co-ordination systems involving pyridine donor groups, including ligand synthesis and X-ray structural studies

Abstract

Reaction of palladium(II) acetate with 1,3-bis[1-(pyridin-2-yl)ethyl] benzene and 1,3-bis(2-pyridinecarbonyl) benzene results in cyclopalladation and formation of 2,6-bis[1-(pyridin-2-yl)ethyl] phenyl and 2,6-bis( 2-pyridinecarbonyl)phenyl complexes [Pd{(NC₅H₄CHMe)₂C₆H₃}(O₂CMe)]·1.5H₂O (1) and [Pd{(NC₅H₄CO)₂C₆H₃}(O₂CMe)](4), respectively. X-Ray crystallographic studies reveal square-planar co-ordination and unidentate acetate groups, with tridentate [N–C–N]^– intramolecular co-ordination involving two six-membered [graphic omitted] palladocycle rings. Proton n.m.r. spectra of complex (1) and its chloro-derivative [Pd{(NC₅H₄CHMe)₂C₆H₃}Cl](2) indicate the presence of meso and rac diastereoisomers, but the crystal of (1) used for X-ray study contained the meso form only [monoclinic, space group C2/c with a= 26.08(1), b= 11.093(6), c= 15.846(7)Å, β= 116.19(3)°, and Z= 8; R= 0.042 for 2 909 ‘observed’ reflections]. Crystals of (4) contain two molecules in the asymmetric unit, with the molecules having different conformations of the tridentate ligand [monoclinic, space group Pc with a= 11.373(6), b= 10.925(6), c= 15.640(10)Å, β= 116.98(4)°, Z= 4; R= 0.043 (both chiralities) for 2 677 ‘observed’ reflections]. Syntheses of the new ligands are described.