CHIRAL METAL-COMPLEXES .41. STEREOISOMERISM IN PALLADIUM(II) COMPLEXES OF A SERIES OF CHIRAL BIS-PICOLYL N-4-TETRADENTATE LIGANDS

A series of linear N-4 tetradentates based on substituted forms of picen (2,5-diaza-1,6-di(2-pyridyl)hexane) has been synthesized. The square-planar coordination mode of these ligands to palladium(II) was investigated by H-1 NMR, chiroptical and crystallographic methods. Varying degrees of stereoselectivity have been achieved by alkyl or aryl substitution on the central carbon atoms, on the secondary nitrogen atoms and at the 6-positions of the pyridyl rings. The 3,4-C-disubstituted forms of the optically active. ligands, which have C-2 point symmetry, are found to coordinate stereospecifically. N,N-Dialkylation of these ligands results in complex mixtures containing two diastereoisomers, with each minor component having C-1 symmetry. Complexes of meso forms of the tetradentates are obtained as isomeric mixtures which vary with the degree and nature of ligand substitution. The X-ray diffraction analysis of two representative complexes is reported. [N,N'-Di(2-picolyl)-N-methyl-2S-amino-methylpyrrolidine]palladium(II) perchlorate, C18H24N4O8Cl2Pd, is monoclinic, space group P2(1), with a = 8.631(2), b = 9.705(2), c = 13.552(3) Angstrom, beta = 101.70(2)degrees and Z = 2. The structure was refined by least-squares methods to R = 0.031 for 1948 diffractometer data. The methyl group and the pyrrole ring are disposed trans to each other and the central five-membered chelate ring has a delta conformation. The average Pd-N(pyridyl) and Pd-N(tert-amine) distances are 2.040(9) and 1.995(10) Angstrom, respectively. [N,N'-Di(2-(6-methyl)picolyl)-1R, 2R-diaminocyclohexane]palladium(II) perchlorate, C20H28N4O8Cl2Pd, is hexagonal, space group P6(5)22, with a = 9.203(2), c = 48.030(17) Angstrom, gamma = 120 degrees and Z = 6. The structure was refined by least-squares methods to R = 0.041 for 2702 diffractometer data. The central five-membered chelate ring has a lambda configuration and the Pd-N(pyridyl) and the Pd-N(sec-amine) distances are 2.080(3) and 1.944(3) Angstrom, respectively. The packing of the ions in the lattice gives rise to an interesting left-handed helical structure, stabilized by pi-pi interactions between pyridyl rings of adjacent molecules.