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 pic en (2,5-diaza-1,6-di(2-pyridyl)hexane) has been synthesized. The squar e-planar coordination mode of these ligands to palladium(II) was inves tigated by H-1 NMR, chiroptical and crystallographic methods. Varying degrees of stereoselectivity have been achieved by alkyl or aryl subst itution on the central carbon atoms, on the secondary nitrogen atoms a nd at the 6-positions of the pyridyl rings. The 3,4-C-disubstituted fo rms of the optically active. ligands, which have C-2 point symmetry, a re found to coordinate stereospecifically. N,N-Dialkylation of these l igands results in complex mixtures containing two diastereoisomers, wi th each minor component having C-1 symmetry. Complexes of meso forms o f the tetradentates are obtained as isomeric mixtures which vary with the degree and nature of ligand substitution. The X-ray diffraction an alysis of two representative complexes is reported. -N-methyl-2S-amino -methylpyrrolidine]palladium(II) perchlorate, C18H24N4O8Cl2Pd, is mono clinic, 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 ref ined by least-squares methods to R = 0.031 for 1948 diffractometer dat a. The methyl group and the pyrrole ring are disposed trans to each ot her and the central five-membered chelate ring has a delta conformatio n. 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 c entral five-membered chelate ring has a lambda configuration and the P d-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 give s rise to an interesting left-handed helical structure, stabilized by pi-pi interactions between pyridyl rings of adjacent molecules.