Chiral metal complexes 41. Stereoisomerism in palladium(II) complexes of a series of chiral bis-picolyl N₄-tetradentate ligands

A series of linear N₄ 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 ¹H NMR, chiroptical and crystallographic methods. Varying degress of stereoselectivity hav been achieved by alkyl or aryl substitution on the central carbon atoms. on the secondary nitrogen atoms and at the 6-position of the pyridyl rings. The 3,4-C-disubstituted forms of the optically active ligands, which have C₂ 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 C₁ 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 P2₁, with a = 8.631(2), b = 9.705(2), c = 13.552(3)Å, β = 101.70(2)° 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 δ conformation. The average Pd-N(pyridyl) and Pd-N(tert-amine) distances are 2.040(9) and 1.995(10) Å, respectively. [N,N′-Di(2-(6-methyl)picolyl-1R,2R-diaminocyclohexane]palladium(II) perchlorate, C₂₀H₂₈N₄O₈Cl₂Pd, is hexagonal, space group P6₅22, with a = 9.203(2), c = 48.030(17) A ̊,γ = 120° 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 λ configuration and the Pd-N(pyridyl) and the Pd-N(secamine) distances are 2.080(3) and 1.944(3) Å, respectively. The packing of the ions in the lattice gives rise to an interesting left-handed helical structure, stabilized by π-π interactions between pyridyl rings of adjacent molecules.