Chiral metal complexes 41. Stereoisomerism in palladium(II) complexes of a series of chiral bis-picolyl N 4-tetradentate ligands

Abstract

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 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 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-2 S-amino-methylpyrrolidine] palladium(II) perchlorate, C 18H 24N 4O 8Cl 2Pd, is monoclinic, space group P2 1, 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-1 R,2 R-diaminocyclohexane]palladium(II) perchlorate, C 20H 28N 4O 8Cl 2Pd, is hexagonal, space group P6 522, 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.