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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)A, 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) A, 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) A, 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. |
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