Chiral metal complexes 35. Unusual terdentate behaviour in the crystal structure of chloro[N,N′-di(2-picolyl)-N-(2-pyridinecarboxamide)-1,2-diaminocyclohexane]palladium(II) perchlorate hemihydrate

An improved method of synthesis for the tetradentate ligand rac-N,N′-di(2-picolyl)-1,2-diaminocyclohexane, (rac-picchxn), is described. The controlled reaction of 2-pyridinecarboxylic acid with this compound results in the potentially pentadentate N₅ ligand N,N′-di(2-picolyl)-N-(2-pyridinecarboxamide)-1,2-diaminocyclohexane, (rac-picchxnam). A palladium(II) complex of this ligand was isolated and its crystal structure determined by X-ray diffraction. [Pd(picchxnam)Cl]ClO₄· 1 2H₂O, C₂₄H₂₈N₅O_5.5Cl₂Pd, crystallises in the monoclinic space group Pn (non-standard No. 7) with a=12.006(2), b=8.207(2), c=13.477(2) Å, β=100.80(1)°, Z=2. The structure was refined to R=0.039 by least-squares techniques for 2066 reflections. The ligand coordinates as a terdentate through the two picolyl pyridine-N atoms and the secondary amine-N atom. This results in a five-membered chelate ring and an unusual eight-membered ring which contains the tertiary amide nitrogen atom. The pyridinecarboxamide group does not coordinate, but is non-planar and is orientated away from the PdN₃Cl square-plane. The PdCl, PdN(amine) and PdN(pyridyl) distances are 2.309(3), 2.079(7), 2.091(8) and 2.029(8) Å, respectively.