Studies on the metal-amide bond. XVI. The coordination chemistry of the di-tertiary amide ligand N,N′-dimethyl-N,N′-bis(2′-pyridinecarboxamide)-1,2-ethane, [bpenMe₂], including the crystal structure of trans-[Pd(bpenMe₂)Cl₂]

The isolation and characterization of a number of transition metal complexes of the di-tertiary-amide ligand N,N′-dimethyl-N,N′-bis(2′-pyridinecarboxamide)-1,2-ethane, bpenMe₂, is described. Six-coordinate polymeric complexes involving coordination as a [NO]₂-bis-bidentate through pyridyl-N and amide-O atoms were obtained with bivalent copper, nickel, cobalt and zinc. N₂-Bidentate function through pyridlyl-N atoms alone is concluded for complexes of Ag(I), Pt(II) and Pd(II). The crystal structure of [Pd(bpenMe₂)Cl₂] was determined by X-ray diffraction methods. C₁₆H₁₈N₄O₂Cl₂Pd is orthorhombic, space group P2₁2₁₂, with a = 12.229(10), b = 17.053(13), c = 8.757(9) Å and Z = 4. These structure was refined to R 0.072 by least-squares techniques for 1360 photographic reflexions. The coordination geometry about Pd is square planar and the molecule possesses an approximate two-fold rotation axis. The ligand functions as a rare N₂-trans-bidentate, coordinating through two pyridyl-N atoms alone [average Pd-N 2.00(1) Å]. The two Cl atoms complete the coordination sphere [average Pd-Cl 2.311(7) Å]. The two picolinamide groups are non-planar, the dihedral angles between the amide groups planes and their corresponding pyridine ring planes being 69.7 and 86.1°. The central ethane link has a staggered conformation, the NCCN torsion angle being 58.8°. This rigid structure, with a two-fold rotation axis, is retained in solution. The complex 200 MHz proton n.m.r. spectrum in CDCl₃ may be simulated assuming such a structure and HCCH torsion angles calculated from the observed vicinal coupling constants confirm the gauche conformation of the central ethane bond in solution. The separate chemical shifts of these methylene protons are explained in terms of the non-planar structure of the ligand in the chelate molecule.