Equilibrium and structural studies of copper and nickel complexes with a pentadentate ligand providing amide, amine and pyridyl nitrogen donors. Crystal structure of the dinuclear nickel(II) complex

Abstract

A novel potentially pentadentate ligand 1,7-bis(2-pyridylcarbamoyl)-1,4,7-triazaheptane (pycdien) has been synthesised as its trihydrochloride salt; its protonation constants and the stability constants of the copper(II) and nickel(II) chelates have been determined by potentiometric and spectrophotometric techniques. Amide groups deprotonation allows the formation of [M(pycdienH −1)] + and [M(pycdienH −2)] (M = Cu or Ni) species in all cases, whereas the detection of the complexes of the neutral or the protonated forms of the ligand depends on the nature of the metal ions. In the solid state, mononuclear complexes {[Cu(pycdienH)](ClO 4) 2Cl ( 1) and [Cu(pycdien)](ClO 4) 2 ( 2)} and dinuclear compounds {[Cu 2(pycdienH −2) 2] · H 2O ( 3) and [Ni 2(pycdienH −2) 2(OH 2) 2] · 6H 2O ( 4)} have been synthesised and characterised by spectroscopic techniques. X-ray crystal determination shows that the dimeric [Ni 2(pycdienH −2) 2 (OH 2) 2] · 6H 2O ( 4) crystallizes in the triclinic space group P 1 ¯ , and each nickel(II) center resides in a octahedral geometry generated by the N pyridyl, N amido, N amine portion of one ligand and one pyridine-2-carboxamido end of the other. The hexacoordination of each Ni 2+ ion is achieved by a water molecule. The complexes obtained with the deprotonated form of the ligand imply the coordination through the nitrogen atoms of the amidic groups.