Electrochemical Investigation of Transition—Metal Complexes of the Ligand Tetrakis(2-Pyridylmethyl)-Ethylenediamine (Tpen). Crystal Structure of [Ni(tpen)](CIO4)2·2/3H2O

Abstract The potentially hexadentate ligand N,N,N′,N'-tetrakis(2-pyridylmethyl)ethylenediamine (C26H28N6, tpen) reacts in ethanolic solution with M(CIO4)2.6H2O (stoichiometry 1:1; M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II)) yielding the corresponding [M(tpen)](ClO4)2 complexes. The new complex [Cr(tpen)](PF6)3 was also prepared from a dimethyl sulfoxide solution of tpen and CrCl3.6H2O. The crystal structure of the violet, [Ni(tpen)](ClO4)2-2/3H2O complex has been determined by X-ray crystallography. Crystal data: monoclinic, space group C2/c; a=41.205(5), b = 9.492(2), c = 23.930(3) Å, β = 108.02(3)°, V = 8,900(6) Å3, Z = 12, D cal = 1.55g cm−3; final R = 0.067 from 3379 observed reflections. The structure consists of two distinct distorted octahedral Ni(H) cations, perchlorate anions and waters of crystallization. Electrochemical behaviour (cyclic voltammetry) and spectroelectrochemistry in acetonitrile shows a reversible or quasi-reversible one electron transfer process. The nickel(II) and copper(II) complexes can be also reduced reversibly to the corresponding (formal) nickel(I) and copper(I) complexes. Stabilization of different oxidation states is discussed in terms of the flexibility and π-acceptor capability of the ligand tpen. Key Words: X-ray crystal structureelectrochemistrytetrakis(2-pyridylmethyl)ethylenediaminenickel(II)