Chelation versus binucleation: metal complex formation with the hexadentate...

The hexadentate ligand all-cis-N(1),N(2)-bis(2,4,6-trihydroxy-3,5-diaminocyclohexyl)ethane-1,2-diamine (L(e)) was synthesized in five steps with an overall yield of 39% by using [Ni(taci)(2)]SO(4).4H(2)O as starting material (taci=1,3,5-triamino-1,3,5-trideoxy-cis-inositol). Crystal structures of [Na(0.5)(H(6)L(e))](BiCl(6))(2)Cl(0.5).4H(2)O (1), [Ni(L(e))]Cl(2).5H(2)O (2), [Cu(L(e))](ClO(4))(2).H(2)O (3), [Zn(L(e))]CO(3).7H(2)O (4), [Co(L(e))](ClO(4))(3) (5c), and [Ga(H(-2)L(e))]NO(3).2H(2)O (6) are reported. The Na complex 1 exhibited a chain structure with the Na(+) cations bonded to three hydroxy groups of one taci subunit of the fully protonated H(6)(L(e))(6+) ligand. In 2, 3, 4, and 5c, a mononuclear hexaamine coordination was found. In the Ga complex 6, a mononuclear hexadentate coordination was also observed, but the metal binding occurred through four amino groups and two alkoxo groups of the doubly deprotonated H(-2)(L(e))(2-). The steric strain within the molecular framework of various M(L(e)) isomers was analyzed by means of molecular mechanics calculations. The formation of complexes of L(e) with Mn(II), Cu(II), Zn(II), and Cd(II) was investigated in aqueous solution by using potentiometric and spectrophotometric titration experiments. Extended equilibrium systems comprising a large number of species were observed, such as [M(L(e))](2+), protonated complexes [MH(z)(L(e))](2+z) and oligonuclear aggregates. The pK(a) values of H(6)(L(e))(6+) (25 degrees C, mu=0.10 M) were found to be 2.99, 5.63, 6.72, 7.38, 8.37, and 9.07, and the determined formation constants (log beta) of [M(L(e))](2+) were 6.13(3) (Mn(II)), 20.11(2) (Cu(II)), 13.60(2) (Zn(II)), and 10.43(2) (Cd(II)). The redox potentials (vs. NHE) of the [M(L(e))](3+/2+) couples were elucidated for Co (-0.38 V) and Ni (+0.90 V) by cyclic voltammetry.