Chelation versus Binucleation: Metal Complex Formation with the Hexadentate all‐cis‐N 1 , N 2 ‐Bis(2,4,6‐trihydroxy‐3,5‐diaminocyclohexyl)ethane‐1,2‐diamine

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 ⋅4 H 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 ⋅4 H 2 O ( 1 ), [Ni(L e )]‐ Cl 2 ⋅5 H 2 O ( 2 ), [Cu(L e )](ClO 4 ) 2 ⋅H 2 O ( 3 ), [Zn(L e )]CO 3 ⋅7 H 2 O ( 4 ), [Co(L e )](ClO 4 ) 3 ( 5 c ), and [Ga(H −2 L e )]‐ NO 3 ⋅2 H 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 5 c , 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 p K a values of H 6 (L e ) 6+ (25 °C, μ =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  β ) 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.