Anion Binding by Dimetallic Nickel(II) and Nickel(III) Complexes of a Face-to-Face Bicyclam: Looking for a Bimacrocyclic Effect

The dinickel(II) complex of the face-to-face bicyclam ligand α,α′-bis(5,7-dimethyl-1,4,8,11-tetraazacyclotetradecan-6-yl)-o-xylene (L∩L) in a dimethyl sulfoxide solution exists as a mixture of high- and low-spin forms and uptakes up to three halide and pseudohalide ions (X-), according to stepwise equilibria, whose constants were determined through spectrophotometric titration experiments. In the case of halides, the first anion goes into the intermetallic cavity, whereas pseudohalides first coordinate the metal center from outside. Comparison with equilibrium data for the complex with the macrocycle 5,7-dimethyl-6-benzyl-1,4,8,11-tetraazacyclotetradecane (L) shows that the dinuclear complex [NiII 2(L∩L)]4+ displays an affinity for the first halide distinctly higher than the mononuclear complex [NiII(L)]2+, thus disclosing the existence of a bimacrocyclic effect for anion binding. Differential pulse voltammetry studies typically showed a three-peak profile: the most anodic pertaining to the [NiII 2(L∩L)]4+ → NiIII 2(L∩L)]6+ two-electron process, then one originating from the [NiII 2(L∩L)X]3+ → NiIII 2(L∩L)X]5+ two-electron process, and one deriving from the two two-electron half reactions [NiII 2(L∩L)X2]2+ → NiIII 2(L∩L)X2]4+ and [NiII 2(L∩L)X3]+ → NiIII 2(L∩L)X3]3+, taking place at nearly the same potential. The crystal structure of the [NiII 2(L∩L)(μ-NCO)(NCO)2]ClO4·2.5H2O complex salt showed a caterpillar arrangement of the three metal-bound cyanate ions.