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 [Ni(II)2(L∩L)](4+) displays an affinity for the first halide distinctly higher than the mononuclear complex [Ni(II)(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 [Ni(II)2(L∩L)](4+) → Ni(III)2(L∩L)](6+) two-electron process, then one originating from the [Ni(II)2(L∩L)X](3+) → Ni(III)2(L∩L)X](5+) two-electron process, and one deriving from the two two-electron half reactions [Ni(II)2(L∩L)X2](2+) → Ni(III)2(L∩L)X2](4+) and [Ni(II)2(L∩L)X3](+) → Ni(III)2(L∩L)X3](3+), taking place at nearly the same potential. The crystal structure of the [Ni(II)2(L∩L)(µ-NCO)(NCO)2]ClO4·2.5H2O complex salt showed a caterpillar arrangement of the three metal-bound cyanate ions.

The disproportionation of [Ni(tacn)]2+ in Ni2+ and [Ni(tacn)2]2+ crystallographically demonstrated (tacn = 1,4,7-triazacyclononane).

From an EtOH/H2O solution, 0.3 M each of Ni(2+) and cyclic triamine tacn (2; tacn = 1,4,7-triazacyclononane), after 10 min refluxing and cooling at room temperature, copious and comparable amounts of blue crystals (containing the complex [Ni(tacn)(H2O)3](2+)) and pink crystals (containing in the same cell both [Ni(H2O)6](2+) and [Ni(tacn)2](2+)) precipitated. This unusual behaviour is ascribed to the fact that at the refluxing temperature the three species are present at the equilibrium in similar concentrations, which are frozen on cooling, due to the inertness of the macrocyclic complexes [Ni(tacn)(H2O)3](2+) and [Ni(tacn)2](2+).

The solution stability of copper(I) and silver(I) complexes with N-heterocyclic carbenes.

The tris-benzimidazolium cage LH(3)(3+), in MeCN solution, in the presence of OH(-), forms with Cu(I) and Ag(I) ions complexes of formula [M(I)(LH)](2+), in which each metal is linearly coordinated by two carbenes and one imidazolium N-H fragment remains intact. To achieve two-coordination, the two N-heterocyclic moieties of the cage make a saloon-door type motion, with a conformationally costless rotation of ca. 30° each. The two [M(I)(LH)](2+) complexes show high thermodynamic stability and are inert with respect to metal substitution, due to the mechanical constraints imposed by the ligating framework. Complexation with Cu(I) and Ag(I) with the reference unidentate carbene ligand Q, derived from the benzimidazolium precursor QH(+), was studied for comparison. Both metals in MeCN form 1:1 and 1:2 complexes with the carbene ligand Q according to two stepwise equilibria. Q complexes of both metals are labile with respect to metal substitution and those of Ag(I) are more stable than those of Cu(I). A significant cooperative effect has been observed with the formation of the [Ag(I)Q(2)](+) complex.