Metallomacrocycles: Synthesis, X-ray Structure, Electrochemistry, and ESR Spectroscopy of Mononuclear and Heterodinuclear Complexes

A number of novel heterodinucleating ligands have been synthesized by Ba2+-templated (1:1) macrocyclization of the dialdehydes 7, 8, and 20 with the diamines 9 and 21. The ligands have both a cavity suitable for complexation of transition-metal cations and a cavity for complexation of alkali- or alkaline-earth-metal cations. The resulting barium complexes could be converted into the heterodinuclear complexes 1–3, 10–13, 23–25, and 28 upon reaction with nickel, copper, or zinc acetate. The mono-transition-metal complexes 14–17, 26, 27, and 29 could be obtained by removing the barium salt from the polyether cavity. Their 1H NMR, IR, and mass spectra are discussed, and four heterodinuclear complexes have been analyzed by X-ray crystallography. The nickel/barium complex 3•MeOH (C35H44BaCl2NiO19) crystallizes in the monoclinic system, space group P21/n, with a = 15.096 (4) Å, b = 20.278 (4) Å, c = 13.818 (4) Å, β= 93.61 (1)°, and Z = 4. The nickel/barium complex 10-H2O crystallizes in the triclinic system, space group P1, with a = 9.096 (3) Å, b = 11.491 (7) Å, c = 17.119 (5) Å, a = 92.38 (2)°, β = 102.27 (1)°, γ = 98.63 (2)°, and Z = 2. The nickel/sodium complex 14.sodium picrate crystallizes in the monoclinic system, space group P21/c, with a = 14.697 (5) Å, b = 14.574 (7) Å, c = 14.782 (3) Å, β = 96.08 (1)°, and Z = 4. The zinc/barium complex 25.3DMF crystallizes in the monoclinic system, space group C2/c, with a = 24.301 (5) Å, b = 26.232 (4) Å, c = 22.468 (7) Å, β= 119.06 (2)°, and Z = 8. In the complexes 3.MeOH, 10.H2O, and 14-sodium picrate the coordination of the nickel cation is square planar. The zinc cation in 25.3DMF has a square-pyramidal coordination with a DMF molecule at the axial position. The distance between the two metal ions in the complexes 3•MeOH, 10-H2O, 14.sodium picrate, and 25.3DMF is 3.63-3.70 Å. The electrochemical properties of the complexed transition-metal cations in the heterodinuclear complexes and mono-transition-metal cation complexes have been investigated by polarography and cyclic voltammetry. The half-wave potential is dependent on the nature of the transition-metal cation and the mode of coordination. Complexation of alkali-metal (Li+, Na+, and K+) or alkaline-earth-metal (Ba2+) cations in the polyether cavity resulted in anodic shifts of the half-wave potential up to 213 mV; the bivalent Ba2+induced the largest shifts. The shifts of the half-wave potential are also dependent on the ring size and rigidity of the polyether cavity. Cyclic voltammetry of the nickel/barium complex 10 and the copper/barium complex 12 revealed a chemically reversible but electrochemically irreversible reduction at scan rates of 0.5-6 and 1–6 V/s, respectively. A relatively slow adsorption process was observed for the nickel complex 15, and the reduction was chemically reversible but electrochemically irreversible at scan rates of 50 mV /s to 2 V/s. The zinc/barium complex 25 undergoes an irreversible two-electron reduction at E1/2= -1.466 V, whereas the nickel and copper complexes 10–17, 23, 24, 28, and 29 undergo a one-electron reduction. The ESR spectra of a number of copper containing and heterodinuclear complexes are in line with the redox properties