Metallomacrocycles: synthesis, x-ray structure, electrochemistry, and ESR spectroscopy of mononuclear and heterodinuclear complexes

Abstract

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 E_1/2= -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.