Synthetic, spectroscopic, and solution studies of imidazolate-bridged dicopper(II) complexes

Article abstract of DOI:10.1021/ic50223a037

The synthesis and spectroscopic characterization of discrete, water soluble, imidazolate-bridged copper(II) complexes are reported. Binuclear imidazolate-bridged complexes were crystallized from basic aqueous or methanolic solutions of Cu(pip)²⁺, pip = 2-[(2-(2-pyridyl)ethylimino)methyl]pyridine, or Cu(PMDT)²⁺, PMDT = 1,1,4,7,7-pentamethyldiethylenetriamine, and various substituted imidazoles. The condensation of imidazole-4,5-dicarboxaldehyde with (2-(2-aminoethyl)-pyridine)copper(II) afforded the Cu₂bpim³⁺ cation, where bpim is 4,5-bis[(2-(2-pyridyl)ethylimino)methyl]imidazolate. Addition of imidazole and sodium hydroxide to aqueous solutions of this cation yielded the tetranuclear complex [Cu₂bpim(im)]₂(NO₃)₄·4H ₂O (im = imidazolate). Potentiometric titrations and solution electron spin resonance spectroscopic studies showed that the integrity of the imidazolate bridge in the Cu₂bpim³⁺ cation is maintained in the range 3.5 < pH < 11.5. Similar studies of the [Cu(pip)]₂im³⁺ complex revealed that the bridge is broken both by protonation of the imidazolate group at low pH and by competition with hydroxide ions at high pH. Four protons are reversibly titrated in solutions of [Cu₂bpim(im)]₂⁴⁺ corresponding to protonation of the imidazolate rings that link the two Cu₂bpim³⁺ units in this complex to form 2 equiv of imidazolium ion.