Pyrazine-imide complexes: reversible redox and MOF building blocks

Abstract

The synthesis of the symmetric pyrazine imide ligand, N-(2-pyrazylcarbonyl)-2-pyrazinecarboxamide, (Hdpzca) and five new first row transition metal complexes of it are reported: [M^II(dpzca)₂], M^II = Fe, Cu, Zn; [Cu^II(dpzca)(H₂O)₂]BF₄, [Cu^II(dpzca)(H₂O)₃]₂SiF₆. The crystal structures of Hdpzca, [Co^II(dpzca)₂], [Cu^II(dpzca)₂], {[Co^III(dpzca)₂](BF₄)}₂·5CH₃CN and [Cu^II(dpzca)(H₂O)₃]₂SiF₆·2H₂O were determined and reveal an orthogonal positioning of the ‘spare’ pyrazine nitrogen atoms and ‘spare’ pairs of imide oxygen atoms. The [M^II(dpzca)₂] complexes are therefore useful six–coordinate building blocks for producing larger supramolecular assemblies. Two examples of secondary assembly of [M^II(dpzca)₂] complexes, with M = Co and Ni, with silver nitrate gave single crystals; {[Co^III(dpzca)₂Ag](NO₃)₂·2H₂O}_n and {([Ni^II(dpzca)₂Ag^I_1/2](1/2NO₃)(xH₂O}_n were structurally characterised. The redox processes of [M^II(dpzca)₂], with M^II = Fe, Ni, Cu and Zn, are reported and, as seen for M^II = Co, reversible metal- and ligand-based redox processes are observed, with E_m(M^II/M^III) values 0.15–0.24 V higher than for the analogous complexes of Hpypzca (non-symmetric pyridine/pyrazine imide ligand), and 0.35–0.36 V higher than for the complexes of Hbpca (symmetric pyridine imide ligand).