Cycle contraction and symmetrisation in redox-active ligands: from alloxazine to isoimidazolonequinoxaline derivatives and their electrochemical and coordination studies

Abstract

The formation of two parent divergent ligands derived from 1,4-bis(pyrid-3-yl)benzene is reported. The synthetic route involves condensation of alloxan with a dibromodiamine precursor, followed by benzylation, leading, after the Suzuki–Miyaura cross-coupling reaction, to the formation of two ligands: L1, bearing the well-known pteridine-dione moiety, and L2, in which ring contraction and symmetrisation occur, resulting in an imidazopyrazinone core. The synthesis of L1 and L2, along with their characterization in solution and in the solid state, is reported. Electrochemical studies of L1 and L2 solutions revealed analogous two-electron reduction processes, with the first reduction step leading to radical species, as confirmed by EPR spectroelectrochemistry. For L1, the first and second reductions occur at E_Red1 = −0,80 V and E_Red2 = −1.35 V vs. Ag/AgCl, in agreement with the values reported for other pteridine-dione species, whereas L2 displays more negative reduction potentials, shifted by approximately 0.7 V. These observations were confirmed by DFT calculations. The coordination abilities of L1 and L2 were investigated. Single-crystal X-ray diffraction (SCXRD) revealed the formation of a pillared 3D compound, L2-Zn, obtained by mixing L2, 4,4′-biphenyldicarboxylic acid (H₂bpdc) and a Zn²⁺ salt under solvothermal conditions. A series of powdered isostructural L2-M compounds (M = Zn, Ni, Co) was synthesized and characterized by powder X-ray diffraction (PXRD). Under the same conditions, using L1 instead of L2 led to the formation of poorly diffracting crystals, which nevertheless exhibited a three-dimensional pillared architecture. A complete series of powdered isostructural L1-M compounds (M = Co, Ni, Cu and Zn) was evidenced. The solid-state electrochemical behavior of the L2-M analogues (M = Zn and Co) was preliminarily investigated, revealing ligand-based reduction processes occurring within the three-dimensional pillared structure for both L2-Zn and L2-Co.