Synthesis of anionic alkaline earth metal methanesulfonates bearing photo-responsive methylviologen

Abstract

Anionic coordination frameworks stabilized by methylviologen (MV²⁺; 1,1′-dimethyl 4,4′-bipyridinium) as a photochromic module are a distinct class of functional materials. Herein, we report the first examples of alkaline earth metal methanesulfonates [MV{Mg(OSO₂Me)₃(H₂O)₂}₂] (1), [MV{Ca(OSO₂Me)₃(H₂O)₂}₂] (2), [2{Mg(OSO₂Me)₂(H₂O)₄}MV·2OSO₂Me] (3), and [Me₄-pip{Ca₂(OSO₂Me)₆(H₂O)₂}]_n (4) incorporating methylviologen or tetra-N-methylpiperazinium (Me₄-pip) as a charge-balancing cation. X-ray crystallographic studies of 1–3 emphasize the role of isolated ion-pairs for the construction of hydrogen-bonded supramolecular assemblies and the ability of methanesulfonates to donate electrons (centroid⋯O = 3.09–3.15 Å) to promote photoinduced one-electron reduction of MV²⁺ to an MV˙⁺ radical cation in the solid state. The inorganic framework in 4 adopts a layered motif with an inter-lamellar spacing of 11.8 Å, providing spatial expanse to accommodate the organic cations.