Alkali‐Metal‐Mediated Frameworks Based on Bis(2,6‐pyridinedicarboxylate)cobalt(II) Species

Abstract

Metal–organic frameworks (MOFs) with compositional multiplicity show unique topologies as well as enhanced chemical and physical properties. The construction of mixed‐metal frameworks containing more than one type of metal element can provide further structural versatility and diversity. The combination of transition‐ and alkali‐metal elements in a framework can result in interesting materials with substantial advantages. Herein, we report a synthetic strategy for the mixed transition‐ and alkali‐metal frameworks [Na2Co(PDA)2] (1, PDA = 2,6‐pyridinedicarboxylate) and [K2Co(PDA)2] (2). The solid‐state structures of these frameworks show that two PDA ligands chelate to a cobalt ion to form a Co(PDA)22– species, which in turn undergoes further assembly mediated by alkali‐metal cations (Na+ or K+) to give mixed‐metal framework structures. The {[KCo(PDA)2]–·[NH2(CH3)2]+} framework (3) is also prepared by a change in stoichiometry. The multiple coordination of alkali metal atoms to the carboxylate oxygen atoms of the PDA ligands of the Co(PDA)22– species generates unique three‐dimensional architectures. As 2 and 3 react with NaOH, the coordinated K+ exchanges readily with Na+ in aqueous solution, and finally 1 crystallizes. The strong ionic character and small size of Na+ account for the facile transformations of 2 and 3. Temperature‐dependent magnetization studies reveal that 1 exhibits distinct magnetic susceptibility relative to those of 2 and 3.