Modulating the microporosity of cobalt phosphonates via positional isomerism of co-linkers

Abstract

By incorporating co-ligands 1,2-bis(imidazol-1-ylmethyl)benzene (1,2-bix), 1,3-bis(imidazol-1-ylmethyl)benzene (1,3-bix) and 1,4-bis(imidazol-1-ylmethyl)benzene (1,4-bix), three isomeric cobalt phosphonates Co₅(1,2-bix)₂(pbtc)₂(H₂O)₄·8H₂O (1), Co₅(1,3-bix)₂(pbtc)₂(H₂O)₄·7.5H₂O (2), and Co₅(1,4-bix)₂(pbtc)₂(H₂O)₄·6H₂O (3) (pbtcH₅ = 5-phosphonatophenyl-1,2,4-tricarboxylic acid) have been isolated successfully. Compound 1 shows a layer structure in which chains containing alternately arranged Co₃O₂ trimers and Co₂ dimers are connected by pbtc⁵⁻ ligands. The 1,2-bix co-ligand adopts a cis-mode, and coordinates to the cobalt atoms from the same chain. Upon dehydration, compound 1 undergoes single-crystal-to-single-crystal (SC–SC) structural transformation forming a new crystal phase Co₅(1,2-bix)₂(pbtc)₂(H₂O)₄ (1a). The process is reversible upon rehydration, showing a breathing effect. In compounds 2 and 3, similar layers composed of cobalt phosphonate chains and pbtc⁵⁻ linkages are also found, which are further cross-linked by 1,3-bix or 1,4-bix to form 3D framework structures. The three compounds show different microporosities, as confirmed by their different N₂ gas adsorption behaviors.