Fecht's acid revisited: a spirocyclic dicarboxylate for non-aromatic MOFs

Abstract

Spiro[3.3]heptane-2,6-dicarboxylic acid (Fecht's acid, H₂SHDC) is examined as a non-aromatic terephthalic acid isostere for the first time. The rigid spirocyclic backbone provides greater steric bulk than conventional aromatic dicarboxylates with consequences for pore chemistry and control of interpenetration, presented here in the structures of two new MOFs. Complex 1 is a three-dimensional rod packed structure consisting of Yb-carboxylate chains bridged by SHDC linkers which, although non-porous, exhibits a surprisingly high thermal stability for a spirocyclic cyclobutane derivative. Complex 2 is a co-ligand complex of SHDC with trans-1,2-bis(4-pyridyl)ethene (bpe) which contains linear solvent channels despite fourfold interpenetration. Although the framework does not retain its structure following evacuation, a clear difference is observed in the extended structure compared to the structurally related terephthalate species. This observation suggests the non-aromatic backbone of Fecht's acid and other rigid aliphatic linkers may prove an effective means to disfavour deleterious close inter-framework contacts which prevail in interpenetrated aromatic MOFs.