The non-porous crystalline state of a trinuclear iron cluster containing compound ijFe 3 IJμ 3 -O)IJμ 2 -CH 3 COO) 6 IJC 5 H 5 NO) 2 IJH 2 O)]ClO 4 ·3H 2 O (1) demonstrates a series of crystalline state reactions that can be stimulated by an external substrate, such as methanol or pyridine. This simple trinuclear {μ 3 -O} cluster ijFe 3 IJμ 3 -O)IJμ 2 -CH 3 COO) 6 IJC 5 H 5 NO) 2 IJH 2 O)] 1+ exhibits reversible gas-solid reactions driven by the ligand exchange at one of its metal centres that involve Fe-OH 2 bond breaking and Fe-NIJpyridine) bond formation in the solid state in a reversible crystal to crystal transformation. The gas-solid interface reactions have been confirmed by IR spectroscopy and powder X-ray diffraction studies. A classical trinuclear iron {μ 3 -O} cluster with a slight modification in its basic molecular structure opens up new possibilities that can find applications in the development of multi-functional materials. The reversible solid to solid transformations in the present article have been established by various spectroscopic arguments. CrystEngComm This journal is Scheme 1 (a) Basic carboxylate (trinuclear Fe 3 {μ 3 -O} carboxylate cluster), R = CH 3 , Ph, ClCH 2 , Cl 3 C etc.; L = neutral monodentate ligands, e.g., H 2 O, pyridine, methanol etc.; M = Zn 2+ , Cr 3+ , Fe 2+ , Fe 3+ etc. (b) ijFe 3 IJμ 3 -O)IJμ 2 -CH 3 COO) 6 IJC 5 H 5 NO) 2 IJH 2 O)] + (1C), the basic carboxylate of the present work, an unusual trinuclear Fe 3 {μ 3 -O} carboxylate cluster.