Guest dependent reversible single-crystal to single-crystal structural transformation in a flexible Gd(III)-coordination polymer

The ligand 2,6,2′,6′-tetranitro-biphenyl-4,4′-dicarboxylic acid (H₂L) reacts solvothermally with [Gd(NO₃)₃]·6H₂O to produce a flexible and porous metal–organic framework, {[Gd₂(L)₃(DMF)₄]·(4DMF)·(3H₂O)}_n (1) (DMF = N,N′-dimethylformamide). An X-ray crystallographic study reveals that compound 1 contains a 3D framework structure with two different 1D channels (A and B) that are occupied by solvent DMF and water molecules. Crystals of 1 when kept in a dichloromethane solution of 4-chlorobenzaldedhyde (4-ClPhCHO) afford the daughter product {[Gd₂(L)₃(DMF)₄]·(4-ClPhCHO)·(4DMF)}_n (1a), via single-crystal to single-crystal (SC–SC) transformation, where lattice water molecules of channel B are replaced by guest aldehyde molecules. Likewise, exposure of 4-fluorobenzaldehyde (4-FPhCHO) and 4-methylbenzaldehyde (4-MePhCHO) vapors to fresh crystals of 1 afforded two isostructural daughter frameworks, {[Gd(L)_1.5(DMF)(H₂O)₃]·(4-FPhCHO)·(DMF)·(3H₂O)}_n (1b) and {[Gd(L)_1.5(DMF)(H₂O)₃]·(4-MePhCHO)·(2DMF)·(H₂O)}_n (1c), respectively. Here, the guest aldehyde molecules occupy both the channels of the framework. Interestingly, the latter transformations exhibit a drastic rearrangement of the framework channels followed by several ‘carboxylate-shift’ processes, and concomitant movement of the water molecules from the cavity to the metal center. Importantly, all the host–guest complexes revert back to the as-synthesized crystal when kept in fresh DMF, rendering the mother framework a flexible and dynamic container for the aromatic aldehydes. All these transformations transpire through an SC–SC fashion under ambient conditions, pointing to the high flexibility of the framework and “guest-responsive fitting” of the channels. All the compounds are characterized by X-ray crystallography, thermogravimetry, elemental analysis, powder X-ray diffraction measurements and infrared spectroscopy.