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A Dynamic Open Framework Exhibiting Guest- and/or Temperature-Induced Bicycle-Pedal Motion in Single-Crystal to Single-Crystal Transformation

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journal contribution
posted on 07.03.2011, 00:00 by Manish K. Sharma, Parimal K. Bharadwaj
A ligand bis(4-imidazol-1-yl-phenyl)diazene (azim) incorporating an azo moiety at the center and two imidazole groups at the terminals forms two coordination polymers {[Co(azim)2(DMF)2]·(ClO4)2·2DMF}n (1) and {[Cd(azim)2(DMF)2]·(ClO4)2·2DMF}n (2) (DMF = N,N′-dimethylformamide) at room temperature. Both 1 and 2 are isostructural with rhombic two-dimensional sheets stacking in ABAB... fashion resulting in large voids that contain DMF and ClO4¯ as guests. In 1, the azo groups and phenyl rings are disordered over two positions and as in usual cases, the pedal motion cannot be discerned. Upon heating, 1 turns amorphous. In the case of 2, however, heat treatment does not lead to loss of crystallinity. Thus, when a crystal of 2 (mother crystal) is heated slowly, it causes substantial movement or escape of both metal-bound and lattice DMF besides movement of ClO4¯ anions to give daughter crystals 2a, 2b, and 2c without losing crystallinity (single-crystal to single-crystal (SC−SC) transformation). Most interestingly, the X-ray structures of 2 and its daughter products reveal stepwise reversible bicycle-pedal or crankshaft motion of the azo group. When a crystal of 2c is kept in DMF for 10 h, crystal 2′ is formed whose structure is similar to that of 2 with slight changes in the bond distances and angles. Also, crystals of 2 are converted to 3 and 4 upon being kept in acetone or DEF (DEF = N,N′-diethylformamide), respectively, for 10 h at ambient temperature in SC−SC transformation. In 3, each lattice DMF molecule is replaced by an acetone molecule, leaving the two coordinated DMF molecules intact. However, in 4, all lattice and coordinated DMF molecules are replaced by equal number of DEF molecules. Both in 3 and 4, the azo moieties show bicycle-pedal motion. Thus, bicycle-pedal motion that normally cannot be observed is shown here to be triggered by heat as well as guest molecules in SC−SC fashion.