Coordination Polymers of Copper and Zinc Ions with a Linear Linker Having Imidazole at Each End and an Azo Moiety in the Middle: Pedal Motion, Gas Adsorption, and Emission Studies

The imidazole donor ligand bis(4-imidazol-1-yl-phenyl)-diazene (azim) reacts readily with M(ClO4)(2)center dot 6H(2)O [M(II) = Zn(II) and Cu(II)] salts at room temperature to afford {[Zn(azim)(2)]center dot(ClO4)(2)center dot 3DMF}(n) (1) and {[Cu(azim)(2)(DMF)(2)]center dot(ClO4)(2)center dot 2DMF}(n) (2) (DMF = N,N'-dimethylformamide). With MCl2 center dot 6H(2)O [M(II) = Zn(II) and Cu(II)] salts under solvothermal conditions, compounds {[Zn(azim)(HCOO)(2)]}(n) (3) and {[Cu(azim)(Cl)]}(n) (4) are obtained in moderate yields. Complexes 1 and 2 crystallize as 2D four-connected 4(4) sql networks with rectangular and rhombus grids, respectively. The frameworks are stabilized by an intricate array of hydrogen bonding interactions with guest molecules that also results in overall 3D frameworks. On the other hand, complexes 3 and 4 form 1D zigzag infinite chain structures. All compounds have been characterized by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric analysis (TGA), and powder X-ray diffraction (PXRD) measurements. The TGA and PXRD measurements reveal that the integrity of the frameworks 1 and 2 is maintained upon guest removal. N-2 and CO2 adsorption measurements were carried out for evacuated frameworks 1 and 2, and the adsorption studies show CO2 selectivity over N-2 at low pressure in the case of 2. Interestingly, heat and guest induced bicycle pedal motion of the azo moiety can be observed in 1 and 4 without loss of crystallinity. Upon excitation at 304 nm, compounds 1-4 and 4' exhibit solid-state luminescence at room temperature.