Structural Flexibility and Sorption Properties of 2D Porous Coordination Polymers Constructed from Trinuclear Heterometallic Pivalates and 4,4′-Bipyridine

The temperature dependence of crystal structure parameters of isostructural coordination polymers Fe2MO(Piv)(6)(bipy)(1.5)center dot 2DMF [1.2DMF and 2.2DMF, where M = Ni-II or Co-II, respectively, Piv = (CH3)(3)CCO2-, bipy = 4,4-bipyridine and DMF is N,N-dimethylformamide] was studied at five temperatures in the range from 120 to 296 K. The X-ray structure of the N, N-diethylformamide (DEF) solvate of the same coordination polymer 1, Fe2NiO(Piv)(6)(bipy)(1.5)center dot 2DEF(1.2DEF), was studied at 173 and 260 K. Both the DMF and DEF solvates possess similar crystal structures, consisting of a series of parallel 2D polymeric layers with honeycomb voids. These series of parallel layers intersect, leading to their interpenetration and formation of zig-zag channels. It was found that the change of temperature and/or solvent molecule in the pores causes significant structural rearrangement associated with a change of the separation between parallel layers and the angle between intersecting layers. These structural changes lead to a variation of the calculated solvent-accessible volume from 0.21 cm(3) g(-1) (120 K, 2.2DMF) to 0.30 cm(3) g(-1) (260 K, 1.2DEF) for a probe molecule with r = 1.4 angstrom. These changes can model structural transformations which can occur at desolvation and adsorption of guests. Adsorption of n-hexane, n-octane, methanol and ethanol by Fe2MO(Piv)(6)(bipy)(1.5) was studied. The sorption isotherms of alkanes were typical for microporous sorbents and in this case the behaviour of the coordination polymers resembled that which is typical for sorbents with a rigid structure. In a contrast, alcohol sorption capacity continuously grew at increasing pressure which may be caused by structural transformations, similar to temperature-induced structural transformations. Among the studied sorbates, sorption capacity grew in order methanol < ethanol < n-hexane < n-octane which may be caused by the hydrophobic nature of the channels (due to the presence of tert-butyl groups).