Soft Secondary Building Unit: Dynamic Bond Rearrangement on Multinuclear Core of Porous Coordination Polymers in Gas Media

A new synthetic approach to prepare flexible porous coordination polymers (PCPs) by the use of soft secondary building units (SBUs) which can undergo multiple reversible metal-ligand bonds breaking is reported. We have prepared a zinc paddle-wheel-based two-fold interpenetrated PCP, {[Zn-2(tp)(2)(L-2)]center dot 2.5DMF center dot 0.5water}(n) (2a, H(2)tp = terephthanlic acid; L-2 = 2,3-difluoro-1,4-bis(4-pyridyl)benzene), showing dynamic structural transformations upon the removal and rebinding of guest molecules. The X-ray structures at different degrees of desolvation indicate the highly flexible nature of the framework. The framework deformations involve slippage of the layers and movement of the two interpenetrated frameworks with respect to each other. Interestingly, the coordination geometry of a zinc paddle-wheel unit (one of the popular SBUs) is considerably changed by bond breaking between zinc and oxygen atoms during the drying process. Two zinc atoms in the dried form 24 reside in a distorted tetrahedral geometry. Compound 24 has no void volume and favors the uptake of O-2 over Ar and N-2 at 77 K The 02 and Ar adsorption isotherms of 24 show gate-opening-type adsorption behaviors corroborating the structure determination. The CO2 adsorption isotherm at 195 K exhibits multiple steps originating from the flexibility of the framework The structural transformations of the zinc dusters in the framework upon sorption of guest molecules are also characterized by Raman spectroscopy in which the characteristic bands corresponding to v(sym) (COO-) vibration were used.