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, {[Zn₂(tp) ₂(L²)]•2.5DMF•0.5water}_n (2a, H ₂tp = terephthanlic acid; L² = 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 2d reside in a distorted tetrahedral geometry. Compound 2d has no void volume and favors the uptake of O₂ over Ar and N₂ at 77 K. The O₂ and Ar adsorption isotherms of 2d 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 clusters in the framework upon sorption of guest molecules are also characterized by Raman spectroscopy in which the characteristic bands corresponding to ν_sym(COO^-) vibration were used.