Post-Synthetic Reversible Incorporation of Organic Linkers into Porous Metal-Organic Frameworks through Single-Crystal-to-Single-Crystal Transformations and Modification of Gas-Sorption Properties

The porous metal-organic framework (MOF) {[Zn-2(TCPBDA)(H2O)(2)]center dot 30DMF center dot 6H(2)O}(n) (SNU-30; DMF=N,N-dimethylformamide) has been prepared by the solvothermal reaction of N,N,N',N'-tetrakis(4-carboxyphenyl)biphenyl-4,4'-diamine (H(4)TCPBDA) and Zn(NO3)(2)center dot 6H(2)O in DMF/tBuOH. The post-synthetic modification of SNU-30 by the insertion of 3,6-di(4-pyridyl)-1,2,4,5-tetrazine (bpta) affords single-crystalline {[Zn-2(TCPBDA)(bpta)]center dot 23DMF center dot 4H(2)O}(n) (SNU-31SC), in which channels are divided by the bpta linkers. Interestingly, unlike its pristine form, the bridging bpta ligand in the MOF is bent due to steric constraints. SNU-31 can be also prepared through a one-pot solvothermal synthesis from Zn-II, TCPBDA(4-), and bpta. The bpta linker can be liberated from this MOF by immersion in N,N-diethylformamide (DEF) to afford the single-crystalline SNU-30SC, which is structurally similar to SNU-30. This phenomenon of reversible insertion and removal of the bridging ligand while preserving the single crystallinity is unprecedented in MOFs. Desolvated solid SNU-30' adsorbs N-2, O-2, H-2, CO2, and CH, gases, whereas desolvated SNU-31' exhibits selective adsorption of CO2 over N-2, O-2, H-2, and CH4, thus demonstrating that the gas adsorption properties of MOF can be modified by post-synthetic insertion/removal of a bridging ligand.