In Situ X-ray Absorption Spectroscopy Studies of Kinetic Interaction between Platinum(II) Ions and UiO-66 Series Metalâ•fiOrganic Frameworks

The interaction of guest Pt(II) ions with UiO-66−X (X = NH2, H, NO2, OMe, F) series metal−organic frameworks (MOFs) in aqueous solution was investigated using in situ X-ray absorption spectroscopy. All of these MOFs were found to be able to coordinate with Pt(II) ions. The Pt(II) ions in UiO-66−X MOFs generally coordinate with 1.6−2.4 Cl and 1.4−2.4 N or O atoms. We also studied the time evolution of the coordination structure and found that Pt(II) maintained a coordination number of 4 throughout the whole process. Furthermore, the kinetic parameters of the interaction of Pt(II) ions with UiO-66−X series MOFs (X = NH2, H, NO2, OMe, F) were determined by combinational linear fitting of extended X-ray absorption fine structure (EXAFS) spectra of the samples. The Pt(II) adsorption rate constants were found to be 0.063 h−1 for UiO-66−NH2 and 0.011−0.017 h−1 for other UiO-66−X (X = H, NO2, OMe, F) MOFs, which means that Pt(II) adsorption in UiO-66−NH2 is 4−6 times faster than that in other UiO-66 series MOFs. FTIR studies suggested that the carboxyl groups could be the major host ligands binding with Pt(II) ions in UiO-66 series MOFs, except for UiO-66−NH2, in which amino groups coordinate with Pt(II) ions. ■ INTRODUCTION Metal−organic frameworks (MOFs) are interesting coordination polymers due to their potential applications as permanently porous host materials for guest ions, molecules, and gases. Postsynthetic modifications can be used to change the structure and composition of the organic linker and therefore change the pore volume, surface area, and chemical properties of the MOF. These changes, along with the addition of guest metal ions to the MOF, can be used to solve application-specific problems, such as hydrogen gas storage, catalysis, chemical adsorption and separation, sensing of organic molecules, gases, and ions, as well as drug delivery. The loading of guest metal ions into MOFs is mostly carried out in the solution phase with a conventional wetchemistry strategy. Functional groups on MOF linkers, such as −NH2, −SH, and −COOH, are generally considered as the key factors for efficient encapsulation of metal ions. The coordination of metal ions with common organic ligands, dendrimers, and polymers has been extensively studied. For example, Crooks and co-workers studied the binding of PtCl4 2− anions with hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers with UV−vis spectroscopy. From their study, the formation of PtCln(H2O)4−n 2−n (n ≤ 3) is the key step that leads to the coordination of Pt(II) with the amino group in the PAMAM dendrimer. Considering the importance and broad applications of MOFs as host materials for guest metal ions, no detailed studies have been carried out on the coordination mechanism of metal ions within MOFs, particularly the coordination kinetics. We chose the UiO-66 MOF, which consists of Zr6O4(OH)4 clusters and 1,4-benzenedicarboxylate (BDC) linkers, as the host material in our studies because UiO-66 has superior thermal and chemical stability. It has been reported that UiO66 could maintain its crystalline structure up to 540 °C and tolerate most common neutral solvents. A series of isoreticular UiO-66 MOFs could be synthesized by using functionalized BDC linkers with various functional groups, such as −NH2, −NO2, −F, and −OMe. Functionalized BDC linkers can be introduced into the MOF either before the synthesis or via postsynthetic modifications of UiO-66 MOFs. In our previous study, we found that Pt(II) ions could be efficiently adsorbed inside of the cages of UiO-66−NH2 via a conventional wetness impregnation approach in aqueous solution. Potassium tetrachloroplatinate (K2PtCl4) was found to be a good metal precursor, and the amino group in the MOF (UiO66−NH2) was essential for the efficient loading of Pt(II) into the MOF. We also found that both the functional groups and MOF’s porous structure are important in controlling the coordination structure of metal ions. The loaded Pt(II) ions were further reduced to metallic Pt nanoclusters that had the Special Issue: Spectroscopy of Nanoand Biomaterials Symposium Received: July 3, 2014 Revised: August 18, 2014 Published: August 21, 2014 Article