Characterization of a New Porous Pt-Containing Metal-Organic Framework Containing Potentially Catalytically Active Sites: Local Electronic Structure at the Metal Centers
2007-01-23T00:00:00Z (GMT) by
A crystalline and thermally stable metal-organic framework (MOF) with Pt<sup>II</sup> and Gd<sup>III</sup> sites incorporated in the structure has been recently reported. This material has been synthesized with the aim to develop a heterogeneous Pt<sup>II</sup> counterpart to homogeneous metal-organic complexes having C−H activating properties. The first account focused on the MOF synthesis and on structural and stability characterization of the material. In the present work, a multitechnique approach has been adopted to investigate the effect of solvent removal and the reversibility of this process. Structural features have been investigated by means of powder X-ray diffraction and extended X-ray absorption fine structure spectroscopy at both Pt and Gd L<sub>3</sub>-edges. Electronic properties have been studied with diffuse reflectance surface UV−vis and X-ray absorption near-edge spectroscopies. Finally, IR spectroscopy has been used to determine the vibration properties. Thermogravimetric methods have been used to quantify the water loss. X-ray absorption spectroscopy has been used to compare the Pt environment in the periodic MOF structure, in related molecular complexes, and in the linkers. Our results demonstrate that the environment around Pt is more or less unaffected by the incorporation of the Pt centers in the molecular complexes into the 3D extended framework of the Pt−Gd MOF. The principle of using known homogeneous complexes as building blocks for the construction of single-site heterogeneous catalysts therefore seems applicable in the present case. Removal of solvent water molecules from the internal voids of the as-prepared MOF presents an opportunity to attain a porous material with accessible Pt<sup>II</sup> sites. We observe that the structure undergoes a reversible loss of long-range order upon dehydration at ambient temperature. The environment of Gd is somewhat perturbed in the dehydration/hydration process, while that of Pt is almost unaffected. When a total dehydration is achieved, the original structure is only partially recovered upon rehydration.