Direct Identification of Mixed-Metal Centers in Metal–Organic Frameworks: Cu<sub>3</sub>(BTC)<sub>2</sub> Transmetalated with Rh<sup>2+</sup> Ions
Kamolrat Metavarayuth, Otega A. Ejegbavwo, Gavin A. McCarver, Michael L. Myrick, Thomas M. Makris, Konstantinos D. Vogiatzis, Sanjaya D. Senanayake, Olivia M. Manley, Amani M. Ebrahim, Anatoly I. Frenkel, Sooyeon Hwang, Thayalan Rajeshkumar, Juan D. Jiménez, Kexun Chen, Natalia B. Shustova, Donna A. Chen
Abstract
Raman spectroscopy was used to establish direct evidence of heterometallic metal centers in a metal–organic framework (MOF). The Cu3(BTC)2 MOF HKUST-1 (BTC3– = benzenetricarboxylate) was transmetalated by heating it in a solution of RhCl3 to substitute Rh2+ ions for Cu2+ ions in the dinuclear paddlewheel nodes of the framework. In addition to the Cu–Cu and Rh–Rh stretching modes, Raman spectra of (CuxRh1–x)3(BTC)2 show the Cu–Rh stretching mode, indicating that mixed-metal Cu–Rh nodes are formed after transmetalation. Density functional theory studies confirmed the assignment of a Raman peak at 285 cm–1 to the Cu–Rh stretching vibration. Electron paramagnetic resonance spectroscopy experiments further supported the conclusion that Rh2+ ions are substituted into the paddlewheel nodes of Cu3(BTC)2 to form an isostructural heterometallic MOF, and electron microscopy studies showed that Rh and Cu are homogeneously distributed in (CuxRh1–x)3(BTC)2 on the nanoscale.