Direct Identification of Mixed-Metal Centers in Metal–Organic Frameworks: Cu₃(BTC)₂ Transmetalated with Rh²⁺ Ions

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 (BTC^3- = 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 (Cu_<i>x</i>Rh_1-<i>x</i>)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 (Cu_<i>x</i>Rh_1-<i>x</i>)₃(BTC)₂ on the nanoscale.