A trimetallic supramolecular complex of osmium(II) and rhodium(III) displaying MLCT transitions in the near-IR

The mixed-metal supramolecule [{(tpy)OsCl(dpp)}2RhCl2](PF6)3 and the monometallic synthon [(tpy)OsCl(dpp)] (tpy = 2,2′:6′,2″-terpyridine, dpp = 2,3-bis(2-pyridyl)pyrazine) have been synthesized and studied by exploring redox and light absorbing properties. The complex [(tpy)OsCl(dpp)](PF6) is synthesized by the reaction of [(tpy)OsCl3] and dpp in ethylene glycol. The supramolecular complex [{(tpy)OsCl(dpp)}2RhCl2](PF6)3 is prepared by reaction of the monometallic synthon with RhCl3 · 3H2O in ethanol/water. Cyclic voltammetry shows an irreversible reduction for [{(tpy)OsCl(dpp)}2RhCl2](PF6)3 at −0.51 V (vs. Ag/AgCl) corresponding to the RhIII/II/I couple. The RhIII/II/I couple is anodic to the first dpp0/− couple, predicting a lowest lying metal-to-metal charge transfer electronic excited state. Both the monometallic and trimetallic complexes absorb in the near-IR, with [{(tpy)OsCl(dpp)}2RhCl2](PF6)3 absorbing strongly throughout the 200–1000 nm region.