Synthesis and study of Ru,Rh,Ru triads: modulation of orbital energies in a supramolecular architecture.

Abstract

Supramolecular trimetallic complexes [((tpy)RuCl(BL))(2)RhCl(2)](3+) where tpy = 2,2':6',2' '-terpyridine and BL = dpp or bpm [dpp = 2,3-bis(2-pyridyl)pyrazine and bpm = 2,2'-bipyrimidine] have been synthesized and characterized. The mixed-metal complexes couple a reactive rhodium(III) center to two ruthenium(II) light absorbers to form a light absorber-electron collector-light absorber triad. The variation of the bridging (dpp and bpm) and terminal (tpy in lieu of bpy) ligands has some profound effects on the properties of these complexes, and they are remarkably different from the previously reported [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+) system. The electrochemical data for both title trimetallics consist of overlapping Ru(III/II) couples for both terminal metals at 1.12 V versus the Ag/AgCl reference electrode. Cathodically an irreversible Rh(III/I) reduction followed by bridging ligand reductions is seen. This is indicative of highest occupied molecular orbitals (HOMO) localized on the terminal ruthenium metal centers and a lowest unoccupied molecular orbital (LUMO) residing on the rhodium. This rhodium-based LUMO is in contrast to the bpy analogue [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+), which has a bpm(pi) localized LUMO. This orbital inversion by terminal ligand variation illustrates the similar energy of these Rh(dsigma) and bpm(pi) orbitals within this structural motif. Both title trimetallics possess broad, low-energy Ru --> BL charge transfer absorbances at 540 nm (dpp) and 656 nm (bpm). A comparison of the spectroscopic, electrochemical, and spectroelectrochemical properties of these trimetallic complexes is presented.