Ruthenium(II)-Polyazine Light Absorbers Bridged to Reactive cis-Dichlororhodium(III) Centers in a Bimetallic Molecular Architecture

Bimetallic complexes of the form [(bpy)(2)Ru(BL)RhCl2(phen)](PF6)(3), where bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, and BL = 2,3-bis(2-pyridyi)pyrazine (dpp) or 2,2'-bipyrimidine (bpm), were synthesized, characterized, and compared to the [{(bpy)(2)Ru(BL)}(2)RhCl2](PF6)(5) trimetallic analogues. The new complexes were synthesized via the building block method, exploiting the known coordination chemistry of Rh(III) polyazine complexes. In contrast to [{(bpy)(2)Ru(dpp)}(2)RhCl2](PF6)(5) and [{(bpy)(2)Ru(bpM)}(2)RhCl2](PF6)(5), [(bpy)(2)Ru(dpp)RhCl2(phen)](PF6)(3) and [(bpy)(2)Ru(bpm)RhCl2(phen)](PF6)(3) have a single visible light absorber subunit coupled to the cis-(RhCl2)-Cl-III moiety, an unexplored molecular architecture. The electrochemistry of [(bpy)(2)Ru(dpp)RhCl2(phen)](PF6)(3) showed a reversible oxidation at 1.61 V (vs Ag/AgCl) (Ru-III/II), quasi-reversible reductions at -0.39 V, -0.74, and -0.98 V. The first two reductive couples corresponded to two electrons, consistent with Rh reduction. The electrochemistry of [(bpy)(2)Ru(bpm)RhCl2(phen)](PF6)(3) exhibited a reversible oxidation at 1.76 V (Ru-III/II). A reversible reduction at -0.14 V (bpm(0/-)), and quasi-reversible reductions at -0.77 and -0.91 V each corresponded to a one electron process, bpm(0/-), Rh-III/II, and Rh-II/I. The dpp bridged bimetallic and trimetallic display Ru(d pi)-> dpp(pi*) metal-to-ligand charge transfer (MLCT) transitions at 509 nm (14,700 M-1 cm(-1)) and 518 nm (26,100 M-1 cm(-1)), respectively. The bpm bridged bimetallic and trimetallic display Ru(d pi)-> bpm(pi*) charge transfer (CT) transitions at 581 nm (4,000 M-1 cm-1) and 594 nm (9,900 M-1 cm(-1)), respectively. The heteronuclear complexes [(bPY)(2)Ru(dpp)RhCl2(phen)](PF6)(3) and [{(bpy)(2)Ru(dpp)}(2)RhCl2](PF6)(5) had (MLCT)-M-3 emissions that are Ru(d pi)-> dpp(pi*) CT in nature but were redshifted and lower intensity than [(bpy)(2)Ru(dpp)Ru(bpy)(2)](PF6)(4). The lifetimes of the (MLCT)-M-3 state of [(bpy)(2)Ru(dpp)RhCl2(phen)](PF6)(3) at room temperature (30 ns) was shorter than [(bpy)(2)Ru(dpp)Ru(bpy)(2)](PF6)(4), consistent with favorable electron transfer to Rh(III) to generate a metal-to-metal charge-transfer ((MMCT)-M-3) state. The reported synthetic methods provide means to a new molecular architecture coupling a single Ru light absorber to the Rh(III) center while retaining the interesting cis-RhIIICl2 moiety.