Molecular Design of Asymmetric Metal Dinuclear Complexes towards Molecular Switching of Intramolecular Electron Transfer

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Supramolecular assemblies composing of oligonuclear metal complexes with bridging ligands can be considered to have a potential as a building block for synthesizing a molecular level chip such as molecular size "wires, " and "rectifiers". We focus our attention on the "molecular switching" by proton transfer in metal complexes, since the proton transfer can be regarded as a trigger signal to change the metal-metal interaction or the structure of the complex. In order to get the basic information for the design of molecular-level devices such as a molecular switch, new mononuclear and hetero-dinuclear complexes, [M(L)_2(bpbimH_2)]^<2+> and [M(L^1)_2(bpbimH_2)M'(L^2)_2]^<4+> (M and M' = Ru, Rh, and/or Os ; L^1 or L^2 = 2, 2'-bipyridine (bpy), 1, 10-phenanthroline (phen), or 4.4'-dimethylbipyridine (dmbpy)), containing dinucleating ligand 2, 2'-bis- (2-pyridyl)-bibenzimidazole (bpbimH_2) have been prepared. Both absorption spectra and oxidation potential are strongly dependent on the solu … More tion pH, which is responsible for the N-H deprotonation on the coordinated bpbimH_2 ligand. The mononuclear Ru and Os complexes act not only as a basic acid but also as a diacidic base, while hetero-dinuclear complexes essentially act as a dibasic acid. The proton-tcoupled electron transfer reaction is proved by E_<1/2> - pH plot (Pourbaix diagram). The number of electron participated in the proton- coupled electron transfer reaction essentially depends on the redox activity on the second metal site. The pK_a values of the complexes reflect on both metal and its oxidation states, M (II) and M (III). The pK_a values of the mixed-valence dinuclear complexes are quite close to those of each M (II) and M (III) component, which suggests that the mixed-valence complexes have a localized valence structure. The mixed-valence complexes bridged by the protonated ligand bpbimH_2 exhibit the intervalence (IT) band at 7300 cm^<-1> for M = Ru, and at 9100 cm^<-1> for M = Os, respectively. When the bridging ligand is deprotonated, this IT band is shifted to the lower energy at 5880 cm^<-1> for M = Ru, and 7700 cm^<-1> for M = Os, and intensified. The degree of metal-metal interaction of the deprotonated dinuclear complexes becomes 4-6 times larger than that of the protonated complexes. This proton-induced change of metal-metal interaction can be rationalized by change of HOMO energy levels on deprotonation or protonation in the bridging ligands. Thus, the proton transfer in the bpbimH_2 bridging dinuclear complexes can be elucidated to serve as a trigger signal for switching the metal-metal minteraction. Less