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

由具有桥联配体的寡核金属络合物组成的超分子组装体可以被认为是合成分子水平芯片的潜在构件，如分子尺寸的“导线”和“整流器”。由于质子转移可以被视为改变金属-金属相互作用或络合物结构的触发信号，因此我们将注意力集中在金属络合物中质子转移所产生的“分子开关”上。为了获得分子开关等分子级器件的基本信息，新的单核和异双核配合物[M(L)_2(BpbimH_2)]^&lt；2+&gt；和[M(L^1)_2(BpbimH_2)M‘(L^2)_2]^&lt；4+&gt；(M和M’=Ru，Rh，和/或Os；合成了含二核配体2，2‘-二(2-吡啶)-联苯并咪唑(BpbimH_2)的L^1或L^2=2，2’-联吡啶(Bpy)、1，10-邻菲咯啉(Phen)或4.4‘-二甲基联吡啶(Dmbpy)。吸收光谱和氧化电位都强烈依赖于溶液的…PH值对配位的bpbimH_2配体上的N-H去质子化起主要作用。单核Ru和Os配合物不仅作为碱性酸，而且还作为二酸碱，而异双核配合物本质上是二元酸。用E&lt；1/2&p H图(Pourbai x图)证明了质子耦合的电子转移反应。参与质子耦合电子转移反应的电子数主要取决于第二金属位上的氧化还原活性。络合物的pK_a值反映了金属及其氧化态M(II)和M(III)。混价双核配合物的pK_a值与M(II)和M(III)各组分的pK_a值非常接近，表明混价配合物具有定域价结构。由质子化配体bpbimH_2桥联的混价络合物在M=Ru和M=Os时分别在7300 cm^&lt；-1和9100 cm^&lt；-1&gt；处出现价间(IT)带。当桥联配体去质子化时，该带向较低能量方向移动，M=Ru为5880 cm；-1，M=Os为7700 cm~(-1)。去质子化的双核配合物的金属-金属相互作用程度是质子化的配合物的4-6倍。这种由质子引起的金属-金属相互作用的变化可以通过桥联配体中去质子化或质子化的HOMO能级的改变来解释。因此，bpbimH_2桥联双核配合物中的质子转移可以被解释为金属-金属间相互作用的触发信号。较少

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