Rocha Reginaldo C, Brown Mac G, Londergan Casey H, Salsman J Catherine, Kubiak Clifford P, Shreve Andrew P
Bioscience Division, MS G755, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
J Phys Chem A. 2005 Oct 13;109(40):9006-12. doi: 10.1021/jp051482+.
Resonance Raman spectroelectrochemistry (RR-SEC) at -20 degrees C has been performed on the pyrazine-bridged dimer of mu-oxo-centered trinuclear ruthenium-acetate "clusters"--[(dmap)(CO)(mu-OAc)6(mu3-O)Ru3(mu-L(b))Ru3(mu3-O)(mu-OAc)6(CO)(dmap)]n (where dmap = 4-(dimethylamino)pyridine and L(b) = pyrazine-h4 and pyrazine-d4)-in three oxidation states: n = 0, -1, and -2. In the one-electron reduced, "mixed-valent" state (overall -1 charge and a single odd electron; formal oxidation states [II, II, III]-[III, III, II] on the metal centers), the Raman excitation at 800 nm is resonant with a cluster-to-cluster intervalence charge-transfer (IVCT) band. Under these conditions, scattering enhancement is observed for all four totally symmetric vibrational modes of the bridging pyrazine ligand (nu8a, nu9a, nu1, and nu6a) in the investigated spectral range (100-2000 cm(-1)), and there is no evidence of activity in non-totally symmetric vibrations. Resonantly enhanced Raman peaks related to peripheral pyridyl (dmap) ligand modes and low-frequency features arising from the trigonal Ru3O cluster core and the cluster[Ru]-[N]ligand vibrations were also observed in the spectra of the intermediate-valence (n = -1) cluster dimer. The vibrational assignments and interpretations proposed in this work were reinforced by observation of characteristic isotopic frequency shifts accompanying deuteration of the bridging pyrazine. The results reveal that the fully symmetric (A(g)) vibrational motions of the organic bridge are coupled to the nominally metal cluster-to-metal cluster fast intramolecular electron transfer (ET) and provide validation of the near-delocalized description according to a predicted three-site/three-state (e.g., metal-bridge-metal) vibronic coupling model, in which the important role of the bridging ligand in mediating electronic communication and delocalization between charge centers is explicitly considered. Further compelling evidence supporting an extended five-state model, which incorporates the peripheral cluster-bound pyridyl ligands, is also presented.
在-20℃下，对以μ-氧为中心的三核醋酸钌“簇”的吡嗪桥连二聚体——[(dmap)(CO)(μ-OAc)6(μ3-O)Ru3(μ-L(b))Ru3(μ3-O)(μ-OAc)6(CO)(dmap)]n（其中dmap = 4-(二甲氨基)吡啶，L(b) = 吡嗪-h4和吡嗪-d4）——的三种氧化态（n = 0、-1和-2）进行了共振拉曼光谱电化学（RR-SEC）研究。在单电子还原的“混合价”态（总电荷为-1且有一个单奇电子；金属中心的形式氧化态为[II, II, III]-[III, III, II]）下，800 nm处的拉曼激发与簇间价态电荷转移（IVCT）带共振。在这些条件下，在所研究的光谱范围（100 - 2000 cm(-1)）内，观察到桥连吡嗪配体的所有四种全对称振动模式（ν8a、ν9a、ν1和ν6a）的散射增强，并且没有非全对称振动活性的证据。在中间价态（n = -1）簇二聚体的光谱中还观察到了与外围吡啶基（dmap）配体模式相关的共振增强拉曼峰以及由三角Ru3O簇核心和簇[Ru]-[N]配体振动产生的低频特征。这项工作中提出的振动归属和解释通过观察桥连吡嗪氘代伴随的特征同位素频率位移得到了加强。结果表明，有机桥的全对称（A(g)）振动运动与名义上的金属簇到金属簇的快速分子内电子转移（ET）耦合，并根据预测的三位点/三态（例如，金属-桥-金属）振动耦合模型验证了近离域描述，其中明确考虑了桥连配体在介导电荷中心之间的电子通信和离域中的重要作用。还提供了支持扩展五态模型的进一步有力证据，该模型纳入了外围簇结合的吡啶基配体。
