Photochemical Reduction of Carbon Dioxide Catalyzed by a Ruthenium-Substituted Polyoxometalate
Photochemical Reduction of Carbon Dioxide Catalyzed by a Ruthenium-Substituted Polyoxometalate
复制标题
DOI：
10.1002/chem.200901673
复制
发表时间：
2010-01-01
影响因子：
4.3
通讯作者：
Neumann, Ronny
中科院分区：
文献类型：
--
作者：
Khenkin, Alexander M.;Efremenko, Irena;Neumann, Ronny
用Ru-III取代的Keggin结构的聚氧乙烯酸盐，（6）Q（5）[Ru-III（H2O）SiW 11 O 39]，其中（6）Q=（C6 H 13）（4）N+，催化用叔胺，优选Et 3 N作为还原剂将CO2光还原为CO。CO_2与（6）Q（5）[Ru-III（H_2 O）SiW_（11）O_（39）]配位的研究表明：1）加入CO_2后，UV/维斯光谱发生变化; 2）EPR谱中出现菱形信号（g（x）=2.146，g（y）=2.100，g（z）=1.935）; C-13 NMR谱在105.78ppm处具有加宽的结合CO2峰（Delta（1/2）=122 Hz）。得出的结论是，在存在和不存在Et 3 N的情况下，CO2与Ru-III活性位点配位，得到（6）Q（5）[Ru-III（CO2）SiW 11 O39]。电化学测量表明，在（6）Q（5）[Ru-III-（CO2）SiW 11 O39]中，Ru-III还原为Ru-II，相对于SCE为-0.31 V，但在（6）Q（5）[Ru-III（H2O）SiW 11 O39]中没有观察到这种还原。在M06/PC 1//PBE/AUG-PC 1//PBE/PC 1-DF理论水平上优化的DFT计算的几何结构表明，CO2优选地通过形成Ru-O键以侧边方式与聚氧乙烯酸盐中的Ru-III配位，通过CO2的亲电碳原子与聚氧乙烯酸盐的氧原子的相互作用进一步稳定。末端CO2与Ru-III的键合在能量上是不太有利的，但CO2是相当弯曲的，因此有利于在碳原子上的亲核攻击，从而稳定碳sp（2）杂化状态。O2 C-NMe 3两性键的形成反过来又导致CO2的弯曲，并增强碳sp（2）杂化。这两种相互作用的协同作用稳定了Ru-O和C-N相互作用，并可能决定了胺对[Ru-III（H2O）SiW 11 O39]（5-）活化CO2的促进作用。电子结构分析表明，聚氧乙烯酸盐参与了CO2和Et 3 N的活化。根据实验和计算结果，提出了CO2光还原的机理途径。
A polyoxometalate of the Keggin structure substituted with Ru-III, (6)Q(5)[Ru-III(H2O)SiW11O39] in which (6)Q=(C6H13)(4)N+, catalyzed the photoreduction of CO2 to CO with tertiary amines, preferentially Et3N, as reducing agents. A study of the coordination of CO2 to (6)Q(5)[Ru-III(H2O)SiW11O39] showed that 1) upon addition of CO2 the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (g(x)=2.146, g(y)=2.100, and g(z)=1.935), and 3) the C-13 NMR spectrum had a broadened peak of bound CO2 at 105.78 ppm (Delta(1/2)=122 Hz). It was concluded that CO2 coordinates to the Ru-III active site in both the presence and absence of Et3N to yield (6)Q(5)[Ru-III(CO2)SiW11O39]. Electrochemical measurements showed the reduction of Ru-III to Ru-II in (6)Q(5)[Ru-III-(CO2)SiW11O39] at -0.31 V versus SCE, but no such reduction was observed for (6)Q(5)[Ru-III(H2O)SiW11O39]. DFT-calculated geometries optimized at the M06/PC1//PBE/AUG-PC1//PBE/PC1-DF level of theory showed that CO2 is preferably coordinated in a side-on manner to Ru-III in the polyoxometalate through formation of a Ru-O bond, further stabilized by the interaction of the electrophilic carbon atom of CO2 to an oxygen atom of the polyoxometalate. The end-on CO2 bonding to Ru-III is energetically less favorable but CO2 is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp(2) hybridization state. Formation of a O2C-NMe3 zwitterion, in turn, causes bending of CO2 and enhances the carbon sp(2) hybridization. The synergetic effect of these two interactions stabilizes both Ru-O and C-N interactions and probably determines the promotional effect of an amine on the activation of CO2 by [Ru-III(H2O)SiW11O39](5-). Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO2 and Et3N. A mechanistic pathway for photoreduction of CO2 is suggested based on the experimental and computed results.