Synthesis of Heterometallic Bismuth/Molydenum Alkoxides and Their Behavior on Silica Surfaces^§

The reaction of [(C3H5)Mo(CO)2(CH3CN)2Cl], 2, with [Bi(OCH2CH2OCH3)3]2 on a large scale leads to the novel molybdenum/bismuth alkoxide [(C3H5)Mo(CO)2(μ-κO,2κO‘-OCH2CH2OCH3)2(μ-κO-OCH2CH2OCH3)BiCl], 6, as the main product as well as to [(C3H5)Mo(CO)2(μ-κO,2κO‘-OCH2CH2OCH3)2(μ-Cl)BiCl], 4, as a byproduct. Both compounds were characterized by elemental analysis, IR, and NMR spectroscopy as well as by X-ray diffraction. If 6 is brought into contact with a large excess of silica gel, aggregation and condensation reactions are initiated, which led to clusters of ca. 200 nm size spread over the silica surface. When the resulting material is calcinated at 350 °C in the presence of O2, all organic ligands are eliminated and the metal oxo units rearrange: SEM/EDX measurements showed afterward Mo-free bismuth oxo clusters with sizes between 30 and 1000 nm, which are distributed together with molybdenum oxo particles of lower nuclearity over the silica surface. If such a material is employed as a potential catalyst for the propene oxidation under technical conditions, no activity is observed. If, however, the process is performed under very low pressures, a conversion of 5% is found. This result is discussed in the context of the mechanism proposed for the technical oxidation of propene to acrolein on bismuthmolybdate catalysts.将化合物2 [(C₃H₅)Mo(CO)₂(CH₃CN)₂Cl]与二聚体[Bi(OCH₂CH₂OCH₃)₃]₂进行大规模反应，可得到新型钼/铋醇盐化合物6 [(C₃H₅)Mo(CO)₂(μ-κO,2κO’-OCH₂CH₂OCH₃)₂(μ-κO-OCH₂CH₂OCH₃)BiCl]作为主产物，同时副产物为化合物4 [(C₃H₅)Mo(CO)₂(μ-κO,2κO’-OCH₂CH₂OCH₃)₂(μ-Cl)BiCl]。两种化合物均通过元素分析、红外光谱（IR）、核磁共振（NMR）波谱以及X射线衍射完成结构表征。将化合物6与大量过量硅胶接触后，会引发聚集与缩合反应，最终在硅胶表面形成粒径约200 nm的团簇。将所得材料在氧气氛围下于350 ℃煅烧后，所有有机配体均被脱除，金属氧单元发生重排：扫描电子显微镜/能量色散X射线光谱（SEM/EDX）表征结果显示，此时硅胶表面分布着粒径介于30~1000 nm之间的无钼铋氧团簇，以及核数更低的钼氧颗粒。若将该材料作为丙烯氧化的潜在催化剂在工业条件下测试，则未观测到催化活性；但当反应在极低压力下进行时，丙烯转化率可达5%。该结果可结合工业上以钼酸铋为催化剂、将丙烯氧化为丙烯醛的反应机理展开讨论。