Synthesis of Mixed-Metal Metallocenoid Clusters

混合金属簇引起了极大的关注，因为通过金属-金属键彼此直接连接的不同过渡金属元素的协同效应可以在金属簇的物理性质或反应性方面提供优势。尽管已经做出了很多努力来将不同的金属结合到金属簇骨架中，但是仍然难以实现具有计划组成的三核或更高核的混合金属簇的选择性和有效的合成，其中每种金属周围的配体环境是准等价的。控制金属化学计量的困难，即，M_3、M_2M ′、MM ′_2和M ′_3的形成不仅是热力学的因素，而且是动力学的因素：由于M-M、M-M ′和M ′-M ′键的强度和反应活性不同，M与M ′的结合速率有本质的不同。此外，M-L和M '-L解离/缔合的容易性（其也可能参与三金属核的形成）可影响混合金属核形成的选择性。因此，需要一个精心设计的合成策略来选择性地构建M_2M'和MM'_2核。在本项目中，我们实现了在一个共同的三明治框架中选择性地构建三角形PdPt_2和Pd_2Pt核，其中一个关键是针对不同的复合三明治使用不同的碳环配体;即，对于PdPt_2，环庚三烯基;对于Pd_2Pt，环庚三烯基。在Pd_2Pt核的构建后，环庚三烯配体可以转化为环庚三烯基配体，并保持了（碳环配体）-（金属三角）-（碳环配体）的三明治结构。

The mixed metal clusters have attracted great attention because a synergetic effect of different transition metal elements which are directly connected with each other via metal-metal bonds may give an advantage in physical properties or reactivity of metal clusters. Although much efforts have been made to incorporate different metals in a metal cluster framework, there remains a difficulty to attain a selective and efficient synthesis of tri- or higher nuclear mixed-metal clusters having a planned composition, in which the ligand environment around each metal is quasi-equivalent. The difficulty of controlling the metal stoichiometry, i.e., M_3, M_2M’, MM’_2, and M’_3, may arise not only from the thermodynamic but the kinetic factor : The incorporation rate of M to the triangular trimetal core is inherently different from that of M’, due to the different strength and reactivity of M-M, M-M’, and M’-M’ bonds. Furthermore, the ease of M-L and M’-L dissociation/association which might also be involved in the formation of trimetallic core can affect the selectivity of the mixed-metal core formations. Thus, a well-planned synthetic strategy is needed for the selective construction of M_2M’ and MM’_2 cores. In this project, we realized selective construction of the triangular PdPt_2 and Pd_2Pt cores in a common sandwich framework, where a key is to use different carbocyclic ligands for the different composite sandwich ; i.e., cycloheptatrienyl for PdPt_2, and cycloheptatriene for Pd_2Pt. After construction of the Pd_2Pt core, the cycloheptatriene ligands can be converted to cycloheptatrienyl ligands where a (carbocyclic ligand)-(metal triangle)-(carbocyclic ligand) sandwich structure was retained.