Synthesis of Mixed-Metal Metallocenoid Clusters

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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.