Design and synthesis of macrocyclic multidentate polyphosphine ligands with nano-sized cavities for the encapsulation of vertex-sharing polyicosahedral mixed-metal nanoclusters

Elucidation of the stereochemical and bonding principles of mixed-metal clusters in the nanosize regime (containing tens to hundreds of metal atoms) will lead to better control of their size, shape, composition, and structure, and hence their physical and chemical properties. Ultimately, such information will result in a better understanding of, for example, alloy formation and site preference in multimetallic systems or phases and reactivities and selectivities of multimetallic catalysts as well as lead to new materials of technological importance. Our work in this area gave rise to the synthesis and structural characterization of a series of nanosized AuAg clusters whose metal frameworks can be described as vertex-sharing polyicosahedra. We refer to these high-nuclearity mixed-metal clusters as ‘clusters of clusters’. This ‘cluster of clusters’ growth pathway follows a well-defined growth sequence, from a single icosahedron with 13 atoms (s1(13)) to an icosahedron of 13 icosahedra with 127 atoms (s13(127)). Using monodentate phosphine ligands such as triphenylphosphine, the most successful synthetic route to the polyicosahedral clusters (sn(N)) is based on a spontaneous but stepwise agglomeration of icosahedral cluster units via a progressive reduction. Beyond four icosahedral units, however, it becomes increasingly difficult to crystallize and chararacterize the compound. In this paper, the design and synthesis of a new series of macrocyclic polyphosphine ligands, containing multiple units of dibenzophosphole (DBP) as building blocks, are described. By analogy to the guest-host chemistry such as the (crown ether)-(alkali metal) complexes, these large polyphosphine ring systems, with cavity sizes ranging from 10 to 30 Å in diameter, can be used to stabilize or encapsulate the larger vertex-sharing polyicosahedral mixed-metal (e.g. AuAg and AuAgPt) nanoclusters. Conversely, specifically designed macrocyclic multidentate polyphosphine ligands can be used as templates for the ‘growth’ of polyicosahedral metal cluster of particular size and shape. The suitability of the DBP ligands in large metal cluster synthesis was illustrated by our successful synthesis and structural characterization of a number of AuAg nanoclusters based on phenyldibenzophosphole (PDBP) ligand. While these macrocyclic polyphosphine ligands were originally designed for vertex-sharing polyicosahedral metal clusters, they can also be used to synthesize other polyhedral metal nanoclusters of various sizes and shapes.