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 clu sters in the nanosize regime (containing tens to hundreds of metal atoms) w ill lead to better control of their size, shape, composition, and structure , and hence their physical and chemical properties. Ultimately, such inform ation will result in a better understanding of, for example, alloy formatio n and site preference in multimetallic systems or phases and reactivities a nd selectivities of multimetallic catalysts as well as lead to new material s of technological importance. Our work in this area gave rise to the synth esis and structural characterization of a series of nanosized Au-Ag cluster s whose metal frameworks can be described as vertex-sharing polyicosahedra. We refer to these high-nuclearity mixed-metal clusters as 'clusters of clu sters'. This 'cluster of clusters' growth pathway follows a well-defined gr owth sequence, from a single icosahedron with 13 atoms (s(1)(13)) to an ico sahedron of 13 icosahedra with 127 atoms (s(13)(127)). Using monodentate ph osphine ligands such as triphenylphosphine, the most successful synthetic r oute to the polyicosahedral clusters (s(n)(N)) is based on a spontaneous bu t stepwise agglomeration of icosahedral cluster units via a progressive red uction. Beyond four icosahedral units, however, it becomes increasingly dif ficult to crystallize and chararacterize the compound. In this paper, the d esign and synthesis of a new series of macrocyclic polyphosphine ligands, c ontaining 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 cav ity sizes ranging from 10 to 30 Angstrom in diameter, can be used to stabil ize or encapsulate the larger vertex-sharing polyicosahedral mixed-metal (e .g. Au-Ag and Au-Ag-Pt) nanoclusters. Conversely, specifically designed mac rocyclic multidentate polyphosphine ligands can be used as templates for th e 'growth' of polyicosahedral metal cluster of particular size and shape. T he suitability of the DBP ligands in large metal cluster synthesis was illu strated by our successful synthesis and structural characterization of a nu mber of Au-Ag nanoclusters based on phenyldibenzophosphole (PDBP) ligand. W hile these macrocyclic polyphosphine ligands were originally designed for v ertex-sharing polyicosahedral metal clusters, they can also be used to synt hesize other polyhedral metal nanoclusters of various sizes and shapes. (C) 2001 Elsevier Science B.V. All rights reserved.