Construction of the Tetrahedral Trifluorophosphine Platinum Cluster Pt4(PF3)8 from Smaller Building Blocks

The experimentally known but structurally uncharacterized Pt-4(PF3)(8) is predicted to have an S-4 structure with a central distorted Pt-4 tetrahedron having four short Pt=Pt distances, two long Pt-Pt distances, and all terminal PF3 groups. The structures of the lower nuclearity species Pt(PF3)(n) (n = 4, 3, 2), Pt-2(PF3)(n) (n = 7, 6, 5, 4), and Pt-3(PF3)(6) were investigated by density functional theory to assess their possible roles as intermediates in the formation of Pt-4(PF3)(8) by the pyrolysis of Pt(PF3)(4). The expected tetrahedral, trigonal planar, and linear structures are found for Pt(PF3)(4), Pt(PF3)(3), and Pt(PF3)(2), respectively. However, the dicoordinate Pt(PF3)(2) structure is bent from the ideal 180 linear structure to approximately 160 degrees. Most of the low-energy binuclear Pt-2(PF3)(n) (n = 7, 6, 5) structures can be derived from the mononuclear Pt(PF3)(n) (n = 4, 3, 2) structures by replacing one of the PF3 groups by a Pt(PF3)(4) or Pt(PF3)(3) ligand. In some of these binuclear structures one of the PF3 groups on the Pt(PF3), ligand becomes a bridging group. The low-energy binuclear structures also include symmetrical [Pt(PF3)(n)](2) dimers (n = 2, 3) of the coordinately unsaturated Pt(PF3) (n = 3, 2). The four low-energy structures for the trinuclear Pt-3(PF3)(6) include two structures with central equilateral Pt-3 triangles and two structures with isosceles Pt-3 triangles and various arrangements of terminal and bridging PF3 groups. Among these four structures the lowest-energy Pt-3(PF3)(6) structure has an unprecedented four-electron donor eta(2)-mu(3)-PF3 group bridging the central Pt-3 triangle through three Pt-P bonds and one Pt-F bond. Thermochemical studies on the aggregation of these Pt-PF3 complexes suggest the tetramerization of Pt(PF3)(2) to Pt-4(PF3)(8) to be highly exothermic regardless of the mechanistic details.