Synthesis and Structural/Theoretical (DFT) Analysis of the Geometrically Unprecedented Au–Pt Cluster, Pt₇(μ ₂-CO)₈(PPh₃)₄(μ ₄-AuPPh₃)₂: Structure-to-Synthesis Appraisal Concerning Its Formation

Abstract

In ongoing attempts to synthesize nanosized gold-platinum carbonyl phosphine clusters either directly or indirectly by initially obtaining intermediate-sized ones as potential precursors, the neutral Au–Pt CO/PR₃-ligated cluster, Pt₇(μ ₂-CO)₈ (PPh₃)₄(μ ₄-AuPPh₃)₂ (1), was isolated and characterized by low-temperature CCD X-ray diffraction, IR, and mass spectrometric measurements. The heretofore unknown 9-atom metal architecture of the Au₂Pt₇ core may be envisioned as a Pt₇ adduct of a Pt₄ butterfly and an edge-opened Pt₃ triangle that is additionally linked by two tetracapping AuPPh₃ units. Each of the two butterfly-hinged (basal) Pt atoms is connected to two of the three edge-opened triangular Pt atoms thereby giving rise to an essentially coplanar Pt₅ fragment consisting of three edge-fused bonding triangles; addition of two AuPPh₃ units to the Pt₇ adduct completes the Au₂Pt₇ framework via formation of the two symmetry-equivalent square-pyramidal (μ ₄-Au)Pt₄ moieties, each resulting from the tetracapping of a AuPPh₃ unit to the central Pt₃ triangle of the coplanar Pt₅ fragment and to one of the two out-of-plane wingtip Pt atoms. Six PPh₃ ligands are coordinated to the two Au atoms, the two wingtip Pt atoms of the Pt₄butterfly-shaped fragment, and the two outer Pt atoms of the edge-opened Pt₃ triangle. Four of the eight doubly bridging μ ₂-COs link the four edges connecting the two out-of-plane wingtip Pt atoms with the two hinged (basal) Pt atoms, while the other four μ ₂-COs span four of the five Pt–Pt bonding edges of the coplanar Pt₅ fragment (i.e., the fifth Pt–Pt bonding edge is spanned by the two out-of-plane wingtip Pt atoms). The resulting molecular configuration (without P-attached phenyl substituents) possesses crystallographic C₂ (2) and pseudo-C_2v symmetry. An optimized geometry of the PH₃- and PMe₃-models of 1, obtained from gradient-corrected (scalar-relativistic) DFT calculations is in reasonably good agreement with the crystallographically determined geometry of 1.A structure-to-synthesis approach is presented as part of extensive but unsuccessful efforts to design a high-yield reproducible preparative route to 1 in order to enable physical/chemical studies as well as to utilize it as a preformed cluster for further PPh₃/CO deligation reactions.