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Cationic, phosphine-ligated Pt−Au cluster compounds, [Pt(AuPPh3)8](NO3)2 (1) and [(PPh3)Pt(AuPPh3)6](NO3)2 (2), have been immobilized on silica and alumina supports. Characterization of the supported clusters by 31P MAS NMR, IR, and UV−visible spectroscopy and study by chemisorption and temperature-programmed desorption of CO show that the clusters are immobilized intact without measurable fragmentation or irreversible transformation. This is confirmed by quantitative desorption of the clusters. The reactivity of the supported clusters with CO and H2 was found to be similar to the known reactivities in solution and molecular solid phases. The cluster [(PPh3)Pt(H)(AuPPh3)7](NO3)2 (3) was also immobilized on silica and found to be intact by MAS 31P NMR and desorption experiments. The turnover rate (TOR) for H2−D2 equilibration was determined for these supported clusters at room temperature. The clusters 1 and 2 were good catalysts on silica and alumina supports after treatment at 135 °C under vacuum (TOR = 10−20 s-1), but showed significant activation by treatment at 110 °C under H2 (TOR = 85−220 s-1). Cluster 3/SiO2 gave a similar rate (TOR = 170 s-1) with no thermal activation. All samples subjected to the thermolysis conditions given above were found to be intact by 31P MAS NMR and could be quantitatively desorbed. The high activities of the supported Pt−Au clusters are believed to result from support-promoted, partial PPh3 dissociation. Desorption experiments show that this dissociation is reversible.

The title cluster ion can be prepared by the reaction of Hz with Au(PPh,)+ and Pt(PPh,), as well as by the reaction of Pt-(AuPPh3)82+ with PPh3 and an acid. Mossbauer spectra show that the central atom is Pt. The structure was determined by X-ray diffraction of P~(H)(PP~,)(AUPP~~)~(PF~)~.~/~CH~CI~ (triclinic, Pi, a = 17.085 (6) A, b = 17.301 (8) A, c = 29.401 (12) A, a = 94.62 (IO)', 0 = 96.26 (S)', y = 118.24 ( 5 ) O , V = 7523.2 A', Z = 2, R = 0.085 and R , = 0.1 15 for 15 921 unique reflectionsand 596 variables, Cu K a radiation). In the metal cluster the central Pt is surrounded by seven Au atoms and one PPh3 in a distorted cubic geometry, each Au being attached to a PPh,. The H is close to Pt and at least two Au atoms, as can be concluded from deformations in the metal frame and from Orpen calculations. Detailed ,'P, 19sPt, and IH NMR spectra are presented. The different Au-PPh, sites, present in the crystal, are equilibrated in solution at room temperature.

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