MODELS FOR OXIDE INTERACTIONS IN BIMETALLIC CATALYSTS - OXO CLUSTERS BY STEPWISE OXIDATION OF A PT3RE CLUSTER

Oxidation of the complex [Pt-3{Re(CO)(3)}(mu-dppm)(3)](+) (1, dppm = P h(2)PCH(2)PPh(2)) with the oxidants Me(3)NO, PhIO, O-2, or H2O2 under mild conditions gives the series of oxo clusters [Pt-3{Re(CO)(3)}(mu(3 )-O)(n)(mu-dppm)(3)](+) (2, 1; 3, n = 2; 4, n = 3). In this series, th e cluster electron count increases from 54 to 58, 62, and 66 in 1, 2, 3, and 4, respectively; there is a corresponding decrease in metal-met al bonding in this series and 4 contains no metal-metal bonding. The c luster 3 has been characterized by an X-ray structure determination an d the other complexes by spectroscopic methods. Crystals of 3[PF6]. OE t(2) are monoclinic, space group P2(1)/n, a = 17.1603(18) Angstrom, b = 23.2822(17) Angstrom, c = 19.7021(9) Angstrom, beta = 94.160(6)degre es; R = 0.047, R(w) = 0.042 for 11822 unique reflections with I greate r than or equal to 2 sigma(I) and theta(Mo K alpha) less than or equal to 30 degrees. The P-31 NMR spectra are shown to reflect the extent o f Pt-Pt bonding. Oxidation of 4 by and H2O2 or of 1 or 3 at high tempe rature by O-2 can also give [Pt-3(ReO3)(mu(3)-O)(3)(mu-dppm)(3)](+) (5 ) and [Pt-3(ReO3)(mu-dppm)(3)](+) (6), respectively, the first example s of a metal-metal bonded cluster containing metals in widely differen t oxidation states. Cluster 6 is also prepared by reaction of [Pt-3(mu (3)-H)(mu-dppm)(3)](+) with MeReO(3). The cluster 3 reacts with donor ligands to give [Pt-3{Re(CO)(2)L}(mu(3)-O)(2)(mu-dppm)(3)](+) (7). The nature of the bonding in clusters 1-6 is discussed on the basis of XP S spectra; the oxidation of 1 to 4 causes very little change in the Re 4f(7/2) binding energy but an increase in the Pt 4f(7/2) binding ener gy whereas there is a large increase in Re 4f(7/2) binding energy on o xidation to 5 and 6. EHMO calculations indicate that the ReO3+ group i s a stronger acceptor of electron density from the Pt-3(mu-dppm) group than is Re(CO)(3)(+) and support an analogy between the donors C5H5- and Pt-3(mu-dppm)(3). The relevance of these clusters as models of bim etallic Pt/Re/Al2O3 catalysts is discussed.