RHENIUM-PLATINUM MIXED-METAL CLUSTERS - CHARACTERIZATION IN SOLUTION AND DYNAMIC BEHAVIOR OF THE ISOMERS OF [RE3PT(MU-H)3(CO)14] - AN EXAMPLE OF A LABILE METAL FRAGMENT THAT UNDERGOES INTERMOLECULAR EXCHANGE

The cluster (Re3Pt(mu-H)3(CO)14] (1), which contains a [ReH(CO)5] molecule bound to the Pt atom of a Re2Pt triangular cluster, exists in solution at low temperature as two isomers, with a ratio of ca. 19:1. The values of the H-1-Pt-195 coupling constants and the dynamic behavior suggest that in both the species [ReH(CO)5] is bound to Pt, but with different locations with respect to the H-bridged Re-Pt triangular edge: cis in the major one (1a), as found in the solid state structure, and trans in the minor isomer (1b). H-1 2D EXSY experiments at 180 K showed the occurrence of two dynamic processes: one interconverts the two isomers, while the other one leads to intermolecular exchange of the [ReH(CO)5] fragment. The addition of [ReH-CO)5] caused a dramatic increase of the rate of this exchange even at 180 K and showed that this latter process is operative only in the major species 1a. The isomerization rate, on the contrary, is insensitive to the addition of [ReH(CO),5], indicating its intramolecular nature. The rate constants of the two processes obtained at 180 K allowed the calculation of the activation parameters DELTAG(double dagger)180 = 43.0 kJ/mol and DELTAG(double dagger)180 = 41.9 kJ/mol for the isomerization and for [ReH-CO)5] exchange, respectively. Several indirect thermodynamic and kinetic evidences indicate that above 210 K one or more species, in fast exchange with the two isomers, are present in solution. At 264 K, a H-1 2D EXSY experiment showed the onset of a new exchange process, implying the cleavage of the H-Re bond of the [ReH(CO)5] fragment, likely via oxidative addition of [ReH(CO)5] on Pt.