Mixed-metal rhenium-platinum cluster compounds. Synthesis and characterization of three isomers of the triangular cluster complex [Re2Pt(μ-H)2(CO)8(PPh3) 2]

The reaction between [Re2(μ-H)2(CO)8] and [Pt(PPh3)2(C2H4)] at 273 K results in the insertion of the [Pt(PPh3)2] fragment into a Re(μ-H)Re bond, giving the triangular cluster [Re2Pt(μ-H)2(CO)8(PPh3) 2], characterized spectroscopically. On increasing the temperature, an irreversible transformation occurs, due to the exchange of one phosphine and one carbonyl between platinum and rhenium. Two other isomers are formed in this way, characterized by 1H and 31P NMR, which differ in the location of the phosphine bound to rhenium. They equilibrate slowly on the NMR time scale, and variable-temperature experiments allowed the estimation of the thermodynamic parameters of this isomerization: ΔH⊖ = -1.72 ± 0.04 kcal mol-1, ΔS⊖ = -4.56 ± 0.14 cal mol-1 K-1. NMR 2D reverse correlation experiments were performed to detect 195Pt chemical shifts of the three isomers and to measure the passive couplings with 31P not observable in the 1D spectra. The relative signs of these couplings are discussed. One of the isomers has been investigated by X-ray single-crystal analysis. It gives monoclinic crystals, space group P21/c (No. 14), with a = 14.044 (3), b = 19.343 (5), c = 16.391 (3) Å, β = 93.69 (2)°, Z = 4. The refinements, based on 5306 significant observations, gave a final R value of 0.031. It contains a PtRe2 triangle with the Re-Re edge and one of the Pt-Re edges supposed to be bridged by hydride ligands. The Pt atom bears one PPh3 and one CO ligand and exhibits approximately a square-planar coordination. The two Re atoms show octahedral environments, one bearing four CO and the second one three CO and a PPh3 ligand, coordinated in the axial direction with respect to the cluster plane. The hydrogen-bridged Re-Re bond is 3.203 (1) Å long; the two Pt-Re bond lengths are 2.906 (1) and 2.788 (1) Å, for the hydrogen-bridged edge and for the unbridged one, respectively. © 1990 American Chemical Society.