Proton-transfer reactions on hexanuclear platinum clusters: reversible heterolytic cleavage of H2 and C-H activation...

The hexanuclear cluster {Pt(6)}H(2) (2) contains a sterically hindered and chemically stable {Pt(6)} = Pt(6)(mu-PtBu(2))(4)(CO)(4) core, with the six metals forming an edge-bridged tetrahedron. The two hydrides are the reactive sites of the cluster and lie on opposite sides of the cluster, terminally bonded to the two "apical" edge-bridging platinum centres. Indeed, cluster 2 reacts with acids of different acidity (HA = CF(3)SO(3)H, HBF(4), p-CH(3)-C(6)H(4)-SO(3)H, CF(3)COOH, PhCOOH and CH(3)COOH), affording, after evolution of two equivalents of dihydrogen, the corresponding anion-substituted clusters {Pt(6)}A(2) (4). We suggest that the reaction proceeds through a mechanism similar to the one generally accepted for the analogous protonation of mononuclear hydrides, with some of the intermediates partially characterised at low temperature. Interestingly, the reverse reaction, the heterolytic splitting of H(2) by clusters 4, occurs readily under mild conditions. The anions in clusters 4a and 4b (4a: A = CF(3)SO(3), 4b: A = BF(4)) are bonded in the solid state but very easily dissociate in solution and may be substituted under mild conditions by weak ligands, such as CH(2)Cl(2) or CH(3)CN. With dialkyl ethers, the reaction proceeds further with the heterolytic splitting of a C-H bond of the ethereal ligand. This process allowed us to isolate the polymer [{Pt(6)}(CH(2)OCH(2)CH(2)OCH(2))](x) (8), in which the {Pt(6)} cluster units are connected by insulating spacers arising from dimethoxyethane. The results of single-crystal X-ray diffraction studies on 4a and 8 are also reported.