Proton-Transfer Reactions on Hexanuclear Platinum Clusters: Reversible Heterolytic Cleavage of H₂ and CH Activation Affording a Linear, Cluster-Containing Polymer

摘要

The hexanuclear cluster {Pt₆}H₂ (2) contains a sterically hindered and chemically stable {Pt₆}=Pt₆(μ-PtBu₂)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=CF₃SO₃H, HBF₄, p-CH₃C₆H₄SO₃H, CF₃COOH, PhCOOH and CH₃COOH), affording, after evolution of two equivalents of dihydrogen, the corresponding anion-substituted clusters {Pt₆}A₂ (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 H₂ by clusters 4, occurs readily under mild conditions. The anions in clusters 4 a and 4 b (4 a: A=CF₃SO₃, 4 b: A=BF₄) are bonded in the solid state but very easily dissociate in solution and may be substituted under mild conditions by weak ligands, such as CH₂Cl₂ or CH₃CN. With dialkyl ethers, the reaction proceeds further with the heterolytic splitting of a CH bond of the ethereal ligand. This process allowed us to isolate the polymer [{Pt₆}(CH₂OCH₂CH₂OCH₂)]_x (8), in which the {Pt₆} cluster units are connected by insulating spacers arising from dimethoxyethane. The results of single-crystal X-ray diffraction studies on 4 a and 8 are also reported.