Formation and characterization of the hexanuclear platinum cluster [Pt-6(mu PBu2t)(4)(CO)(6)](CF3SO3)(2) through structural, electrochemical, and computational analyses

The reaction between equimolar amounts of Pt3(μ-PBut2)3(H)(CO)2, Pt3H, and CF3SO3H under CO atmosphere affords the triangular species [Pt3(μ-PBut2)3(CO)3]X, [Pt3(CO)3+]X (X = CF3SO3-), characterized by X-ray crystallography, or in an excess of acid, [Pt6(μ-PBut2)4(CO)6]X2, [Pt62+]X2. Structural determination shows the latter to be a rare hexanuclear cluster with a Pt4 tetrahedral core formed by joining the unbridged sides of two orthogonal Pt3 triangles. The dication Pt62+ features also extensive redox properties as it undergoes two reversible one-electron reductions to the congeners [Pt6(μ-PBut2)4(CO)6]+ (Pt6+, E1/2 = −0.27 V) and Pt6(μ-PBut2)4(CO)6 (Pt6, E1/2 = −0.54 V) and a further quasi-reversible two-electron reduction to the unstable dianion Pt62- (E1/2 = −1.72 V). The stable radical (Pt6+) and diamagnetic (Pt6) species are also formed via chemical methods by using 1 or 2 equiv of Cp2Co, respectively; further reduction of Pt62+ causes fast decomposition. The chloride derivatives [Pt6(μ-PBut2)4(CO)5Cl]X, (Pt6Cl+)X, and Pt6(μ-PBut2)4(CO)4Cl2, Pt6Cl2, observed as side-products in some electrochemical experiments, were prepared independently. The reaction leading to Pt3(CO)3+ has been analyzed with DFT methods, and identification of key intermediates allows outlining the reaction mechanism. Moreover, calculations for the whole series Pt62+ → Pt62-afford the otherwise unknown structures of the reduced derivatives. While the primary geometry is maintained by increasing electron population, the system undergoes progressive and concerted out-of-plane rotation of the four phosphido bridges (from D2d to D2 symmetry). The bonding at the central Pt4 tetrahedron of the hexanuclear clusters (an example of 4c-2e- inorganic tetrahedral aromaticity in Pt62+) is explained in simple MO terms.