Formation and characterization of the hexanuclear platinum cluster [Pt6(μPBut2)4(CO)6](CF3SO3)2 through structural, electrochemical, and computational analyses

The reaction between equimolar amounts of Pt-3(mu-PBu'(2))3(H)(CO)(2), Pt3H, and CF3SO3H under CO atmosphere affords the triangular species [Pt-3(mu-PBu'(2))(3)(CO)(3)]X, [Pt-3(CO)(3)(+)]X (X = CF3SO3-), characterized by X-ray crystallography, or in an excess of acid, [Pt-6(mu-PBu'(2))(4)(CO)(6)]X-2, [Pt-6(2+)]X-2. Structural determination shows the latter to be a rare hexanuclear cluster with a Pt-4 tetrahedral core formed by joining the unbridged sides of two orthogonal Pt-3 triangles. The dication Pt-6(2+) features also extensive redox properties as it undergoes two reversible one-electron reductions to the congeners [Pt-6(muPBu'(2))(4)(CO)(6)](+) (Pt-6(+), E-1/2 = -0.27 V) and Pt-6(mu-PBu'(2))(4)(CO)(6) (Pt-6, E-1/2 = -0.54 V) and a further quasi-reversible two-electron reduction to the unstable dianion Pt-6(2-) (E-1/2 = -1.72 V). The stable radical (Pt-6(+)) and diamagnetic (Pt-6) species are also formed via chemical methods by using 1 or 2 equiv of Cp2CO, respectively; further reduction of Pt-6(2+) causes fast decomposition. The chloride derivatives [Pt-6(mu-PBu'(2))(4)(CO)(5)Cl]X, (Pt6Cl+)X, and Pt-6(mu-PBu2)(4)(CO)(4)Cl-2, Pt6Cl2, observed as side-products in some electrochemical experiments, were prepared independently. The reaction leading to Pt-3(CO)(3)(+) has been analyzed with DFT methods, and identification of key intermediates allows outlining the reaction mechanism. Moreover, calculations for the whole series Pt-6(2+) --> Pt-6(2-) 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 D-2d to D-2 symmetry). The bonding at the central Pt-4 tetrahedron of the hexanuclear clusters (an example of 4c-2e(-) inorganic tetrahedral aromaticity in Pt-6(2+)) is explained in simple MO terms.