Formation and Characterization of the Hexanuclear Platinum Cluster [Pt_6(mu-PBu~t_2)_4(CO)_6](CF_3SO_3)_2 through Structural,Electrochemical,and Computational Analyses

摘要

The reaction between equimolar amounts of Pt_3(mu-PBu_2~t)_3(H)(CO)_2,Pt_3H,and CF_3SO_3H under CO atmosphere affords the triangular species [Pt_3(mu-PBu_2~t)_3(CO)_3]X,[Pt_3(CO)_3~+]X(X = CF_3SO_3~-),characterized by X-ray crystallography,or in an excess of acid,[Pt_6(mu-PBu_2~t)_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(M-PBu'2)_4(CO)_6]~+(Pt_6~+,E_(1/2)= -0.27 V)and Pt_6(mu-PBu_2~t)_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 Cp_2Co,respectively;further reduction of Pt_6~(2+)causes fast decomposition.The chloride derivatives [Pt_6(mu-PBu_2~t)_4(CO)_5CI]X,(Pt_6CI~+)X,and Pt_6(mu-PBu_2~t)_4(CO)_4Cl_2,Pt_6CI_2,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.