Synthesis and characterization of a series of diphosphine ligated Pt-Au cluster compounds

A series of diphosphine {Ph₂P(CH₂)_nPPh₂, n = 2-4, abbreviated P-n-P} ligated platinum-gold cluster compounds has been synthesized in high yield by ligand exchange reactions with PPh₃ ligated, hydrido Pt-Au clusters. The exchange reaction between [(H)(PPh₃)Pt(AuPPh₃)₇]²⁺ and the P-2-P and P-3-P ligands gives the clusters [(PPh₃)PtAu₈(P-n-P)₄]²⁺ (1, 2), and with P-4-P the cluster [(P-4-P) PtAu₆(P-4-P)₃]²⁺ (3) is formed. The higher nuclearity cluster [PtAU₁₀(P-3-P)₅]²⁺ (8) was prepared by reaction of [(H)Pt(AuPPh₃)₉]²⁺ with P-3-P. All of these clusters have the 18-electron configuration and thus are unreactive with small electron donor molecules such as CO, Hg and H₂. Clusters 2,3 and 8 react with the Lewis acids MX (MX = CuCl and AgNO₃) giving tri-metallic compounds of higher nuclearity. The products of the MX addition to 1 and 2 are the four new clusters [(PPh₃)Pt(MCl)Au₈(P-n-P)₄]²⁺ where n = 2 and 3 and M = Cu and Ag (4-7). The reaction of MX with 8 gives only the double addition products [Pt(MCl)₂Au₁₀(P-3-P)₅]²⁺ (9, 10). The molecular structures of clusters 2, 3, 5, 7 and 8 were determined by single crystal X-ray diffraction (Mo Kα, λ = 0.71037 Å, 173 K; crystal data for 2: P1̄, a= 17.1484(4), b = 20.8146(5), c = 23.1098(5), α = 71.284(1) Å, β = 70.717(1), γ = 71.632(1)°, V = 7169.6(3) ų, Z = 2, R = 0.10; 3: P1̄, a = 14.833(1), b = 15.905(2), c = 27.068(3) Å, α = 81.175(7), β = 77.088(12), γ = 67.742(6)°, V = 5743.7(10) ų,Z = 2, R = 0.051; 5: P2₁/c, a = 24.353(3), b = 26.326(3), c = 20.554(2) Å, β = 108.082(6)°, V = 12526(2) ų, Z = 4, R = 0.067; 7: P1̄, a = 17.4401(3), b = 18.4505(2), c = 23.1111(3) Å, α = 97.615(1), β = 104.731(1), γ = 104.275(1)°, V = 6817.3(2) ų, Z = 2, R = 0.087; 8: P1̄, a = 16.048(3), b = 16.015(3), c = 30.790(5) Å, α = 83.408(12), β = 89.677(12), γ = 65.135(13)°, V = 7124(2) ų Z = 2, R = 0.096). All have Pt-centered, icosahedral fragment geometries for their PtMAu₈ metal frames with the diphosphine ligands bridging adjacent Au atoms and also chelated to Pt in 3. The diphosphine ligated clusters are less labile and less fluxional than their PPh₃ analogs due to constraints on the number of skeletal rearrangement pathways imposed by the bridging diphosphine ligands.