H⁺/AuPPh₃⁺ Exchange for the Hydride Complexes CpMoH(CO)₂(L) (L = PMe₃, PPh₃, CO). Formation and Structure of [Cp(CO)₂(PMe₃)Mo(AuPPh₃)₂]⁺[BF₄]^-

The reaction of CpMoH(CO)₂L with AuPPh₃⁺BF₄^- in THF at −40 °C proceeds directly to the MoAu₂ cluster compounds [CpMo(CO)₂L(AuPPh₃)₂]⁺BF₄^- (L = PMe₃ (1), PPh₃ (2)) with release of protons. A 1:1 reaction leaves 50% of the starting hydride unreacted. At lower temperature, however, the formation of a [CpMo(CO)₂(PMe₃)(μ-H)(AuPPh₃)]⁺ intermediate is observed. This compound evolves to the cation of 1 and CpMoH(CO)₂(PMe₃) upon warming and is deprotonated by 2,6-lutidine to afford CpMo(CO)₂(PMe₃)(AuPPh₃). The X-ray structure of 1 can be described as a four-legged piano stool with the PMe₃ and the “η²-(AuPPh₃)₂” ligands occupying relative trans positions. [Cp(CO)₂(PMe₃)Mo(AuPPh₃)₂]⁺[BF₄]^- (M_r = 1298.41):  monoclinic, space group P2₁/n, a = 18.1457(13) Å, b = 9.7811(7) Å, c = 26.096(2) Å, β = 105.086(5)°, V = 4472.0(5) ų, Z = 4. The reaction of CpMoH(CO)₂(PMe₃) with 3 equiv of AuPPh₃⁺ affords a MoAu₃ cluster, [CpMo(CO)₂(PMe₃)(AuPPh₃)₃]²⁺ (3), in good yields under kinetically controlled conditions. Under thermodynamically controlled conditions, 3 dissociates extensively into 1 and free AuPPh₃⁺. It is proposed that the hydride ligand helps build higher nuclearity Mo−Au clusters. The difference in reaction pathways for the interaction of AuPPh₃⁺ with CpMoH(CO)₂L when L = PR₃ or CO and for the interaction of CpMoH(CO)₂(PMe₃) with E⁺ when E = H, Ph₃C or AuPPh₃ is discussed. The lower acidity and greater aurophilicity of the [CpMo(CO)₂L(μ-H)(AuPPh₃)]⁺ intermediate when L = PMe₃ favor attack by AuPPh₃⁺ before deprotonation.