Reaction of H2FeRu3(CO)13 with Tertiary Phosphines and Phosphites. Influence of Ligand Size and Basicity on Substitution Site Preference. Crystal and Molecular Structure of H2FeRu3(CO)12(PMe2Ph)

Reaction of the mixed-metal cluster H₂FeRu₃(CO)₁₃ with a series of tertiary phosphines and phosphites (L = PPh₃, PPh₂Me, PPhMe₂, PPhEt₂, P(OMe)₃, P(OEt)₃, P(OEt)₂Ph, PMe₃, P(i-Pr)₃) leads to the formation of substituted clusters of the form H₂FeRu₃(CO)_13-nL_n (n = 1, 2) in 20-30% yield. Small quantities of trisubstituted clusters have also been isolated from the reactions employing PPh₃, P(OMe)₃, and P(OEt)₃. The infrared spectra of the substituted clusters are similar to that of H₂FeRu₃(CO)₁₃ and indicate that no severe structural changes occur in the cluster upon substitution. Detailed ³¹P and NMR studies show that the monosubstituted clusters can exist in two isomeric forms of C_s and C₁ symmetries. The ratio of these two isomers for a given phosphorus donor ligand is dependent on both the ligand size and basicity. Large ligands give exclusively the C_s isomer. With small ligands, basicity becomes the controlling factor with the more basic ligands preferring the C₁ isomer. Kinetic measurements indicate that the substitution reaction of H₂FeRu₃(CO)₁₃ with PPh₃ proceeds by a first-order, [PPh₃]-independent path, presumably involving CO dissociation from H₂FeRu₃(CO)₁₃ in the rate-determining step, with activation parameters ∆H±* = 25.3 ± 0.9 kcal/mol and ∆S±* = 4.9 ± 3.1 cal/(mol K). The structure of the C, isomer of H₂FeRu₃(CO)₁₂(PMe₂Ph) has been determined by X-ray diffraction. It crystallizes in the space group P1-C_i¹ (No. 2) with a = 9.958 (4) Å, b = 9.970 (3) Å, c = 14.705 Å, a = 87.36 (2)°, β = 104.88 (3)°, and = 103.76 (3)° with Z = 2. The structure was refined to R₁ = 0.034 and R₂ = 0.035 for 5490 independent reflections having I > 3.0σ (I). The overall geometry of H₂FeRu₃(CO)₁₂(PMe₂Ph) is similar to that of the parent H₂FeRu₃(CO)₁₃ cluster, implying that little distortion occurs upon substitution. The hydrides were located and refined in the H₂FeRu₃-(CO)₁₂(PMe₂Ph) structure and bridge two of the three Ru-Ru bonds in accord with the suggestion made earlier for H₂FeRu₃(CO)₁₃ on the basis of bond length arguments. The phosphorus atom is attached to the Ru atom that is also ligated by both hydrides and lies 2.11 A below the Ru₃ plane.