Further Studies of Cluster-Bound Imido Ligands. Imido—Acyl Coupling and Promotion of the Formation and Carbonylation of Imido Ligands by Halides

Halides, cyanide, and hydride ligands have been shown to promote the formation of imido ligands from nitrosobenzene as evidenced by the rapid reaction which occurs to form the cluster anions [Ru₃(μ₃-NPh)(X)(CO)₉]⁻ (X = Cl, Br, I, CN, H) when nitrosobenzene is added to solutions of [Ru₃(X)(CO)_n]⁻(n = 10, 11). The halide and cyanide derivatives also result from addition of the appropriate [(Ph₃P)₂N]X salt to the imido cluster Ru₃(μ₃-NPh)(CO)₁₀. The salt [Na(18-crown-6)][Ru₃(μ₃-NPh)(I)(CO)₉] has been structurally characterized: P1*, a = 10.369 (3), b = 13.335 (3), c = 13.738 (3) Å, α = 79.54 (2), β = 77.03 (2), γ = 86.76 (2)°, V = 1820.1 (6) ų, Z = 2, R(F) = 4.55%, R(wF) = 5.20% for 5663 reflections (4σ(F_o)). Its structure is similar to that of the parent cluster Ru₃(μ₃-NPh)(CO)₉(μ₃-CO), with the iodide having replaced a CO in a position trans to the μ₃-CO ligand. The anion promotion of the nitrosobenzene deoxygenation reaction is believed to result from the anions promoting loss of CO and formation of an intermediate with a coordinated nitrosobenzene which quickly deoxygenates to form CO₂ and the imido ligand. Support for this suggestion comes from the observation that the cluster Ru₃(μ₃-NPh)(CO)₁₀ rapidly forms in high yield upon addition of PhNO to Ru₃(CO)₁₁(CH₃CN). Similar reactions of PhNO and Bu^tNO with acetonitrile-substituted clusters have been used to prepare Os₃(μ₃-NPh)(CO)₁₀, the new heteronuclear clusters Fe₂Ru(μ₃-NPh)(CO)₁₀ and FeRu₂(μ₃-NPh)(CO)₁₀, and the mixed-substituent bis(imido) cluster Ru₃(μ₃-NPh)(μ₃-NBu^t)(CO)₉. The heteronuclear cluster anion [CoRu₂(μ₃-NPh)(CO)₉]⁻ has also been prepared by a metal-exchange reaction via addition of [Co(CO)₄]⁻ to Ru₃(μ3-NPh)(CO)₁₀. Protonation of this anion gives the hydride cluster HCoRu₂(μ₃-NPh)(CO)₉ which has been structurally characterized: P1*, a = 8.609 (2), b = 15.547 (4), c = 15.721 (4) Å, α = 85.07 (2), β = 75.97 (2), γ = 79.58 (2)°, V = 2005.8 (9) ų, Z = 4, R(F) = 4.17%, R(wF) = 5.26% for 5172 reflections (3σ(Fo)). The cluster consists of a CoRu₂ triangle with a triply bridging imido ligand and with the hydride bridging the two Ru atoms. Halides have also been found to promote the carbonylation of imido ligands to form isocyanates as illustrated by the rapid reactions which occur to form PhN═C═O when the cluster anions [Ru₃(μ₃-NPh)(X)(CO)₉]⁻ (X = Cl, Br, I) are placed under 1 atm of CO at 22 °C. In contrast, the cluster anions [Ru₃(μ₃-NPh)(X)(CO)₉]⁻ (X = CN, H) do not react with CO under these mild conditions, although the anion [CoRu₂(μ₃-NPh)(CO)₉]⁻ readily carbonylates to form Ru₃(CO)₁₂, [Co(CO)₄]⁻, and [PhNCO]₂ under slightly more forcing conditions. Acyl-substituted clusters [Ru₃(μ₃-NPh)(CO)₉(C{O}R)]⁻ (R = Me, Ph) form upon reacting Ru₃(μ₃-NPh)(CO)₁₀ with the appropriate RLi reagent, and the acyl ligands in these clusters migrate to the imido ligands when placed under CO. The resultant amido clusters have been protonated to form the hydride species HRu₃{μ₂-N(Ph)C(O)R}(CO)₁₀ which undergo reductive elimination of the amide PhNHC{O}R when placed under a CO atmosphere. The possible relevance of these various imido transformations to nitroaromatic carbonylation catalysis is discussed.