Synthesis and Structures of (C₅H₅)₂Mo₂Fe_xTe₂(CO)₇(x = 1, 2). Cluster Assembly Mechanisms and the Role of the Tellurium

The compound Cp₂Mo₂FeTe₂(CO)₇ (Cp = η⁵-C₅H₅), 2, is formed in high yield from the reaction of Fe₃Te₂(CO)₉ and Cp₂Mo₂(CO)₆ in hexane under CO. The product was characterized by spectroscopic methods and its structure determined by X-ray crystallography. Compound 2 crystallizes in the P1 space group with a = 13.216 (2) Å, b = 15.962 (3) Å, c = 21.710 (5) Å, α = 100.31 (2)°, β = 104.74 (2)°, γ = 94.20 (1)°, Z = 8, and ρ_calcd = 2.547 g cm^-3. The structure was solved by direct methods. Blocked cascade refinement on 9947 reflections (F_o ≥ 3σF_o) produced the final residuals R_F = 0.0354 and R_WF = 0.0394 The four molecules per asymmetric unit are quite similar, each consisting of a CpMo(CO)₂ fragment bridging the Te wing tips of a Cp(CO)₅MoFeTe₂ butterfly. The 3.13-Å Te⋯Te distance is well within bonding distance and is proposed to be chemically significant. This interaction is discussed in the context of other nonmetal-containing cluster compounds. On the basis of these data, it is proposed that such intracluster nonmetal-nonmetal interactions can have a significant influence on the structures and reactivity of compounds in this class of clusters. The mechanism of formation of 2 is discussed in light of the recently reported compound Co₂Fe₂S₂(CO)₁₁. Thermolysis of 2 affords metallatetrahedrane Cp₂Mo₂FeTe(CO)₇, Cp₂Mo₂Fe₂Te₃(CO)₆, and Cp₂Mo₂Fe₂Te₂(CO)₇, 3. Compound 3 crystallizes in the orthorhombic space group Pn2₁a with a = 13.617 Å, b = 12.939 Å, c = 12.199 Å, Z = 4°, (10)°, and ρ_calcd = 2.73 g cm^-3. The structure was solved by direct methods. Blocked cascade refinement on 1837 reflections (F₀ > 3σF₀) produced R_F = 0.0256 and R_WF = 0.0249. The molecule of approximately C_2v symmetry consists of a tetrahedral Mo₂Fe₂ core with each Mo₂Fe face capped by a μ₃-Te atom. The Fe-Fe distance of 2.433 Å is one of the shortest known single Fe-Fe bonds. The mechanism of formation of 3 from 2 was shown to involve no scrambling of Mo moieties (Cp-labeling experiments) and proceeds optimally (60%) in the presence of 10 equiv of Fe(CO)₅. Compound 2 also reacts with CpCo(CO)₂ to give two isomers of Cp₃Mo₂CoFeTe₂(CO)₅. These results imply that the recently reported isomers of Cp₂Mo₂Fe₂S₂(CO)₈ are formed via a five-vertex Mo₂FeS₂ intermediate.