Synthesis and Oxygen to Iron Methyl Migration Reaction of the Heterodinuclear Methoxycarbyne Complex Cp(CO)Fe(μ-COCH₃)(μ-CO)Cr(CO)(η⁶-C₆H₆)

Reaction of CpFe(CO)₂^-+·∼0.5THF and (η⁶-C₆H₆)Cr(CO)₂(CH₃CN) gives the new heterodinuclear anion Cp(CO)Fe(μ-CO)₂Cr(CO)(η⁶-C₆H₆)^-Na⁺ (6-Na⁺) in 77% yield. Metathesis of 6-Na⁺ with PPN⁺Cl^-[PPN⁺ = bis(triphenylphosphine)nitrogen(+1)] gives 6-PPN⁺ in 73% yield. Alkylation of 6-Na⁺ was accomplished with Me₃O⁺BF₄^- to give the new methoxycarbyne complex Cp(CO)Fe(μ-COCH₃)(μ-CO)Cr(CO)(η⁶-C₆H₆) (7) in 29% yield. Cis/trans isomerization of 6-Na⁺ was observed by ¹H and ¹³C NMR to occur remarkably rapidly, with ΔG^⧧(300 K) = 11.5 ± 0.6 kcal/mol in CD₃CN. Isomerization of 7 is slower but still rapid by comparison to related compounds, with ΔG^⧧(300 K) = 14.3 ± 1.8 kcal/mol in toluene-d₈ and 15.2 ± 1.9 kcal/mol in CD₂Cl₂. Thermal decomposition of 7 provides the fourth example of oxygen to metal methyl migration from a methoxycarbyne complex. However, it is the first example in which a methoxycarbyne follows kinetics in which the rate constants are independent of the phosphine concentration, giving k = 8.1 ± 0.6 × 10^-5 s^-1 at 50 °C in C₆D₆ for [PPh₃] = 0 to 0.59 M. In the presence of PPh₃, the products include primarily (C₆H₆)Cr(CO)₃ and CpFe(CO)(PPh₃)CH₃ but also smaller amounts of (C₆H₆)Cr(CO)₂PPh₃ and CpFe(CO)₂CH₃. In the absence of PPh₃, relatively low yields of (C₆H₆)Cr(CO)₃ and CpFe(CO)₂CH₃ form.