Synthesis and Oxygen to Iron Methyl Migration Reaction of the Heterodinuclear Methoxycarbyne Complex Cp(CO)Fe(μ-COCH3)(μ-CO)Cr(CO)(η6-C6H6)
journal contributionposted on 1997-09-16, 00:00 authored by Wei Luo, Raymond H. Fong, William H. Hersh
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.