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 ...