Transition metal–cyanocarbon chemistry. Part 8. Cyano-substituted buta-1,3- dienylidene bridged di-iron complexes from unprecedented cyanoalkyne insertion into the carbon–hydrogen bond of a bridging alkenylidene ligand

Abstract

The reactions of [{Fe(cp)(CO)}₂(µ-CO)(µ-CCHR)][cp =µ⁵-C₅H₅; R = H(I), CH(CH₃)₂(3), or CH₃(4)] with monocyanoethyne H–CC–CN at 340 K lead to the substituted µ-4-cyanobuta-1,3- dienylidene complexes [{Fe(cp)(CO)}₂(µ-CO){µ-CCR–CHC(CN)H}][R = H (2) CH(CH₃)₂(5), or CH₃(6)]via an unprecedented formal insertion of the alkyne into a vinylic carbon–hydrogen bond. The reactions are regio- but not stereo-selective: the nitrile is attached to the Cδ carbon and both Z and E isomers (with respect to the C_γC_δ double bond) are formed. Complex (1) reacts at 210 K with dicyanoethyne NC–CC–CN to yield the µ-3,4-dicyanobuta-1,3- dienylidene complex [{Fe(cp)(CO)}₂(µ-CO){p-CCH–C(CN)C(CN)H)](7) similarly, with Z and E isomers again being formed. Reaction of (2–Z) with Ph₂PCH₂PPh₂(dppm) in refluxing toluene gives the triply bridged complex [(Fe(cp)}₂(µ-CO){µ-CCH–CHC(CN) H}(µ-dppm)](8) in both Z and E forms. A possible mechanism of the insertion reaction is discussed.