Formation of C-C bonds in diiron complexes by carbanions addition to alkynyl(methoxy)carbene ligands

Addition of cyanide ions to the alkynyl(methoxy)carbene complexes [Fe2{μ-CN(Me)(R)}(μ-CO)(CO){Cα(OMe)CβCγR}- (Cp)2]+ (R = Xyl, R = Tol, 1a; R = Xyl, R = Ph, 1b; R = Xyl, R = Me3Si, 1c; R = Me, R = Tol, 1d; R = Me, R = Ph, 1e) occurs selectively at Cα to afford the 1,1-disubstituted σ-propargyl complexes [Fe2{μ-CN(Me)(R)}(μ-CO)(CO){C(OMe)- (CN)(CCR)}(Cp)2] (R = Xyl, R = Tol, 2a; R = Xyl, R = Ph, 2b; R = Xyl, R = Me3Si, 2c; R = Me, R = Tol, 2d; R = Me, R = Ph, 2e). Conversely, the stabilised carbanaions [CH(R)2]– (R = CN, CO2Me) add at the Cγ position with subsequent hydrogen migration to Cβ to give the 1-σ-buta-1,3- dienyl complexes [Fe2{μ-CN(Me)(Xyl)}(μ-CO)(CO){Cα- (OMe)=CβHCγ(Tol)=Cδ(R)2}(Cp)2] (R = CN, 3a; CO2Me, 3b). No migration is possible in the absence of hydrogen atoms at Cδ, therefore addition of [C(Me)(CO2Me)2]– to 1a results in the formation of the σ-allenyl complex [Fe2{μ-CN(Me)-(Xyl)}(μ-CO)(CO){Cα(OMe)=Cβ=Cγ(Tol)Cδ(Me)(CO2Me)2}- (Cp)2] (4). Protonation of the neutral complexes 3b and 4 affords the vinyl(methoxy)carbene complexes [Fe2{μ-CN- (Me)(Xyl)}(μ-CO)(CO){Cα(OMe)CβH=Cγ(Tol)Cδ(R)(CO2- Me)2}]+ (R = H, 5; Me, 6), which exist in solution as mixtures of (E)- and (Z)-isomers in dynamic equilibrium, as shown by VT NMR studies. The cationic complex 6 shows electrophilic behaviour. Thus, addition of CN– results in the σ-allyl complex [Fe2{μ-CN(Me)(Xyl)}(μ-CO)(CO){Cα(OMe)(CN)- [CβH=Cγ(Tol)Cδ(Me)(CO2Me)2]}(Cp)2] (7), whereas the reaction with Me2NH affords the vinyl acyl complex [Fe2{μ- CN(Me)(Xyl)}(μ-CO)(CO){Cα(O)CβH=Cγ(Tol)Cδ(Me)(CO2- Me)2}(Cp)2] (8). The crystal structures of 2b and 3a·CH2Cl2 have been determined.