Stereochemistry of the insertion of disubstituted alkynes into the metal aminocarbyne bond in diiron complexes

Terminal alkynes (HCCR′) (R′=COOMe, CH2OH) insert into the metal–carbyne bond of the diiron complexes [Fe2{μ-CN(Me)(R)}(μ-CO)(CO)(NCMe)(Cp)2][SO3CF3] (R=Xyl, 1a; CH2Ph, 1b; Me, 1c; Xyl=2,6-Me2C6H3), affording the corresponding μ-vinyliminium complexes [Fe2{μ-σ:η3-C(R′)CHCN(Me)(R)}(μ-CO)(CO)(Cp)2][SO3CF3] (R=Xyl, R′=COOMe, 2; R=CH2Ph, R′=COOMe, 3; R=Me, R′=COOMe, 4; R=Xyl, R′=CH2OH, 5; R=Me, R′=CH2OH, 6). The insertion is regiospecific and C–C bond formation selectively occurs between the carbyne carbon and the CH moiety of the alkyne. Disubstituted alkynes (R′CCR′) also insert into the metal–carbyne bond leading to the formation of [Fe2{μ-σ:η3-C(R′)C(R′)CN(Me)(R)}(μ-CO)(CO)(Cp)2][SO3CF3] (R′=Me, R=Xyl, 8; R′=Et, R=Xyl, 9; R′=COOMe, R=Xyl, 10; R′=COOMe, R=CH2Ph, 11; R′=COOMe, R=Me, 12). Complexes 2, 3, 5, 8, 9 and 11, in which the iminium nitrogen is unsymmetrically substituted, give rise to E and/or Z isomers. When iminium substituents are Me and Xyl, the NMR and structural investigations (X-ray structure analysis of 2 and 8) indicate that complexes obtained from terminal alkynes preferentially adopt the E configuration, whereas those derived from internal alkynes are exclusively Z. In complexes 8 and 9, trans and cis isomers have been observed, by NMR spectroscopy, and the structures of trans-8 and cis-8 have been determined by X-ray diffraction studies. Trans to cis isomerization occurs upon heating in THF at reflux temperature. In contrast to the case of HCCR′, the insertion of 2-hexyne is not regiospecific: both [Fe2{μ-σ:η3-C(CH2CH2CH3)C(Me)CN(Me)(R)}(μ-CO)(CO)(Cp)2][SO3CF3] (R=Xyl, 13; R=Me, 15) and [Fe2{μ-σ:η3-C(Me)C(CH2CH2CH3)CN(Me)(R)}(μ-CO)(CO)(Cp)2][SO3CF3] (R=Xyl, 14, R=Me, 16) are obtained and these compounds are present in solution as a mixture of cis and trans isomers, with predominance of the former.