Iminium substituent directs cyanide and hydride additions to triiron vinyliminium complexes

Abstract

The reactivity of diiron bis-cyclopentadienyl cationic complexes containing a vinyliminium ligand coordinated to a diiron scaffold and decorated with a ferrocenyl group, [1a–g]CF₃SO₃, was investigated towards NBu₄CN (in CH₂Cl₂) and NaBH₄ (in THF). Thus, (amino)cyano-allylidene derivatives, 2a–2d, were synthesized in good yields by cyanide attack to the iminium carbon C¹ in the respective precursors. However, this outcome is influenced by the nature of one iminium substituent (R), when the other one is methyl: when RCH₂CHCH₂, a μ-alkylidene-alkenyl by-product (3), is obtained, while R4-C₆H₄OMe determined the concurrent cyanide addition to the vinyl carbon C² leading to a bis-alkylidene complex (4d). The analogous bis-alkylidene 4e was selectively prepared in the case of R2,6-C₆H₃(Me)(Cl) (Xyl^Cl), and this substituent exerted steric protection on C¹. Similarly, hydride addition took place at either C¹ (complex 5a, R4-C₆H₄OMe) or C² (6, RXyl^Cl). The reactions of [1g]CF₃SO₃, containing two benzyl groups as iminium substituents, with NBu₄CN and NaBH₄ led to the selective formation of the C¹ addition derivatives, 2e and 5b. All products were purified by alumina chromatography and fully characterized using analytical and spectroscopic methods, while DFT calculations were extensively carried out to shed light on structural and thermodynamic aspects. The representative complexes 2e, 4e, 5b and 6 were further characterized by cyclic voltammetry and IR-spectroelectrochemistry studies.