Iminoborylene complexes: evaluation of synthetic routes towards BN-allenylidenes and unexpected reactivity towards carbodiimides

Abstract

The synthetic and reaction chemistries of cationic iminoborylene complexes [L_nMBNCR₂]⁺, which feature a unique heterocumulene structure, have been systematically investigated. Precursors of the type CpFe(CO)₂B(Cl)NCAr₂ (Ar = p-Tol/Mes, 5c/d) have been generated by B-centred substitution chemistry using CpFe(CO)₂BCl₂ and suitable lithiated ketimines – a reaction which is found to be highly sensitive to the steric bulk at both the metal fragment and the ketimino group. Carbonyl/phosphine exchange (using PCy₃ or PPh₃), followed by halide abstraction allows for the generation of the cationic iminoborylenes [CpFe(PR₃)(CO)(BNCAr₂)]⁺[BAr^X₄]⁻ (R = Cy, Ar = p-Tol/Mes, 12c/d; R = Ph, Ar = Mes, 13d; Ar^X = 3,5-X₂C₆H₃ where X = Cl, CF₃) which have been characterized spectroscopically and by X-ray crystallography. The reactivity of these iminoborylene systems towards a range of nucleophiles and unsaturated substrates has been investigated. The latter includes the first examples of MB metathesis reactivity with a carbodiimide, and results in FeB cleavage and formation of the isonitrile complexes [CpFe(PCy₃)(CO)(CNR)]⁺[BAr^Cl₄]⁻ (R = ⁱPr/Cy, 16/17).