Converging and Diverging Synthetic Strategies to Tetradentate (N,N′)-Diaminomethyl,(P,P′)-Ferrocenyl Ligands: Influence of tert-Butyl Groups on Ferrocene Backbone Conformation

Hexasubstituted hybrid tetradentate (N,N′,P,P′)-ferrocenes bearing phosphino and aminomethyl groups, plus hindering tert-butyl moieties, were synthesized by using two different strategies: a “diverging” synthesis involving successive functionalization of preformed di-tert-butylated ferrocene and a “converging” assembly of the species from appropriately substituted cyclopentadienyl rings. While the new cyclopentadienyl salts formed are of interest, their assembly with iron dichloride used as a “converging” way to produce tetradentate ferrocene ligands presented several drawbacks. Conversely, the synthesis of new tert-butylated (aminomethyl)­ferrocene derivatives was found convenient to further form (N,N′)-aminomethyl,(P,P′)-tert-butylated-ferrocenyl diphosphines by N-directed ortho-metalation. The novel N₂-didentate and N₂P₂-tetradentate tert-butylated ferrocene compounds were all synthesized in good to high yields (48–96%) and tolerated aryl, alkyl, and heteroaryl phosphino groups as substituents on nitrogen and phosphorus atoms. They were characterized by X-ray diffraction and multinuclear NMR (¹H, ¹³C, ³¹P, ¹⁵N). We observed the conformation control provided to rac-(N,N′)-diaminomethyl-(P,P′)-tert-butylated-ferrocenyldiphosphines with in particular the systematic near-eclipsed conformation of aminomethyl groups. This conformation is at the origin of the unexpected formation at RT of a zwitterionic cyclopalladate from an (aminomethyl)­ferrocene derivative, arising from intramolecular Cp-proton transfer to the proximate free amino group by simple C–H activation reaction in the presence of palladium dichloride.