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 cydopentadienyl 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 N-2-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 (H-1, C-13, P-31, N-15). 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.