Palladacycle from Cyclometalation of the Unsubstituted Cyclopentadienyl Ring in Ferrocene: Synthesis, Characterization, Theoretical Studies, and Application to Suzuki–Miyaura Reaction

The
ferrocenylimines of general formula [(η5-C5H5)­Fe­(η5-C5H4)-CH2NCH-C­(R)CH-C6H5] with
R = H (2a) and CH3 (2b) were
conveniently prepared from ferrocenylmethylamine. Reaction of 2a,b with lithium tetrachloropalladate in methanol
in the presence of anhydrous sodium acetate resulted in the formation
of the di-μ-chloro-bridged heteroannular cyclopalladated complexes 3a,b via the unsubstituted ferrocenyl C–H
bond activation of the related ligands. Treatment of 3a,b with triphenylphosphine gave Pd­{[(η5-C5H4)­Fe­(η5-C5H4)­CH2NCH-CHCH-C6H5]}­ClPPh3 (4a) and Pd­{[(η5-C5H4)­Fe­(η5-C5H4)-CH2NCH-C­(CH3)CH-C6H5]}­ClPPh3 (4b), respectively.
The crystal structures of 4a,b confirmed
the formation of a carbon–palladium bond by using a carbon
atom in the unsubstituted cyclopentadienyl ring. Additionally, theoretical
studies using density functional theory calculations were carried
out in order to account for the regioselectivity of cyclometalation.
As for the catalysts, using 0.1% of palladacycles 4a,b in the presence of K3PO4·7H2O as base exhibited excellent yields in the Suzuki–Miyaura
coupling reaction of aryl bromides with phenylboronic acid.