Palladium Complexes of Ferrocene-Based Phosphine Ligands as Redox-Switchable Catalyst in Buchwald-Hartwig Cross-Coupling Reactions
The synthesis and full characterization of several transition metal complexes of a redox-switchable, biaryl(ArT)-substituted phosphaferrocenophane, FcPArT (L2), in conjunction with some palladium complexes of the mesityl(Mes)-substituted derivative, FcPMes (L1), and the non-bridged diphenylferrocenyl phosphine, Ph2PFc (L3), are reported. Cyclic voltammetry studies on the bimetallic complexes [Ir(κ1P-L2)(cod)Cl] (1), [Ir(κ1P-L2)(CO)2Cl] (2), [AuCl(κ1P-L2)] (3), [Pd(1P-L2)(η3-allyl)Cl] (4), [Pd(κ1P-L3)(η3-allyl)Cl] (5), [Pd(κ1P-L3)(2-aminobiphenyl)(SO3CH3)] (6), and [Pd(κ1P-L1)(2-aminobiphenyl)(SO3CH3)] (7) provided evidence for a good electronic communication between the metal atoms. In order to confirm that the ferrocenophane or 1-ferrocenyl unit might be able to electrochemically influence the reactivity of the coordinated transition metal, the palladium complexes 4 – 7 were employed as redox-switchable catalyst (RSC) in Buchwald-Hartwig cross-coupling reactions. The catalytic activity depends on the ligand and decreases in the series L3 > L2 > L1. In stirred solutions, the reaction rate can be influenced by addition of an oxidizing reagent. Some reactions were accelerated with in situ generated, cationic complexes as a catalyst as compared to their neutral analogs; in some cases, the activity was found to be lower. Furthermore, steric effects play a predominant role, among other factors.