Palladium complexes of ferrocene-based phosphine ligands as redox-switchable catalysts in Buchwald–Hartwig cross-coupling reactions

Abstract

The synthesis and full characterization of several transition metal complexes of a redox-switchable, biaryl(Ar^T)-substituted [1]phosphaferrocenophane, FcPAr^T (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(κ¹P-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 indications for a good electronic communication between the metal atoms. In order to confirm that the [1]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 catalysts (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 catalysts 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.