Cyclopalladation of a ferrocene acylphosphine and the reactivity of the C–H activated products

Abstract

Acylphosphines are an attractive subclass of phosphine ligands with specific reactivity and ligating properties. This study describes the synthesis of a ferrocene-based acylphosphine, FcC(O)PPh₂ (1, Fc = ferrocenyl), the corresponding phosphine chalcogenides FcC(O)P(E)Ph₂ (E = O, S and Se), and palladium(II) complexes resulting from the orthometallation of 1, viz. [Pd(μ-X)(1 − H)]₂ (2-X, where X = Cl, Br and I), which in turn serve as a convenient entry point to a range of monopalladium complexes. Thus, the cleavage of 2-X with phosphines, 4-(dimethylamino)pyridine and in situ-generated 1,3-dimesitylimidazolin-2-ylidene (L) provided complexes [PdX(L)(1 − H)] (3–5), which are typically obtained as single isomers by crystallisation but undergo spontaneous isomerisation in solution leading to equilibrium mixtures of cis and trans isomers. Upon treatment with sodium acetylacetonate (Na(acac)), 2-Cl was transformed into [Pd(acac)(1 − H)] (6), which reacted with (diphenylphosphino)acetic acid under proton transfer to give [Pd(Ph₂PCH₂CO₂-κ²O,P)(1 − H)] (7), while the reaction with allylating agents produced the π-allyl complex [Pd(η³-C₃H₅)(1 − H)] (9). Compound [PdCl(PMe₃)(1 − H)] was further used to prepare a series of diorganopalladium complexes [Pd(R)(PMe₃)(1 − H)] (8; R = Me, 4-C₆H₄Me, 4-C₆H₄CF₃, CHCHC₆H₄Me-4 and CCC₆H₄Me-4). All compounds were structurally characterised using spectroscopic methods and, in most cases, also by X-ray diffraction analysis. The structural data indicated the rigidity of the Pd(1 − H)⁺ fragment and revealed variations in the Pd-donor distances dictated by an interplay between the trans influence and steric demands of the ligands surrounding Pd(II). Moreover, some compounds were studied by DFT to rationalise their isomerisation equilibria and electrochemical properties, which were examined by cyclic voltammetry.