Versatile coordination and C–H activation of a multi-donor phosphinoferrocene carboxamide ligand in Pd(ii) complexes

Abstract

A multi-donor phosphinoferrocene carboxamide, FcCONHCH₂CH₂PPh₂ (1, Fc = ferrocenyl), was prepared, converted into the corresponding phosphine oxide 1O and phosphine selenide 1Se and, mainly, studied as a ligand in Pd(II) complexes. In its native form, amide 1 preferentially coordinated soft Pd(II) as a simple phosphine, giving rise to mixtures of cis and trans-[PdX₂(1-κP)₂] (2; X = Cl (a), Br (b), and I (c)), wherein the isomer ratios depended on the auxiliary halide ligand or, alternatively, to the complex [(L^NC)PdCl(1-κP)] (6, L^NC = 2-[(dimethylamino)methyl-κN]phenyl-κC¹). This coordination mode was nevertheless easily changed when creating a vacant coordination site at the palladium. Thus, treatment of 2a with NH₄[PF₆] in the presence of free 1 produced [PdCl(1-κP)₃][PF₆] (3), while complete halogen removal with a Ag(I) salt led to cationic complexes cis-[Pd(1-κ²O,P)₂]X₂ (4, X = CF₃SO₃ (a), ClO₄ (b), BF₄ (c)) or [(L^NC)Pd(1-κ²O,P)]X (7a and 7b), containing seven-membered O,P-chelate rings. In contrast, amide nitrogen deprotonation with KOt-Bu followed by spontaneous intramolecular halogen substitution resulted in the transformation of 6 into the chelate complex [(L^NC)Pd{(1 – H)-κ²N,P}] (8) featuring a five-membered N,P-chelate ring, and in the conversion of 2a and 2b into the product of C–H bond activation [Pd{Fe(η⁵-C₅H₃CONCH₂CH₂PPh₂-κ³C,N,P)(η⁵-C₅H₅)}(1-κP)] (5), with doubly chelating deprotonated 1. Importantly, complexes 2–4–5 and 6–7–8 were mutually interconverted in triads (by protonation/deprotonation and by halide addition/abstraction), which highlights the flexible coordination and chemical stability of ligand 1. The crystal structures of 1O·½H₂O, trans-2a·MeCN, trans-2b·3C₂H₄Cl₂, trans-2c·2.5C₂H₄Cl₂, 4a·CH₂Cl₂, 5·3CHCl₃·Et₂O, and 8 were determined by single-crystal X-ray diffraction analysis, and the representative compounds were studied by cyclic voltammetry.