Tailoring the Fe → Pd interaction in cationic Pd(ii) complexes via structural variation of the ligand scaffold of sterically demanding dppf-analogs and their P,N-counterparts

Abstract

Two 1,1′-azaphospha substituted dppf-analogues Fc′(NMe₂)(PPh₂) (Ph = C₆H₅, Fc′ = 1,1′-ferrocenediyl, 3a) and Fc′(NMe₂)(PMes₂) (Mes = 2,4,6-Me₃C₆H₂, 3b) have been prepared, via reductive amination, followed by salt-metathesis (of 2), starting from 1,1′-azabromoferrocene 1. Their donor properties have been explored using heteronuclear NMR spectroscopy based on their ¹J_P–Se coupling, and the formation of PdCl₂-complexes in comparison to a set of related dppf analogs with gradual steric variation, such as Fc′(PMes₂)(PPh₂) (5) and Fc′(PMes₂)(P^tBu₂) (6). Chloride abstraction from these complexes, namely Fc′(PMes₂)(PPh₂)·PdCl₂ (7), Fc′(PMes₂)(P^tBu₂)·PdCl₂ (8), and [Fc′(NMe₂)(PPh₂)]₂·PdCl₂ (9) using AgSbF₆ produced the corresponding cationic Pd(II) complexes [Fc′(PMes₂)(P^tBu₂)·PdCl][SbF₆] (10), [Fc′(PPh₂)(NMe₂)·PdCl][SbF₆]₂ (11) and [Fc′(PPh₂)(NMe₂)·Pd(PPh₂C₅H₅)][SbF₆]₂ (12) featuring Fe → Pd interactions. Variation of the counter anion by coordination of 3a to a chloride-free Pd(II) source furnished [Fc′(PPh₂)(NMe₂)·Pd(PPh₃)][BF₄]₂ (13), [Fc′(PPh₂)(NMe₂)·Pd(PPh₂)Fc′(NMe₂)][BF₄]₂ (14), and [Fc′(PPh₂)(NMe₂)·PdP(p-OMe-C₆H₄)₃][BF₄]₂ (15) with similar Fe → Pd interactions. Comparison with previously reported diphospha- and azaphospha- counterparts, revealed that 10 and 11 display the shortest and 15 the longest Fe–Pd bond, within their ligand scaffold congeners. DFT calculations performed on compounds 10–15 were further able to verify their intrinsic structural features and trends and shed light on the nature of the Fe → Pd bonding interactions which are furthermore consistent with CV measurements.