Pd(II) Trifluoroacetate Complexes Containing Chelating Phosphine Ligands: Synthesis, Structures, and Catalysis

Abstract

Carboxylates coordinate to transitionmetals in a terminal, chelating, or bridging mode through their oxygen atoms, and serve as active ligands in various catalytic reactions. Transition-metal complexes having a trifluoroacetate (TFA) ligand are well known to have higher solubility and crystallinity than the corresponding acetate counterparts depending on the metal identity. In particular, TFA complexes of late transitionmetals are efficient catalysts in several organic reactions, including polymerization of isocyanide or butadiene and copolymerization of CO and olefin. Moreover, such complexes exhibit higher efficiency than the corresponding acetate complexes in the aerobic oxidation of organic substrates, enamide formation by vinyl transfer, or oxidative cyclization of organic substrates in the presence of Pd catalysts. Although many catalytic Suzuki–Miyaura C C coupling reactions have been reported, those involving late-transition-metal–TFA complexes are still rare. For this reason, we attempted to develop a new synthetic route to Pd–TFA complexes possessing chelating phosphine ligands for use in such coupling reactions. In this work, we prepared a series of Pd(II)–TFA–(chelating diphosphine) complexes and examined their catalytic C C coupling reactions of aryl halides and arylboronic acids. We first prepared a Pd(II)–TFA–TMEDA complex (TMEDA = N,N,N0,N0-dimethyl ethylene diamine), [Pd (TMEDA)(TFA)2] (1), by themetathesis of [PdCl2(TMEDA)] and 2 equiv of [Ag(TFA)]. Subsequently, the ligand replacement of complex 1 was investigated with one or two equivalents of chelating phosphine, including DMPE, DEPE, 1,2Ph2P(CH2)nPPh2 (n = 2, DPPE; n = 3, DPPP; n = 4, DPPB), and the ferrocene-bridged phosphines DPPF and DIPPF (1,10-bis(diphenyl or diisoporpyl)phosphine ferrocene). These reactions afforded ionic bis(diphosphine) Pd(II) complexes [Pd(P~P)2](TFA)2 (type A, 2–6) or neutral mono(diphosphine) Pd(II)–TFA complexes [Pd(P~P) (TFA)2] (type B, 7 and 8) in high yields (Scheme 1). Chemical compositions of complexes2–8 strongly indicate that sterically hindered and less basic chelating phosphines such as DPPF or DIPPF prefer to form mono(diphosphine) compounds. Isolated bis(chelating phosphine)–Pd(II)–TFA complexes are poorly soluble in organic solvents. All products were characterized by spectroscopy (IR and NMR) and elemental analysis, together with X-ray crystallography for complexes 2 and 7. The IR spectra display characteristic stretching C O and C F bands at 1671–1711 and at 1105–1190 cm, respectively. Details on crystal data of complexes are summarized in Supporting Information (see Table S1). Figure 1 shows the cationic part of complex 2 H2O, in which two CF3COO counterions and one lattice water are omitted for clarity. The molecular structure of 7 is given in Figure 2, which shows a Pd(II) metal, a DPPF ligand, and two CF3COO ligands. TwoCp rings in theDPPF ligand are not perfectly parallel,with a dihedral angle of 4.4(4) . ThePd Fe separation of 4.1932(8)A indicates no direct interactions between these two metals. We examined the ligand exchange reaction of complex 8 with 2 equiv of NaN3 in CH2Cl2 (Scheme 2). The reaction readily proceeded to give a Pd(II) azide complex, [(DIPPF) Pd(N3)2] (9), as an orange crystal in quantitative yield, which was characterized by spectroscopic data and elemental analysis. The IR spectrum of 9 shows a strong absorption band at 2045 cm, a characteristic peak of the N3 group. Recently, several Suzuki–Miyaura C C coupling reactions employing Pd(II)–TFA complexes containing C–N donor or pincer-type (PCP) ligands were reported. However, the corresponding reactions with bis(chelating phosphine) analogs are relatively rare. In this context, we evaluated the catalytic activity of several complexes (1, 2, 6–8, and 9) for the coupling reactions (catalysts in Chart 1 and Scheme 3). In order to find the optimum conditions for Scheme 1. Syntheses of Pd(II) trifluoroacetates. Note DOI: 10.1002/bkcs.10590 K.-W. Kim et al. BULLETIN OF THE KOREAN CHEMICAL SOCIETY