Catalysis by Palladium Pincer Complexes

Aryl-based pincer metal complexes with anionic terdentate ligands have been widely applied in organic synthesis, organometallic catalysis and other related areas. Synthetically, the most simple and convenient method for the construction of these complexes is the direct metal-induced Caryl–H bond activation, which can be fulfilled by choosing the appropriate functional donor groups in the two side arms of the aryl-based pincer preligands. In this perspective, we wish to summarize some results achieved by our group in this context. Successful examples include symmetrical chiral bis(imidazoline) NCN pincer complexes with Ni(II), Pd(II) and Pt(II), bis(phosphinite) and bis(phosphoramidite) PCP pincer Pd(II) complexes, unsymmetrical (pyrazolyl)phosphinite, (amino)phosphinite and (imino)phosphinite PCN pincer Pd(II) complexes, chiral (imidazolinyl)phosphinite and (imidazolinyl)phosphoramidite PCN pincer complexes with Ni(II) and Pd(II) as well as unsymmetrical (oxazolinyl)amine and (oxazolinyl)pyrazole NCN′ pincer Pd(II) complexes. Among them, the P-donor containing complexes are efficiently synthesized by the “one-pot phosphorylation/metalation” method. The obtained symmetrical and unsymmetrical pincer complexes have been used as catalysts in Suzuki–Miyaura reaction (Pd), asymmetric Friedel–Crafts alkylation of indole with trans-β-nitrostyrene (Pt) as well as in asymmetric allylation of aldehyde and sulfonimine (Pd). In the Suzuki couplings conducted at 40–50 °C, some unsymmetrical Pd complexes exhibit much higher activity than the related symmetrical ones which can be attributed to their faster release of active Pd(0) species resulting from the hemilabile coordination of the ligands. Literature results on the synthesis of some related pincer complexes as well as their activities in the above catalytic reactions are also presented.

Symmetrical and unsymmetrical pincer complexes with group 10 metals: synthesis via aryl C-H activation and some catalytic applications.

Unsymmetrical Chiral PCN Pincer Palladium(II) and Nickel(II) Complexes with Aryl-Based Aminophosphine–Imidazoline Ligands: Synthesis via Aryl C–H Activation and Asymmetric Addition of Diarylphosphines to Enones

Cross-coupling reactions catalysed by palladium pincer complexes. A review of recent advances

Unsymmetrical Chiral PCN Pincer Palladium(II) and Nickel(II) Complexes of (Imidazolinyl)aryl Phosphinite Ligands: Synthesis via Ligand C−H Activation, Crystal Structures, and Catalytic Studies

Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors and their complexes with palladium(II)

Structural stability, electronic and magnetic properties for half-metallic quaternary Heusler alloys CrLaCoZ

Catalytic studies on the air- and moisture-stable triphenylphosphine (PPh3) adducts of heteroannular cyclopalladated ferrocenylimines [PdCl{C5H4FeC5H4CHRN=CHC4H3X}(Ph3P)] (R = H, CH3; X = S, O) indicate that they are all effective catalysts for the Suzuki–Miyaura coupling reaction of activated aryl bromides with phenylboronic acid in water under air atmosphere. Typically, using 1 mol‰ of catalyst in the presence of 1.2 equivalents of K3PO4 as base in water at 100 °C provided the corresponding biaryls in good to excellent yields. Meanwhile, the structure of [PdCl{C5H4FeC5H4CH(CH3)N=CHC4H3S}(PPh3)] has been established by the single-crystal X-ray diffraction technique. According to the crystal structure, the palladium atom is bound to the unsubstituted Cp ring, showing the four-coordinate structure typical of palladium complexes.

Catalysis of the Suzuki–Miyaura coupling reaction in water by heteroannular cyclopalladated ferrocenylimine complexes

The unsymmetrical, achiral and chiral NCN pincer ligand precursors (3a-3d) with oxazoline and pyrazole as N donors as well as (3e) which has oxazolinyl and amino group have been synthesized in a facile manner in four steps starting from commercially available isophthalaldehyde. Direct C2 metallation of the precursors (3a–3e) with Pd(OAc)2 in refluxing HOAc, followed by treatment with LiCl at room temperature provided convenient access to the corresponding pincer palladium(II) complexes (4a–4e). The molecular structure of complex 4e has been determined by X-ray single-crystal diffraction. The obtained Pd complexes exhibited good activities in the Suzuki reactions of aryl bromides and activated aryl chlorides with phenylboronic acid.

Jun-Rui Liu

Papers

Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors and their complexes with palladium(II)

Structural stability, electronic and magnetic properties for half-metallic quaternary Heusler alloys CrLaCoZ

Catalysis of the Suzuki–Miyaura coupling reaction in water by heteroannular cyclopalladated ferrocenylimine complexes

Unsymmetrical, Oxazolinyl‐Containing Achiral and Chiral NCN Pincer Ligand Precursors and Their Complexes with Palladium(II).