Nickel(II) and Palladium(II) Complexes Bearing an Unsymmetrical Pyrrole-Based PNN Pincer and Their Norbornene Polymerization Behaviors versus the Symmetrical NNN and PNP Pincers

Copper(I) complexes bearing pyrrole-bridged S,N and N-donor ligands as catalysts for tandem hydroamination-alkynylation: effect of anions on product formation

In this study, the reaction between 2-(3,5-dimethylpyrazolylmethyl)-5-(dimethylaminomethyl)pyrrole and thiophenol under heating conditions afforded the new ligand 2-(3,5-dimethylpyrazolylmethyl)-5-(phenylthiomethyl)pyrrole 2. The reaction of 2 with meta-chloroperbenzoic acid provided sulfoxide 3 and sulfone 4 group-containing ligands. The reaction of 2 with copper(I) halides provided the binuclear complexes [Cu(μ-X){μ-C4H3N-2-(CH2Me2pz)-5-(CH2SPh)-κ2-S,N}]2 (X = Cl, Br and I, 5-7) in high yields. Conversely, the analogous reaction of 4 with copper(I) halides yielded two types of complexes, three coordinate bi- and mononuclear of the type [Cu(μ-Cl){C4H3N-2-(CH2Me2pz)-5-(CH2SO2Ph)-κ1-N}]28 and [CuX{C4H3N-2-(CH2Me2pz)-5-(CH2SO2Ph)-κ1-N}2], X = Br, 9 and I, 10. When the reaction was carried out in the presence of KPF6, the two-coordinate complex [Cu{C4H3N-2-(CH2Me2pz)-5-(CH2SO2Ph)-κ1-N}2]PF6-11a was isolated, whereas its BF4- analogue 11b was synthesized by the reaction of 8 with AgBF4. The structures of these complexes were determined using single-crystal X-ray crystallography. These copper complexes catalyzed the hydroamination-alkynylation reaction between several secondary amines and alkyl and aryl terminal alkynes. Using 1 mol% of complexes 5-10 as catalysts, both tri- and tetra-substituted propargylamines were isolated. Alternatively, phenylacetylene and different secondary amines afforded the corresponding trisubstituted propargylamines as the major products and alkyl terminal alkynes gave the tetrasubstituted products in excellent yields. In addition, the role of counter anions such as TfO-, PF6-, BF4-, PO43- and Ph4B- on the product selectivity was studied. When fluorinated anions such as TfO-, PF6-, and BF4- were present with the copper complexes, the hydroamination-hydrovinylation product 1-aminodiene 23 was observed, which was not formed with PO43-, Ph4B- or halide ions. Specifically, TfO- and PF6- favored the formation of 23, while BF4- favored the tetrasubstituted product as the major product. This was further supported by the isolated copper(I) complexes containing PF6- and BF4- and by other specific reactions. The peaks for enamines and [LCu]+ species in the HRMS spectra of the reaction mixtures and the isolation of the morpholinium copper(I) salt support the proposed mechanism.

Norbornene Copolymerization with Polar Monomers Catalyzed by Palladium Catalysts Containing Imidazolidin-2-imine/Guanidine Ligands

The development of a palladium catalyst that has enhanced catalytic performance, such as low aluminum cocatalyst loading, good copolymerization ability, high molecular weight, and excellent solubility of the (co)polymers, is still a challenge in norbornene copolymerizations. Here, a series of PdCl2 and PdMeCl complexes containing differently substituted anilines and imidazolidin-2-imine/guanidine ligands was successfully synthesized and characterized. X-ray diffraction analysis results revealed that these Pd complexes adopted an almost square-planar geometry, and the six-membered chelate ring showed structural distinctions as compared to traditional N^N-based α-diimine and β-diimine Pd complexes. These Pd complexes were activated by EtAlCl2 and then exhibited moderate activity (104-105 g mol-1 h-1) and good thermal stability (up to 90 °C) for norbornene polymerization to produce high-molecular-weight PNBs (Mn up to 96.4 kg mol-1) with narrow polydispersities (PDI as low as 1.39). These Pd complexes also exhibited good polar group tolerance in the copolymerization of norbornene with methyl 5-norbornene-2-carboxylate and methyl 10-undecenoate, in which the activity was achieved up to 7.04 × 104 g mol-1 h-1. It furnished polar functionalized norbornene-based copolymers with high molecular weight (Mn up to 63.1 kg mol-1), narrow PDI, reasonable polar monomer incorporation, and good solubility. These Pd catalysts exhibited an enhanced copolymerization ability to produce PNB or NB-based copolymers, representing significant progress in this field.

Cationic and Neutral PdII and PtII Pincer Complexes of Phosphinamino-Triazolyl-Pyridine [PN(H)N]: Pincer Ligand-Stabilized Palladium Nanoparticles and Their Catalytic Annulation of Internal Alkynes to Indenones

We describe the synthesis of a triazolyl-pyridine-based aminophosphine, N-(diphenylphosphaneyl)-6-(1-phenyl)-1H-(1,2,3-triazol-4-yl)pyridine-2-amine [2,6-{(PPh₂)-N(H)(C₅H₃N)(C₂HN₃C₆H₅)}] [1, PN(H)N hereafter], and its palladium and platinum complexes and their catalytic application. The reaction of 1 with [M(COD)Cl₂] (M = Pd or Pt) afforded the cationic complex [(MCl){PN(H)N}-κ³-P,N,N]Cl [M = Pd (2) or Pt (3)]. Alternatively, compounds 2 and 3 were also synthesized by treating [2,6-{H₂N(C₅H₃N)(C₂HN₃C₆H₅)}] (A) with [M(COD)Cl₂] (M = Pd or Pt), followed by the addition of stoichiometric amounts of PPh₂Cl and Et₃N. The neutral, dearomatized complexes [(MCl){PNN}-κ³-P,N,N] [M = Pd (4) or Pt (5)] were prepared by the deprotonation of the NH of 2 and 3 with 1 equiv of ^tBuOK. Compounds 4 and 5 were also synthesized stepwise by treating [2,6-{H₂N(C₅H₃N)(C₂HN₃C₆H₅)}] (A) with [M(COD)Cl₂] (M = Pd or Pt) to give intermediate complexes [{MCl₂}2,6-{NH₂(C₅H₃N)(C₂HN₃C₆H₅)-κ²-N,N}] [M = Pd (B) or Pt (C)], which were subsequently phosphinated. The in situ-generated PNN ligand-stabilized Pd nanoparticles from compound 2 catalyzed the annulation of o-bromobenzaldehyde with alkynes to yield indenone derivatives. Mechanistic investigations suggested that the reaction was catalyzed by Pd nanoparticles (Pd@2) generated from compound 2 and proceeded through sequential oxidative addition, alkyne insertion, and reductive elimination steps to produce indanone products.

Catalytic performance of tridentate versus bidentate Co(ii) complexes supported by Schiff base ligands in vinyl addition polymerization of norbornene

A series of Co(ii) complexes supported by Schiff base ligands, L_A-L_C, where L_A, L_B, and L_C are (E)-3-methoxy-N-(quinolin-2-ylmethylene)propan-1-amine, (E)-N ¹,N ¹-dimethyl-N ²-(pyridin-2-ylmethylene)ethane-1,2-diamine, and (E)-N ¹,N ¹-dimethyl-N ²-(thiophen-2-ylmethylene)ethane-1,2-diamine, respectively, were designed and synthesized. Structural studies revealed a distorted trigonal bipyramidal geometry for [L_BCoCl₂] and a distorted tetrahedral geometry for [L_CCoCl₂]. After activation with modified methyl aluminoxane (MMAO), all the Co(ii) complexes catalyzed the polymerization of norbornene (NB) to yield vinyl-type polynorbornenes (PNBs) with activities of up to 4.69 × 10⁴ g_PNB mol Co^-1 h^-1 at 25 °C. High-molecular-weight (M _n of up to 1.71 × 10⁵ g mol^-1) soluble PNBs with moderate molecular-weight distributions (MWD) were obtained. The activity of the Co(ii)/MMAO catalytic system is influenced by the steric hindrance and electronic properties of the ligands.

Polymerisation of styrene using pincer type amine functionalized azo aromatic complexes of Co(II) as catalysts

In the present report, three mononuclear azo-aromatic complexes of Co(II), 1-3, and an imine-based Co(II) complex, 4, were synthesized through a reaction of respective amine-functionalized pincer-like ligands, HL1-4, with CoCl₂·6H₂O in the ligand-to-metal ratio of 1 : 1. All the complexes, 1-4, were thoroughly characterized using various physicochemical characterization techniques, single-crystal X-ray structure determination, and density functional theory (DFT) calculations. Complexes 1-4 were explored for the catalytic styrene polymerisation reaction separately in the presence of modified methyl aluminoxane (MMAO). All the complexes, 1-4, are indeed active for the polymerisation of styrene under mild conditions at room temperature upon activation with MMAO. Among the azo-aromatic complexes 1-3, complex 3 is the most efficient. The activity of the imine complex 4 is poor compared to those of the azo-aromatic complexes 1-3. The weight average molecular weight (M_w) of the isolated polystyrene ranges from 32.9 to 144.0 kg mol^-1, with a polydispersity index (Đ) in the range of 1.1-1.8. Microstructural analysis of the isolated polymer from complexes 1-4 was carried out by ¹³C NMR spectroscopy, infrared spectroscopy, and powder X-ray diffraction studies. Their thermal properties were scrutinized by differential scanning calorimetry and thermogravimetric analysis. These studies have shown the atactic and amorphous nature of the polymers. The mechanical strength of the polymers was measured by a nanoindentation technique which has shown the good plastic/soft nature of the polymers.

Recent Advances in Chemistry of Unsymmetrical Phosphorus-Based Pincer Nickel Complexes: From Design to Catalytic Applications

Pincer complexes play an important role in organometallic chemistry; in particular, their use as homogeneous catalysts for organic transformations has increased dramatically in recent years. The high catalytic activity of such bis-cyclometallic complexes is associated with the easy tunability of their properties. Moreover, the phosphorus-based unsymmetrical pincers showed higher catalytic activity than the corresponding symmetrical analogues in several catalytic reactions. However, in modern literature, an increasing interest in the development of catalysts based on non-precious metals is observed. For example, nickel, which is an affordable and sustainable analogue of platinum and palladium, known for its low toxicity, has attracted increasing attention in the catalytic chemistry of transition metals in recent years. Thus, this mini-review is devoted to the recent advances in the chemistry of unsymmetrical phosphorus-based pincer nickel complexes, including the ligand design, the synthesis of nickel complexes and their catalytic applications.

Tridentate diarylamido-based pincer complexes of nickel and palladium: sidearm effects in the polymerization of norbornene

The sidearm effects were found in the polymerization of norbornene catalyzed by the Ni and Pd complexes bearing new diarylamido based unsymmetrical quinolino [NNN^ox] and phenylthio [SNN^ox] pincer ligands.