Pyridine-imine Ligands Research Articles

Semistable Half‐Sandwich Cp*Co(II)(P‐X) (X=N, O) Complexes: Synthesis, Characterization and Electrochemical Properties

AbstractThree Cp*Co(II)(P−X) (X=N, O) complexes were synthesized using cyclopentadienyl anionic and [P N]/[P O] bidentate ligands. Crystallographic analysis revealed five‐coordinate, 17‐electron unsaturated structures with low‐spin d⁷ Co(II) configurations. Electrochemical studies showed reversible Co(II)/Co(III) redox couples and an irreversible Co(II)/Co(I) process of Cp*Co(II)(P−O) complexes, while complex Cp*Co(II)(P−N) exhibited irreversible Co(II)/Co(III) process due to the pyridine imine ligand. Oxidation with AgBF4 produced stable Co(III) complexes, confirmed by crystallographic data and NMR data. These complexes exhibit significant electrochemical properties and stability, indicating potential for catalytic and organic molecule binding applications.

Promotion of chain growth and suppression of chain transfer by 2-methoxy group in diarylmethyl moiety in pyridine-imine nickel- and palladium-catalyzed ethylene (co)polymerization

Diarylmethyl substituents are commonly used to prevent chain transfer in late-transition metal-catalyzed ethylene polymerization. However, this substituent is not effective in inhibiting chain transfer in the pyridine-imine system. In this study, we introduced substituents at the 2-position of the diarylmethyl moiety to synthesize high molecular weight polyethylene in the pyridine-imine system. Using this strategy, we synthesized two pyridine-imine ligands with either ethyl or methoxy at the 2-position diarylmethyl moiety, along with their corresponding nickel and palladium complexes. These complexes were then utilized for ethylene (co)polymerization. The catalysts with OMe exhibited significantly higher polymerization activity (an order of magnitude, 105 vs. 104 g/(mol Ni·h)) compared to the corresponding Et catalysts. In addition, it was found that polyethylene produced from nickel and palladium catalysts with OMe has higher molecular weights than that produced from ethyl nickel and palladium catalysts. The nickel system produces polyethylene with a molecular weight up to an order of magnitude higher (an order of magnitude, 13.0–13.6 vs. 3.1–4.0 kg/mol). Similar results were also observed in the palladium-catalyzed copolymerization of ethylene with MA. Notably, the palladium catalyst with OMe has a significantly higher capacity for MA insertion (three times, 1.68–4.74 vs. 5.00–11.73 mol%) than the ethyl palladium catalyst, despite their similar spatial steric structures. The observed phenomenon may be attributed to the weak interaction between OMe and the metal center. In addition, 13C NMR analysis showed that the palladium-catalyzed polyethylene and E-MA copolymers are hyperbranched polymers, with the polar groups located at the ends of the branches.

A Versatile Strategy for the Controlled Synthesis of Atomically Precise Palladium Nanoclusters.

The study of the structures, applications, and structure-property relationships of atomically precise metal nanoclusters relies heavily on their controlled synthesis. Although great progress has been made in the controlled synthesis of Group 11 (Cu, Ag, Au) metal nanoclusters, the preparation of Pd nanoclusters remains a grand challenge. Herein, a new, simple, and versatile synthetic strategy for the controlled synthesis of Pd nanoclusters is reported with tailorable structures and functions. The synthesis strategy involves the controllable transformations of Pd4(CO)4(CH3COO)4 in air, allowing the discovery of a family of Pd nanoclusters with well-defined structure and high yield. For example, by treating the Pd4(CO)4(CH3COO)4 with 2,2-dipyridine ligands, two clusters of Pd4 and Pd10 whose metal framework describes the growth of vertex-sharing tetrahedra have been selectively isolated. Interestingly, chiral Pd4 nanoclusters can be gained by virtue of customized chiral pyridine-imine ligands, thus representing a pioneering example to shed light on the hierarchical chiral nanostructures of Pd. This synthetic methodology also tolerates a wide variety of ligands and affords phosphine-ligated Pd nanoclusters in a simple way. It is believed that the successful exploration of the synthetic strategy would simulate the research enthusiasm on both the synthesis and application of atomically precise Pd nanoclusters.

Preparation of branched oligoethylene and polar functionalized oligoethylene using unsymmetrical arylamine based cationic Ni(II) and Pd(II) iminopyridyl catalysts

In this contribution, three unsymmetrical arylamine based pyridine-imine ligands and the corresponding nickel and palladium complexes were prepared and used for ethylene oligomerization. The nickel complexes are highly active (level of 106 g·mol−1·h−1) in catalyzing ethylene oligomerization to produce highly branched (66–89/1000C) oligoethylenes (1.4–3.2 kg/mol). The corresponding palladium complexes presented moderate catalytic activities (level of 104 g·mol−1·h−1) and low molecular weight (0.4–0.6 kg/mol) oligoethylenes with high branching densities (103–126/1000C) were yielded in ethylene oligomerization. Moreover, high incorporation ratio (6.86–14.16 mol%) ester functionalized oligoethylenes with low molecular weights (ca. 0.4 kg/mol) were yielded with these palladium complexes in ethylene–methyl acrylate co-oligomerization. Methyl branching comprises the majority of all the branching for the oligoethylenes yielded by nickel complexes and branching of C3+ comprises the majority with palladium ones. Hyperbranched architecture was identified for all the produced oligoethylenes and ester functionalized oligoethylenes. Most interestingly, compared with the reported pyridine-imine catalysts with fully substituted benzothiophenes, the partially benzothiophene substituted ones exhibited significantly higher activity and yielded much higher molecular weight oligoethylenes.

Reversible C-C Bond Formation, Halide Abstraction, and Electromers in Complexes of Iron Containing Redox-Noninnocent Pyridine-imine Ligands.

The exploration of pyridine-imine (PI) iron complexes that exhibit redox noninnocence (RNI) led to several interesting discoveries. The reduction of (PI)FeX2 species afforded disproportionation products such as (dmpPI)2FeX (dmp = 2,6-Me2-C6H3, X = Cl, Br; 8-X) and (dippPI)2FeX (dipp = 2,6-iPr2-C6H3, X = Cl, Br; 9-X), which were independently prepared by reductions of (PI)FeX2 in the presence of PI. The crystal structure of 8-Br possessed an asymmetric unit with two distinct electromers, species with different electronic GSs: a low-spin (S = 1/2) configuration derived from an intermediate-spin S = 1 core antiferromagnetically (AF) coupled to an S = 1/2 PI ligand, and an S = 3/2 center resulting from a high-spin S = 2 core AF-coupled to an S = 1/2 PI ligand. Calculations were used to energetically compare plausible ground states. Polydentate diazepane-PI (DHPI) ligands were applied to the synthesis of monomeric dihalides (DHPI)FeX2 (X = Cl, 1-Cl2; X = Br, 1-Br2); reduction generated the highly distorted bioctahedral dimers (DHPA)2Fe2X2 ((3-X)2) containing a C-C bond formed from imine coupling; the monomers 1-X2 could be regenerated upon Ph3CX oxidation. Dihalides and their reduced counterparts were subjected to various alkyl halides and methyl methacrylate (MMA), generating polymers with little to no molecular weight control, indicative of simple radical-initiated polymerization.

Platinum complexes from pyridine-imine ligands with pendent functional groups

Notable differences are observed in reactions of platinum(II) precursor complexes with pyridine-imine ligands of formula 2-C5H4NCH = NCH2-C6H4OR with R = Me or H. The ligand 2-C5H4NCH = N-2-C6H4OMe, L1, gave simple complexes [PtXY(L1)], XY = Me,Me; Me,Cl; Cl,Cl. However, the ligand 2-C5H4NCH = NCH2-2-C6H4OH, L2, gave either [PtCl(κ3-N,N,O-2-C5H4NCH2N = CH-2-C6H4O)], in which the ligand L2 had undergone a proton migration and a deprotonation, or a more complex product arising from reaction with solvent. The ligand 2-C5H4NCH = NCH2-4-C6H4OH, L3, gave a complex [PtCl2(L3)], which formed a polymer in the solid state through intermolecular OH..Cl hydrogen bonding.

Ligand‐Regulated Regiodivergent Hydrosilylation of Isoprene under Iron Catalysis

AbstractA method for the regiodivergent and stereoselective hydrosilylation of the basic industrial feedstock isoprene with unactivated silanes has been developed using earth‐abundant iron catalysts. The manipulation of regioselectivity relies on fine modification of the coordination geometry of the iron center. While a bidentate pyridine imine ligand promoted the formation of allylic silanes through 4,1‐addition, selectivity for the 3,4‐adduct homoallylic silanes was observed with a tridentate nitrogen ligand. Experimental studies and analysis were carried out to elucidate the reaction mechanism and the factors enabling manipulation of the regioselectivity. This study contributes to the art of regioselectivity control in alkene hydrofunctionalization.

Ligand-Regulated Regiodivergent Hydrosilylation of Isoprene under Iron Catalysis.

A method for the regiodivergent and stereoselective hydrosilylation of the basic industrial feedstock isoprene with unactivated silanes has been developed using earth-abundant iron catalysts. The manipulation of regioselectivity relies on fine modification of the coordination geometry of the iron center. While a bidentate pyridine imine ligand promoted the formation of allylic silanes through 4,1-addition, selectivity for the 3,4-adduct homoallylic silanes was observed with a tridentate nitrogen ligand. Experimental studies and analysis were carried out to elucidate the reaction mechanism and the factors enabling manipulation of the regioselectivity. This study contributes to the art of regioselectivity control in alkene hydrofunctionalization.

Synthesis and characterization of iron, cobalt and nickel complexes bearing para-phenylene-linked pyridine imine ligand and their catalytic properties for ethylene oligomerization

The (6E,7E)-N1,N4-bis((pyridin-2-yl)methylene)benzene-1,4-diamine ligand L-0 was prepared in order to synthesize the corresponding pyridine imine metal (iron, cobalt and nickel) complexes Fe-1, Co-2 and Ni-3. The structures of the ligand and its corresponding metal complexes were characterized by element analysis, NMR, IR, UV–Vis and ESI–MS. Upon activation with cocatalyst, the metal complexes Fe-1, Co-2 and Ni-3 exhibited moderate catalytic activity, and the oligomer products were mainly 1-butene. The influence of reaction parameters, such as solvent type, Al/M (M = Fe, Co, Ni) molar ratio, reaction temperature, reaction pressure and cocatalyst type, on catalytic properties was investigated in detail. Ethylene oligomerization data suggested that metal center had an important impact on catalytic performance. In most cases, attributed to electron effects, Fe-1 had better catalytic activity than Co-2 and Ni-3 at the same reaction conditions, and Ni-3 had better catalytic selectivity toward high-carbon olefins (≥ 1-decene). The density functional theory calculations were utilized to optimize the geometry structure for exploring the relationship between structure and catalytic properties, which provides a possibility of predicting the catalytic selectivity in ethylene oligomerization. Moreover, the catalytic mechanism was further studied for better understanding of ethylene oligomerization process catalyzed by Fe-1, Co-2 and Ni-3. The para-phenylene-linked pyridine imine metal (iron, cobalt and nickel) complexes have the ability of catalyzing ethylene oligomerization. Attributed to electron effects, their catalytic properties including activity and selectivity are different.

Ethylene oligomerization studies utilizing nickel complexes bearing pyridine-imine ligands

Two pyridine-imine ligands (6E,7E)-N1,N4-bis((pyridin-2-yl)methylene)benzene-1,4-diamine (L1) and (6E,13E)-N1,N4-bis(1-(pyridin-2-yl)ethylidene)benzene-1,4-diamine (L2) were synthesized in good yields. Reactions with L1 and L2 with Ni salts afforded nickel complexes NiCl2{(6E,7E)-N1,N4-bis((pyridin-2-yl)methylene)benzene-1,4-diamine}NiCl2 (C1) and NiCl2{(6E,13E)-N1,N4-bis(1-(pyridin-2-yl)ethylidene)benzene-1,4-diamine}NiCl2 (C2). The compounds were characterized through NMR (1H NMR and 13C NMR), IR, UV–Vis, MS and elemental analysis. Upon activation with a co-catalyst, C1 and C2 both displayed good catalytic activity (0.63 × 104–16.66 × 104 g/mol Ni h and 0.11 × 105–23.70 × 105 g/mol Ni h, respectively) in ethylene oligomerization experiments, the oligomer products followed the classic Schulz-Flory distribution with mostly 1-C4= oligomers. The catalytic performance was obviously affected by the reaction parameters including reaction temperature, reaction pressure, Al/Ni molar ratio, reaction time and co-catalyst type. The experimental data suggested that C1 and C2 generally had quite different catalytic performance under the same reaction conditions indicating the catalyst structure had a significant influence on the catalytic performance. The density functional theory (DFT) calculation was utilized to optimize the geometry structure for exploring the relationship between structure and catalytic properties. Combined with a proposed mechanism, reaction intermediates and frontier molecular orbitals were simulated for further studying the intermediates structure and energy variation in ethylene oligomerization process.

Syntheses, structures and catalytic activities of molybdenum carbonyl complexes based on pyridine-imine ligands

Thermal treatment of pyridine imines [C5H4N-2-C(H)=N-C6H4-R] [R = H (1), CH3 (2), OMe (3), CF3 (4), Cl (5), Br (6)] with Mo(CO)6 in refluxing toluene provided six novel mononuclear molybdenum carbonyl complexes of the type [(η2-2-C5H4N)CH=N(C6H4-4-R)]Mo(CO)4 [R = H (7); CH3 (8); OMe (9); CF3 (10); Cl (11); Br (12)]. All of these complexes were separated by chromatography and fully characterized by elemental analysis, IR, and NMR spectroscopy. The crystal structures of complexes 7, 8 and 10 were determined by X-ray crystal diffraction analysis. In addition, the catalytic performance of these complexes was also tested, and it was found that these complexes had obvious catalytic activity on Friedel–Crafts reactions of aromatic compounds with a variety of acylation reagents.

Construction of iridium and rhodium cyclometalated macrocycles based on p-carborane and N,N'-donor bridging ligands.

Six tetranuclear half-sandwich iridium and rhodium complexes bridged by neutral N,N'-donor pyridyl-imine ligands and 1,12-dicarbadodecaborane(12)-1,12-dicarboxylate (p-CDC) were controllably synthesized and fully characterized. The results revealed that the precursors, binuclear complexes [(Cp*M)2{1,4-bis(2-pyridylmethyleneamino)benzene}Cl2][OTf]2 (Cp* = η(5)-pentamethylcyclopentadienyl, M = Ir (2a), Rh (2b)), have different preferential configurations, and tetranuclear complexes [(Cp*M)4{1,4-bis(2-pyridylmethyleneamino)benzene}2(p-CDC)2][OTf]4 (M = Ir (3a), Rh (3b)), which were prepared using monodentate p-carborane dicarboxylate and 2a or 2b, showed highly twisted backbones. Likewise, homologous regular rectangular compounds [(Cp*M)4{1,4-{(2-C5H4N)HC=N}2-2,3,5,6-Me4C6}2(p-CDC)2][OTf]4 (M = Ir (4a), Rh (4b)) and [(Cp*M)4{1,4-{(2-C5H4N)HC=N}2-1,5-naphthalene}2(p-CDC)2][OTf]4 (M = Ir (5a), Rh (5b)) were obtained following a similar synthetic route. X-ray determination confirmed that 4b and 5b have stacking channels as well.

Studies of electronic effects of modified pyridine-imine ligands utilized in cobalt-catalyzed meta-selective Diels–Alder reactions

The regioselectivity of cobalt-catalyzed Diels–Alder reactions can be controlled by the choice of ligands on the cobalt center. Ligands of the pyridine-imine type favor the formation of the 1,3-disubstitution pattern on the dihydroaromatic product. The investigation was aimed to elucidate the factors for controlling the regioselectivities and reactivity induced by electronic effects of the pyridine-imine ligands in the cobalt-catalyzed Diels–Alder reaction. For that, electron-withdrawing as well as electron donating substituents were introduced in the 4-position on the pyridine moiety and on the 4′-position of the aniline derivative used in the imine subunit of the ligands. In close synergy DFT calculations and the comparison with experimental results proved that electronic variation of the substituents at both positions have a negligible influence on the regioselectivity. However, the kinetic data for the cobalt-catalyzed Diels–Alder reactions revealed that there are great differences in the lengths of the induction periods when different cobalt pyridine-imine complexes are applied. These results could be elucidated by conductivity experiments showing that ionic homoleptic complexes Co(L)22+/CoBr42− are in equilibrium with their corresponding neutral heteroleptic complexes of type Co(L)Br2 in solution. The equilibrium position depends on the electronic characteristics of the pyridine-imine ligands (L), thereby influencing the length of the induction period.

Cyclopentadienyl chromium diimine and pyridine-imine complexes: ligand-based radicals and metal-based redox chemistry

Paramagnetic CpCr(III) complexes with antiferromagnetically-coupled anionic radical diimine and pyridine-imine ligands were prepared and characterized. The diimine chloro CpCr[(ArNCR)(2)]Cl complexes (1: Ar = 2,6-iPr(2)C(6)H(3) (Dpp), R = H; 2: Ar = 2,6-Me(2)C(6)H(3) (Xyl), R = Me; 3: Ar = 2,4,6-Me(3)C(6)H(2) (Mes), R = Me) were synthesized by treatment of previously reported Cr(diimine)(THF)(2)Cl(2) precursors with NaCp. Reduction of 1 with Zn gives CpCr[(DppNCH)(2)], 4, resulting from reduction of Cr(III) to Cr(II) with retention of the ligand-based radical. Alkoxide complexes CpCr[(DppNCH)(2)](OCR(2)R') (5: R = Me, R' = Ph; 6: R = iPr, R' = H) were synthesized by protonolysis of Cp(2)Cr with HOCR(2)R' in the presence of the neutral diimine and catalytic base. The corresponding radical pyridine-imine complexes CpCr(PyCHNMes)Cl (9), CpCr(PyCHNMes) (8), and CpCr(PyCHNMes)(OCMe(2)Ph) (11), were prepared by analogous routes. Oxidation of 8 with iodine gives CpCr(PyCHNMes)I (10) where oxidation of Cr(II) to Cr(III) again occurs with retention of the anionic pyridine-imine radical ligand. The molecular structures of complexes 1, 2, 4-8, 10 and 11 were determined by single-crystal X-ray diffraction. Unusual low energy bands were observed in the UV-vis spectra of the reported complexes, with particularly strong transitions observed for the Cr(II) complexes 4 and 8. The electronic structure of pyridine-imine complexes 8 and 10 were investigated by theoretical calculations.

Atropisomerism in a thermally switchable, cyclometallated iridium complex

Two stable diastereomeric atropisomers of a cyclometallated iridium complex containing a pyrene functionalized pyridine imine ligand have been isolated. These are the first fully characterized examples of metal containing atropisomers in which the rotational axis is not between two chelating atoms. The atropisomers can be converted thermally via a rocking motion of the pyrene moiety.

Regiocontrolled Cobalt-Catalyzed Diels−Alder Reactions of Silicon-Functionalized, Terminal, and Internal Alkynes

The efficient control of the regiochemistry of the Diels-Alder adducts which are formed in excellent yields from 1,3-dienes and alkynylsilanes can be realized utilizing cobalt complexes with a pyridine-imine ligand or a dppe ligand, respectively. The application of 2-trimethylsilyloxy-1,3-butadiene leads to a very interesting cyclohexenone derivative suitable for further transformations.

CpRu-Catalyzed Asymmetric Carroll Rearrangement

Despite the great interest devoted to the Ru-catalyzed allylic substitution (C. Bruneau, J. L. Renaud, B. Demerseman Chem. Eur. J. 2006, 12, 5178-5187), only a single report of an enantio­selective process has appeared (M. D. Mbaye, J. L. Renaud, B. Demerseman, C. Bruneau Chem. Commun. 2004, 1870-1871). In the present paper, the authors describe the first ruthenium-catalyzed enantioselective Carroll rearrangement using a chiral pyridine imine ligand.

An Enantioselective CpRu‐Catalyzed Carroll Rearrangement

Simple ligands, catalyst, and conditions: The combination of readily prepared unsymmetrical pyridine–imine ligands and [CpRu(CH3CN)3][PF6] (Cp=C5H5) affords regio- and enantioselective Carroll rearrangements (see scheme; b/l= branched-to-linear ratio). This reaction represents the first example of Ru-catalyzed enantioselective CC bond-forming allylic substitution. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2007/z604573_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Preparation of Fluorinated Copolymers by Copper-Mediated Living Radical Polymerization

Copper-mediated living radical polymerization mediated with pyridine imine ligands has been used for the synthesis of poly(methacrylate)s and polystyrene incorporating fluorinated moieties as either initiators or monomers as well as in the presence of fluorinated solvents. It was found that fluorinated organics do not interfere detrimentally in the polymerization mechanism. α-Perfluoroalkyl initiators have been used for the polymerization of MMA initiated by both 2-perfluoroalkyl ethyl 2-bromoisobutyrate (ZONYL BA initiator) and 2-perfluoroalkyl ethyl-co-poly(ethylene glycol) 2-bromoisobutyrate (ZONYL FSO initiator) initiators which occurs at an almost identical rate to that initiated by ethyl 2-bromoisobutyrate (EIBr). The Mn increased linearly with conversion, and the polydispersity indices were low throughout all polymerizations (<1.20). Polymerization of MeOPEG-MA initiated by 2-perfluoroalkyl ethyl 2-bromoisobutyrate (ZONYL BA initiator) at 60 °C results in a water-soluble/dispersible polymer with Mn = 6700 g mol-1 and PDI = 1.07. Perfluoroalkylethyl methacrylate (FEMA) proves difficult to homopolymerize due to solubility problems but has been used to form statistical copolymers with MMA and styrene. Synthesis of AB block copolymers using fluorinated macroinitiators with a methacrylate B block gives both hydrophobic, MMA, and hydrophilic, MeOPEG-MA, polymers with well-defined α-terminal block copolymers and PDI as low as 1.10. Copper-mediated living radical polymerization was found to be an effective method for the synthesis of fluorine-containing synthetic polymers.

The pH dependent phase transfer of an organometallic complex: synthesis, characterization, and crystal structure of a W(0) tetracarbonyl with an acidic pyridine–imine ligand

Reaction of the asymmetrical pyridine–imine ligand L (where L=benzoic acid, 3-[(2-pyridinylmethylene)amino]) with W(CO) 3(EtCN) 3 and CO (g) affords a W(0) tetracarbonyl in which the Schiff-base ligand coordinates in a bidentate fashion. The complex was characterized by IR, NMR, and UV–visible spectroscopy and elemental analysis. The solid-state structure of W(CO) 4( L), which crystallized in the orthorhombic space group Pbca with unit cell parameters a=22.497(4) Å, b=14.112(6) Å, and c=13.675(3) Å was established by X-ray crystallography. Deprotonation of the –CO 2H group on the ligand with base results in the formation of a water-soluble organometallic complex which can be partitioned between organic and aqueous phases in a biphasic system by pH adjustment.