Imino Pyridine Ligands Research Articles

Asymmetric dihydroboration of allenes enabled by ligand relay catalysis

Catalytic asymmetric hydroboration of unsaturated bonds has been recognized as the most straightforward method for the construction of chiral organoboron compounds. Although catalytic asymmetric hydroboration of alkenes has been well-developed, enantioselective hydroboration of allenes still remains rare probably due to the challenges in controlling the enantio-, stereo-, and regioselectivity. Additionally, the hydroboration products might go through over-borohydride, making the catalytic asymmetric dihydroboration of allenes challenging. Here, we report a cobalt-catalyzed asymmetric dihydroboration of allenes using a ligand relay strategy with two simple ligands. This protocol shows excellent enantio-, stereo-, and regioselectivity with positive functional group compatibilities in the construction of chiral 1,4-diboronate products. The applications of this reaction are demonstrated by various product derivatizations, gram-scale reactions, and the preparation of artigenin analogues. Mechanistic studies indicate that the achiral ligand controls the first hydroboration of allenes, and the chiral oxazoline iminopyridine ligand is responsible for the subsequent isomerization and asymmetric hydroboration.

Synthesis, structure, characterization and one pot catalytic activity of half-sandwich iridium(III) complexes

A series of stable mononuclear half-sandwich iridium(III) complexes of the general formula [(η5-C5Me5)Ir(L)Cl](BPh4) has been synthesized in dry methanol using different imino pyridine ligands (L1–L6) and [(η5-C5Me5)2Ir2(µ-Cl)2Cl2] in a 2:1 molar ratio. All complexes were fully characterized by melting point, CHN analysis, FT-IR, UV–visible, 1H NMR, 13C NMR spectroscopy and mass spectrometry. The structure of complex 3 [(η5-C5Me5)Ir(L3)Cl](BPh4) was determined by single crystal X-ray structure analysis, showing a chelating coordination mode of the imino pyridine L3. One pot organic transformations of aldehyde to amide were carried out by complexes 1–6 in toluene at 110 °C in the presence of hydroxyl amine hydrochloride and sodium bicarbonate. The catalytic activity of complex 3 was found to be excellent, showing high conversion with catalytic turnover frequency up to 500.

High−molecular weight poly(2-substituted 1,3-diene)s using iminopyridine iron precatalysts at high polymerization temperature and low iron loading

The synthesis of a series of iron(II) dichloride complexes bearing iminopyridine (ImPy) ligands, differing in the substituents at the pyridine, bridging carbon and N−aryl ring is described (1-LHPh−7-LHS). The X-ray structures of some of them are presented. All the iron compounds and, for comparison, the already reported 8-LHPhP, that features bulky ortho isopropyl N−aryl substituents, have been investigated as precatalysts for the polymerization of 2-substituted 1,3-dienes, i.e., isoprene and β-myrcene, using a low excess of methylaluminoxane (MAO) as alkylating reagent, iron loading as low as 0.025 mol% and variable temperature up to 100 °C. The goal is to have insights into ligand effects on catalyst reactivity and selectivity and to recognize the most robust iron precatalysts to access high-molecular weight (MW) polymers using low iron loading and high polymerization temperature, which meets current demands of mild, operationally simple approach. Particularly, the ketimine 4-LMePh and 5-LPhPh are more stable and least prone to chain termination than all the aldimine parents. 4-LMePh and 5-LPhPh produce cis-1,4/3,4 ultra-high-MW poly(isoprene)s and solid high-MW poly(β-myrcene)s, while maintaining narrow and unimodal molecular weight distribution. In this study we intentionally kept the monomer conversion low to provide a valuable assessment of catalyst stability, uncorrupted by mass or heat transport limitations. Furthermore, the synthesis of the unsubstituted aminopyridine (AmPy) iron(II) dichloride complex 9-LAPh, analogue to 1-LHPh, is described. 9-LAPh has been demonstrated to be a valuable probe for stability studies. Given the variety of catalytic transformations and substrates for which (ImPy)FeCl2 precatalysts have promising reactivity and selectivity, those developed in this study offer promising applications also beyond the polymerization of 1,3-dienes.

Synthesis, characterization, and anticancer properties of iminopyridine platinum(II) complexes containing long aliphatic chains

A series of iminopyridine ligands were prepared from the condensation reaction of 6-methylpyridine-2-carboxaldehyde and long-chain aliphatic primary amines. Square planar platinum(II) complexes were synthesized from the reaction of [PtCl2(η2-coe)]2 (coe = cis-cyclooctene) with two equivalents of ligand. The synthesis of the platinum compounds appears to go through a five-coordinate trigonal pyramidal species where coe remains coordinated initially but eventually dissociates to afford the four-coordinate complexes. Ligands and platinum complexes were characterized using multi-nuclear NMR and FT-IR spectroscopies as well as an X-ray diffraction study for the platinum complex derived from n-hexylamine. The cytotoxic properties of the platinum complexes against the ovarian cancer cell line SKOV-3 using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide colorimetric method were examined. Relative cytotoxicities of the platinum complexes follow a trend of greater potency with increasing chain length against the SKOV-3 cell line, with the longest aliphatic hydrocarbon chain (C16) exhibiting anticancer effects comparable to that of cisplatin.

Doublet Ground State in a Vanadium(II) Complex: Redox and Coordinative Noninnocence of Tripodal Ligand Architecture

We report on the geometric and electronic structures of a series of V2+/3+ tren-bridged iminopyridine complexes [tren = tris(2-aminoethyl)amine] that enable the observation of an unexpected doublet ground state for a nominally 3d3 species. Tren undergoes condensation reactions with picolinaldehyde or methyl-6-formylnictonate to form the respective tripodal ligand sets of (py)3tren and (5-CO2Mepy)3tren. The (py)3tren ligand is coordinated to V2+ and V3+ metal centers to form complex salts [1-H](OTf)2 and [1-H](OTf)3, respectively (OTf- = CF3SO3-). For [1-H]2+, strong metal-ligand π-covalency with respect to the V2+ (3d3) and iminopyridine ligands weakens its interelectronic repulsion. For [1-H]3+, the bridgehead nitrogen of the tren scaffold forms a seventh coordinate covalent bond with a V3+ (3d2) metal center. The coordination of (5-CO2Mepy)3tren to a V2+ metal center results in the redox noninnocent and heptacoordinate compound [1-CO2Me](OTf)2 with a doublet (S = 1/2) ground state that we support with magnetic susceptibility and spectroscopy measurements. The complexes are uniformly characterized experimentally with single-crystal X-ray diffraction, electronic absorbance, and electrochemistry, and electronic structures are corroborated by computational techniques. We present a new computational procedure that we term the spin-optimized approximate pair (SOAP) method that enables the visualization and quantification of electron-electron interactions.

Synthesis, electronic structures, and reactivity of mononuclear and dinuclear low-valent molybdenum complexes in iminopyridine and bis(imino)pyridine ligand environments

Molybdenum in redox non-innocent ligand environments features prominently in biological inorganic systems. While Holm and coworkers, along with many other researchers, have thoroughly investigated formally high-oxidation-state molybdenum (Mo(IV)-Mo(VI)) ligated by dithiolenes, less is known about molybdenum in other formal oxidation states and/or different redox-active ligand environments. This work focuses on the investigation of low-valent molybdenum in four different redox non-innocent nitrogen ligand type environments (mononucleating and dinucleating iminopyridine, mononucleating and dinucleating bis(imino)pyridine). The reaction of iminopyridine N-(2,6-diisopropylphenyl)-1-(pyridin-2-yl)methanimine (L1) with Mo(CO)3(NCMe)3 produced Mo(L1)(CO)3(NCMe). Mo(L1)(CO)3(NCMe) undergoes transformation to Mo(L1)(CO)4 upon treatment with CS2 or prolonged stirring in dichloromethane. The reaction of the open-chain dinucleating bis(iminopyridine) ligand N,N′-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine) (L2) similarly produced an hexacarbonyl complex Mo2(L2)(CO)6(NCMe)2 which also underwent transformation to the octacarbonyl Mo2(L2)(CO)8. Both complexes featured anti-parallel geometry of the chelating units. The oxidation of Mo(L1)(CO)3(NCMe) with I2 resulted in Mo(L1)(CO)3I2. The reaction of mononucleating potentially tridentate bis(imino)pyridine ligand (L3) (N-mesityl-1-(6-((E)-(mesitylimino)methyl)pyridin-2-yl)methanimine) with both Mo(CO)3(NCMe)3 and Mo(CO)4(NCMe)2 produced complexes Mo(L3)(CO)3(NCMe) and Mo(L3)(CO)4 in which L3 was coordinated in a bidentate fashion, with one imino sidearm unbound. The reaction of dinucleating macrocyclic di(bis(imino)pyridine) analogue (L4) led to the similar chemistry of Mo2(L4)(CO)6(NCMe)2 and Mo2(L4)(CO)8 complexes. Treatment of Mo(L3)(CO)3(NCMe) with I2 formed a mono(carbonyl) complex Mo(L3)(CO)I2 in which molybdenum was formally oxidized and L3 underwent coordination mode change to tridentate. The electronic structures of formally Mo(0) complexes in iminopyridine and bis(imino)pyridine ligand environments were investigated by density functional theory calculations.

Structural evolution of iminopyridine support for nickel/palladium catalysts in ethylene (oligo)polymerization.

The interest in the late transition metal catalyst based design of new architectures of polyethylene (PE) has continuously been increasing over the last few years. The structure of these catalysts is predominantly important in controlling the morphological and architectural properties of the resulting polyethylene. Particularly, iminopyridine is a versatile bidentate support for Ni and Pd catalysts in ethylene (oligo)polymerization providing a wide variety of products ranging from volatile oligomers to ultra-high molecular weight polyethylene. Extensive structural modifications have been induced in the iminopyridine ligand through steric and electronic substitution, tuning the catalyst behavior in terms of activity and properties of the resulting polymer. Carbocyclic-fused iminopyridine and N-oxide iminopyridine are the new state of the art iminopyridine ligand designs. In this review, we aim to summarize all the developments in mononuclear iminopyridine-nickel and -palladium catalysts for ethylene (oligo)polymerization since the first report published in 1999 to present, focusing on the correlation among the pre-catalyst, co-catalyst type, thermal stability and polymer/oligomer structure. For comparison, the structural variations in the binuclear iminopyridine-nickel catalysts are also described. The detailed comparison of the structural variations in these catalysts with respect to their polymerization performance will give deep understanding in the development of new efficient catalyst designs.

Температурные зависимости давления насыщенного пара комплексов олова(IV) с полидентатными лигандами

The present paper is a continuation of the previously pursued studies of thermal properties of the Sn(IV) complexes with tridentate iminopyridine ligands. The study of thermal properties including temperature dependences of the saturated vapor pressure for Sn(IV) complexes with the tridentate iminopyridine ligands by differential scanning calorimetry (DSC) and the Knudsen effusion method leads to the use of the studied compounds as luminescent materials, as well as components for nonlinear optics. Coordination compounds of tin (IV) containing tridentate Schiff bases were chosen as objects of research: 2,4-di-tert-butyl-6-((phenyl (pyridin-2-yl)methylene)amino)phenolatotrichlorotin(IV) (1), 4-methyl-2-((phenyl (pyridin-2-yl)methylene)amino)phenolatotrichlorotin(IV) (2), 4-chloro-2-((phenyl (pyridin-2-yl)methylene)amino) phenolatotrichlorotin(IV) (3). These compounds were obtained by tem-plate synthesis from tin tetrachloride, various o-aminophenols, and α-carbonyl substituted pyri-dines; they contain an iminopyridine function and are capable of covalently binding to a metal by a phenolic group. Phase transitions of compounds 1–3 were studied by DSC in the temperature range 25–500 °C. For all studied complexes an endothermic transition correlated to melting was revealed. The melting points of the studied compounds were taken to be the temperature values corresponding to the onset of the transition, according to the standard Netzsch Software Proteus method: 300, 320 and 330 °C for 1–3, respectively. The temperature dependences of the saturated vapor pressure were obtained for the studied complexes, using the Knudsen effusion method with the weight recording of the amounts of sublimated substances. As a result of the experiments it was found that compounds 1–3 sublimated in the temperature ranges of 239–244, 228–260 and 226–256 °C, respectively. Thermodynamic parameters of sublimation processes were calculated.

Synthesis of Branched Polyethylene and Ethylene-MA Copolymers Using Unsymmetrical Iminopyridyl Nickel and Palladium Complexes

It is usually challenging to obtain high-molecular-weight polyethylene in the iminopyridyl Ni(II) and Pd(II) system due to the only one side axial steric structure of the iminopyridine ligand. In t...

(Iminopyridine)Pd (II) complexes as versatile catalysts for copolymerization and terpolymerization of vinyl arene, ethylene, and carbon monoxide

AbstractPalladium catalysts modified with iminopyridine ligands were employed for ethylene/CO and styrene/CO copolymerization and ethylene/styrene/CO terpolymerization. Our studies revealed that the steric and electronic effects of the catalysts play key, yet distinct roles in these polymerization reactions. The thermal properties of the polyketones were engineered by varying the styrene content in terpolymers derived from ethylene, styrene, and CO sources, with increased styrene ratios yielding reduced polyketone crystallinity.

Iron- and Cobalt-Catalyzed Asymmetric Hydrofunctionalization of Alkenes and Alkynes.

Transition metal catalyzed asymmetric hydrofunctionalization of readily available unsaturated hydrocarbons presents one of the most straightforward and atom-economic protocols to access valuable optically active products. For decades, noble transition metal catalysts have laid the cornerstone in this field, on account of their superior reactivity and selectivity. In recent years, from an economical and sustainable standpoint, first-row, earth-abundant transition metals have received considerable attention, due to their high natural reserves, affordable costs, and low toxicity. Meanwhile, the earth-abundant metal catalyzed hydrofunctionalization reactions have also gained much interest and been investigated gradually. However, since chiral ligand libraries for earth-abundant transition-metal catalysis are limited to date, the development of highly enantioselective versions remains a significant challenge.This Account summarizes our recent efforts in developing suitable chiral ligands for iron and cobalt catalysts and their applications in the highly enantioselective hydrofunctionalization reactions (hydroboration and hydrosilylation) of alkenes and alkynes. In ligand design, we envisioned that chiral unsymmetric NNN-tridentate (UNT) ligand scaffolds could promote these enantioselective transformations with earth-abundant metals. Therefore, several types of chiral UNT ligands were designed and prepared in our laboratory, utilizing readily available natural amino acids as chiral sources. In the very beginning, chiral oxazoline iminopyridine (OIP) ligands were proposed and investigated through the rational combination of nitrogen-containing ligand scaffolds. After a systematic survey of the ligand effects, the imine moiety in the rigid OIP ligands was replaced by a conformationally more flexible amine unit, leading to the construction of reactive oxazoline aminoisopropylpyridine (OAP) ligands. Subsequently, imidazoline iminopyridine (IIP) and thiazoline iminopyridine (TIP) ligands were prepared by altering the oxygen atom of oxazoline with nitrogen and sulfur linkers, respectively. To further expand the chiral ligand library, other tridentate ligands containing a twisted pincer, anionic, and nonrigid backbone were also designed and synthesized, including iminophenyl oxazolinyl phenylamine (IPOPA) and imidazoline phenyl picolinamide (ImPPA). The efficacy of these chiral UNT ligands for asymmetric induction in iron and cobalt catalysis has been demonstrated through asymmetric hydrofunctionalization of alkenes and asymmetric sequential hydrofunctionalization of alkynes, which exhibit excellent reactivity as well as high chemo-, regio-, and stereoselectivity with broad functional group tolerance. Notably, highly regio- and enantioselective hydrofunctionalization of challenging substrates, such as 1,1-disubstituted aryl alkenes and terminal aliphatic alkenes, was also achieved. Furthermore, the development of asymmetric sequential isomerization/hydroboration of internal alkenes and sequential hydrofunctionalization of alkynes further demonstrates the synthetic power of these catalytic systems. The chiral enantioenriched products obtained by these methodologies could be potentially utilized in organic synthesis, medicinal chemistry, and materials science. We believe that our continuous efforts in this field would be beneficial to the development of asymmetric earth-abundant metal catalysis.

Rational design and synthesis of AIE active cationic Ir(III) complexes featuring iminopyridine ligand with dibenzosuberane core

Novel cationic iridium(III) complexes (Ir1–Ir4) composed of iminopyridine ligands with dibenzosuberane core were synthesized. All the designed complexes Ir1–Ir4 were fully characterized through FT-IR, elemental analysis, 1H, 13C, 19F, and 31P NMR. Molecular structures of Ir1 and Ir2 are unambiguously determined by single-crystal XRD studies and thus confirm the distorted-octahedral geometry around the Ir(III) centers. Photophysical properties such as absorption, emission, and photoluminescence quantum yields (PLQYs) of Ir1–Ir4 can be accessibly finely-tuned by varying the substituents of ancillary ligands (N^N). Cyclic voltammetry (CV), and thermogravimetric analysis (TGA) were then performed to investigate the electrochemical and thermal properties of Ir1–Ir4. Solution-state emission spectra of the complexes Ir1–Ir4 showed blue-shifted emission in the range of 433 – 436 nm (PLQYs = 0.19 – 0.62 %) whereas the same complexes displayed red-shifted emission (λ max 626 – 638 nm, PLQYs = 0.31 – 0.65 %) of their solid-state emission spectra. Aggregation-induced emission (AIE) properties of Ir1–Ir4 were analyzed with dichloromethane and hexane solvent mixture. Upon aggregation (99 % of hexane), the emission intensities of Ir1–Ir4 were constantly increased with the increase of hexane concentration. Similarly, the PLQYs and lifetimes of aggregated species of Ir1–Ir4 (PLQYs = 18 – 28 % and τ = 78.8 – 101.3 ns) were excellently improved in contrast to those in pure dichloromethane solutions. Interestingly, the scanning electron microscope (SEM) images of aggregates of Ir2 exposed the hollow polymer type of morphology and such a kind of morphology derived from Ir(III) complex is not reported.

Synthesis, characterization, and ethylene oligomerization with star iminopyridine nickel(II) complexes

A series of star iminopyridine ligands, containing different groups (H, CH3, Br) in the pyridine ring, have been synthesized and characterized by FT-IR, 1H NMR, and UV-vis. The corresponding nickel(II) complexes have been prepared in good yields. The nature of the Ni complexes has been established by FT-IR, UV-vis, ESI-MS, and ICP. Upon activation with methylaluminoxane (MAO), the nickel complexes were able to oligomerize ethylene to produce C4–C18 fractions at activity of up to 105 g (mol Ni h)−1. The influence of substituent, catalyst structure, solvent, co-catalyst, and reaction conditions on the catalytic activity and product selectivity was investigated. The results showed that substitution on the pyridine ring had large influence on the catalytic activity and product selectivity. The catalytic activity increased at first and then decreased with the increase of reaction temperature and Al/Ni molar ratio. However, the catalytic activity continuously increased with increasing reaction pressure. When toluene was used as solvent and MAO was used as co-catalyst, catalytic activity of 4.39 × 105 g (mol Ni h)−1 and 31.93% selectivity of higher carbon olefins (C8–C18) were obtained at 10 °C, Al/Ni molar ratio 500, and the reaction pressure 1.0 MPa.

NEt3-Triggered Synthesis of UHMWPE Using Chromium Complexes Bearing Non-innocent Iminopyridine Ligands

Chromium complexes bearing non-innocent iminopyridine ligands were investigated as precatalysts for the polymerization of ethylene using different aluminum cocatalysts and Lewis base NEt3 as additive. Beyond confirming the key role of the chromium to ligand synergy in accessing the active complex, other factors that play a crucial role are (i) the nature of the aluminum activator that influences the ion pair generated, (ii) the presence of the additive that boosts the synthesis of UHMWPE with narrow and unimodal molecular weight distribution even at 40 °C, and (iii) the polymerization temperature that affects the polymerization catalysis and the polymer molecular weight. UV–vis–NIR and Fourier transform infrared (FT-IR) spectroscopic techniques have been applied to inspect the activation process and to understand the mode of action by which NEt3 affects the catalytic conversion of ethylene. NEt3 interacts with the aluminum cocatalyst (rather than with the chromium complex) to form an Et3N*AlRx adduct, thus affecting the catalytic ion pair.

A computational study of isoprene polymerization catalyzed by iminopyridine-supported iron complexes: Ligand-controlled selectivity

DFT calculations have been conducted for investigating the relation between the electronic states and catalytic properties of iminopyridine-ligated cationic iron complexes in the stereoselective polymerization of isoprene. Iminopyridine ligand is found to be redox-inert in this catalytic system. The neutral pre-catalyst and active cationic species present as a high spin quintet and the open-shell unpaired 3d-electrons reside on Fe center. The trans and cis polymerization of isoprene catalyzed by the complexes [(octyl-iminopyridine)FeII(Me)]+ (A1) and [(supermesityl-iminopyridine)FeII(Me)]+ (A2) could be attributed to the electronic and steric effects in the transition states, respectively.

Fluorescent zwitterionic Iridium(III) complexes containing sulfonate groups: synthesis, biological activity and tracking in live cells

Fluorescent zwitterionic iridium complexes have been rarely reported. In this work, four fluorescent zwitterionic cyclometalated iridium complexes bearing different sulfonated iminopyridine ligands (Ir1–Ir4) were synthesized. The solid-state molecular structure of Ir1 was established by X-ray crystallography. These zwitterionic complexes are very stable in aqueous solution and have a detectable fluorescence. The structure-activity relationship toward cancer cells was observed for these complexes. Further, the fluorescence properties of these zwitterionic complexes make it possible to determine their microscopic mechanism by the cell imaging. It is noteworthy that complex Ir3 can specifically and effectively target lysosomes and induce cell death by damaging lysosomes Further, the energy-dependent pathway was involved in the uptake of Ir3 in cancer cells. In addition, the modes of cell death were also studied through flow cytometry experiments. It seems that Ir3 displayed their biological efficacy by cell-cycle arrest and the induction of cell apoptosis.

Cobalt-Catalyzed Migrational Isomerization of Styrenes.

An efficient cobalt-catalyzed migrational isomerization of styrenes was developed using the thiazoline iminopyridine (TIP) ligand. This reaction is operationally simple and atom-economical using readily available starting materials to access trisubstituted alkenes. Even when using a 0.1 mol % catalyst loading, the reaction could be conducted in neat and completed in 1 h with excellent conversion and high E stereoselectivity.

Температурные зависимости давления насыщенного пара комплексов олова(IV) с тридентатными иминопиридиновыми лигандами

Температурные зависимости давления насыщенного пара комплексов олова(IV) с тридентатными иминопиридиновыми лигандами

Monoligated vs Bisligated Effect in Iminopyridyl Nickel Catalyzed Ethylene Polymerization

We developed an efficient method to synthesize monoligated and the corresponding bisligated iminopyridyl Ni(II) catalysts containing the same iminopyridine ligands. Subsequently, effects of monolig...

Heterospin Copper(II) Catecholate Complex with the TEMPO–Iminopyridine Ligand

The heterospin copper(II) complex, ((pyridin-2-ylmethylene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl)-3,6-di-tert-butylcatecholatocopper(II) (I), is synthesized and characterized by IR spectroscopy, magnetochemistry, EPR, and X-ray diffraction analysis. The one-electron oxidation of complex I by AgBF4 affords the biradical copper(I) complex: bis[((pyridin-2-ylmethylene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl)]copper(I) (II). An analysis of the parameters of isotropic EPR spectra of complexes I and II indicates that they are biradicals with the fast (I) and intermediate (II) exchange interaction between the radical centers.