Imino Pyridine Ligands Research Articles

Platinum(II) complexes with bidentate iminopyridine ligands: Synthesis, spectral characterization, properties and structural analysis

Four platinum(II) complexes, [PtCl 2L] (L = (4-fluorophenyl)pyridin-2-ylmethylene-amine, 1; (4-chlorophenyl)pyridin-2-ylmethyleneamine, 2; (4-bromophenyl)pyridin-2-ylmethyleneamine, 3 and (4-iodophenyl)pyridin-2-ylmethyleneamine, 4) have been synthesized and characterized by CHN analysis, IR and UV–Vis spectroscopy. The crystal structures of 1 and 2 were determined using single crystal X-ray diffraction. The coordination polyhedron about the platinum (II) center in the complexes is best described as distorted square planar. The complexes undergo stacking to form a zigzag Pt···Pt···Pt chain containing both short (3.57(7) Å in 1 and 3.62(8) Å in 2) and long (5.16(7) Å in 1 and 5.41(9) Å in 2) Pt···Pt separations through the crystal. The compounds absorb moderately in the visible region, owing to a charge-transfer-to-diimine electronic transition. The redox potentials are approximately insensitive to the substituents on the phenyl ring of the ligands.

Combining Dynamic Heteroleptic Complex Formation with Constitutional Dynamic Synthesis: A Facile Way to M3LL′ Cage Assemblies

The quantitative preparation of heteroleptic copper(I) complexes arising from a combination of 2,9-diarylphenanthrolines and iminopyridines (Schiff bases) is described. This strategy was applied to construct mono- and binuclear complexes but equally a discrete three-dimensional M(3)LL' cage. By means of a constitutional dynamic synthesis, the heteroleptic aggregates were equally prepared from four-component mixtures using the copper(I) center as a catalyst for the in situ generation of the iminopyridine ligands.

Structures of Co, Pd and Ni complexes with iminopyridine ligands having an hydroxymethyl or acrylate pendant group

Co(II), Pd(II) and Ni(II) complexes with iminopyridine ligands have different coordination structures in the solid state, depending on the metal, pendant group of the pyridyl ring and recrystallization conditions. The iminopyridine ligand having an acrylate group, Ar–N CH-2,6-C 5H 3N–CH 2–OCO–CH CH 2 ( L 1 : Ar = 2,6-diisopropylphenyl), forms a Co(II) complex whose metal center has a trigonal pyramidal coordination with two chloride ligands and an N, N’-bidentate ligand L 1 . Both L 1 and the iminopyridine ligand with a CH 2OH pendant on the pyridyl ring, Ar–N CH-2,6-C 5H 3N–CH 2–OH ( L 2 : Ar = 2,6-diisopropylphenyl), coordinate to the PdCl 2 moiety, giving complexes with a square–planar coordination. The reaction of L 1 with [NiBr 2(H 2O) 3] and recrystallization of the products forms crystals which contain mono- and dinuclear complexes with aqua ligands and the N, N’-bidentate ligand. L 2 is coordinated to the Ni(II) center as an N, N’, O-tridentate ligand. Recrystallization of the products of the reaction of L 2 with [NiBr 2(H 2O) 3] with and without excess H 2O in the solution yields crystals of the cationic octahedral complex with two aqua ligands [NiBr(H 2O) 2( L 2 )]Br and the neutral square pyramidal complex [NiBr 2( L 2 )], respectively.

Polymetallic Carbosilane Dendrimers Containing N,N'‐Iminopyridine Chelating Ligands: Applications in Catalysis

AbstractThis overview covers the most relevant results obtained by our group in the last few years concerning the synthesis and catalytic applications of metal complexes anchored to the surface of carbosilane dendrimers functionalized with iminopyridine ligands. Nickel dibromide complexes activated with methylaluminoxane (MAO) have been studied for the polymerization of ethylene, whereas cat‐ionic palladium(ii) derivatives have been applied to the copolymerization of carbon monoxide and tert‐butylstyrene. In addition, palladium dendrimers have been tested in the Heck reaction between methyl acrylate and iodotoluene. Correlations between the catalyst activities or reaction products (product distributions, polymer molecular weights, etc.) and the dendrimer generation are discussed and tentative explanations for these correlations provided.

Mercury(I) complexes with bidentate iminopyridine ligands: Synthesis, spectral characterization and structural analysis

Four mercury(I) complexes, [L 2HgHgL 2](NO 3) 2 (L 2 = (4-fluorophenyl)pyridin-2-ylmethyleneamine, 1; (4-chlorophenyl)pyridin-2-ylmethyleneamine, 2; (4-bromophenyl)pyridin-2-ylmethyleneamine, 3 and (4-iodophenyl)pyridin-2-ylmethyleneamine, 4) have been synthesized and characterized by CHN analysis, IR and UV–Vis spectroscopy. The crystal structures of 1 and 3 were determined by single crystal X-ray spectroscopy. These complexes contain the Hg 2 2 + moiety with a mercury–mercury bonded core, in which one diimine ligand is coordinated to each mercury atom. The Hg atoms have an additional axial interaction with the oxygen atom of the NO 3 - anion in both complexes. Complex 1 shows a step in the Hg–Hg bond due to the parallel configuration of the planar ligands. However, the ligand planes in the complex 3 are nearly planar and these planes are approximately perpendicular to each other. Supramolecular structures were built in both complexes via π–π stacking of the diimine ligands.

Structure-dependent spectroscopic and redox properties of copper(I) complexes with bidentate iminopyridine ligands

A series of iminopyridine ligands; cyclopropylpyridin-2-ylmethyleneamine ( A), cyclopentylpyridin-2-ylmethyleneamine ( B), cyclohexylpyridin-2-ylmethyleneamine ( C), and cycloheptylpyridin-2-ylmethyleneamine, ( D) and their copper(I) complexes, [Cu(L) 2] + ( 1a– 1d) and [Cu(L)(PPh 3) 2] + ( 2a– 2d) have been synthesized and characterized by CHN analyses, 1H NMR and IR and UV–Vis spectroscopy. Structures of 1a, 1b, 1c and 2a were determined by X-ray crystallography. The coordination polyhedron about the Cu I center in the complexes is best described as a distorted tetrahedron. The dihedral angles between the least-squares planes of the chelate ligands show considerable variation from 86.1° in 1a to 68.3° in 1b, indicating the importance of packing forces in the crystalline environment. The UV–Vis spectra of the complexes are characterized by first metal to ligand charge transfer bands increasing in wavelength with increasing size of the ring substituents in the ligands, except for the cyclopropyl compounds ( 1a and 2a), in good agreement with the variation of the dihedral angles between the ligand planes. Cyclic voltammetry of the complexes indicates a quasireversible redox behavior for the complexes. The bulkier ligands (PPh 3) inhibit the geometric distortion within the oxidized form and the redox potentials of complexes 2a– 2d are shifted to more positive values, therefore.

Self-assembly of silver(i) and bis-bidentate N,N-donor ligands: from a tetranuclear complex to coordination polymers

The supramolecular self-assembly structures of one tetranuclear silver(i) complex and three silver(i) coordination polymers derived from bis-bidentate imino-pyridine ligands and their reduced N-methyl derivatives are discussed. Six new ligands were synthesised, and the X-ray structural analysis of L(1), L(2) and L(5) with silver(i) ions indicated the formation of 1D coordination polymers, namely {[AgL(1)]BF(4)}(n) (1), {[AgL(2)]ClO(4)}(n) (2), and {[AgL(5)]ClO(4).H(2)O}(n) (5), whereas ligand L(4) led to a tetranuclear complex {[AgL(4)]ClO(4)}(4) (4). L(1) is the Schiff-base derived from the condensation of alpha,alpha'-diamino-m-xylene and pyridine-2-carboxaldehyde and L(4) that resulting from the reaction of 2-(4-aminophenyl)ethylamine with 6-methyl-pyridine-2-carboxaldehyde, while L(2) and L(5) are the reduced N-methyl derivatives. The structural determination shows achiral syndiotactic polymers for 1 and chiral isotactic for 2 and 5. The 1D coordination chains of 1 are stabilised by intramolecular pi-pi stacking interactions between the phenyl ring of a spacer with the pyridine of a symmetry related adjacent ligand, while the crystal packing of 5 show inter-chain pi-pi interactions. Compound 4 comprises discrete tetranuclear [AgL(4)](4)(4+) complex cations and the rhomboid disposition of the metal ions leads to a globular arrangement of the silver(i) complex. The results provide an insight about the effect of ligand spacer length and flexibility in stabilising the metallo-supramolecular architecture.

Design and electrochemical characterization of a new cobalt(II)–cyclodextrin complex. Evidence for a supramolecular stabilization of the Co(I) state

The electrochemical behavior of a new cobalt–cyclodextrin (CD) complex was investigated, in dimethylformamide, from CoX 2 (X = Br and BF 4) in the presence of 1 equiv. 6-Deoxy-6- N-(2-methyliminopyridine)-β-cyclodextrin as ligand. Under these conditions, it was demonstrated for the first time, that the electrogenerated cobalt(I) species can be kinetically and thermodynamically stabilized. The electrochemical study of CoX 2 in the presence of a related iminopyridine ligand (2-pyridyl- N-benzylmethylimine), in which the cyclodextrin (CD) group was replaced by a simple aryl moiety, allowed to highlight the crucial role of the CD in this unexpected stabilization. Importantly, this unprecedented result was only observed when both the iminepyridine and the CD moieties were together covalently attached. Importantly, the supramolecular stabilized low-valent cobalt species remained fairly reactive towards aromatic halides despite its intrinsic stability. This original work opens new opportunities for the development of more selective catalytic processes both in organic and aqueous media.

Sterically Demanding Iminopyridine Ligands

AbstractTwo sterically demanding iminopyridine ligands, (2,6‐diisopropylphenyl)[6‐(2,4,6‐triisopropylphenyl)pyridin‐2‐ylmethylene]amine and (2,6‐diisopropylphenyl)[6‐(2,6‐dimethylphenyl)pyridin‐2‐ylmethylene]amine, were prepared by a two‐step process: first, condensation of 6‐bromopyridine‐2‐carbaldehyde with an equimolecular amount of 2,6‐diisopropylaniline, and second, Kumada‐type coupling of in‐situ‐formed Grignard compounds of 1‐bromo‐2,6‐dimethylphenyl and 1‐bromo‐2,4,6‐triisopropylphenyl. Dichlorido complexes of the ligands were synthesized starting from FeCl2, [PdCl2(cod)], [NiCl2(dme)], and CoCl2 (cod = 1,5‐cyclooctadiene, dme = dimethoxyethane). X‐ray crystal structure analyses of a Fe, Pd, and Co complex were determined. Ethylene polymerization/oligomerization behavior of the dichlorido complexes after activation with methylaluminoxane or triethylaluminum was studied. Ethylene dimerization selectivity greater than 95 % was observed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Synthesis, structure, and redox properties of copper(I) complexes with phenylpyridin-2-ylmethyleneamine derivatives

Four copper(I) complexes, [Cu(diimine)2]ClO4 (1 – 4, Diimine[dbnd](4-fluorophenyl)pyridin-2-ylmethylene-amine, (1) (4-chlorophenyl)pyridin-2-ylmethyleneamine, (2), (4-bromophenyl)pyridin-2-ylmethyleneamine, (3), and (4-iodophenyl)pyridin-2-ylmethyleneamine, (4)) have been synthesized and characterized by CHN analyses, 1H NMR, IR, and UV – vis spectroscopy. The crystal structure of [Cu(B)2]ClO4 (2) was determined from single crystal X-ray diffraction. The coordination polyhedron about the copper(I) center in 2 is best described as a distorted tetrahedron. A quasi-reversible redox behavior is observed for the complexes. The spectral, structural, and redox data of these complexes are compared with similar iminopyridine copper(I) complexes. The variation of substituent on the phenyl ring of iminopyridine ligands did not show significant change in the absorption and redox properties of these types of complexes.

The use of iminopyridines as efficient ligands in the palladium(II)-catalyzed cyclization of ( Z)-4′-acetoxy-2′-butenyl 2-alkynoates

Iminopyridines were found to be a sort of efficient bidentate ligands for the palladium(II)-catalyzed cyclization of ( Z)-4′-acetoxy-2′-butenyl 2-alkynoates in acetic acid to afford the α-( Z)-acetoxyalkylidene-β-vinyl-γ-butyrolactones. The iminopyridine ligands could not only inhibit β-hydride elimination but also stabilize the vinyl-palladium intermediate in acetic acid in the reaction.

Synthesis, spectral characterization, properties and structures of copper(I) complexes containing novel bidentate iminopyridine ligands

Four new ligands, (4-methyl-phenyl)-pyridin-2-ylmethylene-amine (A), (2,3-dimethyl-phenyl)-pyridin-2-ylmethylene-amine (B), (2,4-dimethyl-phenyl)-pyridin-2-ylmethylene-amine (C) and (2,5-dimethyl-phenyl)-pyridin-2-ylmethylene-amine (D), and their corresponding copper(I) complexes, [Cu(A) 2]ClO 4 ( 1a), [Cu(B) 2]ClO 4 ( 1b), [Cu(C) 2]ClO 4 ( 1c), [Cu(D) 2]ClO 4 ( 1d), [Cu(A)(PPh 3) 2]ClO 4 ( 2a), [Cu(B)(PPh 3) 2]ClO 4 ( 2b), [Cu(C)(PPh 3) 2]ClO 4 ( 2c) and [Cu(D)(PPh 3) 2]ClO 4 ( 2d), have been synthesized and characterized by CHN analyses, 1H and 13C NMR, IR and UV–Vis spectroscopy. The crystal structures of [Cu(B) 2]ClO 4 ( 1b), [Cu(C) 2]ClO 4 ( 1c) and [Cu(A)(PPh 3) 2]ClO 4 · 1/2CH 3CN ( 2a) were determined from single crystal X-ray diffraction. The coordination polyhedron about the copper(I) center in the three complexes is best described as a distorted tetrahedron. A quasireversible redox behavior is observed for the complexes.

Modular iminopyridine ligands. Application to the enantioselective copper(II)-catalyzed Henry reaction

Chiral iminopyridines prepared in a modular fashion from monoterpenic (camphor-derived) ketones and pyridinylalkylamines catalyze the enantioselective Henry (nitro aldol) reaction between nitromethane and o-anisol in the presence of copper(II) acetate, with high yields and good ee (up to 86%) under straightforward experimental conditions without the need for air or moisture exclusion.

Synthesis and characterization of random and block copolymers with pendant rhenium diimine complexes by controlled radical polymerization

AbstractWe report the polymerization of rhenium‐containing methacrylates by atom transfer radical polymerization. The structure of the monomer was confirmed by X‐ray crystallography, which showed the bulkiness of the metal‐complex moiety. The rhenium complexes were polymerized in the presence of copper(I) bromide, 1,1,4,7,7‐pentamethyldiethylenetriamine, and methyl 2‐bromopropionate. They were copolymerized with methyl methacrylate in different monomer ratios. An ABA triblock copolymer was also synthesized with poly(methyl methacrylate) as the macroinitiator. When 2,2′‐bipyridine was used as the ligand for the copper catalyst in the polymerizations, it underwent a ligand exchange process with the iminopyridine ligand in the monomer. The neutral rhenium complex in the homopolymers and copolymers could be converted into ionic forms by the replacement of the chloride with an imidazole ligand, and the solubility of the resulting ionic polymers was greatly enhanced. The photosensitizing properties of the doped and undoped polymer films were investigated by the measurement of the photocurrent response under an externally applied electric field. The photoconductivities of the polymers were approximately 10−12–10−13 Ω−1 cm−1. The experimental quantum efficiencies were simulated with Onsager's theory, and they showed that the initial quantum yield and thermalization distance were 10−3 and 1.7 nm, respectively. Transmission electron microscopy showed that the rhenium complexes aggregated to form domains with dimensions of approximately 20–30 nm. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1292–1308, 2005

Synthesis and reactivity of water-soluble platinum(II) complexes containing nitrogen ligands

A series of water-soluble platinum(II) complexes containing bidentate imino pyridine ligands L of the general formula LPtX 2 (X=Cl or Me) have been prepared. The dichloro complexes are very stable in water or dimethyl sulfoxide (DMSO), even at elevated temperatures, whereas the dimethyl complexes are less stable in these strongly polar solvents. In DMSO, an equilibrium between the complex LPtMe 2 and (DMSO) 2PtMe 2 is observed, whereas in water decomposition is observed within 1 day at room temperature.

Iron complexes bearing iminopyridine and aminopyridine ligands as catalysts for atom transfer radical polymerisation

A series of iron(II) complexes bearing iminopyridine and aminopyridine ligands with N-alkyl and N-aryl substituents has been synthesised. X-Ray crystal structures of n-propyl- and 2,6-diisopropylphenyl-iminopyridine derivatives reveal dinuclear structures with halide bridges; a derivative carrying a cyclododecyl imine substituent and an α-methylpyridine unit is mononuclear. In studies of their atom transfer radical polymerisation behaviour, the mononuclear iminopyridine derivative is found to catalyse the polymerisation of styrene and methyl methacrylate while the dinuclear catalysts polymerise styrene only. The aminopyridine complexes are more efficient ATRP catalysts for styrene polymerisation than their unsaturated iminopyridine relatives.

Excimer emission induced by metal ion coordination in 1,8-naphthalimide-tethered iminopyridine ligands

Ligands 2–5, containing the light-emitting subunit 1,8-naphthalimide, have been prepared and their photophysical properties studied by absorption and emission spectroscopy. Ligand 2 interacts in solution with several cations according to a 1 ∶ 2 (metal ∶ ligand) stoichiometry, the 1 ∶ 3 species being not favoured probably because of steric hindrance, while ligands 3–5 form 1 ∶ 3 adducts with all the investigated metal ions. The interaction of ligands 2–5 with metal ions induces considerable variations on the photophysical properties of the light-emitting subunit. The coordination of genuine transition metal ions (FeII, CoII, NiII, CuII) causes the emission intensity to decrease in all the investigated systems, while ZnII or CdII induce a fluorescence enhancement (system 2) or the formation of a new band in the emission spectra (systems 3–5) which can be ascribed to an intramolecular excimeric species. Excimeric emission is not observed in the complexes of 2, possibly because the ethylenic chain bridging the naphthalimide and the iminopyridine units is too short to allow the intramolecular interaction. The excimeric species disappears on increasing the metal ion (ZnII or CdII) concentration, as a result of the “disassembling” of the 1 ∶ 3 complexes and the consecutive formation of 1 ∶ 2 and 1 ∶ 1 species, in which the intramolecular interaction is less probable or no longer possible. The appearance and disappearance of an excimer band in the emission spectrum can be described as a convenient way to monitor a metal-driven assembling/disassembling process.