Imino Pyridine Ligands Research Articles

Triphenylantimony(V) catecholato complexes with 4-(2,6-dimethylphenyliminomethyl)pyridine. Structure, redox properties: The influence of pyridine ligand

The complexation of triphenylantimony(V) catecholates (3,6-DBCat)SbPh3 (1), (4,5-pip-3,6-DBCat)SbPh3 (2) and (4,5-Cl2-3,6-DBCat)SbPh3 (3) with neutral 4-(2,6-dimethylphenyliminomethyl)pyridine (Py-CHN–Ar) leads to the formation of hexacoordinated complexes (3,6-DBCat)SbPh3·(Py-CHN–Ar) (4), (4,5-pip-3,6-DBCat)SbPh3·(Py-CHN–Ar) (5) and (4,5-Cl2-3,6-DBCat)SbPh3·(Py-CHN–Ar) (6) (where 3,6-DBCat is 3,6-di-tert-butyl-catecholate, 4,5-pip-3,6-DBCat is 4,5-(N,N′-piperazine-1,4-diyl)-3,6-di-tert-butyl-catecholate, 4,5-Cl2-3,6-DBCat is 4,5-dichloro-3,6-di-tert-butyl-catecholate) containing NPyridine-coordinated neutral donor ligand Py-CHN–Ar. In the absence of donor ligands chlorine-containing catecholate 3 undergoes rearrangement in acetonitrile to form ionic complex [Ph4Sb]+[(4,5-Cl2-3,6-DBCat)2SbPh2]- (7). Complexes have been isolated and characterized by spectroscopic methods and cyclic voltammetry. The pyridine-containing catecholate (3,6-DBCat)SbPh3·Py (8) was also synthesized in order to compare it's the electrochemical behaviour with those of iminopyridine complexes 4–6. The presence of NPyridine-coordinated iminopyridine ligand changes the mechanism of catecholate oxidation in 4 and 6: the first oxidation wave (at Eox1p = 0.94 V for 4 and 0.99 V for 6) is two-electron and corresponds to the oxidation of dianionic catecholate ligand to a coordinated o-benzoquinone. In contrast to catecholates 4 and 6, the electrochemical behaviour of piperazine-containing catecholate 5 does not practically differ from that one for initial catecholate 2. The molecular structures of complexes 3–7 in crystals have been determined by single-crystal X-ray analysis.

Template Synthesis of Tin(IV) Complexes with Tridentate Iminopyridine Ligands

Six-coordinate metal complexes containing tridentate Schiff bases with an ONN-chelating moiety were prepared by template synthesis on tin tetrachloride using various aminophenols and α-carbonyl-substituted pyridines. The structures of five compounds were studied by X-ray diffraction (CIF files CCDC nos. 849152–1849156). The UV/Vis spectra of the obtained series of compounds were found to be highly sensitive to the nature of substituents in the moieties of organic ligand.

Heterospin bis(dioxolene)manganese complexes with iminopyridine ligands. The effect of ancillary ligand on the charge distribution in the complex

Three novel bis-dioxolene manganese complexes (dioxolene = the reduced forms of 3,6-di-tert-butyl-o-quinone) with iminopyridine ligands having various substituents (cyclohexyl, 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) and phenyl) at the imine nitrogen atom have been synthesized. The X-ray structural investigation has shown that the obtained compounds are characterized by different charge distribution. In spite of this complexes demonstrate close magnetic behavior. Compounds based on phenyl- and TEMPO-substituted iminopyridines containing anion-radical form of dioxolene are characterized by strong antiferromagnetic exchange interaction between unpaired electrons on the manganese ions and o-semiquinonato ligands.

Anionic guest-dependent slow magnetic relaxation in Co(ii) tripodal iminopyridine complexes.

We report the syntheses and magnetic property characterizations of four mononuclear cobalt(ii) complex salts featuring a tripodal iminopyridine ligand with external anion receptor groups, [CoL5-ONHtBu]X2 (X = Cl (1), Br (2), I (3) and ClO4 (4)). While all four salts exhibit anion binding through pendant amide moieties, only in the case of 1 is field-induced slow relaxation of magnetisation observed, whereas in the other salts this phenomenon is absent at the limits of our instrumentation. The effect of chloride inducing a seventh Co-N interaction and concomitant structural distortion is hypothesized as the origin of the observed dynamic magnetic properties observed in 1. Ab initio computational studies carried out on a 7-coordinate Co(ii) model species survey the complex interplay of coordination number and trigonal twisting on the sign and magnitude of the axial anisotropy parameter (D), and identify structural features whose distortions can trigger large switches in the sign and magnitude of magnetic anisotropy.

Synthesis, structure and magnetic properties of a series of Ln(iii) complexes with radical-anionic iminopyridine ligands: effect of lanthanide ions on the slow relaxation of the magnetization.

We report the investigation of synthesis, structure and magnetic properties of a series of homoleptic Ln(iii) complexes coordinated by radical-anionic iminopyridine ligands of general formula [Ln(IPy)3]·solv (IPy = iminopyridine; Ln = Tb, Dy, Er, Y, Gd). The dysprosium analogue exhibits a zero-field Single-Molecule Magnet (SMM) behavior.

Crystal structure of cationic η3-methallylpalladium complexes bearing aliphatic iminopyridine ligands

Three aliphatic ligands are synthesized and fully characterized by IR, 1H and 13C NMR spectroscopy. These ligands react with a zerovalent compound Pd(dba)2 in the presence of methallyloxytris(dimethylamino)phosphonium hexafluorophosphate salt [C4H7OP(NMe2)3]+PF6– as an allylating agent for the synthesis of cationic η3-methallylpalladium complexes to give three cationic mononuclear η3-methallylpalladium complexes in high yields. These formed complexes are characterized by IR, 1H NMR and 13C NMR spectroscopy. One of them is characte­rized by X-ray diffraction. A DFT-optimized structure is also discussed.

Copper complexes for the promotion of iminopyridine ligands derived from β-alanine and self-aldol additions: relaxivity and cytotoxic properties.

In the study presented herein, we explore the ability of copper complexes with coordinated pyridine-2-carboxaldehyde (pyca) or 2-acetylpyridine (acepy) ligands to promote the addition of amines (Schiff condensation) and other nucleophiles such as alcohols (hemiacetal formation). Distinct reactivity patterns are observed: unlike pyca complexes, acepy copper complexes can promote self-aldol addition. The introduction of a flexible chain via Schiff condensation with β-alanine allows the possibility of chelate ring ring-opening processes mediated by pH. Further derivatization of the complex [CuCl(py-2-C(H)[double bond, length as m-dash]NCH2CH2COO)] is possible by replacing its chloride ligand with different pseudohalogens (N3-, NCO- and NCS-). In addition to the change in their magnetism, which correlates with their solid-state structures, more unexpected effects in their cytotoxicity and relaxitivities are observed, which determines their possibility to be used as MRI contrast agents. The replacement of a chloride by another pseudohalogen, although a simple strategy, can be used to critically change the cytotoxicity of the Schiff base copper(ii) complex and its selectivity towards specific cell lines.

Concerted Electron Transfer in Iminopyridine Chromium Complexes: Ligand Effects on the Polymerization of Various (Di)olefins

A study of reactions among CrCl2, CrCl3(THF)3, and iminopyridine ligands differing in the nature of the substituents at the iminic carbon and at the ortho positions of the aryl ring (2,6-R12C6H3N═C...

Cooperative Binding of SO2 under M–O and C–S Bond Formation in a Rhenium(I) Complex with Activated Amino- or Iminopyridine Ligand

Metal–ligand cooperative activation of C=O and CN multiple bonds in transition metal complexes with pyridine-based ligands is a currently active field of research. We herein expand the substrate scope to S=O bonds. The anionic complex K [Re(amidopy)(CO)3] readily reacts with a SO2 source to give the sulfinate fac-K[Re(amidopy-OSO)(CO)3] under Re–O and C–S bond formation as a diastereomeric mixture, which shows mutual dynamic interconversion even at ambient temperature. The chemical exchange was studied by 2D 1H 1H EXSY NMR spectroscopy. Activation parameters are obtained via VT 1H NMR and spectral line shape analysis.

Azobased iminopyridine ligands and their rhenium metal complexes: Syntheses, spectroscopic, trans-cis photoisomerization and theoretical studies

The reaction of rhenium(I) pentacarbonyl chloride Re(CO)5Cl with N,N-dimethyl-4-((E)-(pyridin-2-ylmethylene) amino)phenyl) diazenyl) aniline L1 and (E)-4-((E)-(4-nitrophenyl)diazenyl)-N-(pyridin-2-ylmethylene) aniline L2 affords two rhenium metal complexes [ReL1(CO)3Cl] (ReL1) and [ReL2(CO)3Cl] (ReL2), respectively. These Re(I) complexes were characterized in detail, including their single crystal structures, absorption spectra and electronic structures using DFT calculations. Trans to cis photoisomerization of the free ligands and complexes was investigated experimentally and rationalized theoretically. Interestingly, light-induced photoisomerization of the azo fragment is observed for free ligand L1 while inhibition of this process is produced upon coordination to Re metal cation.

Influences of Fluorine Substituents on Iminopyridine Fe(II)- and Co(II)-Catalyzed Isoprene Polymerization.

A series of iminopyridine complexes of Fe(II) and Co(II) complexes bearing fluorinated aryl substituents were synthesized for the polymerization of isoprene. The structures of complexes 3a, 2b and 3b were determined by X-ray diffraction analysis. Complex 3a contained two iminopyridine ligands coordinated to the iron metal center forming an octahedral geometry, whereas 2b adopted a chloro-bridged dimer, and 3b featured with two patterns of cobalt centers bridged via chlorine atoms. Complexes 2b and 3b represented rare examples of chlorine bridged bimetallic Co(II) complexes. The fluorine substituents effects, particularly on catalytic activity and polymer properties such as molecular weight and regio-/stereo-selectivity were investigated when these complexes were employed for isoprene polymerization. Among the Fe(II)/methylaluminoxane (MAO) systems, the 4-CF3 substituted iminopyridine Fe(II) complex 1a was found as a highly active isoprene polymerization catalyst exhibiting the highest activity of 106 g·(mol of Fe)−1·h−1. The resultant polymer displayed lower molecular weight (Mn = 3.5 × 104 g/mol) and moderate polydispersity index (PDI = 2.1). Furthermore, the ratio of cis-1,4-/3,4 was not affected by the F substituents. In the series of Co(II)/AlEt2Cl binary systems, complexes containing electron-withdrawing N-aryl substituents (R = 4-CF3, 2,6-2F) afforded higher molecular weights polyisoprene than that was obtained by the complex containing electron-donating N-alkyl substituents (R = octyl). However, ternary components system, complex/MAO/[Ph3C][B(C6F5)4] resulted in low molecular weight polyisoprene (Mn < 2000) with high trans-1,4-unit (>95%).

Cobalt(II) complexes of a benzimidazole functionalized tridented nitrogen-donor imino-pyridine ligand: Synthesis, characterization and phenoxazinone synthase activity studies

Cobalt(II) complexes of a benzimidazole functionalized tridented nitrogen-donor imino-pyridine ligand: Synthesis, characterization and phenoxazinone synthase activity studies

Enantioselective Cobalt-Catalyzed Sequential Nazarov Cyclization/Electrophilic Fluorination: Access to Chiral α-Fluorocyclopentenones.

A newly designed thiazoline iminopyridine ligand for enantioselective cobalt-catalyzed sequential Nazarov cyclization/electrophilic fluorination was developed. Various chiral α-fluorocyclopentenones were prepared with good yields and diastereo- and enantioselectivities. Further derivatizations could be easily carried out to provide chiral cyclopentenols with three contiguous stereocenters. Furthermore, a direct deesterification of fluorinated products could afford chiral α-single fluorine-substituted cyclopentenones.

How Do Ring Size and π-Donating Thiolate Ligands Affect Redox-Active, α-Imino-N-heterocycle Ligand Activation?

Considerable effort has been devoted to the development of first-row transition-metal catalysts containing redox-active imino-pyridine ligands that are capable of storing multiple reducing equivalents. This property allows abundant and inexpensive first-row transition metals, which favor sequential one-electron redox processes, to function as competent catalysts in the concerted two-electron reduction of substrates. Herein we report the syntheses and characterization of a series of iron complexes that contain both π-donating thiolate and π-accepting (α-imino)-N-heterocycle redox-active ligands, with progressively larger N-heterocycle rings (imidazole, pyridine, and quinoline). A cooperative interaction between these complementary redox-active ligands is shown to dictate the properties of these complexes. Unusually intense charge-transfer (CT) bands, and intraligand metrical parameters, reminiscent of a reduced (α-imino)-N-heterocycle ligand (L•-), initially suggested that the electron-donating thiolate had reduced the N-heterocycle. Sulfur K-edge X-ray absorption spectroscopic (XAS) data, however, provides evidence for direct communication, via backbonding, between the thiolate sulfur and the formally orthogonal (α-imino)-N-heterocycle ligand π*-orbitals. DFT calculations provide evidence for extensive delocalization of bonds over the sulfur, iron, and (α-imino)-N-heterocycle, and TD-DFT shows that the intense optical CT bands involve transitions between a mixed Fe/S donor, and (α-imino)-N-heterocycle π*-acceptor orbital. The energies and intensities of the optical and S K-edge pre-edge XAS transitions are shown to correlate with N-heterocycle ring size, as do the redox potentials. When the thiolate is replaced with a thioether, or when the low-spin S = 0 Fe(II) is replaced with a high-spin S = 3/2 Co(II), the N-heterocycle ligand metrical parameters and electronic structure do not change relative to the neutral L0 ligand. With respect to the development of future catalysts containing redox-active ligands, the energy cost of storing reducing equivalents is shown to be lowest when a quinoline, as opposed to imidazole or pyridine, is incorporated into the ligand backbone of the corresponding Fe complex.

Zinc and copper complexes of stilbene iminopyridine ligands with η2-Olefin binding mode

Two stilbene based iminopyridine ligands (L1-L2) synthesized by a condensation reaction between N,N-Dimethyl-4,4′-azodianiline and 2-pyridinecarboxaldehyde or 2,6-pyridinedicarboxaldehyde, with 73% and 65% yield, are described. The two ligands have been characterized by elemental analysis and spectroscopic techniques. The complexation of ligand L1 with ZnCl2 afforded neutral tetrahedral zinc(II) metal complex C1 formulated as [ZnL1Cl2] with a 2D supramolecular architecture reinforced by π···π stacking and hydrogen bonding in the solid state. Interestingly in the case of copper(I) complex C2, ligand L1 acts as a ditopic ligand since it coordinates one Cu(I) with an iminopyridyl fragment and a second metal center with an η2-olefin binding mode giving rise to a 1D-polymeric structure. The coordination sphere is completed with an acetonitrile solvent molecule leading to a distorted tetrahedral geometry around copper cation and the resulting coordination polymer can be formulated as {[Cu(L1)2CH3CN]BF4}. In addition complexation of L2 with zinc chloride afforded complex C3 formulated as [ZnL2Cl2]. DFT and TDDFT computations permitted to investigate the frontier MOs of all species and to assign their UV–visible absorption bands.

10 gram-scale synthesis of a chiral oxazoline iminopyridine ligand and its applications

This method reports on the preparation of a chiral oxazoline iminopyridine (OIP) ligand and its applications on a scale of 10 grams.

Steric control in the metal–ligand electron transfer of iminopyridine–ytterbocene complexes

A systematic study of reactions between Cp*2Yb(THF) (Cp* = η5-C5Me5, 1) and iminopyridine ligands (IPy = 2,6-iPr2C6H3N[double bond, length as m-dash]CH(C5H3N-R), R = H (2a), 6-C4H3O (2b), 6-C4H3S (2c), 6-C6H5 (2d)) featuring similar electron accepting properties but variable denticity and steric demand, has provided a new example of steric control on the redox chemistry of ytterbocenes. The reaction of the unsubstituted IPy 2a with 1, either in THF or toluene, gives rise to the paramagnetic species Cp*2YbIII(IPy)˙- (3a) as a result of a formal one-electron oxidation of the YbII ion along with IPy reduction to a radical-anionic state. The reactions of 1 with substituted iminopyridines 2b-d, bearing aryl or hetero-aryl dangling arms on the 6 position of the pyridine ring occur in a non-coordinating solvent (toluene) only and afford coordination compounds of a formally divalent ytterbium ion, coordinated by neutral IPy ligands Cp*2YbII(IPy)0 (3b-d). The X-ray diffraction studies revealed that 2a-c act as bidentate ligands; while the radical-anionic IPy in 3a chelates the YbIII ion with both nitrogens, neutral IPy ligands in 3b and 3c participate in the metal coordination sphere through the pyridine nitrogen and O or S atoms from the furan or thiophene moieties, respectively. Finally, in complex 3d the neutral IPy ligand formally adopts a monodentate coordination mode. However, an agostic interaction between the YbII ion and an ortho C-H bond of the phenyl ring has been detected. Imino-nitrogens in 3b-d are not involved in the metal coordination. Variable temperature magnetic measurements on 3a are consistent with a multiconfigurational ground state of the Yb ion and suggest that the largest contribution arises from the 4f13-radical configuration. For complexes 3b and 3c the data of magnetic measurements are indicative of a YbII-closed shell ligand electronic distribution. Complex 3d is characterized by a complex magnetic behavior which does not allow for an unambiguous estimation of its electronic structure. The results are rationalized using DFT and CSSCF calculations. Unlike diazabutadiene analogues, 3a does not undergo a solvent mediated metal-ligand electron transfer and remains paramagnetic in THF solution. On the other hand, complexes 3b-d readily react with THF to afford 1 and free IPy 2b-d.

Cobalt-Catalyzed Asymmetric Sequential Hydroboration/Hydrogenation of Internal Alkynes.

A highly regio- and enantioselective cobalt-catalyzed hydroboration/hydrogenation of internal alkynes with HBpin and a hydrogen balloon in one pot was developed. A new type of chiral imidazoline iminopyridine (IIP) ligand was introduced for the first time in this novel and efficient strategy. This protocol used relatively simple and available starting materials with good functional group tolerance to construct more valuable chiral secondary organoboronates. The primary mechanistic studies illustrated that the cobalt-catalyzed regioselective hydroboration of alkynes did initially occur followed by HBpin-promoted and cobalt-catalyzed enantioselective hydrogenation of alkenylboronates.

Iminopyridine ligand complexes of group 14 dihalides and ditriflates – neutral chelates and ion pair formation

Abstract Reaction of the chelating imino-pyridine ligand SIMPY, (SIMPY=2-(DippN=CH)-C5H4N), Dipp=2,6- i Pr2-C6H3, with germanium(II) and tin(II) halides provides the respective neutral complexes [SIMPY·EX2] (EX2: E=Ge, X=Cl, Br; E=Sn, X=Cl, Br, I). The method is readily extendable to give the tin(II) triflate complex [SIMPY·Sn(OTf)2] (OTf, triflate=CF3SO3 −). In the solid state, the neutral compounds [SIMPY·EX2] exist as monomers, in which the four-coordinate tetrel atoms feature a slightly distorted disphenoidal geometry around germanium and tin. Reaction of the tridentate imino-pyridine ligand DIMPY, (DIMPY=2,6-(DippN=CH)2-C5H3N) with Sn(OTf)2 provided access to a neutral tin(II) complex. Similar to the previously reported reactions leading to the germanium and tin chloride complexes [DIMPY·SnCl]+[SnCl3]−, and [Me2DIMPY·EX]+[EX3]− (Me2DIMPY=2,6-(DippN=C(Me))2-C5H3N, E=Ge, Sn; X=Cl), the reactions of DIMPY with GeX2·dioxane (X=Cl, Br) and SnX2 (X=Br, I) yielded Ge(II) and Sn(II) based ion pairs [DIMPY·EX]+[EX3]− (E=Ge, X=Cl, Br; E=Sn, X=Br, I) as a consequence of spontaneous dissociation of the group 14 dihalides. The tetrel atoms in the cationic parts in [DIMPY·EX]+[EX3]− are four-coordinate as one halide substituent is replaced by the coordination of a second imino donor group from the ligand. The anionic fragments adopt a pyramidally, tri-coordinate geometry. In contrast, the DIMPY tin(II) ditriflate complex crystallizes with two independent, neutral molecules per asymmetric unit, in which one of the tin centers is five- coordinate by interaction with three donor sites of the chelating bis(imino)pyridine ligand and two additional contacts towards the oxygen atoms of the triflate counter-anions. In the second crystallographically independent complex the tin atom is six-coordinate with a slightly distorted octahedral geometry via interaction with THF as an additional donor molecule. All compounds reported were studied by means of multinuclear NMR spectroscopy. In addition, the solid state structures of the complexes [SIMPY·EX2] (EX2: E=Ge, X=Cl, Br; E=Sn, X=Cl, Br, I), the ion pairs [DIMPY·EX]+[EX3]− (E=Ge, X=Cl; E=Sn, X=Br) and the tin(II) ditriflate [DIMPY·Sn(OTf)2] were authenticated by means of single-crystal X-ray diffraction analyses. Moreover, [DIMPY·Sn(OTf)2] was investigated by 119Sn Mössbauer spectroscopy.

Coordination entities of a pyrene-based iminopyridine ligand: Structural and photophysical properties

A pyrene-based iminopyridine ligand L has been prepared and displays the absorption and emission properties expected for pyrene-based derivatives in solution. Ligand L, as well as two neutral and one monocationic coordination entities, respectively formulated as [ZnLCl2] 1, [ReLCl(CO)3] 3 and [CuL2](BF4) 2, have been crystallized and analyzed by single crystal X-ray diffraction analysis. The corresponding crystal structures indicate the formation of supramolecular architectures generated by offset π⋯π stacking between pyrene fragments and strong C–H⋯π interactions in coordination entity 1. For the cationic coordination entity 2, the crystal packing reveals the presence of C–H⋯F and C–H⋯π interactions and numerous C–H⋯π contacts interconnecting the molecules into a 3D network. As for coordination entity 3, hydrogen bonding and π⋯π stacking link the molecules in a three dimensional manner. Zinc(II) and copper(I) coordination entities have also been studied through isothermal titration calorimetry, which indicate a strong binding and a different stoichiometry for both coordination entities. Photophysical studies of the ligand and corresponding coordination entities show a monomer type pyrene emission and a higher fluorescence quantum yield for the zinc coordination entity 1 as compared with copper 2 and rhenium 3 coordination entities.