N-oxide Ligands Research Articles

Synthesis and crystal structure of diisothiocyanatotetrakis(4-methylpyridine N-oxide)cobalt(II) and diisothiocyanatotris(4-methylpyridine N-oxide)cobalt(II) showing two different metal coordination polyhedra

The reaction of Co(NCS)2 with 4-methylpyridine N-oxide (C6H7NO) leads to the formation of two compounds, namely, tetrakis(4-methylpyridine N-oxide-κO)bis(thiocyanato-κN)cobalt(II), [Co(NCS)2(C6H7NO)4] (1), and tris(4-methylpyridine N-oxide-κO)bis(thiocyanato-κN)cobalt(II), [Co(NCS)2(C6H7NO)3] (2). The asymmetric unit of 1 consists of one CoII cation located on a centre of inversion, as well as one thiocyanate anion and two 4-methylpyridine N-oxide coligands in general positions. The CoII cations are octahedrally coordinated by two terminal N-bonding thiocyanate anions in trans positions and four 4-methylpyridine N-oxide ligands. In the extended structure, these complexes are linked by C—H...O and C—H...S interactions. In compound 2, two crystallographically independent complexes are present, which occupy general positions. In each of these complexes, the CoII cations are coordinated in a trigonal–bipyramidal manner by two terminal N-bonding thiocyanate anions in axial positions and by three 4-methylpyridine N-oxide ligands in equatorial positions. In the crystal, these complex molecules are linked by C—H...S interactions. For compound 2, a nonmerohedral twin refinement was performed. Powder X-ray diffraction (PXRD) reveals that 2 was nearly obtained as a pure phase, which is not possible for compound 1. Differential thermoanalysis and thermogravimetry data (DTA–TG) show that compound 2 start to decompose at about 518 K.

Synthesis, crystal structure and properties of poly[(μ-2-methyl-pyridine N-oxide-κ2O:O)bis-(μ-thio-cyanato-κ2N:S)cobalt(II)

The title compound, [Co(NCS)2(C6H7NO)]n or Co(NCS)2(2-methyl-pyridine N-oxide), was prepared by the reaction of Co(NCS)2 and 2-methyl-pyridine N-oxide in methanol. All crystals obtained by this procedure show reticular pseudo-merohedric twinning, but after recrystallization, one crystal was found that had a minor component with only a very few overlapping reflections. The asymmetric unit consists of one CoII cation, two thio-cyanate anions and one 2-methyl-pyridine N-oxide coligand in general positions. The CoII cations are octa-hedrally coordinated by two O-bonding 2-methyl-pyridine N-oxide ligands, as well as two S- and two N-bonding thio-cyanate anions, and are connected via μ-1,3(N,S)-bridging thio-cyanate anions into chains that are linked by μ-1,1(O,O) bridging coligands into layers. No pronounced directional inter-molecular inter-actions are observed between the layers. The 2-methyl-pyridine coligand is disordered over two orientations and was refined using a split model with restraints. Powder X-ray diffraction (PXRD) indicates that a pure sample was obtained and IR spectroscopy confirms that bridging thio-cyanate anions are present. Thermogravimetry and differential thermoanalysis (TG-DTA) shows one poorly resolved mass loss in the TG curve that is accompanied by an exothermic and an endothermic signal in the DTA curve, which indicate the decomposition of the 2-methyl-pyridine N-oxide coligands.

N-oxide ligands for selective separations of lanthanides: insights from computation.

Preorganized ligands such as bis-lactam-1,10-phenanthroline (BLPhen) show unique selectivity trends across the lanthanide series, indicating the synergistic effects of both N and O donors in complexing with lanthanides. We hypothesize that by replacing amide functional groups with an N-oxide functionality would open the door to new ligand architectures with improved selectivities. To test this idea, we computationally examined mixed N,O-donor ligands containing pyridinic N and N-oxide groups and evaluated their relative aqueous La(iii)/Ln(iii) selectivity by computing free energy changes for the exchange reaction between the designed ligands and a reference ligand. Three novel ligands show promise as excellent extractant agents in selectively separating trivalent lanthanides. The extent of conjugation (and hyperconjugation), the complex geometry, and the electron accumulations on the two O-donors of the N-oxide groups are found to be important factors in dictating the selectivity trends.

Evaluation of Heteroleptic Pt (II) β-Diketonate Complexes as Precatalysts for the Photoactivated Curing of Silicone Resins

A library of platinum (II) complexes of general formula (O,O′-β-diketonate)PtLX (L = dimethylsulfoxide, pyridine, triphenylphosphine; X = chloride, γ-acetylacetonate) has been prepared, using synthetic methodologies available in the literature. The library has been supplemented with a novel platinum (II) complex bearing a triazenido N-oxide ligand. The complexes have been characterized and tested as precatalysts for the photoactivated curing of silicone resins. Correlations have been established between the nature of the employed ligands, the ultraviolet-visible (UV-Vis) absorption spectrum of the complexes and their catalytic performance, which enable the tailored preparation of complexes with improved performance in view of possible technological applications.

Low-dimensional compounds containing bioactive ligands. Part XVI: Halogenated derivatives of 8-quinolinol N-oxides and their copper(II) complexes

Four N-oxides, 8-quinolinol N-oxide (8-HQNO), 5,7-dichloro-8-quinolinol N-oxide (HdClQNO), 5,7-dibromo-8-quinolinol N-oxide (HdBrQNO) and 7-iodo-8-quinolinol N-oxide (HIQNO) as well as their six copper complexes, CuCl2(8-HQNO)2(H2O) (1), CuCl2(HdClQNO)2(H2O)2 (2), Cu(dClQNO)2(CHCl3) (3), Cu(dClQNO)2(H2O) (4), {[Cu(dBrQNO)2]•2H2O}n (5) and CuCl2(HIQNO)2(H2O)4 (6) were synthesised as possible anticancer agents. Crystal structures of N-oxides contain planar molecules held together via hydrogen bonds involving oxygen atoms of N-oxide groups as acceptors. Crystal structure of 5 represents the first structure of a copper(II) complex with an N-oxide ligand derived from 8-HQNO and is formed by infinite chains. In the chain, the Cu(II) atom coordinates to six oxygen atoms from two bidentate chelating dBrQNO ligands occupying apexes of elongated tetragonal bipyramid with bridging oxygen atoms of N-oxide groups in axial positions. Antiproliferative activity of prepared N-oxides as well as their complexes was studied using in vitro MTT assay against the MDA-MB-231, HCT-116 and A549 cancer cell lines and their selectivity was verified on MSCs cells. Among the tested cancer cell lines, A549 and MDA-MB-231 cells were the most sensitive to the tested complexes. Complex 1 showed the highest cytotoxicity against both tumor cell lines. At concentration, which could be tested in animal models, 1 induced cell death in more than 50% of cancer cells and in 20% of MSCs indicating its selectivity.

Composition-Thermometric Properties Correlations in Homodinuclear Eu3+ Luminescent Complexes.

A family of homodinuclear Ln3+ (Ln3+ = Gd3+, Eu3+) luminescent complexes with the general formula [Ln2(β-diketonato)6(N-oxide)y] has been developed to study the effect of the β-diketonato and N-oxide ligands on their thermometric properties. The investigated complexes are [Ln2(tta)6(pyrzMO)2] (Ln = Eu (1·C7H8), Gd (5)), [Ln2(dbm)6(pyrzMO)2] (Ln = Eu (2), Gd (6)), [Ln2(bta)6(pyrzMO)2] (Ln = Eu (3), Gd (7)), [Ln2(hfac)6(pyrzMO)3] (Ln = Eu (4), Gd (8)) (pyrzMO = pyrazine N-oxide, Htta = thenoyltrifluoroacetone, Hdbm = dibenzoylmethane, Hbta = benzoyltrifluoroacetone, Hhfac = hexafluoroacetylacetone, C7H8 = toluene), and their 4,4′-bipyridine N-oxide (bipyMO) analogues. Europium complexes emit a bright red light under UV radiation at room temperature, whose intensity displays a strong temperature (T) dependence between 223 and 373 K. This remarkable variation is exploited to develop a series of luminescent thermometers by using the integrated intensity of the 5D0 → 7F2 europium transition as the thermometric parameter (Δ). The effect of different β-diketonato and N-oxide ligands is investigated with particular regard to the shape of thermometer calibration (Δ vs T) and relative thermal sensitivity curves: i.e.. the change in Δ per degree of temperature variation usually indicated as Sr (% K–1). The thermometric properties are determined by the presence of two nonradiative deactivation channels, back energy transfer (BEnT) from Eu3+ to the ligand triplet levels and ligand to metal charge transfer (LMCT). In the complexes bearing tta and dbm ligands, whose triplet energy is ca. 20000 cm–1, both deactivation channels are active in the same temperature range, and both contribute to determine the thermometric properties. Conversely, with bta and hfac ligands the response of the europium luminescence to temperature variation is ruled by LMCT channels since the high triplet energy (>21400 cm–1) makes BEnT ineffective in the investigated temperature range.

-Proline N-oxide dihydrazides as an efficient ligand for cross-coupling reactions of aryl iodides and bromides with amines and phenols

A novel catalytic system based on l -proline N-oxide/CuI was developed and applied to the cross-coupling reactions of various N- and O- nucleophilic reagents with aryl iodides and bromides. This strategy featured in the employment of an l -proline derived dihydrazides N-oxide compound as the superior supporting ligand. By using this protocol, a variety of products, including N-arylimidazoles, N-arylpyrazoles, N-arylpyrroles, N-arylamines, and aryl ethers, were synthesized with up to 99% yield. • A novel l -proline based N-oxide ligand has been designed and synthesized. • The developed ligand performed well in copper catalyzed Ullmann-type cross-coupling reactions. • This ligand showed high compatibility in both C–N and C–O bond formation reactions. • A broad substrate scope with respect to both coupling components worked efficiently in high yields.

Ligand-regulated assemblies of three 1D to 3D Cu(II) coordination polymers: Structural diversities and magnetic properties

Three structurally diverse Cu(II) coordination polymers (CPs) from one-dimension (1D) to three-dimension (3D) upon nicotinate N-oxide (NNO) and isonicotinate N-oxide (INO) ligands have been synthesized solvothermally. CP 1 [Cu3(NNO)4(OH)2(H2O)2]n based on NNO ligand displays a 1D wavy double-chain. CP 2 [Cu(INO)(OH)]n based on INO ligand possesses a 2D wavy double-sheet. CP 3 [Cu(INO)(CH3O)]n based on INO ligand exhibits a 3D framework with 1D wavy channels. Structural comparisons indicate that the position of functional group of ligand, the coordination modes of ligand, the solvent system determine these diversified CPs. And Moreover, the structural diversity of CPs 1–3 further result in their different magnetic behaviors.

Structural Disorder in High-Spin {CoII9WV6} (Core)-[Pyridine N-Oxides] (Shell) Architectures.

The combinations of Co(II), octacyanidotungstate(V), and monodentate pyridine N-oxide (pyNO) or 4-phenylpyridine N-oxide (4-phpyNO) led to crystallization of novel crystalline phases {CoII[CoII8(pyNO)12(MeOH)12][WV(CN)8]6} (1) and {CoII[CoII8(4-phpyNO)7(MeOH)17][WV(CN)8]6}·7MeOH·(4-phpyNO)3 (2). In both architectures, metal–cyanide clusters are coordinated by N-oxide ligands in a simple monodentate manner to give the spherical objects of over 1 nm core diameter and about 2.2 nm (1) and 3 nm (2) of the total diameter, terminated with the aromatic rings. The supramolecular architecture is dominated by dense and rich π–π interaction systems. Both structures are characterized by a significant structural disorder in ligand shell, described with the suitable probability models. For 1, the π–π interactions between the pyNO ligands attached to the same metal centers are suggested for the first time. In 2, 4-phpyNO acts as monodentate ligand and as the crystallization molecule. Magnetic studies indicate the high-spin ground state due to the ferromagnetic interactions Co(II)–W(V) through the cyanido bridges. Due to the high symmetry of the clusters, no signature of slow magnetic relaxation was observed. The characterization is completed by solid-state IR and UV–Vis–NIR spectroscopy. The conditions for the stable M9M’6-based crystals formation are synthetically discussed in terms of the type of capping ligands: monodentate, bridging, and chelating. The potential of the related polynuclear forms toward the magnetism-based functional properties is critically indicated.

Zinc complexes supported by pyridine-N-oxide ligands: synthesis, structures and catalytic Michael addition reactions.

New dipyridylpyrrole N-oxide ligands HL1 and HL2 are designed and synthesized via oxidation of 2-(5-(pyridin-2-yl)-1H-pyrrol-2-yl)pyridine (Hdpp) by using 3-chloroperbenzoic acid (m-CPBA) in CH2Cl2. The treatment of ZnEt2 with two equiv. of HL1 and HL2 affords [Zn(L1)2] and [Zn(L2)2] in medium yield, respectively. These ligands and zinc complexes are fully characterized by NMR, IR, UV-vis and ESI-MS spectroscopy and X-ray diffraction analysis. The structure of HL1 and HL2 shows a planar geometry. The intramolecular hydrogen-bond interactions between the imino hydrogen and N-oxide oxygen atom are observed. In [Zn(L1)2] and [Zn(L2)2], two ligands chelate to the zinc metal with a cross perpendicular geometry. The zinc complexes were employed as a highly efficient catalyst for the thiol-Michael addition of thiols to α,β-unsaturated ketones in EtOH at room temperature. The loading of the catalyst is lowered to 0.01 mol%. The catalytic mechanism was proposed based on NMR and ESI-MS experiments.

Unraveling the Mechanism of Catalytic Water Oxidation via de Novo Synthesis of Reactive Intermediate.

Artificial photosynthesis could promise abundant, carbon-neutral energy, but implementation is currently limited by a lack of control over the multi-electron catalysis of water oxidation. Discoveries of the most active catalysts still rely heavily on serendipity. [Ru(tpy)(bpy)(H2O)]2+ (1; bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine) is representative of the largest known class of water oxidation catalysts. We undertook an extensive spectroscopic analysis of the prototypical 1 water oxidation catalyst and its fastest known analog [Ru(EtO-tpy)(bpy)(H2O)]2+ (2), capable of 10 times faster water oxidation, to investigate the mechanism of action and factors controlling catalytic activity. EPR and resonance Raman spectroscopy did not detect the proposed [RuV═O] intermediate in 1 and 2 but indicated the possible formation of N-oxides. A lag phase was observed prior to O2 evolution, suggesting catalyst modification before the onset of catalysis. The reactive intermediate [Ru(tpy)(bpy-NO)(H2O)]2+ (1-NO; bpy-NO = 2,2'-bipyridine-N-oxide) proposed by combined spectroscopic and DFT analysis was de novo synthesized and demonstrated 100-fold greater catalytic activity than 1. Thus, in situ transient formation of small amounts of the Ru complex with N-oxide ligands can significantly activate single-site Ru-based catalysts. Furthermore, the rate of O2 evolution was found to correlate with the redox potential of the ligand. This observation might assist with rational design of new catalysts.

A five-coordinate iron(III) porphyrin complex including a neutral axial pyridine N-oxide ligand.

While six-coordinate iron(III) porphyrin complexes with pyridine N-oxides as axial ligands have been studied as they exhibit rare spin-crossover behavior, studies of five-coordinate iron(III) porphyrin complexes including neutral axial ligands are rare. A five-coordinate pyridine N-oxide-5,10,15,20-tetraphenylporphyrinate-iron(III) complex, namely (pyridine N-oxide-κO)(5,10,15,20-tetraphenylporphinato-κ4N,N',N'',N''')iron(III) hexafluoroantimonate(V) dichloromethane disolvate, [Fe(C44H28N4)(C5H5NO)][SbF6]·2CH2Cl2, was isolated and its crystal structure determined in the space group P-1. The porphyrin core is moderately saddled and the Fe-O-N bond angle is 122.08 (13)°. The average Fe-N bond length is 2.03 Å and the Fe-ONC5H5 bond length is 1.9500 (14) Å. This complex provides a rare example of a five-coordinate iron(III) porphyrin complex that is coordinated to a neutral organic ligand through an O-monodentate binding mode.

Construction and structural diversification of eight rare-earth MIII coordination complexes with 5-bromonicotinic acid N-oxide

A series of eight MIII (PrIII, SmIII, NdIII, EuIII, GdIII, YIII, ErIII, and YbIII) coordination complexes were assembled from the 5-bromonicotinic acid N-oxide ligand (Hbno) under similar method. Single-crystal X-ray diffraction shows that these complexes display 1-D or bimetallacyclic coordination motifs, respectively. These structural discrepancies may be attributed to the radii of MIII ions, as well as their diverse affinities to N-oxide group. Furthermore, these lower-dimensional coordinative arrays can be further extended via additional secondary interactions, such as H-bonding and aromatic stacking interactions. All samples have been characterized by elemental analyses, and powder X-ray diffraction techniques. In addition, the luminescence sensing ability of 4 was investigated in detail.

Comparative Study of Structures and Luminescent Properties of Three Ag(I) Complexes Utilizing an Achiral Bipyridyl-N or Its Axially Chiral N-Oxide Analogue

An achiral 2,2′-bipyridyl-3,3′-dicarboxylate (H2LN) can be oxidized to a C2 axially chiral N-oxide analogy (R,S)-2,2′-bipyridyl-3,3′-dicarboxylate-1,1′-dioxide ((R,S)-H2LNO), converting normal bipyridyl-N to a charge-separated N-oxide group. The achiral H2LN connects the Ag(I) ions into a 4-connected mesomeric 3D network [Ag4(LN)2·(H2O)]·3H2O (1). However, as expected, with the introduction of N-oxide groups, {Ag4[(R,S)-LNO]2·2(H2O)}·H2O (2) and {Ag2[(R,S)-LNO]}·H2O (3) contain right- and left-handedness homochiral layers. Such opposite handedness layers are linked together to give a racemic compound in 2 but meso double-layers in 3. Notably, both ligands in 1–3 support argentophilic interactions. Moreover, a luminescent comparison of 1–3, pyridyl-N, and its functional N-oxide ligands is also investigated in detail.

MOLECULAR ABSORPTION SPECTROSCOPY IN THE UV-VIS REGION FOR THE QUINOLINE-N-OXIDE, NICOTINAMIDE-N-OXIDE AND 2,2-DITHIOBISPYRIDINE-N-OXIDE LIGANDS

The purpose of this study was to analyze and quantify spectral parameters, the absorption bands, specifically in the uv-visible region of some N-oxide ligands. From the spectra, the area under the absorption band, the molar absorptivity, the integrated absorption coefficient and the oscillator strength by the Gaussian approximation method were calculated for each of them, using the proposed evaluation by Drago and Figgs. According to the obtained results, it can be concluded that the heterocyclic compounds studied absorb very well electromagnetic radiation in the ultraviolet region. For the ligands QUI-NO and NIC-NO it was observed that there are two types of bands, primary and secondary, being transitions of type π → π * and n → π*, respectively. For DITBIS-NO, there was only one primary band (transition 1) and two additional shoulders, which had the highest observed absorbance coefficient. These ligands are expected as potential electron donors in complex systems whose purpose is to change the energy levels of the electronic system in order to promote a better utilization of the absorbed energy and therefore a better efficiency.

Cycloplatinated(ii) complexes bearing an O,S-heterocyclic ligand: search for anticancer drugs

Cycloplatinated(ii) complexes containing the 2-mercaptopyridine N-oxide ligand were synthesized and characterized spectroscopically. The biological activities of these complexes were also investigated.

Ligand and Metal Effects on the Stability and Adsorption Properties of an Isoreticular Series of MOFs Based on T-Shaped Ligands and Paddle-Wheel Secondary Building Units.

The synthesis of stable porous materials with appropriate pore size and shape for desired applications remains challenging. In this work a combined experimental/computational approach has been undertaken to tune the stability under various conditions and the adsorption behavior of a series of MOFs by subtle control of both the nature of the metal center (Co2+ , Cu2+ , and Zn2+ ) and the pore surface by the functionalization of the organic linkers with amido and N-oxide groups. In this context, six isoreticular MOFs based on T-shaped ligands and paddle-wheel units with ScD0.33 topology have been synthesized. Their stabilities have been systematically investigated along with their ability to adsorb a wide range of gases (N2 , CO2 , CH4 , CO, H2, light hydrocarbons (C1 -C4 )) and vapors (alcohols and water). This study has revealed that the MOF frameworks based on Cu2+ are more stable than their Co2+ and Zn2+ analogues, and that the N-oxide ligand endows the MOFs with a higher affinity for CO2 leading to excellent selectivity for this gas over other species.

A Highly Water-Tolerant Magnesium(II) Coordination Polymer Derived from a Flexible Layered Structure.

A two-dimensional (2D) layered Mg(II) coordination polymer (CP) with a high tolerance for H2 O was designed, synthesised, and crystallographically characterised. The synthesis was achieved by the introduction of a flexible 2D layered structure composed of Mg(II) ions and isonicotinate N-oxide ligands. Owing to its high H2 O tolerance, the obtained 2D layered structure has the flexibility to repeatedly adsorb a large amount of H2 O associated with interlayer expansion and enable the removal of H2 O from a H2 O/2-propanol mixed vapour. These results indicate that the CP could be an excellent dehydrating agent.

Synthesis, X-ray characterization and DFT study of a novel Fe(III)–pyridine-2,6-dicarboxylic acid N-oxide complex with unusual coordination mode

Our synthetic approach demonstrates that pyridine-2,6-dicarboxylic acid N-oxide can form a stable FeIII complex with the rare 1:3 metal-to-ligand ratio. This study provides more information about packing features of one crystalline compound, (H9a-acr)3[Fe(pydco)3]·6H2O (1) (where H2pydco=pyridine-2,6-dicarboxylic acid N-oxide and 9a-acr=9-aminoacridine) via proton transfer methodology. The structure of 1 was characterized by IR spectroscopy, X-ray diffraction methods and the elemental analysis was performed to check the purity of the compound. On the basis of crystallographic data, compound 1 contains a monomeric anion with distorted octahedral geometry. The metal center is coordinated to six oxygen atoms by three bidentate pyridine-2,6-dicarboxylic acid N-oxide ligands to form a ceiling fan-shaped entity in which only the N-oxide oxygen and one carboxylate oxygen generate a six-membered chelate ring as seen for other N-oxides. Furthermore, the [Fe(pydco)3]3− and (9a-acr)+ units in 1 form 2D anionic and cationic layers. The intermolecular interactions in this novel system consist of electrostatic attraction between ion pairs, different kinds of H-bonding and π-stacking all of which join the 2D layers to create 3D supramolecular architectures. Since pyridine-2,6-dicarboxylic acid N-oxide and pyridine-2,6-dicarboxylic acid have similar frameworks, we also highlighted some areas of current interest in the coordination behavior of the latter ligand in mononuclear monomeric compounds in the reported structures up to now and investigated the influence of the N-oxide moiety on the coordination mode(s) in the crystal lattice as well.

Structures and magnetism of two copper(II) compounds with mixed hydroxido-carboxylate bridges derived from isomeric pyridylbenzoate N-oxide ligands

Two Cu(II) coordination polymers with two isomeric pyridylbenzoate N-oxide ligands, 4-(4-pyridyl)benzoic acid N-oxide (4,4-Hopybz) or 4-(3-pyridyl)benzoic acid N-oxide (4,3-Hopybz), were synthesized and structurally and magnetically characterized. They are formulated as [Cu(4,4-opybz)(OH)]n (1) and [Cu(4,3-opybz)(OH)]n (2). The two compounds show similar layer structures, in which 1D coordination chains with uniform [(μ-OH)(μ-COO)] double bridges are linked by the 4,4-opybz or 4,3-opybz ligands. Magnetic studies revealed that the mixed double bridges transmit ferromagnetic coupling in the compound 1, and the antiferromagnetic interactions exist in the compound 2. The magnetic differences can be related to the Cu–O–Cu angles influenced by isomeric pyridylbenzoate N-oxide ligands.