Azopyridine Ligands Research Articles

A multi-responsive organogel and colloid based on the self-assembly of a Ag(i)-azopyridine coordination polymer.

In this work, through the coordination of C3 symmetric azopyridine ligands and Ag(i), coordination polymers with azo groups on the main chain were prepared. The trans coordination polymer formed an organogel with a network of nanofibers at low critical gelation concentrations, and it exhibited the abilities of self-healing and multi-stimuli response to heating, light, mechanical shearing, and chemicals due to the presence of dynamic coordinating bonds. On the other hand, the cis coordination polymer was found to assemble into nanoparticles to give a responsive colloid, which can produce fibrous precipitation in several days upon visible light irradiation due to the isomerization of the azo groups. This work provides a novel example for the design of a multi-responsive organogel and colloid based on the structural transformation of coordination polymers.

A photoresponsive azopyridine-based supramolecular elastomer for self-healing strain sensors

Optical-stimuli materials have been applied in many fields since it can be activated from external system and localized in time and space. Although, optically healable polymers have been well developed in recent years, to be healed under mild conditions remains a big challenge for these materials. Herein, we report the design and synthesis of a metallo-supramolecular polymer containing azopyridine ligands to address this conundrum. The resulting photo-healable polymer is highly stretchable (up to 735%) with a high toughness (16.33 MJ/m3) and excellent light-healing efficiency of ~90% after three cutting/healing cycles at mild temperature (40 °C) in various harsh conditions (e.g. underwater, subzero temperature). Furthermore, a novel bioinspired microcrack nanostructure design is presented to endow the resulted supramolecular elastomer with outstanding sensitivity to strain, which makes it very suitable for human motion monitoring applications. We believe this work will provide afflatus on future design, fabrication and application of photo-stimuli smart materials.

Ni(II)porphyrins as pH dependent light-driven coordination-induced spin-state switches (LD-CISSS) in aqueous solution

A water-soluble Ni(II)-porphyrin substituted with a covalently attached azopyridine ligand was synthesized. Upon irradiation with violet and green light, the azo unit performs a reversible cis–trans isomerization. This geometry change triggers a coordination/de-coordination of the pyridine nitrogen at the central nickel(II) ion. The concomitant change in coordination number at the Ni ion in turn switches the spin state between high and low spin, a process we coined a “light-driven coordination-induced spin state switch (LD-CISSS). To increase the coordination power of the pyridine, particularly in aqueous environments, we introduced an electron donating OH group in 4-position. With increasing pH, the hydroxyl group is deprotonated, further enforcing coordination. We report on the properties of this pH-dependent spin switch, particularly the magnetic properties.

Ligand-Induced Reductive Elimination of Ethane from Azopyridine Palladium Dimethyl Complexes.

Reductive elimination (RE) is a critical step in many catalytic processes. The reductive elimination of unsaturated groups (aryl, vinyl and ethynyl) from Pd(II) species is considerably faster than RE of saturated alkyl groups. Pd(II) dimethyl complexes ligated by chelating diimine ligands are stable toward RE unless subjected to a thermal or redox stimulus. Herein, we report the spontaneous RE of ethane from (azpy)PdMe2 complexes and the unique role of the redox-active azopyridine (azpy) ligands in facilitating this reaction. The (azpy)PdMe2 complexes are air- and moisture-stable in the solid form, but they readily produce ethane upon dissolution in polar solvents at temperatures from 10 °C to room temperature without the need for an external oxidant or elevated temperatures. Experimental and computational studies indicate that a bimolecular methyl transfer precedes the reductive elimination step, where both steps are facilitated by the redox-active azopyridine ligand.

Pharmaco-genomic investigations of organo-iridium anticancer complexes reveal novel mechanism of action.

Resistance to platinum drugs (used in >50% of cancer chemotherapies) is a clinical problem. Other precious metal complexes with distinct mechanisms of action might overcome this. Half-sandwich organometallic complexes containing arene or cyclopentadienyl (Cp) ligands show promise. We screened two iridium(iii) complexes [Ir(CpXbiph)(ppy)Cl] (ZL49, 1, ppy = phenylpyridine) and [Ir(CpXph)(azpyNMe2)Cl]PF6 (ZL109, 2, azpyNMe2 = N,N-dimethylphenylazopyridine) in 916 cancer cell lines from 28 tissue types. On average, complex 2 was 78× more potent than 1, 36× more active than cisplatin (CDDP), and strongly active (nanomolar) in patient-derived ovarian cancer cell lines. RNA sequencing of A2780 ovarian cells revealed upregulation of antioxidant responses (NRF2, AP-1) consistent with observed induction of reactive oxygen species (ROS). Protein microarrays, high content imaging and cell cycle analysis showed S/G2 arrest, and late-stage DNA damage response without p53 requirement. The triple-negative breast cancer cell line OCUB-M was highly sensitive to 2 as were cell lines with KIT mutations. Complex 2 exhibits a markedly different pattern of antiproliferative activity compared to the 253 drugs in the Sanger Cancer Genome database, but is most similar to osmium(ii) arene complexes which share the same azopyridine ligand. Redox modulation and DNA damage can provide a multi-targeting strategy, allowing compounds such as 2 to overcome cellular resistance to platinum anticancer drugs.

A photo-responsive polymeric azopyridine ligand with metal-complexation sensitivity: application to coordination equilibrium studies on the polymer complexes of a cobalt(ii) Schiff base

A photo-responsive polymeric azopyridine ligand with complexation sensitivity provided coordination equilibrium for complexation with cobalt salen in a dilute solution.

NBO Population Analysis and Electronic Calculation of Four Azopyridine Ruthenium Complexes by DFT Method

The molecular structure, the Natural Bond orbital (NBO) and the Time Dependent-DFT of both isomers cis or γ-Cl and trans or δ-Cl of RuCl2(L)2, where L stands respectively for 2-phenylazopyridine (Azpy), 2,4-dimethyl-6-[phenylazo]pyridine (Dazpy), 2-[(3,5-dimethylphenyl)azopyridine] (Mazpy) and 2-pyridylazonaphtol (Nazpy) were calculated with DFT method at B3LYP/LANL2DZ level. The prediction of the frontier orbitals (Highest Occupied Molecular Orbital or HOMO and Lowest Unoccupied Molecular Orbital or LUMO) shows that the most active complexes suitable for electronic reactions are admitted to be the trans isomers. Moreover, δ-RuCl2 (Azpy)2 is discovered to react more actively as photo-sensitizer since its energy gap is the minimum. Besides, electronic structures of all complexes through NBO calculation indicate that Ru-N bonds are made of delocalization of occupancies from lone pair orbital of N atoms to the ruthenium. Moreover, Ru was assumed to have almost the same charge regardless the structure of the azopyridine ligands in the complex indicating that the ligands provide only a steric effect that is responsible for the ruthenium’s selectivity. Concerning the transition state, NBO analysis also highlights that the transition LP(Ru) π*(N1-N2) does correspond to t2g π*(L). This transition is assumed to correspond to Metal to Ligand Charge Transfer (MLCT) that is responsible for the photo-sensitiveness of the metallic complex. Besides, TDDFT calculation of complexes showed that δ-RuCl2(Nazpy)2 displays the largest band during the absorption. For that reason, it is admitted to be the best photosensitizer due to a large system of conjugation provided by Nazpy ligand.

Redox-induced reversible intramolecular carbon–nitrogen bond formation of an azopyridylruthenium complex: Control of carbonyl ligand photoreactivity caused by structural change of the complex

Intramolecular carbon–nitrogen bond formation between a carbonyl and proximal azopyridine ligand by ligand-based reduction led to the formation of a novel ruthenium complex containing a bidentate carbamoyl ligand. Reversible C–N bond formation and cleavage reactions were confirmed by chemical and electrochemical redox reactions. The change in character of the CO moiety based on intramolecular structural change affects the CO ligand photoreactivity. The molecular structures of the carbamoyl compound and photoreaction products were determined by X-ray crystallography to confirm the present photoreaction process.

Potent organometallic osmium compounds induce mitochondria-mediated apoptosis and S-phase cell cycle arrest in A549 non-small cell lung cancer cells

The problems of acquired resistance associated with platinum drugs may be addressed by chemotherapeutics based on other transition metals as they offer the possibility of novel mechanisms of action. In this study, the cellular uptake and induction of apoptosis in A549 human non-small cell lung cancer cells of three promising osmium(II) arene complexes containing azopyridine ligands, [Os(η(6)-arene)(p-R-phenylazopyridine)X]PF6, where arene is p-cymene or biphenyl, R is OH or NMe2, and X is Cl or I, were investigated. These complexes showed time-dependent (4–48 h) potent anticancer activity with highest potency after 24 h (IC50 values ranging from 0.1 to 3.6 μM). Cellular uptake of the three compounds as quantified by ICP-MS, was independent of their logP values (hydrophobicity). Furthermore, maximum cell uptake was observed after 24 h, with evident cell efflux of the osmium after 48 and 72 h of exposure, which correlated with the corresponding IC50 values. The most active compound 2, [Os(η(6)-p-cymene)(NMe2-phenylazopyridine)I]PF6, was taken up by lung cancer cells predominately in a temperature-dependent manner indicating that energy-dependent mechanisms are important in the uptake of 2. Cell fractionation studies showed that all three compounds accumulated mainly in cellular membranes. Furthermore, compound 2 induced apoptosis and caused accumulation in the S-phase of the cell cycle. In addition, 2 induced cytochrome c release and alterations in mitochondrial membrane potential even after short exposure times, indicating that mitochondrial apoptotic pathways are involved. This study represents the first steps towards understanding the mode of action of this promising class of new osmium-based chemotherapeutics.

Two Zn(II) and Co(II) compounds with dicarboxylates and curved 4,4′-azopyridine ligands: Syntheses, crystal structures and gas sorption properties

Two new compounds Zn(PDA)(AZPY)·4H2O (1) and [Co(NDC)(AZPY)]2·2H2O·DMF (2) (H2PDA=pyridine-2,6-dicarboxylic acid, AZPY=4,4′-azopyridine, H2NDC=2,6-naphthalenedicarboxylic acid) were prepared through solvothermal reactions and characterized. Single crystal X-ray diffraction analysis indicates that compound 1 exhibits a one-dimensional chain structure, while compound 2 features a three-dimensional framework. Notably the bridged AZPY ligands involved in both compounds exhibit rare bent configurations. The thermal stability and gas sorption properties of compound 2 have also been studied. A normal amount (2.8mmolg−1) of hydrogen uptake at 1atm and −196°C for compound 2 were observed, which can be attributed to the presence of micropores.

Poly[[(μ2-benzene-1,4-dicarboxylato-κ4O1,O1′:O4,O4′)(μ2-di-4-pyridyldiazene-κ2N1:N1′)cobalt(II)]N,N-dimethylformamide disolvate hemihydrate

In the title compound, {[Co(C8H4O4)(C10H8N4)]·2C3H7NO·0.5H2O}n, the CoII atom is six-coordinated by four O atoms from two benzene-1,4-dicarboxyl­ate (H2bdc2−) groups and two N atoms from two 4,4′-azopyridine (4,4′-azpy, or di-4-pyridyldiazene) ligands, leading to a distorted octa­hedral geometry. The structure consists of two-dimensional corrugated sheets with a 44 topology in an …ABAB… packing pattern stacking along the a axis. The separation of the adjacent corrugated sheets is ca. 8.561 (2) Å (Co⋯Co distance) along the a axis. The uncoordinated water molecule is half-occupied. The crystal structure is stabilized by O—H⋯N and C—H⋯O hydrogen-bonding inter­actions.

Catalytic organometallic anticancer complexes

Organometallic complexes offer chemistry that is not accessible to purely organic molecules and, hence, potentially new mechanisms of drug action. We show here that the presence of both an iodido ligand and a sigma-donor/pi-acceptor phenylazopyridine ligand confers remarkable inertness toward ligand substitution on the half-sandwich "piano-stool" ruthenium arene complexes [(eta(6)-arene)Ru(azpy)I](+) (where arene = p-cymene or biphenyl, and azpy = N,N-dimethylphenyl- or hydroxyphenyl-azopyridine) in aqueous solution. Surprisingly, despite this inertness, these complexes are highly cytotoxic to human ovarian A2780 and human lung A549 cancer cells. Fluorescence-trapping experiments in A549 cells suggest that the cytotoxicity arises from an increase in reactive oxygen species. Redox activity of these azopyridine Ru(II) complexes was confirmed by electrochemical measurements. The first one-electron reduction step (half-wave potential -0.2 to -0.4 V) is assignable to reduction of the azo group of the ligand. In contrast, the unbound azopyridine ligands are not readily reduced. Intriguingly the ruthenium complex acted as a catalyst in reactions with the tripeptide glutathione (gamma-L-Glu-L-Cys-Gly), a strong reducing agent present in cells at millimolar concentrations; millimolar amounts of glutathione were oxidized to glutathione disulfide in the presence of micromolar ruthenium concentrations. A redox cycle involving glutathione attack on the azo bond of coordinated azopyridine is proposed. Such ligand-based redox reactions provide new concepts for the design of catalytic drugs.

A novel dicyanamide-bridged zinc(II) complex incorporating the 4,4′-azopyridine ligand

A novel complex {[Zn(azpy)2(dca)(H2O)2]·ClO4·azpy·2H2O}n (1) has been synthesized and structurally and spectroscopically characterized, where azpy is the 4,4′-azopyridine and dca the dicyanamide. The crystal structure analysis of 1 shows that in 1 the dca ligands act as μ 1,5-bridges linking the zinc(II) ions into 1D covalent bonding chains. The zinc(II) ion lies in a distorted octahedral environment completed by two nitrogen atoms from two terminal azpy groups, two nitrogen atoms from two dca bridges, and two water molecules. The most striking feature of 1 is that the 1D covalent bonding chains are linked through O–H·N hydrogen bonds and π–π stacking interactions into a 3D supramolecular structure.

Synthesis and structure of the 2-(chlorophenylazo)pyridine chelated tricarbonylrhenium(I) complex and its anion radical derivatives via electrochemical reduction

Reacting Re(CO)5Cl with the azopyridine ligand (1) (L) in boiling benzene afford the complex Re(CO)3Cl(L), (2) in excellent yield [L=2-(p-Cl-C6H4NN)C5H4N]. The chelation of the azopyridine ligand accompanied by displacement of the two carbon monoxide ligands furnish a five-membered chelate ring. Structure determination of complex (2) has revealed a distorted octahedral ReC3N2Cl coordination sphere. The Re–N(pyridine) and, Re–N(azo) distances are 2.158(3) and 2.153(6) A respectively, and the N–N length [1.273(4) A], implicate relatively weak Re-azo(π*) back–bonding. The Re(CO)3Cl(L) lattice consists of C–H...Cl hydrogen bonding and Cl...O non-bonded interactions constituting a supramolecular network. Extended Huckel calculations reveal that the LUMO of Re(CO)3Cl(L) is Ca. 57% azo in character. One-electron quasireversible electrochemical reduction of the complex occurs near −0.3 V versus Saturated Calomel electrode(s.c.e.) The redox orbital is believed to belong to the above noted LUMO. Electrogenerated Re(CO)3Cl(L•–) underwent spontaneous solvolytic chloride displacement in MeCN, resulting in the isolation of Re(CO)3(MeCN)(L•–). The latter complex in turn reacted with imidazole and triphenylphosphine, furnishing Re(CO)3(C3H4N2)(L•–) and Re(CO)3(PPh3)(L•–), respectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO)3Cl(L) but with shifts to lower frequencies by 10–20 cm−1. All three radical anion systems are one-electron paramagnetic (1.7–1.8 μ B ). The unpaired electron is primarily localized on the azoheterocycle ligand in a predominantly azo-π* orbital, but a small metal contribution (185, 187Re, I=5/2) is also present. Thus Re(CO)3(MeCN)(L•–) and Re(CO)3(C3H4N2)(L•–) display six-line e.p.r. spectra (A ˜ 28 G). The line shapes and intensities are characteristic of the presence of g-strain. In the case of Re(CO)3(PPh3)(L•–) seven nearly equispaced lines are observed due to virtually equal coupling between the metal and 31P (I=½) nuclei. The g-values of the radical species are slightly higher than the free-electron value of 2.0023.

A novel HgI2 adduct with an azopyridine ligand: synthesis, structure and optical refractive effect of [HgI2(4,4′-azopyridine)

A novel mercury iodide adduct [HgI2(4,4′-azpy)]n (4,4′-azpy⊕=⊕4,4′-azopyridine) was synthesized in THF. Single crystal X-ray diffraction analysis shows that it exhibits a zigzag chain structure with distorted tetrahedral Hg centers. Its optical response to incident light shows considerable optical absorptive activity and a strong self-focusing effect with α2⊕=⊕1.3⊕×⊕10−11 m W−1, n2⊕=⊕5.39⊕×⊕10−18 m2 W−1.

Ruthenium mononitro and mononitroso terpyridine complexes incorporating azoimine based ancillary ligands. Synthesis, crystal structure, spectroelectrochemical properties and kinetic aspects †

Ruthenium mononitro and mononitroso terpyridine complexes incorporating strong π-acidic azopyridine ligands of the types [RuII(trpy)(L)(NO2)]ClO46–10 and [RuII(trpy)(L)(NO)][ClO4]311–15 [trpy = 2,2′∶6′,2″-terpyridine; L = NC5H4NNC6H4(R), R = H, m-Me/Cl or p-Me/Cl] respectively have been synthesized starting from [RuII(trpy)L(H2O)]2+1–5. The single crystal X-ray structure of the nitro complex 7 has been determined. In acetonitrile solvent the nitro complexes exhibit strong MLCT transitions near 500 nm and the same MLCT bands are observed to be blue shifted near 370 nm in the case of the nitroso derivatives. The nitrosyl complexes exhibit νNO in the range 1960–1950 cm−1. The nitro (6–10) and nitroso (11–15) complexes exhibit ruthenium(II)–ruthenium(III) oxidation in the ranges 1.33–1.47 V and 1.62–1.72 V versus SCE respectively. The nitroso complexes exhibit two successive one-electron reductions near 0.7 and 0.0 V due to reductions of coordinated NO+ → NO and NO → NO− respectively. All the complexes (6–15) systematically display three ligand based (trpy and L) reductions to negative potentials of the SCE. The nitro complexes (6–10) display weak emissions near 700 nm (Φ = 0.83 × 10−2–2.98 × 10−2). The electrochemically generated 6+++++–10+++++ show LMCT transitions near 490 nm and rhombic EPR spectra in acetonitrile at 77 K. The pseudo first order rate constants (k) and the thermodynamic parameters (ΔH‡, ΔS ‡ and K) for formation of the nitro complexes, [RuII(trpy)(L)(H2O)]2+ → [RuII(trpy)(L)(NO2)]+, and the conversion [RuII(trpy)(L)(NO)]3+ → [RuII(trpy)(L)(NO2)]+ have been determined.

Mononuclear and dinuclear ruthenium(II)/(III) salicylates incorporating azoimine functionalities as ancillary ligands. Synthesis, spectroscopic and electron-transfer properties†

Ruthenium–salicylate complexes incorporating azoimine based azopyridine ligands, L, of the type [RuII(L)2(salicylate)] 1–5 [L = NC5H4NNC6H4(R), R = H, m-Me/Cl or p-Me/Cl] have been synthesized and their spectroelectrochemical aspects investigated. The complexes systematically exhibit two 1e− and two 2e− oxidation processes and four successive one-electron reductions. The stepwise electrochemical oxidations were followed by electronic and EPR spectral studies on each oxidation step which indicate that the initial one-electron oxidation process corresponds to stereoretentive oxidation of the ruthenium(II) centre to ruthenium(III), [RuIII(L)2(salicylate)]+1++–5++. The second oxidation step corresponds to oxidation of the coordinated salicylate moiety in 1++–5++ to the ruthenium(III)–salicylate semiquinone cationic radical, [RuIII(L)2(salicylate)]2+E. The electrogenerated ruthenium(III) congeners (1++–5++) exhibit rhombic EPR spectra corresponding to distorted octahedral complexes. The electrogenerated semiquinone salicylate radical [RuIII(L)2(salicylate)]2+E undergoes a radical recombination process which leads to formation of antiferromagnetically coupled dimeric species, [(L)2RuIII(X)RuIII(L)2]4+F [(X = −O2C(O)C6H4C6H4(O)CO2−]. The next two 2e− oxidation processes are associated with oxidation of the bridging moiety of the dimeric species [(L)2RuIII(X′)RuIII(L)2]4+G [(X′ = −O2C(O)C6H3C6H3(O)CO2−] followed by oxidation of the metal centres, [(L)2RuIV(X′)RuIV(L)2]6+H. The chemical oxidation of the complexes 1–5 by HNO3 leads to formation of dimeric complexes, G, straightaway. The complexes display intense charge-transfer bands in the UV-visible region which have been found to be reasonably blue shifted while moving from 1–5 to 1++–5++ to F to G.

Oxidation of octyl α- d-glucopyranoside to octyl α- d-glucuronic acid, catalyzed by several ruthenium complexes, containing a 2-(phenyl)azopyridine or a 2-(nitrophenyl)azopyridine ligand

The oxidation of octyl α- d-glucopyranoside by NaBrO 3, catalyzed by a number of ruthenium polypyridyl complexes was investigated. Both [Ru(azpy) 2(H 2O) 2] 2+ (azpy = 2-(phenyl)azopyridine, system I) and [Ru(naz) 2(H 2O) 2] 2+ (naz = 2-( p-nitrophenyl)azopyridine, system II) proved to be active catalysts yielding octyl glucuronic acid (in acidic medium) and a 2,3-glycol cleavage product (in basic medium) from octyl α- d-glucopyranoside (OGP). A major side-reaction is the hydrolysis of OGP, at the C 1 position. System II appears to be more reactive than system I, which has been explained by the redox properties of the complex. Especially at pH 3 a considerable amount of 1-O-octyl α- d-glucuronic acid is formed without the formation of hydrolysis products. Mechanistic studies, using results from reactions with cyclobutanol in combination with spectroscopic data, indicate that a Ru(IV)=O species is the active catalytic species. Two new complexes were synthesized, [Ru(terpy) (azpy) (H 2O)] (ClO 4) 2 (terpy = terpyridine, system III) and [Ru(terpy) (naz) (H 2O)] (ClO 4) 2 (system IV). Both complexes are less active than system I and II. The origin of the reduced activity can lie in the fact that the ruthenium site is sterically less accessible, or because of oxidative instability of the complex, as indicated by electrolysis experiments.

Metal complexes of azopyridines. Part III. Complexes of nickel(II) and copper(II) with 2,2′-azopyridine

2,2′-Azopyridine has been used to form metal halide complexes of the types MX2,2L, MX2,L, MX2,½L (M = Cu; X = Cl or Br; M = Ni; X = Cl) and NiCl2,½L,H2O; and metal sulphate complexes of the type MSO4,½L,nH2O (M = Cu, n= 1; M = Ni, n= 2). The results of i.r. (450–80 cm.–1) and electronic spectral measurements suggest that in CuX2,2L (X = Cl or Br) and in the complexes of nickel chloride and of both metal sulphates, the metal atoms are six-co-ordinate. For CuX2,½L (X = Cl or Br) the low-frequency i.r. results are compatible with structures in which the azopyridine ligands bridge planar or tetrahedrally co-ordinated Cu2X4 units (the latter geometry being indicated by the electronic spectra), whereas CuX2,L (X = Cl or Br) contain linear CuX2 groupings linked by amine bridges.