Imine-based Ligand Research Articles

Host Spin-Crossover Thermodynamics Indicate Guest Fit.

Spin-crossover (SCO) metal-organic cages capable of switching between high-spin and low-spin states have the potential to be used as magnetic sensors and switches. Variation of the donor strength of heterocyclic aldehyde subcomponents in imine-based ligands can tune the ligand field for a FeII center, which results in both homoleptic and heteroleptic cages with diverse SCO behaviors. The tetrahedral SCO cage built from 1-methyl-1H-imidazole-2-carbaldehyde is capable of encapsulating various guests, which stabilize different cage spin states depending on guest size. Conversely, the SCO tetrahedron exhibits different affinities for guests in different spin states, which is inferred to result from subtle structural differences of the cavity caused by the change in metal center spin state. Examination of SCO thermodynamics across a series of host-guest complexes enabled sensitive probing of guest fit to the host cavity, providing information complementary to binding-constant determination.

Ring-Opening Polymerization of ε-Caprolactone and Styrene Oxide-CO2 Coupling Reactions Catalyzed by Chelated Dehydroacetic Acid-Imine Aluminum Complexes.

A series of chelated dehydroacetic acid–imine-based ligands L1H~L4H was synthesized by reacting dehydroacetic acid with 2-t-butylaniline, (S)-1-phenyl-ethylamine, 4-methoxylbenzylamine, and 2-(aminoethyl)pyridine, respectively, in moderate yields. Ligands L1H~L4H reacted with AlMe3 in toluene to afford corresponding compounds AlMe2L1 (1), AlMe2L2 (2), AlMe2L3 (3), and AlMe2L4 (4). All the ligands and aluminum compounds were characterized by IR spectra, 1H and 13C NMR spectroscopy. Additionally, the ligands L1H~L4H and corresponding aluminum derivatives 1, 3, and 4 were characterized by single-crystal X-ray diffractometry. The catalytic activities using these aluminum compounds as catalysts for the ε-caprolactone ring-opening polymerization (ROP) and styrene oxide-CO2 coupling reactions were studied. The results show that increases in the reaction temperature and selective solvent intensify the conversions of ε-caprolactone to polycaprolactone. Regarding the coupling reactions of styrene oxide and CO2, the conversion rate is over 90% for a period of 12 h at 90 °C. This strategy dispenses the origination of cyclic styrene carbonates, which is an appealing concern because of the transformation of CO2 into an inexpensive, renewable and easy excess carbon feedstock.

Synthesis, crystal structures, optical properties, DFT and TD-DFT studies of Ni (II) complexes with imine-based ligands

Two new square-planar nickel (II) complexes were synthesized from (2-((E)-(9-ethyl-9H-carbazol-7-ylimino)methyl)phenol) and (1-((E)-(9-ethyl-9H-carbazol-7-ylimino)methyl)naphthalen-2-ol) ligands, for complex I and complex II, respectively. They were characterized by IR, UV-Vis spectroscopy and X-ray diffraction. Further measurements were also conducted as cyclic voltammetry and conductivity measurements. Both complexes crystallize in the monoclinic system with the space groups P21/n and P21/c for complex I and complex II, respectively. The electronic absorption spectrum of complex I exhibits the lowest-lying absorption band (LEB) at 381 nm and the highest energy absorption band (HEB) at 269 nm. For complex II, the LEB was located at 431 nm while the HEB was found to be at 260 nm. Theoretical structure geometries, vibrational frequencies and electronic properties of the two complexes were calculated, by means of the density functional theory (DFT). The basis set 6-31g+(d) was chosen, for non-metal atoms (C, H, N and O) and the Stuttgart/Dresden effective core potentials basis set (SDD), for Ni atoms. UV-Vis transitions nature was investigated via a time-dependent density functional theory (TD-DFT) calculation. Overall, theoretical calculations were in accordance with the experimental results.

2-Methoxy-5-(6-methoxypyridin-3-yl-imino-methyl)phenol and its transition metal complexes as potent antibacterial agents: Synthesis, characterization, theoretical investigations and biological evaluation

In this study, we have prepared an imine-based ligand, 2-methoxy-5-((6-methoxypyridin-3-ylimino)methyl)phenol (MIMP) and its Cu(II), Ni(II) and Zn(II) complexes in 2:1 stoichiometric ratio (2MIMP : Metal). The structure of obtained ligand and its metal complexes were elucidated with the aid of FT-IR, UV–Visible, NMR (1H and 13C) and mass spectra. Further, all the structures were analyzed via density functional theory (DFT) approach at B3LYP/LanL2DZ/6-311++G(2d,p) level, with HOMO-LUMO energies, geometric parameters, reactivity properties and electronic excitations obtained through TD-DFT calculations. Antibacterial activity of MIMP ligand and metal complexes have been evaluated via in vitro assays. In addition, the inhibition of the protein DNA gyrase-DNA complex was evaluated using molecular docking calculations, and the results revealed that biological accessibility of the metal complexes was better than ligand.

Biological Potency of New Benzimidazole Derived Imine Based Ligand and its Co(III), Ni(II), Cu(II) and Pt(II) Complexes: Synthesis, Structure, Antimicrobial, Antioxidant and BSA Interaction Studies

In this study, we report the synthesis and characterization of transition metal complexes (MC1, MC2, MC3, MC4) where M= Co(III), Ni(II), Cu(II) and Pt(II), respectively, using a new imine-based ligand, 2-{(E)-[(1H-benzimidazol-2-ylmethyl)imino]methyl}-4-bromophenol (HBMB). The molecular structure of the HBMB ligand and its complexes were confirmed by various analytical techniques such as UV-Vis, FT-IR, 1H, 13C-NMR, ESR, TGA/DTA, LC-MS, molar conductance, and magnetic moment measurements. The spectral data of complexes recommended tridentate binding modes of HBMB ligand and suggested an octahedral geometry for MC1 and MC2 complexes while square planar geometry for MC3 and MC4 complexes. Further, the biological activities viz., antimicrobial, antioxidant, and BSA interaction studies were performed using the prepared compounds. The antimicrobial activity results suggested that all the synthesized transition metal complexes showed significant antimicrobial activity against several bacterial and fungal species compared with standard drug and the parent ligand, HBMB. The interaction of metal complexes with BSA (Bovine serum albumin) was performed. The results strongly recommended among the synthesized complexes, platinum complex (MC4) showed marked interaction with the BSA protein. It also indicates that the probable quenching mechanism of BSA fluorescence by complexes is a static quenching procedure, and the spontaneous binding interaction is mainly entropy driven with either van der Waals force or hydrogen binding reaction. The free radical scavenging ability of test samples was assessed using an in vitro assay viz., DPPH.

Cyclometalation reactions of platinum complexes with azo- and imine-based ligands: Ligand-dependent and water-induced C–H activation

Noncyclometalated and cyclometalated Pt(II) complexes with azo- or imine-based ligands were synthesized and structurally characterized. These Pt(II) complexes had square-planar structures, with cyclometalation resulting in six-membered ring formation and highly distorted square-planar geometry. The Pt(II) complex with an azo-based ligand easily underwent cyclometalation, even at room temperature, while the Pt(II) complex with an imine-based ligand required a higher reaction temperature and a solvent with higher solubility. Cyclometalation in these complexes was also dependent on the presence of water as reaction solvent. Water as an additive was considered to eliminate HCl originating from the phenyl ring and coordinate chloride ions, promoting cyclometalation.

Novel Benzimidazole Derived Imine-Based Ligand and its Co(III), Ni(II), Cu(II) and Pt(II) Complexes: Chemical Synthesis, Structure, Antimicrobial, DNA Interaction Studies and Nuclease Activity

Herein, we report the synthesis of a Schiff base ligand, 2-((E)-((1H-benzo[d]imidazol-2-yl)methylimino)methyl)-4,6-dichlorophenol (BMDC) and its Co(III), Ni(II), Cu(II) and Pt(II) coordinated compounds (henceforth named as M1, M2, M3, and M4, respectively). The synthesized compounds were subjected to various analytical and physical characterization techniques to confirm their molecular structures. Further, the DNA binding and cleavage propensity of the as-prepared metal complexes were evaluated using calf thymus DNA (CT-DNA) and supercoiled plasmid DNA (pBR322). The DNA interaction results revealed that the tested metal complexes bind with DNA through the covalent and non-covalent mode of interaction, which is investigated using absorption and fluorescence spectral studies followed by viscosity measurement. The scission of supercoiled plasmid DNA by the metal complexes suggested the potentiality of the molecules toward the cleavage of pBR322 DNA. Furthermore, the compounds were screened for in vitro antiproliferative activity, tested against the various cell lines such as A549, EAC, SIHA, and NIH3T3. Results revealed that compound M4 exhibited marked anti-proliferative activity against EAC cell line with a significant IC50 value of 10 µm compared to its parent ligand, BMDC, and other metal transition complexes under study. In addition, various hematological parameters (alkaline phosphate, creatinine, urea, RBC, and WBC) were studied, and significant results are obtained from the experiments.

Tumoricidal Potential of Novel Amino-1,10-phenanthroline Derived Imine Ligands: Chemical Preparation, Structure, and Biological Investigations.

Herein we report the synthesis and structural elucidation of two novel imine-based ligands, 2-(1,10-phenanthrolin-5-yl)imino)methyl)-5-bromophenol (PIB) and N-(1,10-phenanthrolin-5-yl)-1-(thiophen-3-yl)methanimine (PTM) ligands. An in vitro cytotoxicity assay of the synthesized molecules was carried out against breast, cervical, colorectal, and prostate cancer cell lines as well as immortalized human keratinocytes. The observations indicated that both the molecules possesses dose-dependent selective cytotoxicity of cancer cells with no detrimental effect on the normal cell lines. Furthermore, the detailed computational analysis of newly synthetized ligands (PIB and PTM) has been conducted in order to identify their most important parts from the perspective of local reactivity. The IC50 values of PIB treatment on MCF-7, HeLa, HCT-116 and PC-3 were 15.10, 16.25, 17.88, 17.55 and 23.86 micromoles, respectively. Meanwhile, the IC50 values of PTM on MCF-7, HeLa, HCT-116, PC-3 and HaCat were observed to be 14.82, 15.03, 17.88, 17.28 and 21.22 micromoles, respectively. For computational analysis, we have employed the combination of Density Functional Theory (DFT) calculations and MD simulations. DFT calculations provided us with information about structure and reactivity descriptors based on the electron distribution. Surfaces of molecular electrostatic potential (MEP) and averaged local ionization energy (ALIE) indicated the sites within studied molecules that are most reactive. These results indicated the importance of nitrogen atoms and OH group. Additionally, the values of bond dissociation for hydrogen abstraction showed that both molecules, especially the PTM, are stable toward the influence of autoxidation mechanism. On the other side, MD simulations gave us an insight how ligands interact with water molecules. Namely, the radial distribution functions (RDF) indicated that the hydrogen atom of the OH group in the case of the PIB has the most pronounced interactions with water.

Simultaneous Generation of a [2 × 2] Grid-Like Complex and a Linear Double Helicate: a Three-Level Self-Sorting Process.

Two constitutional dynamic libraries (CDLs)-each containing two amines, two dialdehydes, and two metal salts-have been found to self-sort, generating two pairs of imine-based metallosupramolecular architectures (sharing no component) each with a [2 × 2] grid-like complex and a linear double helicate. These CDLs provided unique examples of a three-level self-sorting process, as only two imine-based ligand constituents, two metal complexes, and two architectures were selected during their assembly out of all the possible combinations of their initial components. The metallosupramolecular architectures assembled were characterized by NMR, mass spectroscopy, and X-ray crystallography.

Synthesis and structures of mono- and di-nuclear aluminium and zinc complexes bearing α-diimine and related ligands, and their use in the ring opening polymerization of cyclic esters.

A series of organoaluminium imino-amido complexes of the type {[ArNC(Me2)C(Me)[double bond, length as m-dash]NAr]AlMe2} (Ar = 2,6-iPr2C6H3 (1), Ar = 2,6-Et2C6H3 (2); Ar = 2,6-Me2C6H3 (3) have been prepared via reaction of AlR3 and the respective α-diimine. Similar reaction of the bis(α-diimine) [ArN[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]N-]2 (Ar = 2,6-iPr2C6H3) with AlMe3 afforded the bimetallic complex [ArN-C(Me)2C(Me)[double bond, length as m-dash]NAlMe2]2 (4), whilst reaction of the acetyl-imino compound [O[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]NAr] (Ar = 2,6-Et2C6H3) with AlMe3 afforded the bimetallic complex {[OCMe2CH(Me)[double bond, length as m-dash]NAr]AlMe2}2 (5). In related organozinc chemistry, we have isolated {[ArNC(Me)(Et)C(Me)[double bond, length as m-dash]NAr]ZnEt} (Ar = 2,6-iPr2C6H3, 6) and the trinuclear complex {[ArN[double bond, length as m-dash]C(Me)COCHCO(Me)C(Me)[double bond, length as m-dash]NAr][OCH(Me)C(Me)[double bond, length as m-dash]NAr](ZnEt)3} (Ar = 2,6-iPr2C6H3, 7) from reactions of ZnEt2 with ArN[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]NAr or [O[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]NAr], respectively. Reaction of the bis(α-diimine), LiPr-N2-ArCH2Ar-N2, derived from 4,4'-methylenebis(2,6-diisopropylaniline), with ZnCl2 affords [LiPr-N2-ArCH2Ar-N2(ZnCl2)2] (8). The molecular structures of complexes 1-8 are reported. Preliminary results of the ability of 1-8, along with the previously reported metal-metal bonded complex {[ArN[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]NAr]Al(THF)}2 (9), to act as catalysts for the ring opening polymerization (ROP) of the cyclic esters ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and rac-lactide (r-LA) are presented. For ε-CL and δ-VL, best results were obtained using the metal-metal bonded complex 9. For r-LA, the Al-based systems exhibited moderate activity affording only liquid oligomers, whilst the Zn-based systems performed better affording at 80 °C isotactic PLA with Mnca. 10 kDa with conversions of up to 66%. The co-polymerization of ε-CL with δ-VL was also examined, and differing preferences were noted for monomer incorporation.

Optical properties of Eu(III) and Tb(III) complexes with pyridine- and quinoline- based ligands under high hydrostatic pressure

The spectroscopy of nitrate complexes of Eu(III) and Tb(III) with chiral and racemic imine-based [L1 = (N,N’-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine) and L3 = N, N’-bis(2-quinolylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine] and amine-based [L2 = N,N’-bis(2-pyridylmethyl)-1,2-(R,R + S,S)-cyclohexanediamine) and L4 = N,N’-bis(2-quinolylmethyl)-1,2-(R,R + S,S)-cyclohexanediamine] ligands has been studied under high hydrostatic pressure (above 100 kbar). With the increasing pressure, a reduction of the Tb(III) and Eu(III) luminescence intensity is detected for all the complexes, whilst a significant reduction of the Tb(III) and Eu(III) excited state lifetimes has been observed for all Tb-based complexes [L1Tb(NO3)3 → L4Tb(NO3)3] and only for the Eu(III) complexes containing the imine-based ligands [L1Eu(NO3)3 and L3Eu(NO3)3]. This behavior has been rationalized taking into account two main aspects: i) the relative position of the energy levels of the ligands and the metal ions and ii) the change of these position upon compression DFT calculations have been also performed to elucidate the nature of the orbitals involved in the UV electronic absorption transitions (NTO orbitals) upstream of the energy transfer process to the metal ion.

Synthesis and Characterization of Oxidovanadium(IV) Complexes of 2-((E)-(6-Fluorobenzo[d]thiazol-2-ylimino)methyl)-6-methoxyphenol and Their Antimicrobial, Antioxidant, and DNA-Binding Studies.

Two novel oxidovanadium(IV) complexes with a new bidentate (O- and N-) imine-based ligand 2-((E)-(6-fluorobenzo[d]thiazol-2-ylimino)methyl)-6-methoxyphenol (HL) were synthesized under in situ experimental condition where VOSO4 acts as a kinetic template in the ratio 2 : 1 (L : M) and mixed ligand complex using 1,10-phenanthroline (phen) in 1 : 1 : 1 (L : M : phen) ratio. The synthesized compounds were structurally characterized by microanalysis, magnetic susceptibility, FTIR, electronic spectra, TG/DTA, ESR, and molar conductance studies. Based on the spectral studies, the complexes have the general composition [VO(L)2] (C1) and [VO(L)phen] (C2) in a square pyramid geometrical fashion. The synthesized compounds were primarily screened for their in vitro growth inhibiting activity against different strains of bacteria, namely, E. coli, B. subtilis, S. aureus, and P. aeruginosa by the disc diffusion method. Also, the antifungal activity was determined against C. albicans and A. niger by the Bateman poisoned technique. The in vitro antioxidant activity of all the compounds was determined by DPPH free radical-scavenging assay. Intercalative mode of DNA-binding properties of the oxidovanadium(IV) complexes with calf-thymus DNA (CT-DNA) was investigated using UV, fluorescence spectra, and viscosity measurements.

Synthesis and Characterization of Cobalt(III) and Copper(II) Complexes of 2-((E)-(6-Fluorobenzo[d]thiazol-2-ylimino) methyl)-4-chlorophenol: DNA Binding and Nuclease Studies—SOD and Antimicrobial Activities

A bidentate (N- and O-) imine-based ligand (L1) and its metal complexes of types [CuII(L1)2] (C1), [CuII(L1)(Phen)] (C2), [CoIII(L1)2] (C3), and [CoIII(L1)(Phen)] (C4) (L1 = 2-((E)-(6-fluorobenzo[d]thiazol-2-ylimino)methyl)-4-chlorophenol and phen = 1,10-phenanthroline) were synthesized as potential chemotherapeutic drug candidates. The prepared complexes were structurally characterized by spectral techniques (NMR, FT-IR, LC-MS, EPR, and electronic absorption), thermogravimetric analysis (TGA/DTA), magnetic moment, and CHNO elemental analysis. Spectroscopic studies suggested the distorted octahedral structure for all complexes. In vitro bioassay studies include binding and nuclease activities of the ligand and its complexes with target calf thymus- (CT-) DNA were carried out by employing UV-Vis, fluorescence spectroscopy, viscosity, and gel electrophoresis techniques. The extent of binding propensity was determined quantitatively by Kb and Ksv values which revealed a higher binding affinity for C2 and C4 as compared to C1 and C3. In addition, the scavenging superoxide anion free radical (O∙-2) activity of metal complexes was determined by nitroblue tetrazolium (NBT) light reduction assay. Molecular docking studies with DNA and SOD enzyme were also carried out on these compounds. The antimicrobial study has shown that all the compounds are potential antibacterial agents against Gram-negative bacterial strains and better antifungal agents with respect to standard drugs used.

Photoswitchable transition metal complexes with azobenzene-functionalized imine-based ligands: structural and kinetic analysis.

We report on the characterization of two imine type ligands containing photoresponsive azobenzene units as side groups and their transition metal ions complexes. The ligands, both free and in their complexes undergo trans- > cis photoisomerization after irradiation with UV light, but binding of metal ions reduces both the photoisomerisation reaction rates and cis isomer concentrations in the photostationary states. The greatest diminution in the photoisomerisation rate was observed for the complex containing Cd(ii), the heaviest among the various transition metal ions tested in this study.

Luminescent Eu3+ complexes in acetonitrile solution: Anion sensing and effect of water on the speciation

In this paper the trifluoromethansulphonate (CF3SO3−, OTf−, triflate) Eu3+ complexes of a new family of imine-based ligands with 1:1 ligand to metal mole ratio have been employed for luminescence sensing of NO3−, F−, Cl−, Br−, I−, CH3COO− and ClO4−, in non-anhydrous acetonitrile (wet AN=wAN) solution. The ligands show different donor ability and stereochemistry. In particular, they consist of the trans racemic isomers containing pyridine or furan as donating ring [N,N′-bis(2-pyridylmethylidene)-1,2-(R,R+S,S)-cyclohexanediamine, L1; N,N′-bis(2-furanylmethylidene)-1,2-(R,R+S,S)-cyclohexanediamine, L2 and the cis (meso) isomer containing the pyridine ring N,N′-bis(2-pyridylmethylidene)-1,2-(R,S)-cyclohexanediamine, L3]. As far as the sensitivity and the selectivity towards the different anions are concerned, the donor ability of the heteroaromatic ring within the ligand backbone, plays a crucial role: the poorly donating furan-based ligand L2 shows the best sensitivity and selectivity towards nitrate. On the other hand, the stereochemistry of the pyridine-based ligand shows a significant effect on the bromide sensing. The effects of the water content on the stability of these complexes have been also studied. Even though the speciation in non-anhydrous is the same as in anhydrous AN (1:1 EuL and 1:2 EuL2 species), their stability is significantly lower. The reasons for this behavior are proposed to be mainly found in the different solvation of both Eu3+ ion and the ligands.

Magnetite nanoparticles surface-modified with a zinc(II)-carboxylate Schiff base ligand as a sorbent for solid-phase extraction of organochlorine pesticides from seawater

Magnetite nanoparticles were surface-modified with mercaptoacetic acid (MAA), complexed with Zn(II), and then treated with the dual Schiff base (referred to as imine-based ligand; IBL; obtained by reaction of p-aminobenzoic acid and 2,3-butanedione) to give particles with an architecture of type Fe3O4@MAA@IBL. These are shown to be viable sorbents for magnetic solid phase extraction of organochlorine pesticides (OCPs) from seawater samples. Efficient extraction of the OCPs probably is due to lone pair-π, π-complexation and π-interactions. The sorbent was characterized by transmission electron microscopy, scanning electron microscopy, FT-IR and energy-dispersive X-ray spectroscopy. The effects of the volumes of sample, sorbent dosage and eluent, adsorption and desorption times, and the salinity of the sample on the extraction efficiencies were optimized. The OCPs (heptachlor, aldrin, dieldrin, p,p’-DDE and p,p’-DDT) were quantified by gas chromatography with microelectron capture detection. Under optimal conditions, the limit of detections range was between 1.0 and 1.9 ng L−1. The enrichment factors are between 84.1 and 99.9 %. The sorbent was applied to the rapid extraction of trace quantities of OCPs from seawater samples and gave good relative recoveries (78 to 108 %) and relative standard deviations (<8.3 %).

ChemInform Abstract: Non‐Heme Imine‐Based Iron Complexes as Catalysts for Oxidative Processes

AbstractReview: 170 refs.

Tuning of the sensing properties of luminescent Eu(3+) complexes towards the nitrate anion.

A new family of imine-based ligands containing pyridine or furan as an aromatic donating ring [N,N'-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L1; N,N'-bis(2-furanylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L2 and N,N'-bis(2-thienylmethylidene)-1,2-(R,S)-cyclohexanediamine, L3] has been prepared in high yield by means of an easy synthetic protocol. Their trifluoromethansulphonate (CF3SO3(-), OTf(-)) Eu(iii) complexes have been employed for luminescence sensing of the NO3(-) anion in an anhydrous acetonitrile solution. Spectrophotometric titrations have been carried out to define the speciation in the solution and study the formation of ternary species occurring with the addition of NO3(-) anions. The sensing response towards this anion is strongly dependent on the nature of the ligand, the stoichiometry of the complexes and their concentration.

Highly Active, Thermally Stable, Ethylene-Polymerisation Pre-Catalysts Based on Niobium/TantalumImine Systems

The reactions of MCl5 or MOCl3 with imidazole-based pro-ligand L(1)H, 3,5-tBu2-2-OH-C6H2-(4,5-Ph2-1H-)imidazole, or oxazole-based ligand L(2)H, 3,5-tBu2-2-OH-C6H2 (1H-phenanthro[9,10-d])oxazole, following work-up, afforded octahedral complexes [MX(L(1,2))], where MX=NbCl4 (L(1), 1a; L(2), 2a), [NbOCl2(NCMe)] (L(1), 1b; L(2), 2b), TaCl4 (L(1), 1c; L(2), 2c), or [TaOCl2(NCMe)] (L(1), 1d). The treatment of α-diimine ligand L(3), (2,6-iPr2C6H3N=CH)2, with [MCl4(thf)2] (M=Nb, Ta) afforded [MCl4(L(3))] (M=Nb, 3a; Ta, 3b). The reaction of [MCl3(dme)] (dme=1,2-dimethoxyethane; M=Nb, Ta) with bis(imino)pyridine ligand L(4), 2,6-[2,6-iPr2C6H3N=(Me)C]2C5H3N, afforded known complexes of the type [MCl3(L(4))] (M=Nb, 4a; Ta, 4b), whereas the reaction of 2-acetyl-6-iminopyridine ligand L(5), 2-[2,6-iPr2C6H3N=(Me)C]-6-Ac-C5H3N, with the niobium precursor afforded the coupled product [({2-Ac-6-(2,6-iPr2C6H3N=(Me)C)C5H3N}NbOCl2)2] (5). The reaction of MCl5 with Schiff-base pro-ligands L(6)H-L(10)H, 3,5-(R(1))2-2-OH-C6H2CH=N(2-OR(2)-C6H4), (L(6)H: R(1)=tBu, R(2)=Ph; L(7)H: R(1)=tBu, R(2)=Me; L(8)H: R(1)=Cl, R(2)=Ph; L(9)H: R(1)=Cl, R(2)=Me; L(10)H: R(1)=Cl, R(2)=CF3) afforded [MCl4(L(6-10))] complexes (M=Nb, 6a-10a; M=Ta, 6b-9b). In the case of compound 8b, the corresponding zwitterion was also synthesised, namely [Ta(-)Cl5(L(8)H)(+)]·MeCN (8c). Unexpectedly, the reaction of L(7)H with TaCl5 at reflux in toluene led to the removal of the methyl group and the formation of trichloride 7c [TaCl3(L(7-Me))]; conducting the reaction at room temperature led to the formation of the expected methoxy compound (7b). Upon activation with methylaluminoxane (MAO), these complexes displayed poor activities for the homogeneous polymerisation of ethylene. However, the use of chloroalkylaluminium reagents, such as dimethylaluminium chloride (DMAC) and methylaluminium dichloride (MADC), as co-catalysts in the presence of the reactivator ethyl trichloroacetate (ETA) generated thermally stable catalysts with, in the case of niobium, catalytic activities that were two orders of magnitude higher than those previously observed. The effects of steric hindrance and electronic configuration on the polymerisation activity of these tantalum and niobium pre-catalysts were investigated. Spectroscopic studies ((1)H NMR, (13)C NMR and (1)H-(1)H and (1)H-(13)C correlations) on the reactions of compounds 4a/4b with either MAO(50) or AlMe3/[CPh3](+)[B(C6F5)4](-) were consistent with the formation of a diamagnetic cation of the form [L(4)AlMe2](+) (MAO(50) is the product of the vacuum distillation of commercial MAO at +50 °C and contains only 1 mol% of Al in the form of free AlMe3). In the presence of MAO, this cationic aluminium complex was not capable of initiating the ROMP (ring opening metathesis polymerisation) of norbornene, whereas the 4a/4b systems with MAO(50) were active. A parallel pressure reactor (PPR)-based homogeneous polymerisation screening by using pre-catalysts 1b, 1c, 2a, 3a and 6a, in combination with MAO, revealed only moderate-to-good activities for the homo-polymerisation of ethylene and the co-polymerisation of ethylene/1-hexene. The molecular structures are reported for complexes 1a-1c, 2b, 5, 6a, 6b, 7a, 8a and 8c.

Two copper coordination polymers with pyridine imine-based ligand: Synthesis, crystal structure and luminescent properties

Two new copper coordination polymers [CuI2L(SCN)2]n (1) and {[CuIIL2(NO3)2]·2H2O}n (2), have been synthesized and characterized by IR, elemental analysis, XRPD, TG technique and X-ray crystallography, where L is N,N′-bis-(1-pyridin-4-yl-ethylidene)-hydrazine. Polymer 1 displays a new 3D inorganic–organic hybrid network generated through the coordination interactions between 2D inorganic sheets Cu(I)-(1,1,3-μ-SCN−) and bidentate bridging ligands L with two terminal monodentate pyridyl N donors. Each Cu(I) center adopted a distorted tetrahedral environment. The structure of polymer 2 is a 2D layer network containing tetrametallic quadrangular ring units [CuII4L4], in which each copper(II) atom is linked by four terminal monodentate pyridyl N-donors from four twist ligands and two oxygen atoms from two NO3−. The luminescent properties of polymers 1 and 2 were investigated in the solid state at room temperature.