Solid-state high-resolution NMR studies on spin fluctuation associated with valence tautomerism of metal–benzoquinone system: Cobalt complex in the stable and meta-stable phases

Equilibrium between low-spin [Co^III(SQ)(Cat)(N–N)] and high-spin [Co^II(SQ)₂(N–N)] redox isomers, where SQ is semiquinonate (charge: −1, spin: 1/2), Cat is catecholate (charge: −2, spin: 0) and N–N is chelating nitrogen donor ligand, respectively, is a representative valence tautomeric phenomenon. To elucidate independently the spin state of the cobalt ion and that of benzoquinone-derived ligands in the solid state, we measured ¹³C MAS NMR spectrum of 3,5-di-t-butyl-1,2-benzoquinone and ²H MAS NMR spectrum of deuterated 2,2′-bipyridine for [Co(3,5-di-t-butyl-1,2-benzoquinone)₂(2,2′-bipyridine)] · x(C₆H₅CH₃) and its deuterated analogue in a temperature range of 200–350 K. Irreversible change of an effective magnetic moment μ_eff of a virgin sample was observed around 370 K due to a partial loss of crystal solvent and a change of crystal structure, whereas the sample annealed at 390 K showed a crystal structure different from the reported one and a reversible change of μ_eff, which is ascribed to equilibrium between Co(III)-form (S = 1/2) and Co(II)-form (S = 3/2). Based on the shifts and the number of NMR peaks for the annealed sample, we concluded that (1) interconversion between redox isomers occurs faster than NMR time scale (>10⁴ s⁻¹) in the solid state, (2) intraconversion between SQ and Cat in Co(III)-form also occurs much faster than 5 × 10⁴ s⁻¹ even at 198 K and (3) electron spins on SQ ligands in Co(II)-form are quenched probably due to a strong antiferromagnetic coupling between the two SQ ligands. The enthalpy and the entropy of the interconversion were estimated to be 17 kJ/mol and 54 J/(K mol), respectively. For the virgin metastable phase, SQ and Cat were clearly distinguished by ¹³C MAS NMR spectrum. The solid-state high-resolution NMR spectrum is useful to detect independently the change of spin states of benzoquinone-derived radical and metal ion.     

Synthesis and characterization of three Cu(II), Zn(II), Cd(II) supramolecular complexes bridged by biphenyl 2,2′-dicarboxylate

Three complexes, M₂(bpy)₂(bpdc)₂·xH₂O [M = Cu, x = 0; M = Zn or Cd, x = 2], have been hydrothermally synthesized by 1,1′-biphenyl-2,2′-dicarboxylic acid (H₂bpdc) with 2,2′-bipyridine (bpy) to form binuclear molecules. In each, the two bpdc groups align the two opposing planar [M(bpy)]²⁺ cations. The molecules are connected by C–H⋯O hydrogen bonds, π–π stacking, and C–H⋯π interactions to form three dimensional supramolecular networks. Furthermore, at room temperature, complex 3 exhibits strong photoluminescence.     

Two novel Keggin tungstocobaltates grafted by cobaltII complex group(s): K[Co(phen)2(H2O)]2[HCoW12O40]·2H2O and [Co(2,2′-bipy)3]1.5{[Co(2,2′-bipy)2(H2O)][HCoW12O40]}·0.5H2O

Two novel inorganic–organic hybrid compounds composed of Keggin tungstocobaltate framework and cobalt(II)–N coordination complexes, K[Co(phen)₂(H₂O)]₂[HCoW₁₂O₄₀]·2H₂O (1) (phen = 1,10-phenanthroline) and [Co(2,2′-bipy)₃]_1.5{[Co(2,2′-bipy)₂(H₂O)][HCoW₁₂O₄₀]·0.5H₂O (2) (bipy = bipyridine), have been synthesized under hydrothermal conditions by directly using Keggin POMs as starting materials, which were characterized by elemental analyses, IR, TG analyses and X-ray single crystal diffraction. Crystal data for compound 1: C₄₈H₄₁Co₃KN₈O₄₄W_12, triclinic, space group P-1, a = 10.918(5) Å, b = 13.401(5) Å, c = 13.693(5) Å, α = 69.291(5)°, β = 71.568(5)°, γ = 78.421(5)°, V = 1768.9(12) Å³, Z = 1; for compound 2: C₁₃₀H₁₀₄Co₇N₂₆O₈₃W_24, orthorhombic, space group, C2/c, a = 46.839(9) Å, b = 14.347(3) Å, c = 26.147(5) Å, α = β = γ = 90°, V = 17,570(6) Å³, Z = 4. Compound 1 exhibits a pseudo-1D chainlike structure, in which potassium ions act as linkages of Keggin unit doubly grafted by [Co(phen)₂(H₂O)] complex. Compound 2 represents a [Co(2,2′-bipy)₂(H₂O)]²⁺ mono-grafted Keggin tungstocobaltate derivative with 1.5[Co(2,2′-bipy)₃]²⁺ countercations. The cyclic voltammetric behavior of 1-CPE is similar to the parent 3-CPE, but the cyclic voltammetric behavior of Co^II shows a little difference. Variable-temperature magnetic susceptibility measurement of compound 1 demonstrates the presence of antiferromagnetic interactions.     

Novel metallophthalocyanines bearing 3-(p-chlorophenyl)-5-p-tolyl-4H-1,2,4-triazole bulky substituents by microwave irradiation

The synthesis of metallophthalocyanines [6–9; M = Ni(II), Zn(II), Co(II) and Cu(II)] with four 1,2,4-triazole units obtained from 4-{(4-chloro-2-fluorobenzyl)[3-(4-chlorophenyl)-5-(4-methylphenyl)-4H-1,2,4-triazol-4-yl]amino}phthalonitrile (5) in the presence of dimethylaminoethanol and the corresponding anhydrous metal salts is described. The thermal stabilities of the Pc compounds were determined by thermogravimetric analysis. The new compounds were characterized by a combination of IR, ¹H NMR, ¹³C NMR, UV–Vis, elemental analysis.     

Thermal and spectroscopic studies of some metal complexes with a new enaminone ligand 3-chloro-4-((4-methoxyphenyl)amino)pent-3-en-2-one and their investigation as anti-urease and cytotoxic potential drugs

Complexes of transition metals [Co(II), Cd(II) and Mo(0)] with a new enaminone (PA) 3-chloro-4-((4-methoxyphenyl)amino)pent-3-en-2-one were synthesized and afterwards characterized by ¹H NMR, ¹³C NMR, FAB-MS, UV–Vis, ICP-OES, TGA and FTIR. The spectroscopic and conductance data suggested that the ligand (PA) is attached to the metal ions in bidentate, neutral form through the nitrogen atom of amino group and the oxygen of carbonyl group. Metal complexes displayed octahedral geometries. In vitro urease inhibition and cytotoxic activities of all the compounds were evaluated. Results revealed that Co(II) complex (PA-Co) was even more significant than the reference drug thiourea. Analysis of the cytotoxicity indicated that, the Co(II) complex has more cytotoxic effect than enaminone ligand and other complexes when assessed on the human cancer cell lines MCF-7.Molecular docking simulation was also performed to find out the putative binding mode within the target protein.     

Tungstoarsenate(III) polyoxoanions as inorganic ligands for polynuclear copper complexes

A trinuclear copper(II) complex and a tetranuclear copper(II) complex that each incorporate a 9-tungstoarsenate(III) polyoxoanion ligand and several 2,2′-bipyridine (2,2′-bpy) ligands co-crystallize as the acid form: H₄[{Cu(2,2′-bpy)}₃(α-AsW₉O₃₃)] · [{Cu(2,2′-bpy)₂}{Cu(2,2′-bpy)}₃(α-AsW₉O₃₃)] · 38H₂O (1). Compound 1 was obtained from a hydrothermal reaction employing the trivacant lacunary polyoxometalate [α-AsW₉O₃₃]^9? as a precursor. The α-isomeric form of the parent ion is retained in both product anions. Four different coordination modes are observed for the seven copper centers, with unusual structural distortions imposed by the steric bulk of the polyanion. Intra- and intermolecular π-stacking of the 2,2′-bpy ligands are both evident. Variable temperature magnetic susceptibility, magnetization and EPR data are consistent with very weak antiferromagnetic interactions between the copper centers that are likely mediated by the π-stacking of the 2,2′-bpy ligands.     

Synthesis,characterization and biological properties of Co(II), Ni(II), Cu(II) and Zn(II) complexes with an SNO functionalized ligand

Some new metal(II) complexes, ML₂[M = Co, Ni, Cu and Zn], of 2-acetylthiophene benzoylhydrazone ligand (HL) containing a trifunctional SNO-donor system have been synthesized and characterized on the basis of physicochemical data by elemental analysis, magnetic moment, molar conductance, thermogravimetric and spectroscopic (electronic, IR, ¹H NMR and ¹³C NMR) data. The ligand functions as monobasic SNO tridentates where the deprotonated enolic form is preferred in the coordination producing distorted octahedral complexes.     

Synthesis and asymmetric catalytic application of chiral imidazolium–phosphines derived from (1R,2R)-trans-diaminocyclohexane

Reaction between a chiral imidazole–amine precursor derived from (1R,2R)-trans-diaminocyclohexane and P¹Cl (where P¹ = PPh₂, P(1,3,5-Me₃C₆H₃)₂, P(2,2′-O,O′-(1,1′-biphenyl), P((R)-(2,2′-O,O′-(1,1′-binaphthyl))) and P((S)-(2,2′-O,O′-(1,1′-binaphthyl)))) followed by RX (where R = ⁿPr, ⁱPr, CHPh₂, X = Br; R = ⁱPr, X = I), respectively, gives a selection of chiral imidazolium–phosphine compounds. Deprotonation of the imidazolium salt gives the corresponding NHC–P ligands that can be used in metal-mediated asymmetric catalytic applications. Catalytic reactions show that NHC–P ligands give a significantly greater rate of reaction for a palladium catalysed allylic substitution reaction in comparison to analogous di-NHC or NHC–imine ligands and that NHC–P hybrids are also effective for iridium catalysed transfer hydrogenation.     

Synthesis and spectroscopic characterization of new tetradentate Schiff base and its coordination compounds of NOON donor atoms and their antibacterial and antifungal activity

New Schiff base (H₂L) ligand is prepared via condensation of o-phthaldehyde and 2-aminobenzoic acid in 1:2 ratio. Metal complexes are prepared and characterized using elemental analyses, IR, solid reflectance, magnetic moment, molar conductance, ¹H NMR, ESR and thermal analysis (TGA). From the elemental analyses data, the complexes were proposed to have the general formulae [MCl(L)(H₂O)]·2H₂O (where M = Cr(III) and Fe(III)); [M(L)]·yH₂O (where M = Mn(II), Ni(II), Cu(II) and Zn(II), y = 1–2) and [M(L)(H₂O)_n]·yH₂O (where M = Co(II) (n = y = 2), Co(II) (n = y = 1), Ni(II) (n = 2, y = 1). The molar conductance data reveal that all the metal chelates were non-electrolytes. IR spectra show that H₂L is coordinated to the metal ions in a bi-negative tetradentate manner with NOON donor sites of the azomethine-N and carboxylate-O. The ¹H NMR spectral data indicate that the two carboxylate protons are also displaced during complexation. From the magnetic and solid reflectance spectra, it was found that the geometrical structure of these complexes are octahedral (Cr(III), Fe(III), Co(II) and Ni(II)), square planar (Cu(II)), trigonal bipyramidal (Co(II)) and tetrahedral (Mn(II), Ni(II) and Zn(II)). The thermal behaviour of these chelates showed that the hydrated complexes losses water molecules of hydration in the first step followed immediately by decomposition of the ligand molecule in the subsequent steps. The biological activity data show that the metal complexes to be more potent/antibacterial than the parent Shciff base ligand against one or more bacterial species.     

Molecular wiring of LiMnPO4 (olivine) by ruthenium(II)-bipyridine complexes

LiMnPO₄ (olivine) was surface-modified by two different complexes: Ru-bis(4,4′-diethoxycarbonyl-2,2′-bipyridine)(4,4′-dicarboxylate-2,2′-bipyridine) and Ru-bis(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(4,4′-dinonyl-2,2′bipyridine). These complexes have redox potentials of 4.45 and 4.25 V vs. Li/Li⁺, respectively, and are both active for molecular wiring of LiMnPO₄. The surface-confined Ru(II)/Ru(III) redox reaction propagates across the monolayer via hole-hopping, allowing a subsequent chemical delithiation of the underneath olivine towards MnPO₄. The activity of LiMnPO₄ is about half of that of LiFePO₄ (olivine) at similar experimental conditions.     

A high molar extinction coefficient charge transfer sensitizer and its application in dye-sensitized solar cell

A high molar extinction coefficient charge transfer sensitizer tetrabutylammonium [Ru(4,-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(4,4′-di-(2-(3,6-dimethoxyphenyl)ethenyl)-2,2′-bipyridine)(NCS)₂], is developed which upon anchoring onto nanocrystalline TiO₂ films exhibit superior power conversion efficiency compared to the standard sensitizer bistetrabutylammonium cis-dithiocyanatobis(4,4′-dicarboxylic acid-2,2′-bipyridine)ruthenium(II) (N719). The new sensitizer anchored TiO₂ films harvest visible light very efficiently over a large spectral range and produce a short-circuit photocurrent density of 18.84 mA/cm², open-circuit voltage 783 mV and fill factor 0.73, resulting remarkable solar-to-electric energy conversion efficiency (η) 10.82, under Air Mass (AM) 1.5 sunlight. The Time Dependent Density Functional Theory (TDDFT) excited state calculations of the new sensitizer show that the first three HOMOs have ruthenium t_2g character with sizable contribution coming from the NCS ligands and the π-bonding orbitals of the 4,4′-di-(2-(3,6-dimethoxyphenyl)ethenyl)-2,2′-bipyridine. The LUMO is a π^* orbital localized on the 4,4′-dicarboxylic acid-2,2′-bipyridine ligand.     

Synthesis,structure and characterization of new 1D and 2D Ni(II) coordination polymers

Two new Ni(II) coordination polymers, namely [Ni₃(Hsdac)₂(sdac)₂(2,2′-bipy)₂] (1) and [Ni₂(sdac)₂(4,4′-bipy)₂]·2H₂O (2) (sdba = 4,4′-sulfonyldibenzoate, 2,2′-bipy = 2,2′-bipyridine, 4,4′-bipy = 4,4′-bipyridine), have been prepared under hydrothermal conditions and characterized by single crystal X-ray diffraction analyses. Compound 1 possesses an interesting chain structure. Adjacent chains are further linked through H-bond interactions between Hsdac ligands to give a two-dimensional (2D) supramolecular architecture. Compound 2 displays an unusual 2D polyrotaxane-like network.     

Solution studies of tris(2-benzylaminoethyl)amine complexes of zinc(II) and copper(II): The catalytic hydrolysis of toxic organophosphate

Solution studies of the tetradentate ligand tris(2-benzylaminoethyl)amine, BzTren with both zinc(II) and copper(II) salts were investigated in aqueous methanol (33% v/v) by means of ¹H NMR, potentiometric, and UV-visible titrations as well as cyclic voltammetry. Subsequently, their zinc(II) and copper(II) complexes [BzTren-M(OH₂)]²⁺ 1 and 2 (M²⁺ = Zn²⁺ and Cu²⁺) were synthesized and fully characterized by using FT-IR spectroscopy, elemental analysis, and thermal analysis. Complexes 1 and 2 are investigated kinetically for the catalytic hydrolysis of the toxic organophosphate parathion at 50 °C in aqueous methanol (33%, v/v). The kinetic results indicate that copper(II) complex 2 is more active than zinc(II) complex 1, presumably a reflection of the effective electron-withdrawing as well as the greatest electrophilicity of copper(II) ion.     

Preparation of a novel Mixed-Ligand divalent metal complexes from solvent free Synthesized Schiff base derived from 2,6-Diaminopyridine with cinnamaldehyde and 2,2′‐Bipyridine: Characterization and antibacterial activities

The solvent-free conditions were employed to synthesise symmetrical Schiff base ligand from 2,6-diaminopyridine with cinnamaldehyde in (1 min) with a fair yield utilizing formic acid as a catalyst. Through coordination chemistry, new heteroleptic complexes of Cu(II), Co(II), Ni(II), Pt(II), Pd(II) and Zn(II) were achieved from Schiff base as a primary chelator (L¹) and 2,2′‐bipyridine (2,2′-bipy) as a secondary chelator (L²). The prepared compounds have been characterized by elemental analysis, molar conductivity, magnetic susceptibility, FTIR, ¹H NMR, UV–visible, mass spectrometry, and thermal gravimetric analysis, and screened in vitro for their potential as antibacterial activity by the agar well diffusion method. The metal complexes were formulated as [M (L¹) (L²) (X)] Y⋅nH₂O, L¹ = Schiff base, L² = 2,2′-bipy, (M = Cu(II), Co(II), Zn(II), Y = 2NO₃, n = 1), (M = Ni(II), X = 2H₂O, Y = 2NO_3, n = 0) and (M = Pd(II) Pt(II), Y = 2Cl, n = 0). Both L¹ and L² act as a neutral bidentate ligand and coordinates via nitrogen atoms of imine and 2,2′-bipy to metal ions. The metal complexes were found to be electrolytic, with square-planar heteroleptic Cu(II), Co(II), Pt(II), and Pd(II) chelates and octahedral Ni(II) complex. As well as tetrahedral geometry, has been proposed for the complex of Zn(II). Furthermore, the biological activity study revealed that some metal chelates have excellent activity than Schiff base when tested against Gram-negative and Gram-positive strains of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Finally, it was found that the Zn(II) and Pd(II) complexes were more effective against both types of bacteria tested than the imine and other metal complexes.     

Mesophase and magnetic behavior in cobalt(II) and iron(II) compounds

The metal complexes with long alkyl chains [Co(C16-terpy)₃](BF₄)₂ (1) and [Fe(C16-terpy)₂](BF₄)₂ (2) were synthesized and the physical properties of the complex were characterized by magnetic susceptibility, Mössbauer spectroscopy, polarizing optical microscopy, differential scanning calorimetry, and X-ray scattering, where C16-terpy is 4′-hexadecyloxy-2,2′:6′,2′′-terpyridine. Variable-temperature magnetic susceptibility measurements and/or Mössbauer studies revealed that the complex 1 exhibited unique spin transition (T_1/2↓ = 217 K and T_1/2↑ = 260 K) induced by structural phase transition, and the complex 2 was in the low-spin state in the temperature region of 5–400 K before the first mesophase transition. The cobalt(II) and iron(II) complexes exhibited liquid-crystal properties in the temperature range of 371–528 K and 466–556 K, respectively. After mesophase transition, the complex 1 exhibited only slight spin transition (T_1/2↓ = 266 K and T_1/2↑ = 279 K), and the complex 2 was in the low-spin state. The compounds with multifunction, i.e., magnetic property and liquid-crystal properties, are important in the development of molecular materials.     

Dimethyl sulfoxide oxidation mediated by a copper(II) diamine complex: A possible source of problem in the synthesis of molecular magnetic compounds

In the present work we report a reaction in which dimethyl sulfoxide, initially used as solvent, undergoes oxidation to form sulfate, which then participates to the formation of a linear one-dimensional copper chain. Indeed, using [Cu(bipy)Cl₂], a building block largely applied in synthesis of molecular magnetic compounds, the coordination compound [Cu(bipy)(H₂O)₂(SO₄)]_n, where bipy = 2,2′-bipyridine was obtained. Magnetic characterization of complex shows a weak antiferromagnetic interaction between the copper(II) ions with J = ?0.53 cm^?1. DFT calculations demonstrate that the pathway for the weak antiferromagnetic interaction is through the sulfate bridge.     

A novel HMP-2 dye of high extinction coefficient designed for enhancing the performance of ZnO platelets

Newly designed cis-[Ru(H₂dcbpy) (L) (NCS)₂ (HMP-2), where L is 4-(4-(N,N-di-(p-hexyloxyphenyl)-amino)styryl)-4′-methyl-2,2′-bipyridine, sensitizing dye of high extinction coefficient than the routinely preferred ruthenium (II) cis-di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) (N3) sensitizing dye is explored. Electrochemically synthesized ZnO (plates of 4–5 μm in height and 100–150 nm in width) electrodes are dipped intentionally in N3 and HMP-2 dyes for 20 h so as to observe large surface agglomeration effect for ZnO/N3 system. Due to the local inhomogeneity resulting from the Zn²⁺/dye complex layer formed between ZnO platelets and N3 dye, its solar-to-electrical conversion efficiency is inferior than aggregation-free ZnO/HMP-2.     

An oxidovanadium(IV) complex having a perrhenato ligand: An efficient catalyst for aerobic oxidation reactions of benzylic and propargylic alcohols

An oxidovanadium(IV) complex having a perrhenato ligand [VO(ReO₄)(4,4′-^tBubpy)₂][0.25SO₄·0.5ReO₄] (4,4′-^tBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine) efficiently catalyzes not only dehydrogenative oxidation of benzylic and propargylic mono-alcohols but also oxidative CC bond cleavage of meso-1,2-diaryl-1,2-ethanediols under atmospheric molecular oxygen, affording the corresponding carbonyl compounds in good yield.     

Nonadiabatic molecular dynamics with solvent effects: A LR-TDDFT QM/MM study of ruthenium (II) tris (bipyridine) in water

The previously derived trajectory-based nonadiabatic molecular dynamics scheme [E. Tapavicza, I. Tavernelli, U. Rothlisberger, Phys. Rev. Lett. 98 (2007) 023001] is extended to include the coupling of the quantum system with a classically described environment. The dynamics is performed using LR-TDDFT energies and forces computed on-the-fly together with the nonadiabatic coupling vectors needed for the propagation of the nuclear coefficients according to Tully’s fewest-switches surface hopping algorithm. The resulting LR-TDDFT-QM/MM approach is applied to the study of the ultrafast relaxation of the photoexcited singlet metal-to-ligand-charge-transfer state (¹MLCT) of [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) in water. The observed intersystem crossing dynamics with the triplet MLCT is in good agreement with available experimental results.     

Syntheses,structural studies and spectroscopic characterisation of pyridyl–phthalimide complexes of fac-(CO)3ReI-diimines

The mono-dentate ligands, 3-aminomethyl-N-phthalimido-pyridine (L¹) and 3-amino-N-phthalimido-pyridine (L²), were synthesised using a solvent-free melt method. These ligands were then used to access three pairs of functionalised luminescent Re^I complexes of the generic type fac-{Re(CO)₃(diimine)(Lⁿ)}(BF₄) [where diimine = 4,4′-dimethyl-2,2′-bipyridine (dmb); 2,2′-bipyridine (bpy); 1,10-phenanthroline (phen)]. X-ray crystallography has been used to structurally characterise five of the complexes showing that in the cases of the L¹ species the phthalimide unit is adjacent to and co-planar with the coordinated diimine ligand. Solution state UV–Vis absorption, electrochemistry and IR studies confirm that the proposed formulations and coordination modes exist in solution. The photophysical studies show that the visible emission from each of the six complexes is ³MLCT at room temperature. Within each pair of complexes the precise energy of the emission was subtly dependent upon the axial ligand, Lⁿ with luminescence lifetimes in the range 121–288 ns.