The Magnetic Properties of Transition Metal Complexes

1- and 2-Tetrazolylacetonitrile as Versatile Ligands for Laser Ignitable Energetic Coordination Compounds

1- and 2-Tetrazolylacetonitrile (1- and 2-TAN) have been synthesized by the reaction of chloroacetonitrile with 1H-tetrazole under basic conditions. They further were reacted with sodium azide in the presence of zinc(II) chloride to form 5-((1H-tetrazol-1-yl)methyl)-1H-tetrazole (1-HTMT) and 5-((2H-tetrazol-2-yl)methyl)-1H-tetrazole (2-HTMT). The nitrogen-rich compounds have been applied as ligands for Energetic Coordination Compounds (ECCs) and show interesting coordinative behavior due to different bridging modes. The structural variability of the compounds has been proved by low-temperature X-ray analysis. The ECCs were analyzed for their sensitivities to provide information about the safety of handling and their capability to serve as primary explosives in detonator setups to replace the commonly used lead styphnate and azide. All colored ECCs were evaluated for their ignitability by laser initiation in translucent polycarbonate primer caps. In addition, the spin-crossover characteristics of [Fe(1-TAN)6](ClO4)2 were highlighted by the measurement of the temperature-dependent susceptibility curve.

Water-soluble chiral coordination polymers of Li+, Na+, K+, and Ba2+ with an anionic iron(III) complex of a L-threonine derivative and a significant red shift of visible spectra with Al3+ salt

Four salts of an anionic iron(III) bis-complex, [Fe(LL-thr)2]1-, were synthesized from water or methanol. H2LL-thr is a tridentate ligand derived from the L-threonine amino acid, and the cations used are Li+ (1), Na+ (2), K+ (3), and Ba2+ (4). Single-crystal X-ray diffraction showed that all the complexes are coordination polymers of different dimensionalities. The iron(III) complex binds to cations through its coordinated phenolate and non-coordinated carboxylate oxygen atoms. While Li+ forms a linear chain, all others have a pair of bridged cations intervening the iron(III) complexes. The 3D network of Ba2+ salt has a sizeable solvent-accessible space occupied by aquated chloride ions. The differences in circular dichroism (CD) spectra and significantly lower conductance values in water and methanol support partial retention of the polymeric nature in methanol. The visible spectra of 4 in methanol or water showed an ∼10 nm shift of the charge transfer bands from 3. However, the addition of Al3+ salt to 2 showed a significant colour shift. Further investigation confirmed that the colour shift is due to partial protonation of the complex with protons generated from salt hydrolysis. Most reports on visual aluminium detection consider aluminium's binding as the shift's source. The present results show that protonation due to hydrolysis of aluminium salt can skew the observation.

Further Perspectives on the Teflate versus Fluoride Analogy: The Case of a Co(II) Pentafluoroorthotellurate Complex

The pentafluoroorthotellurate group (teflate, OTeF5) is considered as a bulky analogue of fluoride, yet its coordination behavior in transition metal complexes is not fully understood. By reaction of [CoCl4]2- and neat ClOTeF5, we synthesized the first cobalt teflate complex, [Co(OTeF5)4]2-, which exhibits moisture-resistant Co-OTeF5 bonds. Through a combined experimental and theoretical (DFT and NEVPT2) study, the properties and electronic structure of this species have been investigated. It exhibits a distorted tetrahedral structure around the cobalt center and can be described as a d7 system with a quartet (S = 3/2) ground state. A comparative bonding analysis of the (pseudo)tetrahedral [CoX4]2- anions (X = OTeF5, F, Cl) revealed that the strength of the Co-X interaction is similar in the three cases, being the strongest in [Co(OTeF5)4]2-. In addition, an analysis of the charge of the Co center reinforced the similar electron-withdrawing properties of the teflate and fluoride ligands. Therefore, the [Co(OTeF5)4]2- anion constitutes an analogue of the polymeric [CoF4]2- in terms of electronic properties, but with a monomeric structure.

Electrocatalytic hydrogen generation using tripod containing pyrazolylborate-based copper(ii), nickel(ii), and iron(iii) complexes loaded on a glassy carbon electrode

Three transition metal complexes (MC) namely, [TpMeMeCuCl(H2O)] (CuC), [TpMeMeNiCl] (NiC), and [TpMeMeFeCl2(H2O)] (FeC) {TpMeMe = tris(3,5-dimethylpyrazolyl)borate} were synthesized and structurally characterized. The three complexes CuC, NiC, and FeC-modified glassy carbon (GC) were examined as molecular electrocatalysts for the hydrogen evolution reaction (HER) in alkaline solution (0.1 M KOH). Various GC-MC electrodes were prepared by loading different amounts (ca. 0.2-0.8 mg cm-2) of each metal complex on GC electrodes. These electrodes were used as cathodes in aqueous alkaline solutions (0.1 M KOH) to efficiently generate H2 employing various electrochemical techniques. The three metal complexes' HER catalytic activity was assessed using cathodic polarization studies. The charge-transfer kinetics of the HER at the (GC-MC)/OH- interface at a given overpotential were also studied using the electrochemical impedance spectroscopy (EIS) technique. The electrocatalyst's stability and long-term durability tests were performed employing cyclic voltammetry (repetitive cycling up to 5000 cycles) and 48 h of chronoamperometry measurements. The catalytic evolution of hydrogen on the three studied MC surfaces was further assessed using density functional theory (DFT) simulations. The GC-CuC catalysts revealed the highest HER electrocatalytic activity, which increased with the catalyst loading density. With a low HER onset potential (E HER) of -25 mV vs. RHE and a high exchange current density of 0.7 mA cm-2, the best performing electrocatalyst, GC-CuC (0.8 mg cm-2), showed significant HER catalytic performance. Furthermore, the best performing electrocatalyst required an overpotential value of 120 mV to generate a current density of 10 mA cm-2 and featured a Tafel slope value of -112 mV dec-1. These HER electrochemical kinetic parameters were comparable to those measured here for the commercial Pt/C under the same operating conditions (-10 mV vs. RHE, 0.88 mA cm-2, 108 mV dec-1, and 110 mV to yield a current density of 10 mA cm-2), as well as the most active molecular electrocatalysts for H2 generation from aqueous alkaline electrolytes. Density functional theory (DFT) simulations were used to investigate the nature of metal complex activities in relation to hydrogen adsorption. The molecular electrostatic surface potential (MESP) of the metal complexes was determined to assess the putative binding sites of the H atoms to the metal complex.

Oxidoborates Templated by Cationic Nickel(II) Complexes and Self-Assembled from B(OH)3

Several oxidoborates, self-assembled from B(OH)3 and templated by cationic Ni(II) coordination compounds, were synthesized by crystallization from aqueous solution. These include the ionic compounds trans-[Ni(NH3)4(H2O)2][B4O5(OH)4].H2O (1), s-[Ni(dien)2][B5O6(OH)4]2 (dien = N-(2-aminoethyl)-1,2-ethanediamine (2), trans-[Ni(dmen)2(H2O)2] [B5O6(OH)4]2.2H2O (dmen = N,N-dimethyl-1,2-diaminoethane) (3), [Ni(HEen)2][B5O6(OH)4]2 (HEen = N-(2-hydroxyethyl)-1,2-diaminoethane) (4), [Ni(AEN)][B5O6(OH)4].H2O (AEN = 1-(3-azapropyl) -2,4-dimethyl-1,5,8-triazaocta-2,4-dienato(1-)) (5), trans-[Ni(dach)2(H2O)2][Ni(dach)2] [B7O9(OH)5]2.4H2O (dach = 1,2-diaminocyclohexane) (6), and the neutral species trans-[Ni(en)(H2O)2{B6O7(OH)6}].H2O (7) (en = 1,2-diaminoethane), and [Ni(dmen)(H2O){B6O7(OH)6}].5H2O (8). Compounds 1–8 were characterized by single-crystal XRD studies and by IR spectroscopy and 2, 4–7 were also characterized by thermal (TGA/DSC) methods and powder XDR studies. The solid-state structures of all compounds show extensive stabilizing H-bond interactions, important for their formation, and also display a range of gross structural features: 1 has an insular tetraborate(2-) anion, 2–5 have insular pentaborate(1-) anions, 6 has an insular heptaborate(2-) anion (‘O+’ isomer), whilst 7 and 8 have hexaborate(2-) anions directly coordinated to their Ni(II) centers, as bidentate or tridentate ligands, respectively. The Ni(II) centers are either octahedral (1–4, 7, 8) or square-planar (5), and compound 6 has both octahedral and square-planar metal geometries present within the structure as a double salt. Magnetic susceptibility measurements were undertaken on all compounds.

Manganese-Catalyzed Hydroboration of Terminal Olefins and Metal-Dependent Selectivity in Internal Olefin Isomerization-Hydroboration

In the past decade, the use of earth-abundant metals in homogeneous catalysis has flourished. In particular, metals such as cobalt and iron have been used extensively in reductive transformations including hydrogenation, hydroboration, and hydrosilylation. Manganese, on the other hand, has been considerably less explored in these reductive transformations. Here, we report a well-defined manganese complex, [Mn(^iPrBDI)(OTf)₂] (2a; BDI = bipyridinediimine), that is an active precatalyst in the hydroboration of a variety of electronically differentiated alkenes (>20 examples). The hydroboration is specifically selective for terminal alkenes and occurs with exclusive anti-Markovnikov selectivity. In contrast, when using the analogous cobalt complex [Co(^iPrBDI)(OTf)₂] (3a), internal alkenes are hydroborated efficiently, where a sequence of isomerization steps ultimately leads to their hydroboration. The contrasting terminal versus internal alkene selectivity for manganese and cobalt was investigated computationally and is further discussed in the herein-reported study.

Models and materials for generalized Kitaev magnetism

The exactly solvable Kitaev model on the honeycomb lattice has recently received enormous attention linked to the hope of achieving novel spin-liquid states with fractionalized Majorana-like excitations. In this review, we analyze the mechanism proposed by Jackeli and Khaliullin to identify Kitaev materials based on spin-orbital dependent bond interactions and provide a comprehensive overview of its implications in real materials. We set the focus on experimental results and current theoretical understanding of planar honeycomb systems (Na₂IrO₃, α-Li₂IrO₃, and α-RuCl₃), three-dimensional Kitaev materials (β- and γ-Li₂IrO₃), and other potential candidates, completing the review with the list of open questions awaiting new insights.

Dinuclear first-row transition metal complexes with a naphthyridine-based dinucleating ligand

A series of dinuclear and tetranuclear first-row transition metal complexes were synthesized with the dinucleating ligand 2,7-bis(di(2-pyridyl)fluoromethyl)-1,8-naphthyridine (DPFN). The coordination pocket and rigidity of the DPFN ligand enforces pseudo-octahedral geometries about the metal centers that contain chloro, hydroxo, and aqua bridging ligands forming a "diamond" shaped configuration with metal-metal distances varying from 2.7826(5) to 3.2410(11) Å. Each metal center in the dinuclear complexes has an additional open coordination site that accommodates terminal ligands in a syn geometry of particular interest in catalyst design. The complexes are characterized by electronic spectroscopy, electrochemistry and potentiometric titration methods.

Synthesis, characterization, structural analysis of metal(II) complexes of N'-[(E)-3-Bromo-5-Chloro-2-hydroxybenzidene]-4-hydroxybenzohydrazide-Multisubstituted Schiff base as a F(-) and Cu(2+) ions selective chemosensor

New colorimetric chemosensor, N'-[(E)-3-Bromo-5-Chloro-2-hydroxybenzidene]-4-hydroxybenzohydrazide, containing OH and NH groups as binding sites have been synthesized and characterized by spectral UV, IR, NMR and ESR. The molecular structure of ligand is determined by X-ray crystallography and it has the monoclinic space group P21/c with cell parameters a=15.1058(6), b=14.3433(6), c=17.5800(8)Å and Z=8. The electronic spectral measurements show that Co(2+), Ni(2+) and Zn(2+) complexes have tetrahedral geometry, while Cu(2+) complex has square planar geometry. Magnetic measurements show that Cu(2+), Co(2+) and Ni(2+) complexes have paramagnetic behavior and Zn(2+) complex has diamagnetic behavior. Anion binding studies carried out using (1)H NMR and UV-visible spectrophotometric titrations revealed that these receptors exhibit selective recognition towards F(-) over other halide anions. The selectivity for F(-) among the halides is attributed mainly to the hydrogen-bond interaction of the receptor with F(-). Receptor (5 × 10(-5)M) shows color change from colorless to yellow in the presence of tetrabutylammonium fluoride (TBAF, 1.5 × 10(-3)M). Moreover, F(-)-induced color changes remain the same even in the presence of large excess of Cl(-), Br(-) and I(-). The binding constant is found to be higher towards F(-) ion and this may be due to presence of OH group, which offers extra binding site. Chromogenic receptor undergoes distinct color changes from colorless to green on gradual addition of Cu(2+) can be used as colorimetric probes for spectrophotometric and visual analysis of Cu(2+) in the presence of other transition metal ions such as Co(2+), Ni(2+) and Zn(2+).

Synthesis, DNA binding, and photonuclease activity of new tetraaza macrocyclic constrained isoxazole rings as subunit in metal complexes

The DNA-binding and photonuclease activity of newly synthesized tetra-azamacrocyclic ligand L (C(32)H(32)N(8)O(4)) and its complexes of type [MLCl(2)] and [ML]Cl(2) (where M = Co(II), Fe(II) and Cu(II); L = N,N'-[3-(4-{5-[(2-amino-ethylamino)-methyl]-isoxazol-3yl}-phenyl)-isoxazol-5-yl methyl-ethane-1,2-diamine] are specified. An octahedral geometry has been proposed for Fe(II) and Co(II) complexes, while the Cu(II) complex has a square planar environment. The absorption spectral results indicate that the complexes bind with the base pairs of DNA, with an intrinsic binding constant K(b) of Fe(II), Co(II), and Cu(II) complexes found to be 3.2 × 10(4) M(-1), 5.3 × 10(4) M(-1), and 4.2 × 10(4) M(-1), respectively, in 5 mM Tris-HCl/50 mM NaCl buffer at pH 7.2. The large enhancement in the relative viscosity of DNA on binding to the complexes supports the proposed DNA binding modes. The viscosity and thermal denaturation studies sustain the effective intercalation with DNA. The DNA photocleavage studies demonstrated that compounds exhibit significant photonuclease activity by a concentration dependent on singlet oxygen mediated mechanism.

Spectroscopic, electrochemical, and alkylation reactions: tert-butyl N-(2-mercaptoethyl)carbamate copper(II) and nickel(II) complexes as structural mimics for the active site of thiolate-alkylating enzymes

Two new dithiolate copper(II) and nickel(II) complexes with the ligand tert-butyl N-(2-mercaptoethyl)-carbamate (Boc-SH) were prepared. Their structures were established to be [(Boc-S)2M], where M=Cu (1) and Ni (2) by using elemental analysis, thermal analysis, molar conductivity, FT-IR, Raman, UV/VIS, and ESR as well as EI-mass spectroscopic methods. The X-ray structure of the ligand Boc-SH was also determined. Spectral data showed that the carbamate ligand act as anioinic bidentate through one immine nitrogen and one thiolate sulfur donor atoms. The spectral techniques suggest that both complexes appear to have square planar geometries. The very low electrical conductance of the two complexes supports their neutral nature. The redox behaviors of the obtained complexes were also investigated by cyclic voltammetry. The monomeric nature of both complexes was assessed from their magnetic susceptibility values. The thermoanalytical data evidence that complex (2) is stable up to 165°C and undergo complete decomposition, resulting in NiO as a residual product. The TEM image of the obtained oxide residue showed its nanosize cluster, suggesting that complex (2) may be used as a precursor for the formation of nanooxide. The methylation reactions of the two dithiolate complexes (1) and (2) with methyl iodide appear to occur intramolecularly at the metal(II)-bound dithiolates, forming the metal(II)-bound dithioether complexes [M(Boc-SCH3)2]I2 with clean second-order constants of 7.95×10(-2) and 10.59×10(-2) M(-1) s(-1), respectively.

Spectral, coordination and thermal properties of 5-arylidene thiobarbituric acids

Synthesis of 5-arylidine thiobarbituric acids containing different functional groups with variable electronic characters were described and their Co(2+), Ni(2+) and Cu(2+) complexes. The stereochemistry and mode of bonding of 5-(substituted benzylidine)-2-TBA complexes were achieved based on elemental analysis, spectral (UV-VIS, IR, (1)H NMR, MS), magnetic susceptibility and conductivity measurements. The ligands were of bidentate and tridentate bonding through S, N and O of pyrimidine nucleolus. All complexes were of octahedral configuration. The thermal data of the complexes pointed to their stability. The mechanism of the thermal decomposition is discussed. The thermodynamic parameters of the dissociation steps were evaluated and discussed.

Synthesis, ESR, UV-Visible and reactivity studies of new bis(N-dimethoxyaniline-3,5-(t)Bu2-salicylaldiminato)copper(II) complexes

Several new copper(II) complexes (4-6) with N-dimethoxyphenyl-3,5-(t)Bu(2)-salicylaldimine ligands abbreviated as N-R-3,5-DTBS, where R=2,4-dimethoxyphenyl (1), 2,5-dimethoxyphenyl (2) and 3,5-dimethoxyphenyl (3) have been prepared and their spectroscopic (IR, (1)H NMR, UV/Vis, ESR), magnetic and redox reactivity are described. The ESR spectra of frozen-solution 4-6 and solid state 4 and 5 are indicative of axially symmetric g-tensor (g([parallel])>g([perpendicular])>2.03). The powder ESR spectrum of 6 is unusual for CuN(2)O(2) type systems and exhibits "reversed" type (g([parallel])<g([perpendicular])) signal with a rhombic g-tensor g(3) (2.235)>g(2) (2.128)>g(1) (2.019) pattern indicative of a d(z)(2) ground state. Chemical oxidation of 4 and 6, as supported by ESR and UV/Vis techniques, generates new Cu(II) species and Cu(II)-phenoxyl radicals. The CV studies have shown that the complexes possess ligand-centered and copper(II)-centered quasi-reversible and irreversible responses.

Synthesis, spectroscopic characterization, antimicrobial, DNA binding and oxidative-induced DNA cleavage activities: new oxovanadium(IV) complexes of 2-(2-hydroxybenzylideneamino)isoindoline-1,3-dione

A mononuclear complex [(phen)VO(bid)]SO(4)·3H(2)O (1), (phen=1,10-phenanthroline) and a binuclear [(VObid)(2)]·H(2)O (2), with Hbid [(E)-2-(2-hydroxybenzylideneamino)isoindoline-1,3-dione] were prepared and characterized by elemental analysis, IR, mass, UV-Vis spectral studies, ESR, molar conductance and thermogravimetric analysis. The DNA-binding properties of the complexes 1 and 2 were investigated by UV-spectroscopy, fluorescence spectroscopy and viscosity measurements. The superoxide dismutase-like activity of the complexes has been determined. The spectral results suggest that the complexes 1 and 2 can bind to DNA by intercalation. The cleavage properties of these complexes with super coiled pUC19 have been studied using the gel electrophoresis method, where both complexes 1 and 2 displayed chemical nuclease activity in the absence and presence of H(2)O(2)via an oxidative mechanism. All the complexes inhibit the growth of both Gram positive and Gram negative bacteria to competent level. Minimum inhibitory concentration (MIC) was determined by microtiter method.

Supramolecular structure, mixed ligands and substituents effect on the spectral studies of oxovanadium(IV) complexes of bioinorganic and medicinal relevance

An interesting series of heterocyclic mixed ligand of oxovanadium(IV) complexes have been synthesized by the reaction of vanadium(IV) sulfate with rhodanine azo (HL(n)) in the presence of β-diketon (LH). The elemental analysis, magnetic moments, spectral (UV-Vis, IR, (1)HNMR and ESR) with thermal studies were used to characterize the isolated complexes. The IR showed that the ligands (HL(n) and LH) act as a monobasic bidentate through the (NN), oxygen keto moiety and oxygen atom of the two enolate groups thereby forming a six-membered. The molar conductivities show that all the complexes are non-electrolytes. The ESR spectra indicate that the free electron is in d(xy) orbital. The calculated bonding parameter indicates that in-plane σ-bonding is more covalent than in-plane π-bonding. The coordination geometry around oxovanadium(IV) in all complexes is a hex-coordinated trans octahedral, with one bidentate ligand (L(n)), and one bidentate ligand (L). Electronic and magnetic data proposed the octahedral structure for all complexes under investigation. ESR spectra of VO(2+) reveal data that confirmed the proposed structure. The value of covalency factor (β(1)(∗))(2) and orbital reduction factor K accounts for the covalent nature of the complexes. All electronic transitions were assigned. The Hammett's constant is also discussed.

Synthesis, DNA-binding, DNA-photonuclease profiling and antimicrobial activity of novel tetra-aza macrocyclic Ni(II), Co(II) and Cu(II) complexes constrained by thiadiazole

A new tetra-aza macrocyclic ligand, L (C(24)H(16)N(12)O(2)S(4)) and its complexes of type, [MLCl(2)] and [CuL]Cl(2) (where M=Ni(II), Co(II); L=N,N'-(benzene-1,3-diyldi-1,3,4-thiadiazole-5,2-diyl)bis{2-[(5-benzene-1,3-diyl-1,3,4-thiadiazol-2-yl)amino]acetamide}) were synthesized and characterized by the spectral and analytical techniques. An octahedral geometry has been proposed for Ni(II) and Co(II) complexes while Cu(II) complex exhibit a square planar geometry. All the synthesized metal complexes were screened for their in vitro antimicrobial activity against selected species of pathogenic bacteria and fungi. The binding property of the complexes with CT-DNA was studied by absorption spectral analysis, followed by viscosity measurement and thermal denaturation studies. The photo induced cleavage studies revealed that the complexes possess photonuclease property against pUC19 DNA under UV-visible irradiation.