Transition metal atoms grafted on the nanodiamonds surface: Identification and guest–host spin–spin interactions

This survey describes recent achievements in creating a new type of materials – nanodiamonds grafted with atoms of transition metals. Structural features of some selected chelate complexes studied by density functional theory, their scope and limitations as well as possible applications are discussed. Using the example of copper ions, their location relative to subsurface defects of detonation diamond is investigated by the method of electron paramagnetic resonance (EPR).     

Interpretation of the parameters of the EPR spectra of complexes containing ns 1 ions

The parameters of the EPR spectra of complexes containing paramagnetic ions with an unpaired ns electron (ns ¹ ions) were interpreted. The effect of the ligand spin-orbital coupling on the parameter of the Zeeman splitting was discussed. The effect of spin polarization on the parameters of hyperfine and ligand hyperfine couplings was considered. The reasons for the anomalous behavior of the EPR parameters were noted. The character of the covalent bonding was analyzed from the EPR spectra. The anomalous behavior of the parameters of ligand hyperfine couplings in tetragonal complexes with ns ¹ ions was discussed.     

ESR study of weak exchange interactions in binuclear copper(ii) complexes with acyldihydrazones of fluorinated β-diketones

A series of binuclear copper(ii) complexes with acyldihydrazones of aliphatic dicarboxylic acids (from malonic to adipic) and fluorinated -diketones (trifluoro- and hexafluoroacetylacetone) of composition Cu₂L·2Py (H₄L is acyldihydrazone) were studied by ESR spectroscopy. The ESR spectra of solutions of complexes with trifluoroacetylacetone acyldihydrazones have an isotropic signal with a seven-line hyperfine structure from two equivalent copper nuclei (g = 2.112, a _Cu = (39—40)·10^–4 cm^–1), which is indicative of weak exchange interactions between the paramagnetic centers due to spin density delocalization through a chain of the -bonds of the polymethylene bridge. On going to hexafluoroacetylacetone derivatives, the coupling is suppressed and the ESR spectra of solutions of such complexes show a signal with a four-line hyperfine structure (g = 2.121—2.131, a _Cu = (55—63)·10^–4 cm^–1) typical of mononuclear copper complexes.     

Dinuclear Mesogens with Antiferromagnetic Properties

Herein, two new groups of isomeric bimetallic nickel(II) and copper(II) complexes containing pyrazine or pyrimidine rings are synthesized and examined. The complexes exhibit liquid‐crystalline columnar phases in a broad temperature range. For the copper(II) complexes, super‐exchange coupling between two Cu^II ions is observed. For the pyrimidine derivative in which the paramagnetic Cu^II ions are separated only by three atoms, an antiferromagnetic spin alignment is detected. When the distance between Cu^II ions increases to four atoms in the pyrazine derivative, the magnetic interaction becomes significantly weaker.     

Solid‐State NMR Correlation Experiments and Distance Measurements in Paramagnetic Metalorganics Exemplified by Cu‐Cyclam

We show how to record and analyze solid‐state NMR spectra of organic paramagnetic complexes with moderate hyperfine interactions using the Cu‐cyclam complex as an example. Assignment of the ¹³C signals was performed with the help of density functional theory (DFT) calculations. An initial assignment of the ¹H signals was done by means of ¹H–¹³C correlation spectra. The possibility of recording a dipolar HSQC spectrum with the advantage of direct ¹H acquisition is discussed. Owing to the paramagnetic shifting the resolution of such paramagnetic ¹H spectra is generally better than for diamagnetic solid samples, and we exploit this advantage by recording ¹H–¹H correlation spectra with a simple and short pulse sequence. This experiment, along with a Karplus relation, allowed for the completion of the ¹H signal assignment. On the basis of these data, we measured the distances of the carbon atoms to the copper center in Cu‐cyclam by means of ¹³C R₂ relaxation experiments combined with the electronic relaxation determined by EPR.     

Nuclear magnetic resonance relaxometry as a spectroscopic probe of the coordination sphere of a paramagnetic metal bound to a humic acid mixture

Protons on water molecules are strongly affected by paramagnetic ions. Since the acid-base properties of water facilitate rapid proton exchange, a single proton nuclear magnetic resonance (NMR) signal is seen in aqueous solutions of paramagnetic ions. Proton relaxation times are significantly affected by paramagnetic species and the readily detectable single signal serves as a powerful amplifier of the information contained concerning the protons in the paramagnetic environment. Where water molecules coordinated to free paramagnetic ions and to metal complexes of ligands that form non-labile (on the NMR time scale) complexes, the effects on water in the two environments can be distinguished. This can provide information on the nature of the ligand binding sites. The example of Cu²⁺ bound to the Laurentian humic acid mixture reported here using convenient low field NMR relaxometers shows that the information can enrich our understanding of complexation and speciation in the presence of complex mixture ligands characteristic of natural water systems. In this case, the data underline the role of aggregation and conformation in defining the complexation sites.     

A nickel-substituted form of nanodiamonds and its catalytic activity in decomposition of hydrogen peroxide

Adsorption of nickel (II) ions with nanodiamonds obtained by the detonation synthesis was studied. A nickel-substituted form of nanodiamonds was obtained. The catalytic activity exhibited by nickel ions (II) in the form of the surface complexes with nanodiamond functional groups in decomposition of hydrogen peroxide was determined.     

Synthesis and study of binuclear vanadyl complexes with acyldihydrazones of salicylaldehyde and dicarboxylic acids

The binuclear vanadyl(ii) complexes [(VO)₂·2Py·2EtOH]·mH₂O with acyldihydrazones of salicylaldehyde (H₄L) and dicarboxylic acids were synthesized and studied. In these complexes, two chelate vanadyl(ii) complexes with the tridentate bicyclic ligands are linked to each other by the polymethylene bridges —(CH₂) _n — of different lengths varying from one to four units. The ESR spectra of solutions of these complexes, unlike those of analogous copper(ii) complexes, have an isotropic signal with an eight-line hyperfine structure (g = 1.972, a _V = 93·10^–4 cm^–1) typical of monomeric vanadyl complexes, which indicates that no exchange interactions occur between the paramagnetic centers through the polymethylene chain.     

Magnetic interactions in spin-labelled glycine complexee to cupric ions

Summary The interactions between cupric ions and a paramagnetic aminoacid ligand labelled with a nitroxide group were investigated by visible and e.p.r. spectroscopies. Glycine substituted on the carboxylic acid function with 4-amino-2,2,6,6-tetramethylpiperidineN-oxide forms complexes with cupric ions which show strong magnetic interactions between the paramagnetic centers. At high pH values, averaged g and hyperfine coupling parameters are observed with g=2.05 and a=30 gauss, corresponding to a complex formed with two ligands coordinated to the cupric ions. The presence of other species is discussed.     

An analysis of the EPR spectral parameters and the electronic state of the low-symmetry complexes of 67Zn+, 111Cd+, and 205Tl2+ns1 ions in crystals with the potassium sulfate structure

The electronic structures of the low-symmetry complexes of 4s ¹-⁶⁷Zn⁺, 5s ¹-¹¹¹Cd⁺, and 6s ¹-²⁰⁵Tl²⁺ ions in isostructural crystals of potassium sulfate, rubidium sulfate, and potassium selenate were studied by the EPR data. The sp-mixing of the atomic orbitals of the paramagnetic ion in the molecular orbital of the ground state was established. The parameters of the EPR spectra were analyzed. The sp-mixing was shown to result in negative shifts of the Zeeman interaction parameters. The signs of components of the hyperfine coupling tensor (A tensor) were considered. The determining role of the contribution from the isotropic hyperfine coupling to the components of the A tensor was found. The anomalously low observed values of the hyperfine coupling parameters of the thallium complexes were interpreted.     

Natural limit of the number of copper ions chemisorbed on the surface of a detonation nanodiamond

The upper limit of the amount of copper ions that can be grafted to the detonation nanodiamond surface has been calculated. A maximum of 34 wt% copper can be grafted to carboxylated and hydroxylated 5 nm nanodiamonds. The results of calculations have been compared with the experimental and published data.     

Iron(II), cobalt(III), nickel(II) and copper(II) chelates of furan and thiophene azo-oximes

Summary Complexes of furan and thiophene azo-oximes with iron(II), cobalt(III), nickel(II) and copper(II) have been prepared and characterised. Iron(II), cobalt(III) and copper(II) complexes are diamagnetic in the solid state. The diamagnetism of the copper(II) chelates is suggestive of antiferromagnetic interaction between two copper centres.¹H n.m.r. spectral data suggest atrans-octahedral geometry for the tris-chelates of cobalt(III). Nickel(II) complexes are paramagnetic, in contrast to the diamagnetism of the analogous complexes of arylazooximes. The electronic spectra are suggestive of octahedral geometry for the iron(II), cobalt(III) and nickel(II) complexes, andD _4h -symmetry for copper(II). Infrared data indicate N-bonding of the oximino-group to the metal ions.     

Electronic structure of binuclear mixed valence copper azacryptates derived from integrated advanced EPR and DFT calculations

Binuclear, mixed valence copper complexes with a [Cu(+1)(.5), Cu(+1)(.5)] redox state and S = (1)/(2) can be stabilized with rigid azacryptand ligands. In this system the unpaired electron is delocalized equally over the two copper ions, and it is one of the very few synthetic models for the electron mediating Cu(A) site of nitrous oxide reductase and cytochrome c oxidase. The spatial and electronic structures of the copper complex in frozen solution were obtained from the magnetic interactions, namely the g-tensor and the (63,65)Cu, (14)N, (2)H, and (1)H hyperfine couplings, in combination with density functional theory (DFT) calculations. The magnetic interactions were determined from continuous wave (CW) electron paramagnetic resonance (EPR), pulsed electron nuclear double resonance (ENDOR), two-dimensional TRIPLE, and hyperfine sublevel correlation spectroscopy (HYSCORE) carried out at W-band or/and X-band frequencies. The DFT calculated g and Cu hyperfine values were in good agreement with the experimental values showing that the structure in solution is indeed close to that of the optimized structure. Then, the DFT calculated hyperfine parameters were used as guidelines and starting points in the simulations of the various experimental ENDOR spectra. A satisfactory agreement with the experimental results was obtained for the (14)N hyperfine and quadrupole interactions. For (1)H the DFT calculations gave good predictions for the hyperfine tensor orientations and signs, and they were also successful in reproducing trends in the magnitude of the various proton hyperfine couplings. These, in turn, were very useful for ENDOR signals assignments and served as constraints on the simulation parameters.     

Spacered dinuclear copper(<Emphasis Type="SmallCaps">II</Emphasis>) complexes with acyldihydrazones of aliphatic dicarboxylic acids and 2-hydroxy-5-nitroacetophenone

Dinuclear copper(II) complexes with acyldihydrazones of 2-hydroxy-5-nitroacetophenone (H₄L) of the composition Cu₂(Py)_xL·mEtOH were synthesized and characterized. In these complexes, the coordination polyhedra of the copper atoms are linked to each other by a polymethylene chain of different lengths, from one to five monomer units. The structure of the [Cu₂L·4Mrf] complex (Mrf is morpholine) based on acyldihydrazone of malonic acid was established by X-ray diffraction. The copper(II) atoms in this complex are [4+1]-coordinated and are spaced by 6.94 Å. At room temperature, the signal in the ESR spectra of solutions of the complexes based on acyldihydrazones of malonic, succinic, glutaric, and adipic acids has a seven-line hyperfine structure with the constant of (35.3–38.8)·10⁻⁴ cm⁻¹ (g = 2.109–2.112) due to exchange interactions between unpaired electrons and two equivalent copper nuclei. An increase in the length of the polymethylene chain to five monomer units hinders exchange interactions, and the ESR signal of the complex based on acyldihydrazone of pimelic acid has a four-line hyperfine structure with a _Cu = 72.7·10⁻⁴ cm⁻¹ typical of mononuclear copper(II) complexes. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 229–234, February, 2007.     

Multi‐frequency (S,X, Q and W‐band) EPR and ENDOR Study of Vanadium(IV) Incorporation in the Aluminium Metal–Organic Framework MIL‐53

Doping the well‐known metal–organic framework MIL‐53(Al) with vanadium(IV) ions leads to significant changes in the breathing behaviour and might have repercussions on the catalytic behaviour as well. To understand the properties of such a doped framework, it is necessary to determine where dopant ions are actually incorporated. Electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) are applied to reveal the nearest environment of the paramagnetic vanadium(IV) dopant ions. EPR spectra of as‐synthesised vanadium‐doped MIL‐53 are recorded at S‐, X‐, Q‐ and W‐band microwave frequencies. The EPR spectra suggest that at low dopant concentrations (1.0–2.6 mol %) the vanadium(IV) ions are well dispersed in the matrix. Varying the vanadium dopant concentration within this range or the dopant salt leads to the same dominant EPR component. In the ENDOR spectra, hyperfine (HF) interactions with ¹H, ²⁷Al and ⁵¹V nuclei are observed. The HF parameters extracted from simulations strongly suggest that the vanadium(IV) ions substitute Al in the framework.     

EPR spectra and structures of dinuclear copper(<Emphasis Type="SmallCaps">II</Emphasis>) complexes with acyldihydrazones of benzenedicarboxylic acids

Spacer-armed dinuclear copper(II) complexes with condensation products of isophthalic and terephthalic acid dihydrazides with salicylaldehyde and 2-hydroxyacetophenone were synthesized and studied by EPR and X-ray diffraction. The compositions and structures of most of the complexes were determined by elemental analysis, thermogravimetric analysis, and IR spectroscopy. The structure of the copper(II) complex with acyldihydrazone of salicylaldehyde and 1,3-benzenedicarboxylic acid (H₄L) with the composition [Cu₂L¹·2morph·MeOH] (morph is morpholine) was established by X-ray diffraction. The Cu^II atoms are spaced by 10.29 Å and are structurally nonequivalent. One copper cation has a square-planar coordination formed by donor atoms (2 N + O) of the doubly deprotonated acylhydrazine fragment and the N atom of the morpholine molecule. The second copper atom is additionally coordinated by a methanol molecule through the oxygen atom, so that this copper atom is in a tetragonal-pyramidal coordination with the oxygen atom in the axial position. The EPR spectra of liquid solutions of the complexes based on 1,4-benzenedicarboxylic acid acyldihydrazones and 1,3-benzenedicarboxylic acid bis(salicylidene)hydrazone at room temperature show a four-line hyperfine structure with the constant a _Cu = 54.4–67.0·10⁻⁴ cm⁻¹ (g = 2.105–2.147), which is indicative of the independent behavior of the paramagnetic centers. The EPR spectrum of a solution of the complex based on isophthalic acid and 2-hydroxyacetophenone shows the seven-line hyperfine structure corresponding to two equivalent copper nuclei (g = 2.11, a _Cu = 36.5·10⁻⁴ cm⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1898–1905, October, 2007.     

Copper Complexes Stabilized by Chitosans: Peculiarities of the Structure,Redox, and Catalytic Properties

Peculiarities of the structure and physicochemical properties of copper–chitosan complexes prepared by different methods were studied by IR, UV-visible, ESR spectroscopy, and electron microscopy. The catalytic activity of redox copper centers stabilized by the chitosan matrix in the reactions of oxidation of o- and p-dihydroxybenzenes in an aqueous medium was determined. Quantitative ESR measurements provide evidence for the localization of virtually all copper ions introduced in the initial heterogeneous chitosan samples with copper contents below 1.5 wt % in the form of isolated Cu²⁺ ions in square planar coordination. The chitosan matrix was shown to strongly bind copper ions under conditions of redox transformations in the catalytic tests or upon prolonged heating in boiling water. Reoxidation of the samples with H₂O₂ results in quantitative restoration of the initial ESR signal of Cu(II). Heterogenized copper–chitosan samples exhibited high activity and stability in the catalytic oxidation of dihydroxybenzenes into quinones, whereas the homogeneous system was characterized by irreversible poisoning due to formation of copper–hydroquinone complexes. Preparation of the binary composite system with a thin heterogeneous copper–chitosan film supported on a macroporous silica allows one to dramatically enhance the specific catalytic activity and the efficiency of the active component. Such an approach may turn out to be useful in the synthesis of supported chitosan catalyst with a low noble metal content.     

Physico‐Chemical Studies on Cu(II) Complexes of Acrylate Chelating Polymers

Abstract

Metal chelating polymers containing amide and carboxylic groups were prepared by gamma‐radiation polymerization of acrylic acid (AA) monomers in the presence of polyacrylamide (PAM). The resins obtained were loaded by copper ions and characterized by FT‐IR spectroscopy, electron spin resonance (ESR), thermogravimetric analysis (TGA), and differential scanning calorimetery (DSC). The IR spectra indicated a lower frequencies shift in the carbonyl bands due to copper ion chelation with carbonyl groups in the polymer resins. Also, the IR spectra reveal a splitting in the band at 3600–3200?cm^?1 that due to the coordination of the NH and OH groups with copper ions. The ESR spectrum was anisotropic with hyperfine structure having the following values 2.3808 and 2.07218 for g _∥ and g _⊥, respectively. These spectra for copper ions have square planar coordination with two nitrogen and two oxygen atoms. TGA and DSC studies show that radiation crosslinking and complexation with copper ion increase the thermal stability of PAM–AA resins. Meanwhile, resin complexes with copper ion showed a higher thermal stability than pure resin. The increase in thermal stability may be correlated with the metal ions coordination with NH and OH groups; this coordination prevents the splitting of ammonia and water molecules. Also, the metal ions providing a coordination crosslink between polymer chains could increase thermal stability.     

Direct Detection of 17O in [Gd(DOTA)]− by NMR Spectroscopy

The ¹⁷O NMR spectrum of the non‐coordinated carboxyl oxygen in the Gd^III–DOTA (DOTA=tetraazacyclododecanetetraacetic acid) complex has been observed experimentally. Its line width is essentially unaffected by paramagnetic relaxation due to gadolinium, and is only affected by the quadrupole pathway. The results are supported by the relevant parameters (hyperfine and quadrupole coupling constants) calculated by relativistic DFT methods. This finding opens up new avenues for investigating the structure and reactivity of paramagnetic Gd^III complexes used as contrast agents in magnetic resonance imaging.     

Exogenous bridging and nonbridging in Cu(II) complexes of Mannich base ligands: Synthesis and physical properties

Preparation of pentadentate ligands L¹, L², L³ and L⁴, where L¹ = 4-chloro-3-methyl-2[(prolin-1-yl)methyl]-6-[N-phenyl piperazin-1-yl)methyl]phenol, L² = 4-ethyl-2-[(prolin-1-yl)methyl]-6-[(N-phenyl piperazin-1-yl)methyl]phenol, L³ = 4-chloro-3-methyl-2-[(prolin-1-yl)methyl]-6-[N-methyl piperazin-1-yl]methyl phenol, L⁴ = 4-methoxy-2-[(prolin-1-yl)methyl]-6-[(N-phenyl piperazin-1-yl)methyl]phenol is described together with that of the corresponding Cu(II) complexes with various bridging motifs like OH, OAc and NO₂. The complexes are characterized by elemental analysis, electrochemical and electron paramagnetic spectral studies. Redox properties of the complexes in acetonitrile are highly quasireversible due to the chemical or/and stereochemical changes subsequent to electron transfer. The complexes show resolved copper hyperfine EPR at room temperature, indicating the presence of weak antiferromagnetic coupling between the copper atoms. Strengths of the antiferromagnetic interactions are in the order NO₂>OAc>OH.