X-band EPR Spectra Research Articles

Phase-dependent polymerization isomerism in the coordination complexes of a flexible bis(β-diketonato) ligand.

First prepared in the late 70s, the pro-ligand 1,3-bis(3,5-dioxo-1-hexyl)benzene (H2bdhb) contains two acetoacetyl terminations linked to a central 1,3-phenylene unit through dimethylene bridges. Since each termination can be either in diketonic or keto-enolic form, in organic solution it exists as a mixture of three spectroscopically resolvable tautomers. In the presence of pyridine, Co2+ and the bdhb2- anion form a crystalline dimeric compound with formula [Co2(bdhb)2(py)4] (2) and a Co⋯Co separation of more than 11 Å. Complex 2 contains two pseudo-octahedrally coordinated and non-interacting high-spin cobalt(II) ions (S = 3/2) displaying a large easy-plane anisotropy (D ∼ 70 cm-1), as consistently indicated by magnetic measurements, X-band EPR spectra, and complete active space self-consistent field/N-electron valence state perturbation theory (CASSCF/NEVPT2) calculations. At cryogenic temperatures (T < 7 K) and in an applied static magnetic field, the compound shows detectably slow magnetic relaxation, which occurs through direct and Raman mechanisms. Combined mass spectrometry, UV-Vis, and 1H/2H NMR data, including an isotopic labelling experiment and a determination of molecular weight by diffusion ordered spectroscopy (DOSY), show that 2 rearranges to monomeric high-spin [Co(bdhb)(py)x] species (x = 0, 1, or 2) in organic solution (CH2Cl2, THF) with concomitant partial dissociation of the py ligands. The X-band EPR spectra in a frozen CH2Cl2/toluene matrix concurrently suggest a significant alteration of the coordination environment upon dissolution. These observations are fairly well reproduced by density functional theory (DFT) and CASSCF/NEVPT2 calculations on the lowest Gibbs free energy conformers of each species, as provided by an extensive conformational search based on meta-dynamics simulations and semiempirical tight-binding methods. After the vanadyl analogue, compound 2 provides the second example of polymerization isomerism in the 1 : 1 adducts of bdhb2- with divalent metal ions.

Quantum spin coherence and electron spin distribution channels in vanadyl-containing lantern complexes.

We herein investigate the heterobimetallic lantern complexes [PtVO(SOCR)4] as charge neutral electronic qubits based on vanadyl complexes (S = 1/2) with nuclear spin-free donor atoms. The derivatives with R = Me (1) and Ph (2) give highly resolved X-band EPR spectra in frozen CH2Cl2/toluene solution, which evidence the usual hyperfine coupling with the 51V nucleus (I = 7/2) and an additional superhyperfine interaction with the I = 1/2 nucleus of the 195Pt isotope (natural abundance ca. 34%). DFT calculations ascribe the spin density delocalization on the Pt2+ ion to a combination of π and δ pathways, with the former representing the predominant channel. Spin relaxation measurements in frozen CD2Cl2/toluene-d8 solution between 90 and 10 K yield Tm values (1-6 μs in 1 and 2-11 μs in 2) which compare favorably with those of known vanadyl-based qubits in similar matrices. Coherent spin manipulations indeed prove possible at 70 K, as shown by the observation of Rabi oscillations in nutation experiments. The results indicate that the heavy Group 10 metal ion is not detrimental to the coherence properties of the vanadyl moiety and that Pt-VO lanterns can be used as robust spin-coherent building blocks in materials science and quantum technologies.

Copper-assisted anticancer activity of hydroxycinnamic acid terpyridine conjugates on triple-negative breast cancer.

The development of active therapeutic agents to treat highly metastatic cancer while minimizing damage to healthy cells is of utmost importance. Due to potential antioxidant properties, hydroxycinnamic acid derivatives (caffeic acid and p-coumaric acids) were found to inhibit highly metastatic breast cancer cell growth both in vitro and in vivo without much effect on normal cells. Especially due to the structure-activity relationships, ester and amide derivatives of hydroxycinnamic acids are reported to gain much higher radical scavenging ability than their naked hydroxycinnamic acid analogs like caffeic acid and p-coumaric acid. These results prompted us to design a set of ligands by incorporating an amide moiety on caffeic acid and p-coumaric acid to achieve the least toxicity towards healthy cell lines. Further, the Cu(II) complexes of amide-coupled caffeic acid and p-coumaric acid ligands have been explored for their therapeutic activity on triple-negative breast cancer and other cancer cells like colon, and prostate cancer. The Cu(II) complexes (4 & 5) were characterized by UV-Vis spectroscopy, FTIR, and X-band EPR spectroscopy. The trigonal bipyramidal geometry of complexes was confirmed by the X-band EPR spectra recorded in solution state at liquid N2 temperature. The purity of the complexes was determined by elemental analysis and HPLC traces. Initially, Calf thymus DNA (ct-DNA) binding studies with the complexes were explored. The results suggested the complexes (4 & 5) bind majorly through an intercalative mode of binding with ct-DNA, whereas no significant binding was observed for the bare organic ligands (2 & 3). The intercalation binding modes of 4 and 5 were further supported by UV-visible spectroscopy, ct-DNA melting point analysis, and CD spectroscopy. Moreover, these complexes showed better activity towards cisplatin-resistant TNBC cell lines (4T1, a TNBC cell line derived from the mammary gland tissue of a mouse). The combination of antioxidants and Cu(II) as the metal center made the complexes more cytotoxic toward cancer cell lines (4T1) (IC50 ∼ 3.5 ± 2.5 μM) and the least toxic toward healthy cells (L929) (IC50 ∼ 15 ± 5 μM). Finally, the mechanism of cell death was studied using JC-1 staining and a cell colony formation assay. These studies might help in designing safer anticancer drugs for treating more aggressive types of cancer.

Syntheses of two copper(II) coordination polymers from a morpholine-based tridentate Schiff base ligand: crystal structures and magnetic properties

Two new one-dimensional (1-D) end-to-end azido and thiocyanato bridged Cu(II) coordination polymers, [Cu(L)(µ-1,3-N3)] n (ClO4) n (1) and [Cu(L)(µ-1,3-NCS)] n (ClO4) n (2) with a morpholine-based tridentate N,N,N-donor Schiff base ligand (1-methyl-1H-imidazol-2-yl)-N-[2-(morpholin-4-yl)ethyl]methanimine, L) have been synthesized. Compounds 1 and 2 have been characterized by elemental analyses and spectroscopic techniques. The X-ray single crystal structures of 1 and 2 show the formation of regular zigzag chains with µ-1,3-azido and µ-1,3-thiocyanate bridges, respectively. Variable temperature (2–300 K) magnetic studies indicate that 1 shows a ferromagnetic intrachain exchange interaction with g = 2.135(2) and J = 2.13(2) cm−1, while in 2 this interaction is antiferromagnetic with g = 2.1412(3) and J = –0.277(2) cm−1. X-band EPR spectra of 1 and 2 in frozen (143 K) DMF solution corroborate our magnetic studies.

Mechanism of Radical Initiation and Transfer in Class Id Ribonucleotide Reductase Based on Density Functional Theory.

Class Id ribonucleotide reductase (RNR) is a newly discovered enzyme, which employs the dimanganese cofactor in the superoxidized state (MnIII/MnIV) as the radical initiator. The dimanganese cofactor of class Id RNR in the reduced state (inactive) is clearly based on the crystal structure of the Fj-β subunit. However, the state of the dimanganese cofactor of class Id RNR in the oxidized state (active) is not known. The X-band EPR spectra have shown that the activated Fj-β subunit exists in two distinct complexes, 1 and 2. In this work, quantum mechanical/molecular mechanical calculations were carried out to study class Id RNR. First, we have determined that complex 2 contains a MnIII-(μ-oxo)2-MnIV cluster, and complex 1 contains a MnIII-(μ-hydroxo/μ-oxo)-MnIV cluster. Then, based on the determined dimanganese cofactors, the mechanism of radical initiation and transfer in class Id RNR is revealed. The MnIII-(μ-oxo)2-MnIV cluster in complex 2 has not enough reduction potential to initiate radical transfer directly. Instead, it needs to be monoprotonated into MnIII-(μ-hydroxo/μ-oxo)-MnIV (complex 1) before the radical transfer. The protonation state of μ-oxo can be regulated by changing the protein microenvironment, which is induced by the protein aggregation and separation of β subunits with α subunits. The radical transfer between the cluster of MnIII-(μ-hydroxo/μ-oxo)-MnIV and Trp30 in the radical-transfer chain of the Fj-β subunit (MnIII/MnIV ↔ His100 ↔ Asp194 ↔ Trp30 ↔ Arg99) is a water-mediated tri-proton-coupled electron transfer, which transfers proton from the ε-amino group of Lys71 to the carboxyl group of Glu97 via the water molecule Wat551 and the bridging μ-hydroxo ligand through a three-step reaction. This newly discovered proton-coupled electron-transfer mechanism in class Id RNR is different from those reported in the known Ia-Ic RNRs. The ε-amino group of Lys71, which serves as a proton donor, plays an important role in the radical transfer.

Triazole-based C3-symmetric multivalent dendritic architecture as Cu(II) ion sensor

C3-symmetric triazole-based multivalent dendritic architecture having uniform aromatic core and branches has been employed to study its metal binding ability towards Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II) and Zn(II) ions. The dendritic architecture is found to exhibit a high affinity for Cu(II) ions and formed a solid complex with a blue shift in the dd-band of Cu(II) chloride. X-band EPR spectra at low temperature has supported a distorted tetrahedral geometry for the complex. The complex has displayed quasi-reversible redox waves in cyclic voltammetry (CV). DFT calculations have shown that Cu(II) has a high affinity for the dendritic structure, leading to high complexation energies of the order of -25 eV, showing that the complexation reactions are highly exothermic. The binding constant (K) for the Cu(II) complex has been determined using a fluorescence titration method.

The powder X-band electron paramagnetic resonance spectroscopy of septet pyridyl-2,4,6-trinitrene.

The first X-band EPR spectrum containing only non-overlapping signals of septet pyridyl-2,4,6-trinitrene and triplet pyridylnitrenes is reported. This spectrum was recorded after photolysis of 2,4,6-triazidopyridine in solid argon at 5 K. The zero-field splitting (ZFS) parameters of this trinitrene as well as of intermediate triplet mononitrenes and quintet dinitrenes formed at early stages of the photolysis were determined using the combination of modern computer line-shape spectral simulations and density functional theory (DFT) calculations. It was found that septet pyridyl-2,4,6-trinitrene has the record negative parameter DS = -0.1031 cm-1 among all known to date septet pyridyl-2,4,6-trinitrenes and may be of interest as a model multi-qubit spin system for investigations of quantum computation processing.

A New Reaction for Improved Calibration of EPR Rapid-Freeze Quench Times: Kinetics of Ethylene Diamine Tetraacetate (EDTA) Transfer from Calcium(II) to Copper(II).

The kinetics of the transfer of the chelate, ethylenediamine tetraacetate (EDTA), from Calcium(II) to Copper(II) in imidazole (Im) buffers near neutral pH, corresponding to the conversion, [Cu(II)Im4]2+→ [Cu(II)EDTA]2-, are characterized with stopped-flow absorption spectroscopy and implemented as a tool for calibrating the interval between mixing and freezing, the freeze-quench time (t Q ), of a rapid freeze-quench (RFQ) apparatus. The kinetics of this reaction are characterized by monitoring changes in UV-visible spectra (300 nm) due to changes in the charge-transfer band associated with the Cu2+ ions upon EDTA binding. Stopped-flow measurements show that the rates of conversion of the Cu2+ ions exhibit exponential kinetics on millisecond time scales at pH values less than 6.8. In parallel, we have developed a simple but precise method to quantitate the speciation of frozen solution mixtures of [Cu(II)(EDTA)]2- and tetraimidazole Cu(II) ([Cu(Im)4]2+) in X-band EPR spectra. The results are implemented in a simple high-precision 'recipe' for determining t Q . These procedures are more accurate and precise than the venerable reaction of aquometmyoglobin with azide for calibrating RFQ apparatus, with the benefit of avoiding high-concentrations of toxic azide solutions.

Novel Lanthanide(III) Porphyrin-Based Metal-Organic Frameworks: Structure, Gas Adsorption, and Magnetic Properties.

The present work focuses on the hydrothermal synthesis and properties of porous coordination polymers of metal–porphyrin framework (MPF) type, namely, {[Pr4(H2TPPS)3]·11H2O}n (UPJS-10), {[Eu/Sm(H2TPPS)]·H3O+·16H2O}n (UPJS-11), and {[Ce4(H2TPPS)3]·11H2O}n (UPJS-12) (H2TPPS = 4,4′,4″,4‴-(porphyrin-5,10,15,20-tetrayl)tetrakisbenzenesulfonate(4-)). The compounds were characterized using several analytical techniques: infrared spectroscopy, thermogravimetric measurements, elemental analysis, gas adsorption measurements, and single-crystal structure analysis (SXRD). The results of SXRD revealed a three-dimensional open porous framework containing crossing cavities propagating along all crystallographic axes. Coordination of H2TPPS4– ligands with Ln(III) ions leads to the formation of 1D polymeric chains propagating along the c crystallographic axis. Argon sorption measurements at −186 °C show that the activated MPFs have apparent BET surface areas of 260 m2 g–1 (UPJS-10) and 230 m2 g–1 (UPJS-12). Carbon dioxide adsorption isotherms at 0 °C show adsorption capacities up to 1 bar of 9.8 wt % for UPJS-10 and 8.6 wt % for UPJS-12. At a temperature of 20 °C, the respective CO2 adsorption capacities decreased to 6.95 and 5.99 wt %, respectively. The magnetic properties of UPJS-10 are characterized by the presence of a close-lying nonmagnetic ground singlet and excited doublet states in the electronic spectrum of Pr(III) ions. A much larger energy difference was suggested between the two lowest Kramers doublets of Ce(III) ions in UPJS-12. Finally, the analysis of X-band EPR spectra revealed the presence of radical spins, which were tentatively assigned to be originating from the porphyrin ligands.

Metallocomplexes, exhibiting catecholate binding mode for o-quinone, annulated with dithiete cycle

Novel catecholate complexes (ditCat)SbPh3 (1), (ditCat)Ni(2,2′-bpy) (2) and (3,6-tBu-ditCat)VCp2 (3), containing annulated dithiete cycle, were synthesized by reactions of bifunctional o-quinone, annulated with dithiete ring (ditQ) with SbPh3, (2,2′-bpy)Ni(CO)2 and VCp2, respectively. The structures of complexes 1 and 2 were confirmed by X-ray analysis. X-band EPR spectrum of complex 3 in THF has typical of d1-complexes of vanadium (IV) 8-lined pattern. Complex 1 in toluene reversibly binds molecular dioxygen with formation of spiroendoperoxide (3,6-tBu-ditCat)SbPh3∙O2∙C7H8 (4).

Di-2-pyridyl ketone-based ligands as evergreen “trees” in the “forest” of manganese chemistry: Mononuclear Mn(III) complexes from the use of MnF3

The syntheses, crystal and electronic structures are reported of three new mononuclear Mn(III) complexes with the formulae [Mn{(py)2C(OMe)(O)}2(MeOH)2](PF6) (1), [Mn{(py)2C(OMe)(O)}2](PF6) (2) and [Mn{(py)2C(OMe)(O)}2](ClO4) (3), where (py)2C(OMe)(O)− is the monoanion of the hemiacetal form of di-2-pyridyl ketone, (py)2CO. They were obtained from the reactions of (py)2CO with MnF3 in the presence of (NH4)PF6 (1, 2) or NaClO4 (3) in refluxing MeOH. The complex cations are centrosymmetric pseudo-octahedral with a rhombic arrangement of the donor atoms; the anion (py)2C(OMe)(O)− behaves as a N,O-bidentate chelating (η1:η1) ligand in 1 and as a N,O,N-tridentate chelating (η1:η1:η1) ligand in 2 and 3. The 3D architectures of 1–3 are characterized by hydrogen-bonded chains of the cations surrounded by columns of the counterions and are formed through an extensive network of cation–anion hydrogen bonds. The three compounds are the first structurally characterized Mn(III) complexes with (py)2CO-based ligation. Variable-temperature magnetic susceptibility, magnetization isotherms and X-band EPR spectra of 1 and 2 reveal high-spin MnIII atoms (S = 2), anisotropy effects and a positive value of ~ +4 cm−1 of the zero--field splitting parameter D; moreover, EPR spectroscopy indicates the contribution of rhombic and/or higher order terms.

Mononuclear copper(II) complexes with (Z)-N′-{(2-hydroxynapthalen-1-yl}methylene)acetohydrazide: X-ray single-crystal structures, Hirshfeld analysis, X-band epr spectra, DFT calculations and SOD mimetic activity

The 1:1 reaction of (Z)-Ń-{(2-hydroxynapthalen-1-yl}methylene)acetohydrazide (HL) and copper(II) salt in methanol yields a series of new mononuclear complexes, [Cu(HL)(OH2)]ClO41, [Cu(HL)(OH2]ClO42, [Cu(HL)(OH2)]NO33 and [Cu(L)(Cl)] 4. The room temperature magnetic moments of the complexes 1–4 are 1.79, 1.78, 1.80 and 1.76B.M. respectively. The absorption bands emerging in the UV region (337–341 nm) are characteristic of the HL ligand. In these complexes, the LMCT bands were observed in the range of 503–506 nm and the d-d band at 617–623 nm. The molecular structures of all four complexes have been solved using single-crystal X-ray analysis. The X-ray analysis shows the formation of supramolecular structures through various non-covalent interactions viz., CH∙∙∙π and π···π intermolecular interactions. The detailed Hirshfeld surface and fingerprint plots yielded a comparative picture of the mode of non-covalent interactions. The DFT calculations were performed and the results are in good agreement with experimental results. X-band epr spectral measurements have been carried out to authenticate the paramagnetic behavior of all complexes. The stability of the copper(II) centre was examined using cyclic and differential pulse voltammetry. The IC50 and SOD activity values for all complexes reveal that they are good models among the best values reported for low molecular weight mononuclear copper(II) complexes.

Non-covalent interactions governing the supramolecular assembly of copper(II) complexes with hydrazone-type ligand: Experimental and quantum chemical study

A series of two new mono- and one binuclear µ-nitrato bridged copper(II) complexes [Cu(L)(HL)]ClO4 (1), [Cu(HL)(NO3)(H2O)]2NO3∙H2O (2) and [Cu2(L2)(µ-NO3)2] (3), with an unsymmetrical NNO donor Schiff base (HL) have been synthesized and characterized by elemental analysis, FTIR, CV, UV–vis and EPR spectroscopy. Their molecular structures were also determined by single crystal X-ray crystallography. In the binuclear complex 3, the Cu⋯ Cu distance is 3.494 Å. In 1, 2 and 3, the Cu(II) centers have distorted square pyramidal geometry (τ5 = 0.05–0.17). Evidence of weak π⋯π stacking intermolecular interactions along with other non-covalent interactions (hydrogen bonding) was observed by analyzing the respective crystal structures of the complexes. Thus, these hydrogen bonds, π⋯π stacking interactions and other weak intermolecular interactions establish in the form of supramolecular architectures a crystalline “network” environment. The non-covalent interactions were also investigated by employing Hirshfeld Analysis. The room temperature magnetic moments of the mononuclear complexes are less than the spin only values which are indicative of small interactions. Also, significant magnetic interactions were not exhibited by binuclear copper(II) complex 3 in the variable temperature magnetic measurements. The X-band EPR spectra of all three complexes exhibit copper(II) hyperfine structures as well as zero-field splitting which are appropriate for the triplet states of dimers. In complexes 1 and 2, the presence of pseudo dipolar interactions is proposed. Quantum chemical calculations (DFT) were carried out on complexes 1–3 to explore the electronic and spectral properties of these newly synthesized complexes. These complexes show significant antiproliferative and SOD activity. The SOD activity measured in terms of kMcCF is in the range 4.94–12.31 (mol L)−1s−1)× 104.

CuII Complexes and Coordination Polymers with Pyridine or Pyrazine Amides and Amino Benzamides—Structures and EPR Patterns

Isonicotine amide, picoline amide, pyrazine 2-amide, 2- and 4-amino benzamides and various CuII salts were used to target CuII complexes of these ligands alongside with 1D and 2D coordination polymers. Under the criterion of obtaining crystalline and single phased materials a number of new compounds were reliably reproduced. Remarkably, for some of these compounds the ideal Cu:ligand ratio of the starting materials turned out to be very different from Cu:ligand ratio in the products. Crystal and molecular structures from single-crystal XRD were obtained for all new compounds; phase purity was checked using powder XRD. We observed exclusively the Oamide and not the NH2amide function binding to CuII. In most of the cases; this occurred in chelates with the second pyridine, pyrazine or aminophenyl N function. µ-O,N ditopic bridging was frequently observed for the N = pyridine, pyrazine or aminophenyl functions, but not exclusively. The geometry around CuII in these compounds was very often axially elongated octahedral or square pyramidal. X-band EPR spectra of powder samples revealed various spectral symmetry patterns ranging from axial over rhombic to inverse axial. Although the EPR spectra cannot be unequivocally correlated to the observed geometry of CuII in the solid state structures, the EPR patterns can help to support assumed structures as shown for the compound [Cu(Ina)2Br2] (Ina = isonicotine amide). As UV-vis absorption spectroscopy and magnetic measurement in the solid can also be roughly correlated to the surrounding of CuII, we suggest the combination of EPR, UV-vis spectroscopy and magnetic measurements to elucidate possible structures of CuII compounds with such ligands.

Influence of V2O5 on physical and spectral (optical, EPR & FTIR) studies of SrO-TeO2-TiO2-B2O3 glasses

Physical and spectral studies of vanadium doped strontium titanium boro-tellurite glasses have investigated by XRD, SEM, optical, EPR & FTIR spectroscopies. The optical absorption spectra demonstrate two transitions related to 2B1g→2B2g and 2B1g→2E2g energies, which are characteristic features of VO2+ ions. From the optical edge and Tauc relation, the Urbach energies and optical band gap values have calculated. The X-band EPR spectra (experimental and simulated) confirms the non-linear variation of the ligand field around the V4+ ions. From perturbation theory, the theoretical Hamiltonian parameters have been calculated and compared with experimental EPR results. FTIR spectroscopy is evident that the role of SrO on the local structure of strontium titanium boro-tellurite glasses containing vanadium ions.

One-dimensional coordination polymers constructed from copper(II) ions and chromato bridges: Synthesis, crystal structures and thermal analysis

Three 1D coordination polymers, 1∞[Cu(NH3)4CrO4] (1) and 1∞[CuL(H2O)(CrO4)](H2O)(C2H5OH)0.5 with L = bipy (2) and phen (3), have been assembled using chromate(VI) ions as spacers (μ-ĸO:ĸO’ bridging mode). Their crystal structures have been solved. The copper ions in compound 1, whose structure consists of linear chains, show an elongated octahedral geometry, with four ammonia molecules, forming the equatorial plane, and two oxygen atoms arising from two chromato bridges into the apical positions. The ammonia ligands are involved in intramolecular hydrogen bonds with two chromate oxygen atoms, and intermolecular hydrogen bonds with two chromato oxygen atoms from another chain. The crystal structures of compounds 2 and 3 consist of helicoidal chains and solvent molecules. The copper(II) ions in 2 and 3 are five-coordinated (square-pyramidal geometry), with the apical positions occupied by the oxygen atoms from the chromato bridges. Helicoidal chains with opposite chirality are interconnected by π-π stacking interactions established between the organic ligands, resulting in supramolecular layers. The X-band and Q-band EPR spectra of compounds 1–3 are discussed. The total thermal decomposition of these complexes leads to a mixture of CuCr2O4 and CuO, for 1, and of CuCrO2, CuO and Cr2O3, for 2 and 3.

A Room-Temperature Stable Y(II) Aryloxide: Using Steric Saturation to Kinetically Stabilize Y(II) Complexes.

The utility of the bulky aryloxide ligands 2,6-Ad2-4-Me-C6H2O- (Ad,Ad,MeArO-) and 2,6-Ad2-4-t-Bu-C6H2O- (Ad,Ad,t-BuArO-; Ad = 1-adamantyl) for stabilizing the Y(II) ion is reported and compared with the results with 2,6-t-Bu2-4-Me-C6H2O- (Ar'O-). In contrast to the reduction product obtained from reducing Y(OAr')3 with potassium graphite, which is only stable in solution for 60 s at room temperature, KC8 reduction of Y(OArAd,Ad,t-Bu)3 in THF in the presence of 2.2.2-cryptand (crypt) produces the room-temperature stable, crystallographically characterizable Y(II) aryloxide [K(crypt)][Y(OArAd,Ad,t-Bu)3]. The X-band EPR spectrum at 77 K shows an axial pattern with resonances centered at g⊥ = 1.97 and g∥ = 2.00 and hyperfine coupling constants of A⊥ = 156.5 G and A∥ = 147.8 G and at room temperature shows an isotropic pattern with giso = 1.98 and Aiso = 153.3 G, which is consistent with an S = 1/2 spin system with nuclear spin I = 1/2 for the 89Y isotope (100% natural abundance).

Proton-Coupled Oxidation of Aldimines and Stabilization of H-Bonded Phenoxyl Radical-Phenol Skeletons.

Stable phenoxyl radicals H-bonded to phenols were successfully isolated. The effect of the intermolecular H-bonding to the concerted proton coupled electron transfer (CPET) reactions of the aldimines and the stability and spin distribution of the H-bonded phenoxyls are reported. Salts of iminium-phenol derivatives as cations and the corresponding imine-phenolato derivatives coordinated to zinc(II) as anions, [ZnII(ArRO)Cl2]-[ArROH2]+, were isolated, where ArOH are aldimine derivatives. Notably, [ZnII(ArRO)Cl2]-[ArROH2]+ salts undergo CPET reactions in air affording phenoxyl analogues, [ZnII(ArOH)Cl2].ArO●·CH3CN. [ZnII(ArRO)Cl2]-[ArROH2]+ salts incorporate intermolecular iminium-phenolato, [(Zn)Ar-O--+HN═CH-] H-bonds, while [ZnII(ArOH)Cl2].ArO●·CH3CN moieties contain intermolecular phenoxyl-phenol, [Ar-O-HO-Ar]●, H-bonds. The phenoxyls are presented in two forms, [(Zn)Ar-O●---HO-Ar (zinc phenoxyl) ↔ (Zn)Ar-OH---●O-Ar (free phenoxyl)]. In crystals, the spin density scatters on both phenolic fragments corresponding to a delocalized state, while in solution the latter form has been calculated as a ground electronic state. The X-band EPR spectra of crystals, solutions and frozen glasses were analyzed. The powder spectra at g = 2.0030 ± 0.0005 and the frozen glass spectra at g = 2.0075 ± 0.0003 follow the hyperfine patterns due to 14N (I = 1) nuclei. In fluid solutions, the g values of the hyperfine signals due to 14N and 1H nuclei are 2.0078 ± 0.0001. 1-3 exhibit absorption bands at 350-390 nm due to π → π* intraligand charge transfer (ILCT) transitions, while the radical species, in addition to π → π transitions at 405-440 nm, display phenol to phenoxyl intervalence charge transfer (IVCT) transitions at 600-650 nm. The cyclic voltammograms (CVs) of 1-3 depend on the scan rates; at lower scan rates (100-400 mV/s) the CPET reactions occur at -0.92 to -0.96 V versus Fc+/Fc couple, whereas at higher scan rates (1000-2400 mV/s), the oxidation occurs by the electron transfer (ET) path at 0.05-0.12 V. Thus, a potential shift of ∼1.0 V is recorded due to CPET reactions facilitated by H-bonding.

Experimental and theoretical studies on isatin-Schiff bases and their copper(II)-complexes: Syntheses, spectroscopy, tautomerism, redox potentials, EPR, PXRD and DFT/TDDFT

Abstract The isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) react with the copper(II) acetate or nitrate to give the bis[3-(o/p-tolylimino)indole-2-olato-κ2N,O]copper(II) (1 and 2) via lactam (L)- to enol (E)-tautomerism in solution. IR spectra show solely the L-tautomer at solid state, while both tautomers in solution. 1H NMR results indicate a dynamic tautomerization equilibria between the L- and E-tautomers in solution with varying ratios of ca. 83/17, 60/40 and 54/46 (L/E) within 0.5, 5 and 24 h, respectively. Cyclic voltammograms demonstrate two quasi reversible one electron charge transfer processes for the Cu(II)/Cu(I) and Cu(I)/Cu(0) couples in acetonitrile, respectively. PXRD patterns at 5–50° (2θ) correspond well to the crystalline nature with various degrees of crystallinity for the ligands and complexes. X-band EPR spectrum shows an intense broad band at relatively high field region with isotropic nature. Magnetic moment values (μeff. = 1.56–1.63 μB) indicate paramagnetic nature of the copper(II)-complexes with one unpaired electron in methanol. Optimized structures by DFT show the L-form relatively stable than the E-form by 19.76 kcal/mol, while compounds HL2 and 2 are slightly stable than HL1 and 1 by 0.35 and 0.79 kcal/mol, respectively. Electronic spectra by TDDFT strongly support the experimental results in methanol.

On the Optimum Aging Time: Magnetic Resonance Study of Asphaltene Adsorption Dynamics in Sandstone Rock

Wettability is a key factor defining ultimate hydrocarbon recovery. Extensive experimentation is required to replicate the wetting state of reservoir rocks. This involves a rock sample wettability restoration procedure, including an aging step and a concept of an optimum aging time, in which asphaltene chemistry plays a major role. There are numerous reports on significance of various crude oil components and elements of asphaltene structure to their tendency to interact with the solid phase, though subject evidence is contradictory. We investigate a possible relationship between the composition of oil, kinetics of an aging process, and a change of sandstone rock wettability. Wettability state of the cores was monitored using low-field NMR relaxometry. The composition and key components of accumulated deposits were established by matching 1H solution-state NMR and X-band EPR spectra of deposits to the spectra of SARA (saturates-aromatics-resins-asphaltenes) fractions of aging fluids. We determined adsorpt...