Zero-field Splitting Effects Research Articles

Synthesis, Crystal Structure and Magnetic Properties of the Helical-Chain Compounds [M2(O2CC12H8CO2)2(H2O)8] [M = Cobalt(II), Nickel(II)

New 1D complexes are obtained from the reaction of 1,1′-biphenyl-2,2′-dicarboxylic acid (diphenic acid) with KOH and [Co(NO3)2·6H2O] or [Ni(NO3)2·6H2O] under hydrothermal conditions. Compounds 1 and 2 are isostructural and their structure consists of stacked helical chains of octahedral metal(II) ions. Magnetic measurements show that the CoII atoms in compound 1 are quasi-isolated. In compound 2, the NiII chains exhibit a ferromagnetic behaviour. From a simple model derived from the scaling theory of phase transition, the exchange coupling between neighbouring NiII atoms is evaluated at J = +1.06 K. A zero-field splitting effect is observed below 16 K.

Skirting the oxo-wall: characterization and catalytic reactivity of binuclear Co 2+/3+ 1,2-bis(2-hydroxybenzamido)benzene complexes with comparison to their isostructural Fe 2+/3+ analogs. Implications of d-electron count on oxygen atom transfer catalysis

The reaction of Co 2+( N-MeIm) 2(Cl) 2 ( 1), with Li 2H 2Hbab (H 4Hbab=1,2-bis(2-hydroxybenzamido)benzene) affords the binuclear complex [Co 2 2+ (H 2Hbab) 2( N-MeIm) 2] ( 2). Single crystal X-ray crystallography shows that the two Co 2+ centers in 2 are related by a crystallographically imposed center of symmetry with the coordination sphere around each metal center composed of amide oxygens, terminal and bridging phenolate oxygen atoms and a single nitrogen from N-methylimidazole. 2 is isostructural to the Fe 2+ complex, [Fe 2(H 2Hbab) 2( N-MeIm) 2] ( 5), previously reported from our laboratory (A. Stassinopoulos, G. Schulte, G.C. Papaefthymiou, J.P. Caradonna, J. Am. Chem. Soc. 113 (1991) 8686–8697). Stoichiometric iodometric oxidation of 2 yields the mixed valence complex [Co 2+Co 3+(H 2Hbab) 2( N-MeIm) 2] + ( 3), and the oxidized complex [Co 2 3+ (H 2Hbab) 2( N-MeIm) 2] 2+ ( 4). The UV–Vis spectrum of 2 shows ligand field transitions at 600 nm ( ε M=160) and 540 nm ( ε M=120) and a phenolate-to-Co 2+ ligand-to-metal charge-transfer (LMCT) band at 327 nm ( ε M=23 600). Oxidation of the dicobalt core results in a bathchromic shift of the LMCT band ( 3: 305 nm ( ε M=17 400), 334 nm (sh, ε M=14 500); 4: 295 nm ( ε M=21 000), 357 nm ( ε M=15 000)). EPR spectra of 2 and 3 at 4 K show broad resonances from 500 to 4500 Gauss indicative of the presence of strong zero-field splitting effects, while 4 is EPR silent. Solution magnetic susceptibility measurements for 2 ( S 1= S 2=3/2; μ B=6.90) are consistent with a weakly interacting high-spin Co 2+ dimer. Analogous measurements for 3 ( S 1=3/2, S 2=0; μ B=4.78) and 4 ( S 1= S 2=0; μ B=0.25) indicate the presence of a single high-spin Co 2+ for 3 and a diamagnetic core for 4, consistent with the latter’s 1H NMR spectrum. Complexes 2– 4 exhibit the ability to catalytically oxidize a variety of organic substrates (olefins, sulfides) using iodosylbenzene (OIPh) as oxygen atom donor. Reactions with olefin substrates (norbornene, cyclooctene, styrene) yield solely epoxide products while cyclohexene afforded 85% epoxide with small amounts (15%) of allylic oxidation products. Oxygen atom transfer to olefins occurs with high but not exclusive retention of stereochemistry. The mechanistic implications of the significantly different product distributions obtained with the Co 2+ (d 7) dimer, 2, versus the isostructural Fe 2+ (d 6) dimer, 5 (A. Stassinopoulos, J.P. Caradonna, J. Am. Chem. Soc. 112 (1990) 7071–7073), are presented. These data strongly infer the operation of two independent mechanisms for the Co 2+ and Fe 2+ catalyzed reactions.

Multi-frequency EPR determination of zero field splitting of high spin species in liquids: Gd(III) chelates in water

Multi-frequency EPR spectroscopy at 9.5, 35, 94, and 249 GHz has been employed to investigate the zero field splitting (ZFS) of high spin ions in liquids. In particular, experiments are reported on aqueous solutions of DTPA and DOTA chelates of Gd(III), and on the uncomplexed ion, which are relevant to the effectiveness of paramagnetic contrast agents for magnetic resonance imaging (MRI). The field dependence of the centroid of the resonance line, characterized by an effective g factor, geff, has been analysed in order to determine δ1, the trace of the square of the ZFS matrix. Analysis of the variation in transverse electron spin relaxation (T 2e) with experimental frequency provides yet another route to measure δ2 from EPR data. This analysis also gives δv, a correlation time describing the time-dependent ZFS effect. The ZFS parameters so obtained agree well with results obtained by the analysis of proton nuclear magnetic relaxation dispersion. At 94 GHz, partially resolved spectra from chelated and unchelated Gd(III) were observed. The shifts in resonance field for Gd(III) in these two compounds are due primarily to differences in the magnitude of ZFS. The spectral resolution as a function of frequency exhibits a maximum in the range of our experiments; the resolution disappeared at either higher or lower resonance frequency. Study of ZFS by EPR at multiple high fields offers a new and sensitive route to probe water interactions and chelate dynamics in biologically relevant systems having high spin ions.

A new one-dimensional thiocyanato-bridged bimetallic compound [Cu(en)2Mn(NCS)4(H2O)2]n. Synthesis, crystal structure, and magnetic properties

A bimetallic compound [Cu(en)2Mn(NCS)4(H2O)2]n (en = ethylenediamine) has been prepared and its structure determined. It crystallizes in space group , with a = 7.432(1), b = 7.492(1), c = 9.751(2) Å, α = 91.00(3)°, ß = 99.14(3)°, γ = 112.51(3)°, and Z = 1. The structure consists of one-dimensional chains in which alternating Mn(II) and Cu(II) ions are bridged by NCS ligands. The coordination environment is octahedral for Mn(II) and elongated octahedral for Cu(II). From magnetic susceptibility measurements the compound was found to exhibit a ground state of low-spin multiplicity. Comparison between theory and experimental has been made using 1-D model and taking into account the zero-field splitting effect. The decrease in chimT observed at low temperature for the compound is attributed to a dominant zero-field splitting of the single Mn(II) ions with a weak antiferromagnetic interaction superimposed between the Mn(II) ions via the NCS-Cu-SCN pathway within each chain. Key words: crystal structure, copper(II) complexes, manganese(II) complexes, thiocyanate bridges, bimetallic complexes.

Influence of the counterion on the coordinating properties of (2,2′-bipyridyl)bis(oxalato)chromate(III) anion: crystal structures and magnetic properties of AsPh4[Cr(bipy)(ox)2]·H2O and [NaCr(bipy)(ox)2(H2O)]·2H2O

Two new chromium(III)-containing complexes of formula AsPh4[Cr(bipy)(ox)2]·H2O 1 and [NaCr(bipy)(ox)2(H2O)]· 2H2O 2 (bipy = 2,2′-bipyridine and ox = oxalate dianion) have been synthesized and characterized by single-crystal X-ray diffraction. The structure of 1 consists of discrete [Cr(bipy)(ox)2]– mononuclear anions, tetraphenylarsonium cations and uncoordinated water molecules. The structure of 2 reveals a novel two-dimensional framework which is made up of oxalato-bridged bimetallic CrIII–NaI helical chains which are interconnected through centrosymmetric Na2O2 units. Within the chain, a regular alternation of the metal ions is observed, the oxalate group acting as a bis(chelating) ligand. In addition to this coordination mode, two oxalates each act as monodentate ligands towards a sodium atom of a neighbouring chain thus leading to a sheetlike polymeric structure. The chromium environment is distorted octahedral in both complexes: two nitrogen atoms from a bidentate bipy ligand and four oxygen atoms from either two chelating (1) or two bis(chelating) (2) oxalate groups build the coordination polyhedron around the chromium atom. The Cr–N bond lengths [values in the ranges 2.077(3)–2.057(3) (1) and 2.067(4)–2.058(4) A (2)] are somewhat longer than the Cr–O ones [values in the ranges 1.960(2)–1.946(2) (1) and 1.968(3)–1.949(3) A (2)]. The sodium atom in 2 is also six-coordinated: a coordinated water molecule [2.371(5) and 2.325(4) A for Na(1)–O(17) and Na(2)–O(18), respectively] and five oxygens from three oxalate groups [values of the Na–O (ox) bonds in the ranges 2.511(4)–2.331(4) and 2.481(4)–2.364(4) A around Na(1) and Na(2), respectively] build a distorted octahedral NaO6 environment. The intralayer chromium–sodium and sodium–sodium distances through bridging oxalate in 2 vary in the ranges 5.657(4)–5.579(2) and 3.534(3)–3.497(4) A, respectively. Variable-temperature magnetic susceptibility measurements of 1 and 2 reveal the occurrence of very weak antiferromagnetic interactions together with zero-field splitting effects in both compounds. The use of the [Cr(bipy)(ox)2]– unit as a ligand towards different univalent and divalent metal ions aimed at designing new heterobimetallic systems is analysed and discussed in the light of available structural data.

Effects of non-collinear fine and hyperfine structure on the zero-field electron magnetic resonance of [Cu2(pmdien)2(N3)2][BPh4

The zero-field electron magnetic spectrum of [Cu2(pmdien)2(N3)2][BPh4]2 has been analysed in terms of an electron spin triplet interacting with two equivalent nuclear spin quartets. The effect of non-collinear zero-field splitting and hyperfine tensors on the transition frequencies is emphasized for this particular spin system. The Euler rotation angles α and γ have little effect on the spectrum whereas β ≠ 0 leads to an effective reduction in Az for the D – E and 2E branches of the triplet spectrum but has no effect on the D + E branch. Least-squares analysis produced spin-Hamiltonian parameters |D| = 2546.3 MHz, |E| = 450.1 MHz, |Az| = 261.5 MHz and the angle β = 23.9°.

Metal pyrazolate polymers. Part 4. Synthesis, characterization, and magnetic properties of [Co(pz*)2]x complexes (where pz* = pyrazolate, 3-methylpyrazolate, and 4-substituted 3,5-dimethylpyrazolate)

The title compounds were prepared and characterized by X-ray powder diffraction, scanning electron microscopy, vibrational and electronic spectroscopy, and magnetic susceptibility measurements. Indirect evidence supports polymeric one-dimensional chain structures involving double pyrazolate bridges linking pseudo-tetrahedrally coordinated metal centres in all six compounds. The spectroscopic studies indicate a greater degree of distortion of the CoN4 chromophore from regular stereochemistry in the 4-substituted 3,5-dimethylpyrazolate complexes. The magnetic studies reveal anisotropy in the susceptibilities and the presence of significant antiferromagnetic exchange. Magnetic modelling employing both Ising and Heisenberg models suggests the exchange behaviour is probably of the Heisenberg type with zero-field splitting effects producing Ising-like exchange at low temperatures. The derived J values (~ −2 to −6 cm−1) have been used to qualitatively rank the magnitude of exchange as follows: 4-Hpz ≈ 3-Mepz [Formula: see text] 4-Medmpz ≈ 4-Cldmpz ≈ 4-Brdmpz ≈ 4-Hdmpz.

HIP technology — prospects for the 1990s

Two neutral nickel(II) coordination polymers [Ni(en)(dca)2]n (1) and [Ni(dmen)(dca)2]n (2) (en = ethylenediamine; dmen = N,N-dimethylethylenediamine; dca = dicyanamide) have been synthesized and X-ray crystallographically characterized. Each nickel(II) center in 1/2 adopts a distorted octahedral coordination environment with a NiN6 chromophore ligated by two amine N atoms of the bidentate amine (en/dmen) and four nitrile N atoms of μ1,5 bridged dca. The metal(II) centers are connected with each other through single μ1,5 M–NCNCN–M bridges, resulting in a 2D layer structure with a (4,4) topology in 1 and a 3D network of topology (6,6) in 2. Multiple lateral N–H···N and C–H···N hydrogen bondings promote dimensionality. The magnetic susceptibility results of 1 and 2 at very low temperature support the zero-field splitting effect of the nickel(II) ions.

Electron spin resonance studies of "FeO6" tris chelate complexes: models for the effects of zero-field splitting in distorted S = 5/2 spin systems

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectron spin resonance studies of "FeO6" tris chelate complexes: models for the effects of zero-field splitting in distorted S = 5/2 spin systemsDavid Collison and Anne K. PowellCite this: Inorg. Chem. 1990, 29, 23, 4735–4746Publication Date (Print):November 1, 1990Publication History Published online1 May 2002Published inissue 1 November 1990https://doi.org/10.1021/ic00348a030RIGHTS & PERMISSIONSArticle Views341Altmetric-Citations45LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

The rhombic effect of zero-field splitting on the nuclear relaxation times. Quenching of the electron spin angular momentum

The effect of zero-field splitting (ZFS) on the nuclear relaxation rates, T 1M −1 and T 2M −1, is investigated under the condition that the static ZFS is larger than the energy uncertainty of the ZFS levels. It is shown that the rhombic term E of ZFS drastically reduces the nuclear relaxation rates of a paramagnetic molecule with an integer electron spin quantum number. In this case the nuclear relaxation time is independent of the external magnetic field strength. This signifies quenching of an electron spin angular momentum, which was predicted by N. Bloembergen and L. O. Morgan ( J. Chem. Phys. 34, 842, 1961). It is suggested that the true value for the correlation time of lanthanide ion complexes could be much longer than the accepted value, 10 −13 s.

The electron-nucleus dipolar coupling in slow rotating systems. 4. The effect of zero-field splitting and hyperfine coupling when [formula omitted] and [formula omitted

Solvent proton relaxation rates as a function of magnetic field have been calculated for slow rotating high-spin manganese(II)-containing systems. The static zero-field splitting of the S = 5 2 manifold has been included in the spin Hamiltonian and found to be quite meaningful with respect to understanding the experimental data. The effect of the hyperfine coupling between the manganese nuclear spin and the electron spin has been evaluated and found to be minor. The available experimental data on manganese(II) concanavalin A have been used to test the influence of the above spin-Hamiltonian terms on the parameters describing the electron-nucleus coupling.

The electron-nucleus dipolar coupling. The effect of zero-field splitting of an [formula omitted] manifold

An equation is derived to describe the water 1H T 1 − NMR values when water is interacting with high-spin cobalt(II) with S = 3 2 under slow rotation conditions. In particular the effect of the static zero-field splitting of the S manifold is taken into consideration. Comparison with the experimental data on the pseudotetrahedral cobalt(II)-substituted human carbonic anhydrase I is highly encouraging.

Spin–spin interactions in polynuclear nickel(II) complexes: susceptibility and low-temperature magnetisation studies of tris[bis(pentane-2,4-dionato)nickel(II)

The magnetic susceptibility and magnetisation of the trinuclear complex [Ni3(pd)6](pd = pentane-2,4-dionate) have been reinvestigated in the range 1.8–300 K. Detailed magnetisation studies have been performed to low temperatures to evaluate the properties of the septet (S= 3) ground state of the molecule. The results have been analysed in terms of the Heisenberg–Dirac–Van Vleck isotropic exchange Hamiltonian including contributions from single-ion zero-field splittings. Ferromagnetic coupling between adjacent ions is confirmed, although the magnitude of both this interaction and the antiferromagnetic interaction between the terminal ions is found to be half that previously reported. This diminution is due to the effect of zero-field splitting on the low-temperature magnetic properties, the ground-state septet (S= 3) having a splitting of –1.3 cm–1.

Electron and nuclear spin relaxation in paramagnetic transition-metal complexes with S = 1. The effects of a non-zero average zero-field splitting on the spin energy levels

The effects of a non-zero average zero-field splitting on electron spin relaxation in paramagnetic (S = 1) complexes is treated theoretically. The spin-lattice interaction is postulated to be a simple scalar P(t). S process with correlation time Ti. This process is assumed not to modulate the zero-field splitting which, however, is modulated by the molecular tumbling. The frequency dependence of the nuclear relaxation time (Tl) now depends on the magnitude of the zero-field splitting constant (D), and, for large values of D, the value of T1 is independent of the applied field strength.

Magnetic Circular Dichroism of Cytochrome c

We have measured the magnetic circular dichroism of ferrocytochrome c from 350 to 1000 nm and the MCD of ferricytochrome c from 350 to 750 nm. We present expressions for the shapes of MCD and absorption spectra in the presence of a magnetic field of randomly oriented molecules for the general case in which the absorption band is split in zero field, and the two components of the split band have different intensities. After correcting for the effects of unresolved zero-field splitting, we find that the ``angular momentum'' of the Q00 band of Fe(II) cyt c is 7.9 μB. We show that the 23-fold decrease in the MCD of the Q00 band upon oxidation of Fe(II) to Fe(III) is due to about a fivefold increased bandwidth while the oscillator strength and angular momentum remain essentially unchanged. The increase in bandwidth may reflect greater mixing of the porphyrin orbitals with the d levels of the iron ion in the oxidized state. The MCD of Fe(II) cyt c in the near infrared (600–1000 nm) can be decomposed using essentially the same components previously used to decompose the CD if some components are presumed degenerate. The angular momenta of the degenerate components are in the range expected for d→ d transitions. The CD and MCD of the 695 nm band of Fe(III) cyt c are both relatively weak and suggest, respectively, that the transition is electric dipole allowed and nondegenerate.

Dielectric Properties of Interacting Ion-Impurity Systems in KCl

The theory of the dielectric properties of assemblies of interacting impurity ions in solids at low temperatures is discussed, with particular reference to KCl:${\mathrm{OH}}^{\ensuremath{-}}$, KCl:${\mathrm{Li}}^{+}$, and KCl:${\mathrm{Li}}^{+}$. A quantum-mechanical approach in which the effects of zero-field splitting in the motional spectrum of the ions are included is maintained throughout. An analytical study of the properties of an especially simple model with features relevant to the problem of interacting impurity dipoles is presented. The dielectric virial-expansion formalism is introduced, and numerical results for the theoretical second dielectric virial coefficient $B(T)$ for each of the three species are given. The question of convergence of the virial expansion is considered, and it is shown that convergence fails for the specific case of KCl:${\mathrm{OH}}^{\ensuremath{-}}$ at impurity densities so far studied. An alternative theory for this system is presented which gives numerical predictions in satisfactory agreement with experiment. The basic notion of this theory is that neighboring impurity ions are locked into coherent ferroelectric pairs. At sufficiently low temperatures, this pairing is partially disrupted by largely antiferro-electric interactions between distinct pairs.