Rafael Calvo

EPR measurements in copper saccharinate single crystals

Abstract Electron paramagnetic resonance measurements have been performed in single crystals of copper(II) saccharinate, [Cu(C7H4NO3S)2(H2O)4].2H2O, as a function of the magnetic field orientation. The characteristics of the spectra are dominated by the exchange narrowing phenomenon which partially collapses the hyperfine structure, and the resonances due to magnetically different copper ions in the unit cell. The data are used to estimate the magnitude of the exchange couplings between copper ions in the compound and the results are discussed in terms of the structural data.

New copper(II)-radical one dimensional chain: Synthesis, crystal structure, EPR, magnetic properties and DFT calculations

The novel chain compound [Cu(Phtfac)(2)(NITpPy)](n) (where NITpPy = 4-pyridyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and Phtfac = 4,4,4-trifluoro-1-phenylbutane-1,3-dione) was synthesized and characterized structurally, magnetically and by EPR. The compound contains two non equivalent Cu(II) ions, Cu1 and Cu2, located at inversion centers and bridged by a NITpPy ligand coordinating Cu1 through the pyridine donor atom, and Cu2 through a N-O group, resulting in a head-to-head chain structure. The chain exhibits an unusual spin topology with two alternating pairs of magnetic coupling constants. The magnetic behavior was modeled considering a 16-membered ring with alternating exchange couplings. The best fit parameters indicate a ferromagnetic (J(1) = 29.4 cm(-1)), and antiferromagnetic (J(2) = -4.6 cm(-1)) couplings and an average g = 2.05, corresponding to a ground state with three parallel and one anti-parallel spin for each Cu(2)NITpPy(2) unit. DFT calculations allowed assigning the ferromagnetic coupling to Cu-O-NITpPy and the antiferromagnetic coupling to Cu-N(Py)-NITpPy. Single crystal EPR spectra display only one resonance for most field orientations, as a consequence of the collapse of the signals of the different spins produced by the exchange interactions. The observed g-tensor of this resonance is related to those expected for the Cu(II) and radical ions. Comparison of this compound with other Cu-NIT radicals chains bearing different substituents in the organic radicals, highlights that the beta-diketonate ligand plays an important role in determining the final architecture. Moreover, we show how a knowledge of the spin density distribution in the initial building blocks is essential to rationalize the magnetic behavior of the resulting product.

EPR spectroscopy and exchange interaction parameters in Cu(glycine)2·H2O

Abstract EPR measurements have been performed in single crystals of Cu(glycine) 2 ·H 2 O, at 9.7 and 33 GHz and at room temperature. this orthorhombic crystal has four chemically identical but magnetically non-equivalent copper ions per unit cell. The EPR data at both microwave frequencies show two partially resolved EPR lines for some of the field directions in the ab plane, while only a single line is observed along any direction in the other two crystallographic planes. This indicates that the four copper ions are arranged in two weakly coupled and symmetry-related subsets , each involving two non-equivalent and interacting copper sites. The decoupling of the spectra leads to g 1 = 2.256(2), g 2 = 2.071(2) and g 3 = 2.061(2) for the principal values of the molecular g -factors. The EPR data, used in conjunction with the classical Andersons model and the Kubo and Tomitas theory, allow to evaluate the exchange interaction parameters between the magnetically non-equivalent copper ions. The Andersons model and the observed EPR line positions data lead to | J ′/ k | = 19(3) mK for the intersubset exchange coupling. The analysis of the angular variation of the line width in terms o the Kubo and Tomitas theory, gives | J ′/ k | = 21(2) mK and | J / k | = 57(1) mK for the intersubset and intrasubset exchange couplings, respectively. These values are discussed in terms of the crystal structure of the compound and compared with the results obtained by other authors.

Magnetic interactions in aqua(l-aspartato)-(2,2′-bipyridine)copper(II) trihydrate

We report magnetic susceptibility measurements in polycrystalline samples and EPR measurements at X-band in single crystal samples of the ternary complex of copper with aspartic acid and bipyridine, aqua(l-aspartato)(2,2′-bipyridine)copper(II) trihydrate, Cu(asp)bpy. This compound exhibits a layered structure for the copper ions. Within each layer, these ions are connected by a chemical path that involves a carboxylate bridge and a hydrogen bond, and by the stacking of the aromatic rings of the bipyridyl ligands. They provide the pathways for magnetic superexchange. Magnetic susceptibility data indicate a weakly ferromagnetic Curie-Weiss behavior in the studied temperature range (2–300 K), and molecular field theory gives J/kB = 0.12(4) K for the mean value of the exchange parameter between copper ions. We observed a single exchange collapsed EPR line in Cu(asp)bpy for any orientation of the magnetic field. The g tensor is nearly diagonal and axially symmetric, in agreement with the crystal structure. The linewidth ΔBPP(≡,φ) exhibits a behavior characteristic of a two-dimensional system, which originates in the layered structure of Cu(asp)bpy. From the EPR data we estimate ∥J∥ = ∥(Jc+JB)/2∥ ≈ ∥Jc∥ ≈ ∥JB∥ ≈ 0.20(3) K, where JC is the exchange parameter associated with the chemical path involving the carboxylate bridge and the hydrogen bond, and JB is that associated with the stacking of the bipyridyl ligands. There is a reasonable agreement between the exchange parameters obtained from susceptibility and EPR measurements. The JB value indicates that the stacking of the aromatic rings may act as a sizeable exchange interaction path.

Structure of bis(l-alaninato)zinc(II) and single crystal EPR spectra of Cu(II) impurities

The title compound (CHCO 2 CH 3 NH 2 ) 2 Zn(II), abbreviated as Zn( l -alanine) 2 , crystallizes in the monoclinic space group P 2 1 with a = 8.637(1), b = 5.334(1), c = 9.581(1) A, β = 90.36(1)°, and Z = 2. The structure was solved employing 1206 independent X-ray reflections with I > 2 gs ( I ) by Patterson and Fourier methods and refined by full-matrix least squares to R 1 = 0.031. The Zn(II) ion is fivefold coordinated with two bidentale amino acid ligands and a carboxylate oxygen of another symmetry-related alaninate ion. EPR spectra of copper impurities were measured with the magnetic field in three orthogonal planes of a single crystal sample. The g and hyperfine coupling tensors were evaluated from the spectra. These values are discussed in terms of the structure of Zn( l -alanine) 2 and compared with those reported for the quasi-isomorphic compound Cu( l -alanine) 2 . The distortion of the Zn site when occupied by copper(II) impurities is analyzed.

Structure and magnetic properties of binuclear Cu2(O2CCHCHCH3)4(DMF)2: a carboxylate-bridged Cu(II) spin dimer

Abstract The complex Cu2(O2CCHCHCH3)4(DMF)2 was synthesized from Cu(OH)2, trans-2-butenoic acid, and dimethylformamide. An X-ray structure determination showed that the complex has the familiar Cu2(O2CR)4L2 dimer structure. trans-2-Butenoate groups in molecules related by a center of symmetry have a short contact between the α-carbon atoms (3.611 A), suggesting a study of the solid-state reactivity of this complex. Irradiation of the complex (60Co X-rays, 219 kGy), did not produce any detectable product. Magnetic susceptibility measurements in the range between 2 and 350 K in a powder sample, and EPR measurements in powder and single-crystal samples were performed. The data show the dimeric magnetic structure of the compound, with an antiferromagnetic exchange coupling J/kB=−220 K and a fine structure coupling D/h=−10.04 GHz within the excited spin triplet. Hyperfine structure with the two Cu nuclei is clearly observed. These results are discussed in terms of the structure.

Magnetic properties of the M(PO3)3 (M=Ti, V) metaphosphates

Two metaphosphates with the formula M(PO3)3 (M=Ti and V) have been synthesized and characterized. Their crystal structures consist of isolated MO6 octahedra linked through infinite [PO3 ]∞ chains of PO4 tetrahedra. The IR and UV–VIS results for these compounds are consistent with chain structures in which the TiIII and VIII cations exhibit octahedral symmetry. The EPR spectrum of the titanium metaphosphate at 4.2 K shows a nearly Lorentzian signal centred at g=1.77, which disappears at 100 K. EPR results obtained for the doped Sc(PO3)3:0.1% TiIII metaphosphate are discussed on the basis of an effective trigonal symmetry. For the vanadium metaphosphate, the EPR spectrum exhibits a weak signal attributed to V3+ ions, with a zero-field splitting parameter D estimated at between 2 and 8 cm–1 . Magnetic measurements show ferromagnetic and antiferromagnetic interactions for the titanium and vanadium metaphosphates, respectively. A value of J/k=–0.92(2) K has been obtained by fitting the experimental magnetic curve for V(PO3)3 to an analytical expression deduced for a three-dimensional Heisenberg model.

Magnetic and structural properties of trans-bis (D,L-isoleucine) copper(II)

A magnetic and structural characterization of single crystals of the copper derivative of the amino acid D,L-isoleucine, Cu(NH{sub 2}(CH){sub 2}CH{sub 2}(CH{sub 3}){sub 2}CO{sub 2}){sub 2}, was performed by EPR and X-ray diffraction techniques. The complex crystallizes in the orthorhombic space group Aba2, with a = 11.165(3) {angstrom}, b = 11.111(3) {angstrom}, c = 25.985(6) {angstrom}, and Z = 8. The copper ions occupy sites of point symmetry C{sub 2}. The position and peak-to-peak linewidth of the single EPR line observed were measured at 9.7 GHz and 293 K in three perpendicular planes of the sample. The gyromagnetic tensor {lvec {rvec g}} has near axial symmetry around {cflx c}, with a small anisotropy in the perpendicular plane, in agreement with the orthorhombic symmetry indicated by the crystallographic results. The principal values of {lvec {rvec g}} are g{sub 1} = 2.0607(5), g{sub 2} = 2.0616(5), and g{sub 3} = 2.2619(3), with principal directions parallel to the crystal axes. The observed angular variation of the linewidth suggests a layered arrangement of the copper ions.

A Novel Structure of (l-Aspartato)(1,10-phenanthroline)copper(II) Hydrate

The title compound, [Cu(C 4 H 5 NO 4 )(C 12 H 8 N 2 )].3.5H 2 O, consists of two types of different pentacoordinate copper complex: one monomeric, with the aspartate group acting as a tridentate ligand, and the other polymeric, with the aspartate group bridging two different copper centers. The whole structure is held together by a complex hydrogen-bonding scheme involving the water molecules of solvation

Molecular structure and single crystal EPR spectra of bis(L-Valinato)copper(II) monohydrate, Cu[H2NCH(CH3)2CHCO2]2·H2O

Abstract A magnetic-structural study of the title compound, Cu(L-Val)2·H2O, employing EPR spectroscopy and X-ray diffraction methods is presented. The complex crystallizes in the space group C2 with a = 21.314(5) A , b = 9.586(2) A , c = 7.417(2) A , β = 108.89(2)°, and Z=4. The Cu(II) ions, which are in a five-fold pyramidal coordination, with two valine molecules and a water oxygen (Ow), are arranged in layers parallel to (100). Neighboring Cu(L-Val)2·H2O molecules in a layer are linked to each other by a net of Ow-H…O and N-H…O hydrogen bonds. The single EPR line observed arises from the collapse of the resonances due to the two magnetically inequivalent Cu(II) ions in the lattice caused by the exchange interaction. The obtained g⌈ = 2.254(1) and g⊥ = 2.061(1) principal values of the molecular gyromagnetic tensor point to a d(x2−y2) orbital for the unpaired electron. The orientation of this orbital was also obtained from our EPR data. The angular variation of the resonance line width has contributions arising from the dipolar interaction modulated by exchange effects in a two-dimensional magnetic lattice, and from the incomplete collapse of the hyperfine structure. The EPR results are compared to those obtained in other copper amino acid complexes.