EPR as a necessary complement of magnetic measurements in exchange coupled systems

Bencini, Alessandro; Benelli, C.; Gatteschi, Dante; Zanchini, C.

doi:10.1007/bf02886278

Cited by 6 publications

(5 citation statements)

References 33 publications

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“…These weak interactions indeed play a crucial role in self-organization of molecules, supramolecular chemistry, and molecular recognition. − There are many papers describing magnetic interactions in structures of copper(II) complexes mediated via mono- and multiatomic bridged ligands. − However, studies on complexes wherein the interactions are primarily via weak intermolecular in nature are rather scarce. In such systems EPR spectroscopy is an ideal, complementary technique to study magnetic properties . With this point in view, we report here synthesis and magnetic resonance (single-crystal EPR and 1 H NMR) spectroscopic studies of [Cu( l -phe)(phen)(H 2 O)]ClO 4 ( 1 ), [Cu( l -phe)(bpy)(H 2 O)]ClO 4 ( 2 ), and [Cu( l -his)(bpy)]ClO 4 ·1.5H 2 O ( 3 ) complexes.…”

Section: Introductionsupporting

confidence: 75%

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Conformational Isomerism and Weak Molecular and Magnetic Interactions in Ternary Copper(II) Complexes of [Cu(AA)L‘]ClO₄·nH₂O, Where AA =l-Phenylalanine andl-Histidine, L‘ = 1,10-Phenanthroline and 2,2-Bipyridine, andn= 1 or 1.5: Synthesis, Single-Crystal X-ray Structures, and Magnetic Resonance Investigations

et al. 2001

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Weak molecular and magnetic exchange interactions in ternary copper(II) complexes, viz., [Cu(L-phe)(phen)(H(2)O)]ClO(4) (1), [Cu(L-phe)(bpy)(H(2)O)]ClO(4) (2), and [Cu(L-his)(bpy)]ClO(4).1.5H(2)O (3), where L-phe = L-phenylalanine, L-his = L-histidine, phen = 1,10-phenanthroline, and bpy = 2,2'-bipyridine, have been investigated. Single-crystal X-ray structures reveal that complex 2 crystallizes in a monoclinic space group P2(1), with unit cell parameters a = 7.422(7) A, b = 11.397(5) A, c = 12.610(2) A, beta = 102.10(5) degrees, V = 1043.0(11) A(3), Z = 2, R = 0.0574, and R(w) = 0.1657. Complex 3 crystallizes in a monoclinic space group C2, with a = 18.834(6) A, b = 10.563(4) A, c = 11.039(3) A, beta = 115.23(2) degrees, V = 1986.6(11) A(3), Z = 4, R = 0.0466, and R(w) = 0.1211. Molecules of 2, in the solid state, are self-assembled via weak intra- and intermolecular pi-pi stacking and H-bonding interactions. Molecules of 3 exhibit intermolecular dimeric association with the Cu.Cu separation being 3.811 A. X-ray structures and (1)H NMR studies reveal conformational isomerism in both solid and liquid states of complexes 1 and 2. The aromatic side chain of L-phe in 1 and 2 adopts either a "folded" (A) or an "extended" (B) conformation. Variable-temperature (1)H NMR and spin lattice relaxation measurements point out interconversion between conformations A and B at temperatures above 323 K. The change in molecular conformation induces a change in the electron density at the site of copper and band gap energy between HOMO and LUMO orbitals. Interestingly, in spite of paramagnetic nature, complexes 1 and 2 are amenable for both EPR and (1)H NMR spectroscopic studies. Single-crystal EPR spectra of 2 in three orthogonal planes are consistent with three-dimensional magnetic behavior. Intramolecular exchange dominates the dipolar interactions. The EPR spectra of 3 correspond to weak magnetic interactions between associated dimeric units. The structural and magnetic resonance investigations together reveal that the weak pi-pi stacking interactions are the electronic pathways for magnetic interactions in 1-3.

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Section: Introductionsupporting

confidence: 75%

“…In such systems EPR spectroscopy is an ideal, complementary technique to study magnetic properties. 13 With this point in view, we report here synthesis and magnetic resonance (single-crystal (3) complexes. Single-crystal X-ray structures of 2 and 3 are also reported.…”

Section: Introductionmentioning

confidence: 99%

Conformational Isomerism and Weak Molecular and Magnetic Interactions in Ternary Copper(II) Complexes of [Cu(AA)L‘]ClO₄·nH₂O, Where AA =l-Phenylalanine andl-Histidine, L‘ = 1,10-Phenanthroline and 2,2-Bipyridine, andn= 1 or 1.5: Synthesis, Single-Crystal X-ray Structures, and Magnetic Resonance Investigations

et al. 2001

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“…EPR spectroscopy has been proved to be a complementary technique to magnetic measurements in such cases. [20][21][22][23] EPR of Polycrystals and Single Crystals. The spectra for the polycrystalline samples of Cu(salEen) 2 (ClO 4 ) 2 at 298 and 77 K are similar and correspond to a rhombically distorted g tensor.…”

Section: Resultsmentioning

confidence: 99%

“…The exchange interaction is expected to be weak and the intrachain exchange interaction is mediated via counterion perchlorate and H-bonding. EPR spectroscopy has been proved to be a complementary technique to magnetic measurements in such cases. − …”

Section: Resultsmentioning

confidence: 99%

Synthesis, X-ray Structure, Single-Crystal EPR and¹H NMR Studies of a Distorted Square Planar Cu(salEen)₂(ClO₄)₂Complex in a Novel Bilayered Architecture: salEen =N,N-Diethylethylenesalicylidenamine

2000

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Single-crystal X-ray structure and spectroscopic characterizations of Cu(salEen)2(ClO4)2, are reported; salEen is a Schiff base condensation product of equimolar amounts of salicylaldehyde and N,N-diethylethylenediamine. The complex crystallizes in a triclinic space group P1 with a = 12.055(4) A, b = 10.652(3) A, c = 12.701(3) A, alpha = 90.54(2) degrees, beta = 94.39(2) degrees, gamma = 91.35(3) degrees, and Z = 2. The coordination geometry around Cu(II) ion is tetrahedrally distorted square planar with salEen coordinating as a neutral bidentate ligand through N and O donor atoms (average Cu-N and Cu-O distances are 2.004 and 1.880 A, respectively). The counterion ClO4- makes H-bonding contacts with the neighboring cation moieties and forms a one-dimensional layered arrangement of the molecules. The pendent, N,N-diethylethylenediamine groups of salEen (from the centrosymmetrically related molecules) in adjacent layers interpenetrate, forming novel bilayered architectures, which are further held together by pi-pi stacking interactions. EPR studies on single crystals, in three mutually orthogonal planes, yield a rhombic g tensor (gx = 2.041(1), gy = 2.073(1), and gz = 2.220(1)) consistent with the distorted square planar geometry of the CuN2O2 moiety. The peak-to-peak line width of the EPR signal exhibits a |3 cos2 theta - 1|4/3 variation, attributable to one-dimensional magnetic exchange behavior of the complex. The analysis suggests that both the dipole-dipole and exchange interactions contribute to the line width. Interestingly, the complex is amenable for both EPR and NMR studies at ambient temperatures. The proton NMR signals are narrow in CD3CN solutions and the HOMO correlation spectroscopy (COSY) studies reveal the 1H connectivities. Nuclear spin lattice relaxation time (T1) measurements, using inversion recovery method, indicate that T1 values for all the protons are remarkably long compared to those of other mononuclear Cu(II) complexes.

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“…In these cases, single crystal EPR spectroscopy can provide the fundamental piece of information to obtain unequivocal answers about magnetic dimensionality, magnetic behavior and dominant interaction pathways. 7 A typical example of the importance of this technique has been provided in the past by the investigation of the copper(II) complex [Cu(hfac) 2 TEMPOL] n , where TEMPOL is the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, for which the crystalline structure suggested an alternating one-dimensional magnetic spin topology. 8 However, EPR studies demonstrated that the compound behaves as a magnetic dimer, 9 due to the long distance between the nitroxide group and the paramagnetic center coordinated by the donor atom (hydroxyl group) in the 4-position of the piperidine ring.…”

Section: Introductionmentioning

confidence: 99%

Determination of the relevant magnetic interactions in low-dimensional molecular materials: the fundamental role of single crystal high frequency EPR

et al. 2011

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A new one-dimensional copper(II) complex with formula [Cu(hfac)(2)(N(3)TEMPO)](n) (hfac = hexafluoroacetylacetonate and N(3)TEMPO = 4-azido-2,2,6,6-tetramethylpiperidine-1-oxyl) has been synthesized and investigated by X-ray crystallography, magnetometry and multifrequency single crystal EPR. The system crystallizes in the P1 space group with two non equivalent copper(II) ions in the unit cell, the two nitroxide radicals being coordinated to Cu(1) in axial positions. The copper(II) ions are bridged by N(3)TEMPO radicals resulting in a zig-zag chain structure. The magnetic susceptibility data were at first satisfactorily modeled assuming an alternating spin chain along the monodimensional covalent skeleton, with a ferromagnetic interaction between Cu(1) and the nitroxide moieties and a weaker antiferromagnetic interaction between these and Cu(2) (J(1) = -13.8 cm(-1), J(2) = +2.4 cm(-1)). However, single crystal EPR studies performed at the X- and W-band clearly demonstrate that the observed magnetic monodimensional character of the complex is actually due to the intermolecular contacts involving N(3)TEMPO ligands. This prompted us to fit the magnetic data using a consistent model, pointing out the fundamental role of single crystal EPR data in defining a correct model to describe the magnetic properties of molecular low dimensional systems.

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EPR as a necessary complement of magnetic measurements in exchange coupled systems

Cited by 6 publications

References 33 publications

Synthesis, X-ray Structure, Single-Crystal EPR and¹H NMR Studies of a Distorted Square Planar Cu(salEen)₂(ClO₄)₂Complex in a Novel Bilayered Architecture: salEen =N,N-Diethylethylenesalicylidenamine

Determination of the relevant magnetic interactions in low-dimensional molecular materials: the fundamental role of single crystal high frequency EPR

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EPR as a necessary complement of magnetic measurements in exchange coupled systems

Cited by 6 publications

References 33 publications

Synthesis, X-ray Structure, Single-Crystal EPR and1H NMR Studies of a Distorted Square Planar Cu(salEen)2(ClO4)2Complex in a Novel Bilayered Architecture: salEen =N,N-Diethylethylenesalicylidenamine

Determination of the relevant magnetic interactions in low-dimensional molecular materials: the fundamental role of single crystal high frequency EPR

Contact Info

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About

Synthesis, X-ray Structure, Single-Crystal EPR and¹H NMR Studies of a Distorted Square Planar Cu(salEen)₂(ClO₄)₂Complex in a Novel Bilayered Architecture: salEen =N,N-Diethylethylenesalicylidenamine