Intramolecular Magnetic Interactions Research Articles

Magnetic Interactions in CuII−LnIII Cyclic Tetranuclear Complexes: Is It Possible to Explain the Occurrence of SMM Behavior in CuII−TbIII and CuII−DyIII Complexes?

An extensive series of tetranuclear CuII2LnIII2 complexes [CuIILLnIII(hfac)2]2 (with LnIII being all lanthanide(III) ions except for the radioactive PmIII) has been prepared in order to investigate the nature of the CuII-LnIII magnetic interactions and to try to answer the following question: What makes the CuII2TbIII2 and CuII2DyIII2 complexes single molecule magnets while the other complexes are not? All the complexes within this series possess a similar cyclic tetranuclear structure, in which the CuII and LnIII ions are arrayed alternately via bridges of ligand complex (CuIIL). Regular SQUID magnetometry measurements have been performed on the series. The temperature-dependent magnetic susceptibilities from 2 to 300 K and the field-dependent magnetizations from 0 to 5 T at 2 K have been measured for the CuII2LnIII2 and NiII2LnIII2 complexes, with the NiII2LnIII2 complex containing diamagnetic NiII ions being used as a reference for the evaluation of the CuII-LnIII magnetic interactions. These measurements have revealed that the interactions between CuII and LnIII ions are very weakly antiferromagnetic if Ln=Ce, Nd, Sm, Yb, ferromagnetic if Ln=Gd, Tb, Dy, Ho, Er, Tm, and negligible if Ln=La, Eu, Pr, Lu. With the same goal of better understanding the evolution of the intramolecular magnetic interactions, X-ray magnetic circular dichroism (XMCD) has also been measured on CuII2TbIII2, CuII2DyIII2, and NiII2TbIII2 complexes, either at the L- and M-edges of the metal ions or at the K-edge of the N and O atoms. Last, the CuII2TbIII2 complex exhibiting SMM behavior has received a closer examination of its low temperature magnetic properties down to 0.1 K. These particular measurements have revealed the unusual very slow setting-up of a 3D order below 0.6 K.

A Dinuclear Ni(II) Complex with Two Types of Intramolecular Magnetic Couplings: Ni(II)−Ni(II) and Ni(II)−TTF•+

A dinuclear Ni(II) complex involving tetrathiafulvalene (TTF) radicals as ligands has been prepared and characterized, [Ni2(mu-Cl)2(L*+)2(I3)4(I2)3.(H2O)2.(C4H8O)3 (1), L = 4,5-bis(2-pyridylmethylsulfanyl)-4',5'-ethylenedithiotetrathiafulvalene. There are two types of intramolecular magnetic exchange interactions, namely one ferromagnetic Ni(II)-Ni(II) and one antiferromagnetic Ni(II)-TTF*+. This study is new in the respect of revealing a magnetic exchange interaction between a TTF*+ radical and a paramagnetic transition metal ion. This is due to the fact of a direct binding of the transition metal ion to the skeleton of the TTF*+ radical.

Structure and Magnetic Properties of the Radical Cation Salt of a TTF-based NiII Complex

Chemical oxidation of a TTF-based NiII complex with I2 produces the corresponding radical cation salt1, [Ni2Cl2(L)2](I3)2(I5)2(I2)(H2O)2(C4H8O)3, (L=4,5-bis(2-pyridylmethylsulfanyl)-4′,5′-ethylenedithiotetrathiafulvalene). The results of magnetic susceptibility measurements show the occurrence of intramolecular magnetic exchange interactions in1. The lack of close S…S contacts, confirmed by crystal structure analysis, results in an insulating behavior.

Low-energy excitations in theS=12molecular nanomagnetK6[V15IVAs6O42(H2O)]∙8H2Ofrom proton NMR andμSR

Zero- and longitudinal-field muon-spin-rotation $(\ensuremath{\mu}\mathrm{SR})$ and $^{1}\mathrm{H}$ NMR measurements on the $S=\frac{1}{2}$ molecular nanomagnet ${\mathrm{K}}_{6}[{\mathrm{V}}_{15}^{IV}{\mathrm{As}}_{6}{\mathrm{O}}_{42}({\mathrm{H}}_{2}\mathrm{O})]∙8{\mathrm{H}}_{2}\mathrm{O}$ are presented. In LF experiments, the muon asymmetry $P(t)$ was fitted by the sum of three different exponential components with fixed weights. The different muon relaxation rates ${\ensuremath{\lambda}}_{i}$ $(i=1,2,3)$ and the low-field $H=0.23\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ $^{1}\mathrm{H}$ NMR spin-lattice relaxation rate $1∕{T}_{1}$ show a similar behavior for $T>50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$: starting from room temperature they increase as the temperature is decreased. The increase of ${\ensuremath{\lambda}}_{i}$ and $1∕{T}_{1}$ can be attributed to the ``condensation'' of the system toward the lowest-lying energy levels. The gap $\ensuremath{\Delta}\ensuremath{\sim}550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ between the first and second $S=\frac{3}{2}$ excited states was determined experimentally. For $T<2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the muon relaxation rates ${\ensuremath{\lambda}}_{i}$ stay constant, independently of the field value $H\ensuremath{\leqslant}0.15\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The behavior for $T<2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ strongly suggests that, at low $T$, the spin fluctuations are not thermally driven but rather originate from quasielastic intramolecular or intermolecular magnetic interactions. We suggest that the very-low-temperature relaxation rates could be driven by energy exchanges between two almost degenerate $S=\frac{1}{2}$ ground states and/or by quantum effects.

Broken-Symmetry Density Functional Theory Investigation on Bis-Nitronyl Nitroxide Diradicals: Influence of Length and Aromaticity of Couplers

A series of nitronyl nitroxide (NN) diradicals with linear conjugated couplers and another series with aromatic couplers have been investigated by the broken-symmetry (BS) DFT approach. The overlap integral between the magnetically active orbitals in the BS state has been explicitly computed and used for the evaluation of the magnetic exchange coupling constant (J). The calculated J values are in very good agreement with the observed values in the literature. The magnitude of J depends on the length of the coupler as well as the conformation of the radical units. The aromaticity of the spacer decreases the strength of the exchange coupling constant. The SOMO-SOMO energy splitting analysis, where SOMO stands for the singly occupied molecular orbital, and the calculation of electron paramagnetic resonance (EPR) parameters have also been carried out. The computed hyperfine coupling constants support the intramolecular magnetic interactions. The nature of magnetic exchange coupling constant can also be predicted from the shape of the SOMOs as well as the spin alternation rule in the unrestricted Hartree-Fock (UHF) treatment. It is found that pi-conjugation along with the spin-polarization plays the major role in controlling the magnitude and sign of the coupling constant.

Structure and magnetic properties of the radical cation salt of a TTF-based NiII complex

Chemical oxidation of a TTF-based NiII complex with I2 produces the corresponding radical cation salt1, [Ni2Cl2(L)2](I3)2(I5)2(I2)(H2O)2(C4H8O)3, (L=4,5-bis(2-pyridylmethylsulfanyl)-4′,5′-ethylenedithiotetrathiafulvalene). The results of magnetic susceptibility measurements show the occurrence of intramolecular magnetic exchange interactions in1. The lack of close S…S contacts, confirmed by crystal structure analysis, results in an insulating behavior.

Theoretical study of the magnetic exchange coupling behavior substituting Cr(III) with Mo(III) in cyano‐bridged transition metal complexes

AbstractMolecular magnetism in a series of cyano‐bridged first and second transition metal complexes has been investigated using density functional theory (DFT) combined with the broken‐symmetry (BS) approach. Several exchange‐correlation (XC) functionals in the ADF package were used to investigate complexes I [−(Me3tacn)2(cyclam)NiMo2(CN)6]2+, II [−(Me3tacn)2(cyclam)Ni‐Cr2(CN)6]2+, III [(Me3tacn)6MnMo6(CN)18]2+, and IV [(Me3tacn)6MnCr6(CN)18]2+ (Me3tacn = N,N′,N‴‐trimethyl‐1,4,7‐triazacyclononane). For models A (the molded structure of complex I) and B (the modeled structure of complex II), all the XCs given qualitatively reasonable results and predict ferromagnetic coupling character between M (M = MoIII for A or CrIII for B) and NiII in coincidence with the experimental results (see Tables I and II). The calculated using Operdew, OPBE, O3LYP, and B3LYP functionals and experimental J values show that substituting CrIII with MoIII will enhance the ferromagnetic exchange coupling interactions. But VWN, PW91, PBE, VSXC, and tau‐HCTH functionals have no way to differentiate the relative strength of the intramolecular magnetic exchange coupling interactions of A and B correctly. For models C (the modeled structure of complex III) and D (the modeled structure of complex IV), all the XCs in ADF and B3LYP in Gaussian 03 with several basis sets show that substituting CrIII with MoIII will enhance the antiferromagnetic exchange coupling interactions. From the above calculations, the substitution of CrIII by MoIII will enhance the magnetic coupling interactions, whether the magnetic coupling interactions are ferro‐ or antiferromagnetic. Moreover, Kahn's model was applied to investigate the above facts. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

New Photoswitching Unit for Magnetic Interaction: Diarylethene with 2,5-Bis(arylethynyl)-3-thienyl Group

Photoswitching of the intramolecular magnetic interaction was demonstrated using diarylethenes with 2,5-bis(arylethynyl)-4-methyl-3-thienyl side group. Two nitroxide radicals were placed at each end of the 2,5-bis(arylethynyl)-4-methyl-3-thienyl group. Three kinds of aryl groups, 2,5-thienylene, p-phenylene, and m-phenylene groups, were used in the arylethynyl moiety. The diarylethene photoswitching units have an extended pi-conjugated chain on one side of the diarylethene. The photochromic reactivity was dependent on the arylethynyl group. Diarylethenes with m-phenylene group showed an efficient photochromic reactivity. Along with the photochromic reaction the diarylethenes showed photoswitching of an ESR spectrum originating from the change in the magnetic interaction between two unpaired electrons. The open-ring isomer showed stronger exchange interaction than the photogenerated closed-ring isomer. The magnetic interaction between two radicals via the pi-conjugated chain was altered by photocyclization due to the change of the hybrid orbital at the 2-position of the thiophene ring from sp(2) to sp(3).

A quantum chemical study of magnetic interactions in phenazine derivatives for chemical or photo-induced organic magnets

Donor-radical model systems for chemical or photo-induced magnets are proposed, and their electronic states and intramolecular magnetic interactions are investigated by Kohn-Sham hybrid density functional theory (KS-HDFT). 5,10-dihydrophenazine is employed as a donor and nitronyl- or imino-nitroxide radicals as stable radical substituents. The models mostly provide ferromagnetic intramolecular spin alignment in hole doping. In addition, the inductive effect on intramolecular magnetic interactions is examined by substituting the 5- and 10-positions of dihydrophenazine by several electron-withdrawing and -donating groups. The model compounds exhibit appropriate magnetic interactions and ionization potentials to allow them to serve as components of chemical or photo-induced magnets.

Intramolecular Magnetic Interaction Controlled by Redox Reaction of Tetraphenylethylene-Based Spin System

In order to investigate the possibility of a redox-switchable intramolecular magnetic interaction converting system, the electronic structures of the model compounds of 1,2-bis[x-( N- tert-butylaminoxyl)phenyl]-1,2-bis[4-{ N, N-bis(4-methoxyphenyl)amino}phenyl] ethylene (x = 4, p, p′- 1; x = 3, m, m′- 1) were examined theoretically with the hybrid DFT method. It was found that these molecular systems can operate as a redox switch and change their electronic structures depending on the substituted position of nitroxide radicals.

Theoretical studies of magnetic interaction in π-radical thiol and Gold hybrid systems

Nanoscale magnetic material is becoming an important topic in nanoscience and magnetism. Especially, gold nanoparticles chemisorbed by alkanethiols have drawn much attention due to the size effect on their electronic structure. However, magnetic interaction of gold nanoparticles derivatized by magnetically active ligands has not been investigated in detail. In order to elucidate the magnetic interaction in gold nanoparticles chemisorbed by alkanethiol systems, small gold cluster and π-radical hybrid model systems were examined using the hybrid DFT method. In this research, the effective exchange integrals (J ab) of model systems were calculated for qualitative understanding of the intramolecular magnetic interaction. From these calculations, it was found that there was a possibility of controlling the magnetic interaction of these hybrid model systems using electron/hole doping.

Ab Initio Quantum Chemical Investigation of Intramolecular Magnetic Interaction in Some Diradical Derivatives of Imino Nitroxide and Nitronyl Nitroxide

The magnetic properties of the monoradicals 2-(4-phenyl acetylene)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidozolyl-oxyl (1) and 2-(4-phenyl acetylene)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide (2) and the diradicals 2,2'-(1,2-ethynediyldi-4,1-phenylene)bis[4,4,5,5-tetramethyl-4,5-dihydro-1H-imidozolyl-oxyl] (3), 2,2'-(1,2-ethynediyldi-4,1 3,1-phenylene)bis[4,4,5,5-tetramethyl-4,5-dihydro-1H-imidozolyl-oxyl] (4), and 2,2'-(1,2-ethynediyldi-4,1 3,1-phenylene)bis[4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide] (5) are investigated by ab initio quantum chemical methods. The rule of spin alternation in the unrestricted Hartree-Fock (UHF) method clearly shows that the radical sites are antiferromagnetically coupled in 3 and ferromagnetically coupled in 4 and 5, which is consistent with a previous experiment. The molecular geometries are optimized at Hartree-Fock levels. This is followed by single-point calculations using the density functional (UB3LYP) treatment and the multiconfigurational complete active space self-consistent field (CASSCF) methodology. Magnetic exchange coupling constants are determined from the broken-symmetry approach. The calculated J values, -3.60 cm(-1) for 3, 0.16 cm(-1) for 4, and 0.67 cm(-1) for 5, are in excellent agreement with the observed values. Because of the very large size of the diradicals 3-5, the CASSCF (10,10) calculations cannot yield realistic J values. Nevertheless, the CASSCF calculations support the antiferromagnetic nature of the magnetic coupling in 3 and the ferromagnetic nature of the coupling in 4 and 5. The existence of an intramolecular magnetic coupling in 3-5 is also confirmed through computations of the isotropic hyperfine coupling constants for monoradicals 1 and 2 as well as diradicals 3-5.

Synthesis of a new diarylethene diradical which has extended π-conjugated chains from the 2,5-position of one thiophene ring

Abstract A diarylethene diradical having a new switching unit for intramolecular magnetic interaction was synthesized. The photoswitching unit has an extended π-conjugated chain in one aryl unit, and two nitronyl nitroxide radical are placed at both ends of the π-conjugated chain. The diarylethene moiety is located in the middle of the chain. This diarylethene is designed to change the hybrid orbital from sp 2 to sp 3 at the 2-position of the thiophene ring when this diarylethene undergoes a photochromic reaction. But the new diradical compound did not undergo photocyclic reaction upon irradiation with UV light. The photochemical behavior is perturbed by a resonant quinoid structure which stabilizes the open-ring isomer.

Effective photoswitching of intramolecular magnetic interaction by diarylethene: Backgrounds and applications

Diarylethene is a photochromic molecule, which is potentially applicable to various optoelectronic devices. In diarylethenes the π-system of the two aryl rings is separated in the open-ring isomer, while the π-system is delocalized throughout the molecule in the closed-ring isomer. Based on this idea the exchange interaction between two nitronyl nitroxide radicals connected by a diarylethene unit was photocontrolled reversibly along with photochromism. The switching efficiency is more than 150-fold and thiophene spacer transmits the interaction more efficiently. When diarylethene dimer is used for the switching unit, the electric circuit-like behavior was observed. Moreover, reversed switching is achieved by reversing the thiophene ring. Magnetochemical and photochemical behavior of the radical-substituted diarylethene is described extensively.

Synthesis and intramolecular magnetic interaction of triphenylamine derivatives with nitronyl nitroxide radicals

Two triphenylamine derivatives bearing nitronyl nitroxide radicals, 1 and 2 , were synthesized. The weak antiferromagnetic interactions exist between the nitronyl nitroxide moieties in 1 and 2 , and furthermore, those radical groups are oxidized much easier than the central triphenylamine moiety. Two triphenylamine derivatives bearing nitronyl nitroxide radicals, tris[4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl]amine ( 1 ) and N , N -bis[4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl]-4-methoxyphenylamine ( 2 ) were synthesized. Variable-temperature magnetic susceptibility measurements showed the weak antiferromagnetic interactions in the powder samples of 1 and 2 . The electrochemical and ESR measurements revealed that the first oxidation process of 1 and 2 corresponds to the oxidation of nitronyl nitroxide moieties.

Theoretical Investigation of Intramolecular Magnetic Interaction through an Ethylenic Coupler

We show that an ethylenic coupler provides a very strong intramolecular magnetic interaction. A recently synthesized nitronyl nitroxide derivative, D-NIT2, is investigated by ab initio quantum chemical methods. The broken symmetry approach yields a coupling constant -541 K that is in good agreement with the observed value in solid state.

Synthesis, crystal structure, and magnetic properties of binuclear Mn III-azido and 1D polymeric Mn II-μ 1,3-thiocyanato novel species based on a neutral hexadentate Schiff base

A binuclear manganese(III) complex [Mn 2(L 1)(N 3) 6] ( 2) and an unprecedented 1D chain of manganese(II) binuclear units bridged by two end-to-end thiocyanato anions { [ Mn 2 II ( L 1 ) ( NCS ) 4 ] · 2 CH 3 CN } n ( 3 · 2CH 3CN) have been prepared from N-(1-pyridin-2-ylbenzylidene)- N′-[2-({2-[(1-pyridin-2-ylbenzylidene)amino]ethyl}amino)ethyl]ethane-1,2-diamine (L), manganese(II) salts and azide/thiocyanate anions in air. L 1, N-(1-pyridin-2-ylbenzylidene)- N-[2-(4-{2-[(1-pyridin-2-ylbenzylidene)amino]ethyl}piperazin-1-yl)ethyl]amine, results from an alkylating cyclization of the flexible hexadentate Schiff base L occurring along the course of the complexation reactions. Both complexes have been characterized by single crystal X-ray diffraction studies. The Mn centres of complexes 2 and 3 are in distorted octahedral ( 2: Mn IIIN 6, 3: Mn IIN 5S) coordination environments with intramolecular Mn⋯Mn distance ( 2: 6.473(2) Å, 3: 6.437(1) Å) consistent with the absence of intramolecular magnetic interactions. Compound 3, exhibits weak intermolecular antiferromagnetic interactions ( J = −1.5 cm −1) between pairs of Mn II ions bridged via two μ 1,3-NCS.

Tetraarylethylene having two nitroxide groups: redox-switching of through-bond magnetic interaction by conformation change

Reversible redox-switching of through-bond magnetic interaction has been achieved by conformation change of the tetraarylethylene moiety upon redox input: intramolecular magnetic interaction between two nitroxide groups is dead after oxidation, whereas it was alive before.

Nuclear spin conversion in the gaseous phase in the presence of a static electric field: Intramolecular magnetic interactions and the role of collisions

When a gaseous sample of $^{13}\mathrm{CH}_{3}\mathrm{F}$ is prepared with a spin-isomer population ratio (ortho and para forms) far from the equilibrium given by nuclear spin statistics, it relaxes towards this equilibrium with an exponential decay rate. This phenomenon, called nuclear spin conversion, is mainly governed by intramolecular spin-spin and spin-rotation interactions which couple two pairs of quasidegenerate ortho-para levels ($J=9,K=3;$ ${J}^{\ensuremath{'}}=11,{K}^{\ensuremath{'}}=1$) and ($J=20,K=3;$ ${J}^{\ensuremath{'}}=21,{K}^{\ensuremath{'}}=1$). The presence of a static electric field can induce the degeneracy for Stark sublevels and yields an increase of the conversion rate. Such a ``conversion spectrum'' has been recorded experimentally. The intensities of the peaks are directly related to the intramolecular magnetic interaction strengths, and their widths depend on how the collisions break the coherence between ortho and para levels which is created by the interactions. Such collision-induced rates are directly determined and compared to the rate of rotationally inelastic molecular collisions.

Structural characterisation of [Cu 2(bptd) (H 2O) Cl 4] and [Ni 2(bptd) 2(H 2O) 4] Cl 4·3H 2O; evidence of null intramolecular exchanges by EPR and magnetic measurements

Using the 2,5-bis(2-pyridyl)-1,3,4-thiadiazole (bptd), we recently prepared [Cu 2(bptd) (H 2O) Cl 4] and [Ni 2(bptd) 2 (H 2O) 4] Cl 4, 3H 2O in which the magnetic centres are connected through one diazine+one chloro and two diazine ligand bridges, respectively. These two compounds are the first examples that show null intramolecular magnetic interactions despite M–M distances close to 3.7 Å within perfectly planar edifices: Down to T t =125 K , [Cu 2(bptd)Cl 4(H 2O)] is paramagnetic while, below T t, half of the Cu 2+ions interact, leading to residual paramagnetism of one Cu 2+/f.u. Magnetic susceptibility measurements, EPR and pulsed EPR study indicate the original intermolecular nature of AF exchanges. [Ni 2(bptd) 2(H 2O) 4]Cl 4·3H 2O susceptibility obeys a Curie-law involving pure paramagnetism. Moreover, its EPR spectrum can be interpreted on the basis of virtual S=1 monomers. Below 70 K, Zero Field Splitting ( D∼275 G) due to dipolar interactions without magnetic exchanges could be responsible for the LT spectra splitting. For both compounds, the thia role is suggested as partially responsible for the null-in-plane magnetic exchanges.