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Ferromagnetic Coupling in an Fe[C(SiMe3)3]2/Ferrihydrite Hetero-Mixture Molecular Magnet
European Journal of Inorganic Chemistry
Abstract
Magnetization and low temperature in-field 57Fe Mössbauer spectroscopy measurements have been performed on a Fe[C(SiMe3)3]2/ferrihydrite hetero-mixture. The results indicate the presence of ferromagnetic coupling of magnetic moments involving Fe[C(SiMe3)3]2 with a hyperfine magnetic field of about 151 T, attributable mainly to the non-frozen atomic orbital contribution. The present findings show the sensitivity of single-ion molecular magnets to local alterations of their lattice-environment and might explain and reconcile some of the differences found in the literature for the observed bulk magnetic properties of the title Fe[C(SiMe3)3]2 compound.
... Molecular magnetism, which is the focus of current scientific interest with possible technological aspects, is mostly present in molecules with unusual geometry and bonding anisotropy [1][2][3]23], such as two-coordinate iron(II) complexes with linear geometry that possess interesting electrical and magnetic properties [24][25][26][27][28][29][30][31][32][33][34]. In order to create such an unusually low coordination number and stabilize low-valent metal centers, a special set of bulky, protective ligands was designed, as shown in Chart 1. ...
... In the Fe II [C(SiMe3)3]2 (Fe(L1)2) molecule, an unusually large 57 Fe hyperfine magnetic field was detected at low temperatures [26][27][28]. At first, we obtained this Fe(L1)2 compound and characterized it using 57 Fe-Mössbauer spectroscopy, which is known for accurate measurement of hyperfine magnetic field strength at the iron site, and by other physical methods that helped to confirm and explain the extremely high magnetic field at the central atom [25,26,33,34]. The results of the geometry optimization and electronic structure determination performed by density functional theory (DFT) calculations were in agreement with the geometry determined by X-ray analysis. ...
... At the same time, the spectrum of the ferrihydrite remained unchanged [35] (Figure 5). Thus, the low-temperature exchange interaction in Fe(L1)2 is confirmed to be of a ferromagnetic type, in agreement with the hysteresis clearly observed in magnetization measurements of the same system [33]. Further exploration of this topic of unusually high hyperfine internal magnetic fields at low temperatures led to the preparation of other related compounds in which the Catom in methanide L1 was replaced by a nitrogen atom in a series of similarly sterically hindered amides L2-L5 (Chart 1). ...
In this mini-review of our research group’s activity, the application of 57Fe Mössbauer spectroscopy in studies of electronic structure, coordination environment, and magnetic interactions in an interesting series of Fe(II/III) compounds selected is discussed. We selected two prominent phenomena that arose during investigations of selected groups of compounds carried out at different periods of time: (1) very high magnetic hyperfine fields observed at low temperatures; (2) changes in the oxidation state of the central iron atom of complexes in the solid state during interactions with gaseous O2/H2O mixtures, resulting in spin crossover (SCO).
... Here we summarize our main results, obtained by Mössbauer spectroscopy, on the spin transitions found either for iron-bis-glioxime complexes [26] and for oxygenated iron phthalocyanines [35], and the results on the observation of low temperature magnetic exchange couplings found either in oxygenated iron phthalocyanines [36] or, together with extraordinary high hyperfine fields, in single molecule magnets [47,48]. ...
A multifunctional transducer for applications in Mössbauer spectrometry has been developed. The transducer design and performance is presented in detail. The described transducer is able to carry an object weighing up to approximately 1.5 kg. Thus it is able to operate while being loaded with a scintillation detector. This ability offers a number of interesting measuring configurations, with the most beneficial being the possibility of measuring several Mössbauer spectra while using a single radiation source. The capabilities of our transducer are demonstrated by acquiring spectra of 12μm α-Fe in different velocity ranges and at different operational frequencies using a triangular and saw velocity profile. The linewidth obtained under optimal conditions was 0.21 mm/s.
Standard magnetic measurements cannot be routinely carried out in cluster samples. The magnetic behavior of these clusters needs to be sensitive to spectroscopic techniques in order to elicit important information about them. Such a highly selective technique is Mossbauer spectroscopy. With regard to molecular magnetism, the most appropriate element that this technique can be applied on is iron. Therefore, this chapter is devoted to the application of this technique in iron‐based molecular magnets. It highlights the critical parameters that are relevant to molecular magnetism, and specifically focuses on the issues pertaining to slow magnetic relaxation. Applications of Mossbauer spectroscopy to study iron‐based oligo‐ and polynuclear single‐molecule magnets (SMMs) are presented. Often for powder samples powder X‐ray diffraction (pXRD) measurements are performed in order to test the purity of the sample and the possible existence of other phases.
In Fe[N(SiPh2Me)2]2 molecular magnet, an extraordinary B ≈ 92 T hyperfine field was found in the 5 K ⁵⁷Fe Mössbauer spectrum under an external magnetic field of 0.1 T. This evidences the presence of an unquenched orbital angular moment at the central iron atom. Fe[N(SiPh2Me)2]2 complex is thus shown to represent a further example of low-coordinate iron complexes where quasi free-ion magnetism visualizes itself through an unquenched orbital moment. Magnetization measurements and hysteresis in magnetization indicated exchange coupling and nanosized magnetic units.
Graphical abstractThe Fe[N(SiPh2Me)2]2 molecular magnet complex was successfully prepared and found to display extraordinary B ≈ 92 T iron hyperfine magnetic field in response to an external magnetic field of 0.1 T at 5 K, evidencing an unquenched orbital angular moment at the central iron atom. In-field 57Fe Mossbauer spectroscopy, magnetization and magnetic hysteresis measurements indicate exchange coupling and the presence of nanosized magnetic units of Fe[N(SiPh2Me)2]2.
The electronic structure and magnetic anisotropy of six complexes of high-spin Fe-II with linear FeX2 (X = C, N, O) cores, Fe[N(SiMe3)(Dipp)](2) (1), Fe[C(SiMe3)(3)](2) (2), Fe[N(H)Ar'](2) (3), Fe[N(H)Ar*](2) (4), Fe[O(Ar')](2) (5), and Fe[N(t-Bu)(2)](2) (7) [Dipp = C6H3-2,6-Pr-2(i); Ar' = C6H3-2,6-(C6H3-2,6-Pr-2(i))(2); Ar* = C6H3-2,6-(C6H2-2,4,6-Pr-2(i))(2); Ar-# = C6H3-2,6-(C6H2-2,4,6-Me-3)(2)], and one bent (FeN2) complex, Fe[N(H)Ar-#](2) (6), have been studied theoretically using complete active space self-consistent field (CASSCF) wavefunctions in conjunction with N-Electron Valence Perturbation Theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for the treatment of magnetic field and spin-dependent relativistic effects. Mossbauer studies on compound 2 indicate an internal magnetic field of unprecedented magnitude (151.7 T) at the Fe-II nucleus. This has been interpreted as arising from first order angular momentum of the (5)Delta ground state of Fe-II center (J. Am. Chem. Soc. 2004, 126, 10206). Using geometries from X-ray structural data, ligand field parameters for the Fe-ligand bonds were extracted using a 1 : 1 mapping of the angular overlap model onto multireference wavefunctions. The results demonstrate that the metal-ligand bonding in these complexes is characterized by: (i) strong 3d(z2)-4s mixing (in all complexes), (ii) pi-bonding anisotropy involving the strong pi-donor amide ligands (in 1, 3-4, 6, and 7) and (iii) orbital mixings of the sigma-pi type for Fe-O bonds (misdirected valence in 5). The interplay of all three effects leads to an appreciable symmetry lowering and splitting of the (5)Delta (3d(xy), 3d(x2-y2)) ground state. The strengths of the effects increase in the order 1 < 5 < 7 similar to 6. However, the differential bonding effects are largely overruled by first-order spin-orbit coupling, which leads to a nearly non-reduced orbital contribution of L - 1 to yield a net magnetic moment of about 6 mu(B). This unique spin-orbital driven magnetism is significantly modulated by geometric distortion effects: static distortions for the bent complex 6 and dynamic vibronic coupling effects of the Renner-Teller type of increasing strength for the series 1-5. Ab initio calculations based on geometries from X-ray data for 1 and 2 reproduce the magnetic data exceptionally well. Magnetic sublevels and wavefunctions were calculated employing a dynamic Renner-Teller vibronic coupling model with vibronic coupling parameters adjusted from the ab initio results on a small Fe(CH3)(2) truncated model complex. The model reproduces the observed reduction of the orbital moments and quantitatively reproduces the magnetic susceptibility data of 3-5 after introduction of the vibronic coupling strength (f) as a single adjustable parameter. Its value varies in a narrow range (f = 0.142 +/- 0.015) across the series. The results indicate that the systems are near the borderline of the transition from a static to a dynamic Renner-Teller effect. Renner-Teller vibronic activity is used to explain the large reduction of the spin-reversal barrier U-eff along the series from 1 to 5. Based upon the theoretical analysis, guidelines for generating new single-molecule magnets with enhanced magnetic anisotropies and longer relaxation times are formulated.
Fe MSssbauer spectra of two synthetic samples of ferrihydrite, recorded at 4.2 K in applied fields of up to 9 T, have been analysed by a mean-field model. The samples exhibit two and six X-ray diffraction peaks. It is shown that only one ferric ion site is present in the mineral, and that in this site the ions are octahedrally coordinated. The spectra show the presence of different magnetic states: ferri- magnetism in two-line ferrihydrite, and antiferromagnetism in six-line ferrihydrite. The ferrimagnetism in two-line ferrihydrite is analysed in terms of random fluctuations arising from the small numbers of ferric ions per particle, and it is shown that the different magnetic states may arise purely as a result of these fluctuations.
Nanomagnetism is a rapidly expanding area of research which appears to be able to provide novel applications. Magnetic molecules are at the very bottom of the possible size of nanomagnets and they provide a unique opportunity to observe the coexistence of classical and quantum properties. The discovery in the early 90's that a cluster comprising twelve manganese ions shows hysteresis of molecular origin, and later proved evidence of quantum effects, opened a new research area which is still flourishing through the collaboration of chemists and physicists. This book is the first attempt to cover in detail the new area of molecular nanomagnetism, for which no other book is available. In fact research and review articles, and book chapters are the only tools available for newcomers and the experts in the field. It is written by the chemists originators and by a theorist who has been one of the protagonists of the development of the field, and is explicitly addressed to an audience of chemists and physicists, aiming to use a language suitable for the two communities.
A series of two-coordinate complexes of iron(II) were prepared and studied for single-molecule magnet behavior. Five of the compounds, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar0]2 (3), Fe[N(H) Ar*]2 (4), and Fe(OAr0)2 (5) feature a linear geometry at the FeII center, while the sixth compound, Fe [N(H)Ar#]2 (6), is bent with an N–Fe–N angle of 140.9(2)� (Dipp 1⁄4 C6H3-2,6-Pri2; Ar0 1⁄4 C6H3-2,6-(C6H3- 2,6-Pri2)2; Ar* 1⁄4 C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# 1⁄4 C6H3-2,6-(C6H2-2,4,6-Me3)2). Ac magnetic susceptibility data for all compounds revealed slow magnetic relaxation under an applied dc field, with the magnetic relaxation times following a general trend of 1 > 2 > 3 > 4 > 5 [ 6. Arrhenius plots created for the linear complexes were fit by employing a sum of tunneling, direct, Raman, and Orbach relaxation processes, resulting in spin reversal barriers of Ueff 1⁄4 181, 146, 109, 104, and 43 cm�1 for 1–5, respectively. CASSCF/NEVPT2 calculations on the crystal structures were performed to explore the influence of deviations from rigorous DNh geometry on the d-orbital splittings and the electronic state energies. Asymmetry in the ligand fields quenches the orbital angular momentum of 1–6, but ultimately spin–orbit coupling is strong enough to compensate and regenerate the orbital moment. The lack of simple Arrhenius behavior in 1–5 can be attributed to a combination of the asymmetric ligand field and the influence of vibronic coupling, with the latter possibility being suggested by thermal ellipsoid models to the diffraction data.
IN one of the pioneering studies of magnetism at the microscopic level, Heisenberg1 concluded that ferromagnetism could not exist in compounds consisting only of light elements. A few well defined2, purely organic materials (consisting only of the elements carbon, hydrogen, oxygen and nitrogen) have subsequently been found3–5 to exhibit evidence of ferromagnetic interactions at low tem-peratures, with a small number of these showing a transition to a true ferromagnetic state. For example, the p-nitrophenyl nitronyl nitroxide radical has a Curie (transition) temperature of 0.60 K (ref. 6), and a possible ferromagnetic transition has been identified7,8 in a C60-based ionic complex. More recently, it was reported9 that the magnetization and magnetic susceptibility behaviour of a crystalline nitroxide biradical, N,N′-dioxy-l,3,5,7-tetramethyl-2,6-diazaadamantane, are indicative of ferromagnetic interactions. Here we report the existence of a ferromagnetic transition in this system, with a Curie temperature of 1.48 ± 0.02 K. As yet, this is the highest transition temperature found for a purely organic, non-ionic material.
The calibration of two popular density functionals (B3LYP and BP86) for the prediction of isomer shifts (IS) in Fe-57 Mossbauer (MB) spectra is reported. A linear correlation between non-relativistically calculated theoretical electron densities and experimentally measured ISs is established. Both functionals lead to accurate predictions of MB ISs with standard deviations on the order of approximately 0.1 mm s(-1) over a range of approximately 2 mm s(-1). The correlation holds for complexes of different total charge, total spin, valence state at the iron, coordination number and coordination geometry. A detailed analysis of the contributions to the IS is presented. It is shown that the important contributions are: (a),the variation of the 3s-contribution which is mainly caused by differing shielding according to the d-population of the metal; and (b) variations in the valence shell contribution. The latter contribution accounts for approximately 70% of the totally observed variation. The interpretation of the valence contribution is complex and involves contributions from changes in covalencies, orbital distortions due to bonding and shielding effects. The changes in metal-ligand distances are also an important factor, which influences the shape of the 4s- orbital and therefore modulates ISs. (C) 2002 Elsevier Science B.V. All rights reserved.
In the last two decades, various molecule-based magnets have been synthesized, and their chemical and physical properties have been elucidated from microscopic and macroscopic standpoints by use of various experimental methods. Among them, thermodynamic methods are unique in the sense that the energetic and entropic aspects inherent in materials can be directly observed. However, the number of scientific papers concerning thermodynamic research is not so many in comparison to other spectroscopic and structural studies. This paper surveys calorimetric studies on molecule-based magnets published in the last two decades and reveals the important roles played by calorimetric studies. Focus is on organic free radicals, one-dimensional magnets, two-dimensional magnets, three-dimensional magnets, single-molecule magnets, spin crossover phenomena, intramolecular electron transfer in mixed-valence compounds and thermochromic complexes.
A revolution in electronics is in view, with the contemporary evolution of the two novel disciplines of spintronics and molecular electronics. A fundamental link between these two fields can be established using molecular magnetic materials and, in particular, single-molecule magnets. Here, we review the first progress in the resulting field, molecular spintronics, which will enable the manipulation of spin and charges in electronic devices containing one or more molecules. We discuss the advantages over more conventional materials, and the potential applications in information storage and processing. We also outline current challenges in the field, and propose convenient schemes to overcome them.
Below 360 mK, Fe magnetic molecular clusters are in the pure quantum relaxation regime and we show that the predicted square-root time relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measure as a function of applied field H the statistical distribution P(\xi_H) of magnetic energy bias \xi_H$ acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby digging a hole in P(\xi_H) (around the resonant condition \xi_H = 0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins. Comment: 4 pages, 6 figures
Several advances in the coordination chemistry of stable free-radical species over the past six years are documented in this review article. Specifically, a number of recent reports focused on the coordination chemistry of chelating nitroxide ligands are highlighted, with an emphasis on enhanced magnetic or optical properties in these complexes. Furthermore, very intriguing recent magnetic and optical studies with one-dimensional nitroxide chain complexes (new "Glauber" chains and chiral magnets) are also discussed. The verdazyls are another family of stable radicals whose coordination chemistry was literally unexplored prior to 1997. A summary of recent reports discussing metal-verdazyl coordination complexes is also presented, followed by an eye to the future of stable radical design and the coordination chemistry of these interesting molecules.
The unpolarized absorption and circular dichroism spectra of the
fundamental vibrational transitions of the chiral molecule,
4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields
are obtained using Density Functional Theory (DFT), MP2, and SCF
methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use
the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP)
density functionals. Mid-IR spectra predicted using LSDA, BLYP, and
B3LYP force fields are of significantly different quality, the B3LYP
force field yielding spectra in clearly superior, and overall excellent,
agreement with experiment. The MP2 force field yields spectra in
slightly worse agreement with experiment than the B3LYP force field. The
SCF force field yields spectra in poor agreement with experiment.The
basis set dependence of B3LYP force fields is also explored: the 6-31G
and TZ2P basis sets give very similar results while the 3-21G basis set
yields spectra in substantially worse agreements with experiment.
This book provides an assessment of the unique contribution Mössbauer spectroscopy can make to the study of the bonding, structural, magnetic, dynamical and time-dependent properties of various systems. It comprises an edited collection of chapters by an international body of distinguished scientists writing on their individual areas of specialisation. The editors and authors aim to give the reader an in-depth understanding of the technique of Mössbauer spectroscopy in terms of the types of information it can provide, together with an appreciation of how this information can be related to that obtained from complementary techniques. This book will be of value to postgraduate students and research workers with no previous knowledge of the technique, but also to those experienced in the use of Mössbauer spectroscopy in a particular area wishing to gain either a wider overview of the technique or information about its use in another area.
[Fe(C5Me5)2]˙+[TCNE]˙– has been characterized by magnetic susceptibility to possess dominant ferromagnetic interactions; its structure has been determined by X-ray crystallography.
Mössbauer spectra of Garfield nontronite H33a were taken at temperatures between 2·5 and 37 K under longitudinally applied magnetic fields up to 9 T. While no magnetic order was apparent in the absence of an applied field above 7 K, the application of external magnetic fields led to the induction of magnetic hyperfine splitting up to at least 19 K. Variation of the applied fields allowed determination of hyperfine fields of bulk nontronite, and indicated the bulk sample to have a Néel temperature of about 20 K. The non-ideal behaviour of this nontronite, leading to the lack of magnetic order in the absence of applied magnetic fields, is in line with the frustration of antiferromagnetic order in the octahedral sheets due to the presence of about 6% of the iron content in tetrahedral sites, although some influence of magnetic dilution cannot be excluded.
Terbium(III) bis-phthalocyaninato neutral complex, a robust and evaporable Single Molecule Magnet (SMM) with a record height of the anisotropy barrier, has recently attracted a great interest as an active unit in single molecule electronics, but at the same time its magnetic hysteresis has been found to be strongly affected when the environment is different from the crystalline phase. Here we present a systematic investigation of the magnetization dynamics in different environments, obtained by magnetic dilution, thermal treatment and sublimation of the molecules, to shed some light on the origin of the evanescence of the hysteretic behavior of this unique SMM.
The first two families of polyoxometalate-based single-molecule magnets (SMMs) are reported here. Compounds of the general formula [Ln(W(5)O(18))(2)](9-) (Ln(III) = Tb, Dy, Ho, and Er) and [Ln(SiW(11)O(39))(2)](13-) (Ln(III) = Tb, Dy, Ho, Er, Tm, and Yb) have been magnetically characterized with static and dynamic measurements. Slow relaxation of the magnetization, typically associated with SMM-like behavior, was observed for [Ln(W(5)O(18))(2)](9-) (Ln(III) = Ho and Er) and [Ln(SiW(11)O(39))(2)](13-) (Ln(III) = Dy, Ho, Er, and Yb). Among them, only the [Er(W(5)O(18))(2)](9-) derivative exhibited such a behavior above 2 K with an energy barrier for the reversal of the magnetization of 55 K. For a deep understanding of the appearance of slow relaxation of the magnetization in these types of mononuclear complexes, the ligand-field parameters and the splitting of the J ground-state multiplet of the lanthanide ions have been also estimated.
Fe[C(SiMe3)3]2 (1) is prepared by the reaction of FeCl2 with 2 equiv. of [(Me3Si)3C]Li(THF)2. X-ray crystallography revealed that 1 is a linear, monomeric complex with D3d symmetry.
The compound Fe[C(SiMe3)3]2 has been prepared and investigated by the means of powder X-ray diffractometry and 57Fe Mössbauer spectroscopy. The compound’s unique geometry, in which iron is linearly coordinated by the two C(SiMe3)3 ligands, results in a unusual electronic structure of iron, which is visualized as an extreme high hyperfine magnetic field
of 157.5(8) T as sensed by the 57Fe nucleus at T = 20 K. In order to obtain information on the electronic structure of iron and on the bonds to the ligands, DFT (density
functional theory) calculations were carried out on Fe[C(SiMe3)3]2. The high-spin state of iron was found to be energetically favored: an Fe(II) electron configuration of 3d5.83 4s0.72 is predicted, where the 4s electron density is only slightly polarized, and most of the unpaired electrons have 3d character.
By assuming a linear crystal field, and associated 3d level scheme as a starting point, it is suggested that the extreme high
hyperfine magnetic field, observed along with an apparently negative quadrupole splitting, is perpendicular to the C–Fe(II)–C
bond axis, and can be decomposed mainly into contact (B
c ≈ 44 T), dipolar (B
d ≈ 14 T), and orbital (B
L ≈ 99 T) hyperfine magnetic field contributions.
Basefreies Tris(trimethylsilyl)methyl-Lithium, Tsi–Li, reagiert mit den Trihalogeniden der Erdmetalle (MHal3 mit M = Al, Ga, In und Hal = Cl, Br, I) primär unter Bildung der Metallate [Tsi–MHal3]Li. Vor allem mit den schwereren Halogeniden von Ga und In findet aber mit überschüssigem Tsi–Li zusätzlich zur simplen Metathese gleichzeitig auch eine Methylierung statt, die in unterschiedlichem Maße die Mono- und Dimethylverbindungen Tsi–M(Me)Hal (M = Ga, In; Hal = I), Tsi–MMe2 (M = Ga) bzw. das Bis-(trisyl)derivat (Tsi)2InMe sowie stets als Nebenprodukt noch 1,3-Disilacyclobutan ergibt. Durch mehrmalige, fraktionierende Kristallisationen oder Sublimationen konnten Vertreter dieser Verbindungstypen aus den Reaktionsmischungen isoliert und mit spektroskopischen Methoden (¹H, ¹³C, ²⁹Si-NMR; IR, Raman) sowie zum Teil röntgenographisch charakterisiert werden. FeCl3 reagiert mit Tsi–Li (Verhältnis 1 : 3) in Toluol bei 55–60 °C unter Reduktion, wobei rotviolettes Fe(Tsi)2, 1,1,1-Tris(trimethylsilyl)-2-phenylethan und geringe Mengen an Tsi–Cl gebildet werden. Fe(Tsi)2 ist monomer, kristallisiert in der monoklinen Raumgruppe C2/c und hat ein lineares C–Fe–C-Skelett mit Fe–C-Bindungsabständen von 204,5(4) pm.
MAGNETIC materials of mesoscopic dimensions (a few to many thousands of atoms) may exhibit novel and useful properties such as giant magnetostriction, magnetoresistivity and magnetocaloric effects14. Such materials also allow one to study the transition from molecular to bulk-like magnetic behaviour. One approach for preparing mesoscopic magnetic materials is to fragment bulk ferromagnets; a more controllable method is to take a 'bottom-up' approach, using chemistry to grow well defined clusters of metal ions5,6. Lis7 has described a twelve-ion manganese cluster in which eight of the Mn ions are in the +3 oxidation state (spin S=2) and four are in the +4 state (S=3/2). These ions are magnetically coupled to give an S=10 ground state8, giving rise to unusual magnetic relaxation properties8,9. Here we report that the magnetization of the Mn12 cluster is highly anisotropic and that the magnetization relaxation time becomes very long below a temperature of 4 K, giving rise to pronounced hysteresis. This behaviour is not, however, strictly analogous to that of a bulk ferromagnet, in which magnetization hysteresis results from the motion of domain walls. In principle, a bistable magnetic unit of this sort could act as a data storage device.
AMORPHOUS materials have no preferred crystal axes, but it might be thought that shape or strain could define an antiferromagnetic axis in a compound containing a large concentration of magnetic ions with negative nearest-neighbour exchange interactions. We have studied such an amorphous compound, and find an ordered state where spins are coupled with their directions statistically distributed within a very fine particle. This spin structure is quite different from normal antiferromagnetism, although there are some similarities in the magnetic properties.
Fe[C(SiMe3)3]2 compound, in which iron is coordinated by two carbons, was prepared using the reaction of FeCl2 with (Me3Si)3CLi, and investigated by XRD, 57Fe Mössbauer spectroscopy and DFT calculations. 157.5T hyperfine magnetic field was found at the site of the iron nucleus
of this compound at 20K. DFT calculations predict the quintet states to be clearly favored energetically over the lower spin
states. The population analysis reveals considerable 4s as well as large unpaired 3d electron contributions, which can be responsible for the extremely high hyperfine field.
This chapter describes in detail a class of heteroleptic complexes associating cyclopentadienyl (Cp) and dithiolene (dt) ligands,
essentially known with the Cp2M(dt), CpM(dt)2, [CpM(dt)]2 and CpM(dt) stoichiometries. The three first classes are reported with early transition metals (groups 4−7), while the latter
is restricted to group 9 and 10 metal centers. Depending on the metal and the complex oxidation state, paramagnetic species
can be isolated, as for example the formally d
1 species Cp2V(dt), [Cp2Mo(dt)]+ or [CpMo(dt)2], or the formally d
7 CpNi(dt). They all exhibit a rich structural chemistry and variable spin density distribution between Cp, M and dt fragments.
Their magnetic properties in the solid state are a combination of an original spin density distribution and specific intermolecular
interactions in the crystalline phase. They lead to a variety of behaviors, from independent spins with their associated Curie-type
law, singlet-triplet systems and their extension to alternated spin chains and spin ladders, uniform spin chains or ordered
antiferromagnetic ground states. Besides the S• • •S and S• • •M interactions which control most of the antiferromagnetic
interactions observed so far in these magnetic organometallic complexes, the contribution of direct Cp• • •Cp and Cp• • •S
interactions is shown to control in some cases the magnetic behavior of the complexes, an original feature in organometallic
chemistry.
KeywordsDithiolene complexes–Intermolecular interactions–Metallocenes–Molecular magnetism
57Fe electric and magnetic hyperfine parameters were calculated for a series of 10 iron model complexes, covering a wide range of oxidation and spin states. Employing the B3LYP hybrid method, results from nonrelativistic density functional theory (DFT) and quasi-relativistic DFT within the zero-order regular approximation (ZORA) were compared. Electron densities at the iron nuclei were calculated and correlated with experimental isomer shifts. It was shown that the fit parameters do not depend on a specific training set of iron complexes and are, therefore, more universal than might be expected. The nonrelativistic and quasi-relativistic electron densities gave fit parameters of similar quality; the ZORA densities are only shifted by a factor of 1.32, upward in the direction of the four-component Dirac-Fock value. From a correlation of calculated electric field gradients and experimental quadrupole splittings, the value of the 57Fe nuclear quadrupole moment was redetermined to a value of 0.16 barn, in good agreement with other studies. The ZORA approach gave no additional improvement of the calculated quadrupole splittings in comparison to the nonrelativistic approach. The comparison of the calculated and measured 57Fe isotropic hyperfine coupling constants (hfcc's) revealed that both the ZORA approach and the inclusion of spin-orbit contributions lead to better agreement between theory and experiment in comparison to the nonrelativistic results. For all iron complexes with small spin-orbit contributions (high-spin ferric and ferryl systems), a distinct underestimation of the isotropic hfcc's was found. Scaling factors of 1.81 (nonrelativistic DFT) and 1.69 (ZORA) are suggested. The calculated 57Fe isotropic hfcc's of the remaining model systems (low-spin ferric and high-spin ferrous systems) contain 10-50% second-order contributions and were found to be in reasonable agreement with the experimental results. This is assumed to be the consequence of error cancellation because g-tensor calculations for these systems are of poor quality with the existing DFT approaches. Excellent agreement between theory and experiment was found for the 57Fe anisotropic hfcc's. Finally, all of the obtained fit parameters were used for an application study of the [Fe(H2O)6]3+ ion. The calculated spectroscopic data are in good agreement with the Mossbauer and electron paramagnetic resonance results discussed in detail in a forthcoming paper.
Mössbauer spectroscopy is a profound analytical method which has nevertheless continued to develop. The authors now present a state-of-the art book which consists of two parts. The first part details the fundamentals of Mössbauer spectroscopy and is based on a book published in 1978 in the Springer series 'Inorganic Chemistry Concepts' by P. Gitlich, R. Link and A.X. Trautwein. The second part covers useful practical aspects of measurements, and the application of the techniques to many problems of materials characterization. The update includes the use of synchroton radiation and many instructive and illustrative examples in fields such as solid state chemistry, biology and physics, materials and the geosciences, as well as industrial applications. Special chapters on magnetic relaxation phenomena (S. Morup) and computation of hyperfine interaction parameters (F. Neese) are also included. The book concentrates on teaching the technique using theory as much as needed and as little as possible. The reader will learn the fundamentals of the technique and how to apply it to many problems of materials characterization. Transition metal chemistry, studied on the basis of the most widely used Mössbauer isotopes, will be in the foreground.
The experimental and theoretical study of the electron spin dynamics in the anionic form of a
single-ion molecule magnet (SIMM), the bis-phthalocyaninato terbium (III) molecule [Pc2Tb]-[TBA]+, has
been addressed by means of solid state 1H NMR spectroscopy. The magnetic properties of the caged
Tb3+ metal center were investigated in a series of diamagnetically diluted preparations, where the excess
of tetrabutylamonium bromide ([TBA]Br)n salt was used as diamagnetic matrix complement. We found that
a high temperature activated spin dynamics characterizes the systems, which involved phonon-assisted
transitions among the crystal field levels in qualitative agreements with literature results. However, the
activation barriers in these processes range from 641 cm-1 for the diamagnetically diluted samples to 584
cm-1 for those undiluted; thus, they exhibit barriers 2-3 times larger than witnessed in earlier (230 cm-1)
reports (e.g., Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. J. Am. Chem. Soc. 2003, 125,
8694-8695). At cryogenic temperatures, fluctuations are driven by tunneling processes between the m )
+6 and -6 low-energy levels. We found that the barrier Δ and the tunneling rates change from sample to
sample and especially the diamagnetically diluted [Pc2Tb]- molecules appear affected by the sample’s
magneto/thermal history. These observations emphasize that matrix arrangements around [Pc2Tb]- can
appreciably alter the splitting of the crystal field levels, its symmetry, and hence, the spin dynamics.
Therefore, understanding how small differences in molecular surroundings (as for instance occurring by
depositing on surfaces) can trigger substantial modifications in the SIMM property is of utmost importance
for the effective operation of such molecules for single-molecule data storage, for example.
A study was conducted to investigate stable two-coordinate, open-shell (d 1-d 9) transition metal complexes. Investigations revealed that such two-coordinate complexes were gaining popularity due to their magnetic properties. The first structural characterization of a two-coordinate molecular species in the solid state was investigated in 1985 when the synthesis and structure of the dialkyl Mn{C(SiMe 3) 3} 2 were published. The d 1-d 9 electron configurations predicted on the basis of a simple ligand field approach in linear coordination were demonstrated along with their ground states. Investigations of such two-coordinate transition metal hydrides and halides also provided important insights that were applicable to stable two-coordinate complexes.
Transition to a ferromagnetic long-range ordered state was found at 0.60 K in the orthorhombic β-phase crystal of p-nitrophenyl nitronyl nitroxide (or 2-(4′-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-N-oxide in the IUPAC nomenclature). This finding follows our recent discovery of the first organic bulk ferromagnet in the triclinic γ-phase crystal of the same compound.
Magnetic moments: The orientation of the title single-molecule magnet was investigated by magnetic single crystal and luminescence characterization, supported by ab initio calculations, and was found to be governed by the position of the hydrogen atoms of the apical water molecules. This finding suggests that simple magneto-structural correlations can give misleading clues for research in molecular magnetism as well as in the design of MRI contrast agents.
Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
The Ph(4)P(+) salt of the tetrahedral complex [Co(SPh)(4)](2-), possessing an S = (3)/(2) ground state with an axial zero-field splitting of D = -70 cm(-1), displays single-molecule magnet behavior in the absence of an applied magnetic field. At very low temperatures, ac magnetic susceptibility data show the magnetic relaxation time, τ, to be temperature-independent, while above 2.5 K thermally activated Arrhenius behavior is apparent with U(eff) = 21(1) cm(-1) and τ(0) = 1.0(3) × 10(-7) s. Under an applied field of 1 kOe, τ more closely approximates Arrhenius behavior over the entire temperature range. Upon dilution of the complex within a matrix of the isomorphous compound (Ph(4)P)(2)[Zn(SPh)(4)], ac susceptibility data reveal the molecular nature of the slow magnetic relaxation and indicate that the quantum tunneling pathway observed at low temperatures is likely mediated by intermolecular dipolar interactions.
The mononuclear high-spin iron(II) complex 1, coordinated by a pentaisopropylcyclopentadienide and an aryl group (see graphic), has been prepared and characterized by X-ray diffraction analysis. AC magnetic studies have revealed that this species exhibits slow magnetic relaxation and has the characteristics of a single-molecule magnet.
Selective memory: Using actinides in designing molecular nanomagnets could provide better performance and higher anisotropy barriers, owing to the peculiar properties of the 5f electron shell. Neptunocene is found to display an open magnetic hysteresis cycle at low temperatures (see picture), and interaction with the hyperfine degrees of freedom determines whether the magnetic relaxation is fast or slow at a given field value.
A new nitronyl nitroxide bridged one-dimensional lanthanide complex [Tb(3)(hfac)(9)(NIT-2thien)(3)](n) (1) (NIT-2thien = 2-(2'-thienyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) has been successfully prepared. Single crystal X-ray crystallographic analysis reveals that complex 1 consists of linear chains built up by Tb(hfac)(3) units bridged by NIT-2thien radicals through their NO groups; the chains run along the crystallographic a-axis. The magnetic behavior of complex 1 is quite unusual. The complex shows the concomitant existence of slow magnetic relaxation and three-dimensional magnetic ordering.
A model example for size effects on the dynamic susceptibility behavior is provided by the chain compound [{Dy(hfac)(3)NitPhIm(2)}Dy(hfac)(3)] (NitPhIm = 2-[4-(1-imidazole)phenyl]nitronyl nitroxide radical). The Arrhenius plot reveals two relaxation regimes attributed to SMM (Delta = 17.1 K and tau(0) = 17.5 x 10(-6) s) and SCM (Delta = 82.7 K and tau(0) = 8.8 x 10(-8) s) behaviors. The ferromagnetic exchange among the spin carriers has been established for the corresponding Gd derivative.
Slow magnetic relaxation is observed for [(tpa(Mes))Fe](-), a trigonal pyramidal complex of high-spin iron(II), providing the first example of a mononuclear transition metal complex that behaves as a single-molecule magnet. Dc magnetic susceptibility and magnetization measurements reveal a strong uniaxial magnetic anisotropy (D = -39.6 cm(-1)) acting on the S = 2 ground state of the molecule. Ac magnetic susceptibility measurements indicate the absence of slow relaxation under zero applied dc field as a result of quantum tunneling of the magnetization. Application of a 1500 Oe dc field initiates slow magnetic relaxation, which follows a thermally activated tunneling mechanism at high temperature to give an effective spin-reversal barrier of U(eff) = 42 cm(-1) and follows a temperature-independent tunneling mechanism at low temperature. In addition, the magnetic relaxation time shows a pronounced dc-field dependence, with a maximum occurring at approximately 1500 Oe.
Five density functionals including GGA (generalized gradient approximation) (BP86), meta-GGA (TPSS), hybrid meta-GGA (TPSSh), hybrid (B3LYP), and double-hybrid functionals (B2PLYP) were calibrated for the prediction of 57Fe Mössbauer isomer shifts on a set of 20 iron-containing molecules. The influence of scalar relativistic effects and the basis set dependence of the predictions were investigated.
Current gradient-corrected density-functional approximations for the exchange energies of atomic and molecular systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy density. Here we report a gradient-corrected exchange-energy functional with the proper asymptotic limit. Our functional, containing only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of atomic systems with remarkable accuracy, surpassing the performance of previous functionals containing two parameters or more.
Langreth and Mehl (LM) and co-workers have developed a useful spin-density functional for the correlation energy of an electronic system. Here the LM functional is improved in two ways: (1) The natural separation between exchange and correlation is made, so that the density-gradient expansion of each is recovered in the slowly varying limit. (2) Uniform-gas and inhomogeneity effects beyond the randomphase approximation are built in. Numerical results for atoms, positive ions, and surfaces are close to the exact correlation energies, with major improvements over the original LM approximation for the ions and surfaces.
A correlation-energy formula due to Colle and Salvetti [Theor. Chim. Acta 37, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the local kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
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Double-decker phthalocyanine complexes with Tb3+ or Dy3+ showed slow magnetization relaxation as a single-molecular property. The temperature ranges in which the behavior was observed were far higher than that of the transition-metal-cluster single-molecule magnets (SMMs). The significant temperature rise results from a mechanism in the relaxation process different from that in the transition-metal-cluster SMMs. The effective energy barrier for reversal of the magnetic moment is determined by the ligand field around a lanthanide ion, which gives the lowest degenerate substate a large |Jz| value and large energy separations from the rest of the substates in the ground-state multiplets.
Mössbauer spectroscopy and dc magnetization measurements have been used to characterize the low temperature magnetism of a rigorously linear, two-coordinate complex of high-spin Fe(II), Fe[(C(Si(CH3)3]2 (1). The local C-Fe-C chromophore of 1 exhibits novel slow, single-ion paramagnetic relaxation and fully resolved magnetic hyperfine splitting of its zero field Mössbauer spectrum over the range approximately 100 to approximately 50K. The hyperfine field at 4.2 K is 152 T! This is the largest magnetic hyperfine field observed for iron to date regardless of spin, oxidation state, or coordination environment. This observation is attributable to the large unquenched orbital angular momentum corresponding to the degenerate ground (dxy, dx2-y2) orbital pair of 1 in local Dinfinityh symmetry. Maintenance of the ground-state degeneracy is required by the Jahn-Teller theorem leading to the unprecedented result that the magnitude of the magnetic moment of 1's 5Deltag ground state is essentially that of the parent free ion (5D4) ground term.
(Figure Presented) Single-chain magnets: A rational approach to decrease magnetic dipolar interactions led to the chain compound [Dy(hfac) 3{NIT(C6H4OPh)}] (see structure; NIT(R): 2-(4′-R)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), the first rare-earth-radical-based single-chain magnet. The compound shows slow relaxation behavior, a dynamical crossover resulting from finite-size effects, and a very rich static magnetic behavior.
The first family of rare-earth-based single chain magnets is presented. Compounds of general formula [M(hfac)3(NITPhOPh)], where M = Eu, Gd, Tb, Dy, Ho, Er, or Yb, and PhOPh is the nitronyl-nitroxide radical (2,4'-benzoxo-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), have been structurally characterized and found to be isostructural. The characterization of both static and dynamic magnetic properties of the whole family is reported. Dy, Tb, and Ho compounds display slow relaxation of the magnetization, and ac susceptibility shows a thermally activated regime with energy barriers of 69, 45, and 34 K for Dy, Tb, and Ho compounds, respectively, while only a frequency-dependent susceptibility is observed for Er below 2.0 K. In Gd and Yb derivatives, antiferromagnetic interactions dominate. The pre-exponential factors differ by about 4 orders of magnitude. Finite size effects, due to naturally occurring defects, affect the static and dynamic properties of the compounds differently.