Article

Ferromagnetic Coupling in an Fe­[C(SiMe3)3]2/Ferrihydrite Hetero-Mixture Molecular Magnet

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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.

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... 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). ...
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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]. ...
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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.
Chapter
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.
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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.
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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.
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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.
Article
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.
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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.
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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.
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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.
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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.
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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.
Article
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.
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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.
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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.
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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.
Article
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.
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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.
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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.
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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.
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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.
Article
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.
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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.
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(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.
Article
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.