Mihail Atanasov

Slow magnetization dynamics in a series of two-coordinate iron(II) complexes

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)Ar′]2 (3), Fe[N(H)Ar*]2 (4), and Fe(OAr′)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 = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = 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 = 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 D∞h 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.

A theoretical analysis of chemical bonding, vibronic coupling, and magnetic anisotropy in linear iron(II) complexes with single-molecule magnet behavior

The electronic structure and magnetic anisotropy of six complexes of high-spin FeII 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-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)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 FeII nucleus. This has been interpreted as arising from first order angular momentum of the 5Δ ground state of FeII 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 3dz2–4s mixing (in all complexes), (ii) π-bonding anisotropy involving the strong π-donor amide ligands (in 1, 3–4, 6, and 7) and (iii) orbital mixings of the σ–π 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 5Δ (3dxy, 3dx2−y2) ground state. The strengths of the effects increase in the order 1 < 5 < 7 ∼ 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 μB. 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 Ueff 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.

A four-coordinate cobalt(II) single-ion magnet with coercivity and a very high energy barrier

Single-molecule magnets display magnetic bistability of molecular origin, which may one day be exploited in magnetic data storage devices. Recently it was realised that increasing the magnetic moment of polynuclear molecules does not automatically lead to a substantial increase in magnetic bistability. Attention has thus increasingly focussed on ions with large magnetic anisotropies, especially lanthanides. In spite of large effective energy barriers towards relaxation of the magnetic moment, this has so far not led to a big increase in magnetic bistability. Here we present a comprehensive study of a mononuclear, tetrahedrally coordinated cobalt(II) single-molecule magnet, which has a very high effective energy barrier and displays pronounced magnetic bistability. The combined experimental-theoretical approach enables an in-depth understanding of the origin of these favourable properties, which are shown to arise from a strong ligand field in combination with axial distortion. Our findings allow formulation of clear design principles for improved materials.

Detailed Ab Initio First-Principles Study of the Magnetic Anisotropy in a Family of Trigonal Pyramidal Iron(II) Pyrrolide Complexes

A theoretical, computational, and conceptual framework for the interpretation and prediction of the magnetic anisotropy of transition metal complexes with orbitally degenerate or orbitally nearly degenerate ground states is explored. The treatment is based on complete active space self-consistent field (CASSCF) wave functions in conjunction with N-electron valence perturbation theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for treatment of magnetic field- and spin-dependent relativistic effects. The methodology is applied to a series of Fe(II) complexes in ligand fields of almost trigonal pyramidal symmetry as provided by several variants of the tris-pyrrolylmethyl amine ligand (tpa). These systems have recently attracted much attention as mononuclear single-molecule magnet (SMM) complexes. This study aims to establish how the ligand field can be fine tuned in order to maximize the magnetic anisotropy barrier. In trigonal ligand fields high-spin Fe(II) complexes adopt an orbitally degenerate (5)E ground state with strong in-state spin-orbit coupling (SOC). We study the competing effects of SOC and the (5)E⊗ε multimode Jahn-Teller effect as a function of the peripheral substituents on the tpa ligand. These subtle distortions were found to have a significant effect on the magnetic anisotropy. Using a rigorous treatment of all spin multiplets arising from the triplet and quintet states in the d(6) configuration the parameters of the effective spin-Hamiltonian (SH) approach were predicted from first principles. Being based on a nonperturbative approach we investigate under which conditions the SH approach is valid and what terms need to be retained. It is demonstrated that already tiny geometric distortions observed in the crystal structures of four structurally and magnetically well-documented systems, reported recently, i.e., [Fe(tpa(R))](-) (R = tert-butyl, Tbu (1), mesityl, Mes (2), phenyl, Ph (3), and 2,6-difluorophenyl, Dfp (4), are enough to lead to five lowest and thermally accessible spin sublevels described sufficiently well by S = 2 SH provided that it is extended with one fourth order anisotropy term. Using this most elementary parametrization that is consistent with the actual physics, the reported magnetization data for the target systems were reinterpreted and found to be in good agreement with the ab initio results. The multiplet energies from the ab initio calculations have been fitted with remarkable consistency using a ligand field (angular overlap) model (ab initio ligand field, AILFT). This allows for determination of bonding parameters and quantitatively demonstrates the correlation between increasingly negative D values and changes in the σ-bond strength induced by the peripheral ligands. In fact, the sigma-bonding capacity (and hence the Lewis basicity) of the ligand decreases along the series 1 > 2 > 3 > 4.

New insights into the effects of covalency on the ligand field parameters: a DFT study

Abstract A new, non-empirical, DFT based ligand field (LF) model is proposed. The calculation involves two steps: (i) an average of configuration (AOC), with equal occupation of the d-orbitals is carried out, (ii) with these Kohn–Sham orbitals kept frozen, the energies of all single determinants (SD) within the whole LF-manyfold is performed. These energies are then used to estimate all the Racah- and LF-parameters needed in a conventional LF-calculation. The results of this first-principle prediction are in very good agreement with the experimental values. Test calculation of tetrahedral Cr (IV) and Ni (II) complexes are used to validate the new model and to analyze the parameters of the LF.

Combined Ligand Field and Density Functional Theory Analysis of the Magnetic Anisotropy in Oligonuclear Complexes Based on FeIII-CN-MII Exchange-Coupled Pairs

Magnetic anisotropy in cyanide-bridged single-molecule magnets (SMMs) with Fe(III)-CN-M(II) (M = Cu, Ni) exchange-coupled pairs was analyzed using a density functional theory (DFT)-based ligand field model. A pronounced magnetic anisotropy due to exchange was found for linear Fe(III)-CN-M(II) units with fourfold symmetry. This results from spin-orbit coupling of the [Fe(III)(CN)6](3-) unit and was found to be enhanced by a tetragonal field, leading to a (2)E g ground state for Fe(III). In contrast, a trigonal field (e.g., due to tau 2g Jahn-Teller angular distortions) led to a reduction of the magnetic anisotropy. A large enhancement of the anisotropy was found for the Fe(III)-CN-Ni(II) exchange pair if anisotropic exchange combined with a negative zero-field splitting energy of the S = 1 ground state of Ni(II) in tetragonally compressed octahedra, while cancellation of the two anisotropic contributions was predicted for tetragonal elongations. A recently developed DFT approach to Jahn-Teller activity in low-spin hexacyanometalates was used to address the influence of dynamic Jahn-Teller coupling on the magnetic anisotropy. Spin Hamiltonian parameters derived for linear Fe-M subunits were combined using a vector-coupling scheme to yield the spin Hamiltonian for the entire spin cluster. The magnetic properties of published oligonuclear transition-metal complexes with ferromagnetic ground states are discussed qualitatively, and predictive concepts for a systematic search of cyanide-based SMM materials are presented.

The ligand field of the azido ligand: insights into bonding parameters and magnetic anisotropy in a Co(II)-azido complex.

The azido ligand is one of the most investigated ligands in magnetochemistry. Despite its importance, not much is known about the ligand field of the azido ligand and its influence on magnetic anisotropy. Here we present the electronic structure of a novel five-coordinate Co(II)-azido complex (1), which has been characterized experimentally (magnetically and by electronic d-d absorption spectroscopy) and theoretically (by means of multireference electronic structure methods). Static and dynamic magnetic data on 1 have been collected, and the latter demonstrate slow relaxation of the magnetization in an applied external magnetic field of H = 3000 Oe. The zero-field splitting parameters deduced from static susceptibility and magnetizations (D = -10.7 cm(-1), E/D = 0.22) are in excellent agreement with the value of D inferred from an Arrhenius plot of the magnetic relaxation time versus the temperature. Application of the so-called N-electron valence second-order perturbation theory (NEVPT2) resulted in excellent agreement between experimental and computed energies of low-lying d-d transitions. Calculations were performed on 1 and a related four-coordinate Co(II)-azido complex lacking a fifth axial ligand (2). On the basis of these results and contrary to previous suggestions, the N3(-) ligand is shown to behave as a strong σ and π donor. Magnetostructural correlations show a strong increase in the negative D with increasing Lewis basicity (shortening of the Co-N bond distances) of the axial ligand on the N3(-) site. The effect on the change in sign of D in going from four-coordinate Co(II) (positive D) to five-coordinate Co(II) (negative D) is discussed in the light of the bonding scheme derived from ligand field analysis of the ab initio results.

Electronic structures of octahedral Ni(II) complexes with "click" derived triazole ligands: a combined structural, magnetometric, spectroscopic, and theoretical study.

The coordination complexes of Ni(II) with the tripodal ligands tpta (tris[(1-phenyl-1H-1,2,3-triazol-4-yl)methyl]amine), tbta ([(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine), and tdta (tris[(1-(2,6-diisopropyl-phenyl)-1H-1,2,3-triazol-4-yl)methyl]amine) and the bidentate ligand pyta (1-(2,6-diisopropylphenyl)-4-(2-pyridyl)-1,2,3-triazole), [Ni(tpta)2](BF4)2 (1), [Ni(tbta)2](BF4)2 (2), [Ni(tdta)2](BF4)2 (3), and [Ni(pyta)3](BF4)2 (4), were synthesized from Ni(BF4)2·6H2O and the corresponding ligands. Complexes 2 and 4 were also characterized structurally using X-ray diffraction and magnetically via susceptibility measurements. Structural characterization of 2 that contains the potentially tetradentate, tripodal tbta ligand revealed that the Ni(II) center in that complex is in a distorted octahedral environment, being surrounded by two of the tripodal ligands. Each of those ligands coordinate to the Ni(II) center through the central amine nitrogen atom and two of the triazole nitrogen donors; the Ni-N(amine) distances being longer than Ni-N(triazole) distances. In case of 4, three of the bidentate ligands pyta bind to the Ni(II) center with the binding of the triazole nitrogen atoms being stronger than those of the pyridine. Temperature dependent susceptibility measurements on 2 and 4 revealed a room temperature χ(M)T value of 1.18 and 1.20 cm(3) K mol(-1), respectively, indicative of S = 1 systems. High-frequency and -field EPR (HFEPR) measurements were performed on all the complexes to accurately determine their g-tensors and the all-important zero-field splitting (zfs) parameters D and E. Interpretation of the optical d-d absorption spectra using ligand field theory revealed the B and Dq values for these complexes. Quantum chemical calculations based on the X-ray and DFT optimized geometries and their ligand field analysis have been used to characterize the metal-ligand bonding and its influence on the magnitude and sign of the zfs parameters. This is the first time that such extensive HFEPR, LFT, and advanced computational studies are being reported on a series of mononuclear, distorted octahedral Ni(II) complexes containing different kinds of nitrogen donating ligands in the same complex.

Trinuclear {M1}CN{M2}2 Complexes (M1 = CrIII, FeIII, CoIII; M2 = CuII, NiII, MnII). Are Single Molecule Magnets Predictable?

The reaction of the hexacyanometalates K3[M(1)(CN)6] (M(1) = Cr(III), Fe(III), Co(III)) with the bispidine complexes [M(2)(L(1))(X)](n+) and [M(2)(L(2))(X)](n+) (M(2) = Mn(II), Ni(II), Cu(II); L(1) = 3-methyl-9-oxo-2,4-di-(2-pyridyl)-7-(2-pyridylmethyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylic acid dimethyl ester; L(2) = 3-methyl-9-oxo-7-(2-pyridylmethyl)-2,4-di-(2-quinolyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylic acid dimethyl ester; X = anion or solvent) in water-methanol mixtures affords trinuclear complexes with cis- or trans-arrangement of the bispidine-capped divalent metal centers around the hexacyanometalate. X-ray structural analyses of five members of this family of complexes (cis-Fe[CuL(2)]2, trans-Fe[CuL(1)]2, cis-Co[CuL(2)]2, trans-Cr[MnL(1)]2, trans-Fe[MnL(1)]2) and the magnetic data of the entire series are reported. The magnetic data of the cyanide bridged, ferromagnetically coupled cis- and trans-Fe[ML]2 compounds (M = Ni(II), Cu(II)) with S = 3/2 (Cu(II)) and S = 5/2 (Ni(II)) ground states are analyzed with an extended Heisenberg Hamiltonian which accounts for anisotropy and zero-field splitting, and the data of the Cu(II) systems, for which structures are available, are thoroughly analyzed in terms of an orbital-dependent Heisenberg Hamiltonian, in which both spin-orbit coupling and low-symmetry ligand fields are taken into account. It is shown that the absence of single-molecule magnetic behavior in all spin clusters reported here is due to a large angular distortion of the [Fe(CN)6](3-) center and the concomitant quenching of orbital angular momentum of the Fe(III) ((2)T2g) ground state.

Mössbauer spectroscopy as a probe of magnetization dynamics in the linear iron(I) and iron(II) complexes [Fe(C(SiMe3)3)2](1-/0.).

The iron-57 Mössbauer spectra of the linear, two-coordinate complexes, [K(crypt-222)][Fe(C(SiMe3)3)2], 1, and Fe(C(SiMe3)3)2, 2, were measured between 5 and 295 K under zero applied direct current (dc) field. These spectra were analyzed with a relaxation profile that models the relaxation of the hyperfine field associated with the inversion of the iron cation spin. Because of the lifetime of the measurement (10(-8) to 10(-9) s), iron-57 Mössbauer spectroscopy yielded the magnetization dynamics of 1 and 2 on a significantly faster time scale than was previously possible with alternating current (ac) magnetometry. From the modeling of the Mössbauer spectral profiles, Arrhenius plots between 5 and 295 K were obtained for both 1 and 2. The high-temperature regimes revealed Orbach relaxation processes with U(eff) = 246(3) and 178(9) cm(-1) for 1 and 2, respectively, effective relaxation barriers which are in agreement with magnetic measurements and supporting ab initio calculations. In 1, two distinct high-temperature regimes of magnetic relaxation are observed with mechanisms that correspond to two distinct single-excitation Orbach processes within the ground-state spin-orbit coupled manifold of the iron(I) ion. For 2, Mössbauer spectroscopy yields the temperature dependence of the magnetic relaxation in zero applied dc field, a relaxation that could not be observed with zero-field ac magnetometry. The ab initio calculated Mössbauer hyperfine parameters of both 1 and 2 are in excellent agreement with the observed hyperfine parameters.