Large spin differences in structurally related Fe6 molecular clusters and their magnetostructural explanation

Spin relaxation in antiferromagnetic Fe-Fe dimers slowed down by anisotropic Dy(III) ions

By using Mössbauer spectroscopy in combination with susceptibility measurements it was possible to identify the supertransferred hyperfine field through the oxygen bridges between Dy(III) and Fe(III) in a {Fe4Dy2} coordination cluster. The presence of the dysprosium ions provides enough magnetic anisotropy to "block" the hyperfine field that is experienced by the iron nuclei. This has resulted in magnetic spectra with internal hyperfine fields of the iron nuclei of about 23 T. The set of data permitted us to conclude that the direction of the anisotropy in lanthanide nanosize molecular clusters is associated with the single ion and crystal field contributions and (57)Fe Mössbauer spectroscopy may be informative with regard to the the anisotropy not only of the studied isotope, but also of elements interacting with this isotope.

Linking [M(III)3] triangles with "double-headed" phenolic oximes

Strapping two salicylaldoxime units together with aliphatic α,Ω-aminomethyl links in the 3-position gives ligands which allow the assembly of the polynuclear complexes [Fe(7)O(2)(OH)(6)(H(2)L1)(3)(py)(6)](BF(4))(5)·6H(2)O·14MeOH (1·6H(2)O·14MeOH), [Fe(6)O(OH)(7)(H(2)L2)(3)](BF(4))(3)·4H(2)O·9MeOH (2·4H(2)O·9MeOH) and [Mn(6)O(2)(OH)(2)(H(2)L1)(3)(py)(4)(MeCN)(2)](BF(4))(5)(NO(3))·3MeCN·H(2)O·5py (3·3MeCN·H(2)O·5py). In each case the metallic skeleton of the cluster is based on a trigonal prism in which two [M(III)(3)O] triangles are tethered together via three helically twisted double-headed oximes. The latter are present as H(2)L(2-) in which the oximic and phenolic O-atoms are deprotonated and the amino N-atoms protonated, with the oxime moieties bridging across the edges of the metal triangles. Both the identity of the metal ion and the length of the straps connecting the salicylaldoxime units have a major impact on the nuclearity and topology of the resultant cluster, with, perhaps counter-intuitively, the longer straps producing the "smallest" molecules.

Complete vs Restricted Active Space Perturbation Theory Calculation of the Cr2 Potential Energy Surface

In this paper, we calculate the potential energy surface (PES) and the spectroscopic constants of the chromium dimer using the recently developed restricted active space second-order perturbation (RASPT2) method. This approach is benchmarked against available experimental measurements and the complete active space second-order perturbation theory (CASPT2), which is nowadays established as one of the most accurate theoretical models available. Dissociation energies, vibrational frequencies, and bond distances are computed at the RASPT2 level using several reference spaces. The major advantage of the RASPT2 method is that with a limited number of configuration state functions, it can reproduce well the equilibrium bond length and the vibrational frequency of the Cr dimer. On the other hand, the PES is well described only at short distances, while at large distances, it compares very poorly with the CASPT2. The dissociation energy is also ill-behaved, but its value can be largely improved using a simple workaround that we explain in the text. In the paper, we also address the effect of the Ionization Potential Electron Affinity (IPEA) shift (a parameter introduced in the zeroth-order Hamiltonian in the CASPT2 method to include the effect of two-electron terms) and show how its default value of 0.25 is not suitable for a proper description of the PES and of the spectroscopic parameters and must be changed to a more sound value of 0.45.

Polynuclear cation-cation complexes of pentavalent uranyl: relating stability and magnetic properties to structure

Reaction of {[UO2Pys][mu-KI2Py2]}n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl ([UO2(dbm),2]2[mu-K(Py)2]2[mu8-K(Py)]}2I2 x Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[mu-K(MeCN)2][mu8-K]}2 (3) (in 40% yield) in which four UO2+ are mutually coordinated (T-shaped "cation-cation" interaction). The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO2+...K+ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO2(dbm)2K(18C6)]2 (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield. 1H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO2+...UO2+ and UO2+...K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)4] and [UO2(dbm)2] species. The UO2+...K+ interaction plays an important synergistic role in the stabilization of the UO2+...UO2+ interactions. Accordingly, the lower affinity of (K(18C6))+ for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K+ and therefore in a weakened UP2+...UO2+ interaction. The UO24+...UO2+ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in chi versus T at approximately 5 K. The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups.

Molecular magnets containing wheel motifs

An overview of some compounds with structures that can be regarded as derived from an {Fe(7)(mu(3)-OR)(6)(mu-OR)(6)} motif is presented. Many of these compounds act as single-molecule magnets. In addition, the results of a comprehensive study on the single-crystal structures, magnetism, and spin structures for five new iron compounds with structural components related to this are given. This serves to illustrate the methodology that can be used to unravel the complicated magnetic behavior of such systems. Thus, the alkoxo- and phenoxo-bridged multinuclear iron complexes, [Fe(II)(7)(Hbmsae)(6)(OMe)(6)]Cl(2) x 6 H(2)O (6), [Fe(II)Fe(III)(6)(bmsae)(6)(mu(3)-OMe)(6)]Cl(2) x 23 H(2)O (7), [NaFe(III)(6)(bmsae)(6)(mu(3)-OMe)(6)]Cl x 30 H(2)O (8), [Fe(III)(3)(bmsae)(3)Cl(2)(MeOH)(OMe)] x MeOH x Et(2)O (9), and [Fe(III)(2)(Hbmsae)(2)(OMe)(2)Cl(2)][Fe(3)(bmsae)(3)(OMe)Cl(2)(MeOH)](2) x 1.5 MeOH (10), were prepared by the reactions of iron sources with Schiff base ligands (H(2)bmsae = 5-bromo-3-methoxysalicylideneaminoethanol). Heptanuclear complexes of 6 and 7 have wheel structures in which six iron(II) and six iron(III) ions locate on the rim with the central iron(II) ions, respectively, while 8 also has a wheel structure with a diamagnetic sodium ion in the center. 9 is a trinuclear ferric complex with an incomplete cubic structure. In 10, di- and trinuclear complexes exist in the crystal, where the trinuclear complex has an incomplete cubic structure similar to that of 9. Magnetic susceptibility measurements revealed that 6 has an S = 10 spin ground state, while the antiferromagnetic interactions dominate in 7-10.

Comparison of Semiempirical ZILSH and DFT Calculations of Exchange Constants in Fe4 Butterfly Complexes

Magnetic interactions in a series of tetranuclear Fe(3+) complexes with the butterfly core structure have been studied with semiempirical ZILSH and density functional theory (DFT) calculations (B3LYP functional). A theoretical analysis of a previously used method of estimating exchange constants from a restricted number of spin configurations reveals systematic errors arising from asymmetry in the complexes, which cause large variations in results with different choices of spin configurations. Correction factors are derived that yield the correct results obtained from full configuration space (FCS) calculations. Exchange constants obtained from DFT FCS calculations for the "body-body" interaction were large and ferromagnetic, in disagreement with values obtained from empirical fits of magnetic susceptibility data for the complexes, established magnetostructural correlations in polynuclear Fe(3+) complexes, and ZILSH calculations. DFT calculations also gave unreasonably large antiferromagnetic exchange constants for interaction between "wingtip" ions that are not directly bridged, again in disagreement with ZILSH calculations. Estimates of exchange constants for interaction of body and wingtip ions obtained with ZILSH and DFT were similar, with the ZILSH values in slightly better agreement with empirical fits. Considering all interactions, the ZILSH method provides results in better accord with experiment than DFT for these complexes. Additional comparisons of exchange constants obtained with different spin coupling schemes showed that values appropriate for two-center spin eigenfunctions gave consistently better results than values calculated with the local spin operator. The effect of basis set was found to be very small. A brief analysis of these findings is given.

A Three-Dimensional Hydrogen-Bonded Network of Triiron(III) and Dilanthanum(III) Pivalate Complexes

A cationic tri-nuclear iron(iii) complex and an anionic di-nuclear lanthanum(iii) complex assemble to form a three-dimensional hydrogen-bonded supramolecular 3d-4f network [Fe3(μ3-O)(μ-O2CtBu)6(im)3]2[La2(μ-O2CtBu)4(η-O2CtBu)4]·2C2H5OH (im = imidazole).