Single-Ion Magnetic Anisotropy and Isotropic Magnetic Couplings in the Metal–Organic Framework Fe₂(dobdc)

The metal–organic framework Fe₂(dobdc) (dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate), often referred to as Fe-MOF-74, possesses many interesting properties such as a high selectivity in olefin/paraffin separations. This compound contains open-shell Fe^II ions with open coordination sites which may have large single-ion magnetic anisotropies, as well as isotropic couplings between the nearest and next nearest neighbor magnetic sites. To complement a previous analysis of experimental data made by considering only isotropic couplings [Bloch et al. <i>Science</i> <b>2012</b>, <i>335</i>, 1606], the magnitude of the main magnetic interactions are here assessed with quantum chemical calculations performed on a finite size cluster. It is shown that the single-ion anisotropy is governed by same-spin spin–orbit interactions (i.e., weak crystal-field regime), and that this effect is not negligible compared to the nearest neighbor isotropic couplings. Additional magnetic data reveal a metamagnetic behavior at low temperature. This effect can be attributed to various microscopic interactions, and the most probable scenarios are discussed

Thorium and Uranium Carbide Cluster Cations in the Gas Phase: Similarities and Differences between Thorium and Uranium

Laser ionization of AnC₄ alloys (An = Th, U) yielded gas-phase molecular thorium and uranium carbide cluster cations of composition An_<i>m</i>C_<i>n</i>⁺, with <i>m</i> = 1, <i>n</i> = 2–14, and <i>m</i> = 2, <i>n</i> = 3–18, as detected by Fourier transform ion-cyclotron-resonance mass spectrometry. In the case of thorium, Th_<i>m</i>C_<i>n</i>⁺ cluster ions with <i>m</i> = 3–13 and <i>n</i> = 5–30 were also produced, with an intriguing high intensity of Th₁₃C_<i>n</i>⁺ cations. The AnC₁₃⁺ ions also exhibited an unexpectedly high abundance, in contrast to the gradual decrease in the intensity of other AnC_<i>n</i>⁺ ions with increasing values of <i>n</i>. High abundances of AnC₂⁺ and AnC₄⁺ ions are consistent with enhanced stability due to strong metal–C₂ bonds. Among the most abundant bimetallic ions was Th₂C₃⁺ for thorium; in contrast, U₂C₄⁺ was the most intense bimetallic for uranium, with essentially no U₂C₃⁺ appearing. Density functional theory computations were performed to illuminate this distinction between thorium and uranium. The computational results revealed structural and energetic disparities for the An₂C₃⁺ and An₂C₄⁺ cluster ions, which elucidate the observed differing abundances of the bimetallic carbide ions. Particularly noteworthy is that the Th atoms are essentially equivalent in Th₂C₃⁺, whereas there is a large asymmetry between the U atoms in U₂C₃⁺

CO₂ Adsorption in Fe₂(dobdc): A Classical Force Field Parameterized from Quantum Mechanical Calculations

Carbon dioxide adsorption isotherms have been computed for the metal–organic framework (MOF) Fe₂(dobdc), where dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate. A force field derived from quantum mechanical calculations has been used to model adsorption isotherms within a MOF. Restricted open-shell Møller–Plesset second-order perturbation theory (ROMP2) calculations have been performed to obtain interaction energy curves between a CO₂ molecule and a cluster model of Fe₂(dobdc). The force field parameters have been optimized to best reproduced these curves and used in Monte Carlo simulations to obtain CO₂ adsorption isotherms. The experimental loading of CO₂ adsorbed within Fe₂(dobdc) was reproduced quite accurately. This parametrization scheme could easily be utilized to predict isotherms of various guests inside this and other similar MOFs not yet synthesized

Role of the Metal in the Bonding and Properties of Bimetallic Complexes Involving Manganese, Iron, and Cobalt

A multidentate ligand platform is introduced that enables the isolation of both homo- and heterobimetallic complexes of divalent first-row transition metal ions such as Mn­(II), Fe­(II), and Co­(II). By means of a two-step metalation strategy, five bimetallic coordination complexes were synthesized with the general formula M₁M₂Cl­(py₃tren), where py₃tren is the triply deprotonated form of <i>N</i>,<i>N</i>,<i>N</i>-tris­(2-(2-pyridylamino)­ethyl)­amine. The metal–metal pairings include dicobalt (<b>1</b>), cobalt–iron (<b>2</b>), cobalt–manganese (<b>3</b>), diiron (<b>4</b>), and iron–manganese (<b>5</b>). The bimetallic complexes have been investigated by X-ray diffraction and X-ray anomalous scattering studies, cyclic voltammetry, magnetometry, Mössbauer spectroscopy, UV–vis–NIR spectroscopy, NMR spectroscopy, combustion analyses, inductively coupled plasma optical emission spectrometry, and ab initio quantum chemical methods. Only the diiron chloride complex in this series contains a metal–metal single bond (2.29 Å). The others show weak metal–metal interactions (2.49 to 2.53 Å). The diiron complex is also distinct with a septet ground state, while the other bimetallic species have much lower spin states from <i>S</i> = 0 to <i>S</i> = 1. We propose that the diiron system has delocalized metal–metal bonding electrons, which seems to correlate with a short metal–metal bond and a higher spin state. Multiconfigurational wave function calculations revealed that, indeed, the metal–metal bonding orbitals in the diiron complex are much more delocalized than those of the dicobalt analogue

Giant Ising-Type Magnetic Anisotropy in Trigonal Bipyramidal Ni(II) Complexes: Experiment and Theory

This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni­(II) complexes, [Ni­(Me₆tren)­Cl]­(ClO₄) (<b>1</b>) and [Ni­(Me₆tren)­Br]­(Br) (<b>2</b>). High-field, high-frequency electron paramagnetic resonance spectroscopy performed on a single crystal of <b>1</b> shows a giant uniaxial magnetic anisotropy with an experimental <i>D</i>_expt value (energy difference between the <i>M</i>_s = ± 1 and <i>M</i>_s = 0 components of the ground spin state S = 1) estimated to be between −120 and −180 cm^–1. The theoretical study shows that, for an ideally trigonal Ni­(II) complex, the orbital degeneracy leads to a first-order spin–orbit coupling that results in a splitting of the <i>M</i>_s = ± 1 and <i>M</i>_s = 0 components of approximately −600 cm^–1. Despite the Jahn–Teller distortion that removes the ground term degeneracy and reduces the effects of the first-order spin–orbit interaction, the <i>D</i> value remains very large. A good agreement between theoretical and experimental results (theoretical <i>D</i>_theor between −100 and −200 cm^–1) is obtained