5 research outputs found
Single-Ion Magnetic Anisotropy and Isotropic Magnetic Couplings in the MetalāOrganic Framework Fe<sub>2</sub>(dobdc)
The metalāorganic framework
Fe<sub>2</sub>(dobdc) (dobdc<sup>4ā</sup> = 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<sup>II</sup> 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
spināorbit 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 AnC<sub>4</sub> alloys (An = Th, U) yielded gas-phase molecular thorium and uranium
carbide cluster cations of composition An<sub><i>m</i></sub>C<sub><i>n</i></sub><sup>+</sup>, with <i>m</i> = 1, <i>n</i> = 2ā14, and <i>m</i> =
2, <i>n</i> = 3ā18, as detected by Fourier transform
ion-cyclotron-resonance mass spectrometry. In the case of thorium,
Th<sub><i>m</i></sub>C<sub><i>n</i></sub><sup>+</sup> cluster ions with <i>m</i> = 3ā13 and <i>n</i> = 5ā30 were also produced, with an intriguing high
intensity of Th<sub>13</sub>C<sub><i>n</i></sub><sup>+</sup> cations. The AnC<sub>13</sub><sup>+</sup> ions also exhibited an
unexpectedly high abundance, in contrast to the gradual decrease in
the intensity of other AnC<sub><i>n</i></sub><sup>+</sup> ions with increasing values of <i>n</i>. High abundances
of AnC<sub>2</sub><sup>+</sup> and AnC<sub>4</sub><sup>+</sup> ions
are consistent with enhanced stability due to strong metalāC<sub>2</sub> bonds. Among the most abundant bimetallic ions was Th<sub>2</sub>C<sub>3</sub><sup>+</sup> for thorium; in contrast, U<sub>2</sub>C<sub>4</sub><sup>+</sup> was the most intense bimetallic
for uranium, with essentially no U<sub>2</sub>C<sub>3</sub><sup>+</sup> 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 An<sub>2</sub>C<sub>3</sub><sup>+</sup> and An<sub>2</sub>C<sub>4</sub><sup>+</sup> cluster ions, which elucidate the observed differing abundances
of the bimetallic carbide ions. Particularly noteworthy is that the
Th atoms are essentially equivalent in Th<sub>2</sub>C<sub>3</sub><sup>+</sup>, whereas there is a large asymmetry between the U atoms
in U<sub>2</sub>C<sub>3</sub><sup>+</sup>
CO<sub>2</sub> Adsorption in Fe<sub>2</sub>(dobdc): A Classical Force Field Parameterized from Quantum Mechanical Calculations
Carbon dioxide adsorption isotherms
have been computed for the metalāorganic framework (MOF) Fe<sub>2</sub>(dobdc), where dobdc<sup>4ā</sup> = 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
MĆøllerāPlesset second-order perturbation theory (ROMP2)
calculations have been performed to obtain interaction energy curves
between a CO<sub>2</sub> molecule and a cluster model of Fe<sub>2</sub>(dobdc). The force field parameters have been optimized to best reproduced
these curves and used in Monte Carlo simulations to obtain CO<sub>2</sub> adsorption isotherms. The experimental loading of CO<sub>2</sub> adsorbed within Fe<sub>2</sub>(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 MnĀ(II), FeĀ(II), and CoĀ(II). By means
of a two-step metalation strategy, five bimetallic coordination complexes
were synthesized with the general formula M<sub>1</sub>M<sub>2</sub>ClĀ(py<sub>3</sub>tren), where py<sub>3</sub>tren is the triply deprotonated
form of <i>N</i>,<i>N</i>,<i>N</i>-trisĀ(2-(2-pyridylamino)Āethyl)Āamine.
The metalāmetal pairings include dicobalt (<b>1</b>),
cobaltāiron (<b>2</b>), cobaltāmanganese (<b>3</b>), diiron (<b>4</b>), and ironāmanganese (<b>5</b>). The bimetallic complexes have been investigated by X-ray
diffraction and X-ray anomalous scattering studies, cyclic voltammetry,
magnetometry, MoĢssbauer spectroscopy, UVāvisāNIR
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 metalāmetal single bond (2.29 Ć
). The others show weak
metalāmetal interactions (2.49 to 2.53 Ć
). 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 metalāmetal bonding electrons, which seems
to correlate with a short metalāmetal bond and a higher spin
state. Multiconfigurational wave function calculations revealed that,
indeed, the metalāmetal 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 NiĀ(II) complexes, [NiĀ(Me<sub>6</sub>tren)ĀCl]Ā(ClO<sub>4</sub>) (<b>1</b>) and [NiĀ(Me<sub>6</sub>tren)ĀBr]Ā(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><sub>expt</sub> value (energy difference between the <i>M</i><sub>s</sub> = Ā± 1 and <i>M</i><sub>s</sub> = 0 components of the ground spin state S = 1) estimated to be between
ā120 and ā180 cm<sup>ā1</sup>. The theoretical
study shows that, for an ideally trigonal NiĀ(II) complex, the orbital
degeneracy leads to a first-order spināorbit coupling that
results in a splitting of the <i>M</i><sub>s</sub> = Ā±
1 and <i>M</i><sub>s</sub> = 0 components of approximately
ā600 cm<sup>ā1</sup>. Despite the JahnāTeller
distortion that removes the ground term degeneracy and reduces the
effects of the first-order spināorbit interaction, the <i>D</i> value remains very large. A good agreement between theoretical
and experimental results (theoretical <i>D</i><sub>theor</sub> between ā100 and ā200 cm<sup>ā1</sup>) is obtained