2 research outputs found
Ligand Effect on the Single-Molecule Magnetism of Tetranuclear Co(II) Cubane
A clear dependence
on the ligand has been observed for the magnetic properties of a closely
related series of Co(II) cubane structures, viz. [Co<sub>4</sub>(<i>mbm</i> or <i>bm</i>)<sub>4</sub>(ROH)<sub>4</sub>Br<sub>4</sub>] (<b>1-MeOH</b>, <b>1-EtOH</b>, <b>2-MeOH</b>, and <b>2-EtOH</b>, where <b>1</b> = [Co<sub>4</sub>(<i>mbm</i>)<sub>4</sub>Br<sub>4</sub>], <b>2</b> = [Co<sub>4</sub>(<i>bm</i>)<sub>4</sub>Br<sub>4</sub>], <i>bm</i> = (1<i>H</i>-benzo[<i>d</i>]imidazol-2-yl)methanolate. and <i>mbm</i> = 1-Me-<i>bm</i>.) The [Co<sub>4</sub>(OR)<sub>4</sub>] cubane core consists
of an octahedral Co<sup>II</sup> center chelated by the alkoxide oxygen
and imidazole nitrogen atoms from monoanionic <i>bm</i> or <i>mbm</i> and coordinated by methanol/alcohol and bromine. Interestingly,
electrospray ionization mass spectrometry (ESI-MS) indicates that <b>1-MeOH</b> and <b>2-MeOH</b> are unstable in methanol and
transformed to the butterfly [Co<sub>4</sub>L<sub>6</sub>]<sup>2+</sup> but that <b>1-EtOH</b> and <b>2-EtOH</b> are stable
in ethanol. Their magnetic susceptibilities suggest ferromagnetic
coupling between the nearest cobalt centers to give a theoretical <i>S</i> = 4 × 3/2 ground state with considerable magneto-crystalline
behavior. The packing and intermolecular interactions appear to influence
the geometry of the cubes and thus the anisotropy of cobalt, which
leads to different blocking temperatures (<i>T</i><sub>B</sub>). Consequently, the compounds with <i>mbm</i>, <b>1-MeOH</b> and <b>1-EtOH</b>, exhibit <i>T</i><sub>B</sub> >
2 K as shown by the relaxation of magnetization in zero applied dc
field where the barriers <i>U</i><sub>eff</sub>/<i>k</i><sub>B</sub> are respectively 27 and 21 K and relaxation
times are τ<sub>0</sub> = 1.3 × 10<sup>–9</sup> and
9.7 × 10<sup>–9</sup> s. However, the compounds with <i>bm</i>, <b>2-MeOH</b> and <b>2-EtOH</b>, remain
paramagnetic above 2 K and do not show nonlinear response of the ac
susceptibilities. These findings reaffirm the subtle dependence of
single-molecule magnetism on coordination geometry and intermolecular
interaction
Hierarchical Assembly and Aggregation-Induced Enhanced Emission of a Pair of Isostructural Zn<sub>14</sub> Clusters
Information of solid-state and solution
structures is crucial in the characterization of molecular clusters
and in advancing the understanding of their diverse properties. [Et<sub>3</sub>NH]<sub>2</sub>[Zn<sub>14</sub>(hmq)<sub>8</sub>(OH)<sub>4</sub>X<sub>10</sub>] [X = Cl and Br; H<sub>2</sub>hmq = 2-(hydroxymethyl)quinolin-8-ol]
consist of a peanut-shaped Zn<sub>10</sub>O<sub>12</sub> core, in
which the Zn atoms occupy the faces and corners of an octahedron and
are protected by bonded halogen atoms and bulky organic ligands. Observation
of the [Zn<sub>14</sub>(hmq)<sub>8</sub>(OH)<sub>4</sub>X<sub>10</sub>]<sup>2–</sup> fragment in electrospray ionization mass spectrometry
(ESI-MS) suggests that the cluster is stable in solution. ESI-MS analyses
from dissolved crystals and mother liquors reveal that Zn(hmq) self-assembles
to Zn<sub>5</sub>(hmq)<sub>4</sub>Cl, then dimerizes through four
[OH]<sup>−</sup> bridges to Zn<sub>10</sub>(hmq)<sub>8</sub>(OH)<sub>4</sub>Cl<sub>2</sub>, and progressively captures four ZnCl<sub>2</sub> one-by-one to [Zn<sub>14</sub>(hmq)<sub>8</sub>(OH)<sub>4</sub>Cl<sub>10</sub>]<sup>2–</sup>. Because the supramolecular
interactions between the anion and cation in the solid suppress the
rotation/vibration of the halogen atoms and confine the movable organic
ligands on the rigid Zn–O core, both crystal phases exhibit
intense photoluminescence, much stronger than that in solution. This
is the first coordination cluster to exhibit “aggregation-induced
enhanced emission”. In addition, preliminary tests indicate
that these coordination clusters are promising for organic-light-emitting-diode
applications