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. [Co₄(<i>mbm</i> or <i>bm</i>)₄(ROH)₄Br₄] (<b>1-MeOH</b>, <b>1-EtOH</b>, <b>2-MeOH</b>, and <b>2-EtOH</b>, where <b>1</b> = [Co₄(<i>mbm</i>)₄Br₄], <b>2</b> = [Co₄(<i>bm</i>)₄Br₄], <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 [Co₄(OR)₄] cubane core consists of an octahedral Co^II 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 [Co₄L₆]²⁺ 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>_B). Consequently, the compounds with <i>mbm</i>, <b>1-MeOH</b> and <b>1-EtOH</b>, exhibit <i>T</i>_B > 2 K as shown by the relaxation of magnetization in zero applied dc field where the barriers <i>U</i>_eff/<i>k</i>_B are respectively 27 and 21 K and relaxation times are τ₀ = 1.3 × 10^–9 and 9.7 × 10^–9 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 Zn₁₄ 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. [Et₃NH]₂[Zn₁₄(hmq)₈(OH)₄X₁₀] [X = Cl and Br; H₂hmq = 2-(hydroxymethyl)­quinolin-8-ol] consist of a peanut-shaped Zn₁₀O₁₂ 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 [Zn₁₄(hmq)₈(OH)₄X₁₀]^2– 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 Zn₅(hmq)₄Cl, then dimerizes through four [OH]⁻ bridges to Zn₁₀(hmq)₈(OH)₄Cl₂, and progressively captures four ZnCl₂ one-by-one to [Zn₁₄(hmq)₈(OH)₄Cl₁₀]^2–. 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