Huge Dielectric Response and Molecular Motions in Paddle‐Wheel [Cu^II₂(Adamantylcarboxylate)₄(DMF)₂]⋅(DMF)₂

<jats:title>Abstract</jats:title><jats:p>The temperature‐dependent dynamic properties of [Cu<jats:sup>II</jats:sup><jats:sub>2</jats:sub>(ADCOO)<jats:sub>4</jats:sub>(DMF)<jats:sub>2</jats:sub>]⋅(DMF)<jats:sub>2</jats:sub> (<jats:bold>1</jats:bold>) and [Cu<jats:sup>II</jats:sup><jats:sub>2</jats:sub>(ADCOO)<jats:sub>4</jats:sub>(AcOEt)<jats:sub>2</jats:sub>] (<jats:bold>2</jats:bold>) crystals were examined by X‐ray crystallography, <jats:sup>1</jats:sup>H NMR spectroscopy, and measurements of the dielectric constants and magnetic susceptibilities (ADCOO=adamantane carboxylate, DMF=<jats:italic>N</jats:italic>,<jats:italic>N</jats:italic>‐dimethylformamide, and AcOEt=ethyl acetate). In both crystals, four ADCOO groups bridged a binuclear Cu<jats:sup>II</jats:sup>Cu<jats:sup>II</jats:sup> bond, forming a paddle‐wheel [Cu<jats:sup>II</jats:sup><jats:sub>2</jats:sub>(ADCOO)<jats:sub>4</jats:sub>] structure. The oxygen atoms of two DMF molecules in crystal <jats:bold>1</jats:bold> and two AcOEt molecules in crystal <jats:bold>2</jats:bold> were coordinated at axial positions of the [Cu<jats:sup>II</jats:sup><jats:sub>2</jats:sub>(ADCOO)<jats:sub>4</jats:sub>] moiety, forming [Cu<jats:sup>II</jats:sup><jats:sub>2</jats:sub>(ADCOO)<jats:sub>4</jats:sub>(DMF)<jats:sub>2</jats:sub>] and [Cu<jats:sup>II</jats:sup><jats:sub>2</jats:sub>(ADCOO)<jats:sub>4</jats:sub>(AcOEt)<jats:sub>2</jats:sub>], respectively. Two additional DMF molecules were included in the unit cell of crystal <jats:bold>1</jats:bold>, whereas AcOEt was not included in the unit cell of crystal <jats:bold>2</jats:bold>. The structural analyses of crystal <jats:bold>1</jats:bold> at 300 K showed three‐fold rotation of the adamantyl groups, whereas rotation of the adamantyl groups of crystal <jats:bold>2</jats:bold> at 300 K was not observed. Thermogravimetric measurements of crystal <jats:bold>1</jats:bold> indicated a gradual elimination of DMF upon increasing the temperature above 300 K. The dynamic behavior of the crystallized DMF yielded significant temperature‐dependent dielectric responses in crystal <jats:bold>1</jats:bold>, which showed a huge dielectric peak at 358 K in the heating process. In contrast, only small frequency‐dependent dielectric responses were observed in crystal <jats:bold>2</jats:bold> because of the freezing of the molecular rotation of the adamantyl groups. The magnetic behavior was dominated by the strong antiferromagnetic coupling between the two <jats:italic>S</jats:italic>=1/2 spins of the Cu<jats:sup>II</jats:sup>Cu<jats:sup>II</jats:sup> site, with magnetic exchange energies (<jats:italic>J</jats:italic>) of −265 K (crystal <jats:bold>1</jats:bold>) and −277 K (crystal <jats:bold>2</jats:bold>).</jats:p>