Molecular Magnetic Materials Based on {CoIII (Tp*)(CN)3 }- Cyanidometallate: Combined Magnetic, Structural and 59 Co NMR Study.

The cyanidocobaltate of formula fac-PPh4 [CoIII (Me2 Tp)(CN)3 ] ⋅ CH3 CN (1) has been used as a metalloligand to prepare polynuclear magnetic complexes (Me2 Tp=hydrotris(3,5-dimethylpyrazol-1-yl)borate). The association of 1 with in situ prepared [FeII (bik)2 (MeCN)2 ](OTf)2 (bik=bis(1-methylimidazol-2-yl)ketone) leads to a molecular square of formula {[CoIII {(Me2 Tp)}(CN)3 ]2 [FeII (bik)2 ]2 }(OTf)2 ⋅ 4MeCN ⋅ 2H2 O (2), whereas the self-assembly of 1 with preformed cluster [CoII 2 (OH2 )(piv)4 (Hpiv)4 ] in MeCN leads to the two-dimensional network of formula {[CoII 2 (piv)3 ]2 [CoIII (Me2 Tp)(CN)3 ]2 ⋅ 2CH3 CN}∞ (3). These compounds were structurally characterized via single crystal X-ray analysis and their spectroscopic (FTIR, UV-Vis and 59 Co NMR) properties and magnetic behaviours were also investigated. Bulk magnetic susceptibility measurements reveal that 1 is diamagnetic and 3 is paramagnetic throughout the explored temperature range, whereas 2 exhibits sharp spin transition centered at ca. 292 K. Compound 2 also exhibits photomagnetic effects at low temperature, selective light irradiations allowing to promote reversibly and repeatedly low-spin⇔high-spin conversion. Besides, the diamagnetic nature of the Co(III) building block allows us studying these compounds by means of 59 Co NMR spectroscopy. Herein, a 59 Co chemical shift has been used as a magnetic probe to corroborate experimental magnetic data obtained from bulk magnetic susceptibility measurements. An influence of the magnetic state of the neighbouring atoms is observed on the 59 Co NMR signals. Moreover, for the very first time, 59 Co NMR technique has been successfully introduced to investigate molecular materials with distinct magnetic properties.

Pressure-Induced Conversion of a Paramagnetic FeCo Complex into a Molecular Magnetic Switch with Tuneable Hysteresis.

A key challenge in the design of magnetic molecular switches is to obtain bistability at room temperature. Here, we show that application of moderate pressure makes it possible to convert a paramagnetic FeIII 2 CoII 2 square complex into a molecular switch exhibiting a full dia- to paramagnetic transition: FeII CoIII ⇔ FeIII CoII . Moreover, the complex follows a rare behavior: the higher the pressure, the broader the magnetic hysteresis. Thus, the application of an adequate pressure allows inducing a magnetic bistability at room temperature with predictable hysteresis width. The structural studies at different pressures suggest that the pressure-enhanced bistability is due to the strengthening of intermolecular interactions upon pressure increase. An original microscopic Ising-like model including pressure effects is developed to simulate this unprecedented behavior. Overall, this study shows that FeCo complexes could be very sensitive piezo switches with potential use as sensors.