XAS and XMCD Reveal a Cobalt(II) Imide Undergoes High-Pressure-Induced Spin Crossover
Résumé
The spin crossover (SCO) transition is investigated in two molecular Co 2+ bisimide compounds, Co(dpzca) 2 and Co(pypzca) 2 (where Hdpzca = N-(2-pyrazylcarbonyl)-2pyrazinecarboxamide and Hpypzca = N-(2-pyrazylcarbonyl)-2-pyridinecarboxamide). They crystallize solvent-free with similar crystal structures but have been reported to exhibit different temperature-dependent magnetic behaviors. Using temperature-and pressure-dependent element selective X-ray absorption spectroscopy (XAS and XMCD) measurements, it is revealed herein that although Co(pypzca) 2 does not afford a temperature-induced SCO, it undergoes a reversible pressure-induced SCO transition that is less abrupt, and is complete at a higher pressure (3.5 GPa), than for Co(dpzca) 2 (0.5 GPa). Wave function-based calculations performed on isolated complexes confirm the LS state nature of Co(dpzca) 2 at low temperatures, and the values of spin and orbital magnetic moments are determined. Calculations show the similarity of groundstate properties for HS Co(dpzca) 2 and HS Co(pypzca) 2 and the existence of a double-well HS-LS in the potential energy surface for both compounds. It is concluded that the observed, significant differences in pressure-and temperature-induced SCO transitions of Co(dpzca) 2 versus Co(pypzca) 2 are probably due to different intermolecular interactions between Co(dpzca) 2 and Co(pypzca) 2 , which would hamper the temperature-induced SCO in Co(pypzca) 2 .
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