XAS and XMCD Reveal a Cobalt(II) Imide Undergoes High-Pressure-Induced Spin Crossover br

The spin crossover (SCO) transition is investigated in two molecular Co2+bis-imide compounds, Co(dpzca)2and Co(pypzca)2(where Hdpzca=N-(2-pyrazylcarbonyl)-2-pyrazinecarboxamide 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 elements elective X-ray absorption spectroscopy (XAS and XMCD) measurements, it is revealed hereinthat although Co(pypzca)2does not afford a temperature-induced SCO, it undergoes a reversiblepressure-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 onisolated complexes confirm the LS state nature of Co(dpzca)2at low temperatures, and the valuesof spin and orbital magnetic moments are determined. Calculations show the similarity of ground-state properties for HS Co(dpzca)2and HS Co(pypzca)2and the existence of a double-well HS-LSin 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)2versusCo(pypzca)2are probably due to different intermolecular interactions between Co(dpzca)2andCo(pypzca)2, which would hamper the temperature-induced SCO in Co(pypzca)2