Deciphering the Unusual Pressure-Induced Electron Transfer in the Molecular Switch {[Fe(Tp)(CN)₃]₂[Co(vbik)₂]₂}·(BF₄)₂·2MeOH

Cyanide-bridged FeCo coordination clusters featuring reversible electron transfer are appealing molecular switches whose physical properties can be controlled by a variety of external stimuli. In this work, we reveal the origins of the atypical pressure-induced electron transfer-coupled transition (ETCST) in the compound {[Fe(Tp)(CN)3]2[Co(vbik)2]2}·(BF4)2·2MeOH (1), which is characterized by (i) an efficient and complete transition of the diamagnetic FeIILSCoIIILS pairs into paramagnetic FeIIILSCoIIHS pairs at moderate pressure and (ii) the occurrence of a p-enhanced cooperativity. Compound 1 which appears paramagnetic in the 2–400 K temperature range at ambient pressure is actually shown to be kinetically trapped in the FeIII–CN–CoII state. Magnetic measurements at a low scan rate (0.01 K/min) actually reveal an inflection point near 140 K and a transition toward the diamagnetic state on cooling. The pressure-induced ETCST of 1 at room temperature was probed by synchrotron X-ray diffraction on a single crystal. The variation of the unit cell parameters versus pressure (p) permits determination of the lattice elastic properties. We find a remarkably strong pressure dependence of the bulk modulus for p→ 0 (dB/dp), which was used, through a simple continuum mechanics model, to explain the unusual switching behavior. To go further, we demonstrated that dB/dp is related probably to the specific distortion of the core structure of 1. We also made the first steps in the correlation of the bulk elastic properties and the microscopic structure of the molecular materials. Two determining intermolecular interactions are revealed in 1 through Hirshfeld surface analysis: the first ones are created by the short contacts of a pair of blocking ligands in π–π stacking on the Co site which runs alongside the a direction, and the second ones are mediated by BF4– anions running in the c direction. Interestingly, the linear bulk moduli in the a and c directions could be correlated to these two intermolecular interactions. The variation of the linear bulk moduli versus pressure indicates that the connectivity of BF4– counteranions seems to play a primordial role at low pressure (<0.46 GPa), while the π–π stacking becomes predominant at higher pressure. This reveals the complexity of large molecular systems and the need for physical models that take it into account.