Pressure Tuning of Coupled Structural and Spin State Transitions in the Molecular Complex [Fe(H2B(pz)2)2(phen)]

The large volume change, which accompanies the molecular spin crossover (SCO) phenomenon in some transition metal complexes, prompts frequently the coupling of the SCO with other instabilities. Understanding the driving mechanism(s) of such coupled phase transitions is not only important for fundamental reasons but also provides scope for the development of multifunctional materials. The general theoretical expectation is that the coupling has elastic origin, and the sequence of transitions can be tuned by an externally applied pressure, but dedicated experiments remain scarce. Here, we used high-pressure and low-temperature single-crystal X-ray diffraction to investigate the high-spin (HS) to low-spin (LS) transitions in the molecular complexes [Fe-II(H2B(pz)(2))(2)(bipy)] and [Fe-II(H2B(pz)(2))(2)(phen)]. In the bipyridine complex, the SCO is continuous and isostructural over the whole T, P-range (100-300 K, 0-2 GPa). In the phenanthroline derivative, however, the SCO is concomitant with a symmetry-breaking transition (C2/c to P (1) over bar). Structural analysis reveals that the coupling between the two phenomena can be tuned by external pressure from a virtually simultaneous HSC2/cLSP (1) over bar transition to the sequence of HSC2/c-LSC2/c-LSP (1) over bar transitions. The correlation of spontaneous strain and order parameter behaviors highlights that the separated transitions remain still connected via strain coupling, whereas the simultaneous transitions are partially split.

Automated summary

This study investigated the high-spin to low-spin transitions in two transition metal complexes using high-pressure and low-temperature single-crystal X-ray diffraction, revealing that external pressure can tune the coupling between the two phenomena and change the sequence of transitions. The correlation between spontaneous strain and order parameter behaviors highlights the interconnected nature of separated transitions and the partial split in simultaneous transitions.