A Series of Linear {FeIII2FeII} Complexes with Paramagnetic Building‐Block‐Modified Spin Crossover Behaviors

Tuning of the spin crossover (SCO) behavior through paramagnetic building blocks with different steric hindrance effects is of great interest in terms of the synergy between SCO and magnetic interactions. Herein, the steric effect of specified FeIII building blocks is modified, from the large Tp* (hydridotris(3,5‐dimethylpyrazol‐1‐yl)borate) analogue to a small Tp (hydrotris(pyrazolyl)borate) derivative; the FeII SCO unit and FeIII paramagnetic ions are incorporated into three well isolated trinuclear complexes featuring thermally induced and light‐induced SCO properties. Reanalysis of the structures reveals that π–π stacking interactions play a key role in the thermal hysteresis and anomalous octahedral distortion parameter Σ around the FeII ion. The Tp* ligand showing the largest steric hindrance induces elongated FeII−N bond lengths and bending of the C≡N−FeII angle in 1, as well as having a relatively large electron donor effect, which leads to the lowest thermal transition temperature among the three compounds. Spin crossover tuning: A series of isomorphic trinuclear complexes with thermally and light‐induced spin crossover (SCO) behaviors are synthesized. The SCO properties are influenced significantly by π–π interactions, and the transition temperature is controlled to a large extent by the steric effect of the building blocks (see figure).