Insight into Structure: Function Relationships in a Molecular Spin-Crossover Crystal, from a Related Weakly Cooperative Compound

The ClO4– salt of [FeL2]2+ {L = 2,6‐bis(3‐methylpyrazol‐1‐yl)pyridine} undergoes very gradual thermal spin‐crossover centered just below room temperature. In contrast, the BF4– salt of the same complex exhibits an abrupt and structured spin‐transition at lower temperature, with a complicated structural chemistry. The difference can be attributed to a much larger change in molecular structure between the spin states of the complex in the more cooperative BF4– salt, leading to an increased kinetic barrier for their interconversion. Consistent with that suggestion, the high‐spin and low‐spin structures of weakly cooperative [FeL2][ClO4]2 are almost superimposable. The ClO4– salt of [FeL2]2+ {L = 2,6‐bis(3‐methylpyrazol‐1‐yl)pyridine} undergoes very gradual thermal spin‐crossover. In contrast, the BF4– salt of the same complex exhibits an abrupt and structured spin‐transition, with a complicated structural chemistry. The difference can be attributed to a much larger change in molecular structure between the spin states of the complex in the more cooperative BF4– salt (the weakly cooperative ClO4– salt is pictured).