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.