Manipulating Electron-Transfer Events in [Fe4Co4] Cubes via a Mixed-Ligand Approach: The Impact of Elastic Frustration

The engineering of intermolecular interaction is challenging but critical for magnetically switchable molecules. Here, we prepared two cyanide-bridged [Fe4Co4] cube complexes via the alkynyl- and alcohol-functionalized trispyrazoyl capping ligands. The alkynyl-functionalized complex 1 exhibited a thermally-induced incomplete metal-to-metal electron transfer (MMET) behaviour at around 220 K, while the mixed alkynyl/alcohol-functionalized cube of 2 showed a complete and abrupt MMET behaviour at 232 K. Remarkably, both compounds showed a long-lived photo-induced metastable state up to 200 K. The crystallographic study demonstrated that the incomplete transition of 1 was likely due to the possible elastic frustration originating from the competition between the anion-propagated elastic interactions and inter-cluster alkynyl-alkynyl & CH-alkynyl interactions, whereas the latter are eliminated in 2 as a result of the partial substitution by the alcohol-functionalized ligand. Additionally, the introduction of chemically distinguishable cobalt centers within the cube unit of 2 did not lead to a two-step but a one-step transition, possibly because of the strong ferroelastic intramolecular interaction through the cyanide bridges.

Automated summary

The preparation of two cyanide-bridged [Fe4Co4] cube complexes using different functionalized ligands was studied. The first complex showed an incomplete metal-to-metal electron transfer behavior at around 220 K, while the second complex exhibited a complete and abrupt transfer at 232 K. Both complexes demonstrated a long-lived photo-induced metastable state. The crystallographic study revealed that the incomplete transition in the first complex was likely due to the competition between elastic interactions and alkynyl-alkynyl & CH-alkynyl interactions, which were eliminated in the second complex due to partial substitution by the alcohol-functionalized ligand. The introduction of chemically distinguishable cobalt centers did not result in a two-step transition in the second complex, possibly due to strong ferroelastic intramolecular interaction through the cyanide bridges.