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 1 exhibited a reversible thermal-induced incomplete metal-to-metal electron transfer (MMET) behavior at around 220 K, while the mixed alkynyl/alcohol-functionalized cube of 2 showed a complete and abrupt MMET behavior at 232 K, and remarkably, both showed a long-lived photo-induced metastable state near 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 ligands. 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.