54 K Spin Transition Temperature Shift in a Fe₆L₄ Octahedral Cage Induced by Optimal Fitted Multiple Guests

Spin-crossover (SCO) coordination cages are at the forefront of research for their potential in crafting next-generation molecular devices. However, due to the scarcity of SCO hosts and their own limited cavities, the interplay between the SCO host and the multiple guests binding has remained elusive. In this contribution, we present a family of pseudo-octahedral coordination cages (M₆L₄, M = Zn^II, Co^II, Fe^II, and Ni^II) assembled from a tritopic tridentate ligand L with metal ions. The utilization of Fe^II ion leads to the successful creation of the Fe₆L₄-type SCO cage. Host-guest studies of these M₆L₄ cages reveal their capacity to encapsulate four adamantine-based guests. Notably, the spin transition temperature T_1/2 of Fe₆L₄ is dependent on the multiple guests encapsulated. The inclusion of adamantine yields an unprecedented T_1/2 shift of 54 K, a record shift in guest-mediated SCO coordination cages to date. This drastic shift is ascribed to the synergistic effect of multiple guests coupled with their optimal fit within the host. Through a straightforward thermodynamic cycle, the binding affinities of the high-spin (HS) and low-spin (LS) states are separated from their apparent binding constant. This result indicates that the LS state has a stronger binding affinity for the multiple guests than the HS state. Exploring the SCO thermodynamics of host-guest complexes allows us to examine the optimal fit of multiple guests to the host cavity. This study reveals that the T_1/2 of the SCO host can be manipulated by the encapsulation of multiple guests, and the SCO cage is an ideal candidate for determining the multiple guest fit.