A Molecular Playground for Spin-State Ice and Coupled Electron-Spin Dynamics

The development of molecular materials that combine spin-crossover (SCO) and metal-to-metal electron transfer (MMET) in a single system remains a fundamental challenge due to their strongly coupled nature and ultrashort-lived intermediate states. Here we present a series of cyanide-bridged {W₆Co₉} clusters, [Co@{W(CN)₈}₆{Co(L)}₆{Co(H₂O)_x(MeOH)_3-x}₂]·sol {L = 2,2,2-tris(1H-pyrazolyl)ethanol, x = 3, sol = MeOH (1) and EtOH (3); L = 1,1',1″-(2-(allyloxy)ethane-1,1,1-triyl)tris(1H-pyrazole), x = 2, sol = 4H₂O (2)}, that exhibit unprecedented coexistence of reversible SCO, MMET, and photoinduced slow magnetic relaxation. By strategically modulating ligand fields and supramolecular packing, we achieve distinct switching behaviors: one-step incomplete transitions in elastically frustrated triangular-packed systems (1 and 3) and a two-step complete transition in a nonfrustrated grid-like system (2). Remarkably, the frustrated systems exhibit "spin-state-ice-like" behavior, with each triangular unit adopting either a two high-spin/one low-spin (2HS/1LS) or 1HS/2LS configuration, representing the first experimental observation of such behavior in molecular clusters. These findings establish a new paradigm for designing multistable magnetic materials with coupled electronic and spin transitions, offering insights into the interplay between elastic frustration and cooperative spin-state switching.