Regulation of Multistep Spin Crossover Across Multiple Stimuli in a 2-D Framework Material.

We investigate the effects of a broad array of external stimuli on the structural, spin-crossover (SCO) properties and nature of the elastic interaction within the two-dimensional Hofmann framework material [Fe(cintrz)Pd(CN)]·guest (cintrz = -cinnamalidene 4-amino-1,2,4-triazole; ·guest; guest = 3HO, 2HO, and Ø). This framework exhibits a delicate balance between ferro- and antiferro-elastic interaction characters; we show that manipulation of the pore contents across guests = 3HO, 2HO, and Ø can be exploited to regulate this balance. In ·3HO, the dominant antiferroelastic interaction character between neighboring Fe sites sees the low-temperature persistence of the mixed spin-state species {HS-LS} for {Fe1-Fe2} (HS = high spin, LS = low spin). Elastic interaction strain is responsible for stabilizing the {HS-LS} state and can be overcome by three mechanisms: (1) partial (2HO) or complete (Ø) guest removal, (2) irradiation via the reverse light-induced excited spin-state trapping (LIESST) effect (λ = 830 nm), and (3) the application of external hydrostatic pressure. Combining experimental data with elastic models presents a clear interpretation that while guest molecules cause a negative chemical pressure, they also have consequences for the elastic interactions between metals beyond the simple chemical pressure picture typically proposed.