Molecule-based spin state switching materials that display ambient temperature transitions with accompanying wide thermal hysteresis offer an opportunity for electronic switching, data storage, and optical technologies but are rare in existence. Here, we present the first 2D Hofmann-type materials to exhibit the elusive combination of ambient temperature spin crossover with wide thermal hysteresis (ΔT = 50 and 65 K). Combined structural, magnetic, spectroscopic, and theoretical analyses show that the highly cooperative transition behaviours of these layered materials arise due to strong host–host interactions in their interdigitated lattices, which optimises long-range communication pathways. With the presence of water molecules in the interlayer pore space in the hydrated phases, competing host–host and host–guest interactions occur, whilst water removal dramatically increases the framework cooperativity, thus affording systematic insight into the structural features that favour optimal spin crossover properties.

A four-stepped cascade of Fe(II) high spin (HS) to low spin (LS) states is demonstrated in a family of 2-D Hofmann materials, [Fe₃ ^II(saltrz)₆(M^II(CN)₄)₃]·8(H₂O) (M^II = Pd (1^Pd), Pt (1^Pt); saltrz = (E)-2-(((4H-1,2,4-triazol-4-yl)imino)methyl)phenol). Alongside the fully HS and LS Fe(II) states, fractional spin state stabilization occurs at HS/LS values of 5/6, 2/3, and 1/6. This unconventional spin state periodicity is driven by the presence of multiple spin crossover (SCO) active Fe(II) sites which are in subtly distinct environments driven by a network of antagonistic host–host and host–guest interactions. Alternating long- and short-range magnetostructural ordering is achieved over the five distinct spin state ratios HS^1.0LS^0.0, HS^0.833LS^0.167, HS^0.667LS^0.333, HS^0.167LS^0.833, and HS^0.0LS^1.0 owing to the flexibility of this 2-D interdigitated lattice topology interconnected by intermolecular interactions. A distinct wave-like spin state patterning is structurally evidenced for each intermediate phase.

The abrupt and hysteretic two-step spin crossover in a new triazole-based 2-D Hofmann-type complex shows a record breaking 120 K intermediate plateau (IP) region stabilized by negative cooperative interactions.