We
probe, here, a family of 2D Hofmann-type frameworks, [FeII(Pd(CN)4)(bztrzX)2]·nH2O [X·nH2O; X = F, Cl, Br; n = 1 (X = Cl, Br) and 3 (X =
F); bztrzX = (E)-1-(2-Xphen-1-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine], with halogen-appended
ligands. In all cases, there are two crystallographically distinct
FeII sites, ({Fe1–Fe2}), driven by the presence
of a range of host–host and host–guest interactions.
We find that lattice modification through X variation influences the
elastic coupling between the FeII sites, the emergence
of ferroelastic or antiferroelastic interactions between these sites,
and the relative spin-state stabilization/destabilization at each
site. In Cl·H2O, the FeII sites
show strong elastic coupling, as evidenced by both FeII sites undergoing a spin transition in a single cooperative step,
as driven by the volume strain over the high-spin (HS)-to-low-spin
(LS) transition. The FeII sites in F·3H2O are also elastically coupled; however, the change of the
X atom characteristics and increased guest molecules in the pores
result in an antiferroelastic interaction characteristic between Fe1
and Fe2 and a resultant two-step spin-state transition. The change
of the X atom to Br in Br·H2O results
in the FeII sites being decoupled due to halogen atom steric
bulk, resulting in the independent spin-state transition of Fe1 and
Fe2 sites and a two-step spin-state transition pathway. Uniquely,
all three possible spin-state transition pathways of a two-site switching
system are observed in this family [(1) {HS–HS} ↔ {HS–LS}
↔ {LS–LS} for Br·H2O, (2)
{HS–HS} ↔ {LS–HS} ↔ {LS–LS} for F·3H2O, and (3) {HS–HS} ↔ {LS–LS}
for Cl·H2O for {Fe1–Fe2}]. Overall,
these findings broadly support recent theoretical models but highlight
that additional structural and topological complexities are needed
to form a holistic picture of the drivers of elastic frustration.