Thermal- and Light-Induced Spin-Crossover Bistability in a Disrupted Hofmann-Type 3D Framework

The
expected 3D and 2D topologies resulting from combining approximately
linear bis- or monopyridyl ligands with [FeIIMII(CN)4] (MII = Pt, Pd, Ni) 4,4-grid sheets are
well established. We show here the magnetic and structural consequences
of incorporating a bent bispyridyl linker ligand in combination with
[FeIIPtII(CN)4] to form the material
[Fe­(H2O)2Fe­(DPSe)2(Pt­(CN)4)2]·3EtOH (DPSe = 4,4′-dipyridylselenide).
Structural investigations reveal an unusual connectivity loosely resembling
a 3D Hofmann topology where (1) there are two distinct local iron­(II)
environments, [FeIIN6] (Fe1) and
[FeIIN4O2] (Fe2), (2)
as a consequence of axial water coordination to Fe2,
there are “holes” in the [FeIIPtII(CN)4] 4,4 sheets because of some of the cyanido ligands
being terminal rather than bridging, and (3) bridging of adjacent
sheets occurs only through one in two DPSe ligands, with the other
acting as a terminal ligand binding through only one pyridyl group.
The magnetic properties are defined by this unusual topology such
that only Fe1 is in the appropriate environment for a
high-spin to low-spin transition to occur. Magnetic susceptibility
data reveal a complete and abrupt hysteretic spin transition (T1/2↓ = 120 K and T1/2↑ = 130 K) of this iron­(II) site; Fe2 remains high-spin. This material additionally exhibits a photomagnetic
response (uncommon for Hofmann-related materials), showing light-induced
excited spin-state trapping [LIESST; T(LIESST) =
61 K] with associated bistability evidenced in a hysteresis loop (T1/2↓ = 60 K and T1/2↑ = 66 K).