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 [Fe^IIM^II(CN)₄] (M^II = 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 [Fe^IIPt^II(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, [Fe^IIN₆] (<b>Fe1</b>) and [Fe^IIN₄O₂] (<b>Fe2</b>), (2) as a consequence of axial water coordination to <b>Fe2</b>, there are “holes” in the [Fe^IIPt^II(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 <b>Fe1</b> 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 (<i>T</i>_1/2↓ = 120 K and <i>T</i>_1/2↑ = 130 K) of this iron­(II) site; <b>Fe2</b> remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; <i>T</i>(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (<i>T</i>_1/2↓ = 60 K and <i>T</i>_1/2↑ = 66 K)