Metallo-Hydrogen-Bonded Organic Frameworks (MHOFs) Integrating Tunable Spin Crossover Properties and Proton Conduction

The emergence of functional hydrogen-bonded organic frameworks (HOFs) has created new opportunities in materials science. However, the development of functional metallo-HOFs (MHOFs) featuring switchable magnetic properties remains a significant challenge. Herein, we report three new magnetic MHOFs built from spin-crossover (SCO)-active [Fe(2-pic) 3 ] 2+ cation and two sulfonate anions, 1,5-naphthalenedisulfonate (NDS 2– ) and 1,3,6,8-pyrenetetrasulfonate (PTS 4– ). These compounds, formulated as [Fe(2-pic) 3 ]·NDS·2H 2 O ( 1 · 2H 2 O ) and [Fe(2-pic) 3 ]·0.5PTS· n H 2 O ( n = 2, 3; denoted as 2 · 2H 2 O and 3 · 3H 2 O ), were fully characterized by single-crystal X-ray diffraction, magnetic measurements, and proton conduction studies. Notably, complex 1 · 2H 2 O undergoes a single-crystal-to-single-crystal transformation upon dehydration, which directly modifies its hydrogen-bonded network, thereby switching its gradual and complete SCO to a fully HS state. Complexes 2·2H 2 O and 3·3H 2 O exhibit distinct SCO profiles due to their different Fe 2+ coordination geometries ( meridional in 2·2H 2 O, facial in 3·3H 2 O ) and hydrogen-bonded networks. While 2·2H 2 O exhibits a gradual two-step SCO, 3·3H 2 O displays hysteresis SCO with a loop width of approximately 10 K. Proton conduction studies revealed that these complexes show moderate proton conduction (with σ up to 2.70 × 10 –3 S cm –1 at 95% RH and 45 °C), with 3·3H 2 O performing the best. These complexes represent rare multifunctional MHOFs integrating switchable SCO and proton transport.