Materials that display multiple stepped spin crossover (SCO) transitions with accompanying hysteresis present the opportunity for ternary, quaternary, and quinary electronic switching and data storage but are rare in existence. Herein, we present the first report of a four-step hysteretic SCO framework. Single-crystal structure analysis of a porous 3D Hofmann-like material showed long-range ordering of spin states: HS, HS LS , HS LS , HS LS , and LS. These detailed structural studies provide insight into how multistep SCO materials can be rationally designed through control of host-host and host-guest interactions.

Atomically dispersed metal catalysts anchored on nitrogendoped (N-doped) carbons demand attention due to their superior catalytic activity relative to that of metal nanoparticle catalysts in energy storage and conversion processes. Herein, we introduce a simple and versatile strategy for the synthesis of hollow N-doped carbon capsules that contain one or more atomically dispersed metals (denoted as H−M−N x −C and H−M mix −N x −C, respectively, where M = Fe, Co, or Ni). This method utilizes the pyrolysis of nanostructured core−shell precursors produced by coating a zeolitic imidazolate framework core with a metal−tannic acid (M−TA) coordination polymer shell (containing up to three different metal cations). Pyrolysis of these core−shell precursors affords hollow N-doped carbon capsules containing monometal sites (e.g., Fe−N x , CoN x , or Ni−N x ) or multimetal sites (Fe/Co−N x , Fe/Ni−N x , Co/Ni−N x , or Fe/Co/Ni−N x ). This inventory allowed exploration of the relationship between catalyst composition and electrochemical activity for the oxygen reduction reaction (ORR) in acidic solution.and H−FeCoNi−N x −C were particularly efficient ORR catalysts in acidic solution. Furthermore, the H− Fe−N x −C catalyst exhibited outstanding initial performance when applied as a cathode material in a proton exchange membrane fuel cell. The synthetic methodology introduced here thus provides a convenient route for developing nextgeneration catalysts based on earth-abundant components.

Covalent post-synthetic modification is a versatile method for gaining high-level synthetic control over functionality within porous metal-organic frameworks and for generating new materials not accessible through one-step framework syntheses. Here we apply this topotactic synthetic approach to a porous spin crossover framework and show through detailed comparison of the structures and properties of the as-synthesised and covalently modified phases that the modification reaction proceeds quantitatively by a thermally activated single-crystal-to-single-crystal transformation to yield a material with lowered spin-switching temperature, decreased lattice cooperativity, and altered color. Structure-function relationships to emerge from this comparison show that the approach provides a new route for tuning spin crossover through control over both outer-sphere and steric interactions.

Molecular crystals with guest-adaptable crystalline structures and properties are comparatively rare owing to their inherent reduced structural stability and malleability to support molecular variation. To overcome this intrinsic challenge, here we introduce structural stabilizing supramolecular interactions into a dinuclear material and henceforth demonstrate a dynamic structural and spin crossover property interchange between solvated (A·3MeOH) and desolvated (A·Ø) products (A = [FeII 2(o-NTrz)5(NCS)4]; 4-(o-nitrobenzyl)­imino-1,2,4-triazole). Relatively uncommon for molecular species, the guest molecules in A·3MeOH are evolved (A·Ø) via a single-crystal to single-crystal transformation with affiliated phase transition resulting in a reversible transformation from one- to two-step spin crossover (SCO) transition character. We additionally present the water-saturated product (A·3H2O), which distinctly shows an abrupt one-step SCO character with a 22 K wide thermal hysteresis loop. Detailed structure–property analysis highlights that the substantial structural malleability and guest-adaptable SCO properties of this dinuclear species are afforded by the supportive, yet flexible, supramolecular interaction pathways derived from the ligand functionalization.