Mapping the cooperativity pathways in spin crossover complexes

Abstract

The abruptness of spin crossover (SCO) is related to intermolecular energy changes occurring over the course of an SCO transition. Crossover is abrupt when SCO-induced strain is accommodated synergistically in a few key interactions.

Hierarchical Spin‐Crossover Cooperativity in Hybrid 1D Chains of FeII‐1,2,4‐Triazole Trimers Linked by [Au(CN)2]− Bridges

“…This is in line with structural studies on [Fe II (R‐trz) 3 ](A) 2 ⋅ x H 2 O systems [4g,h] . The finding that the coupling here via [Au(CN) 2 ] − coordination bridges is more moderate than via short, rigid triazole bridges, gives some indication that the strong cooperativity observed in Hofmann frameworks arises from a bulk effect rather than from local structural features [8, 14] …”

“…Structural information attained from a low‐resolution crystal structure of one member of the [Fe II (R‐trz) 3 ](A) 2 ⋅ x H 2 O family, [4g,h] indicates a dense network of interchain interactions involving the triazole ligands, the anions, and the water molecules. Given that the interchain interactions are necessary to reproduce the abrupt and hysteretic transitions, [2a,b, 5] it is important to establish their specific molecular‐scale role [14] . Qualitatively, the variation to SCO behavior in the [Fe II (R‐trz) 3 ](A) 2 ⋅ x H 2 O series [4] derives from electrostatic contributions [5] and the ability of the chains to interact most efficiently, hence why ligand steric bulk has a negative effect as do larger anions [4f] .…”

“…[4g,h] The finding that the coupling here via [Au(CN) 2 ] À coordination bridges is more moderate than via short, rigidt riazole bridges, gives some indication that the strongcooperativity observed in Hofmann frameworks arises from ab ulk effect rather than from local structuralf eatures. [8,14] Beyond establishing the nature and relative magnitude of intra-trimer (0D) and inter-trimer (1D) coupling, the cationic natureo ft he chains, the presence of anions and solvent in the lattice and the stability of A•guest to solvent variation [A•2(H 2 O•MeOH), A•4 H 2 O, and A•Ø] provides an ovel added platform to substantiate the relative role of different interchain (intermolecular) interactions on SCO character.N otably,a st he [Fe II (R-trz) 3 ](A) 2 •x H 2 Of amily are poorly crystalline, this information is not directly attainable, other than via comparison of SCO characters of anion and guest varied species. Structuralinformation attained from al ow-resolution crystal structure of one member of the [Fe II (R-trz) 3 ](A) 2 •x H 2 Of amily, [4g,h] indicates a dense network of interchaini nteractions involving the triazole ligands, the anions, and the water molecules.…”

“…Given that the interchaini nteractions are necessary to reproducet he abrupt and hysteretic transitions, [2a,b, 5] it is important to establisht heir specific molecular-scale role. [14] Qualitatively, the variation to SCO behavior in the [Fe II (R-trz) 3 ](A) 2 •x H 2 Os eries [4] derives from electrostatic contributions [5] and the ability of the chains to interact most efficiently,h ence why ligand steric bulk has an egative effect as do larger anions. [4f] Indeed,i n A•4 H 2 O, which shows the greatest thermal hysteresis of the A•guest family, structurala nalysis shows that the chains in are more efficiently packed;t his is likely driven by the achievement of shorter host-guest interactions and reduced guest bulk than for example A•2(H 2 O•MeOH).…”

“…Furthermore, including inter-chain coupling (J ## )a tt he mean-field level induces the hysteresis seen experimentally,a nd we find that its energy-scalei sa gain diminishedc ompared to that of the intra-trimer andi nter-trimer interactions (Table S5). This is clear evidencet hat the intrachain connectivityd ominates the sharp spin-state transition and that the transition character (such as smooth versus abrupt,w ith or without thermal hysteresis) is defined by subtleties of inter-chain interactions, in particular inter-chain coupling; [14] all of which has thus far only been theorized in [Fe II (Rtrz) 3 ](A) 2 •x H 2 Om aterials. [2a,b, 5] The (mean-field) Slichter-Drickamer model has previously been used to demonstrate ac orrelation in the [Fe II (Rtrz) 3 ](A) 2 •x H 2 Of amilyb etween DH and the cooperativity parameter G ¼ 4J II foro ur quasi-1D model.…”