Steric Control over Heteroleptic Trigonal Prisms: From Integrative Self-Sorting to Programmable Spin-State Modulation through Cage Transformations

The construction of well-defined heteroleptic assemblies via integrative self-sorting presents a persistent challenge in metal–organic cage (MOC) chemistry. Herein, we employ backbone steric constraints and precise edge-length complementarity to achieve a quantitative formation of heteroleptic trigonal-prismatic cages with the general formula [M 6 L A 2 L′ 3 ] 12+ (M = Zn 2+, Fe 2+; L′ = L B, L C, and L D ). The C 3 -symmetric tris-aniline ligand (L A ) sterically directs a horizontal orientation of the three complementary rectangular ligands that thermodynamically disfavors the formation of competing homoleptic structures. The embedded tricoordinate sp 2 -boron centers render the Zn 2+ -based cage ( 1a-Zn ) chemically addressable, enabling reversible fluoride binding to form a metastable adduct, which is quantitatively reversed through treatment with Ca 2+ ions, demonstrating chemically gated cage-to-cage interconversion. Ligand exchange transforms a low-spin (LS) homoleptic cubic cage ( 4b-Fe, T 1/2 = 371 K) into a heteroleptic trigonal-prismatic cage ( 2b-Fe, T 1/2 = 272 K), resulting in an unusual ambient-temperature LS-to-HS spin transition. Furthermore, the structurally analogous imidazole-based cage ( 2a-Fe ) exhibits permanent HS character, underscoring the tunability of spin states through subtle ligand modifications. To the best of our knowledge, 2b-Fe represents the first example of a SCO-active heteroleptic trigonal prism, thereby introducing a new structural archetype for hexanuclear Fe(II)-based SCO materials that extends beyond conventional coordination assemblies. These findings establish backbone steric bulk as a general approach to integrative self-sorting, demonstrate chemically triggered cage-to-cage transformations, and offer a versatile blueprint for encoding stimulus-responsive magnetism into discrete multinuclear cages.