Trigonal Antiprismatic Mononuclear Cr(II) Spin-Crossover Complexes

The engineering of Cr(II) complexes for spin crossover (SCO) phenomena remains poorly understood, primarily due to the limited number of reported examples and the associated challenges in their synthesis. In the context of Cr(II) complexes with octahedral geometry, the pronounced Jahn-Teller distortion in the high-spin (HS) state significantly restricts the selection of co-ligands necessary for SCO to occur. In this study, we successfully synthesized a series of air-stable and solvent-free Cr(II) complexes, [Cr(TpmR)2]X2 (TpmR = Tpm: X = [BF4]-, 1; X = [ClO4]-, 2; TpmR = Tpm*: X = [BF4]-, 3; X = [ClO4]-, 4), where the Cr(II) ions adopt a trigonal antiprism (aTPR) geometry rather than the conventional octahedral configuration. Notably, the Tpm-based complexes 1 and 2, with Cr(II) ions in a nearly ideal aTPR geometry, exhibit complete SCO behavior with transition temperatures (T1/2) of 190 K for 1 and 194 K for 2, respectively. In contrast, the Tpm*-based complexes 3 and 4 display considerable axial elongation within the aTPR structure, leading to abrupt but incomplete SCO transition at a much lower temperature for 3 (T1/2 = 142 K) while 4 remains predominantly in the HS state. Further structural analysis reveals that substituting [BF4]- with [ClO4]- in 4 results in increased elongation and a denser packing arrangement that disfavour the spin transition. Our findings suggest that the aTPR configuration is advantageous for facilitating spin crossover, while the dynamics of spin transitions in these complexes are closely linked to the structural distortions of the [Cr(TpmR)2]2+ cations. Furthermore, all complexes demonstrate remarkable resistance to oxygen and maintain air stability, which can be attributed to their compact space-filling structures that effectively impede the diffusion of oxygen into the interstitial regions of the crystal lattice.