Environmental Control of the Magnetic Behavior of Transition Metal Complexes: Density Functional Theory Study of Zeolite Y Embedded Complexes [M(bpy)₃]²⁺@Y (M = Fe²⁺, Co²⁺)

Using the supramolecular approach developed for the study of the guest-host interactions in the zeolite Y encapsulated [Fe(bpy)3]2+ compound: [Fe(bpy)3]2+@Y (bpy = 2,2'-bipyridine) [Vargas et al., J. Chem. Theory Comput. 2009, 5, 97-115], we apply density functional theory (DFT) to the study of the influence of zeolite Y encapsulation on the structural and energetic properties of [Co(bpy)3]2+ in the low-spin (LS) and high-spin (HS) states, while revisiting [Fe(bpy)3]2+@Y. Although the accurate prediction of the HS-LS energy difference ΔEHLel remains challenging for current DFT methods, they give accurate estimates of its variation Δ(ΔEHLel) in a series of complexes of a given transition metal ion. Therefore, denoting [M(bpy)3]2+@YSM as the supramolecular model of the inclusion compounds, the values of ΔEHLel for the bpy complexes in the gas phase and in the supercage of zeolite Y were determined by combining the DFT estimates of Δ(ΔEHLel) in the series {[M(NCH)6]2+, [M(bpy)3]2+, and [M(bpy)3]2+@YSM}, with accurate CCSD(T) estimates of ΔEHLel in the benchmark complexes [M(NCH)6]2+ (M = Fe, Co) [Lawson Daku et al., J. Chem. Theory Comput., 2012, 8, 4216-4231]. Generalized gradient approximations as well as global and range-separated hybrids were employed. In order to better account for the key role of dispersion, they were also augmented with the semiempirical D2, D3BJ, and D3BJM dispersion corrections when available. The use of the D3BJ and D3BJM corrections led to similar results, and this is only with the use of the D2 scheme that (i) the free and encapsulated [Fe(bpy)3]2+ are correctly predicted as LS species and that (ii) the encapsulation of both complexes translates into a destabilization of their HS state with respect to their LS state. The increase of the HS-LS energy difference is smaller for [Co(bpy)3]2+ than [Fe(bpy)3]2+ because the HS-LS molecular volume difference ΔVHL in [Co(bpy)3]2+ is ∼50% smaller than in [Fe(bpy)3]2+. Periodic DFT calculations performed on crystalline [M(bpy)3]2+@Y show that the employed [M(bpy)3]2+@YSM supramolecular model allows the influence of encapsulation on the geometry and the spin-state energetics of [M(bpy)3]2+ (M = Fe, Co) to be quantitatively captured.