Anion encapsulation promoted by anionπ interactions in rationally designed hexanuclear antiferromagnetic wheels: synthesis, structure and magnetic properties

The reaction of Kpymca (pymca = pyrimidine-2-carboxylato anion) with MX₂·6H₂O and tmda (N,N,N′,N′-tetramethylethylenediamine) in 1:3:3 molar ratio, using a MeOH/water mixture (3:1) as solvent, afforded the hexanuclear complexes [M₆(µ-pymca)₆(tmda)₆]X₆·4H₂O (M = Ni^II, X = ClO₄⁻1, BF₄⁻2; M = Co^II, X = ClO₄⁻3). The reaction of pymca with either Cu(ClO₄)₂·6H₂O or Cu(BF₄)₂·6H₂O did not lead to any definite product. However, in the presence of strong coordinating chloride anions the linear tetranuclear complex [Cu₄(µ-pymca)₃Cl(H₂O)](ClO₄)₄·4.5H₂O 4 could be obtained. The structures of 1–3 are very similar and consist of wheel-shaped hexanuclear [M₆(µ-pymca)₆(tmda)₆]⁶⁺ cations, with pseudo-D_3d symmetry in which the ligand acts in a bis(chelating)/bridging mode. The pyrimidine rings of the ligands converge alternatively above and below the plane of the wheel toward the normal to this plane, leading to the formation of a bowl-shaped capsule, which is occupied by one ClO₄⁻ (1 and 3) or BF₄⁻ (2) anion. There exist tight contacts between ClO₄⁻/BF₄⁻ and the π-electron density of the pyrimidine rings of the ligands. The tetranuclear cations of 4 exhibit a zig-zag linear conformation, in which copper ions are bridged by three bis(chelating)/bridging pymca ligands. Variable-temperature magnetic susceptibility studies reveal that complexes 1 and 4 show moderate antiferromagnetic coupling between the metal ions through the pymca ligands with J = −26.3 cm⁻¹ for 1, and J₁ = −16.3 cm⁻¹ and J₂ = −43.78 cm⁻¹ for 4. The tetranuclear complex 4 has two different magnetic coupling constants accounting for the external–central (J₁) and central–central (J₂) copper interactions. The magnitude of the J₁ and J₂ values for 4 has been discussed on the basis of the geometrical factors affecting the magnetic pathways. Moreover, DFT calculations have been performed to support the relative magnitude of the J parameters in 4. A model accounting for spin–orbit coupling (λ), orbital reduction factors (Ak), axial zero-field splitting (Δ) and magnetic exchange coupling (J) was used to analyse the magnetic data of 3. The values of these parameters after the diagonalisation process were: Ak = 1.15, λ = −122 cm⁻¹, Δ = 650 cm⁻¹ and J = −5.2 cm⁻¹.