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Design and understanding of solid-state and crystalline materials



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CrystEngComm, 2009, 11, 2054 - 2064, DOI: 10.1039/b906382j

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

Enrique Colacio, Hakima Aouryaghal, Antonio J. Mota, Joan Cano, Reijo Sillanpää and A. Rodríguez-Diéguez

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 = NiII, X = ClO4-1, BF4-2; M = CoII, 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-D3d 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 -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 (), orbital reduction factors (Ak), axial zero-field splitting () 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, = -122 cm-1, = 650 cm-1 and J = -5.2 cm-1.

Graphical abstract image for this article (ID: b906382j)