Further synthetic investigation of the general lanthanoid(iii) [Ln(iii)]/copper(ii)/pyridine-2,6-dimethanol/carboxylate reaction system: {CuLn} coordination clusters (Ln = Dy, Tb, Ho) and their yttrium(iii) analogue.

In addition to previously studied {CuGd ₆ }, {CuGd ₄ }, {CuLn ₇ } and {CuLn ₈ } coordination clusters (Ln = trivalent lanthanide) containing pdm ^2- or Hpdm ^- ligands (H ₂ pdm = pyridine-2,6-dimethanol) and ancillary carboxylate groups (RCO ₂ ^- ), the present work reports the synthesis and study of three new members of a fifth family of such complexes. Compounds [Cu ₅ Ln ₄ O ₂ (OMe) ₄ (NO ₃ ) ₄ (O ₂ CCH ₂ Bu ^t ) ₂ (pdm) ₄ (MeOH) ₂ ] (Ln = Dy, 1; Ln = Tb, 2; Ln = Ho, 3) were prepared from the reaction of Ln(NO ₃ ) ₃ ·xH ₂ O (x = 5, 6), CuX ₂ ·yH ₂ O (X = ClO ₄ , Cl, NO ₃ ; y = 6, 2 and 3, respectively), H ₂ pdm, Bu ^t CH ₂ CO ₂ H and Et ₃ N (2 : 2.5 : 2 : 1 : 9) in MeCN/MeOH. Rather surprisingly, the copper(ii)/yttrium(iii) analogue has a slightly different composition, i.e. [Cu ₅ Y ₄ O ₂ (OMe) ₄ (NO ₃ ) ₂ (O ₂ CCH ₂ Bu ^t ) ₄ (pdm) ₄ (MeOH) ₂ ] (4). The structures of 1·4MeCN·1.5MeOH and 4·2MeOH were solved by single-crystal X-ray crystallography. The five Cu ^II and four Dy ^III centres in 1 are held together by two μ ₅ -O ^2- , four μ-MeO ^- , two syn,synη ¹ :η ¹ :μ Bu ^t CH ₂ CO ₂ ^- , four η ² :η ¹ :η ² :μ ₃ pdm ^2- (each of these groups chelates a Cu ^II atom and simultaneously bridges two Dy ^III atoms through its two -CH ₂ O ^- arms) and two μ-MeOH ligands. The four terminal nitrato groups each chelate (η ¹ :η ¹ ) a Dy ^III centre. The five Cu ^II atoms are co-planar (by symmetry) forming a bow-tie arrangement; the four outer Cu ^II atoms form a rectangle with edges of 3.061(1) and 6.076(1) Å. The four Dy ^III centres also form a rectangle that lies above and below the plane of the Cu ^II centres, with edges of 3.739(1) and 5.328(1) Å. The two strictly planar rectangles are almost perpendicular. Two trigonal bipyramidal μ ₅ -O ^2- groups link the perpendicular Cu ₅ and Dy ₄ frameworks together. The molecule 4 has a very similar structure to that of 1, differences being the replacement of the two chelating nitrato groups of 1 by two chelating Bu ^t CH ₂ CO ₂ ^- ligands in 4 and the coordination polyhedra of the Ln ^III and Y ^III atoms (Snub diphenoids in 1 and biaugmented trigonal prisms in 4). Dc magnetic susceptibility data (χ _M ) on analytically pure samples of 1-3, collected in the 300-2 K range, indicate that ferromagnetic exchange interactions dominate leading to large spin ground states. The χ _M T vs. T data for 4 suggest moderately strong antiferromagnetic Cu ^II Cu ^II exchange interactions. Studies of the dynamic magnetic properties of the {Cu ₅ Ln ₄ } clusters show that 1 behaves as a SMM at zero field and 2 is a very weak field-induced SMM, while 3 exhibits only weak tails in the χ'' _M vs. T plots at various ac frequencies at zero dc field.