Further synthetic investigation of the general lanthanoid(iii) [Ln(iii)]/copper(ii)/pyridine-2,6-dimethanol/carboxylate reaction system: {Cu II5Ln III4} coordination clusters (Ln = Dy, Tb, Ho) and their yttrium(iii) analogue

Abstract

In addition to previously studied {Cu^II₃Gd₆}, {Cu^II₈Gd₄}, {Cu^II₁₅Ln₇} and {Cu^II₄Ln₈} coordination clusters (Ln = trivalent lanthanide) containing pdm²⁻ or Hpdm⁻ ligands (H₂pdm = pyridine-2,6-dimethanol) and ancillary carboxylate groups (RCO₂⁻), the present work reports the synthesis and study of three new members of a fifth family of such complexes. Compounds [Cu₅Ln₄O₂(OMe)₄(NO₃)₄(O₂CCH₂Bu^t)₂(pdm)₄(MeOH)₂] (Ln = Dy, 1; Ln = Tb, 2; Ln = Ho, 3) were prepared from the reaction of Ln(NO₃)₃·xH₂O (x = 5, 6), CuX₂·yH₂O (X = ClO₄, Cl, NO₃; y = 6, 2 and 3, respectively), H₂pdm, Bu^tCH₂CO₂H and Et₃N (2 : 2.5 : 2 : 1 : 9) in MeCN/MeOH. Rather surprisingly, the copper(II)/yttrium(III) analogue has a slightly different composition, i.e. [Cu₅Y₄O₂(OMe)₄(NO₃)₂(O₂CCH₂Bu^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 μ₅-O²⁻, four μ-MeO⁻, two syn,syn η¹:η¹:μ Bu^tCH₂CO₂⁻, four η²:η¹:η²:μ₃ pdm²⁻ (each of these groups chelates a Cu^II atom and simultaneously bridges two Dy^III atoms through its two –CH₂O⁻ arms) and two μ-MeOH ligands. The four terminal nitrato groups each chelate (η¹:η¹) 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 μ₅-O²⁻ groups link the perpendicular Cu₅ and Dy₄ 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^tCH₂CO₂⁻ 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_MT vs. T data for 4 suggest moderately strong antiferromagnetic Cu^II⋯Cu^II exchange interactions. Studies of the dynamic magnetic properties of the {Cu₅Ln₄} 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′′_Mvs. T plots at various ac frequencies at zero dc field.