Carboxylate-free Mn^III₂Ln^III₂ (Ln = lanthanide) and Mn^III₂Y^III₂ complexes from the use of (2-hydroxymethyl)pyridine: Analysis of spin frustration effects

The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [Mn^III ₂M^III₂(OH)₂(NO₃) ₄(hmp)₄(H₂O)₄](NO₃) ₂ (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1-4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic Ln^III atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5-300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J ₁ = -0.16(2) cm^-1 and J₂ = -0.12(1) cm ^-1 for the Mn^III•••Mn^III and Mn^III•••Gd^III interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = -0.75(3) cm^-1 for the Mn^III•••Mn^III interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J₁ and J₂.