Carboxylate-Free MnIII2LnIII2 (Ln = Lanthanide) and MnIII2YIII2 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 [MnIII2MIII2(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 LnIII 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, J1 = −0.16(2) cm–1 and J2 = −0.12(1) cm–1 for the MnIII···MnIII and MnIII···GdIII 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 MnIII···MnIII interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J1 and J2.