New Fe4, Fe6, and Fe8 Clusters of Iron(III) from the Use of 2-Pyridyl Alcohols: Structural, Magnetic, and Computational Characterization

The syntheses, crystal structures, magnetochemical characterization, and theoretical calculations are reported for three new iron clusters [Fe6O2(NO3)4(hmp)8(H2O)2](NO3)2 (1), [Fe4(N3)6(hmp)6] (2), and [Fe8O3(OMe)(pdm)4(pdmH)4(MeOH)2](ClO4)5 (3) (hmpH = 2-(hydroxymethyl)pyridine; pdmH2 = 2,6-pyridinedimethanol). The reaction of hmpH with iron(III) sources such as Fe(NO3)3·9H2O in the presence of NEt3 gave 1, whereas 2 was obtained from a similar reaction by adding an excess of NaN3. Complex 3 was obtained in good yield from the reaction of pdmH2 with Fe(ClO4)3·6H2O in MeOH in the presence of an organic base. The complexes all possess extremely rare or novel core topologies. The core of 1 comprises two oxide-centered [Fe3(µ3-O)]7+ triangular units linked together at two of their apexes by two sets of alkoxide arms of hmp− ligands. Complex 2 contains a zigzag array of four FeIII atoms within an [Fe4(µ-OR)6]6+ core, with the azide groups all bound terminally. Finally, complex 3 contains a central [Fe4(µ4-O)]10+ tetrahedron linked to two oxide-centered [Fe3(µ3-O)]7+ triangular units. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1−3 in the 5.0–300 K range. Fitting of the obtained magnetization versus field (H) and temperature (T) data by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) established that 1 possesses an S = 3 ground-state spin, with g = 2.08, and D = −0.44 cm−1. The magnetic susceptibility data for 2 up to 300 K were fit by matrix diagonalization and gave J 1 = −9.2 cm−1, J 2 = −12.5 cm−1, and g = 2.079, where J 1 and J 2 are the outer and middle nearest-neighbor exchange interactions, respectively. Thus, the interactions between the FeIII centers are all antiferromagnetic, giving an S = 0 ground state for 2. Similarly, complex 3 was found to have an S = 0 ground state. Theoretically computed values of the exchange constants in 2 were obtained with DFT calculations and the ZILSH method and were in good agreement with the values obtained from the experimental data. Exchange constants obtained with ZILSH for 3 successfully rationalized the experimental S = 0 ground state. The combined work demonstrates the ligating flexibility of pyridyl-alcohol chelates and their usefulness in the synthesis of new polynuclear Fe x clusters without requiring the copresence of carboxylate ligands.