New Fe₄, Fe₆, and Fe₈Clusters of Iron(III) from the Use of 2-Pyridyl Alcohols: Structural, Magnetic, and Computational Characterization

Abstract

The syntheses, crystal structures, magnetochemical characterization, and theoretical calculations are reported for three new iron clusters [Fe 6O 2(NO3) 4(hmp) 8(H 2O) 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 NEt 3 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 pdmH 2 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(mu3-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 Fe (III) atoms within an [Fe4(mu-OR) 6](6+) core, with the azide groups all bound terminally. Finally, complex 3 contains a central [Fe 4(mu4-O)](10+) tetrahedron linked to two oxide-centered [Fe3(mu3-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 J1=-9.2 cm(-1), J2=-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 Fe(III) 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 Fex clusters without requiring the copresence of carboxylate ligands.

High-Yield Syntheses and Reactivity Studies of Fe10“Ferric Wheels”: Structural, Magnetic, and Computational Characterization of a Star-Shaped Fe8Complex

“…The spin densities have magnitudes close to the formal value of five expected for high spin (d 5 ) Fe 3+ ions but are reduced below this number by spin delocalization, as found with ZILSH for other complexes of Fe 3+ ions. 22,26,30,54 The signs of the local spin densities indicate the relative directions of the spin moments of the iron ions and show that the correct spin distributions were obtained for all the spin components.…”

“…Calcd for 1a · 2MeCN: C, 28. 54;H,5.23;N,1.51%. Found: C,28.43;H,5.27 6 (H 2 O) 3 ](NO 3 ) (1.03 g, 1.0 mmol).…”

Magnetostructural Correlation for High-Nuclearity Iron(III)/Oxo Complexes and Application to Fe5, Fe6, and Fe8 Clusters

“…These have been referred to as (a) planar, (b) twisted-boat, (c) chairlike, (d) parallel triangles, (e) octahedral, (f) fused or extended butterflies, (g) cyclic, and (h) linked-triangles. 29 Such a structural variety has led to many ground-state spin values being determined. Two of these previous Fe 6 complexes, [Fe 6 O 2 (OH) 2 (O 2 CPh) 9 (hep) 4 ] and [Fe 6 O 2 (OH) 2 (O 2 CPh) 10 (hep) 2 ] (hep − is the anion of 2-(2-hydroxyethyl)pyridine), have a core related to 2 and have been obtained with either cis or trans disposition of the monatomic oxo bridges; 9 the core of the latter is trans and has an S = 5 spin ground state, suggesting a possibly similar ground state for 2 (vide infra).…”

Comparison of Semiempirical ZILSH and DFT Calculations of Exchange Constants in Fe4 Butterfly Complexes

“…This was also found to be the case with the complex [Fe 8 O 2 (OH) 12 (tacn) 6 ] 8+ (ref 32) but not for the complex [Fe 14 O 6 (bta) 6 (OMe) 18 Cl 6 ], in which DFT calculations gave the correct ground-state spin while ZILSH calculations did not. 68 Clearly comparisons for additional complexes are needed to clarify if one method is consistently more reliable than the other for estimating exchange constants in polynuclear transition metal complexes.…”

“…It is also very important to note that in other cases we have observed that the larger exchange constants found with spin couplings obtained from the local spin operator are more consistent with experiment. 33,68 Further study from both theoretical and practical perspectives is needed to clarify these issues.…”