Hexanuclear Ferric Complexes Possessing Different Degrees of Spin Frustration

The synthesis, single-crystal X-ray structures, and magnetochemical properties are reported for two new hexanuclear ferric complexes, [Fe₆(µ₃-O)₂(µ₂-OH)₂(µ₂-O₂CCH₃)₁₀(C₇H₁₁N₂O)₂]·4CH₂Cl₂ (9.4CH₂Cl₂) and [Fe₆(µ₃ O)₂(C₆H₆NO)₈Cl₄](ClO₄)₂·4MeCN (10.4MeCN). The ligand C₇H₁₁N₂O⁻ is the anion of 2-(N-methylimidazol-2-yl)-2-hydroxypropane and C₆H₆NO⁻ is the anion of 2-pyrididylcarbinol. The reaction of [Fe₃O(OAc)₆(py)₃]ClO₄ in acetonitrile with 2-(N-methylimidazol-2-yl)-2-hydroxypropane gives a brown oil which can be crystallized via vapor diffusion of CH₂Cl₂ with hexanes to give complex 9.4CH₂Cl₂. The reaction of FeCl₃·6H₂O and 2-pyridylcarbinol in acetonitrile gives, upon addition of NaClO₄, complex 10.4MeCN. Complex 9 has six high-spin Fe^III ions and can be viewed as two trinuclear µ₃-oxo-bridged subunits bridged by two µ₂-hydroxo and µ₂-acetato ligands. Complex 10 can also be viewed as two asymmetric triangular Fe₃-M₃-oxo subunits bridged together by alkoxo groups. However, in complex 9 there is a planar array of six Fe^III ions, whereas in complex 10 the six high-spin Fe^III ions are arranged in a chair conformation. Variable temperature DC magnetic susceptibility data measured at 10.0 kG are presented for both complexes. For complex 9^·CH₂Cl₂ µ_eff/ molecule was found to be 9.10 μ_Β at 300.0 K, and as the temperature is decreased, this value increases to a maximum of 10.55 μ_Β at 30.0 K, whereupon there is a decrease to 9.77 μ_B at 5.00 K. Complex 10-MeCN in a 10.0 kG field gives μ_eff/molecule = 8.82 μ_Β at 320.0 K. In contrast to 9, the μ_eff/ molecule for complex 10-MeCN decreases with decreasing temperature to 6.08 μ_Β at 5.01 K. Least-squares Fitting of the reduced magnetization (Μ/Nμ_B) versus H/T data for paraffin-embedded complex 9·CH₂Cl₂ in the range of 5.00–50.0 kG external field and 2.0–30.0 K clearly shows that complex 9 has a well isolated S_T = 5 ground state. Reduced magnetization versus H/T data are also presented for a parafilm-embedded sample of complex 10·MeCN in external fields of 0.50–50 kG at temperatures of 2–30 K. Fitting the high-field data suggests the presence of a S_T = 3 ground state. However, the fit of the low-field data is not good for just an isolated S_T = 3 ground state. Theoretical calculations were carried out for two of the known Fe^III6 complexes. The energies of all of the 4332 different spin states of a Fe^III6 complex were calculated, taking into account the pairwise magnetic exchange interactions within μ₃-οxο-bridged Fe^III₃ triangular subunits (parameters J₁, J₂, and J₃) and the interaction (J₄) between iron ions in two Fe₃O triangular subunits. The 20–320 K data measured at 10.0 kG for complex 5 (isostructural to 9) could be fit well with a theoretical calculation where J₁ = −5.6(5) cm⁻¹, J₂ = J₃ = −38(1) cm⁻¹, and J₄ = −7.5(1) cm⁻¹. In agreement with experimental data, a well-isolated S_T = 5 ground state is predicted. The theoretical fit of 20–320 K data for complex 10·MeCN in a 10.0 kG field gives fitting parameters of J₁ = J₂ = −18(1) cm⁻¹, J₃ = −52(2) cm⁻¹, and J₄ = −3(2) cm⁻¹. Even though all pairwise exchange interactions are antiferromagnetic, a Fe^III₆ complex can have a ground state with a spin ranging from. S_T = 0 to 5. The ground state found for a given Fe^III complex is dependent upon which pairwise antiferromagnetic interactions are strongest.