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 FeIII 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 FeIII ions, whereas in complex 10 the six high-spin FeIII 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 µeff/ molecule was found to be 9.10 μΒ at 300.0 K, and as the temperature is decreased, this value increases to a maximum of 10.55 μΒ at 30.0 K, whereupon there is a decrease to 9.77 μB at 5.00 K. Complex 10-MeCN in a 10.0 kG field gives μeff/molecule = 8.82 μΒ at 320.0 K. In contrast to 9, the μeff/ molecule for complex 10-MeCN decreases with decreasing temperature to 6.08 μΒ at 5.01 K. Least-squares Fitting of the reduced magnetization (Μ/NμB) 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 ST = 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 ST = 3 ground state. However, the fit of the low-field data is not good for just an isolated ST = 3 ground state. Theoretical calculations were carried out for two of the known FeIII6 complexes. The energies of all of the 4332 different spin states of a FeIII6 complex were calculated, taking into account the pairwise magnetic exchange interactions within μ3-οxο-bridged FeIII3 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 ST = 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 FeIII6 complex can have a ground state with a spin ranging from. ST = 0 to 5. The ground state found for a given FeIII complex is dependent upon which pairwise antiferromagnetic interactions are strongest.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry