Hexanuclear Ferric Complexes Possessing Different Degrees of Spin Frustration

Cheryl A. Christmas, Hui Lien Tsai, Janet M. Kesseiman, Peter K. Gantzel, Raj K. Chadha, Daniel F. Harvey, David N. Hendrickson, Luca Pardi, Dante Gatteschi

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73 Citations (Scopus)

Abstract

The synthesis, single-crystal X-ray structures, and magnetochemical properties are reported for two new hexanuclear ferric complexes, [Fe63-O)22-OH)22-O2CCH3)10(C7H11N2O)2]·4CH2Cl2 (9.4CH2Cl2) and [Fe63 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.

Original languageEnglish
Pages (from-to)12483-12490
Number of pages8
JournalJournal of the American Chemical Society
Volume115
Issue number26
DOIs
Publication statusPublished - 1993 Dec 1

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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