Hexanuclear ferric complexes possessing different degrees of spin frustration

Abstract

The synthesis, single-crystal X-ray structures, and magnetochemical properties are reported for two new hexanuclear ferric complexes, [Fe6(M3-0)2(M2-OH)2(M2-02CCH3)io(C7HnN20)2]4CH2Cl2 (9-4CH2Cl2) and [Fee0*3-0)2(C6H6N0)8C14](C104)24MeCN (KMMeCN). The ligand C7HnN20-is the anion of 2-(7V-methylimidazol-2yl)-2-hydroxypropane and C6H6NO™ is the anion of 2-pyrididylcarbinol. The reaction of [Fe3G(OAc)6(py)3] C104 in acetonitrile with 2-(iV-methylimidazol-2-yl)-2-hydroxypropane gives a brown oil which can be crystallized via vapor diffusion of CH2C12 with hexanes to give complex 94CH2Cl2• The reaction of FeCl3-6H20 and 2-pyridylcarbinol in acetonitrile gives, upon addition of NaC104, complex KMMeCN. Complex 9 has six high-spin Fem ions and can be viewed as two trinuclear /i3-oxo-bridged subunits bridged by two M2-hydroxo and M2-acetato ligands. Complex 10 can also be viewed as two asymmetric triangular ß3-µ3-subunits bridged together by alkoxo groups. However, in complex 9 there is a planar array of six Fem ions, whereas in complex 10 the six high-spin Fem 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 9-3/2CH2Cl2 Meff/molecule was found to be 9.10 mb at 300.0 K, and as the temperature is decreased, this value increases to a maximum of 10.55 mb at 30.0 K, whereupon there is a decrease to 9.77 mb at 5.00 K. Complex 10-MeCN in a 10.0 kG field gives MCff/molecule = 8.82 mb at 320.0 K. In contrast to 9, the Mefr/ molecule for complex 10-MeCN decreases with decreasing temperature to 6.08 mb at 5.01 K. Least-squares Fitting of the reduced magnetization (M/ µb) versus H/T data for paraffin-embedded complex 9-3/2CH2Cl2 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 Sj = 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 Fem6 complexes. The energies of all of the 4332 different spin states of a Fenl6 complex were calculated, taking into account the pairwise magnetic exchange interactions within µ3-bridged Fem3 triangular subunits (parameters J\, /2, and /3) and the interaction (J4) between iron ions in two Fe30 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 J¡ = -5.6(5) cm-1, /2 = /3 = -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 J¡ = 72 = -18(1) cm-1, /j = -52(2) cm-1, and J4 = -3(2) cm-1.Even though all pairwise exc...

Semiempirical local spin: Theory and implementation of the ZILSH method for predicting Heisenberg exchange constants of polynuclear transition metal complexes

“…It was also assumed that J 12 = J 13 due to approximate symmetry. Values obtained from the fit were J 12 = J 13 = −18 cm −1 , J 23 = −52 cm −1 , and J 24 = −3 cm −1 84, indicating that the ground state has S = 0. However, a variable‐field magnetization experiment indicated that the spin of the ground state is three.…”

“…The base of the triangle is bridged only by the central oxide ligand, and has an iron–iron distance of 3.67 Å. The structure is somewhat unusual, as most triangular iron carboxylate complexes are equilateral 84. Exchange constants (Fig.…”

Molecular magnetism

“…Another very well investigated class of molecules is given by spin rings among which iron rings ("ferric wheels") are most popular [27,28,29,30,31,32,33,34]. Iron-6 rings for instance can host alkali ions such as lithium or sodium which allows to modify the parameters of the spin Hamiltonian within some range [16,35].…”

Salicylaldoxime-Supported Nona- and Tetrametallic Fe^III Cages

“…1) in an attempt to introduce molecules that can compete with the phenolic oximes for the metal coordination sites and in the hope that novel coordination cages would form. The co-ligands were chosen based on their previous success in synthesising Fe III clusters, as reported in the literature [30][31][32][33][34][35][36][37][38][39]. For example, the ligand tacn was used in the synthesis of the first recognised Fe III Single-Molecule Magnet (SMM), [Fe 8 ] [40,41], and hmpH and pdmH 2 have both been used extensively in metal cluster synthesis [30-32, 35, 36, 42-51].…”