Syntheses, Structural, and Physical Studies of Basic Cr^III and Fe^III Benzilates and Benzoates: Evidence of Antisymmetric Exchange and Distributions of Isotropic and Antisymmetric Exchange Parameters

Abstract

The synthesis, crystal structures, spectroscopic and magnetic properties studies of four trinuclear oxo‐centered carboxylate complexes are reported and discussed: [Cr3(μ3‐O){O2C(OH)Ph2}6(H2O)3](NO3)·2H2O·3Me2CO (1·2H2O·3Me2CO), [Cr3(μ3‐O)(O2CPh)6(H2O)3](NO3)·1.9MeCN·0.5H2O (2·1.9MeCN·0.5H2O), [Fe3(μ3‐O){O2C(OH)Ph2}6(H2O)3](NO3)·5.69H2O (3·5.69H2O), and [Fe3(μ3‐O)(O2CPh)6(H2O)3](NO3)·3MeCN (4·3MeCN). From a crystallographic point of view, the three metal ions form an isosceles triangle in 1 and 3 (benzilato complexes) and an almost equilateral one in 2 and 4 (benzoato complexes). Magnetic susceptibility measurements reveal metal‐dependent isotropic exchange, for the CrIII clusters (1, 2), with the “magnetic symmetry” resembling that of an almost equilateral triangle. For the FeIII clusters the “magnetic symmetry” resembles that of an isosceles triangle. Solid‐state 1H NMR studies for 1, through the measurements of the temperature variation of the relaxation rate 1/T1, reveal that the first excited spin state with ST = 3/2 is well above (ca. 30 cm–1) the ground state with ST = 1/2, consistent with the magnetic susceptibility data. Mössbauer spectroscopic studies give a larger quadrupole splitting, ΔEQ, value for 3 than for 4, because of a larger charge asymmetry in 3 attributed to the different nature of the carboxylato ligands. EPR experiments at X‐band reveal the presence of antisymmetric exchange within the CrIII clusters and distributions of both isotropic and antisymmetric exchange interactions. In the case of 2 the magnitude and distribution of these interactions are greater. EPR spectroscopy for 3 suggests weak interactions between neighboring units, which could be described assuming a dimer of trimers. For complex 4 the EPR spectra indicate a more complicated picture. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Interactions between H-bonded [CuII3(μ3-OH)] triangles; a combined magnetic susceptibility and EPR study

“…It is well known that under the influence of antisymmetric exchange in trinuclear systems comprising half-integer spin ions, the ground S = 1/2 state is characterized by an axial g-tensor with g ⊥ < g iso (vide infra). 4,[47][48][49][50] Accordingly, here the low saturation value of the magnetization of 1 at 2.0 K is attributed to the anisotropy of the ground state, which is further corroborated by the EPR data. Finally, it should be mentioned that this model reproduces the low temperature isothermal magnetization data without the requirement of inter-cluster superexchange interactions.…”

“…2,3,8,[41][42][43][44][45] It has been demonstrated that the low-lying doublets of antiferromagnetically coupled half-integer spin triangles are non-degenerate, a property that is also referred to as "magnetic Jahn-Teller effect". 46 Usually, this non-degeneracy is rationalized by considering a decrease of the magnetic symmetry and/or the inclusion of an antisymmetric exchange term, characterized by a vector G. 4,[47][48][49][50][51] Whereas a dynamic mechanism has been recently proposed for the lifting of the degeneracy through isotropic interactions in highly symmetric triangles without a lowering of their symmetry, 52 static distortions provide a valid model for less symmetric triangles such as 1. Symmetry-lowering of the isotropic interactions was unsuccessful in fitting the experimental data ( Fig.…”

Determination of the Distributions of the Spin-Hamiltonian Parameters in Spin Triangles: A Combined Magnetic Susceptometry and Electron Paramagnetic Resonance Spectroscopic Study of the Highly Symmetric [Cr3O(PhCOO)6(py)3](ClO4)·0.5py

“…meaning. An empirical analysis of the broadening of the g ⊥eff component was first presented by Sanakis et al 28 and subsequently applied in several other cases [29][30][31][32][33][34][35] where significant antisymmetric exchange was present. In those studies, such signals were analysed through the consideration of a series of fictitious axial S = 1/2 systems, of identical g || and distributed g ⊥ values.…”

Magnetic Properties of Paddlewheels and Trinuclear Clusters with Exposed Metal Sites

“…For the chro- mium acetate, the CrÀm 3 -O distance is in the range of 1.88 to 1.94 and the CrÀCr distance is between 3.28 and 3.36 . [57][58][59][60][61] The differences are a result of the different axial ligands. For example, the replacement of water by a stronger donor like pyridine in the axial site leads to a lenghtening of the trans CrÀm 3 -O bonds.…”

“…Since the ratio j J 1 J À1 0 j is much smaller than one, results from EPR spectroscopy and INS can be fitted to equilateral as well as isosceles models. [61] The splittings of the spin states from the exchange couplings can be summarized as follows: The main exchange parameter J 0 creates five distinguishable states of different total spin S T = S 1 + S 2 + S 3 , …, 1/2, forming a fully discernible spin ladder. The coupling constant J 0 can be easily calculated from energy differences using Equation (3).…”