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

Abstract

The X-ray crystal structure of the CuII complex [Cu3(μ3-OH)(μ-pz)3(PhCOO)3]- (pz- = pyrazolato anion) shows an isosceles triangular core, further forming a hexanuclear H-bonded aggregate. Cleavage of the H-bonds in solution results in isolated trinuclear species. Analysis of variable temperature magnetic susceptibility data of a powder sample shows an antiferromagnetically-coupled Cu3-core with a doublet ground state and isotropic exchange parameters (Jave = -355 cm-1, Hiso = -JijSiSj). The fitting of magnetic data requires the inclusion of antisymmetric exchange, AE (HAE = Gij·Si × Sj) with Gz = 31.2 cm-1 and no detectable inter-Cu3 isotropic exchange. X-band EPR spectroscopy in a frozen tetrahydrofuran solution of the compound indicates isolated Cu3-species with g‖,eff = 2.25, g⊥,eff = 1.67. The small value of g⊥,eff (≪2.0) is consistent with the presence of AE in agreement with the analysis of the magnetic measurements. The parallel component exhibits a hyperfine pattern corresponding to one I = 3/2 nucleus with A‖ = 425 MHz. This implies a specific exchange coupling scheme obeying the order |J12| = |J13| < |J23| consistent with the crystallographically determined two long and one short CuCu distances. The role of AE in modulating the hyperfine parameters in antiferromagnetic Cu3 clusters is studied. EPR spectra at X- and Q-band were performed with powder samples of the cluster at liquid helium temperatures. The spectra in both bands are consistent with two interacting Sa,b = 1/2 species in the point dipolar approximation. Fitting of the spectra reveals that each spin is characterized by g‖ = 2.24, g⊥ = 1.65 which is in agreement with an isolated Cu3 cluster in the ground state. The determined inter-spin distance of 4.4-4.5 Å is very close to the distance between the Cu(1) and Cu(1)' sites of the two trimeric units as imposed crystallographically (4.3 Å). This constitutes further verification of the specific exchange coupling scheme within each trimer. Magnetostructural correlations previously adopted for antiferromagnetically coupled Cu3 clusters are discussed in the light of the combined magnetic measurements and EPR spectroscopy.

Supramolecular Assemblies of Trinuclear Copper(II)-Pyrazolato Units: A Structural, Magnetic and EPR Study

“…The solid-state structure is characterized by a network of possible dipolar interactions along the chains formed by the trinuclear and mononuclear complexes previously described ( Figure S5). Depending on their magnetic symmetry, i.e., |J| > |J′| versus |J| < |J′|, the spin densities of the triangles may be spread out over all three metal sites, or concentrated on one of them, respectively [13]. Our tentative conclusion from DFT calculation points toward the former case, which is also consistent with the magnetic susceptibility analysis, negating the applicability of the point-dipole approximation and hindering a straightforward analysis of dipolar interactions.…”

“…The antiferromagnetic exchange among the Cu II centers of the triangular Cu 3 (µ 3 -O/OH) units has been studied extensively by us and others [7][8][9][10][11][12]. We have recently turned our attention to interactions between weakly-coupled Cu 3 (µ 3 -O/OH) species and shown that the presence of weak dipolar interactions str evident in the magnetic susceptibility and EPR spectra of H-bonded Cu 3 (µ 3 -OH) units [13]. Continuing along the same lines, we report here the structure and magnetochemical studies of a heptanuclear Cu II assembly employing 4-phenyl-pyrazole ligands and of a polymeric structure containing Cu 3 N 6 metallacycles with terminal and bridging azido ligands.…”

Nitrite Reduction by Trinuclear Copper Pyrazolate Complexes: An Example of a Catalytic, Synthetic Polynuclear NO Releasing System

“…Color coding: Cyan, Cu; blue, N; red, O; black, C; pink, H. Selected interatomic distances (Å) and angles (º): CuCu: 3.350(5)-3.404 (6); Cu-( 3 -OH): 1.979(2)-1.993(2); Cu-N: 1.941(3)-1.930(3); Cu-O(NO): 2.026(3), 2.062(4), 2.141(4); Cu-O-Cu: 115.19(1), 118.28(2).To assess the stability of the complexes in solution, EPR spectra in frozen CH 2 Cl 2 were recorded for 1 and 3 at 4.2 K(Figure 3). Both spectra exhibited similar characteristics typical of antiferromagnetically-coupled half-integer spin triangles experiencing antisymmetric exchange (or Dzyaloshinksii-Moriya) interactions, previously manifested in other complexes of this family 30,34. The spectra of 1(3) consist of an absorption-like peak at g = 2.14(2.18) and a derivative feature at g = 1.87(1.84), with a broad trough centered at g = 1.73(1.74).…”

Polyanisotropic Magnetoelectric Coupling in an Electrically Controlled Molecular Spin Qubit

“…These parameters can also be tailored through molecular design. E.g., properly chosen ligand blends can force a low molecular symmetry that will also be reflected on the magnetic one, as has been shown in copper(II), 46 or iron(III) [47][48][49][50] triangles, with the latter type stabilizing S T = 5/2 ground states in cases of extreme distortions. 49,50 However, this criterion does not necessarily exclude high-symmetry…”