Water-mediated isotropic exchange in an extended Cu(II) crystal lattice: a structural, magnetic and EPR study

We report a study to evaluate the relevance of oriented water molecules in the transmission of through-bond isotropic exchange between two distant S = 1/2 spins. The study is carried out by means of a structural, magnetic and EPR characterization of the copper compound (pyridine-2,6-dicarboxylato)-(pyridine-2,6-dicarboxylic acid)-copper(II) monohydrate (CuDPA). The structure of this compound consists of an extended lattice of Cu(II) ions (S = 1/2, I = 3/2) coordinated to two DPA ligands in a tridentate manner, linked by long pathways in which a oriented water molecule connects two covalent moieties. FTIR experiments complemented by DFT calculations confirmed the importance of this water molecule in stabilizing the crystal structure of CuDPA. X- and Q-band CW-EPR experiments on powder and single crystal samples of CuDPA indicated the presence of isotropic exchange interactions strong enough to collapse both the hyperfine structure due to the Cu(II) nuclei and the resonances of magnetically inequivalent Cu(II) ions. Magnetic susceptibility measurements indicated Curie-Weiss behavior in the 1.8-250 K range with antiferromagnetically coupled Cu(II) spins. The crystal EPR data were rationalized using the theory of Kubo and Tomita, considering that the position of the observed Lorentzian exchange-collapsed resonances is determined by the Zeeman Hamiltonian and that their widths arise from perturbations such as dipolar, anisotropic and antisymmetric exchange, hyperfine coupling and g anisotropy, all modulated by isotropic exchange. Although information about the molecular g_i matrices is lost due to exchange averaging, their eigenvalues and eigenvectors could be reconstructed taking into account the symmetry operations in the crystal lattice of CuDPA. The exchange interaction between magnetically inequivalent copper ions was evaluated from the microwave dependent contribution due to the g anisotropy contribution to the linewidth. Our analysis revealed that, despite the strong distortion of the Cu(II) site, the magnetic ground state of the Cu(II) ion is primarily of the d_x²-y² type. We found that water mediates superexchange through a pathway with a through-bond length of ∼10 Å with |J| = 0.0177(7) cm^-1. The latter demonstrates the importance of structural water molecules in mediating exchange interactions over long distances that are strong enough to determine the spin state of the system.