A series of copper compounds were studied with the general formula Cu_nL_m (n = 2, 4, or 5; m = 2 or 3) where L is either a tetradentate or pentadentate pyridyl amidate-based ligand, and the copper atoms are either in a fully cupric state or exist in a mixture of copper(I) and copper(II) centers. The SQUID magnetization of selected compounds was measured from 2 K to 300 K. The electronic structure of each compound was examined using density functional theory calculations. The unbridged dicopper Cu₂L₂ compound exhibited weak ferromagnetic coupling. The di-hydroxy bridged, tetracupric Cu₄L₂ compound exhibited strong antiferromagnetic coupling that was predicted incorrectly with BS-DFT and required multireference methods for accurate prediction. In the case of the mixed-oxidation state Cu₅L₃ compound, a trapped-valence Cu(I)₂Cu(II)₃ state was implicated by analysis of the spin density. In the pentanuclear copper compound the three Cu(II) centers formed a μ³-oxo triangular arrangement and exhibited significant spin-frustration. Adequately fitting the magnetic behavior of this compound required the inclusion of antisymmetric exchange coupling terms in addition to the standard isotropic exchange to account for antiferromagnetic coupling. Another compound contained an unbridged, rectangular arrangement of mixed-valence Cu(I)₂Cu(II) centers whose ground state proved difficult for DFT to describe properly.

Unpaired spin density plot of Cu<sub>5</sub>L<sub>3</sub> superimposed on a reduced magnetic susceptibility plot

Unpaired spin density plot of Cu₅L₃ superimposed on a reduced magnetic susceptibility plot

Brett A Smith

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