Century-known copper salt Cu(OAc)(OMe) proven to be a unique magnetic lattice composed of tetranuclear copper(II) species with a rare binding mode of the acetate anion.

The structure, spectroscopy, and magnetism of a century-old copper salt, Cu(OAc)(OMe), is reported. The crystal structure contains two independent Cu(II) ions, which are both five-coordinated and which are bridged by methoxo and acetate anions to form an infinite 2D network. Thereby the methoxo groups connect Cu1 and Cu2 with their symmetry-generated counterparts Cu1(i) and Cu2(i), respectively, resulting in Cu.Cu distances of 2.9803(10) and 2.9874(10) A. Cu1 and Cu2 themselves are bridged via the carboxylate groups of two acetates leading to a Cu1.Cu2 distance of 2.9473(7) A. The tetranuclear units thus generated are cross-linked via acetate oxygens to form a 2D sheet structure. One of the two independent acetate ligands has a rare binding mode, whereby it acts as a tetradentate syn-anti, syn-anti bridging ligand. The temperature dependence of the magnetic susceptibility was assigned to be dominated by the very strong antiferromagnetic exchange coupling via bis(mu-methoxo) bridges (J(1) = -409(1) cm(-)(1)).