Solid-State Structure of a Degradation Product Frequently Observed on Historic Metal Objects

In the course of the investigation of glass-induced metal corrosion processes, a microcrystalline sodium copper formate hydroxide oxide hydrate, Cu4Na4O­(HCOO)8­(H2O)4(OH)2, was detected on a series of antique works of art, and its crystal structure was determined ab initio from high-resolution laboratory X-ray powder diffraction data using the method of charge flipping, simulated annealing, and difference-Fourier analysis (P42/n, a = 8.425 109(97) Å, c = 17.479 62(29) Å, V = 1240.747(35) Å3, Z = 8). In the crystal structure, the metal cations are interconnected in a two-dimensional metal–organic framework via the oxygen atoms of the formate, hydroxide, and oxide anions. Doublets of face-sharing square pyramidal Cu2+ polyhedra are linked via a single, central oxide oxygen atom to give a paddle-wheel arrangement, while the Na+ cations are organized in Na2O11 moieties with highly disordered, edge-sharing octahedral coordination. In addition, hydrogen bonding plays an important role in stabilizing the crystal structure.