Strong spin frustration from isolated triangular Cu(ii) trimers in SrCu(OH)₃Cl with a novel cuprate layer

Strontium oxocuprates such as Sr₂CuO₂Cl₂ have been extensively investigated for their two-dimensional (2D) planar CuO₂ square layers that are typical of high-temperature cuprate superconductors. A new synthetic strontium cuprate SrCu(OH)₃Cl features a novel cuprate layer with isolated triangular Cu(ii) trimers. The crystal structure of the title compound can be derived from breaking the cuprate planar layer to form isolated, nonplanar triangular copper trimers ({[Cu₃(μ₃-Cl)(OH)₃](OH)₆Cl₂}^6-), which are connected by strontium coordination spheres to form a novel framework structure. The copper trimers exhibit an unprecedented array that has triangular Cu₃-planes perpendicular not only to their own layer but also to their counterparts in the next layers, which is thus different from the parallel array in the Kagomé layer of ZnCu₃(OH)₆Cl₂. This cross-orientation arrangement of the triangular Cu(ii) trimers is proposed to be responsible for a strong antiferromagnetic exchange (J = -233(2) cm^-1) in the triangles characterized by a Weiss temperature _cw = -135 K but virtually zero (or negligible) magnetic coupling between the triangles. Such remarkable magnetic properties make the title compound a Mott insulator-an ordered inorganic solid with Cu spin-1/2 spins forming an intrinsic "molecular magnet".