Magnetic properties of the isostructural series of compounds known as Cu9X2(cpa)6 (cpa=carboxypentonic acid; X=F,Cl,Br) are reported as a function of the μ3-halide ion. The interconnection topology within each layer is a hexagonal arrangement of trimeric units, i.e., a ‘‘triangles-in-triangles’’ Kagomé lattice. Magnetic field dependent susceptibility data in the range 1.7–250 K demonstrate that all three compounds have a magnetic ground state which is highly field dependent at lower temperatures, but that phase transitions to long-range order are not observed down to 1.7 K. Development of 2-D antiferromagnetic models appropriate for the chemical nature and symmetry of the Cu9X2(cpa)6 lattice will require a two J-value solution (intratrimer and intertrimer), and although theoretical results for this type of lattice are as yet unknown, analogies can be clearly drawn to the Kagomé problem. The Curie–Weiss temperatures in the range of −250 K strongly suggest spin frustration in the absence of any traditional long-range order. Magnetization studies exhibit an intermediate saturation level corresponding to a ground state of one unpaired spin per unit cell at field values well below 1.0 kG, supporting the conclusion that the two J values are antiferromagnetic.

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