Monomer-to-trimer comparison of slow magnetic relaxation in weakly antiferromagnetically coupled triangular Co(II) complexes.

We report a comparative study of slow magnetic relaxation in two structurally related Co(II) complexes designed to clarify how triangular topology and geometric spin frustration reshape single-molecule magnet behavior. A mononuclear complex 1, [Co(pdms)(py)2] (H2pdms = 1,2-bis(methanesulfonamido)benzene, py = pyridine) behaves as an isolated and distorted tetrahedral [CoN4] single-ion reference. A star-shaped trinuclear analogue 2, [Co3(L)(py)6] (H6L = 2,3,6,7,10,11-hexakis(methanesulfonamido)triphenylene), was constructed by anchoring three "1-like" Co(II) units onto a rigid triphenylene-based H6L ligand to enforce a Co3-triangular arrangement. The susceptibility reveals markedly different magnetic parameters, with 1 showing stronger axial anisotropy (DCo = -21.3 cm-1) and 2 exhibiting reduced anisotropy (DCo = -8.4 cm-1) together with antiferromagnetic interactions (CW = -9.4 K, JCo-Co = -1.88 K). Field-induced ac susceptibility confirms slow magnetic relaxation for both complexes; however, 1 relaxes more slowly than 2 at the same temperature, resulting in an effective energy barrier Ueff = 37.1 cm-1 for 1 and Ueff = 3.7 cm-1 for 2. This contrast demonstrates that trimerization does not necessarily enhance the SMM performance. The faster relaxation in 2 is more conservatively attributed to weak antiferromagnetic exchange, closely spaced exchange-coupled spin states, and the triangular Co3 topology, while geometric frustration is discussed only as a possible contributing factor rather than a uniquely established origin.