Magnetic properties of a complex with pentagonal bipyramidal Co(II) units forming a triangular spin necklace

We successfully synthesized [Co(m-Py-V)3(NO3)2], in which a Co 2+ ion adopts a seven-coordinate pentagonal bipyramidal (PBP) geometry.Molecular orbital calculations reveal that one of the three verdazyl radicals in the molecule forms a non-magnetic singlet dimer through the strong intermolecular interaction, leading to a triangular spin necklace composed of the remaining spin-1/2 radicals and a spin-3/2 Co 2+ ion.Magnetic susceptibility and specific heat measurements reveal contributions from ferromagnetic and antiferromagnetic exchange interactions, as well as a low-temperature phase transition.Electron spin resonance spectra and magnetization curves are consistently explained using a simplified model that treats the system as an anisotropic S = 3/2 monomer coupled to an isotropic s = 1/2 chain.Our results demonstrate that introducing PBPcoordinated transition-metal centers into radical-based complexes provides an effective route to incorporating strong magnetic anisotropy into designed quantum spin models, establishing a versatile platform for exploring frustrated and anisotropic triangular spin necklaces.PACS numbers: Co-N1 2.16 O1-Co-O2 57.4 Co-N2 2.14 O2-Co-N2 83.4 Co-N3 2.14 N2-Co-O3 85.2 Co-O1 2.24 O3-Co-O4 57.2 Co-O2 2.22 O4-Co-O1 77.0 Co-O3 2.25 N1-Co-O1 82.8 Co-O4 2.19 N1-Co-O2 86.9 N1-Co-N2 89.3 N1-Co-O3 95.7 N1-Co-O4 90.0 N3-Co-O1 93.4 N3-Co-O2 90.6 N3-Co-N2 93.3 N3-Co-O3 87.3 N3-Co-O4 89.6