Realization of a triangular spin necklace in a verdazyl-based Ni complex

We successfully synthesized a verdazyl-based complex, ${(m\text{\ensuremath{-}}\mathrm{Py}\text{\ensuremath{-}}\mathrm{V})}_{3}[\mathrm{Ni}{({\mathrm{NO}}_{3})}_{2}]$, in which ${\mathrm{Ni}}^{2+}$ ions and verdazyl radicals form a one-dimensional, triangular spin necklace consisting of spin-1/2 and spin-1 units. Molecular orbital calculations reveal strong antiferromagnetic (AF) interactions between inversion-related radical pairs that form spin-1/2 singlet dimers. The remaining verdazyl and ${\mathrm{Ni}}^{2+}$ spins form frustrated triangular units, creating a distinctive spin network. Magnetic susceptibility and specific heat measurements identify a phase transition to an AF order. The application of magnetic fields suppresses the phase transition signal, suggesting field-induced decoupling of the spin-1 moments. Electron-spin resonance measurements are used to evaluate the easy-axis anisotropy of spin-1, which may promote the AF order. This work provides a rare example of a geometrically frustrated quantum spin chain realized via molecular design, thereby offering a platform for exploring frustration-driven quantum phases in low-dimensional materials.