The interplay between low-dimensional quantum correlations and collective ordering is a central issue in strongly correlated quantum matter. Here, we report the magnetic properties of the verdazyl-based Ni complex $\left[\mathrm{Ni}\right(p\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}p\text{−}\mathrm{Br}\left){}_2\right(\mathrm{EtOH}){}_2{\left({\mathrm{H}}_2\mathrm{O}\right)}_2]{\left({\mathrm{NO}}_3\right)}_2$, which realizes a three-dimensionally coupled frustrated square lattice. Magnetization and specific-heat measurements reveal antiferromagnetic ordering at low temperatures and a field-induced phase transition. High-field ESR measurements indicate field-induced decoupling of the $S=1$ spins that mediate the three-dimensional magnetic couplings, resulting in an effective spin-$1/2$ frustrated square lattice. Although the resulting lattice contains both Haldane-type and Tomonaga–Luttinger liquid (TLL) building blocks, no clear Haldane-gap behavior is observed. Instead, pronounced nonlinear magnetization persists up to high fields, suggesting an unusual regime in which Haldane and TLL correlations coexist and compete. These results demonstrate a route toward field-controlled dimensional reduction and emergent quantum states in frustrated square lattices.

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• Frustrated magnetism
• High magnetic fields
• Magnetic coupling
• Magnetic phase transitions
• Molecular magnetism
• 2-dimensional systems
• 3-dimensional systems
• Antiferromagnets