One-dimensional (1D) antiferromagnetic chains are fascinating because of their exotic quantum phenomena. However, isolating large-spin S chains remains challenging as even minimal interchain interaction J′ tends to drive unwanted long-range ordering. Here, we report on the synthesis, crystal structure, magnetism, optical, and electronic properties of two isostructural metal–organic frameworks (MOFs), [M₂(mCB-L)₂(μ₂-H₂O)₂(DMF)₄]_n·solv (M = Co(II) (mCB-Co) or Ni(II) (mCB-Ni)), which feature water-bridged Co (S = 3/2) or Ni (S = 1) spin chains that are effectively separated by bulky carborane linkers (1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane, mCBLH₂).

The temperature-dependent susceptibility reveals strong antiferromagnetic interactions with significant intrachain coupling, J_Co/k_B = −4.65 K (mCB-Co) and J_Ni/k_B = −23.36 K (mCB-Ni), yet confirm the absence of long-range order down to 0.3 K due to negligible interchain interactions, as corroborated by specific heat data. This indicates extremely small J′, with J′/J < 4.7 × 10⁻⁴ (3.7 × 10⁻⁵) for Co (Ni) MOFs, making these new materials nearly ideal 1D antiferromagnets. Additionally, optical band gaps were estimated via the Kubelka–Munk method, yielding an increase from 3.83 eV for mCB-Co to 4.20 eV for mCB-Ni, showcasing tunable electronic properties across the two MOFs.