Single-Step Synthesis of An Ideal Chain Antiferromagnet [H₂(4,4′-bipyridyl)](H₃O)₂Fe₂F₁₀ with Spin S=5/2

One-dimensional (1D) magnets are of great interest owing to their intriguing quantum phenomena and potential application in quantum computing. We successfully synthesized an ideal antiferromagnetic spin S=5/2 chain compound [H₂(4,4′-bpy)](H₃O)₂Fe₂F₁₀ (4,4′-bpy=4,4′-bipyridyl) 1, using a single-step low-temperature hydrothermal method under conditions that favors the protonation of the bulky bidentate ligand 4,4′-bpy. Compound 1 consists of well-separated (Fe³⁺−F−)_∞ chains with a large Fe−F−Fe angle of 174.8°. Both magnetic susceptibility and specific heat measurements show that 1 does not undergo a magnetic long-range ordering down to 0.5 K, despite the strong Fe−F−Fe intrachain spin exchange J with J/k_B=−16.2(1) K. This indicates a negligibly weak interchain spin exchange J′. The J′/J value estimated for 1 is extremely small (<2.8×10⁻⁶), smaller than those reported for all other S=5/2 chain magnets. Our hydrothermal synthesis incorporates both [H₂(4,4′-bpy)]²⁺ and (H₃O)⁺ cations into the crystal lattice with numerous hydrogen bonds, hence effectively separating the (Fe³⁺−F−)_∞ spin chains. This single-step hydrothermal synthesis under conditions favoring the protonation of bulky bidentate ligands offers an effective synthetic strategy to prepare well-separated 1D spin chain systems of magnetic ions with various spin values.