[Fe(bpym)(CN)₄]^-: A New Building Block for Designing Single-Chain Magnets

We herein present the preparation, crystal structure, magnetic properties, and theoretical study of new heterobimetallic chains of formula {[Fe^III(bpym)(CN₄)]₂M^II(H₂O)₂}·6H₂O [bpym = 2,2‘-bipyrimidine; M = Zn (2), Co (3), Cu (4), and Mn (5)] which are obtained by using the building block PPh₄[Fe(bpym)(CN)₄]·H₂O (1) (PPh₄⁺ = tetraphenylphosphonium) as a ligand toward the fully solvated M^II ions. The structure of complex 1 contains mononuclear [Fe(bpym)(CN)₄]^- anions. Compounds 2−5 are isostructural 4,2-ribbonlike bimetallic chains where the [Fe(bpym)(CN)₄]^- unit acts as a bis-monodenate ligand through two of its four cyanide ligands toward the M atom. Water hexamer clusters (4) and regular alternating fused six- and four-membered water rings with two dangling water molecules (2, 3, and 5) are trapped between the cyanide-bridged 4,2-ribbonlike chains. 1 and 2 behave as magnetically isolated low-spin iron(III) centers. 3 behaves as a single-chain magnet (SCM) with intrachain ferromagnetic coupling, slow magnetic relaxation, hysteresis effects, and frequency-dependent ac signals at T < 7 K). As expected for a thermally activated process, the nucleation field (H_n) in 3 increases with decreasing T and increasing v. Below 1.0 K, H_n becomes temperature independent but remains strongly sweep rate dependent. In this temperature range, the reversal of the magnetization may be induced by a quantum nucleation of a domain wall that then propagates due to the applied field. 4 and 5 are ferro- and ferrimagnetic chains respectively, with metamagnetic-like behavior (4). DFT-type calculations and QMC methodology provided a good understanding of the magnetic properties of 3−5.