Cyano-Bridged MnIIIMIII Single-Chain Magnets with MIII=CoIII, FeIII, MnIII, and CrIII

A series of isostructural cyano-bridged MnIII(h.s.)-MIII(l.s.) alternating chains, [MnIII(5-TMAMsalen)MIII(CN)6]4H2O (5-TMAMsalen2-=N,N'-ethylenebis(5-trimethylammoniomethylsalicylideneiminate), MnIII(h.s.)=high-spin MnIII, MIII(l.s.)=low-spin CoIII, MnCo; FeIII, MnFe; MnIII, MnMn; CrIII, MnCr) was synthesized by assembling [MnIII(5-TMAMsalen)]3+ and [MIII(CN)6]3-. The chains present in the four compounds, which crystallize in the monoclinic space group C2/c, are composed of an [-MnIII-NC-MIII-CN-] repeating motif, for which the -NC-MIII-CN- motif is provided by the [MIII(CN)6]3- moiety adopting a trans bridging mode between [MnIII(5-TMAMsalen)]3+ cations. The MnIII and MIII ions occupy special crystallographic positions: a C2 axis and an inversion center, respectively, forming a highly symmetrical chain with only one kind of cyano bridge. The Jahn-Teller axis of the MnIII(h.s.) ion is perpendicular to the N2O2 plane formed by the 5-TMAMsalen tetradentate ligand. These Jahn-Teller axes are all perfectly aligned along the unique chain direction without a bending angle, although the chains are corrugated with an Mn-Naxis-C angle of about 144°. In the crystal structures, the chains are well separated with the nearest inter-chain MM distance being relatively large at 9Å due to steric hindrance of the bulky trimethylammoniomethyl groups of the 5-TMAMsalen ligand. The magnetic properties of these compounds have been thoroughly studied. MnFe and MnMn display intra-chain ferromagnetic interactions, whereas MnCr is characterized by an antiferromagnetic exchange that induces a ferrimagnetic spin arrangement along the chain. Detailed analyses of both static and dynamic magnetic properties have demonstrated without ambiguity the single-chain magnet (SCM) behavior of these three systems, whereas MnCo is merely paramagnetic with SMn=2 and D/kB=-5.3K (D being a zero-field splitting parameter). At low temperatures, the MnM compounds with M=Fe, Mn, and Cr display remarkably large M versus H hysteresis loops for applied magnetic fields along the easy magnetic direction that corresponds to the chain direction. The temperature dependence of the associated relaxation time for this series of compounds systematically exhibits a crossover between two Arrhenius laws corresponding to infinite-chain and finite-chain regimes for the SCM behavior. These isostructural hetero-spin SCMs offer a unique series of alternating [-Mn-NC-M-CN-] chains, enabling physicists to test theoretical SC