Cyano-Bridged Mn super(III)--M super(III) Single-Chain Magnets with M super(III)=Co super(III), Fe super(III), Mn super(III), and Cr super(III)

A series of isostructural cyano-bridged Mn super(III)(h.s.)-M super(III)(l.s.) alternating chains, [Mn super(III)(5-TMAMsalen)M super(III)(CN) sub(6)]4H sub(2) O (5-TMAMsalen super(2-)=N ,N'-ethylenebis(5-trimethylammoniomethylsali cylideneiminate), Mn super(III)(h.s.)=high-spin Mn super(III), M super(III)(l.s.)=low-spin Co super(III), Mn--Co; Fe super(III), Mn--Fe; Mn super(III), Mn--Mn; Cr super(III), Mn--Cr) was synthesized by assembling [Mn super(III)(5-TMAMsalen)] super(3+) and [M super(III)(CN) sub(6)] super(3-). The chains present in the four compounds, which crystallize in the monoclinic space group C2/c, are composed of an [-Mn super(III)-NC-M super(III)-CN-] repeating motif, for which the -NC-M super(III)-CN- motif is provided by the [M super(III)(CN) sub(6)] super(3-) moiety adopting a trans bridging mode between [Mn super(III)(5-TMAMsalen)] super(3+) cations. The Mn super(III) and M super(III) ions occupy special crystallographic positions: a C sub(2) axis and an inversion center, respectively, forming a highly symmetrical chain with only one kind of cyano bridge. The Jahn-Teller axis of the Mn super(III)(h.s.) ion is perpendicular to the N sub(2)O sub(2) 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-N sub(axis)-C angle of about 144 degree . In the crystal structures, the chains are well separated with the nearest inter-chain MM distance being relatively large at 9Aa due to steric hindrance of the bulky trimethylammoniomethyl groups of the 5-TMAMsalen ligand. The magnetic properties of these compounds have been thoroughly studied. Mn--Fe and Mn--Mn display intra-chain ferromagnetic interactions, whereas Mn--Cr 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 Mn--Co is merely paramagnetic with S sub(Mn)=2 and D/k sub(B)=-5.3K (D being a zero-field splitting parameter). At low temperatures, the Mn--M 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 seri