Cyano‐Bridged Mn^IIIM^III Single‐Chain Magnets with M^III=Co^III, Fe^III, Mn^III, and Cr^III

Abstract

A series of isostructural cyano-bridged Mn(III)(h.s.)-M(III)(l.s.) alternating chains, [Mn(III)(5-TMAMsalen)M(III)(CN)(6)]⋅4H(2)O (5-TMAMsalen(2-)=N,N'-ethylenebis(5-trimethylammoniomethylsalicylideneiminate), Mn(III)(h.s.)=high-spin Mn(III), M(III)(l.s.)=low-spin Co(III), Mn-Co; Fe(III), Mn-Fe; Mn(III), Mn-Mn; Cr(III), Mn-Cr) was synthesized by assembling [Mn(III)(5-TMAMsalen)](3+) and [M(III)(CN)(6)](3-). The chains present in the four compounds, which crystallize in the monoclinic space group C2/c, are composed of an [-Mn(III)-NC-M(III)-CN-] repeating motif, for which the -NC-M(III)-CN- motif is provided by the [M(III)(CN)(6)](3-) moiety adopting a trans bridging mode between [Mn(III)(5-TMAMsalen)](3+) cations. The Mn(III) and M(III) ions occupy special crystallographic positions: a C(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(III)(h.s.) ion is perpendicular to the N(2)O(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(axis) -C angle of about 144°. In the crystal structures, the chains are well separated with the nearest inter-chain M⋅⋅⋅M distance being relatively large at 9 Å 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(Mn)=2 and D/k(B)=-5.3 K (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 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 SCM models between the Ising and Heisenberg limits.

Neutral Low-Dimensional Assemblies of a Mn(III) Schiff Base Complex and Octacyanotungstate(V): Synthesis, Characterization and Magnetic Properties

“…Such a flexion is typical for the cyanide bridged Mn III -M(CN)n complexes [69][70][71]57,77,82,84,85]. As in the case of related 0D [82] and 1D [48] compounds, [Mn III (SB + )M III (CN)6(H2O)], M III = Fe, Mn, and Cr, the Schiff-base ligand is in an envelope conformation with a torsion angle comprising the Nimine-C-C-Nimine core and a dihedral angle between aromatic rings. In addition, unlike the reported in ref.…”

“…In addition, unlike the reported in ref. 48 assembly by means of a mutual bridging of the {Mn(5TMAMsalen)} units by phenolate oxygen atoms similar to in ref [47] and forming the 7(CN)-W-CN-Mn(-O-)2-Mn-NC-W(CN)7 core. However, in such a case the valence stretches of CN would not experience some significant shifting in higher frequencies as it take place for 2.…”

“…Cyano-bridged metal assemblies have provided a large number of compounds with SMM [31][32][33][34][35][36][37][38] and SCM behavior [39][40][41][42][43][44][45][46][47][48]. The greater part of these materials comprises 3d metal ions as the core magnetic units.…”

“…Very recently, the neutral molecular dimeric complexes were studied [82], as well as a row of chain coordination polymers [48] anion upon and iii) to separate spatially the final 1D-polymers to cancel or reduce interchain magnetic interactions through the joint impact of both charge repulsion and sterical hindrance [48]. In the present report we have extended this tactic onto the synthesis of a dinuclear complex [Mn III (5TMAMsalen)(H2O)W V (CN)8](H2O)4.75CH3CN (1) and a coordination chain polymer [Mn III (5TMAMsalen)W V (CN)8](H2O)8CH3CN (2) composed of equally charged constituents.…”

Assembly of Mn(III) Schiff Base Complexes with Heptacyanorhenate (IV)

“…As the SMMs [11] and SCMs [9,39,40] (6). This certainly happened due to the low solubility of the latter.…”