Observation of two types of magnetization relaxation in a weakly correlated antiferromagnetic chain of Mn(III)2 single-molecule magnets.

A Mn(III)(2)Fe(II) chain with a [-Mn(III)-(O(Ph))(2)-Mn(III)-ON-Fe(II)-NO-] repeat unit was synthesized via the assembling reaction of Mn(III) salen-type dimers and Fe(II) pyridyloximate complexes, where the -(O(Ph))- and -ON- bridges represent a biphenolate bridge and an oximate bridge, respectively. The bulky counter anions, BPh(4)(-), which surround the chain to form a zeolite-like brick wall, successfully isolate the chains from a magnetic point of view. This compound is isostructural with Mn(2)Ni-BPh(4), which is an SCM with S(T) = 3 (H. Miyasaka, A. Saitoh, M. Yamashita and R. Clérac, Dalton Trans., 2008, 2422). Because the Fe(II) unit is diamagnetic, the Mn(III) dimer, which has the potential to be a single-molecule magnet (SMM), is nearly magnetically isolated, although a weak antiferromagnetic interaction with J/k(B)≈-0.1 K is perturbed between the Mn(III) dimers with S(T) = 4 via the Fe(II) unit. Ac susceptibility data shows that two types of relaxation of the magnetization are present, which are attributed to SMM and chain relaxations.

Cyano-bridged Mn(III)-M(III) single-chain magnets with M(III)=Co(III), Fe(III), Mn(III), and Cr(III).

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 Šdue 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.

Three-dimensional antiferromagnetic order of single-chain magnets: a new approach to design molecule-based magnets.

Two one-dimensional compounds composed of a 1:1 ratio of Mn(III) salen-type complex and Ni(II) oximato moiety with different counter anions, PF(6)(-) and BPh(4)(-), were synthesized: [Mn(3,5-Cl(2)saltmen)Ni(pao)(2)(phen)]PF(6) (1) and [Mn(5-Clsaltmen)Ni(pao)(2)(phen)]BPh(4) (2), where 3,5-Cl(2)saltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene)bis(3,5-dichlorosalicylideneiminate); 5-Clsaltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene)bis(5-chlorosalicylideneiminate); pao(-) = pyridine-2-aldoximate; and phen = 1,10-phenanthroline. Single-crystal X-ray diffraction study was carried out for both compounds. In 1 and 2, the chain topology is very similar forming an alternating linear chain with a [-Mn(III)-ON-Ni(II)-NO-] repeating motif (where -ON- is the oximate bridge). The use of a bulky counteranion, such as BPh(4)(-), located between the chains in 2 rather than PF(6)(-) in 1, successfully led to the magnetic isolation of the chains in 2. This minimization of the interchain interactions allows the study of the intrinsic magnetic properties of the chains present in 1 and 2. While 1 and 2 possess, as expected, very similar paramagnetic properties above 15 K, their ground state is antiferromagnetic below 9.4 K and paramagnetic down to 1.8 K, respectively. Nevertheless, both compounds exhibit a magnet-type behavior at temperatures below 6 K. While for 2, the observed magnetism is well explained by a Single-Chain Magnet (SCM) behavior, the magnet properties for 1 are induced by the presence in the material of SCM building units that order antiferromagnetically. By controlling both intra- and interchain magnetic interactions in this new [Mn(III)Ni(II)] SCM system, a remarkable AF phase with a magnet-type behavior has been stabilized in relation with the intrinsic SCM properties of the chains present in 1. This result suggests that the simultaneous enhancement of both intrachain (J) and interchain (J') magnetic interactions (with keeping J >> J'), independently of the presence of AF phase might be an efficient route to design high temperature SCM-based magnets.

A Mn(III)2Ni(II) single-chain magnet separated by a thick isolating network of BPh4- anions.

The assembly reaction of a Mn(III) salen-type dimeric complex, [MnIII2(saltmen)2(H2O)2](ClO4)2 (saltmen2- = N,N'-(1,1,2,2-tetramethylethylene)bis(salycylideneiminate)), and a Ni(II) oximato complex, [Ni(II)(pao)2(phen)] (pao- = pyridine-2-aldoximate; phen = 1,10-phenanthroline), in the presence of NaBPh4 yielded a heterometallic chain of [MnIII2(saltmen)2NiI)(pao)2(phen)](BPh4)2 (Mn2Ni-BPh4) having a [-Mn(III)-ON-Ni(II)-NO-Mn(III)-(OPh)2-] repeating unit surrounded by a "zeolite-like" network of BPh4- anions. Thanks to such bulky ion-walls, each chain is not only structurally separated with a nearest inter-chain metalmetal distance of 14.4 A (MnNi) but also magnetically extremely well isolated. This magnetic isolation and the ferromagnetic interactions between S = 3 anisotropic units constituting the chain play key roles that induce single-chain magnet behavior in this system.

Single-molecule magnet behavior in heterometallic M(II)-Mn(III)(2)-M(II) tetramers (M(II) = Cu, Ni) containing Mn(III) salen-type dinuclear core.

The linear-type heterometallic tetramers, [Mn(III)(2)(5-MeOsaltmen)(2)M(II)(2)(L)(2)](CF(3)SO(3))(2) x 2H(2)O (MII = Cu, 1a; Ni, 2a), where 5-MeOsaltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene) bis(5-methoxysalicylideneiminate), and H(2)L = 3-{2-[(2-hydroxy-benzylidene)-amino]-2-methyl-propylimino}-butan-2-one oxime, have been synthesized and characterized from structural and magnetic points of view. These two compounds are isostructural and crystallize in the same monoclinic P2(1)/n space group. The structure has a [M(II)-NO-Mn(III)-(O)(2)-Mn(III)-ON-M(II)] skeleton, where -NO- is a linking oximato group derived from the non-symmetrical Schiff-base complex [M(II)(L)] and -(O)(2)- is a biphenolato bridge in the out-of-plane [Mn(2)(5-MeOsaltmen)(2)](2+) dimer. The solvent-free compounds, 1b and 2b, have also been prepared by drying of the parent compounds, 1a and 2a, respectively, at 100 degrees C under dried nitrogen. After this treatment, the crystallinity is preserved, and 1b and 2b crystallize in a monoclinic P2(1)/c space group without significant changes in their structures in comparison to 1a and 2a. Magnetic measurements on 1a and 1b revealed antiferromagnetic Mn(III)---Cu(II) interactions via the oximato group and weak ferromagnetic Mn(III)---Mn(III) interactions via the biphenolato bridge leading to an S(T) = 3 ground state. On the other hand, the diamagnetic nature of the square planar Ni(II) center generates an S(T) = 4 ground state for 2a and 2b. At low temperature, these solvated (a) and desolvated (b) compounds display single-molecule magnet behavior modulated by their spin ground state.