MnII Sites Research Articles

55Mn NQR Study at Mn-II Sites in β-Mn Metal: A Possible Effect of Geometrical Frustration

55 Mn NQR measurements at the Mn-II site in a polycrystalline β-Mn have been performed over the wide temperature range up to 300 K. Fine structures in the NQR spectrum are newly observed below 100 K, indicating that a sufficiently large region of the sample has an antiferromagnetic moment with a small magnitude of ∼ 10 -4 µ B . Taking account of the fact that the spectral shape depends on the sample particle size, we suggest that the surfaces of the sample particles affect the electronic ground state of β-Mn and that the staggered moment extends within 3 µm depth from the surface. A role of the geometrical frustration is discussed. The nuclear spin–lattice relaxation rate 1/ T 1 and the spin–spin relaxation rate 1/ T 2 increase divergently at high temperatures, implying quadrupole relaxation arising from lattice vibrations.

Observation of unusual behavior in [formula omitted] NQR for MnII site in β-Mn metal

β-Mn metal is known not to show any magnetic ordering until low temperature, which is ascribed to an itinerant nearly antiferromagnet or a spin liquid state due to the geometrical frustration at MnII site. We have observed the sudden disappearance of the MnII-NQR signal accompanying by the rapid increase of the nuclear spin–lattice relaxation rate, T 1 −1 above about 200 K . The behavior T 1 −1∝ T below about 100 K is consistent with the self-consistent renormalization theory for itinerant nearly antiferromagnet, however, the unusual behavior described above is difficult to explain in this framework.

Substantial increase of the ordering temperature for [MnII/MoIII(CN)7]-based magnets as a function of the 3d ion site geometry: example of two supramolecular materials with Tc = 75 and 106 K.

Two molecule-based magnets, [Mn(2)(tea)Mo(CN)(7)].H(2)O, 1, and [Mn(2)(tea)Mo(CN)(7)], 2 (tea stands for triethanolamine), formed with the 4d ion building block, [Mo(CN)(7)](4)(-), Mn(II) ions, and an additional ligand, tea, have been prepared and structurally characterized by single-crystal X-ray analyses. Whereas 1 is obtained by a self-assembling process in solution, compound 2 is quantitatively formed through a smooth thermal treatment of 1. Their magnetic properties revealed that these compounds exhibit magnetic ordering at T(c) = 75 and 106 K respectively for compounds 1 and 2. The difference for their critical temperature is attributed to the geometry of the coordination sphere of a Mn(II) site found to be square-pyramidal for 1 and tetrahedral for 2.

N(CH3)4]2[Mn(H2O)]3[Mo(CN)7](2).2H2O: a new high Tc cyano-bridged ferrimagnet based on the [MoIII(CN)7]4- building block and induced by counterion exchange.

The title compound was synthesized by slow diffusion of aqueous solutions containing K4[Mo(CN)7].2H2O, [Mn(H2O)6](NO3)2, and [N(CH3)4]Cl. The compound crystallized in monoclinic space group C2/c, a = 25.8546(14), b = 12.3906(7), c = 13.5382(7) A, beta = 116.4170 (10) degrees, Z = 4, R = 0.0353, wR2 = 0.0456. The MoIII site is surrounded by six -C-N-Mn linkages and one terminal cyano group in a distorted capped-prism fashion. There are two pentahedral MnII sites in the structure, both with four -N-C-Mo linkages and one water molecule. The anisotropic three-dimensional structure consists of connected corrugated gridlike sheet layers parallel to the bc plane. Tetramethylammonium counterions ([N(CH3)4]+) located between these layers seem to induce their distortion. The three-dimensional organization may also be viewed as interconnected octagonal channels propagated along the c axis. The void space of these channels is occupied by coordinated and crystalized water molecules. Temperature and field dependence of the magnetization in both the dc and ac modes have been measured on polycrystalline sample. These investigations have revealed that the compound ordered ferrimagnetically at Tc = 86 K, with a small hysteresis effect. These findings have been compared to those reported previously for three- and two-dimensional materials of the same family.

Synthesis and structure of linear hexanuclear manganese (II) benzoate cluster

From a reaction system including benzoic acid and Mn(NO3)2 in alkali medium, two hexanuclear manganese benzoate cluster compounds have been synthesized. A compound [Et4N]2[Mn6(PhCOO)14] has been structurally characterized, which contains hexanuclear MnII moieties extending unlimitedly to form one-dimensional linear structure. Carboxyl oxygen atoms are bridged in variety of modes to the Mn atoms, forming an arrangement like a sinusoid for the Mn atoms. The structural parameters of these compounds were compared with the data obtained from EXAFS determination for the Mn cluster in the OEC of PSII, supporting that the coordination sphere of the Mn site in the OEC may contain carboxyl bridges. The possible combination modes between the carboxyl group and the Mn atoms have been suggested. The NMR signals exhibit widening and shift produced by the influence of the paramagnetic MnII sites. The red-shift of the absorption in IR spectrum was observed to be attributed to the coordination of the carboxyl group to the Mn atom, supporting the result of the study on crystal structure.

Steric Control of Polynuclear Manganese Complexes by the Use of Tripodal Tetradentate Ligands

Abstract Tripodal tetradentate ligands (H2L1: N,N-bis(2-hydroxybenzyl)-N′,N′-dimethylethylenediamine, H2L2: N-(3,5-di-t-butyl-2-hydroxybenzyl)-N-(2-hydroxybenzyl)-N′,N′-dimethylethylenediamine, H2L3: N,N-bis(3,5-di-t-butyl-2-hydroxybenzyl)-N′,N′-dimethylethylenediamine, H2L4: N,N-bis(2-hydroxy-3-methoxybenzyl)-N′,N′-dimethylethylenediamine, H2L5: N-(3,5-di-t-butyl-2-hydroxybenzyl)-N-(2-hydroxy-3-methoxybenzyl)-N′,N′-dimethylethylenediamine) made it possible to prepare sterically controlled polynuclear manganese complexes. In the presence of carboxylate ligands, in contrast to the L3 ligand which gave mononuclear MnIII complexes, the other ligands afforded mixed-valence trinuclear complexes with an MnIII–MnII–MnIII arrangement. In methanol, the general formula of the products is [Mn3(L)2(carboxylato)2(OCH3)2]. While preparations in acetonitrile generated [Mn3(L1)2(ba)4] (ba = benzoate(1−)) and [Mn3(L2)2(ba)2(OH)2]. The structure of [Mn3(L1)2(ba)4] was determined by X-ray analysis. The three manganese cores are arranged linearly, and the central and terminal ions are bridged by a phenolate and two carboxylate groups. In the case of the L5 ligand, dinuclear and tetranuclear complexes were also obtained and structurally characterized. In the dinuclear complex, [Mn(L5)(CH3OH)(OCH3)MnCl2], a distorted octahedral MnIII and a five-coordinated MnII site are bridged by a phenolate and an alkoxo oxygen donor. The tetranuclear complex, [Mn4(L5)2(ba)6], including an MnIII–MnII–MnII–MnIII arrangement, is regarded as a carboxylato bridged dimer of a dinuclear unit, [Mn(L5)(ba)2Mn]+. Because the bulkiness of the L5 ligand lies between the L2 and L3 ligands, the dinuclear unit is favorable. Variable-temperature magnetic susceptibility measurements showed the ferromagnetic spin-exchange coupling for [Mn(L5)(CH3OH)(OCH3)MnCl2] and the antiferromagnetic one for [Mn3(L1)2(ba)4]. The phenolato and alkoxo bridges give rise to the ferromagnetic exchange interactions between MnIIIand MnII ions. On the other hand, the antiferromagnetic interactions stem from carboxylato bridges.

The antiferromagnetic structure of Mn5Si3

The antiferromagnetic structure of Mn5Si3 at low temperatures (Neel point 68 °K) has been determined by neutron diffraction using single crystals. The magnetic structure is complicated, and the model proposed consists of a modulated non-collinear spin structure propagated through the crystal in three hexagonally related directions. This model gives good agreement with the observed intensities of 50 independent magnetic reflections. The mean moments at the two manganese sites are MnI 0 4 ± 0 1 μB, MnII 1 2 ± 0 1 μB. In addition, the spin densities of the two mutually perpendicular spin components have been calculated separately. A considerable deviation from spherical symmetry at the MnII site is shown by the larger component.