Hexanuclear manganese(III) single-molecule magnets from derivatized salicylamidoximes

Four novel hexanuclear manganese(III) complexes based on derivatized salicylamidoximes, [Mn^III₆(μ₃-O)₂(O₂CPh)₂(Me₂N-sao)₆(EtOH)₄] (1), [Mn^III₆(μ₃-O)₂(O₂CPh)₂(Me₂N-sao)₆(ⁱPrOH)₄] (2), [Mn^III₆(μ₃-O)₂(O₂CPh)₂(Et₂N-sao)₆(EtOH)₄] (3) and [Mn^III₆(μ₃-O)₂(O₂CPh)₂(Et₂N-sao)₆(ⁱPrOH)₄] (4) (Me₂N-Hsao = dimethylsalicylamidoxime; Et₂N-Hsao = diethylsalicylamidoxime), have been prepared and characterized. Single-crystal X-ray diffraction allows one to determine that 1·2CHCl₃ and 4 crystallize in the triclinic system with space group P(–1), whereas 3 crystallizes in the monoclinic system with space group P2₁/n. dc and ac magnetic susceptibility measurements of 1-4 reveal ferromagnetic coupling between Mn(III) metal ions and single-molecule magnet behaviour. The anisotropy barriers are 56, 52, 71 and 59 K for 1, 2, 3 and 4, respectively.     

Oxalate-bridged mixed-valence trinuclear manganese(II)-manganese(III)-manganese(II) complexes: synthesis and magnetism

Summary Two novel Mn^II-Mn^III-Mn^II oxalato complexes have been synthesized and characterized, namely [Mn₂Mn(ox)₃(L)₄](ClO₄) [L = 1,10-phenanthroline (phen) or 5-nitro-1,10-phenanthroline (NO₂-phen), respectively], where ox stands for the oxalate dianion. Based on i.r., elemental analyses and electronic spectra, these complexes are proposed to have extended oxalatobridged structures consisting of Mn^II and Mn^III ions, in which Mn^III and each Mn^II ion has a distorted octahedral environment. The temperature dependence of magnetic susceptibility for [Mn₂Mn(ox)₃(phen)₄] (ClO₄) was measured over the 4.1–300 K range and the observed data were successfully simulated by an equation based on the spin-Hamiltonian operator ( = -2J( _{1 2} + _{2 3})), giving the exchange integral J = -1.57cm^–1. This indicates weak antiferromagnetic spin exchange interaction between Mn^II and Mn^III ions.     

Synthesis and magnetism of tetrachlorophthalato-bridged manganese(II) binuclear complexes

Summary Four novel manganese(II) binuclear complexes have been prepared and characterized, namely [Mn₂(TCPHTA)(L)₄]-(ClO₄)₂ [where L is 2,2-bipyridyl (bipy), 1,10-phenanthroline (phen), 4,4-dimethyl-2,2-bipyridyl (Me₂bipy) or 5-nitro-1,10-phenanthroline (NO₂-phen) and TCPHTA is the tetrachlorophthalate dianion]. Based on i.r. spectra, elemental analyses, conductivity measurements, extended tetrachlorophthalato-bridged structures consisting of two manganese(II) ions in which each manganese(II) ion has a distorted octahedral environment are proposed for these structures. The temperature dependence of the magnetic susceptibility for [Mn₂(TCPHTA)(phen)₄]-(ClO₄)₂·H₂O was measured over the 4–300 K range and the observed data were successfully simulated by an equation based on the spin Hamiltonian operator ( = –2J ₁· ₂), giving the exchange integralJ = –1.05 cm^–1. This result is indication of a weak antiferromagnetic spin exchange interaction between the metal ions.     

A unique tetranuclear Er(III)4 cluster exhibiting field-induced single-molecule magnetism

A tetranuclear Er(III) compound chelated with N(2)O(2) donors of a tetradentate Schiff base was produced from the self-assembly of the corresponding chemical species. This intriguing cluster shows field-induced slow relaxation of magnetization.     

Thermal decomposition studies on manganese(III) selenito complexes

The heats of immersion of the systems B₂O₃P₂O₅ and Na₂OB₂O₃P₂O₅ in pyridine have been measured by the use of a solution microcalorimeter. In the composition range R R = atomic ratio, B/P) the heat of immersion was relatively small and was almost independent of R. The heat increased markedly with boron content in R > 1. The increase of heat qualitatively parallels that of surface acidity. Introduction of the Na₂O component reduced both heat of immersion and acidity. MgO was immersed in a benzene solution of benzoic acid. The heat of immersion increased with an increase of surface basicity.     

系列双核Ln(III)配合物的晶体结构和磁性

合成了分子式为[Ln~2(phen)~2L],phen=C~1~2H~8N~2[ALn=Nd,L=(CH~3COO)~4(ONO~2)~2,BLn=Sm,L=(C~6H~5COO)~6,CLn=Eu,L=(C~6H~5COO)~6]3种同双核配合物。用X射线四圆衍射仪测定了3种化合物的结构。在化合物A分子中,2个Nd(III)原子由4个CH~3COO^-基团桥联,以phen和ONO~2^-为端基,构成了一个具有C~2对称性的双核分子。配合物B和C具有完全相同的结构,它们是以4个苯甲酸根为桥,2个phen和2个C~6H~5COO^-为端基的中心对称双核分子,其中6个苯甲酸根的成键状态可分为3种状况。在3种化合物中,每个Ln均为9配位,呈不规则多面体。Ln-Ln距离,A为0.397nm,B和C均为0.405nm。测定了各配合物的变温磁化率,通过对磁性质研究,发现化合物A在低温下具有反铁磁物质行为,并由理论拟合,求得了磁参数g,J值。     

系列双核Ln(III)配合物的晶体结构和磁性

合成了分子式为[Ln~2(phen)~2L],phen=C~1~2H~8N~2[ALn=Nd,L=(CH~3COO)~4(ONO~2)~2,BLn=Sm,L=(C~6H~5COO)~6,CLn=Eu,L=(C~6H~5COO)~6]3种同双核配合物。用X射线四圆衍射仪测定了3种化合物的结构。在化合物A分子中,2个Nd(III)原子由4个CH~3COO^-基团桥联,以phen和ONO~2^-为端基,构成了一个具有C~2对称性的双核分子。配合物B和C具有完全相同的结构,它们是以4个苯甲酸根为桥,2个phen和2个C~6H~5COO^-为端基的中心对称双核分子,其中6个苯甲酸根的成键状态可分为3种状况。在3种化合物中,每个Ln均为9配位,呈不规则多面体。Ln-Ln距离,A为0.397nm,B和C均为0.405nm。测定了各配合物的变温磁化率,通过对磁性质研究,发现化合物A在低温下具有反铁磁物质行为,并由理论拟合,求得了磁参数g,J值。     

Synthesis, structure, and properties of low-spin manganese(III)-poly(pyrazolyl)borate complexes

The manganese(III)-bis[poly(pyrazolyl)borate] complexes, Mn(pzb)2SbF6, where pzb- = tetrakis(pyrazolyl)borate (pzTp) (1), hydrotris(pyrazolyl)borate (Tp) (2), or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) (3), have been synthesized by oxidation of the corresponding Mn(pzb)2 compounds with NOSbF6. The Mn(III) complexes are low-spin in solution and the solid state (microeff = 2.9-3.8 microB). X-ray crystallography confirms their uncommon low-spin character. The close conformity of mean Mn-N distances of 1.974(4), 1.984(5), and 1.996(4) A in 1, 2, and 3, respectively, indicates absence of the characteristic Jahn-Teller distortion of a high-spin d4 center. N-Mn-N bite angles of slightly less than 90 degrees within the facially coordinated pzb- ligands produce a small trigonal distortion and effective D3d symmetry in 1 and 2. These angles increase to 90.0(4)degrees in 3, yielding an almost perfectly octahedral disposition of N donors in Mn(Tp*)2+. Examination of structural data from 23 metal-bis(pzb) complexes reveals systematic changes within the metal-(pyrazolyl)borate framework as the ligand is changed from pzTp to Tp to Tp*. These deformations consist of significant increases in M-N-N, N-B-N, and N-N-B angles and a minimal increase in Mn-N distance as a consequence of the steric demands of the 3-methyl groups. Less effective overlap of pyrazole lone pairs with metal atom orbitals resulting from the M-N-N angular displacement is suggested to contribute to the lower ligand field strength of Tp* complexes. Mn(pzb)2+ complexes undergo electrochemical reduction and oxidation in CH3CN. The electrochemical rate constant (ks,h) for reduction of t2g4 Mn(pzb)2+ to t2g3eg2 Mn(pzb)2 (a coupled electron-transfer and spin-crossover reaction) is 1-2 orders of magnitude smaller than that for oxidation of t2g4 Mn(pzb)2+ to t2g3 Mn(pzb)22+. ks,h values decrease as Tp* > pzTp > Tp for the Mn(pzb)2+/0 electrode reactions, which contrasts with the behavior of the comparable Fe(pzb)2+/0 and Co(pzb)2+/0 couples.     

Multiple-decker phthalocyaninato Tb(III) single-molecule magnets and Y(III) complexes for next generation devices

A new magnetic relaxation phenomenon for an Ising dimer of a Tb-phthalocyaninato triple-decker SMM Tb₂(obPc)₃ (1) is reported. In Argand plots, the magnetic relaxation splits from a one-component system into a two-component system (temperature-independent and temperature-dependent regimes) in a dc magnetic field. There was clear evidence that the magnetic relaxation mechanisms for the Tb³⁺ dimer depended heavily on the temperature and the dc magnetic field. The relationships among the molecular structure, ligand field, ground state, and SMM properties in a direct current (dc) magnetic field are discussed. Furthermore, in order to investigate the stability of the complexes in vacuum evaporation (dry) process and the control of their surface morphology after transferring to a surface, we studied the lanthanoid-phthalocyaninato triple-decker molecule Y₂Pc₃ deposited on a Au(1 1 1) surface using a low-temperature scanning tunneling microscope. It is important to both understand and control the quantum properties of Ln-Pc multiple-decker SMMs with an external field and the monolayer or multi-layer structures on a substrate for next generation devices, such as magnetic information storage.     

Topologically constrained manganese(III) and iron(III) complexes of two cross-bridged tetraazamacrocycles

A family of Mn3+ and Fe3+ complexes of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1) and 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (2) has been prepared by the chemical oxidation of the divalent manganese and iron analogues. The ligands are ethylene cross-bridged tetraazamacrocycles derived from cylam and cyclen, respectively. The synthesis and characterization of these complexes, including X-ray crystal structure determinations, are described. The structural evidence demonstrates that the tetradentate ligands enforce distorted octahedral geometries on the metal ions, with two cis sites occupied by labile ligands. Magnetic measurements reveal that the complexes are high spin with typical magnetic moments. Cyclic voltammetry shows reversible redox processes for the Fe3+/Fe2+ couples of the iron(III) complexes, while Mn3+/Mn2+ and Mn4+/Mn3+ couples were observed for the complexes with manganese(III). The manganese chemistry of 1 was studied in depth. The dichloro manganese(III) cation of 1 undergoes facile ligand substitution reactions at the labile, monodentate sites, for example substituting azide for chloride ligands. Air oxidation of the dichloro complex of Mn (1)2+ in basic solution does not give the expected mu-oxo dimeric product common to manganese. Instead, an unusual manganese(III)-OH complex has been isolated from this reaction and structurally characterized. A similar reaction under slightly different conditions gives a putative MnIII(OH)2 complex that metathesizes to MnIII(OMe)2 upon recrystallization from methanol.     

Zeolite-Y enslaved manganese(III) complexes as heterogeneous catalysts

Traditional catalytic procedures for oxidation of phenol produce environmentally undesirable wastes. As a consequence, there is a clear demand for development of an environmentally benign catalytic route for the selective oxidation of phenol. A series of zeolite-Y enslaved Mn(III) complexes with Schiff bases derived from vanillin furoic-2-carboxylic acid hydrazone (VFCH), vanillin thiophene-2-carboxylic acid hydrazone (VTCH), ethylvanillin thiophene-2-carboxylic acid hydrazone (EVTCH), and/or ethylvanillin furoic-2-carboxylic acid hydrazone have been synthesized and characterized by physico-chemical techniques. Catalytic oxidations of phenol using 30% H₂O₂ as an oxidant over [Mn(VTCH)₂·2H₂O]⁺-Y, [Mn(VFCH)₂·2H₂O]⁺-Y, and [Mn(EVTCH)₂·2H₂O]⁺-Y under mild conditions were studied. These zeolite-Y enslaved Mn(III) complexes are stable and recyclable under current reaction conditions.     

Pyrazolylborates and their importance in tuning single-molecule magnet properties of {Fe(III)2Ni(II)} complexes

A new tricyanoferrate(III) building block and a trinuclear single-molecule magnet derivative are described. The treatment of a 2:1 ratio of [NEt(4)][(Tp*(Bn))Fe(III)(CN)(3)]·H(2)O·MeOH [1; Tp*(Bn) = tris(3,5-dimethyl-4-benzyl)pyrazolylborate] with nickel(II) trifluoromethanesulfonate gives {[(Tp*(Bn))Fe(III)(CN)(3)](2)[Ni(II)(DMF)(4)]}·2DMF (2; DMF = N,N-dimethylformamide). The symmetry-equivalent Fe(III)(LS) ions lead to a favorable alignment of anisotropy tensors (i.e., Fe···B axes) in 2, and an energy barrier of Δ(eff)/k(B) = 16.7 K is found for the S(T) = 2 complex.     

Studies on binuclear hydroxo/carboxylato-bridged manganese(III) complexes and a mononuclear manganese(III) complex involving salen type ligands

Synthesis of six hydroxo-bridged binuclear manganese(III) complexes of formulae [MnL-X-MnL](ClO₄) [X = OH (1–6)] along with a mononuclear manganese(III) complex (7) [Mn(L)(L′)(MeOH)₂] [HL′ = 2-(2-hydroxy-phen-yl)benzimidazole] and two carboxylate-bridged binuclear manganese(III) complexes (8) and (9) are described. The complexes have been characterized by the combination of i.r., u.v.-vis spectroscopy, magnetic moments and by their redox properties. The electronic spectra of all the complexes exhibit almost identical features consisting of two d–d bands at ca. 550 and 600 nm, one MLCT band at ca.400 nm, together with two π–π^* intra-ligand transitions at ca. 250 nm and ca.300 nm. Room temperature magnetic data range from μ = 2.7–3.0 BM indicates some super-exchange between the binuclear metal centers via bridging hydroxo/carboxylato groups. The X-ray crystal structure of the binuclear complex (5) revealed that it has a symmetric Mn^IIIN₂O₂ core bridged by a hydroxyl group. The X-ray analysis of the mononuclear complex (7) showed that the manganese-center possesses a distorted octahedral geometry. Electrochemical properties of hydroxo-bridged manganese(III) complexes (1–6) show identical features consisting of an irreversible and a quasi-reversible reduction corresponding to the Mn₂^III → Mn^IIMn^III → Mn^IIMn^II couples in the voltammogram. It was found that electron withdrawing substituents on the ligand result in easier reduction. Complex (7) displays an irreversible reduction at 0.08 V and a reversible oxidation at 0.45V assignable to the Mn^III → Mn^II reduction and Mn^III → Mn^IV oxidation, respectively. The carboxylate-bridged compound (8) exhibits two irreversible oxidations at 0.4 and 0.6 V, probably due to Mn₂^III → Mn^IIIMn^IV → Mn^IVMn^IV oxidations and shows a quasi-reversible reductive wave at −0.85 V, tentatively assigned to Mn₂^III → Mn^IIMn^III reduction.     

Effects of chloride ions on electrochemical reactions of manganese(III) complexes

Electrochemical reactions of manganese(III) complexes, Mn^III(L)X (L; salen, salpn, 5-NO₂–salen or 5-NO₂–salpn, X; Cl, Br or NO₂) and Mn^III(L’)₂X (L’; N-Bu-sal, N-Oct–sal, N-Oct–5-Br–sal or N-Oct–5-NO₂–sal, X; Cl or Br), were investigated by voltammetry at a glassy carbon electrode in the absence/presence of Cl⁻ in acetonitrile solution. By the addition of Cl⁻, oxidation processes of Mn^III(L)X and Mn^III(L’)₂X have been found to be improved from quasi-reversible to reversible, and their oxidation products, [Mn^IV(L)X]⁺ and [Mn^IV(L’)₂X]⁺, were stabilized by the combination with Cl⁻ resulting in [Mn^IV(L)Cl₂] and [Mn^IV(L’)₂Cl₂], respectively. On the other hand, the reduction processes of Mn^III(L)X and Mn^III(L’)₂Cl were not so significantly affected by Cl⁻ as those observed for their oxidation. Other types of manganese(III) complexes and iron(III) complex were also investigated. The present study may clarify the role of Cl⁻ being involved in OEC (oxygen-evolving center) in photosystem II.