Interplay of Anisotropic Exchange Interactions and Single-Ion Anisotropy in Single-Chain Magnets Built from Ru/Os Cyanidometallates(III) and Mn(III) Complex.

Two novel 1D heterobimetallic compounds {[MnIII(SB2+)MIII(CN)6]·4H2O}n (SB2+ = N,N'-ethylenebis(5-trimethylammoniomethylsalicylideneiminate) based on orbitally degenerate cyanidometallates [OsIII(CN)6]3- (1) and [RuIII(CN)6]3- (2) and MnIII Schiff base complex were synthesized and characterized structurally and magnetically. Their crystal structures consist of electrically neutral, well-isolated chains composed of alternating [MIII(CN)6]3- anions and square planar [MnIII(SB2+)]3+ cations bridged by cyanide groups. These -ion magnetic anisotropy of MnIII centers. These results indicate that the presence of compounds exhibit single-chain magnet (SCM) behavior with the energy barriers of Δτ1/kB = 73 K, Δτ2/kB = 41.5 K (1) and Δτ1/kB = 51 K, Δτ2 = 27 K (2). Blocking temperatures of TB = 2.8, 2.1 K and magnetic hysteresis with coercive fields (at 1.8 K) of 8000, 1600 Oe were found for 1 and 2, respectively. Theoretical analysis of the magnetic data reveals that their single-chain magnet behavior is a product of a complicated interplay of extremely anisotropic triaxial exchange interactions in MIII(4d/5d)-CN-MnIII fragments: -JxSMxSMnx-JySMySMny-JzSMzSMnz, with opposite sign of exchange parameters Jx = -22, Jy = +28, Jz = -26 cm-1 and Jx = -18, Jy = +20, Jz = -18 cm-1 in 1 and 2, respectively) and single orbitally degenerate [OsIII(CN)6]3- and [RuIII(CN)6]3- spin units with unquenched orbital angular momentum in the chain compounds 1 and 2 leads to a peculiar regime of slow magnetic relaxation, which is beyond the scope of the conventional Glaubers's 1D Ising model and anisotropic Heisenberg model.

Very Anisotropic 2D Molecular Magnetic Materials Based on Pentagonal Bipyramidal Heptacyanidorhenate(IV).

The first neutral 2D heterometallic assemblies based on orbitally degenerate heptacyanidorhenate(IV) were prepared and structurally characterized. An analysis of the magnetic data for the polycrystalline samples of Ph4P[{Mn(acacen)}2Re(CN)7]·Solv (1) and PPN[{Mn(acacen)}2Re(CN)7]·Solv (2) have shown that both materials display slow magnetic relaxation at temperatures below 10 and 21 K for 1 and 2, respectively. Despite the presence of the same molecular magnetic modules that make up the anionic layers, the studied 2D networks differ significantly in magnetic anisotropy, having a small coercive field (0.115 T) for 1 and a large one (~2.5 T) for 2 at 2 K. In addition, for both polymers a M(H) value does not saturate at the maximum available field of 7 T, and the material 2 is a metamagnet. This intriguing difference originates from the cooperative anisotropic spin interaction in ReIV-CN-MnIII pairs and the zero field splitting (ZFS) effect of MnIII ions with a noncollinear alignment of the local magnetic axes in crystals of the compounds.

Oxazolidine Nitroxide Transformation in a Coordination Sphere of the Ln3+ Ions.

Upon the interaction of the hydrated lanthanide(III) salts found in acetonitrile solution with a tripodal paramagnetic compound, 4,4-dimethyl-2,2-bis(pyridin-2-yl)-1,3-oxazolidine-3-oxyl (Rad), functionalized by two pyridyl groups, three neutral, structurally characterized complexes with diamagnetic polydentate ligands-[Dy(RadH)(hbpm)Cl2], [Yb2(ipapm)2(NO3)4], and [Ce2(ipapm)2(NO3)4(EtOAc)2]-were obtained. These coordination compounds are minor uncolored crystalline products, which were formed in a reaction mixture due to the Rad transformation in a lanthanide coordination sphere, wherein the processes of its simultaneous disproportionation, hydrolysis, and condensation proceed differently than in the absence of Ln ions. The latter fact was confirmed by the formation of the structurally characterized product of the oxazolidine nitroxide transformation during its crystallization in toluene solution. Such a conversion in the presence of 4f elements ions is unique since no similar phenomenon was observed during the synthesis of the 3d-metal complexes with Rad.

Lanthanide Complexes with a Tripodal Nitroxyl Radical Showing Strong Magnetic Coupling.

A series of isomorphous mononuclear complexes of Ln(III) ions comprising one stable tripodal oxazolidine nitroxyl radical were obtained in acetonitrile media starting from nitrates. The compounds, [LnRad(NO3)3] (Ln = Gd, Tb, Dy, Tm, Y; Rad = 4,4-dimethyl-2,2-bis(pyridin-2-yl)-1,3-oxazolidine-3-oxyl), have a molecular structure. Their coordination polyhedron, LnO7N2, can be described as a tricapped trigonal prism with symmetry not far from D3h. The extracted value of 23 cm-1 for the antiferromagnetic coupling of Gd-Rad established from the DC magnetic and EPR data is a record strength for the complexes of 4f elements with nitroxyl radicals. The terbium derivative displays frequency-dependent out-of-phase signals in zero field, indicating single-molecule magnetic behavior. With an applied field of 0.1 T, an effective barrier of 57 cm-1 is found.

Magnetic relaxation of 1D coordination polymers (X)2[Mn(acacen)Fe(CN)6], X = Ph4P⁺, Et4N⁺.

Substitution of the organic cation X in the 1D polymer, (X)2[Mn(acacen)Fe(CN)6], leads to an essential change in magnetic behavior. Due to the presence of more voluminous Ph4P(+) cations, the polyanion has a more geometrically distorted chain skeleton and, as a consequence, enhanced single chain magnet (SCM) characteristics compared to those for Et4N(+). The Arrhenius relaxation energy barriers, the exchange interaction constant and the zero-field splitting anisotropy of Mn(III) are determined from the analysis of magnetic measurements. The discussion is supported with ligand field calculations for [Fe(CN)6](3-) that unveils the significant anisotropy of Fe magnetic moments.

[Mn(III)(Schiff base)]3[Re(IV)(CN)7], highly anisotropic 3D coordination framework: synthesis, crystal structure, magnetic investigations, and theoretical analysis.

A new highly anisotropic coordination heterobimetallic polymer [Mn(III)(Schiff-base)]3[Re(IV)(CN)7] was synthesized and characterized structurally and magnetically. The single crystal X-ray analysis has revealed that this is the first framework among the complexes composed of homoleptic cyanometallate and Mn(III) complex of the tetradentate Schiff base ligand. A formation of 3D assembly is possible due to both the pentagonal bipyrimidal geometry of the cyanometallate unit and suitable size of constituents: [Re(CN)7](3-) and [Mn(III)(acacen)](+), where acacen = N,N'-ethylenebis(acetylacetoneiminato). The powder and crystal magnetic studies show that the compound undergoes an antiferromagnetic ordering of a complicated character below Neel temperature of 13 K, and exhibits a metamagnetic behavior and strong magnetic anisotropy similar to those observed in related 3D Mn(II)-[Mo(CN)7](4-) systems. Unusual magnetic properties of [Mn(III)(acacen)]3[Re(IV)(CN)7] (1) originate from an interplay of Re-Mn anisotropic spin coupling and ZFS effect of Mn(III) ions with a noncollinear orientation of the local magnetic axes in the cyano-bridged 3D network. A theoretical model of anisotropic spin coupling between orbitally degenerate [Re(IV)(CN)7](3-) complexes and Mn(III) ions is developed, and specific microscopic mechanisms of highly anisotropic spin coupling in Re(IV)-CN-Mn(III) linkages in complex 1 are analyzed in detail.

A single chain magnet involving hexacyanoosmate.

The first single chain magnet (SCM) based on orbitally degenerate hexacyanoosmate(III) was prepared. The high values of energy barriers for the [Mn(acacen)Os(CN)6](2-) complex of 81 and 48.4 K are the result of interplay of three axes anisotropic coupling of [Os(CN)6](3-) with zero field splitted Mn(3+).

2[Mn(acacen)]+ + 1[Fe(CN)5NO]- polynuclear heterobimetallic coordination compounds of different dimensionality in the solid state.

Depending on the synthetic conditions, five heterometallic Mn(III)Fe(II) polynuclear compounds with the same ratio of constituents, 2[Mn(acacen)](+)/[Fe(CN)(5)NO](2-), of different nuclearity and dimensionality (0D, 1D, 2D) were isolated. A [Mn(acacen)MeOH](2)[Fe(CN)(5)NO]·1.5MeOH, 1 complex has been prepared by reaction of Mn(III)/Schiff base (SB) complex, [Mn(acacen)Cl] (H(2)acacen is N,N'-ethylenebis(acetylacetoneimine)) with sodium nitroprusside (NP). Single crystal X-ray diffraction analyses reveal that crystallization of 1 from coordinating or non-coordinating solvents results in different coordination polynuclear materials: from C(2)H(5)OH [{Mn(acacen)H(2)O}(2)Fe(CN)(5)NO]·C(2)H(5)OH, 2, a trinuclear complex is formed; from CH(3)CN [{Mn(acacen)H(2)O}(4)Fe(CN)(5)NO][Fe(CN)(5)NO]·4CH(3)CN, an ionic compound with a pentanuclear bimetallic cation is formed 3; from i-C(3)H(7)OH [{Mn(acacen)}(2)(i-PrOH)Fe(CN)(5)NO](n), a coordination chain polymer 4 is formed; from toluene [{Mn(acacen)}(2)Fe(CN)(5)NO](n), a layered network 5 is formed. As the magnetic measurements show, for all compounds the weak interaction between Mn(III)S = 2 spins through the NP bridge is antiferromagnetic and exhibits no significant photoactivity.

Inclusion of a nitronyl nitroxyl radical and its hydrochloride in cucurbit[8]uril.

The recognition properties of cucurbit[8]uril (CB8) toward nitronyl nitroxide 2-(2-benzimidazolyl)-4,4,5,5-tetramethylimidazolidinyl-3-oxide-1-oxy (1) and its hydrochloride have been investigated. 1·HCl led to 1:1 inclusion complex [1·HCl@CB8], which was characterized both in solution and by single-crystal X-ray diffraction. In this compound only the tetramethylimidazolidinyl fragment is included in the host. The magnetic behavior of the complex corresponds to a Curie law with a large separation of the spin carriers in the solid. In contrast, an insoluble species exhibiting ferromagnetic behavior is formed when pure 1 reacts with acid-free CB8. The formula [(1)(2)@(CB8)(3)], in which two radical guests are arranged in such a way that the phenyl groups of the benzimidazolyl substituents are both stacked into one CB8 and the tetramethyl fragments are each capped by a terminal macrocycle, is proposed, in agreement with microanalysis, spectrophotometric, EPR, and magnetic measurements. According to McConnell's rules, the alternating spin densities within the stacked aromatic fragments result in a ferromagnetic interaction (J=+2.3 cm(-1), H=-2JS(1)S(2)) and a triplet ground spin state for the inclusion complex.

Trends in exchange coupling for trimethylenemethane-type Bis(semiquinone) biradicals and correlation of magnetic exchange with mixed valency for cross-conjugated systems.

A magnetostructural correlation (conformational electron spin exchange modulation) within an isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature electron paramagnetic resonance spectroscopy, zero-field splitting parameters, and variable-temperature magnetic susceptibility measurements were used to evaluate molecular conformation and electron spin exchange coupling in this series of molecules. Our combined results indicate that the ferromagnetic portion of the exchange couplings occurs via the cross-conjugated pi-systems, while the antiferromagnetic portion occurs through space and is equivalent to incipient bond formation. Thus, molecular conformation controls the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange parameter correlates with average semiquinone ring torsion angles via a Karplus-Conroy-type relation. Because of the natural connection between electron spin exchange coupling and electronic coupling related to electron transfer, we also correlate the exchange parameters in the biradical complexes to mixed valency in the corresponding quinone-semiquinone radical anions. Our results suggest that delocalization in the cross-conjugated, mixed-valent radical anions is proportional to the ferromagnetic contribution to the exchange coupling in the biradical oxidation states.

Trends in metal-biradical exchange interaction for first-row M(II)(nitronyl nitroxide-semiquinone) complexes.

We report molecular structures and temperature-dependent magnetic susceptibility data for several new metal complexes of heterospin triplet ground-state biradical ligands. The ligands are comprised of both nitronyl-nitroxide (NN) and semiquinone (SQ) spin carriers. Five compounds are five-coordinate M(II) complexes (M = Mn, Co, Ni, Cu, and Zn), and one is a six-coordinate Ni(II) complex. Five compounds were structurally characterized. During copper complex formation a reaction with methanol occurs to form a unique methoxy-substituted SQ ring. Variable-temperature magnetic susceptibility studies are consistent with strong intraligand (NN-SQ and NN-PhSQ) ferromagnetic exchange coupling. For the five-coordinate Mn, Co, and Ni complexes, the S = 1 ligand is antiferromagnetically coupled to the metal. For both the five-coordinate Cu complex and the six-coordinate Ni complex, the ligand is ferromagnetically coupled to the metal spins in accordance with orbital symmetry arguments. Despite the low molecular symmetries, the predicted trend in metal-ligand exchange interactions is supported by spin dimer analysis based on extended Hückel calculations. For (NN-SQ)NiTp(Cum,Me)() (Tp(Cum,Me)() = hydro-tris(3-cumenyl-5-methylpyrazolyl)borate), an antisymmetric exchange term was required for the best fit of the magnetic susceptibility data. Antisymmetric exchange was less important for the other complexes due to inherently smaller Deltag. Finally, it is shown that intraligand exchange coupling is of paramount importance in stabilizing high-spin states of mixed metal-biradical complexes.

High-spin metal complexes containing a ferromagnetically coupled tris(semiquinone) ligand.

The tris-bidentate ligand 1,3,5-tris(5'-tert-butyl-3',4'-dihydroxyphenyl)benzene ((TBCat)(3)Ph) was synthesized. The reaction of this molecule in basic solution with two paramagnetic acceptors, i.e., a nickel(II)minus signtetraazamacrocyclic ligand complex (Ni(CTH)) (CTH = dl-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) and manganese(II)-hydrotris[3-(4'-cumenyl)-5-methylpyrazolyl]borate (Mn(Tp(Cum,Me))), yielded two complexes whose analytical formulas are consistent with those of trinuclear complexes. Spectroscopic and magnetic measurements suggest that these derivatives contain divalent metal ions coordinated to the tris(semiquinone) form of the ligand. Analysis of the magnetic data shows that the pi-connectivity of the ligand enforces ferromagnetic coupling between the three semiquinone units of the molecule, giving rise to complexes with S = 9/2 (M = Ni(II)) and S = 6 (M = Mn(II)) ground states. The coupling within the tris(semiquinone) unit is quite large (J = -26 cm(-1) for the nickel(II) derivative and J = -40 cm(-1) for the manganese(II) one, using the general exchange Hamiltonian H = sigma J(ij)S(i)S(j)), and it is of the same order of magnitude as that observed in an analogous series of bis(semiquinone) complexes that we recently reported.