A rare ligand bridged ferromagnetically coupled MnIV3 complex with a ground spin state of S = 9/2

An exclusively chelating ligand bridged high-valent [MnIV3] complex has been synthesized, in which all Mn(IV) ions are ferromagnetically-coupled to exhibit an S(T) = 9/2 spin ground state.     

A novel nonanuclear CuII carboxylate-bridged cluster aggregate with an S= 7/2 ground spin state

The Cu(II) aggregate in [Cu(9)(cpida)(6)(MeOH)(6)].6(MeOH)[H(3)cpida = 2-(carboxyphenyl)iminodiacetic acid] is made up of two weakly ferromagnetically coupled carboxylate-bridged Cu(4) units that are antiferromagnetically linked through a central Cu(II) to give a Cu(9) core with an S= 7/2 spin ground state.     

A centred, elongated "ferric tetrahedron" with an S= 15/2 spin ground state

The reaction of anhydrous FeCl(3) with 1H-benzotriazole-1-methanol (Bta-CH(2)OH) in MeOH produces the pentanuclear complex [Fe(5)O(2)(OMe)(2)(Bta)(4)(BtaH)(MeOH)(5)Cl(5)], containing a distorted tetrahedron of four Fe ions centred on a fifth. The central Fe is antiferromagnetically coupled to the peripheral Fe ions resulting in an S= 15/2 spin ground state.     

Spectroscopic demonstration of a large antisymmetric exchange contribution to the spin-frustrated ground state of a D3 symmetric hydroxy-bridged trinuclear Cu(II) complex: ground-to-excited state superexchange pathways

The magnetic and electronic properties of a spin-frustrated ground state of an antiferromagnetically coupled 3-fold symmetric trinuclear copper complex (TrisOH) is investigated using a combination of variable-temperature variable-field magnetic circular dichroism (VTVH MCD) and powder/single-crystal EPR. Direct evidence for a low-lying excited S = (1)/(2) state from the zero-field split ground (2)E state is provided by the nonlinear dependence of the MCD intensity on 1/T and the nesting of the VTVH MCD isotherms. A consistent zero-field splitting (Delta) value of approximately 65 cm(-1) is obtained from both approaches. In addition, the strong angular dependence of the single-crystal EPR spectrum, with effective g-values from 2.32 down to an unprecedented 1.2, requires in-state spin-orbit coupling of the (2)E state via antisymmetric exchange. The observable EPR intensities also require lowering of the symmetry of the trimer structure, likely reflecting a magnetic Jahn-Teller effect. Thus, the Delta of the ground (2)E state is shown to be governed by the competing effects of antisymmetric exchange (G = 36.0 +/- 0.8 cm(-1)) and symmetry lowering (delta = 17.5 +/- 5.0 cm(-1)). G and delta have opposite effects on the spin distribution over the three metal sites where the former tends to delocalize and the latter tends to localize the spin of the S(tot) = (1)/(2) ground state on one metal center. The combined effects lead to partial delocalization, reflected by the observed EPR parallel hyperfine splitting of 74 x 10(-4) cm(-1). The origin of the large G value derives from the efficient superexchange pathway available between the ground d(x2-y2) and excited d(xy) orbitals of adjacent Cu sites, via strong sigma-type bonds with the in-plane p-orbitals of the bridging hydroxy ligands. This study provides significant insight into the orbital origin of the spin Hamiltonian parameters of a spin-frustrated ground state of a trigonal copper cluster.     

An oximate-based hexanuclear mixed-valence Mn(III)4Mn(II)2 edge-sharing bitetrahedral core with an St = 5 spin ground state

The synthesis, structures and magnetic properties of two hexanuclear Mn(6) clusters are reported: Mn(6)(mu(4)-O)(2)(dapdo)(2)(dapdoH)(4)(mu(2)-OH)(2)](ClO(4))(2).6MeCN (.6MeCN) and [Mn(6)(mu(4)-O)(2)(dapdo)(2)(dapdoH)(4)(mu(2)-OCH(3))(2)](ClO(4))(2).2Et(2)O (.2Et(2)O) [dapdo(2-) is the dianion of 2,6-diacetylpyridine dioxime and dapdoH(-) is the monoanion of the aforesaid dioxime ligand]. Both complexes are mixed-valent with two Mn(II) and four Mn(III) atoms disposed in an edge-sharing bitetrahedral core. Both complexes and display the same [Mn(III)(4)Mn(II)(2)(mu(4)-O)(2)(mu(2)-OR)(2)](10+) core in which R = H for and R = Me for . The [Mn(III)(4)Mn(II)(2)] core is rather uncommon compared to the reported [Mn(III)(2)Mn(II)(4)] core in the literature. DC magnetic susceptibility measurements on and reveal the presence of competing exchange interactions resulting in an S(t) = 5 ground spin state. The magnetic behavior of the compounds indicates antiferromagnetic coupling between the manganese(iii) centers, whereas the coupling between the manganese(iii) and manganese(ii) is weakly antiferromagnetic or ferromagnetic depending on the bridging environments. Finally the interaction between the manganese(ii) centers from the two fused tetrahedra is weakly ferromagnetic in nature stabilizing S(t) = 5 ground spin state in compounds and .     

Unusual mixed valent FeIIIFeII complex (St = 3/2) stabilised by a reduced bulky 1,2-diketone

The reduction of the bulky 1,2-diketone bis(2,6-diisopropylphenyl)glyoxal () and FeBr(2) with 1.5 equivalents of Na results in a Class 2 mixed valent H.S. Fe(II) L.S. Fe(III) complex (2) with two five-coordinate Fe centres which are antiferromagnetically exchange-coupled to give a total spin S(t) = 3/2 ground state and an S(t) = 5/2 excited state that are separated by about 25 cm(-1) (for Delta(J) approximately 5J).     

Oxidative cleavage of DNA by a new ferromagnetic linear trinuclear copper(II) complex in the presence of H2O2/sodium ascorbate

A new trinuclear copper(II) complex has been synthesized and structurally characterized: [Cu(3)(L)(2)(HCOO)(2)(OH)(2)](infinity) (HL = (N-pyrid-2-ylmethyl)benzenesulfonylamide). In the complex, the central copper ion is six-coordinated. The coordination spheres of the terminal copper atoms are square pyramidal, the apical positions being occupied by a sulfonamido oxygen of the contiguous trimer. As a consequence, the complex can be considered a chain of trinuclear species. The three copper atoms are in a strict linear arrangement, and adjacent coppers are connected by a hydroxo bridge and a bidentate syn-syn carboxylato group. The mixed bridging by a hydroxide oxygen atom and a bidentate formato group leads to a noncoplanarity of the adjacent basal coordination planes with a dihedral angle of 61.4(2) degrees. Susceptibility measurements (2-300 K) reveal a strong ferromagnetic coupling, J = 79 cm(-1), leading to a quartet ground state that is confirmed by the EPR spectrum. The ferromagnetic coupling arises from the countercomplementarity of the hydroxo and formato bridges. The trinuclear complex cleaves DNA efficiently, in the presence of hydrogen peroxide/sodium ascorbate. tert-Butyl alcohol and sodium azide inhibit the oxidative cleavage, suggesting that the hydroxyl radical and singlet oxygen are involved in the DNA degradation.     

Observation of the structural consequences of a reversible S = 3/2 and S = 1/2 spin crossover in the single crystal

Highly saddle shaped iron(III) porphyrin complex 1 showing a novel spin crossover process between the S = 3/2 and S = 1/2 has been crystallographically analyzed at 298, 180, and 80 K. As the temperature is lowered, the lattice contraction has occurred specifically along the b-axis. Correspondingly, the iron-pyridine bonds, which tilt slightly from the b-axis, have decreased by 7.3%. In contrast, the lattice contractions along the a- and c-axes are much smaller and the iron-porphyrin bonds, which almost coincide with the a- and c-axes, have shown much smaller contraction, ca. 2.2%. The large contraction of the specific bonds caused by packing force raises the energy level of the d(z)2 orbital and induces the spin transition. The detailed structural and lattice changes in the single crystal, which may be regarded as a superstructure parallel to a protein matrix, have been clarified.     

Spin-frustrated trinuclear Cu(II) clusters with mixing of 2(S = 1/2) and S = 3/2 states by antisymmetric exchange. 1. Dzialoshinsky-Moriya exchange contribution to zero-field splitting of the S = 3/2 state

The mixing of the spin-frustrated 2(S = 1/2) and S = 3/2 states by the Dzialoshinsky-Moriya (DM) exchange is considered for the Cu 3(II) clusters with strong DM exchange coupling. In the antiferromagnetic Cu 3 clusters with strong DM interaction, the 2(S = 1/2)-S = 3/2 mixing by the in-plane DM exchange ( G x ) results in the large positive contribution 2 D DM > 0 to the axial zero-field splitting (ZFS) 2 D of the S = 3/2 state. The correlations between the ZFS 2 D DM of the excited S = 3/2 state, sign of G z and chirality of the ground-state were obtained. In the isosceles Cu 3 clusters, the in-plane DM exchange mixing results in the rhombic magnetic anisotropy of the S = 3/2 state. Large distortions result in an inequality of the pair DM parameters, that leads to an additional magnetic anisotropy of the S = 3/2 state. In the {Cu 3} nanomagnet, the in-plane DM exchange (Gx, Gy) mixing results in the 58% contribution 2 D DM to the observed ZFS 2 D of the S = 3/2 state. The DM exchange and distortions explain the experimental observation that the intensities of the electron paramagnetic resonance (EPR) transitions arising from the 2(S = 1/2) group of levels of the {Cu 3} nanomagnet are comparable to each other and are 1 order of magnitude weaker than that of the S = 3/2 state. In the ferromagnetic Cu 3 clusters, the in-plane DM exchange mixing of the excited 2(S = 1/2) and the ground S = 3/2 states results in the large negative DM exchange contribution 2 D DM' < 0 to the axial ZFS 2 D of the ground S = 3/2 state.     

Single-molecule magnets: a Mn25 complex with a record S = 51/2 spin for a molecular species

The reaction of MnCl2.4H2O (3 equiv), pyridine-2,6-dimethanol (pdmH2) (10 equiv), and NaN3 (10 equiv) in MeOH/MeCN (1:2 v/v) with NMe4OH (1 equiv) gave [Mn25O18(OH)2(N3)12(pdm)6(pdmH)6](Cl)2.12MeCN (1.12MeCN) in approximately 30% yield. The cation of complex 1 comprises five Mnx layers of three types in an ABCBA arrangement. Fitting of variable-temperature and -field magnetization data establishes that 1 has an S = 51/2 ground state, the largest value for a molecular species. The complex also displays hysteresis loops below 0.6 K in magnetization vs applied field sweeps, establishing it as the largest spin single-molecule magnet to date.     

Slow magnetic relaxation of a ferromagnetic Ni(II)5 cluster with an S = 5 ground state

Complex [Ni 5{pyCOpyC(O)(OMe)py} 2(O 2CMe) 4(N 3) 4(MeOH) 2].2MeOH.2.6H 2O ( 1.2MeOH.2.6H 2O) was synthesized by the reaction of Ni(O 2CMe) 2.4H 2O with pyCOpyCOpy and NaN 3 in refluxing MeOH. It crystallizes in the monoclinic C2/ c space group and consists of five Ni (II) atoms in a helical arrangement. Direct current magnetic susceptibility studies reveal ferromagnetic interactions between the Ni (II) ( S = 1) ions, stabilizing an S = 5 ground state. Alternating current susceptibility experiments revealed the existence of out-of-phase signals indicative of slow magnetic relaxation. Analysis of the signals showed that they are composite, suggesting more than one relaxation process, while analysis of their magnitudes suggests not all molecules undergo slow magnetic relaxation. Magnetization field-sweep experiments revealed hysteresis at 1.8 K, and magnetization decay experiments clearly verified the appearance of slow magnetic relaxation at that temperature.     

Heterometallic hexanuclear cluster with an S = 8 spin ground state: MnII[MnII(hfac)2]3[NiII(pao)3]2 (hfac- = hexafluoroacetylacetonate,pao- = pyridine-2-aldoximate)

The heterometallic Mn(II)(4)Ni(II)(2) title compound has been synthesized and characterized by X-ray crystallography. The compound consists of a Ni-Mn-Ni linear moiety, [[Ni-(mu-NO)(3)](2)-Mn], linked by oximate bridges and three Mn(II) hfac terminal units attached by oximate oxygens in a di-mu-oxo fashion, forming a novel heterometallic cluster: Mn[Mn(hfac)(2)](3)[Ni(pao)(3)](2) (1). Magnetic measurements reveal the antiferromagnetic nature of the oximate pathway between Mn(II) and Ni(II) metal ions, which imposes an unusual high-spin ground state (S = 8) for 1.     

Theoretical study on spin alignments in ferromagnetic heterospin chains with competing exchange interactions: a generalized ferrimagnetic system containing organic biradicals in the singlet ground state

Spin alignments in heterospin chains are examined from numerical calculations of model spin Hamiltonians. The Hamiltonians of the heterospin chains mimic an open-shell molecular assemblage composed of an organic biradical in a singlet (S = 0) ground state and a doublet (S = 1/2) monoradical, which are coupled by intermolecular ferromagnetic exchange interactions. It is found from numerical calculations of the spin Hamiltonians that the spin value S2 of the ground-state singlet biradical embedded in the exchange-coupled assemblage deviates from zero and contributes to the bulk magnetization. The alternating chain is found to have two kinds of ground spin states, a high- and a low-spin state. All the spins are parallel to each other in the high-spin state, which is characterized by the spin correlation function of (S(i).S(j)) = 0.25. On the other hand, the spin alignment in the low-spin state is found to be dependent on the topology of the intermolecular exchange interactions. The energy preference of the two states depends on the relative amplitude of the exchange interactions in the chain. The intermolecular ferromagnetic couplings are competing in the ground-state singlet biradical with the intramolecular antiferromagnetic interaction. The appearance of the two kinds of ground states is attributed to a quantum spin frustration effect inherent in the triangular motif of the competing interactions. Magnetic properties of a zigzag chain complex composed of a nitronyl nitroxide biradical with a singlet ground state and Cu(hfac)2 are examined on the basis of the theoretical calculations. The vanishing magnetic moments, or the product of susceptibility and temperature chiT, at low temperatures observed for the complex are consistent with those of the low-spin state predicted in the theoretical calculations.     

The spin coupling in the diiron complex [Fe2(hpdta)(H2O)3Cl

Density functional, multireference configuration interaction, and modified valence configuration interaction calculations are used to investigate the electronic structure and spin coupling of the dinuclear [Fe(2)(hpdta)(H(2)O)(3)Cl] complex (H(5)hpdta = Hydroxypropane-1,3-diamine-N,N,N',N'-tetraacetic acid). The density functional calculations give evidence of both, states with local high-spin iron centres and states with local low-spin iron centres, the relative energy of which strongly depends on the functional. The splitting of states due to the spin coupling between the high-spin iron centres varies by more than a factor of two for different functionals. In an attempt to study to what extent it is possible to undertake configuration interaction calculations on such binuclear compounds, multireference configuration interaction calculations are performed on a [Fe(2)(OH)(5)(H(2)O)(3)(NH(3))(2)Cl] model complex. The results show that, when correlating only the ten iron 3d orbitals and the four valence orbitals of the bridging OH group, the calculated splitting is still by a factor of about 3 smaller than the value for the splitting inferred from magnetic susceptibility measurements. Modified valence configuration interaction calculations are performed to approximately take into account the influence of orbital relaxation effects of all occupied orbitals in the excited configurations. The exchange splitting is significantly increased, but still smaller than the experimental value.