Studies of an enneanuclear manganese single-molecule magnet

The reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)] with the tripodal ligand H(3)thme (1,1,1-tris(hydroxymethyl)ethane) affords the enneanuclear complex [Mn(9)O(7)(O(2)CCH(3))(11)(thme)(py)(3)(H(2)O)(2)] 1.1MeCN.1Et(2)O. The metallic skeleton of complex 1 comprises a series of 10 edge-sharing triangles that describes part of an idealized icosahedron. Variable temperature direct current (dc) magnetic susceptibility data collected in the 1.8-300 K temperature range and in fields up to 5.5 T were fitted to give a spin ground state of S = (17)/(2) with an axial zero-field splitting parameter D = -0.29 cm(-)(1). Ac susceptibility studies indicate frequency-dependent out-of-phase signals below 4 K and an effective barrier for the relaxation of the magnetization of U(eff) = 27 K. Magnetic measurements of single crystals of 1 at low temperature show time- and temperature-dependent hysteresis loops which contain steps at regular intervals of field. Inelastic neutron scattering (INS) studies on complex 1 confirm the S = (17)/(2) ground state and analysis of the INS transitions within the zero-field split ground state leads to determination of the axial anisotropy, D = -0.249 cm(-)(1), and the crystal field parameter, B(4)(0) = 7(4) x 10(-)(6) cm(-)(1). Frequency domain magnetic resonance spectroscopy (FDMRS) determined the same parameters as D = -0.247 cm(-)(1) and B(4)(0) = 4.6 x 10(-)(6) cm(-)(1). DFT calculations are fully consistent with the experimental findings of two Mn(II) and four Mn(III) ions "spin up" and three Mn(IV) ions "spin down" resulting in the S = (17)/(2) spin ground state of the molecule, with D = -0.23 cm(-)(1) and U = 26.2 K.     

Structural and magnetic properties of Mn(III) and Cu(II) tetranuclear azido polyoxometalate complexes: multifrequency high-field EPR spectroscopy of Cu4 clusters with S = 1 and S = 2 ground states

Two new azido-bridged polyoxometalate compounds were synthesized in acetonitrile/methanol media and their molecular structures have been determined by X-ray crystallography. The [[(gamma-SiW10O36)Mn2(OH)2(N3)(0.5)(H2O)(0.5)]2(mu-1,3-N3)](10-) (1 a) tetranuclear Mn(III) complex, in which an end-to-end N3- ligand acts as a linker between two [(gamma-SiW10O36)Mn2(OH)2]4- units, represents the first manganese-azido polyoxometalate. The magnetic properties have been studied considering the spin Hamiltonian H = -J1(S1S2+S1*S2*)-J2(S1S1*), showing that antiferromagnetic interactions between the paramagnetic centers (g = 1.98) occur both through the di-(mu-OH) bridge (J1 = -25.5 cm(-1)) and the mu-1,3-azido bridge (J2 = -19.6 cm(-1)). The [(gamma-SiW10O36)2Cu4(mu-1,1,1-N3)2(mu-1,1-N3)2]12- (2 a) tetranuclear Cu(II) complex consists of two [gamma-SiW10O36Cu2(N3)2]6- subunits connected through the two mu-1,1,1-azido ligands, the four paramagnetic centers forming a lozenge. The magnetic susceptibility data have been fitted. This reveals ferromagnetic interactions between the four Cu(II) centers, leading to an S=2 ground state (H = -J1(S1S2+S1*S2*)-J2(S2S2*), J1 = +294.5 cm(-1), J2 = +1.6 cm(-1), g = 2.085). The ferromagnetic coupling between the Cu(II) centers in each subunit is the strongest ever observed either in a polyoxometalate compound or in a diazido-bridged Cu(II) complex. Considering complex 2 a and the previously reported basal-basal di-(mu-1,1-N3)-bridged Cu(II) complexes in which the metallic centers are not connected by other magnetically coupling ligands, the linear correlation J1 = 2639.5-24.95*theta(av) between the theta(av) bridging angle and the J1 coupling parameter has been proposed. The electronic structure of complex 2 a has also been investigated by using multifrequency high-field electron paramagnetic resonance (HF-EPR) spectroscopy between 95 and 285 GHz. The spin Hamiltonian parameters of the S = 2 ground state (D = -0.135(2) cm(-1), E = -0.003(2) cm(-1), g(x) = 2.290(5), g(y) = 2.135(10), g(z) = 2.158(5)) as well as of the first excited spin state S = 1 (D = -0.960(4) cm(-1), E = -0.080(5) cm(-1), g(x) = 2.042(5), g(y) = 2.335(5), g(z) = 2.095(5)) have been determined, since the energy gap between these two spin states is very small (1.6 cm(-1)).     

Studies of a linear single-molecule magnet

Reaction of the dinuclear complex [Mn2O2(bpy)4](ClO4)3 with H3cht (cis,cis-l,3,5-cyclohexanetriol) in MeCN produces the complex [Mn3(Hcht)2(bpy)4](ClO4)3.Et2O.2MeCN (1.Et2O.2MeCN). Dc magnetic susceptibility measurements reveal the existence of weak ferromagnetic exchange between the three Mn ions, leading to a spin ground state of S = 7, with D = -0.23 cm(-1). W-Band (94 GHz) EPR measurements on restrained powdered crystalline samples confirm the S = 7 ground state and determine the ground state zero-field splitting (ZFS) parameters of D = -0.14 cm(-1) and B4(0)= +1.5 x 10(-5) cm(-1). The apparent 4th order behaviour is due to a breakdown of the strong exchange limit approximation (J approximately d, the single-ion ZFS). Single crystal dc relaxation decay and hysteresis loop measurements reveal the molecule to have an appreciable energy barrier to magnetization relaxation, displaying low temperature sweep rate and temperature-dependent hysteresis loops. Density functional studies confirm the ferromagnetic exchange coupling between the Mn ions.     

Ferromagnetic exchange interactions for Cu6(12+) and Mn6(12+) hexagons sandwiched by two B-alpha-[XW9O33]9- (X = As(III) and Sb(III)) ligands in D3d-symmetric polyoxotungstates

Two novel Cu6 and Mn6 hexagon sandwiched polyoxometalates, [(CuCl)6(AsW9O33)2]12- (1a) and [(MnCl)6(SbW9O33)2]12- (2a), have been synthesized and characterized by X-ray single-crystal analysis and magnetic measurements. These complexes are D3d symmetric and were isolated as [n-BuNH3]+ salts from aqueous solutions: (n-BuNH3)12[(CuCl)6(AsW9O33)2].6H2O (1), rhombohedral, R, a = 20.33(1) A, c = 26.35(2) A, Z = 3 and (n-BuNH3)12[(MnCl)6(SbW9O33)2].6H2O (2). Six Cu (or Mn) atoms, each of which shows 5-fold coordination, make an approximately equatorial hexagon with a first-neighboring Cu...Cu (Mn...Mn) distance of 2.913(2) A (3.248(1) A) and a Cu-O-Cu (Mn-O-Mn) bond angle of 94.5(2) degrees (100.4(2) degrees ). The magnetic behavior investigated by magnetic susceptibility measurements shows the ferromagnetic exchange interactions with J/k = +12.7 K (J/hc = +8.82 cm(-1)) and a S = 3 ground state for 1 and J/k = +0.20 K (J/hc = +0.14 cm(-1)) and a S = 15 ground state for 2, when only J refers to the isotropic magnetic-exchange interactions for first-neighbor atoms of the approximately equilateral Cu6(12+) and Mn6(12+) hexagons. The single-crystal ESR spectroscopy of 1 under the orientation of the magnetic field along a Cu6 hexagon's nearly 6-fold axis equal to the c axis on the variation of temperature supports the S = 3 ground state and allows an estimate of the zero-field (fine-structure) energy separation between Sz = 0 and Sz = +/-1 of D = -0.182 K to be obtained.     

Inelastic neutron scattering on three mixed-valence dodecanuclear polyoxovanadate clusters

The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate compounds Na(4)[V(IV)(8)V(V)(4)As(III)(8)O(40)(H(2)O)].23H(2)O, Na(4)[V(IV)(8)V(V)(4)As(III)(8)O(40)(D(2)O)].16.5D(2)O, and (NHEt(3))(4)[V(IV)(8)V(V)(4)As(III)(8)O(40)(H(2)O)].H(2)O were investigated by an inelastic neutron scattering (INS) study using cold neutrons. In addition, the synthesis procedures and the single-crystal X-ray structures of these compounds have been investigated together with the temperature dependence of their magnetic susceptibilities. The magnetic properties below 100 K can be described by simply taking into account an antiferromagnetically exchange coupled tetramer, consisting of four vanadium(IV) ions. Up to four magnetic transitions between the cluster S = 0 ground state and excited states could be observed by INS. The transition energies and the relative INS intensities could be modeled on the basis of the following exchange Hamiltonian: H(ex) = -2J(12)(xy)[S(1x)S(2x)+ S(3x)S(4x)+ S(1y)S(2y)+ S(3y)S(4y)] - 2J(12)(z)[(S(1z)S(2z)+ S(3z)S(4z)] - 2J(23)(xy)[(S(2x)S(3x)+ S(1x)S(4x)+ S(2y)S(3y)+ S(1y)S(4y)] - 2J(23)(z)[(S(2z)S(3z)+ S(1z)S(4z)]. The following sets of parameters were derived: for Na(4)[V(12)As(8)O(40)(H(2)O)].23H(2)O, J(12)(xy)() = J(12)(z)= -0.80 meV, J(23)(xy) = J(23)(z) = -0.72 meV; for Na(4)[V(12)As(8)O(40)(D(2)O)].16.5D(2)O, J(12)(xy) = J(12)(z) = J(23)(xy) = J(23)(z = -0.78 meV; for (NHEt(3))(4)[V(12)As(8)O(40)(H(2)O)].H(2)O, J(12)(xy) = -0.80 meV, J(12)(z) = -0.82 meV, J(23)(xy)() = -0.67 meV, J(23)(z) = -0.69 meV. This study of the same [V(12)As(8)]-type cluster in three different crystal environments allows us to draw some conclusions concerning the applicability on INS in the area of nondeuterated molecular spin clusters. In addition, the effects of using nondeuterated samples and different sample container shapes for INS were evaluated.     

A fourth isolated oxidation level of the [Mn12O12(O2CR)16(H2O)4] family of single-molecule magnets

The Mn12 family of single-molecule magnets (SMMs) has been extended to a fourth isolated member. [Mn12O12(O2CR)16(H2O)4] (1) exhibits three quasi-reversible one-electron-reduction processes at significantly higher potentials than [Mn12O12(O2CMe)16(H2O)4]. This has allowed the previous generation and isolation of the one- and two-electron-reduced versions of 1 to now be extended to the three-electron-reduced complex. For cation consistency and better comparisons, the complete series of complexes has been prepared with NPrn4+ counterions. Thus, complex 1 was treated with 1, 2, and 3 equiv of NPrn4I, and this led to the successful isolation of (NPrn4)[Mn12O12(O2CCHCl2)16(H2O)4] (2), (NPrn4)2[Mn12O12(O2CCHCl2)16(H2O)4] (3), and (NPrn4)3[Mn12O12(O2CCHCl2)16(H2O)4] (4), respectively. Another three-electron-reduced analogue (NMe4)3[Mn12O12(O2CCHCl2)16(H2O)4] (5) was prepared by the addition of 3 equiv of NMe4I to 1. Direct current magnetization data were collected on dried microcrystalline samples of 2-5 and were fit by matrix diagonalization methods to give S = 19/2, D = -0.35 cm(-1), and g = 1.95 for 2; S = 10, D = -0.28 cm(-1), and g = 1.98 for 3; S = 17/2, D = -0.25 cm(-1), and g = 1.91 for 4; and S = 17/2, D = -0.23 cm(-1), and g = 1.90 for 5, where D is the axial zero-field splitting parameter. Thus, the [Mn12]3- complexes 4 and 5 possess significantly decreased absolute magnitudes of both S and D as a result of the three-electron addition to 1, which has S = 10 and D = -0.45 cm(-1). The D value of the series 1-4/5 shows a monotonic decrease with electron addition that is consistent with the progressive loss of MnIII ions, which are the primary source of the molecular anisotropy. Nevertheless, when studied by ac susceptibility techniques, the [Mn12]3- complexes still exhibit frequency-dependent out-of-phase susceptibility signals at < or =2.5 K, indicating them to be single-molecule magnets (SMMs), albeit at lower temperatures compared with 1 (6-8 K range), 2 (4-6 K range), and 3 (2-4 K range); the shifts to lower temperatures reflect the decreasing S and D values upon successive reduction and hence the decreasing energy barrier to magnetization relaxation. Thus, the [Mn12]3- complexes represent a fourth isolated oxidation level of the Mn12 family of SMMs, by far the largest range of oxidation levels yet encountered within single-molecule magnetism.     

The use of high field/frequency EPR in studies of radical and metal sites in proteins and small inorganic models

Low temperature electron paramagnetic resonance (EPR) spectroscopy with frequencies between 95 and 345 GHz and magnetic fields up to 12 T have been used to study radicals and metal sites in proteins and small inorganic model complexes. We have studied radicals, Fe, Cu and Mn containing proteins. For S = 1/2 systems, the high frequency method can resolve the g-value anisotropy. It was used in mouse ribonucleotide reductase (RNR) to show the presence of a hydrogen bond to the tyrosyl radical oxygen. At 285 GHz the type 2 Cu(II) signal in the complex enzyme laccase is clearly resolved from the Hg(II) containing laccase peroxide adduct. For simple metal sites, the systems over S = 1/2 can be described by the spin Hamiltonian: H(S) = BgS + D[Sz2 - S(S + 1)/3 + E/D (Sx2 - Sy2)]. From the high frequency EPR the D-value can be determined directly by, (I) shifts of g(eff) for half-integer spin systems with large D-values as observed at 345 GHz on an Fe(II)-NO-EDTA complex, which is best described as S = 3/2 system with D = 11.5 cm(-1), E = 0.1 cm(-1) and gx = gy = gz = 2.0; (II) measuring the outermost signal, for systems with small D values, distant of (2S - 1) x absolute value(D) from the center of the spectrum as observed in S= 5/2 Fe(III)-EDTA. In Mn(II) substituted mouse RNR R2 protein the weakly interacting Mn(II) at X-band could be observed as decoupled Mn(II) at 285 GHz.     

Magnetic polyoxometalates: anisotropic exchange interactions in the moiety of [(NaOH2)Co3(H2O)(P2W15O56)2]17-

The magnetic exchange interactions in a C0(3)(11) moiety encapsulated in Na(17) [(NaOH(2))Co(3)(H(2)O)(P(2)W(15)O(56))(2)] (NaCo(3)) were studied by a combination of magnetic measurements (magnetic susceptibility and low-temperature magnetization), with a detailed Inelastic Neutron Scattering (INS) investigation. The novel structure of the salt was determined by X-ray crystallography. The ferromagnetic Co(3)O(14) triangular cluster core consists of three octahedrally oxo-coordinated Co(II) ions sharing edges. According to the single-ion anisotropy and spin-orbit coupling usually assumed for octahedral Co(II) ions, the appropiate exchange Hamiltonian to describe the ground-state properties of the isosceles triangular Co(3) spin cluster is anisotropic and is expressed as H = - 2sigma(alpha)(=)(x,y,z)(J(alpha)(12)S(1alpha)S(2alpha) + J(alpha)(23)S(2alpha)S(3alpha) + J(alpha)(13)S(1alpha)S(3alpha)), where J(alpha) are the components of the exchange interactions between the Co(II) ions. To reproduce the INS data, nonparallel anisotropic exchange tensors needed to be introduced, which were directly connected to the molecular symmetry of the complex. The following range of parameters (value +/- 0.5 cm(-1)) was found to reproduce all experimental information while taking magnetostructural relations into account: J(x)(12) = J(y)(13) = 8.6 cm(-1); J(y)(12) = J(x)(13) = 1.4 cm(-1); J(z)(12) = J(z)(13) = 10.0 cm(-1); J(x)(23) = J(y)(23) = 6.5 cm(-1) and = 3.4 cm(-1).     

Half-integer spin heptanuclear single-molecule magnet with an unusual Mn(III)6Mn(II) exchange-coupled core

The synthesis, X-ray crystallography, magnetic properties, and high-field electron paramagnetic resonance (HFEPR) of a new heptanuclear manganese complex [Mn(7)(heamp)(6)](ClO(4))(2)·4CH(2)Cl(2)·H(2)O (complex 2), in which heampH(3) is 2-[N,N-di(2-hydroxyethyl)aminomethyl]phenol (compound 1), is reported. Complex 2 has a hexagonal, disk-shaped topology and contains six Mn(III) ions and a central Mn(II) ion. It crystallizes in the monoclinic space group P2(1)/c with two molecular orientations. Consideration of the cluster topology, together with variable-temperature and variable-field DC magnetic susceptibility data, suggest that complex 2 exists in a half-integer, S = (19)/(2) ± 1 spin ground state, with appreciable uniaxial zero-field splitting (D = -0.16 cm(-1)). AC magnetic susceptibility measurements clearly show out-of-phase signals, which are frequency- and temperature-dependent, indicating slow magnetization relaxation behavior. An analysis of the relaxation data employing the Arrhenius formula yielded an effective relaxation barrier of 12.9 cm(-1). Simulations of HFEPR studies agree with the assignment of an S ≈ (19)/(2) spin ground state, with g = 1.96, D = -4.71 GHz (-0.16 cm(-1)), and a longitudinal fourth-order zero-field splitting parameter B(4)(0) = -2.7 × 10(-4) GHz (-9.0 × 10(-6) cm(-1)).     

Recognition of topological isomerism: synthesis,structure, and magnetic properties of two pentanuclear high-spin molecules of the type [NiI(N-N)2]3[FeIII(CN)6]2

The syntheses, structures, and magnetic properties of two pentanuclear cyanide-bridged compounds are reported. The trigonal bipyramidal molecule [[Ni(tmphen)(2)](3)[Fe(CN)(6)](2)].14H(2)O, (1).14H(2)O (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) crystallizes in the space group P2(1)/c (No. 14) with unit cell parameters a = 19.531(4) A, b = 24.895(5) A, c = 24.522(5) A, beta = 98.68(3) degrees, V = 11787(4) A(3), and Z = 4. The pi-pi interactions between the tmphen ligands provide the closest intermolecular contacts of 3.37 A leading to large intermolecular M...M distances (> 8.68 A). The dc magnetic susceptibility of 1 indicates a ferromagnetically coupled S = 4 ground state best fit to the parameters g = 2.23, J = +4.3 cm(-1), and D(Ni) = +8.8 cm(-1) for the Hamiltonian H = -2J [(S(Fe(1)) + S(Fe(2))).(S(Ni(1)) + S(Ni(2)) + S(Ni(3)))] + D[S(Ni(1))(z)(2) + S(Ni(2))(z)(2) + S(Ni(3))(z)(2)]. The extended square molecule [Ni(bpy)(2)(H(2)O)][[Ni(bpy)(2)](2)[Fe(CN)(6)](2)].12H(2)O, (2).12H(2)O (bpy = 2,2'-bipyridine) crystallizes in the space group P1 (No. 2) with unit cell parameters a = 13.264(3) A, b = 17.607(4) A, c = 18.057(4) A, alpha = 94.58(3) degrees, beta = 103.29(3) degrees, gamma = 95.18(3) degrees, V = 4065(2) A(3), and Z = 2. The pi-pi interactions of 3.29 A between the bpy ligands are the closest intermolecular contacts, and the intermolecular M...M separations are greater than 7.76 A. The dc magnetic susceptibility data for 2 are also in accord with an S = 4 ground state arising from intramolecular ferromagnetic coupling. The data were best fit to the parameters g = 2.25, J = J' = +3.3 cm(-1), and D(Ni) = +5.8 cm(-1) for the Hamiltonian H = -2J[(S(Fe(1)) + S(Fe(2))).(S(Ni(1)) + S(Ni(2)))] - 2J'[(S(Fe(2)).S(Ni(3)))] + D[S(Ni(1))(z)(2) + S(Ni(2))(z)(2) + S(Ni(3))(z)(2)]. No evidence for long-range magnetic ordering was observed for crystalline samples of 1 or 2.     

New routes to polymetallic clusters: fluoride-based tri-, deca-, and hexaicosametallic MnIII clusters and their magnetic properties

The syntheses, structures and magnetic properties of three new MnIII clusters, [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1), [Mn(0O6(OH)2(bta)8(py)8F8] (2) and [NHEt3]2[Mn3O(bta)6F3] (3), are reported (bta=anion of benzotriazole), thereby demonstrating the utility of MnF3 as a new synthon in Mn cluster chemistry. The "melt" reaction (100 degrees C) between MnF(3) and benzotriazole (btaH, C6H5N3) under an inert atmosphere, followed by dissolution in MeOH produces the cluster [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1) after two weeks. Complex 1 crystallizes in the triclinic space group P1, and consists of a complicated array of metal tetrahedra linked by mu3-O2- ions, mu3- and mu2-OH- ions, mu2-MeO- ions and mu2-bta- ligands. The "simpler" reaction between MnF3 and btaH in boiling MeOH (50 degrees C) also produces complex 1. If this reaction is repeated in the presence of pyridine, the decametallic complex [Mn10O6(OH)2(bta)8(py)8F8] (2) is produced. Complex 2 crystallizes in the triclinic space group P1 and consists of a "supertetrahedral" [Mn(III)10] core bridged by six mu3-O2- ions, two mu3-OH- ions, four mu2-F- ions and eight mu2-bta- ions. The replacement of pyridine by triethylamine in the same reaction scheme produces the trimetallic species [NHEt3]2[Mn3O(bta)6F3] (3). Complex 3 crystallises in the monoclinic space group P2(1)/c and has a structure analogous to that of the basic metal carboxylates of general formula [M3O(RCO2)6L3]0/+, which consists of an oxo-centred metal triangle with mu2-bta- ligands bridging each edge of the triangle and the fluoride ions acting as the terminal ligands. DC magnetic susceptibility measurements in the 300-1.8 K and 0.1-7 T ranges were investigated for all three complexes. For each, the value of chi(M)T decreases with decreasing temperatures; this indicates the presence of dominant antiferromagnetic exchange interactions in 1-3. For complex 1, the low-temperature value of chi(M)T is 10 cm(3) K mol(-1) and fitting of the magnetisation data gives S=4, g=2.0 and D=-0.90 cm(-1). For complex 2, the value of chi(M)T falls to a value of approximately 5.0 cm(3) K mol(-1) at 1.8 K, which is consistent with a small spin ground state. For the triangular complex 3, the best fit to the experimental chi(M)T versus T data was obtained for the following parameters: Ja = -5.01 cm(-1), Jb = +9.16 cm(-1) and g=2.00, resulting in an S=2 spin ground state. DFT calculations on 3, however, suggest an S=1 or S=0 ground state with J(a)=-2.95 cm(-1) and J(b)=-2.12 cm(-1). AC susceptibility measurements performed on 1 in the 1.8-4.00 K range show the presence of out-of-phase AC susceptibility signals, but no peaks. Low-temperature single-crystal studies performed on 1 on an array of micro-SQUIDS show the time- and temperature-dependent hysteresis loops indicative of single-molecule magnetism behaviour.     

Family of double-cubane Mn4Ln2 (Ln = Gd, Tb, Dy, Ho) and Mn4Y2 complexes: a new Mn4Tb2 single-molecule magnet

The synthesis and characterization of a family of Mn(2)(III)Mn(2)(II)Ln(III)(2) complexes (Ln = Gd (1), Tb (2), Dy (3), and Ho (4)) of formula [Mn(4)Ln(2)O(2)(O(2)CBu(t))(6)(edteH(2))(2)(NO(3))(2)] are reported, where edteH(4) is N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine. The analogous Mn(4)Y(2) (5) complex has also been prepared. They were obtained from reaction of Ln(NO(3))(3) or Y(NO(3))(3) with Mn(O(2)CBu(t))(2), edteH(4), and NEt(3) in a 2:3:1:2 molar ratio. The crystal structures of representative 1 and 2 were obtained, and their core consists of a face-fused double-cubane [Mn(4)Ln(2)(μ(4)-O(2-))(2)(μ(3)-OR)(4)] unit. Such double-cubane units are extremely rare in 3d metal chemistry and unprecedented in 3d-4f chemistry. Variable-temperature, solid-state dc and ac magnetic susceptibility studies on 1-5 were carried out. Fitting of dc χ(M)T vs T data for 5 gave J(bb) (Mn(III)···Mn(III)) = -32.6(9) cm(-1), J(wb) (Mn(II)···Mn(III)) = +0.5(2) cm(-1), and g = 1.96(1), indicating a |n, 0, n> (n = 0-5) 6-fold-degenerate ground state. The data for 1 indicate an S = 12 ground state, confirmed by fitting of magnetization data, which gave S = 12, D = 0.00(1) cm(-1), and g = 1.93(1) (D is the axial zero-field splitting parameter). This ground state identifies the Mn(II)···Gd(III) interactions to be ferromagnetic. The ac susceptibility data independently confirmed the conclusions about 1 and 5 and revealed that 2 displays slow relaxation of the magnetization vector for the Mn(4)Tb(2) analogue 2. The latter was confirmed as a single-molecule magnet by observation of hysteresis below 0.9 K in magnetization vs dc field scans on a single crystal of 2·MeCN on a micro-SQUID apparatus. The hysteresis loops also displayed well-resolved quantum tunneling of magnetization steps, only the second 3d-4f SMM to do so.     

Heterometallic integer-spin analogues of S = 9/2 Mn4 cubane single-molecule magnets

A family of distorted heterometallic cubanes, [Mn (III) 3Ni (II)(hmp) 3O(N 3) 3(O 2CR) 3], where O 2CR (-) is benzoate ( 1), 3-phenylpropionate ( 2), 1-adamantanecarboxylate ( 3), or acetate ( 4) and hmp (-) is the anion of 2-pyridinemethanol, was synthesized and structurally as well as magnetically characterized. These complexes have a distorted-cubane core structure similar to that found in the S = 9/2 Mn 4 cubane family of complexes. Complexes 1, 3, and 4 crystallize in rhombohedral, hexagonal, and cubic space groups, respectively, and have C 3 molecular symmetry, while complex 2 crystallizes in the monoclinic space group Cc with local C 1 symmetry. Magnetic susceptibility and magnetization hysteresis measurements and high-frequency electron paramagnetic resonance (HFEPR) spectroscopy established that complexes 1-4 have S = 5 spin ground states with axial zero-field splitting (ZFS) parameters ( D) ranging from -0.20 to -0.33 cm (-1). Magnetization versus direct-current field sweeps below 1.1 K revealed hysteresis loops with magnetization relaxation, definitely indicating that complexes 1-4 are single-molecule magnets that exhibit quantum tunneling of magnetization (QTM) through an anisotropy barrier. Complex 2 exhibits the smallest coercive field and fastest magnetization tunneling rate, suggesting a significant rhombic ZFS parameter ( E), as expected from the low C 1 symmetry. This was confirmed by HFEPR spectroscopy studies on single crystals that gave the following parameter values for complex 2: gz = 1.98, gx = gy = 1.95, D = -0.17 cm (-1), B 4 (0) = -6.68 x 10 (-5) cm (-1), E = 6.68 x 10 (-3) cm (-1), and B 4 (2) = -1.00 x 10 (-4) cm (-1). Single-crystal HFEPR data for complex 1 gave g z = 2.02, gx = gy = 1.95, D = -0.23 cm (-1), and B 4 (0) = -5.68 x 10 (-5) cm (-1), in keeping with the C 3 site symmetry of this Mn 3Ni complex. The combined results highlight the importance of spin-parity effects and molecular symmetry, which determine the QTM rates.     

A family of manganese rods: syntheses, structures, and magnetic properties

The reaction of the mixed-valent metal triangles [Mn(3)O(O(2)CR)(6)(py)(3)] (R = CH(3), Ph, C(CH(3))(3)) with the tripodal ligands H(3)thme (1,1,1-tris(hydroxymethyl)ethane) and H(3)tmp (1,1,1-tris(hydroxymethyl)propane) in MeCN, produces a family of manganese rodlike complexes whose structures are all derived from a series of edge-sharing triangles. Variable temperature direct current (dc) magnetic susceptibility data were collected for all complexes in the 1.8-300 K temperature range in fields up to 7.0 T. Complex 1, [Mn(12)O(4)(OH)(2)(PhCOO)(12)(thme)(4)(py)(2)], has an S = 7 ground state with the parameters g = 1.98 and D = -0.13 K. Complex 2, [Mn(8)O(4)((CH(3))(3)CCO(2))(10)(thme)(2)(py)(2)] has a ground state of S = 6, with g = 1.81 and D = -0.36 K. Complex 3, [Mn(7)O(2)(PhCO(2))(9)(thme)(2)(py)(3)], has a spin ground states of S = 7 with the parameters g = 1.78 and D = -0.20 K. The best fit for complex 4, [Mn(6)((CH(3))(3)CCO(2))(8)(tmp)(2)(py)(2)], gave a spin ground state of S = 3 with the parameters g = 1.73 and D = -0.75 K, but was of poorer quality than that normally obtained. The presence of multiple Mn(2+) ions in the structure of 4 leads to the presence of low-lying excited states with energy levels very close to the ground state, and in the case of complex 5, [Mn(6)(CH(3)CO(2))(6)(thme)(2)(H(2)tea)(2)], no satisfactory fit of the data was obtained. DFT calculations on 4 and 5 indicate complexes with spin ground states of S = 4 and S = 0 respectively, despite their topological similarities. Single-crystal hysteresis loop and relaxation measurements show complex 1 to be a SMM.     

Heterometallic cubane single-molecule magnets

Two new heterometallic cubane molecules have been synthesized. High-frequency electron paramagnetic resonance and magnetization measurements indicate that [Mn(3)Ni(hmp)(3)O(N(3))(3)(C(7)H(5)O(2))(3)] (1) displays a well-isolated S = 5 ground state (DeltaE > 120 K), with g = 2.0, D = -0.23 cm(-1), and ferromagnetic Mn-Mn exchange interactions competing with antiferromagnetic Ni-Mn interactions. [Mn(3)Zn(hmp)(3)O(N(3))(3)(C(3)H(5)O(2))(3)] (2) possesses a S = 6 ground state (DeltaE > 105 K), with g = 2.0, D = -0.14 cm(-1), and ferromagnetic Mn-Mn exchange interactions. Magnetization vs magnetic field data for oriented single crystals of 1 and 2 indicate that both complexes are single-molecule magnets.     

An "S"-shaped pentanuclear CuII cluster derived from the metal-assisted hydrolysis of pyCOpyCOpy: structural, magnetic and spectroscopic studies

Reaction of [Cu2(O2CMe)4(H2O)2] with 2,6-di-(2-pyridylcarbonyl)-pyridine (pyCOpyCOpy or dpcp) in MeCN-H2O 10:1, led to the pentanuclear copper(II) complex [Cu5(O2CMe)6{pyC(O)(OH)pyC(O)(OH)py}2] () which crystallizes in the triclinic P1 space group. The copper(II) atoms are arranged in an "S"-shaped configuration, and are bridged by the doubly deprotonated bis(gem-diol) form of the ligand, pyC(O)(OH)pyC(O)(OH)py2-. Magnetic susceptibility data indicate the interplay of both ferro- and antiferromagnetic intramolecular interactions stabilizing an S=3/2 ground state. Fitting of the data according to a next-nearest-neighbour model {H=-[J1(S1S2+S1'S2')+J2(S2S3+S3'S2')+J3(S1S3+S3'S1')+J4(S2S2')]} yields exchange coupling constants J1=+39.7 cm(-1), J2=-15.9 cm(-1), J3=-8.3 cm(-1) and J4=+4.3 cm(-1), leading to an S=3/2 ground state. X-Band EPR spectroscopy indicates a zero-field splitting of the ground state with |D3/2|=0.38 cm(-1).     

Competing interactions in the tetranuclear spin cluster [Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O. An inelastic neutron scattering and magnetic study

The properties of the spin state manifold of the tetranuclear cluster Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O (bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine) are investigated by combining magnetic susceptibility and magnetization measurements with an inelastic neutron scattering (INS) study on an undeuterated sample of Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O. The temperature dependence of the magnetic susceptibility indicates an S = (1)/(2) ground state, which requires antiferromagnetic interactions both between Cr(3+) and Ni(2+) ions and among the Cr(3+) ions. INS reveals potential single-ion anisotropies to be negligibly small and enables an accurate determination of the exchange parameters. The best fit to the experimental energy level diagram is obtained by an isotropic spin Hamiltonian H = J(CrNi)(S(1)().S(4)() + S(2)().S(4)() + S(3)().S(4)()) + J(CrCr)(S(1)().S(2)() + S(1)().S(3)() + S(2)().S(3)()) with J(CrNi) = 1.47 cm(-)(1) and J(CrCr) = 1.25 cm(-)(1). With this model, the experimental intensities of the observed INS transitions as well as the temperature dependence of the magnetic data are reproduced. The resulting overall antiferromagnetic exchange is rationalized in terms of orbital exchange pathways and compared to the situation in oxalato-bridged clusters.     

Direct synthesis, crystal structures, high-field EPR, and magnetic studies of heterometallic polymers containing manganese(II) carboxylates interconnected by [Cu(en)2]2+

Two heterometallic polymers containing cations [Cu(en)2]2+ and either the [Mn(mal)2(H2O)2]2- (1) or [Mn2(succ)2Cl2]n2n- (2) anions (mal=malonate and succ=succinate) were investigated by X-ray crystallography, high-field electron paramagnetic resonance (EPR) spectroscopy, and magnetic susceptibility measurements. Magnetic susceptibility and EPR spectra characteristic of antiferromagnetically coupled Mn2+-Mn2+ pairs were observed in 2, and the exchange integral J=31 cm(-1) (H=JS1S2) as well as the zero-field-splitting parameter D=-3.046 cm(-1) in the triplet state of the dimanganese entity was determined.     

Models for the lower S states of photosystem II: a trinuclear mixed-valent MnII/MnIV/MnII complex

The trinuclear complex Mn(3)(pko)(4)(CH(3)O)(2)(SCN)(2).CH(3)OH, 1, where Hpko is 2,2'-dipyridylketonoxime, is a rare example of a complex simultaneously containing Mn(II) and Mn(IV). X-ray crystallography and XANES spectroscopy clearly distinguish the Mn(II)(2)Mn(IV) valence isomer from the more commonly observed Mn(III)(2)Mn(II) formulation. Fits to variable-temperature magnetic susceptibility data indicate that the Mn(II) and Mn(IV) are ferromagnetically coupled (J = +6.13 cm(-1)) and that 1 has an S = (13)/(2) ground state.     

Magnetic polyoxometalates: anisotropic antiferro- and ferromagnetic exchange interactions in the pentameric cobalt(II) cluster

The ground-state properties of the pentameric Co(II) cluster [Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)](12-) were investigated by combining magnetic susceptibility and low-temperature magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na(12)[Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)].46D(2)O. The encapsulated magnetic Co(5) unit consists of three octahedral and two tetrahedral oxo-coordinated Co(II) ions. Thus, two different types of exchange interactions are present within this cluster: a ferromagnetic interaction between the octahedral Co(II) ions and an antiferromagnetic interaction between the octahedral and the tetrahedral Co(II) ions. As a result of the single-ion anisotropy of the octahedral Co(II) ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co(5) spin cluster is anisotropic and is expressed as H = -2 summation operator(i= x,y,z)J(1)(i)[S(1)(i)S(2)(i) + S(2)(i)S(3)(i)] + J(2)(i)[S(1)(i)S(5)(i) + S(2)(i)S(5)(i) + S(2)(i)S(6)(i) + S(3)(i)S(6)(i)], where J(1)(i) are the components of the exchange interaction between the octahedral Co(II) ions and J(2)(i) are the components of the exchange interaction between the octahedral and tetrahedral Co(II) ions (see Figure 1d). The study of the exchange interactions in the two structurally related polyoxoanions [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10)(-) and [Co(3)W(H(2)O)(2)(ZnW(9)O(34))(2)](12)(-) allowed an independent determination of the ferromagnetic exchange parameters J(1)(x) = 0.70 meV, J(1)(y) = 0.43 meV, and J(1)(z) = 1.51 meV (set a) and J(1)(x) = 1.16 meV, J(1)(y) = 1.16 meV and J(1)(z) = 1.73 meV (set b), respectively. Our analysis proved to be much more sensitive to the size and anisotropy of the antiferromagnetic exchange interaction J(2). We demonstrate that this exchange interaction exhibits a rhombic anisotropy with exchange parameters J(2)(x) = -1.24 meV, J(2)(y) = -0.53 meV, and J(2)(z) = -1.44 meV (set a) or J(1)(x) = -1.19 meV, J(1)(y) = -0.53 meV, and J(1)(z) = -1.44 meV (set b). The two parameter sets reproduce in a satisfactory manner the susceptibility, magnetization, and INS properties of the title compound.