Monoanionic {Mn(NO)}5 and dianionic {Mn(NO)}6 thiolatonitrosylmanganese complexes: [(NO)Mn(L)2]- and [(NO)Mn(L)2]2- (LH2 = 1,2-benzenedithiol and toluene-3,4-dithiol)

The reaction of MnBr(2) and [PPN](2)[S,S-C(6)H(3)-R] (1:2 molar ratio) in THF yielded [(THF)Mn(S,S-C(6)H(3)-R)(2)](-) [R = H (1a), Me (1b); THF = tetrahydrofuran]. Formation of the dimeric [Mn(S,S-C(6)H(3)-R)(2)](2)(2-) [R = H (2a), Me (2b)] was presumed to compensate for the electron-deficient Mn(III) core via two thiolate bridges upon dissolution of complexes 1a and 1b in CH(2)Cl(2). Complex 2a displays antiferromagnetic coupling interaction between two Mn(III) centers (J = -52 cm(-1)), with the effective magnetic moment (mu(eff)) increasing from 0.85 mu(B) at 2.0 K to 4.86 mu(B) at 300 K. The dianionic manganese(II) thiolate complexes [Mn(S,S-C(6)H(3)-R)(2)](2-) [R = H (3a), Me (3b)] were isolated upon the addition of [BH(4)](-) into complexes 1a and 1b or complexes 2a and 2b, respectively. The anionic mononuclear {Mn(NO)}(5) thiolatonitrosylmanganese complexes [(NO)Mn(S,S-C(6)H(3)-R)(2)](-) [R = H (4a), Me (4b)] were obtained from the reaction of NO(g) with the anionic complexes 1a and 1b, respectively, and the subsequent reduction of complexes 4a and 4b yielded the mononuclear {Mn(NO)}(6) [(NO)Mn(S,S-C(6)H(3)-R)(2)](2-) [R = H (5a), Me (5b)]. X-ray structural data, magnetic susceptibility measurement, and magnetic fitting results imply that the electronic structure of complex 4a is best described as a resonance hybrid of [(L)(L)Mn(III)(NO(*))](-) and [(L)(L(*))Mn(III)(NO(-))](-) (L = 1,2-benzenedithiolate) electronic arrangements in a square-pyramidal ligand field. The lower IR v(NO) stretching frequency of complex 5a, compared to that of complex 4a (shifting from 1729 cm(-1) in 4a to 1651 cm(-1) in 5a), supports that one-electron reduction occurs in the {(L)(L(*))Mn(III)} core upon reduction of complex 4a.     

Synthesis, structure, and magnetic properties of a Mn(21) single-molecule magnet

The reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)](ClO(4)) (1; 3Mn(III)) with [Mn(10)O(4)(OH)(2)(O(2)CMe)(8)(hmp)(8)](ClO(4))(4) (2; 10Mn(III)) in MeCN affords the new mixed-valent complex [Mn(21)O(14)(OH)(2)(O(2)CMe)(16)(hmp)(8)(pic)(2)(py)(H(2)O)](ClO(4))(4) (3; 3Mn(II)-18Mn(III); hmp(-) is the anion of 2-(hydroxymethyl)pyridine), with an average Mn oxidation state of +2.85. Complex 3.7MeCN crystallizes in the triclinic space group P. The structure consists of a low symmetry [Mn(21)(micro(4)-O)(4)(micro(3)-O)(12)(micro-O)(16)] core, with peripheral ligation provided by 16 MeCO(2)(-), 8 hmp(-), and 2 pic(-) groups and one molecule each of water and pyridine. The magnetic properties of 3 were investigated by both dc and ac magnetic susceptibility measurements. Fitting of dc magnetization data collected in the 0.1-0.8 T and 1.8-4.0 K ranges gave S = (17)/(2), D approximately -0.086 cm(-)(1), and g approximately 1.8, where S is the molecular spin of the Mn(21) complex and D is the axial zero-field splitting parameter. ac susceptibility studies in the 10-997 Hz frequency range reveal the presence of a frequency-dependent out-of-phase ac magnetic susceptibility (chi(M)' ') signal consistent with slow magnetization relaxation rates. Fitting of dc magnetization decay versus time data to the Arrhenius equation gave a value of the effective barrier to relaxation (U(eff)) of 13.2 K. Magnetization versus applied dc field sweeps exhibited hysteresis. Thus, complex 3 is a new member of the small but growing family of single-molecule magnets.     

A self-assembled complex with a titanium(IV) catecholate core as a potential bimodal contrast agent

A ditopic chelating ligand (H(6)4) that bears catechol and diethylenetriamine-N,N,N',N',N'-pentaacetate (DTPA) has been designed and shown to specifically bind lanthanide(III) ions at the DTPA core ([Ln(H(2)4)(H(2)O)](-)) and further self-assemble with titanium(IV), thereby giving rise to the formation of a supramolecular metallostar complex with a lanthanide(III)-to-titanium(IV) ratio of 3:1, [(Ln4)(3)Ti(H(2)O)(3)](5-) (Ln=La, Eu, Gd). The efficacy of the metallostar complex as a potential bimodal optical/magnetic resonance imaging (MRI) agent has been evaluated. Nuclear magnetic relaxation dispersion (NMRD) measurements for the [(Gd4)(3)Ti(H(2)O)(3)](5-) complex have demonstrated an enhanced r(1) relaxivity that corresponds to 36.9 s(-1) mM(-1) per metallostar molecule at 20 MHz and 310 K, which is a result of a decreased tumbling rate. The ability of the complex to bind to human serum albumin (HSA) was also examined by relaxometric measurements. In addition, upon UV irradiation the [(Gd4)(3)Ti(H(2)O)(3)](5-) complex exhibits broad-band green emission in the range 400-750 nm with a maximum at 490 nm. Taking into account the high relaxivity and luminescence properties, the [(Gd4)(3)Ti(H(2)O)(3)](5-) complex is a good lead compound for the development of efficient bimodal contrast agents.     

Carboxylate-free Mn(III)2Ln(III)2 (Ln = lanthanide) and Mn(III)2Y(III)2 complexes from the use of (2-hydroxymethyl)pyridine: analysis of spin frustration effects

The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [Mn(III)(2)M(III)(2)(OH)(2)(NO(3))(4)(hmp)(4)(H(2)O)(4)](NO(3))(2) (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1-4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic Ln(III) atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5-300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J(1) = -0.16(2) cm(-1) and J(2) = -0.12(1) cm(-1) for the Mn(III)···Mn(III) and Mn(III)···Gd(III) interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = -0.75(3) cm(-1) for the Mn(III)···Mn(III) interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J(1) and J(2).     

Substituent effect on formation of heterometallic molecular wheels: synthesis, crystal structure, and magnetic properties

The reaction of manganese(III) Schiff bases of the type salen(2-) (N,N'-ethylenebis(salicylideneaminato)) with X-substituted (X = CH(3), Cl) pyridinecarboxamide dicyanoferrite(III) [Fe(X-bpb)(CN)(2)](-) gave rise to a series of cyanide-bridged Mn(6)Fe(6) molecular wheels, [Mn(III)(salen)](6)[Fe(III)(bpmb)(CN)(2)](6) x 7H(2)O (1), [Mn(salen)](6)[Fe(bpClb)(CN)(2)](6) x 4H(2)O x 2CH(3)OH (2), [Mn(salen)](6)[Fe(bpdmb)(CN)(2)](6) x 10H(2)O x 5CH(3)OH (3), [Mn(5-Br(salpn))](6)[Fe(bpmb)(CN)(2)](6) x 24H(2)O x 8CH(3)CN (4), and [Mn(5-Cl(salpn))](6)[Fe(bpmb)(CN)(2)](6) x 25H(2)O x 5CH(3)CN (5). Compared with [Fe(bpb)(CN)(2)](-), which always gives rise to 1D or polynuclear species when reacting with Mn(III) Schiff bases, the introduction of substituents (X) to the bpb(2-) ligand has a driving force in formation of the novel wheel structure. Magnetic studies reveal that high-spin ground state S = 15 is present in the wheel compounds originated from the ferromagnetic Mn(III)-Fe(III) coupling. For the first time, the quantum Monte Carlo study has been used to modulate the magnetic susceptibility of the huge Mn(6)Fe(6) metallomacrocycles, showing that the magnetic coupling constants J range from 3.0 to 8.0 K on the basis of the spin Hamiltonian [Formula: see text]. Hysteresis loops for 1 have been observed below 0.8 K, indicative of a single-molecule magnet with a blocking temperature (TB) of 0.8 K. Molecular wheels 2-5 exhibit frequency dependence of alternating-current magnetic susceptibility under zero direct-current magnetic field, signifying the slow magnetization relaxation similar to that of 1. Significantly, an unprecedented archlike Mn(2)Fe(2) cluster, [Mn(5-Cl(salpn))](2)[Fe(bpmb)(CN)(2)](2) x 3H(2)O x CH(3)CN (6), has been isolated as an intermediate of the Mn(6)Fe(6) wheel 5. Ferromagnetic Mn(III)-Fe(III) coupling results in a high-spin S = 5 ground state. Combination of the high-spin state and a negative magnetic anisotropy (D) results in the observation of slow magnetization relaxation in 6.     

Observation of slow magnetic relaxation in triple-stranded lanthanide helicates

Two dinuclear triple-stranded helicates [Ln(2)L(3)](3+) (Ln = Dy and Tb) obtained via self-assembly from the ligand HL (2,6-diformyl-4-methylphenol di(benzoylhydrazone)) and lanthanide perchlorate have been synthesized and characterized. The crystal structural analysis demonstrates that three ligand strands wrap around a pseudo-threefold axis defined by the two metal ions, leading to a 'meso'-relation between the right- (Δ) and left-hand (Λ) configurations of [Ln(2)L(3)](3+) in the crystal. Each Ln(III) ion is coordinated by nine donor atoms in a distorted tricapped trigonal-prismatic arrangement. Alternating current (ac) susceptibility measurements of [Dy(2)L(3)](3+) reveal a frequency-dependent out-of-phase signal under a 700 Oe dc field, indicating the onset of the slow relaxation of magnetization with a roughly estimated activation energy of ～5 K and τ(0) of 10(-7) s.     

Cobalt-lanthanide coordination polymers constructed with metalloligands: a ferromagnetic coupled quasi-1D Dy3+ chain showing slow relaxation

Four types of cobalt-lanthanide heterometallic compounds based on metalloligand Co(2,5-pydc)(3) (3-) (2,5-H(2)pydc=pyridine-2,5-dicarboxylate acid), [Ln(2)Co(2)(2,5-pydc)(6)(H(2)O)(4)](n) 2n H(2)O (1) (Ln=Tb, Dy for 1 a, 1 b respectively), [Tb(2)Co(2)(2,5-pydc)(6)(H(2)O)(4)](n)3n H(2)O (2), [Tb(2)Co(2)(2,5-pydc)(6)(H(2)O)(9)](n)4n H(2)O (3), and [LaCo(2,5-pydc)(3)(H(2)O)(2)](n)2n H(2)O (4) have been synthesized. Compound 1 has a layer structure with well-isolated carboxylate-bridged Ln(3+) chains, compound 2 is a three-dimensional (3D) porous network with Tb(3+) chains that are also well isolated and carboxylate bridged, 3 is a layer structure based on dinuclear units, and 4 is a 3D network with boron nitride (BN) topology. DC magnetic studies reveal ferromagnetic coupling in all the carboxylate-bridged Ln(3+) chains in 1 a, 1 b, and 2. Compared to the silence of the out-of-phase ac susceptibility of 2, above 1.9 K the magnetic relaxation behavior of both 1 a and 1 b is slow like that of a single-chain magnet.     

A cyano-bridged single-molecule magnet: slow magnetic relaxation in a trigonal prismatic MnMo(6)(CN)(18) cluster

We report the synthesis of the first well-documented example of a cyano-bridged single-molecule magnet. An assembly reaction parallel to that employed in producing the trigonal prismatic [(Me(3)tacn)(6)MnCr(6)(CN)(18)](2+) (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane) cluster affords K[(Me(3)tacn)(6)MnMo(6)(CN)(18)](ClO(4))(3) (1), containing an analogous molybdenum(III)-substituted cluster. Fits to the DC magnetic susceptibility and magnetization data for 1 show that the MnMo(6) cluster possesses weak antiferromagnetic coupling (J = -6.7 cm(-1)), leading to an S = (13)/(2) ground state with significantly enhanced magnetic anisotropy (D = -0.33 cm(-1) and E = -0.018 cm(-1)). Consistent with these results, AC magnetic susceptibility measurements show the molecule to exhibit slow magnetic relaxation indicative of a single-molecule magnet with an energy barrier of 10 cm(-1) for spin reversal.     

4,4'-dithiodipyridine as a bridging ligand in osmium and ruthenium complexes: the electron conductor ability of the -S-S- bridge

The compounds [Ru(NH(3))(5)(dtdp)](TFMS)(3), [Os(NH(3))(5)(dtdp)](TFMS)(3), [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](TFMS)(6), [(NH(3))(5)Os(dtdp)Ru(NH(3))(5)](TFMS)(3)(PF(6))(2), and [(NH(3))(5)Os(dtdp)Fe(CN)(5)] (dtdp = 4,4'-dithiodipyridine, TFMS = trifluoromethanesulfonate) have been synthesized and characterized by elemental analysis, cyclic voltammetry, electronic, vibrational, EPR, and (1)H NMR spectroscopies. Changes in the electronic and voltammetric spectra of the ion complex [Os(NH(3))(5)(dtdp)](3+) as a function of the solution pH enable us to calculate the pK(a) for the [Os(NH(3))(5)(dtdpH)](4+) and [Os(NH(3))(5)(dtdpH)](3+) acids as 3.5 and 5.5, respectively. The comparison of the above pK(a) data with that for the free ligand (pK(1) = 4.8) provides evidence for the -S-S- bridge efficiency as an electron conductor between the two pyridine rings. The symmetric complex, [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](6+), is found to exist in two geometric forms, and the most abundant form (most probably trans) has a strong conductivity through the -S-S- bridge, as is shown by EPR, which finds it to have an S = 1 spin state with a spin-spin interaction parameter of 150-200 G both in the solid sate and in frozen solution. Further the NMR of the same complex shows a large displacement of unpaired spin into the pi orbitals of the dttp ligand relative to that found in [Os(NH(3))(5)(dtdp)](3+). The comproportionation constant, K(c) = 2.0 x 10(5), for the equilibrium equation [Os(II)Os(II)] + [Os(III)Os(III)] right harpoon over left harpoon 2[Os(II)Os(III)] and the near-infrared band energy for the mixed-valence species (MMCT), [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](5+) (lambda(MMCT) = 1665 nm, epsilon = 3.5 x 10(3) M(-)(1) cm(-)(1), deltanu(1/2) = 3.7 x 10(3) cm(-)(1), alpha = 0.13, and H(AB) = 7.8 x 10(2) cm(-)(1)), are quite indicative of strong electron delocalization between the two osmium centers. The electrochemical and spectroscopic data for the unsymmetrical binuclear complexes [(NH(3))(5)Os(III)(dtdp)Ru(II)(NH(3))(5)](5+) (lambda(MMCT) = 965 nm, epsilon = 2.2 x 10(2) M(-)(1) cm(-)(1), deltanu(1/2) = 3.0 x 10(3) cm(-)(1), and H(AB) = 2.2 x 10(2) cm(-)(1)) and [(NH(3))(5)Os(III)(dtdp)Fe(II)(CN)(5)] (lambda(MMCT) = 790 nm, epsilon = 7.5 x 10 M(-)(1) cm(-)(1), deltanu(1/2) = 5.4 x 10(3) cm(-)(1), and H(AB) = 2.0 x 10(2) cm(-)(1)) also suggest a considerable electron delocalization through the S-S bridge. As indicated by a comparison of K(c) and energy of the MMCT process in the iron, ruthenium, and osmium complexes, the electron delocalization between the two metal centers increases in the following order: Fe < Ru < Os.     

Cyano-bridged terbium(III)-chromium(III) bimetallic quasi-one-dimensional assembly exhibiting long-range magnetic ordering

A new cyano-bridged Tb(III)-Cr(III) heterometallic complex [Tb(H(2)O)(2)(DMF)(4){Cr(CN)(6)}]·H(2)O (DMF = dimethylformamide) (1), assembled from paramagnetic hexacyanochromium(III) [Cr(CN)(6)](3-) building block and highly anisotropic terbium(III) ion has been prepared and structurally and magnetically characterized. Complex 1 shows one-dimensional (1D) zig-zag chain-like structural motif which is further extended into three-dimensional network through hydrogen-bonding interactions. The long-range magnetic ordering observed in complex 1, which is possibly due to interchain magnetic dipolar interactions, illuminates that this complex is a molecule-based magnet with critical temperature of about 5 K. This higher critical temperature among those of Ln(III)-Cr(III) heterometallic complexes exhibiting long-range magnetic ordering is probably due to the introduction of highly anisotropic terbium(III) ion.     

Slow magnetic relaxation at zero field in the tetrahedral complex [Co(SPh)4]2-

The Ph(4)P(+) salt of the tetrahedral complex [Co(SPh)(4)](2-), possessing an S = (3)/(2) ground state with an axial zero-field splitting of D = -70 cm(-1), displays single-molecule magnet behavior in the absence of an applied magnetic field. At very low temperatures, ac magnetic susceptibility data show the magnetic relaxation time, τ, to be temperature-independent, while above 2.5 K thermally activated Arrhenius behavior is apparent with U(eff) = 21(1) cm(-1) and τ(0) = 1.0(3) × 10(-7) s. Under an applied field of 1 kOe, τ more closely approximates Arrhenius behavior over the entire temperature range. Upon dilution of the complex within a matrix of the isomorphous compound (Ph(4)P)(2)[Zn(SPh)(4)], ac susceptibility data reveal the molecular nature of the slow magnetic relaxation and indicate that the quantum tunneling pathway observed at low temperatures is likely mediated by intermolecular dipolar interactions.     

Interaction between f-electronic systems in dinuclear lanthanide complexes with phthalocyanines

The first detection and characterization of the interactions between the f-electronic systems in the dinuclear complexes of paramagnetic trivalent Tb, Dy, Ho, Er, Tm, and Yb ions with phthalocyanine ligands are presented. The molar magnetic susceptibilities, chi(m), were measured for PcLnPcLnPc* ([Ln, Ln]; Pc = dianion of phthalocyanine, Pc* = dianion of 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine) and PcLnPcYPc* ([Ln, Y]) in the range from 1.8 K to room temperature. The selective synthetic method previously reported for the heterodinuclear complex [Y, Ln] was used to prepare [Ln, Ln] and [Ln, Y] with a modification on the choice of starting materials. The f-f interaction contributions to the magnetic susceptibility are evaluated as Delta(chi)(m)T = chi(m)([Ln, Ln])T - chi(m)([Ln, Y])T - chi(m)([Y, Ln])T, where T refers to temperature on the kelvin scale. The homodinuclear complexes having f(8)-f(10)-systems, namely [Tb, Tb], [Dy, Dy], and [Ho, Ho], show positive Delta(chi)(m)T values in the 1.8-50 K range, indicating the existence of ferromagnetic interaction between the f-systems. The magnitude of the Delta(chi)(m)T increases in the descending order of the number of f-electrons. [Er, Er] gives negative Delta(chi)(m)T values in the 1.8-50 K range, showing the antiferromagnetic nature of the f-f interaction. [Tm, Tm] exhibits small and negative Delta(chi)(m)T values, which gradually decline in the negative direction as the temperature decreases in the range 13-50 K and sharply rise in the positive direction as the temperature falls from 10 to 1.8 K. [Yb, Yb] has extremely small Delta(chi)(m)T values, whose magnitude at 2 K is less than 1% of that of [Tb, Tb]. The ligand field parameters of the ground-state multiplets of the six [Ln, Y] complexes are determined by simultaneous fitting to both the magnetic susceptibility data and paramagnetic shifts of (1)H NMR. The theoretical analysis successfully converged by assuming that each ligand field parameter is a function of the number of f-electrons in each ion. Using these parameters as well as the previously obtained corresponding parameters for the [Y, Ln] series, the interactions between the f-systems in [Ln, Ln] are investigated. All the characteristic observations above are satisfactorily reproduced with the assumption that the magnetic dipolar term is the sole source of the f-f interaction.     

Single-chain magnet (NEt4)[Mn2(5-MeOsalen)2Fe(CN)6] Made of Mn(III)-Fe(III)-Mn(III) trinuclear single-molecule magnet with an S(T) = 9/2 spin ground state

The cyano-bridged trinuclear compound, (NEt(4))[Mn(2)(salmen)(2)(MeOH)(2)Fe(CN)(6)] (1) (salmen(2)(-) = rac-N,N'-(1-methylethylene)bis(salicylideneiminate)), reported previously by Miyasaka et al. (ref 19d) has been reinvestigated using combined ac and dc susceptibility measurements. The strong frequency dependence of the ac susceptibility and the slow relaxation of the magnetization show that 1 behaves as a single-molecule magnet with an S(T) = (9)/(2) spin ground state. Its relaxation time (tau) follows an Arrhenius law with tau(0) = 2.5 x 10(-)(7) s and Delta(eff)/k(B) = 14 K. Moreover, below 0.3 K, tau saturates around 470 s, indicating that quantum tunneling of the magnetization becomes the dominant process of relaxation. (NEt(4))[Mn(2) (5-MeOsalen)(2)Fe(CN)(6)] (2) (5-MeOsalen(2)(-) = N,N'-ethylenebis(5-methoxysalicylideneiminate)) is a heterometallic one-dimensional assembly made of the trinuclear [Mn(III)(SB)-NC-Fe(III)-CN-Mn(III)(SB)] (SB is a salen-type Schiff-base ligand) motif similar to 1. Compound 2 has two types of bridges, a cyano bridge (-NC-) and a biphenolate bridge (-(O)(2)-), connecting Mn(III) and Fe(III) ions and the two Mn(III) ions, respectively. Both bridges mediate ferromagnetic interactions, as shown by modeling the magnetic susceptibility above 10 K with g(av) = 2.03, J(Mn)(-)(Fe)/k(B) = +6.5 K, and J'/k(B) = +0.07 K, where J' is the exchange coupling between the trimer units. The dc magnetic measurements of a single crystal using micro-SQUID and Hall-probe magnetometers revealed a uniaxial anisotropy (D(T)/k(B) = -0.94 K) with an easy axis lying along the chain direction. Frequency dependence of the ac susceptibility and time dependence of the dc magnetization have been performed to study the slow relaxation of the magnetization. A mean relaxation time has been found, and its temperature dependence has been studied. Above 1.4 K, both magnetic susceptibility and relaxation time are in agreement with the dynamics described in the 1960s by R. J. Glauber for one-dimensional systems with ferromagnetically coupled Ising spins (tau(0) = 3.7 x 10(-)(10) s and Delta(1)/k(B) = 31 K). As expected, at lower temperatures below 1.4 K, the relaxation process is dominated by the finite-size chain effects (tau'(0) = 3 x 10(-)(8) s and Delta(2)/k(B) = 25 K). The detailed analysis of this single-chain magnet behavior and its two regimes is consistent with magnetic parameters independently estimated (J'and D(T)) and allows the determination of the average chain length of 60 nm (or 44 trimer units). This work illustrates nicely a new strategy to design single-chain magnets by coupling ferromagnetically single-molecule magnets in one dimension.     

Synthesis, structures, and magnetic properties of face-sharing heterodinuclear Ni(II)-Ln(III) (Ln = Eu, Gd, Tb, Dy) complexes

Heterodinuclear [(Ni (II)L)Ln (III)(hfac) 2(EtOH)] (H 3L = 1,1,1-tris[(salicylideneamino)methyl]ethane; Ln = Eu, Gd, Tb, and Dy; hfac = hexafluoroacetylacetonate) complexes ( 1.Ln) were prepared by treating [Ni(H 1.5L)]Cl 0.5 ( 1) with [Ln(hfac) 3(H 2O) 2] and triethylamine in ethanol (1:1:1). All 1.Ln complexes ( 1.Eu, 1.Gd, 1.Tb, and 1.Dy) crystallized in the triclinic space group P1 (No. 2) with Z = 2 with very similar structures. Each complex is a face-sharing dinuclear molecule. The Ni (II) ion is coordinated by the L (3-) ligand in a N 3O 3 coordination sphere, and the three phenolate oxygen atoms coordinate to an Ln (III) ion as bridging atoms. The Ln (III) ion is eight-coordinate, with four oxygen atoms of two hfac (-)'s, three phenolate oxygen atoms of L (3-), and one ethanol oxygen atom coordinated. Temperature-dependent magnetic susceptibility and field-dependent magnetization measurements showed a ferromagnetic interaction between Ni (II) and Gd (III) in 1.Gd. The Ni (II)-Ln (III) magnetic interactions in 1.Eu, 1.Tb, and 1.Dy were evaluated by comparing their magnetic susceptibilities with those of the isostructural Zn (II)-Ln (III) complexes, [(ZnL)Ln(hfac) 2(EtOH)] ( 2.Ln) containing a diamagnetic Zn (II) ion. A ferromagnetic interaction was indicated in 1.Tb and 1.Dy, while the interaction between Ni (II) and Eu (III) was negligible in 1.Eu. The magnetic behaviors of 1.Dy and 2.Dy were analyzed theoretically to give insight into the sublevel structures of the Dy (III) ion and its coupling with Ni (II). Frequency dependence in the ac susceptibility signals was observed in 1.Dy.     

Electronic and molecular structures of the members of the electron transfer series [Cr(tbpy)3]n (n = 3+, 2+, 1+, 0): an X-ray absorption spectroscopic and density functional theoretical study

The electron transfer series of complexes [Cr((t)bpy)(3)](n)(PF(6))(n) (n = 3+, 2+, 1+, 0 (1-4)) has been synthesized and the molecular structures of 1, 2, and 3 have been determined by single-crystal X-ray crystallography; the structure of 4 has been investigated using extended X-ray absorption fine structure (EXAFS) analysis. Magnetic susceptibility measurements (4-300 K) established an S = 3/2 ground state for 1, an S = 1 ground state for 2, an S = 1/2 ground state for 3, and an S = 0 ground state for 4. The electrochemistry of this series in CH(3)CN solution exhibits three reversible one-electron transfer waves. UV-vis/NIR spectra and Cr K-edge X-ray absorption spectra (XAS) are reported. The same experimental techniques have been applied for [Cr(III)(tacn)(2)]Br(3)·5H(2)O (5) and [Cr(II)(tacn)(2)]Cl(2) (6), which possess an S = 3/2 and an S = 2 ground state, respectively (tacn = 1,4,7-triazacyclononane, a tridentate, pure σ-donor ligand). The Cr K-edge XAS spectra of the corresponding complexes K(4)[Cr(II)(CN)(6)]·10H(2)O (S = 1) (7) and K(3)[Cr(III)(CN)(6)] (S = 3/2) (8) have also been recorded. All complexes have been studied computationally with density functional theory (DFT) using the B3LYP functional. The molecular and electronic structures of the anionic members of the series [Cr(bpy)(3)](1-,2-,3-) have also been calculated. It is unequivocally shown that all members of the electron transfer series 1-4 and [Cr(bpy)(3)](n) (n = 3+, 2+, 1+, 0, 1-, 2, 3-) possess a central Cr(III) ion ((t(2g))(3), S = 3/2). The three N,N'-coordinated neutral (bpy(0)) ligands in the trication 1 and [Cr(III)(bpy)(3)](3+) are one-electron reduced in a stepwise fashion to localized one, two, and three π-radical anions (bpy(?))(1-) in the dicationic, monocationic, and neutral species, respectively. Complexes 2 and [Cr(bpy)(3)](2+) cannot be described as low-spin Cr(II) species; they are in fact best described as [Cr(III)((t)bpy(?))((t)bpy(0))(2)](2+) and [Cr(III)(bpy(?))(bpy(0))(2)](2+) species. Further one-electron reductions yield one, two, and three diamagnetic (bpy(2-))(2-) dianions in the mono-, di-, and trianion. Thus, [Cr(III)(bpy(2-))(3)](3-) is a normal Werner-type Cr(III) (!) species. In all complexes containing (bpy(?))(1-) ligands, the ligand spins are strongly antiferromagnetically coupled to the spins of the central Cr(III) ion (d(3), S(Cr) = 3/2) affording the observed ground states given above. Thus, all redox chemistry of [Cr(bpy)(3)](n) complexes is ligand-based and documents that the ligand 2,2'-bipyridine is a redox noninnocent ligand; it exists in three oxidation levels in these complexes: as N,N'-coordinated neutral (bpy(0)), monoanionic π-radical (bpy(?))(1-), and diamagnetic dianionic (bpy(2-))(2-).     

Nickel(II)-molybdenum(III)-cyanide clusters: synthesis and magnetic behavior of species incorporating [(Me(3)tacn)Mo(CN)(3)

The substitution of Mo(III) for Cr(III) in metal-cyanide clusters is demonstrated as an effective means of increasing the strength of the magnetic exchange coupling and introducing magnetic anisotropy. Synthesis of the octahedral complex [(Me(3)tacn)Mo(CN)(3)] (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane) is accomplished with the addition of precisely 3 equiv of LiCN to a solution of [(Me(3)tacn)Mo(CF(3)SO(3))(3)] in DMF. An excess of LiCN prompts formation of a seven-coordinate complex, [(Me(3)tacn)Mo(CN)(4)](1)(-), whereas less LiCN produces multinuclear species such as [(Me(3)tacn)(2)Mo(2)(CN)(5)](1+). In close parallel to reactions previously performed with [(Me(3)tacn)Cr(CN)(3)], assembly reactions between [(Me(3)tacn)Mo(CN)(3)] and [Ni(H(2)O)(6)](2+) or [(cyclam)Ni(H(2)O)(2)](2+) (cyclam = 1,4,8,11-tetraazacyclotetradecane) afford face-centered cubic [(Me(3)tacn)(8)Mo(8)Ni(6)(CN)(24)](12+) and linear [(Me(3)tacn)(2)(cyclam)NiMo(2)(CN)(6)](2+) clusters, respectively. Generation of the former involves a thermally induced cyanide linkage isomerization, which rapidly leads to a low-spin form of the cluster containing diamagnetic Ni(II) centers. The cyclic voltammagram of this species in DMF reveals a sequence of six successive reduction waves spaced approximately 130 mV apart, suggesting class II mixed-valence behavior upon reduction. The magnetic properties of the aforementioned linear cluster are consistent with the expected ferromagnetic coupling and an S = 4 ground state, but otherwise vary slightly with the specific conformation adopted (as influenced by the packing of associated counteranions and solvate molecules in the crystal). Magnetization data indicate an axial zero-field splitting parameter with a magnitude falling in the range [D] = 0.44-0.72 cm(-1), and fits to the magnetic susceptibility data yield exchange coupling constants in the range J = 17.0-17.6 cm(-1). These values represent significant increases over those displayed by the analogous Cr(III)-containing cluster. When perchlorate is used as a counteranion, [(Me(3)tacn)(2)(cyclam)NiMo(2)(CN)(6)](2+) crystallizes from water in a dimeric form with pairs of the linear clusters directly linked via hydrogen bonding. In this case, fitting the magnetic susceptibility data requires use of two coupling constants: one intramolecular with J = 14.9 cm(-1) and another intermolecular with J' = -1.9 cm(-1). Reacting [(Me(3)tacn)Mo(CN)(3)] with a large excess of [(cyclam)Ni(H(2)O)(2)](2+) produces a [(Me(3)tacn)(2)(cyclam)(3)(H(2)O)(2)Ni(3)Mo(2)(CN)(6)](6+) cluster possessing a zigzag structure that is a simple extension of the linear cluster geometry. Its magnetic behavior is consistent with weaker ferromagnetic coupling and an S = 6 ground state. Similar reactions employing an equimolar ratio of reactants afford related one-dimensional chains of formula [(Me(3)tacn)(cyclam)NiMo(CN)(3)](2+). Once again, the ensuing structure depends on the associated counteranions, and the magnetic behavior indicates ferromagnetic coupling. It is hoped that substitutions of the type exemplified here will be of utility in the design of new single-molecule magnets.     

Conducting dimerized cobalt complexes with tetrathiafulvalene dithiolate ligands

To obtain novel single-component molecular metals, we attempted to synthesize several cobalt complexes coordinated by TTF (tetrathiafulvalene)-type dithiolate ligands. We succeeded in the syntheses and structure determinations of ((n)Bu(4)N)(2)[Co(chdt)(2)](2) (1), ((n)Bu(4)N)(2)[Co(dmdt)(2)](2) (2), [Co(dmdt)(2)](2) (3), and [Co(dt)(2)](2) (4) (chdt = cyclohexeno-TTF-dithiolate, dmdt = dimethyl-TTF-dithiolate, and dt = TTF-dithiolate). Structure analyses of complexes 1-4 revealed that two monomeric [Co(ligand)2]- or [Co(ligand)(2)](0) units are connected by two Co-S bonds resulting in dimeric [Co(ligand)(2)](2)(2-) or [Co(ligand)(2)](2) molecules. Complex 1 has a cation-anion-intermingled structure and exhibited Curie-Weiss magnetic behavior with a large Curie constant (C = 2.02 K x emu x mol(-1)) and weak antiferromagnetic interactions (theta = -8.3 K). Complex 2 also has a cation-anion-intermingled structure. However, the dimeric molecules are completely isolated by cations. Complexes 3 and 4 are single-component molecular crystals. The molecules of complex 3 form two-dimensional molecular stacking layers and exhibit a room-temperature conductivity of sigmart = 1.2 x 10(-2) S.cm(-1) and an activation energy of E(a) = 85 meV. The magnetic behavior is almost consistent with Curie-Weiss law, where the Curie constant and Weiss temperature are 8.7 x 10(-2) K x emu x mol(-1) and -0.85 K, respectively. Complex 4 has a rare chair form of the dimeric structure. The electrical conductivity was fairly large (sigmart = 19 S.cm(-1)), and its temperature dependence was very small (sigma(0.55K)/sigma(rt) = ca. 1:10), although the measurements were performed on the compressed pellet sample. Complex 4 showed an almost constant paramagnetic susceptibility (chi(300) (K) = 3.5 x 10(-4) emu x mol(-1)) from 300 to 50 K. The band structure calculation of complex 4 suggested the metallic nature of the system. Complex 4 is a novel single-component molecular conductor with a dimeric molecular structure and essentially metallic properties down to very low temperatures.     

Single-molecule magnetic behavior in a neutral terbium(III) complex of a picolinate-based nitronyl nitroxide free radical

The terdentate anionic picolinate-based nitronyl nitroxide (picNN) free radical forms neutral and robust homoleptic complexes with rare earth-metal ions. The nonacoordinated Tb(3+) complex Tb(picNN)(3)·6H(2)O is a single-molecule magnet with an activation energy barrier Δ = 22.8 ± 0.5 K and preexponential factor τ(0) = (5.5 ± 1.1) × 10(-9) s. It shows magnetic hysteresis below 1 K.     

Multi-iron silicotungstates: synthesis, characterization, and stability studies of polyoxometalate dimers

The reaction of Fe(III) with Na(+) and K(+) salts of the trivacant [alpha-SiW(9)O(34)](10)(-) ligand have been investigated at pH 6 and pH 1. A new dimer, [(alpha-SiFe(3)W(9)(OH)(3)O(34))(2)(OH)(3)](11-) (1), is synthesized by reacting Na(7)H(3)[alpha-SiW(9)O(34)] or K(10)[alpha-SiW(9)O(34)] with exactly 3 equiv of Fe(III) in a 0.5 M sodium acetate solution (pH 6). The structure of 1, determined by single-crystal X-ray diffraction (a = 22.454(2) A, b = 12.387(2) A, c = 37.421(2), beta = 100.107(8) degrees , monoclinic, C2/c, Z = 4, R(1) = 5.11% based on 12739 independent reflections), consists of two [alpha-SiFe(3)W(9)(OH)(3)O(34)](4-) units linked by three Fe-mu-OH-Fe bonds. Reaction of K(10)[alpha-SiW(9)O(34)] with 3 equiv of Fe(III) in water (pH 1) yields [(alpha-Si(FeOH(2))(2)FeW(9)(OH)(3)O(34))(2)](8)(-2). The structure of 2 was also determined by single-crystal X-ray diffraction (a = 36.903(2) A, b = 13.9868(9) A, c = 21.7839(13) A, beta = 122.709(1) degrees , monoclinic, C2/c, Z = 4, R(1) = 4.57% based on 11787 independent reflections). It consists of two [alpha-Si(FeOH(2))(2)FeW(9)(OH)(3)O(34)](4-) Keggin units linked by a single edge. The terminal ligand on Fe1 in each trisubstituted Keggin unit becomes a mu(2) oxo ligand bridging to a [WO(6)](2-) moiety. The UV-vis spectra of both complexes show the characteristic oxygen-to-metal-charge-transfer bands of polyoxometalates as well as an Fe(III)-centered band at 436 nm (epsilon = 146 M(-1) cm(-1)) and 456 nm (epsilon = 104 M(-1) cm(-1)) for complexes 1 and 2, respectively. Differential scanning calorimetry data show that complex 1 decomposes between 575 and 600 degrees C whereas no decomposition is observed for complex 2 up to temperatures of 600 degrees C.     

Synthesis, structures, and magnetic properties of heteronuclear Cu(II)-Ln(III) (Ln = La, Gd, or Tb) complexes

Facile one-pot reactions led to the formations of dinuclear [CuLn(hmp)2(NO3)3(H2O)2] (Ln = Tb (1.Tb), Gd (1.Gd), or La (1.La)), and trinuclear [Cu2Ln(mmi)4(NO3)(H2O)2](ClO4)(NO3).2H2O (Ln = Tb (2.Tb) or Gd (2.Gd)) and [Cu2La(mmi)4(NO3)2(H2O)](ClO4).2H2O (2.La) with polydentate ligands 2-(hydroxymethyl)-pyridine and 2-hydroxymethyl-1-methyl-imidazole. In these complexes, each pair of Cu(II) and Ln(III) ions is linked by a double mu-alkoxo bridge. The temperature dependences of the magnetic susceptibilities of 1 and 2 were investigated in the range of 2-300 K. The dinuclear and trinuclear Cu-Gd complexes exhibit ferromagnetic interaction. The coupling constant J values in the heterodinuclear Cu-Gd complexes are correlated to values of the dihedral angles alpha between the two O-Cu-O and O-Gd-O fragments of the bridging CuO2Gd networks, with the largest J value associated with the smallest alpha value. The occurrence of a ferromagnetic interaction between Cu(II) and Gd(III) ions of the trinuclear entity is supported by the field dependence of the magnetization. The field dependence of the magnetization at 2 K of 1.Gd and 2.Gd confirms the nature of the ground state and of the Cu(II)-Gd(III) interaction, while alternating current susceptibility measurements demonstrates out-of-phase ac susceptibility signals of 1.Tb, which is the molecule-based magnetic material of the smallest nuclearity which exhibits frequency-dependent behavior within the 3d-4f mixed-metal systems.