Unusual enlargment of magnetic interactions in the lowest excited states of nitroxide radical chromium(III) complexes as revealed by magnetic circular dichroism and luminescence

Magnetic circular dichroism and NIR luminescence of nitroxide radical complexes, [Cr(III)(beta-diketonato)(2)(NIT2py or IM2py)]PF(6), demonstrate that the energy gaps between the singlet ((1)L(D)) and triplet ((3)L(D)) spin coupled levels in the lowest excited (2)E(g),(2)T(1g) states are much larger than those in the ground state. This is the first observation of magnetic interactions in the excited states of radical complexes, which could be elucidated in terms of the exchange mechanism.     

Metallonitrosyl fragment as electron acceptor: intramolecular charge transfer, long range electronic coupling, and electrophilic reactivity in the trans-[NCRu(py)(4)(CN)Ru(py)(4)NO](3+) ion

The new complex trans-[NCRu(py)(4)(CN)Ru(py)(4)NO](PF(6))(3) (I) was synthesized. In acetonitrile solution, I shows an intense visible band (555 nm, epsilon = 5800 M(-1) cm(-1)) and other absorptions below 350 nm, associated with d(pi) --> pi(py) and pi(py) --> pi(py) transitions. The visible band is presently assigned as a donor-acceptor charge transfer (DACT) transition from the remote Ru(II) to the delocalized [Ru(II)-NO(+)] moiety. Photoinduced release of NO is observed upon irradiation at the DACT band. Application of the Hush model reveals strong electronic coupling, with H(DA) = approximately 2000 cm(-1). The difference between the optical absorption energy and redox potentials for the donor and acceptor sites (Ru(III,II), 1.40 V, and NO(+)/NO, 0.50 V, vs Ag/AgCl, 3 M KCl, respectively) (hnu - DeltaE(red)) is 1.33 eV, a large value which probably relates to the significant changes in distances and angles for the Ru-N-O moiety upon reduction. UV-vis absorptions, IR frequencies, and redox potentials are solvent-dependent. Controlled potential reduction (of NO(+)) and oxidation (of Ru(II) associated with the dicyano-chromophore) of I afford stable species, [NCRu(II)(py)(4)(CN)Ru(py)(4)NO](2+) (I(red)) and [NCRu(III)(py)(4)(CN)Ru(py)(4)NO](4+) (I(ox)), respectively, which are characterized by UV-vis and IR spectroscopies. I(red) shows an EPR spectrum characteristic of [Ru(II)-NO(*)] complexes. Compound I is electrophilically reactive in aqueous solution above pH 5: values of the equilibrium constant for the reaction [NCRu(py)(4)(CN)Ru(py)(4)NO](3+)+ 2 OH(-) <--> [NCRu(py)(4)(CN)Ru(py)(4)NO(2)](+) + H(2)O, K = 3.2 +/- 1.4 x 10(15) M(-2), and of the rate constant for the nucleophilic addition of OH(-), k = 9.2 +/- 0.2 x 10(3) M(-1) s(-1)(25 degrees C, I = 1 M), are obtained, with DeltaH = 90.7 +/- 3.8 kJ mol(-1) and DeltaS = 135 +/- 13 J K(-1) mol(-1). The oxidized complex, I(ox), shows an enhanced electrophilic reactivity toward OH(-). This addition reaction is followed by irreversible processes, which most probably lead to disproportionation of bound nitrite and other products.     

Ferro- and antiferromagnetic interactions in face-sharing trioctahedral Ni(II)Mn(II)Ni(II) and Ni(II)Fe(III)Ni(II) complexes with the same 1-5/2-1 spin system

Two heterotrinuclear complexes, [Mn(II)(Ni(II)L)2].2CH3OH (where H3L = 1,1,1-tris(N-salicylideneaminomethyl)ethane) and [Fe(III)(Ni(II)L)2]NO3.C2H5OH, consisting of three face-sharing octahedra have been prepared; although these complexes have closely related structures and have the same 1-5/2-1 spin system, they show completely different magnetic interactions between the adjacent metal ions: ferromagnetic (Ni(II)-Mn(II)) and antiferromagnetic (Ni(II)-Fe(III)).     

Heterometallic MIIRuIII2 compounds constructed from trans-[Ru(Salen)(CN)2]- and trans-[Ru(Acac)2(CN)2]-. Synthesis, structures, magnetic properties, and density functional theoretical study

The synthesis, structures, and magnetic properties of four cyano-bridged M(II)Ru(III)2 compounds prepared from the paramagnetic Ru(III) building blocks, trans-[Ru(salen)(CN)2]- 1 [H2salen = N,N'-ethylenebis(salicylideneimine)] and trans-[Ru(acac)2(CN)2]- (Hacac = acetylacetone), are described. Compound 2, {Mn(CH3OH)4[Ru(salen)(CN)2]2}.6CH3OH.2H2O, is a trinuclear complex that exhibits antiferromagnetic coupling between Mn(II) and Ru(III) centers. Compound 3, {Mn(H2O)2[Ru(salen)(CN)2]2.H2O}n, has a 2-D sheetlike structure that exhibits antiferromagnetic coupling between Mn and Ru, leading to ferrimagnetic-like behavior. Compound 4, {Ni(cyclam)[Ru(acac)2(CN)2]2}.2CH3OH.2H2O (cyclam = 1,4,8,11-tetraazacyclotetradecane), is a trinuclear complex that exhibits ferromagnetic coupling. Compound 5, {Co[Ru(acac)2(CN)2]2}n, has a 3-D diamond-like interpenetrating network that exhibits ferromagnetic ordering below 4.6 K. The density functional theory (DFT) method was used to calculate the molecular magnetic orbitals and the magnetic exchange interaction between Ru(III) and M(II) (Mn(II), Ni(II)) ions.     

Single-molecule magnets constructed from cyanometalates: {[Tp*Fe(III)(CN)3M(II)(DMF)4]2[OTf]2} x 2DMF (M(II) = Co, Ni)

Treatment of several divalent transition-metal trifluoromethanesulfonates [M(II)(OTf)2; M(II) = Mn, Co, Ni] with [NEt4][Tp*Fe(III)(CN)3] [Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate] in DMF affords three isostructural rectangular clusters of {[Tp*Fe(III)(CN)3M(II)(DMF)4]2[OTf]2} x 2DMF (M(II) = Mn, 3; Co, 4; Ni, 5) stoichiometry. Magnetic studies of 3-5 indicate that the Tp*Fe(CN)3(-) centers are highly anisotropic and exhibit antiferromagnetic (3 and 4) and ferromagnetic (5) exchange to afford S = 4, 2, and 3 spin ground states, respectively. ac susceptibility measurements suggest that 4 and 5 exhibit incipient single-molecule magnetic behavior below 2 K.     

Assembly of azido- or cyano-bridged binuclear complexes containing the bulky [Mn(phen)2]2+ building block: syntheses, crystal structures, and magnetic properties

Two new cyano-bridged heterobinuclear complexes, [Mn(II)(phen)2Cl][Fe(III)(bpb)(CN)2] x 0.5CH3CH2OH x 1.5H2O (1) and [Mn(II)(phen)2Cl][Cr(III)(bpb)(CN)2] x 2H2O (2) [phen = 1,10-phenanthroline; bpb(2-) = 1,2-bis(pyridine-2-carboxamido)benzenate], and four novel azido-bridged Mn(II) dimeric complexes, [Mn2(phen)4(mu(1,1)-N3)2][M(III)(bpb)(CN)2]2 x H2O [M = Fe (3), Cr (4), Co (5)] and [Mn2(phen)4(mu(1,3)-N3)(N3)2]BPh4 x 0.5H2O (6), have been synthesized and characterized by single-crystal X-ray diffraction analysis and magnetic studies. Complexes 1 and 2 comprise [Mn(phen)2Cl]+ and [M(bpb)(CN)2]- units connected by one cyano ligand of [M(bpb)(CN)2]-. Complexes 3-5 are doubly end-on (EO) azido-bridged Mn(II) binuclear complexes with two [M(bpb)(CN)2]- molecules acting as charge-compensating anions. However, the Mn(II) ions in complex 6 are linked by a single end-to-end (EE) azido bridging ligand with one large free BPh4(-) group as the charge-balancing anion. The magnetic coupling between Mn(II) and Fe(III) or Cr(III) in complexes 1 and 2 was found to be antiferromagnetic with J(MnFe) = -2.68(3) cm(-1) and J(MnCr) = -4.55(1) cm(-1) on the basis of the Hamiltonian H = -JS(Mn)S(M) (M = Fe or Cr). The magnetic interactions between two Mn(II) ions in 3-5 are ferromagnetic in nature with the magnetic coupling constants of 1.15(3), 1.05(2), and 1.27(2) cm(-1) (H = -JS(Mn1)S(Mn2)), respectively. The single EE azido-bridged dimeric complex 6 manifests antiferromagnetic interaction with J = -2.29(4) cm(-1) (H = -JS(Mn1)S(Mn2)). Magneto-structural correlationship on the EO azido-bridged Mn(II) dimers has been investigated.     

Synthesis, crystal structure, and high-precision high-frequency and -field electron paramagnetic resonance investigation of a manganese(III) complex: [Mn(dbm)2(py)2](ClO4)

The complex [Mn(dbm)(2)(py)(2)](ClO(4)) (dbm = anion of 1,3-diphenyl-1,3-propanedione (dibenzoylmethane), py = pyridine) was synthesized and characterized by X-ray crystallography. It has tetragonally distorted geometry with the axial positions occupied by the py ligands and the equatorial positions by the dbm ligands. This mononuclear complex of high-spin Mn(III) (3d(4), S = 2) was studied by high-frequency and -field electron paramagnetic resonance (HFEPR) both as a solid powder and in frozen dichloromethane solution. Very high quality HFEPR spectra were recorded over a wide range of frequencies. The complete dataset of resonant magnetic fields versus transition energies was analyzed using automated fitting software. This analysis yielded the following spin Hamiltonian parameters (energies in cm(-1)): D = -4.504(2), E = -0.425(1), B(4)(0) = -1.8(4) x 10(-4), B(4)(2) = 7(3) x 10(-4), B(4)(4) = 48(4) x 10(-4), g(x) = 1.993(1), g(y) = 1.994(1), and g(z) = 1.983(1), where the B(4)(n) values represent fourth-order zero-field splitting terms that are generally very difficult to extract, even from single-crystal measurements. The results here demonstrate the applicability of HFEPR at high-precision measurements, even for powder samples. The zero-field splitting parameters determined here for [Mn(dbm)(2)(py)(2)](+) are placed into the context of those determined for other mononuclear complexes of Mn(III).     

Molecular metamagnet [Ni(4ImNNH)(2)(NO(3))(2)] (4ImNNH = 4-imidazolyl nitronyl nitroxide) and the related compounds showing supramolecular H-bonding interactions

A new chelating radical ligand 4ImNNH (2-(4-imidazolyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) was prepared, and complexation with divalent transition metal salts gave complexes, [M(4ImNNH)(2)X(2)], which showed intermolecular ferromagnetic interaction in high probability (7 out of 10 paramagnetic compounds investigated here). The nitrate complexes (X = NO(3); M = Mn (1), Co (2), Ni (3), Cu (4)) crystallize isomorphously in monoclinic space group P2(1)/a. The equatorial positions are occupied with two 4ImNNH chelates and the nitrate oxygen atoms are located at the axial positions. Magnetic measurements revealed that the intramolecular exchange couplings in 1, 2, and 4 were antiferromagnetic, while that in 3 was ferromagnetic with 2J/k(B) = +85 K, where the spin Hamiltonian is defined as H = -2J(S(1).S(2) + S(2).S(3)) based on the molecular structures determined as the linear radical-metal-radical triads. The intramolecular ferromagnetic interaction in 3 is interpreted in terms of orthogonality between the radical pi and metal dsigma orbitals. Compounds 1-3 exhibited intermolecular ferromagnetic interaction ascribable to a two-dimensional hydrogen bond network parallel to the crystallographic ab plane. Complex 3 became an antiferromagnet below 3.4 K and exhibited a metamagnetic transition on applying a magnetic field of 5.5 kOe at 1.8 K. The complexes prepared from metal halides, [M(4ImNNH)(2)X(2)] (X = Cl, Br; M = Mn, Co, Ni, Cu), showed intramolecular antiferromagnetic interactions, which are successfully analyzed based on the radical-metal-radical system. The crystal structures determined here on 1-4, [Mn(4ImNNH)(2)Cl(2)], and [Cu(4ImNNH)(2)Br(2)] always have intermolecular hydrogen bonds of H(imidazole).X(axial ligand)-M, where X = NO(3), Cl, Br. This interaction seems to play an important role in molecular packing and presumably also in magnetic coupling.     

Heterometallic cubane-like {M2Ln2} (M = Ni, Zn; Ln =, Gd, Dy) and {Ni2Y2} aggregates. Synthesis, structures and magnetic properties

The syntheses, structural determinations and magnetic studies of tetranuclear M(II)Ln(III) complexes (M = Ni, Zn; Ln = Y, Gd, Dy) involving an in situ compartmentalized schiff base ligand HL derived from the condensation of o-vanillin and 2-hydrazinopyridine as main ligand are described. Single-crystal X-ray diffraction reveals that all complexes are closely isostructural, with the central core composed of distorted {M(2)Ln(2)O(4)} cubes of the formulas [Ni(2)Ln(2)(μ(3)-OH)(2)(L)(2)(OAc)(4)(H(2)O)(3.5)](ClO(4))(2)·3H(2)O (Ln = Y 1 and Gd 2), [Ni(2)Dy(2)(μ(3)-OH)(2)(L)(2)(OAc)(5)(EtOH)(H(2)O)(1.5)](ClO(4))·EtOH·H(2)O (3) and [Zn(2)Ln(2)(μ(3)-OH)(2)(L)(2)(OAc)(5)(EtOH)(H(2)O)](ClO(4))·2EtOH·1.5H(2)O (Gd 4 and Dy 5). The Ln(III) ions are linked by two hydroxo bridges and each M(II) ion is also involved in a double phenoxo-hydroxo bridge with the two Ln(III) ions, so that each hydroxo group is triply linked to the two Ln(III) and one M(II) ions. The magnetic properties of all complexes have been investigated. Ni(2)Y(2) (1) has a ferromagnetic Ni(II)Ni(II) interaction. A weak ferromagnetic Ni(II)Ln(III) interaction is observed in the Ni(2)Ln(2) complexes (Ln = Gd 2, Dy 3), along with a weak antiferromagnetic Ln(III)Ln(III) interaction, a D zero-field splitting term for the nickel ion and a ferromagnetic Ni(II)Ni(II) interaction. The isomorphous Zn(2)Ln(2) (Ln = Gd 4, Dy 5) does confirm the presence of a weak antiferromagnetic Ln(III)Ln(III) interaction. The Ni(2)Dy(2) complex (3) does not behave as a SMM, which could result from a subtractive combination of the Dy and Ni anisotropies and an increased transverse anisotropy, leading to large tunnel splittings and quantum tunneling of magnetization. On the other hand, Zn(2)Dy(2) (5) exhibits a possible SMM behavior, where its slow relaxation of magnetization is probably attributed to the presence of the anisotropic Dy(III) ions.     

Fe(bipy)(CN)(4)](-) as a versatile building block for the design of heterometallic systems: synthesis, crystal structure, and magnetic properties of PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O, [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O, and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O [bipy = 2,2'-Bipyridine; M = Mn and Zn

The new cyano complexes of formulas PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O (1), [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O with M = Mn (2) and Zn (3), and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O (4) [bipy = 2,2'-bipyridine and PPh(4) = tetraphenylphosphonium cation] have been synthesized and structurally characterized. The structure of complex 1 is made up of mononuclear [Fe(bipy)(CN)(4)](-) anions, tetraphenyphosphonium cations, and water molecules of crystallization. The iron(III) is hexacoordinated with two nitrogen atoms of a chelating bipy and four carbon atoms of four terminal cyanide groups, building a distorted octahedron around the metal atom. The structure of complexes 2 and 3 consists of neutral centrosymmetric [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] heterotrinuclear units and crystallization water molecules. The [Fe(bipy)(CN)(4)](-) entity of 1 is present in 2 and 3 acting as a monodentate ligand toward M(H(2)O)(4) units [M = Mn(II) (2) and Zn(II) (3)] through one cyanide group, the other three cyanides remaining terminal. Four water molecules and two cyanide nitrogen atoms from two [Fe(bipy)(CN)(4)](-) units in trans positions build a distorted octahedron surrounding Mn(II) (2) and Zn(II) (3). The structure of the [Fe(phen)(CN)(4)](-) complex ligand in 2 and 3 is close to that of the one in 1. The intramolecular Fe-M distances are 5.126(1) and 5.018(1) A in 2 and 3, respectively. 4 exhibits a neutral one-dimensional polymeric structure containing two types of [Fe(bipy)(CN)(4)](-) units acting as bismonodentate (Fe(1)) and trismonodentate (Fe(2)) ligands versus the divalent zinc cations through two cis-cyanide (Fe(1)) and three fac-cyanide (Fe(2)) groups. The environment of the iron atoms in 4 is distorted octahedral as in 1-3, whereas the zinc atom is pentacoordinated with five cyanide nitrogen atoms, describing a very distorted square pyramid. The iron-zinc separations across the single bridging cyanides are 5.013(1) and 5.142(1) A at Fe(1) and 5.028(1), 5.076(1), and 5.176(1) A at Fe(2). The magnetic properties of 1-3 have been investigated in the temperature range 2.0-300 K. 1 is a low-spin iron(III) complex with an important orbital contribution. The magnetic properties of 3 correspond to the sum of two magnetically isolated spin triplets, the antiferromagnetic coupling between the low-spin iron(III) centers through the -CN-Zn-NC- bridging skeleton (iron-iron separation larger than 10 A) being very weak. More interestingly, 2 exhibits a significant intramolecular antiferromagnetic interaction between the central spin sextet and peripheral spin doublets, leading to a low-lying spin quartet.     

Effect of Nonmagnetic Substitution on the Magnetic Properties and Charge-Transfer Phase Transition of an Iron Mixed-Valence Complex, (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)] (dto = C(2)O(2)S(2))

The iron mixed-valence complex (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between Fe(II) and Fe(III) occurs reversibly at ～120 K, in addition to the ferromagnetic phase transition at T(C) = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (n-C(3)H(7))(4)N[Fe(II)(1-x)Zn(II)(x)Fe(III)(dto)(3)] (dto = C(2)O(2)S(2); x = 0-1), and carried out magnetic susceptibility and dielectric constant measurements and (57)Fe M?ssbauer spectroscopy. With increasing Zn(II) concentration (x), the CTPT is gradually suppressed and disappears at x ≈ 0.13. On the other hand, the ferromagnetic transition temperature (T(C)) is initially enhanced from 7 K to 12 K between x = 0.00 and 0.05, despite the nonmagnetic nature of Zn(II) ions, and then it decreases monotonically from 12 K to 3 K with increasing Zn(II) concentration. This anomalous dependence of T(C) on Zn(II) concentration is related to a change in the spin configuration of the ferromagnetic state caused by the partial suppression of the CTPT.     

Synthesis and spectroscopic studies on chromium(III) complex containing mixed-valence chrysenesemiquinone-chrysenecatecholate ligands and 2,2'-bipyridine coligand

The synthesis and spectroscopic properties of Cr(bpy)(chrySQ)(chryCat), a complex containing chromium(III) metal ion and chrysenequinone ligand in its partially reduced (chrySQ) and fully reduced (chryCat) forms, are described. The complex has been prepared by two different routes from Cr(CO)6 and Cr(chrySQ)3. Variable temperature magnetic susceptibility measurements indicated a strong antiferromagnetic coupling between Cr(III) (S=3/2) and chrysenesemiquinone radical (S=1/2), giving a magnetic coupling constant J=-342 cm(-1). Ligand-based redox couples were observed in the electrochemical studies that consist of quasi-reversible chrySQ/chryCat and bpy/bpy*- reductions and chryCat/chrySQ oxidation at negative potentials and irreversible chrySQ/chryBQ oxidation at positive potential. However, the metal was inert in the studied potential range. The electronic spectra of the complex revealed interesting properties. In addition to interaligand pi-pi* and n-pi* transitions, other bands corresponding to Cr(t(2g))-->chrySQ(pi*) and Cr(t(2g))-->bpy(pi*) metal-to-ligand charge-transfer MLCT transitions were observed. The infrared spectral analysis was informative in assigning the vibrations due to SQ and Cat ligands. Also, it was a useful tool in confirming the coordination of bpy ligand to chromium metal ion.     

Oxalate-based soluble 2D magnets: the series [K(18-crown-6)]3[M(II)3(H2O)4{M(III)(ox)3}3] (M(III) = Cr, Fe; M(II) = Mn, Fe, Ni, Co, Cu; ox = C2O4(2-); 18-crown-6 = C12H24O6)

The synthesis and magnetic properties of the oxalate-based molecular soluble magnets with general formula [K(18-crown-6)] 3[M (II) 3(H 2O) 4{M (III)(ox) 3} 3] (M (III) = Cr, Fe; M (II) = Mn, Fe, Ni, Co, Cu; ox = C 2O 4 (2-)) are here described. All the reported compounds are isostructural and built up by 2D bimetallic networks formed by alternating M (III) and M (II) ions connected through oxalate anions. Whereas the Cr (III)M (II) derivatives behave as ferromagnets with critical temperatures up to 8 K, the Fe (III)M (II) present ferri- or weak ferromagnetic ordering up to 26 K.     

Hexacyanometalate molecular chemistry: heptanuclear heterobimetallic complexes; control of the ground spin state

Following a bottom-up approach to nanomaterials, we present a rational synthetic route from hexacyanometalates [M(CN)(6)](3-) (M=Cr(III), Co(III)) cores to well-defined heptanuclear complexes. By changing the nature of the metallic cations and using a localised orbital model it is possible to control and to tune the ground state spin value. Thus, with M=Cr(III), d(3), S=3/2, three heptanuclear species were built and characterised by mass spectrometry in solution, by single-crystal X-ray diffraction and by powder magnetic susceptibility measurements, [Cr(III)(CNbondM'L(n))(6)](9+) (M'=Cu(II), Ni(II), Mn(II), L(n)=polydentate ligand), showing spin ground states S(G)=9/2 [Cu(II)], with ferromagnetic interactions J(Cr,Cu)=+45 cm(-1), S(G)=15/2 [Ni(II)] and J(Cr,Ni)=+17.3 cm(-1), S(G)=27/2 [Mn(II)], with an antiferromagnetic interaction J(Cr,Mn)=-9 cm(-1), (interaction Hamiltonian H=-J(Cr,M) [S(Cr)Sigma(i)S(M)(i)], i=1-6). With M=Co(III), d(6), S=0, the heptanuclear analogues [Co(III)(CN-M'L(n))(6)](9+) (M'=Cu(II), Ni(II), Mn(II)) were similarly synthesised and studied. They present a singlet ground state and allow us to evaluate the weak antiferromagnetic coupling constant between two next-nearest neighbours M'-Co-M'.     

Isomorphous replacement of M(II) ions in M(II)-Gd(III) dimers (M(II) = Cu(II), Mn(II), Ni(II), Co(II), Zn(II)): magnetic studies of the products

Complexes [M(II)Gd(III){pyCO(OEt)pyC(OH)(OEt)py}?](ClO?)?·EtOH [M(II) = Cu(II) (1), Mn(II) (2), Ni(II) (3), Co(II) (4) and Zn(II) (5)] crystallize in the monoclinic Cc space group and contain one hexacoordinate M(II) ion and one enneacoordinate Gd(III) ion, bridged by three {pyCO(OEt)pyC(OH)(OEt)py}? ligands. Magnetic susceptibility measurements indicate a ferromagnetic interaction for 1 and antiferromagnetic interactions for 2-4. Using the ? = -J?(Gd(III))?(M(II)) spin Hamiltonian formalism, fits to the magnetic susceptibility data yielded J values of +0.32 cm?1 for 1, -1.7 cm?1 for 2, and -0.22 cm?1 for 3. In complex 4, the orbital contributions of Co(II) precluded the determination of the magnetic coupling. The complex follows the Curie-Weiss law with θ = -2.07 K (-1.44 cm?1).     

Syntheses and magnetic properties of Cu(II)(hfac)2 and Mn(II)(hfac)2 complexes of 4-pyridyl-substituted thioaminyl radicals

Two types of Cu(II)(hfac)2 and Mn(II)(hfac)2 complexes of N-(4-pyridylthio)-4-ethoxycarbonyl-2,6-bis(4-chlorophenyl)phenylaminyl (1) and N-(4-pyridylthio)-2,4,6-tris(4-chlorophenyl)phenylaminyl (2) were prepared and their X-ray crystallographic and magnetic studies were performed. Mixtures of Cu(II)(hfac)2 and 1 and Mn(II)(hfac)2 and 2 in anhydrous heptane-benzene solution gave 1 : 2 complexes of M(II)(hfac)2 (M = Cu, Mn) and 1 or 2 in 73-75% yields. For Cu(II)(hfac)2(1)2 and Mn(II)(hfac)2(2)2 X-ray crystallographic analyses were successfully performed. The magnetic behaviors for the two metal complexes were investigated with a SQUID magnetometer. The analyses for the chimolTvs. T plots of Cu(II)(hfac)2(1)2 were carried out by the numerical diagonalization of the Heisenberg Hamiltonian matrix (4096 x 4096 matrix) for the four repeating units of the complex (12-spin system). The exchange interaction between the copper(II) ion and the thioaminyl radicals is ferromagnetic (J1/kB = +28 K) and the interactions between the complexes is antiferromagnetic (J2/kB = -13 K). The magnetic behavior of Mn(II)(hfac)2(2)2 complexes is well analyzed with the theoretical equation of a 1/2-5/2-1/2 three-spin system taking the intermolecular interaction (theta) into account. The exchange interaction between the Mn(II) ion and the thioaminyl radicals is antiferromagnetic (J/kB = -4.2 K) and theta = -1.0 K. These magnetic behaviors could be well explained in terms of their crystal structures.     

Crystal structures and magnetic properties of new cyano-bridged two-dimensional grid-like bimetallic assemblies [Ni(tn)2]2[Cr(CN)5((NO)]OH*H2O and [NI(tn)2]2[Co(CN)6]NO3*2H2O (tn=1,3-propanediamine)

Two bimetallic assemblies, [Ni(tn)(2)](2)[Cr(CN)(5)(NO)]OH.H(2)O (1) and [Ni(tn)(2)](2)[Co(CN)(6)]NO(3).2H(2)O (2) (tn = 1,3-diaminopropane), have been prepared and structurally and magnetically characterized. Crystal data for 1 (2): space group P1 (P1), a = 8.698(3) (8.937(2)) A, b = 10.001(2) (9.863(1)) A, c = 10.158(2) (10.064(1)) A, alpha = 87.40(2) (86.064(10)) degrees, beta = 65.10(2) (65.489(10)) degrees, gamma = 81.63(2) (81.572(12)) degrees and Z = 1 (1). Both structures consist of two-dimensional grid-like polycations containing Ni-N triple bond C-M linkages (M = Cr or Co) and counteranions (OH, NO(3)). Magnetic studies of 1 showed that the complex displays a metamagnetic behavior originating from intralayer ferromagnetic and interlayer antiferromagnetic interactions. Long-range antiferromagnetic ordering was observed at T(N) = 3.3 K. Complex 2 exhibits intramolecular ferromagnetic interactions through the diamagnetic N triple bond C-Co-N triple bond C bridges, owing to superexchange involving the empty d(sigma) orbital of the diamagnetic Co(III) ion.     

New charge transfer salts based on bis(ethylenedithio)tetrathiafulvalene (ET) and ferro- or antiferromagnetic oxalato-bridged dinuclear anions: syntheses, structures and magnetism of ET5[MM'(C2O4)(NCS)8] with MM' = Cr(III)Fe(III), Cr(III)Cr(III)

Electrochemical combination of the magnetic dinuclear anion [MM'(C2O4)(NCS)8](4-) (MM' = Cr(III)Cr(III), Cr(III)Fe(III)) with the ET organic pi-donor (ET = BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) gives rise to two new isostructural molecular hybrid salts ET5[MM'(C2O4)(NCS)8], with MM' = CrCr (1), CrFe (2). The molecular structure of compound 1 has been determined by single crystal X-ray diffraction. The particular arrangement of the organic units consists of an unprecedented two-dimensional organic sublattice nearly similar to that observed in kappa-phase structures. For both compounds, the magnetic susceptibility measurements indicate (i) the ET radicals do not contribute to the magnetic moment probably due to the presence of strong antiferromagnetic interaction between them, and (ii) in the anion, the magnetic coupling is antiferromagnetic for 1 (J = -3.65 cm(-1)) and ferromagnetic for 2 (J = 1.14 cm(-1), J being the parameter of the exchange Hamiltonian H = -2JS1S2). The field dependence of the magnetization of compound 2 at 2.0 K gives further evidence of the S = 4 ground-state arising from the interaction between S = 3/2 Cr(III) and S = 5/2 Fe(III). EPR measurements confirm the nature of the magnetic interactions and the absence of any contribution from the organic part, as observed from the static magnetic measurement. Conductivity measurements and electronic band structure calculations show that both salts are semiconductors with low activation energies.     

Rational synthesis and magnetic properties of a family of low-dimensional heterometallic Cr-Mn complexes based on the versatile building block [Cr(2,2'-bipyridine)(CN)4]-

Six heterometallic compounds based on the building block [Cr(bpy)(CN)4]- (bpy = 2,2'-bipyridine) with secondary and/or tertiary coligands as modulators, {Mn(H2O)2[Cr(bpy)(CN)4]2}n (1), {Mn(bpy)(H2O)[Cr(bpy)(CN)4]2 x H2O}n (2), [Mn(bpy)2][Cr(bpy)(CN)4]2 x 5H2O (3), {[Mn(dca)(bpy)(H2O)][Cr(bpy)(CN)4] x H2O}n (4) (dca = N(CN)2(-)), {Mn(N3)(CH3OH)[Cr(bpy)(CN)4] x 2H2O}n (5), and {Mn(bpy)(N3)(H2O)[Cr(bpy)(CN)4] x H2O}2 (6), have been prepared and characterized structurally and magnetically. X-ray crystallography reveals that the compounds 1, 2, 4, and 5 consist of one-dimensional (1D) chains with different structures: a 4,2-ribbon-like chain, a branched zigzag chain, a 2,2-CC zigzag chain, and a 3,3-ladder-like chain, respectively. It also reveals that compound 3 has a trinuclear [MnCr2] structure, and compound 6 has a tetranuclear [Mn2Cr2] square structure. Magnetic studies show antiferromagnetic interaction between Cr(III) and Mn(II) ions in all compounds. All of the chain compounds exhibit metamagnetic behaviors with different critical temperatures (Tc) and critical fields (Hc; at 1.8 K): 3.2 K and 3.0 kOe for 1; 2.3 K and 4.0 kOe for 2; 2.1 K and 1.0 kOe for 4; and 4.7 K and 5.0 kOe for 5, respectively. The noncentrosymmetric compound 2 is also a weak ferromagnet at low temperature because of spin canting. The magnetic analyses reveal Cr-Mn intermetallic magnetic exchange constants, J, of -4.7 to -9.4 cm(-1) (H = -JS(Cr) x S(Mn)). It is observed that the antiferromagnetic interaction through the Mn-N-C-Cr bridge increases as the Mn-N-C angle (theta) decreases to the range of 155-180 degrees, obeying an empirical relationship: J = -40 + 0.2theta. This result suggests that the best overlap between t(2g) (high-spin Mn(II)) and t(2g) (low-spin Cr(III)) occurs at an angle of approximately 155 degrees.