New mixed-valence MnII2MnIII2 clusters exhibiting an unprecedented MnII/III oxidation state distribution in their magnetically coupled cores

The synthesis and magnetic properties of the high-spin tetranuclear cluster [Mn(III)(2)Mn(II)(2)(O(2)CC(CH(3))(3))(2)(teaH(2))(2)(teaH)(2)](O(2)CC(CH(3))(3))(2) (1) (where teaH(3) = triethanolamine) is described. Complex 1 is the pivalate analogue of our previously reported family of tetranuclear mixed-valence carboxylate clusters. The teaH(2)(-) and teaH(2-) anions in complex 1 act as oxygen donors in the {Mn(III)(2)Mn(II)(2)O(2)} "butterfly" core. Detailed dc and ac magnetic susceptibility measurements and magnetisation isotherms have been made and show that intra-cluster ferromagnetic coupling is occurring between the S = 2 Mn(III) and S = 5/2 Mn(II) ions to yield a S = 9 ground state and the g, J(bb) and J(wb) parameters have been deduced (b = body, w = wingtip). Incorporation of the acetylacetonate (acac(-)) ligand has led to three new clusters: [Mn(III)(2)Mn(II)(2)(O(2)CPh)(4)(teaH)(2)(acac)(2)].MeCN (2), [Mn(III)(2)Mn(II)(2)(teaH)(2)(acac)(4)(MeOH)(2)](ClO(4))(2) (3) and [Mn(III)(2)Mn(II)(2)(bheapH)(2)(acac)(4)(MeOH)(2)](ClO(4))(2) (4) (where bheapH(3) = 1-[N,N-bis(2-hydroxyethyl)amino]-2-propanol). Unlike any previously reported tetranuclear clusters containing the Mn(II)(2)Mn(III)(2) core, 2, 3, and 4 exhibit a reversal in their Mn(II)(2)Mn(III)(2) oxidation state distribution. In these clusters, the "wing-tip" Mn atoms exhibit Mn(III) (S = 2) oxidation states while the Mn(II) ions occupy the central "body" positions. Furthermore, the cores in 2, 3, and 4 contain at least one mu(2)-oxygen based bridging ion as opposed to the standard two mu(3)-oxygen bridges previously reported. More precisely, cluster 2 exhibits one mu(3)-O bridge and two mu(2)-bridges in a {Mn(II)(2)Mn(III)(2)O(3)} core while clusters 3 and 4 exhibit two mu(2)-O linkers within the {Mn(II)(2)Mn(III)(2)O(2)} core. All display trigonal prismatic coordination around the Mn(II) centres. These structural and oxidation state differences lead to very different magnetic coupling interactions between the four Mn(II/III) centres compared to 1. Direct current magnetic susceptibility measurements and magnetisation isotherms show that clusters 3 and 4 have ground states of S = 1. The g, J(bb) and J(wb) parameters have been deduced.     

New one-dimensional azido-bridged manganese(II) coordination polymers exhibiting alternating ferromagnetic-antiferromagnetic interactions: structural and magnetic studies

Five Mn(II)[bond]azido coordination polymers of formula [Mn(L)(N(3))(2)](n) have been synthesized and crystallographically characterized, and their magnetic properties studied, where L's are the bidentate Schiff bases obtained from the condensation of pyridine-2-carbaldehyde with aniline (1) and its derivatives p-toluidine (2), m-toluidine (3), p-chloroaniline (4), and m-chloroaniline (5). All the complexes consist of the zigzag Mn(II)[bond]azido chains in which the Mn(II) ions are alternately bridged by two end-to-end (EE) and two end-on (EO) azido ligands, the cis-octahedral coordination being completed by the two nitrogen atoms of the Schiff base ligands. Compound 2 is unique in that the Mn[bond](EE-N(3))(2)[bond]Mn ring adopts an unusual twist conformation with the two linear azido bridges crossing each other. By contrast, the rings in the other compounds take the usual chair conformation with the two azido bridges parallel. The double EO bridging fragments in the complexes are similar with the bridging angles (Mn[bond]N[bond]Mn) ranging from 99.6 degrees to 104.0 degrees. Magnetic analyses reveal that alternating ferro- and antiferromagnetic interactions are mediated through the alternating EO and EE azido bridges with the J(F) and J(AF) parameters in the ranges of 4.1-8.0 and -11.8 to -15.4 cm(-1), respectively. Finally, the magnetostructural correlations are investigated. The present complexes follow the general trend that the ferromagnetic interaction through the double EO bridge increases with the Mn[bond]N[bond]Mn bridging angle, while the antiferromagnetic interaction through the double EE bridge is dependent on the distortion of the Mn[bond](N(3))(2)[bond]Mn ring from planarity toward the chair conformation and the Mn[bond]N[bond]N angle.     

Crystal structures and magnetic properties of 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz)-containing copper(II) complexes

Four new copper(II) complexes of formula [Cu(2)(tppz)(dca)(3)(H(2)O)].dca.3H(2)O (1), [Cu(5)(tppz)(N(3))(10)](n)() (2), [[Cu(2)(tppz)(N(3))(2)][Cu(2)(N(3))(6)]](n)() (3), and [Cu(tppz)(N(3))(2)].0.33H(2)O (4) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and dca = dicyanamide anion] have been synthesized and structurally characterized by X-ray diffraction methods. The structure of complex 1 is made up of dinuclear tppz-bridged [Cu(2)(tppz)(dca)(3)(H(2)O)](+) cations, uncoordinated dca anions, and crystallization water molecules. The copper-copper separation across bis-terdentate tppz is 6.5318(11) A. Complex 2 is a sheetlike polymer whose asymmetric unit contains five crystallographically independent copper(II) ions. These units are building blocks in double chains in which the central part consists of a zigzag string of copper atoms bridged by double end-on azido bridges, and the outer parts are formed by dinuclear tppz-bridged entities which are bound to the central part through single end-on azido bridges. The chains are furthermore connected through weak, double out-of-plane end-on azido bridges, yielding a sheet structure. The intrachain copper-copper separations in 2 are 6.5610(6) A across bis-terdentate tppz, 3.7174(5) and 3.8477(5) A across single end-on azido bridges, and from 3.0955(5) to 3.2047(7) A across double end-on azido bridges. The double dca bridge linking the chains into sheets yields a copper-copper separation of 3.5984(7) A. The structure of 3 consists of centrosymmetric [Cu(2)(tppz)(N(3))(2)](2+) and [Cu(2)(N(3))(6)](2)(-) units which are linked through axial Cu.N(azido) (single end-on and double end-to-end coordination modes) type interactions to afford a neutral two-dimensional network. The copper-copper separations within the cation and anion are is 6.5579(5) A (across the bis-terdentate tppz ligand) and 3.1034(6) A (across the double end-on azido bridges), whereas those between the units are 3.6652(4) A (through the single end-on azido group) and 5.3508(4) A (through the double end-to-end azido bridges). The structure of complex 4 is built of neutral [Cu(tppz)(N(3))(2)] mononuclear units and uncoordinated water molecules. The mononuclear units are grouped by pairs to give a rather short copper-copper separation of 3.9031(15) A. The magnetic properties of 1-4 have been investigated in the temperature range 1.9-300 K. The magnetic behavior of complexes 1 and 4 is that of antiferromagnetically coupled copper(II) dimers with J = -43.7 (1) and -2.1 cm(-)(1) (4) (the Hamiltonian being H = -JS(A).S(B)). An overall ferromagnetic behavior is observed for complexes 2 and 3. Despite the structural complexity of 2, its magnetic properties correspond to those of magnetically isolated tppz-bridged dinuclear copper(II) units with an intermediate antiferromagnetic coupling (J = -37.5 cm(-)(1)) plus a ferromagnetic chain of hexanuclear double azido-bridged copper(II) units (the values of the magnetic coupling within and between the hexameric units being +61.1 and +0.0062 cm(-)(1), respectively). Finally, the magnetic properties of 3 were successfully analyzed through a model of a copper(II) chain with regular alternating of three ferromagnetic interactions, J(1) = +69.4 (across the double end-on azido bridges in the equatorial plane), J(2) = +11.2 (through the tppz bridge), and J(3) = +3.4 cm(-)(1) (across the single end-on azido bridge).     

Magneto-structural correlations: synthesis of a family of end-on azido-bridged manganese(II) dinuclear compounds with S = 5 spin ground state

The preparation of a series of multidentate pyridyl-imine ligands, L1-L3, and their reactivity with the Mn(II)/N3- system is described (L1 = [N,N-bis(pyridine-2-yl)benzylidene]ethane-1,2-diamine; L2 = [N,N-bis(pyridine-2-yl)benzylidene]propane-1,3-diamine, and L3 = [N,N-bis(pyridine-2-yl)benzylidene]butane-1,4-diamine). Complexes comprising dinuclear end-on bis(mu-azido)-bridged manganese(II) units of formulas [Mn2(L1)2(N3)4][Mn2(L1)2(N3)2(CH3OH)2](ClO4)2 (two cocrystallized dinuclear units, 1.2), [Mn2(L2)2(N3)2](ClO4)2 (3), and [Mn2(L3)2(N3)2](ClO4)2 (4) have been synthesized. The crystal structures of complexes 1-4 as well as their magnetic properties are presented. Each manganese atom of cocrystallized complexes in compound 1.2 is heptacoordinated, displaying Mn-N-Mn angles, theta, of 102.53(12) and 101.70(12) degrees and Mn...Mn distances of 3.5091(7) and 3.4680(7) A. On the other hand, each manganese center in compounds 3 and 4 is located within an octahedral coordination environment, the complexes displaying theta angles of 104.29(11) and 103.60(18) degrees , respectively, and Mn...Mn vectors of 3.5371(7) and 3.5338(10) A, respectively. Magnetic susceptibility studies revealed the presence of intramolecular ferromagnetic superexchange, yielding an S = 5 spin ground state in all complexes. Fitting of the experimental data led to coupling constants, intermolecular exchange values, and g factors (in the J/zJ'/g format) of 0.77 cm(-1)/0.01 cm(-1)/2.20 (1.2), 2.04 cm(-1)/0.01 cm(-1)/1.99 (3), and 1.75 cm(-1)/-0.05 cm(-1)/2.04 (4), respectively (using H = -2JS1S2 as the convention for the Heisenberg spin-Hamiltonian). These results are consistent with predictions from recent DFT calculations performed on end-on bis(mu-N3-)-bridged Mn(II) dinuclear complexes. A plot of experimental J vs theta, including data from the only preexisting compound of this kind, reveals a linear relationship, which could be the first evidence of a possible magneto-structural correlation between these two parameters.     

Design of high-dimensional copper(II) malonate complexes with exo-polydentate N-donor ligands

Two polymeric malonato-bridged copper(II) complexes of formulas [(H(2)bpe)[Cu(mal)2]]n.4nH2O (1) and [Cu(4")(mal)(4)(bpe)(3)]n.6nH(2)O (2) [mal = malonate dianion; bpe = 1,2-bis(4-pyridyl)ethylene] have been synthesized and characterized by X-ray diffraction. Complex 1 crystallizes in triclinic space group P(-)1, Z = 1, with unit cell parameters a = 4.8831(10) A, b = 9.585(2) A, c = 11.813(2) A, alpha = 77.29(3) degrees, beta = 82.18(3) degrees, and gamma = 84.92(3) degrees, whereas complex 2 crystallizes in the monoclinic space group P2(1)/n, Z = 4, with unit cell parameters a = 13.462(3) A, b = 10.275(5) A, c = 19.579(4) A, and beta = 105.21(3) degrees. The structure of 1 consists of anionic malonato-bridged uniform copper(II) chains which are connected through hydrogen bonds involving malonate-oxygen atoms, noncoordinated water molecules, and H(2)bpe(2+) cations. The intrachain copper-copper separation through carboxylate-malonate bridge in the anti-syn conformation is 4.8831(10) A. Complex 2 possesses a three-dimensional structure made up of neutral corrugated malonated-bridged copper(II) layers linked through bis-monodentate bpe molecules. The copper(II) atoms within each layer are bridged by a double mu-oxo and four carboxylato-malonate bridges with copper-copper separations of 3.4095(7) A (through oxo) and 4.9488(11)-6.5268(13) A (through carboxylato). The shortest interlayer copper-copper separation across bridging bpe is 13.434(3) A. Variable-temperature magnetic measurements (2-290 K) show an overall ferromagnetic behavior for both compounds. The magnetic pathway of complex 1 is through a single carboxylate-malonate bridge connecting apical and equatorial positions of adjacent copper(II) atoms, and the value of the magnetic coupling (J) for 1 through a numerical expression for a ferromagnetic uniform chain of interacting local doublets is J = +0.049(1) cm(-1). The values for the magnetic couplings through the main intralayer exchange pathways in 2 which correspond to carboxylate-malonate bridges connecting equatorial-equatorial (J(1)) and equatorial-apical (J(2)) coordination sites and to the double mu-oxo bridge linking equatorial-apical (J(5)) positions have been determined through a simplified model. The three magnetic couplings are weak, two of them being ferromagnetic (J(1) = +23(1) cm(-1) and J(2) = +6.5(1) cm(-1)) and the other one antiferromagnetic [zJ' = -1.0(1) cm(-1)]. The values of the magnetic couplings in 1 and 2 compare well with those previously reported for similar malonato-bridged copper(II) complexes of different dimensionalities.     

One-dimensional azido-bridged chiral metal complexes with ferromagnetic or antiferromagnetic interactions: syntheses, structures, and magnetic studies

One-dimensional chiral copper(II) and manganese(II) coordination polymers with single asymmetric end-to-end (EE) azide bridges, [Cu(R-L)2(N3)]n(ClO4)n (1), [Cu(S-L)2 (N3)]n(ClO4)n (2), [Mn(R-L)2(N3)]n(ClO4)n (3), and [Mn(S-L)2(N3)]n(ClO4)n (4) (R-L or S-L = R- or S-pyridine-2-carbaldehyde-imine), have been synthesized, using azide ions as bridging groups and chiral Schiff bases as auxiliary ligands, and characterized. The crystal structure determination of complexes 1 and 2 reveals the formation of one-dimensional chiral chains, in which the central Cu(II) ion is six-coordinate in the form of an elongated octahedron. Complex 3 consists of chiral helical polymeric chains, in which the central Mn(II) has a slightly distorted octahedral geometry. They all crystallize in the chiral space group P2(1). Complexes 1 and 2 are rare examples that exhibit ferromagnetic interaction between copper(II) ions through the single end-to-end azido bridge. Fitting the susceptibility data for 1 using a 1D uniform chain model led to the parameters J = 0.70(3) cm(-1), g = 2.06(2), and zj' = 0.07(2) cm(-1). The magnetic studies on 3 and 4 show that there is weak antiferromagnetic coupling between the manganese(II) ions.     

Framework control by a metalloligand having multicoordination ability: new synthetic approach for crystal structures and magnetic properties

By utilizing the novel metalloligand l(Cu), [Cu(2,4-pydca)(2)](2)(-) (2,4-pydca(2)(-) = pyridine-2,4-dicarboxylate), which possesses two kinds of coordination groups, selective bond formation with the series of the first-period transition metal ions (Mn(ii), Fe(ii), Co(ii), Cu(ii), and Zn(ii)) has been accomplished. depending on the coordination mode of 4-carboxylate with Co(ii), Cu(ii), and Zn(ii) ions, L(Cu) forms a one-dimensional (1-d) assembly with a repeating motif of [-M-O(2)C-(py)N-Cu-N(py)-Co(2)-]: {[ZnL(Cu)(H(2)O)(3)(DMF)].DMF}(N)() (2), [ZnL(Cu)(H(2)O)(2)(MeOH)(2)](N)() (3), and {[ML(Cu)(H(2)O)(4)].2H(2)O}(N)() (M = Co (4), Cu (5), Zn (6)). the use of a terminal ligand of 2,2'-bipyridine (2,2'-bpy), in addition to the cu(ii) ion, gives a zigzag 1-d assembly with the similar repeating unit as 4-6: {[Cu(2,2'-bpy)L(Cu)].3H(2)O}(N)() (9). on the other hand, for Mn(ii) and Fe(ii) ions, L(Cu) shows a 2-carboxylate bridging mode to form an another 1-d assembly with a repeating motif of [-M-O-C-O-CU-O-C-O-]: [ML(Cu)(H(2)O)(4)](N)() (M = Mn (7), Fe (8)). this selectivity is related to the strength of lewis basicity and the electrostatic effect of L(Cu) and the irving-williams order on the present metal ions. according to their bridging modes, a variety of magnetic properties are obtained: 4, 5, and 9, which have the 4-carboxypyridinate bridge between magnetic centers, have weak antiferromagnetic interaction, whereas 7 and 8 with the carboxylate bridge between magnetic centers reveal 1-d ferromagnetic behavior (Cu(II)-M(II); M(II) = Mn(II), J/k(B) = 0.69 K for 7; M(II) = Fe(II), J/k(B) = 0.71 K for 8).     

Experimental and theoretical studies on the magnetic properties of manganese(II) compounds with mixed isocyanate and carboxylate bridges

Two manganese(II) isocyanate complexes with different flexible zwitterionic dicarboxylate ligands, [Mn(2)(bcpp)(NCO)(4)](n) (1; bcpp=1,3-bis(N-carboxylatomethyl-4-pyridinio)propane) and [Mn(2)(bcp)(NCO)(4)](n) (2; bcp=bis(N-carboxylatomethyl)-4,4'-bipyridinium, have been synthesized and characterized by X-ray crystallography and magnetic measurements. Both compounds consist of two-dimensional coordination layers in which uniform anionic chains with mixed (NCO)(2)(COO) triple bridges are cross-linked by flexible cationic 4,4'-trimethylenedipyridinium spacers. Magnetic studies revealed antiferromagnetic interactions through the triple bridges (J=-8.0 cm(-1) (1) and J=-8.6 cm(-1) (2)), which are stronger than those in the isoelectronic analogue (N(3))(2)(COO). To complement the experimental data, periodic and finite-cluster DFT and CASPT2 calculations were performed on the dimeric units of the (NCO)(2)(COO) and (N(3))(2)(COO) mixed-bridged systems to support the Heisenberg picture and stress the relative efficiency of the magnetic couplers. It was found that the isocyanate ligand plays a greater role in the conveyance of antiferromagnetic behavior than the azide counterpart, and that both pseudohalide bridges function cooperatively with the carboxylate group.     

Linear NiII-MnIII2-NiII tetramers: an oligomeric component of the MnIII2NiII single-chain magnets

Heterometallic linear tetramers [Mn(5-R-saltmen)Ni(pao)(bpy)(2)](2)(ClO(4))(4) (5-R-saltmen(2-) = N,N'-1,1,2,2-tetramethylethylene bis(5-R-salicylideneiminate); pao(-) = pyridine-2-aldoximate; bpy = 2,2'-bipyridine, R = H, 1; Cl, 2; Br, 3; MeO, 4) have been synthesized and structurally characterized. These compounds exhibit a [Ni(II)-NO-Mn(III)-(O)(2)-Mn(III)-ON-Ni(II)] skeleton where -ON- is an oximate bridge between Mn(III) and Ni(II) ions and -(O)(2)- is a bi-phenolate bridge between Mn(III) ions. These tetramers can be seen as oligomeric units of the heterometallic Mn(III)(2)-Ni(II) chain observed in a family of single-chain magnets (Clérac, R.; Miyasaka, H.; Yamashita, M.; Coulon, C. J. Am. Chem. Soc. 2002, 124, 12837. Miyasaka, H.; Clérac, R.; Mizushima, K.; Sugiura, K.; Yamashita, M.; Wernsdorfer, W.; Coulon, C. Inorg. Chem. 2003, 42, 8203.). Magnetic measurements on these tetramers confirm the nature of the magnetic interactions reported for the Mn(III)(2)-Ni(II) chains: a strong antiferromagnetic Mn(III)/Ni(II) coupling via the oximate bridge (J(Ni-Mn) ranges from -23.7 to -26.1 K) and a weak ferromagnetic Mn(III)/Mn(III) coupling through the bi-phenolate bridge (J(Mn-Mn) ranges from +0.4 to +0.9 K). These magnetic interactions lead to tetramers with an S = 2 ground state.     

Modeling the resting state of oxalate oxidase and oxalate decarboxylase enzymes

In view of the biological and commercial interest in models for Oxalate Decarboxylases (OxDC) and Oxalate Oxidases (OxOx), we have synthesized and characterized three new Mn (II) complexes ( 1- 3) employing N3O-donor amino-carboxylate ligands (TCMA, 1,4,7-triazacyclononane- N-acetic acid; K (i) Pr 2TCMA, potassium 1,4-diisopropyl-1,4,7-triazacyclononane- N-acetate; and KBPZG, potassium N,N-bis(3,5-dimethylpyrazolyl methyl)glycinate). These complexes were characterized by several techniques including X-ray crystallographic analysis, X-band electron paramagnetic resonance (EPR), electrospray ionization mass spectrometry (ESI-MS), and cyclic voltammetry. The crystal structures of 1 and 3 revealed that both form infinite polymeric chains of Mn (II) complexes linked by the pendant carboxylate arms of the TCMA (-) and the BPZG (-) ligands in a syn-antipattern. Complex 2 crystallizes as a mononuclear Mn (II) cation, six-coordinate in a distorted octahedral geometry. Although complexes 1 and 3 crystallize as polymeric chains, all compounds present the same N3O-donor set atoms around the metal center as observed in the crystallographically characterized OxDC and OxOx. Moreover, complex 2 also contains two water molecules coordinated to the Mn center as observed in the active site of OxDC and OxOx. ESI-MS spectrometry, combined with EPR, were useful techniques to establish that complexes 1- 3 are present as mononuclear Mn (II) species in solution. Finally, complexes 1- 3 are able to model the resting state active sites, with special attention focused on complex 2 which provides the first exact first coordination sphere ligand structural model for the resting states of both OxDC and OxOx.     

One-dimensional manganese coordination polymers composed of polynuclear cluster blocks and polypyridyl linkers: structures and properties

The synthesis, crystal structures and magnetic properties of five new manganese compounds are reported. These include a linear trinuclear cluster [Mn(II)(3)(O(2)CCHMe(2))(6)(dpa)(2)].2MeCN (1) (dpa = 2,2'-dipyridylamine), a tetranuclear cluster [Mn(II)(2)Mn(III)(2)O(2)(O(2)CCMe(3))(6)(bpy)(2)] (3) (bpy = 2,2'-bipyridine), and chain coordination polymers composed of cluster blocks such as Mn(3), Mn(3)O, and Mn(4)O(2) bridged by 2,2'-bipyrimidine (bpm) or hexamethylentetramine (hmta) ligands to give ([Mn(II)(3)(O(2)CCHMe(2))(6)(bpm)].2EtOH)(n) (2), [Mn(II)(2)Mn(III)(2)O(2)(O(2)CCHMe(2))(6)(bpm)(EtOH)(4)](n) (4), and (([Mn(II)Mn(III)(2)O(O(2)CCHMe(2))(6)(hmta)(2)].EtOH)(n) (5). The magnetic analysis of the compounds was achieved using a combination of vector coupling and full-matrix diagonalization methods. Susceptibility data for compound 1 was fitted using a vector coupling model to give g = 2.02(1) and 2J/k(B) = -5.38(2) K. To model the trimer chain, we used vector coupling for initial values of J(1) and then diagonalization techniques to estimate J(2) to give g = 1.98(1), 2J(1)/k(B) = -3.3(1) K and 2J(2)/k(B) = -1.0(1) K by approximating the system to a dimer of trimers. The analysis of 3 was made difficult by the mixture of polymorphs and the difficulties of a three-J model, while for 4 an analysis was not possible because of the size of the computation and the relative magnitudes of the three couplings. Compound 5 was modeled using the same techniques as 2 to give g = 1.99(1), 2J(1)/k(B) = +32.5(2) K, 2J(2)/k(B) = -16.8(1) K, and 2J(3)/k(B) = +0.4(1) K. The combination of techniques has worked well for compounds 2 and 5 and thus opens up a method of modeling complex chains.     

Synthesis of the first heterometalic star-shaped oxido-bridged MnCu3 complex and its conversion into trinuclear species modulated by pseudohalides (N3(-), NCS- and NCO-): structural analyses and magnetic properties

A tetra-nuclear, star-shaped hetero-metallic copper(II)-manganese(II) complex, [{CuL(H(2)O)}(2)(CuL)Mn](ClO(4))(2) (1) has been synthesized by reacting the "complex as ligand" [CuL] with Mn(ClO(4))(2) where H(2)L is the tetradentate di-Schiff base derived from 1,3-propanediamine and 2-hydroxyacetophenone. Upon treatment with the polyatomic anions azide, cyanate, or thiocyanate in methanol medium, complex 1 transforms into the corresponding trinuclear species [(CuL)(2)Mn(N(3))(2)] (2), [(CuL)(2)Mn(NCO)(2)] (3) and [(CuL)(2)Mn(NCS)(2)] (4). All four complexes have been structurally and magnetically characterized. In complex 1 the central Mn(II) ion is encapsulated by three terminal [CuL] units through the formation of double phenoxido bridges between Mn(II) and each Cu(II). In complexes 2-4 one of the CuL units is replaced by a couple of terminal azide, N-bonded cyanate or N-bonded thiocyanate ions respectively and the central Mn(II) ion is connected to two terminal Cu(II) ions through a double asymmetric phenoxido bridge. Variable temperature magnetic susceptibility measurements show the presence of moderate ferrimagnetic exchange interactions in all the cases mediated through the double phenoxido bridges with J values (H = -JS(i)S(i + 1)) of -41.2, -39.8 and -12.6 cm(-1) (or -40.5 and -12.7 cm(-1) if we use a model with two different exchange coupling constants) for the tetranuclear MnCu(3) cluster in compound 1 and -20.0, -17.3 and -32.5 cm(-1) for the symmetric trinuclear MnCu(2) compounds 2-4. These ferrimagnetic interactions lead to spin ground states of 1 (5/2 - 3*1/2) for compound 1 and 3/2 (5/2 - 2*1/2) for compounds 2-4.     

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 magnetism of copper(II) and manganese(II) coordination polymers with azide and pyridylbenzoates

Three transition-metal coordination polymers with azide and/or carboxylate bridges have been synthesized from 4-(3-pyridyl)benzoic acid (4,3-Hpybz) and 4-(4-pyridyl)benzoic acid (4,4-Hpybz) and characterized by X-ray crystallography and magnetic measurements. Compound 1, [Cu(4,3-pybz)(N(3))](n), consists of 2D coordination networks in which the uniform chains with (μ-EO-N(3))(μ-COO) double bridges are cross-linked by the 4,3-pybz ligands. Compound 2, [Cu(2)(4,4-pybz)(3)(N(3))](n)·3nH(2)O, consists of 2-fold interpenetrated 3D coordination networks with the α-Po topology, in which the six-connected dinuclear motifs with mixed (μ-EO-N(3))(μ-COO)(2) (EO = end-on) triple bridges are linked by the 4,4-pybz spacers. Compound 3, [Mn(4,4-pybz)(N(3))(H(2)O)(2)](n), contains 2D manganese(II) coordination networks in which the chains with single μ-EE-N(3) bridges (EE = end-to-end) are interlinked by the 4,4-pybz ligands, and the structure also features a 2D hydrogen-bonded network in which Mn(II) ions are linked by double triatomic bridges, (μ-EE-N(3))(O-H···N) and (O-H···O)(2). Magnetic studies indicated that the mixed azide and carboxylate bridges in 1 and 2 induce ferromagnetic coupling between Cu(II) ions and that 3 features antiferromagnetic coupling through the EE-azide bridge. In addition, compound 1 exhibits antiferromagnetic ordering below 6.2 K and behaves as a field-induced metamagnet. A magnetostructural survey indicates a general trend that the ferromagnetic coupling through the mixed bridges decreases as the Cu-N-Cu angle increases.     

New types of heterospin complexes from trans-oxamido-bridged copper(II) binuclear units and nitronyl nitroxide radicals: crystal structure and magnetic characterization

Three new heterospin complexes derived from trans-oxamido-bridged copper(II) binuclear units [Cu(2)(oxen), oxen = N,N-bis(2-aminoethyl)oxamide] and pyridine-substituted nitronyl nitroxides (o-, m-, and p-PYNN) were synthesized and characterized structurally and magnetically. Complexes 1 and 2 are four-spin complexes. Interestingly, it is found that in complex 3, the Cu(II) ions and m-PYNN units are arranged to form 1D double-stranded helical chains, which to the best of our knowledge is the first example of a metal nitronyl nitroxide complex with such a 1D helical structure. The temperature dependencies of the magnetic susceptibilities of 1 and 2 were fitted to the four-spin model with the Hamiltonian H = -2Js(Cu1)s(Cu2) - 2j(s(Cu1)s(rad1) - s(Cu2)s(rad2)), leading to J = -150.5 cm(-)(1) and j = 47.2 cm(-)(1) for complex 1 and J = -191.7 cm(-)(1) and j = -18.9 cm(-)(1) for complex 2. The temperature dependence of the magnetic susceptibility of complex 3 was approximately simulated with a simple model composed of a dimer of Cu(II) ions and two m-PYNN molecules. The best fitting leads to the values of J = -183.0 cm(-)(1) and zJ' = -0.55 cm(-)(1) for the magnetic exchange of two Cu(II) ions through the oxamide bridge and that between the dimer of Cu(II) ions and two m-PYNN molecules, respectively. The antiferromagnetic exchange of oxamido-bridged Cu(II) ions in complexes 1-3 is strong. The strength of such antiferromagnetic interactions is also similar for the three complexes.     

Synthesis,crystal structure,and magnetic properties of ([Cu(oxbe)]Mn(H(2)O)[Cu(oxbe)(DMF)])(n).nDMF.nH(2)O: From dissymmetrical mononuclear entities to a 3D heterometallic supramolecular coordination polymer

Self-assembly of the building block [Cu(oxbe)](-) with Mn(II) led to a novel coordination polymer ([Cu(oxbe)]Mn(H(2)O)[Cu(oxbe)(DMF)])n).nDMF.nH(2)O, where H(3)oxbe is a new dissymmetrical ligand N-benzoato-N'-(2-aminoethyl)oxamido and DMF = dimethylformamide. The crystal forms in the triclinic system, space group Ponemacr;, with a = 9.260(4) A, b = 12.833(5) A, c = 15.274(6) A, alpha = 76.18(3) degrees, beta = 82.7(3) degrees, gamma = 82.31(3) degrees, and Z = 2. The crystal structure of the title complex reveals that the two-dimensional bimetallic layers are constructed of Cu(II)Mn(II)Cu(II) chains linked together by carboxylate bridge and hydrogen bonds help to produce a novel three-dimensional channel-like structure. The magnetic susceptibility measurements (5-300 K) were analyzed by means of the Hamiltonian H = -2JS(Mn)(S(Cu1) + S(Cu2)), leading to J = -17.4 cm(-1).     

Magneto-structural correlations in dinuclear Mn(III) compounds with formula [{Mn(L)(NN)}(μ-O)(μ-2-RC6H4COO)2{Mn(L')(NN)}]n+

Three dinuclear Mn(III) compounds with oxo and carboxylato bridges have been synthesized and characterized by X-ray diffraction: [{Mn(L)(NN)}(μ-2-ClC(6)H(4)COO)(2)(μ-O){Mn(L')(NN)}](n+) with NN = 2,2'-bipyridine (1 and 2) or 1,10-phenanthroline (3). The counteranion is either NO(3)(-) (1 and 3) or ClO(4)(-) (2) and the monodentate positions (L, L') could be occupied by molecules of water or the counteranion. For compound 1, L = H(2)O and L' = NO(3)(-); compound 2 shows two different dinuclear units and L and L' could be H(2)O or ClO(4)(-), and for compound 3 both monodentate positions are occupied by nitrate anions. The magnetic properties of the three compounds have been analyzed using the Hamiltonian H = -JS(1)·S(2). Compound 1 exhibits a dominant ferromagnetic behavior, with J = 3.0 cm(-1), |D(Mn)| = 1.79 cm(-1), |E(Mn)| = 0.60 cm(-1) with intermolecular interactions zJ' = -0.18 cm(-1). Due to the anisotropy of the Mn(III) ions, the ground state S = 4 shows ZFS with |D(4)| = 0.58 cm(-1). Compounds 2 and 3 show antiferromagnetic couplings, with J = -10.9 and -0.3 cm(-1), respectively. The magnetic interaction in this kind of compound depends on several structural factors. In the present work, the distortion around manganese ions, the torsion angle between the phenyl ring and the carboxylate group and the relative disposition of the coordination octahedra have been analyzed.     

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)).     

Novel mixed-valence manganese cluster with two distinct Mn3(II/III/II) and Mn3(III/II/III) trinuclear units in a pseudocubane-like arrangement

The reaction between Mn(ClO 4) 2 and di-(2-pyridyl)-ketone in the presence of the sodium salt of propanediol as a base in MeOH leads to the formation of a hexanuclear manganese cluster. This cluster has been characterized by the formula [Mn(II) 3Mn(III) 3O(OH)(CH 3pdol) 3(Hpdol) 3(pdol)](ClO 4) 4 ( 1). Molecular conductance measurements of a 10 (-3) M solution of compound 1 in CH 3CN, DMSO, or DMF give Lambda m = 529, 135, or 245 muS/cm, respectively, which suggests a 1:4 cation/anion electrolyte. The crystal structure of hexanuclear manganese cluster 1 consists of two distinct trinuclear units with a pseudocubane-like arrangement. The trinuclear units show two different valence distributions, Mn(II)/Mn(III)/Mn(II) and Mn(III)/Mn(II)/Mn(III). Additional features of interest for the compound include the fact that (a) two of the Mn(III) ions show a Jahn-Teller elongation, whereas the third ion shows a Jahn-Teller compression; (b) one bridge between Mn(III) atoms is an oxo (O (2-)) ion, whereas the bridge between Mn(II) and Mn(III) is a hydroxyl (OH (-)) group; and (c) the di-(2-pyridyl)-ketone ligand that is methanolyzed to methyl-Hpdol and R 2pdol (R = CH 3, H) acts in three different modes: methyl-pdol(-1), Hpdol(-1), and pdol(-2). For magnetic behavior, the general Hamiltonian formalism considers that (a) all of the interactions inside the two "cubanes" between Mn(II) and Mn(III) ions are equal to the J 1 constant, those between Mn(II) ions are equal to the J 2 constant, and those between the Mn(III) ions are equal to the J 3 constant and (b) the interaction between the two cubanes is equal to the J 4 constant. The fitting results are J 1 = J 2 = 0.7 cm (-1), J 3 approximately 0.0, J 4 = -6.2 cm (-1), and g = 2.0 (fixed). According to these results, the ground state is S = 1/2, and the next excited states are S = 3/2 and 5/2 at 0.7 and 1.8 cm (-1), respectively. The EPR spectra prove that the spin ground state at a low temperature is not purely S = 1/2 but is populated with the S = 3/2 state, which is in accordance with the susceptibility and magnetization measurements.     

A ferromagnetic methoxido-bridged Mn(III) dimer and a spin-canted metamagnetic μ(1,3)-azido-bridged chain

Two new Mn(III) complexes of formulas [MnL(1)(N(3))(OMe)](2) (1) and [MnL(2)(N(3))(2)](n) (2) have been synthesized by using two tridentate NNO-donor Schiff base ligands HL(1){(2-[(3-methylaminoethylimino)-methyl]-phenol)} and HL(2) {(2-[1-(2-dimethylaminoethylimino)methyl]phenol)}, respectively. Substitution of the H atom on the secondary amine group of the N-methyldiamine fragment of the Schiff base by a methyl group leads to a drastic structural change from a methoxido-bridged dimer (1) to a single μ(1,3)-azido-bridged 1D helical polymer (2). Both complexes were characterized by single-crystal X-ray structural analyses and variable-temperature magnetic susceptibility measurements. The magnetic properties of compound 1 show the presence of weak ferromagnetic exchange interactions mediated by double methoxido bridges (J = 0.95 cm(-1)). Compound 2 shows the existence of a weak antiferromangetic coupling along the chain (J = -8.5 cm(-1)) through the single μ(1,3)-N(3) bridge with a spin canting that leads to a long-range antiferromagnetic order at T(c) ≈ 9.3 K and a canting leading to a weak ferromagnetic long-range order at T(c) ≈ 8.5 K. It also exibits metamagnetic behavior at low temperatures with a critical field of ca.1.2 T due to the weak antiferromagnetic interchain interactions that appear in the canted ordered phase.