Dicopper(II) Complexes Bridged by Single N−N Bonds. Magnetic Exchange Dependence on the Rotation Angle between the Magnetic Planes

A series of dicopper(II) complexes with two tetradentate (N4) diazine ligands (PAHAP (1), PMHAP (2)) is reported, in which the two dx2-z2 type copper centers are bridged by a single N−N bond. Varying the coligands leads to a situation where the angle between the copper planes can be varied. For small angles (<80°) ferromagnetic coupling prevails, whereas at larger angles antiferromagnetic exchange is observed between the copper(II) centers. This is associated with the degree of alignment of the nitrogen p orbitals in the diazine bridge, and is supported by molecular orbital calculations on the complexes and appropriate models. Structures are reported for PAHAP (1) (picolinamide azine), [Cu2(PAHAP)Cl4]·H2O (3), [Cu2(PAHAP)Br4]·H2O (5), [Cu2(PAHAP)(H2O)6](NO3)4 (6), and [Cu2(PMHAP-H)(NO3)3] (8). 1 crystallized in the orthorhombic system, space group Pbca (No. 61), with a = 19.845(4) Å, b = 13.178(5) Å, c = 9.383(8) Å, and Z = 8. 3 crystallized in the monoclinic system, space group C2/c (No. 15), with a = 26.732(6) Å, b = 8.670(9) Å, c = 16.436(4) Å, β = 100.88(2)°, and Z = 8. 5 crystallized in the monoclinic system, space group C2/c (No. 15), with a = 27.336(2) Å, b = 8.859(4) Å, c = 16.795(3) Å, β = 100.78(1)°, and Z = 8. 6 crystallized in the monoclinic system, space group C2/c (No. 15), with a = 20.983(4) Å, b = 7.505(4) Å, c = 17.219(3) Å, β = 104.22(1)°, and Z = 4. 8 crystallized in the triclinic system, space group P1̄ (No. 2), with a = 7.8380(14) Å, b = 8.015(3) Å, c = 15.655(4) Å, α = 99.81(3)°, β = 101.74(2)°, γ = 94.524(17)°, and Z = 2. The antiferromagnetically coupled complexes [Cu2(PAHAP)Cl4] (4) and [Cu2(PAHAP-H)(N3)2(NO3)] (7) are also reported

Magnetic [<i>n</i> × <i>n</i>] (<i>n</i> = 2−5) Grids by Directed Self-Assembly

Polytopic hydrazone-based ligands are discussed in the context of the design attributes of the ligand and the power of self-assembly as a methodology for the synthesis of polymetallic systems with specific and predetermined organization of the metal centers in a closely spaced bridged arrangement. Magnetic exchange coupling occurs as a result of the close proximity of the metal ions. Homometallic, heterometallic, and mixed-spin-state [n × n] (n = 2−5) square grids are highlighted and discussed in terms of their structural and magnetic properties. Antiferromagnetic, ferromagnetic, and ferrimagnetic examples are described

Coordination Compounds of Schiff-Base Ligands Derived from Diaminomaleonitrile (DMN): Mononuclear, Dinuclear, and Macrocyclic Derivatives

Copper(II) and VIVO complexes of an open chain (1:2) Schiff-base ligand (H2L1), derived by the template condensation of diaminomaleonitrile (DMN) and salicylaldehyde, and dicopper(II) complexes of (2:2) macrocyclic Schiff-base ligands derived by template condensation of diformylphenols and diaminomaleonitrile, have been synthesized and studied. Structures have been established for the first time for mononuclear Cu(II) and VIVO derivatives of the open chain ligand H2L1 (1:2), a dinuclear macrocyclic Cu(II) complex derived from a 2:2 macrocyclic ligand (H2M1), and the half-condensed 1:1 salicylaldehyde ligand (H2L2). [Cu(L1)] (1) (L1 = C18H10N4O2) crystallized in the monoclinic system, space group P21/n (No. 14), with a = 11.753(6) Å, b = 7.708(5) Å, c = 16.820(1) Å, and Z = 4. [VO(L1)(DMSO] (2) crystallized in the orthorhombic system, space group Pbca (No. 61), with a = 22.534(9) Å, b = 23.31(1) Å, c = 7.694(5) Å, and Z = 8. H2L2 (C18H8N4O) (3) crystallized in the monoclinic system, space group P21/c (No. 14), with a = 13.004(6) Å, b = 11.441(7) Å, c = 7.030(4) Å, and Z = 4. [Cu2(M3)](CH3COCH3) (4) (M3 = C32H24N8O4) crystallized in the monoclinic system, space group C2/c (No. 15), with a = 38.33(2) Å, b = 8.059(4) Å, c = 22.67(2) Å, and Z = 8. [Cu(L3)(DMSO)] (5) (L3 = C20H14N2O4) crystallized in the triclinic system, space group P1̄ (No. 2), with a = 10.236(4) Å, b = 13.514(4) Å, c = 9.655(4) Å, and Z = 2. 4 results from the unique addition of two acetone molecules to two imine sites in [Cu2(M1)](ClO4)2 (M1 = 2:2 macrocyclic ligand derived from template condensation of DMN and 2,6-diformyl-4-methylphenol). 4 has extremely small Cu−OPh−Cu bridge angles (92.0, 92.8°), well below the expected lower limit for antiferromagnetic behavior, but is still antiferromagnetically coupled (−2J = 25.2 cm-1). This behavior is associated with a possible antiferromagnetic exchange term that involves the conjugated framework of the macrocyclic ligand itself. The ligand L3 in 5 results from hydrolysis of M1 on recrystallization of [Cu2(M1)](ClO4)2 from undried dimethyl sulfoxide

Helical Antiferromagnetic Copper(II) Chains with a Collagen Structural Motif

Helical Antiferromagnetic Copper(II) Chains with a Collagen Structural Moti

Electronic and Magnetic Interactions in π-Stacked Bisthiadiazinyl Radicals

The preparation of two bisthiadiazinyls (7, R1 = Me, Et; R2 = Cl, R3 = Ph), the first examples of a new class of resonance-stabilized heterocyclic thiazyl radical, are reported. Both radicals have been characterized in solution by EPR spectroscopy and cyclic voltammetry, which confirm highly delocalized spin distributions and low electrochemical cell potentials, features which augur well for the use of these materials as building blocks for neutral radical conductors. In the solid state, the radicals are undimerized, crystallizing in slipped π-stack arrays which ensure the availability of electrons as potential charge carriers. However, despite these favorable electrochemical and structural properties, both materials exhibit low conductivities, with σ(300K) -7 S cm-1, a result which can be rationalized in terms of their EHT band electronic structures, which indicate that intermolecular interactions lateral to the π-stacks are limited. The materials are thus very 1-D with low bandwidths, so that a Mott insulating state prevails. When R1 = Me, the intermolecular overlap along the π-stacks is weak and the material is essentially paramagnetic. When R1 = Et, intermolecular π-overlap is greater and variable-temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically coupled system, the behavior of which has been modeled in terms of a molecular-field modified 1-D Heisenberg chain of S = 1/2 centers. Broken-symmetry DFT methods have been used to estimate the magnitude of individual exchange interactions within both structures

Polynuclear Fe_<i>n</i> Complexes (<i>n</i> = 1, 2, 4, 5) of Polytopic Hydrazone Ligands with Fe(II), Fe(III) and Mixed Oxidation State Combinations

The iron coordination chemistry of some polytopic hydrazone based ligands is examined. The complexes derive from a general self-assembly strategy, where ligand design can be used to devise specific polymetallic [n × n] grid architectures. However, as part of any complex equilibrium process, oligomeric entities can also occur, particularly when ligand tautomeric flexibility is considered, and examples of mononuclear, dinuclear, tetranuclear, and pentanuclear complexes have been observed within a related class of ligands. In addition, ligand site donor composition can lead to coordination spheres that stabilize both high spin Fe­(II) and Fe­(III) sites, with evidence for Fe­(II) spin crossover. Structural and magnetic properties are examined, which reveal the presence of antiferromagnetic exchange in the polynuclear systems

Verdazyl Radicals as Oligopyridine Mimics: Structures and Magnetic Properties of M(II) Complexes of 1,5-Dimethyl-3-(2,2‘-bipyridin-6-yl)-6-oxoverdazyl (M = Mn, Ni, Cu, Zn)

The verdazyl radical 1,5-dimethyl-3-(2,2‘-bipyridin-6-yl)-6-oxoverdazyl (3) was prepared, and its homoleptic metal complexes M(3)22+·2X- (5, M = Mn(II); 6, M = Ni(II); 7, M = Cu(II); 8, M = Zn(II); X = ClO4, PF6) were characterized by single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Relevant crystallographic parameters are as follows:  5, monoclinic space group Pna21, a = 18.755(4) Å, b = 11.154(3) Å, c = 16.594(4) Å, α = 90.00°, β = 90.00°, γ = 90.00°, V = 3471.4(13) Å3, and Z = 4; 7, triclinic space group P1̋, a = 9.4638(18) Å, b = 9.8442(19) Å, c = 18.769(4) Å, α = 103.746(3)°, β = 92.925(3)°, γ = 94.869(3)°, V = 1687.8(6) Å3, and Z = 2; 8, triclinic space group P1̋, a = 9.4858(14) Å, b = 9.7919(14) Å, c = 18.889(3) Å, α = 104.196(3)°, β = 92.855(3)°, γ = 94.216(3)°, V = 1692.1(4) Å3, and Z = 2. In all cases, the two verdazyl-based ligands bind almost perpendicular to each other in meridional positions, yielding pseudooctahedral metal complexes whose general structural features are strongly reminiscent of metal bis(terpyridine) complexes. The intramolecular metal−verdazyl magnetic exchange coupling is strongly ferromagnetic in 6 (JNi-vd= +240 cm-1), and strongly antiferromagnetic in 5 (JMn-vd= −93 cm-1). Complex 7 exhibits weak ferromagnetic coupling (JCu-vd = −4.5 cm-1). Intramolecular radical−radical coupling in the zinc complex 8 was found to be weakly antiferromagnetic (Jvd-vd = −8 cm-1). Intramolecular radical−radical exchange was generally weak in the four metal complexes, ranging from −10 cm-1 (for 5) to +2 cm-1 (for 7). The low-temperature magnetic behavior of 7 and 8 is complex, possibly arising from a combination of intra- and intermolecular interactions

Dinuclear and Octanuclear Mn(II) Complexes with μ²-C, μ²-N(Pyrrolide), and μ<i>-</i>η¹:η⁵-(Pyrrolide) Bridges: A Structural and Magnetic Study

Reaction of the dinuclear [(CH2SiMe3)(μ-CH2SiMe3)Mn(THF)]2 (1) with an equivalent amount of 1,1-dipyrrolylcyclohexane afforded two compounds depending on the solvent employed. Reaction carried out in THF afforded the dinuclear {[1,1-(μ-C4H3N)(C4H3N)C6H10]Mn(THF)2}2·2(THF) (2) while reaction in toluene yielded the octanuclear and cyclic cluster {[1,1-(μ,η1:η5-C4H3N)2C6H10]Mn}8·4(toluene) (3). The magnetism in all three cases is dominated by intramolecular antiferromagnetic exchange with strong coupling in 1 (J = −85 cm-1), and in 2 (J = −23.2 cm-1), whereas substantially weaker coupling through the σ/π-bonded dipyrrolide bridges (J = −3.3 cm-1) was observed within the cyclic and octameric 3

The Mixed-Valent Manganese [3 × 3] Grid [Mn(III)₄Mn(II)₅(2poap-2H)₆](ClO₄)₁₀·10H₂O, a Mesoscopic Spin-¹/₂ Cluster

The magnetic susceptibility and low-temperature magnetization curve of the [3 × 3] grid [Mn(III)4Mn(II)5(2poap-2H)6](ClO4)10·10H2O (1) are analyzed within a spin Hamiltonian approach. The Hilbert space is huge (4 860 000 states), but the consequent use of all symmetries and a two-step fitting procedure nevertheless allows the best-fit determination of the magnetic exchange parameters in this system from complete quantum mechanical calculations. The cluster exhibits a total spin S = 1/2 ground state; the implications are discussed

Synthesis, Structure, and Magnetism of Bimetallic Manganese or Nickel Complexes of a Bridging Verdazyl Radical

Two binuclear metal−radical complexes, formed by the reaction of M(hfac)2·2H2O (M = Mn or Ni; hfac = hexafluoroacetylacetonate) with the 1,5-dimethyl-3-(4,6-dimethylpyrimidin-2-yl)-6-oxoverdazyl radical (3), were synthesized. The binuclear Mn complex 5 (i.e., 3[Mn(hfac)2]2) crystallizes in the monoclinic space group C2/c:  C30H17N6O9F24Mn2, a = 29.947(3), b = 17.143(3), c = 16.276(3) Å, β = 123.748(3)°, Z = 4. The compound consists of two pseudo-octahedral Mn(II) ions, both bearing two hfac ancillary ligands, bridged by the bis(bidentate) radical 3. The temperature dependence of the magnetic susceptibility of 5 reveals moderate antiferromagnetic exchange between each of the Mn(II) ions and the verdazyl radical (J = −48 cm-1). The S = 9/2 ground spin state of the complex was corroborated by low-temperature magnetization versus field measurements. In contrast, the magnetic susceptibility versus temperature behavior of 6 (whose molecular structure is presumed to be analogous to that of 5) indicates that the two Ni(II) ions are strongly ferromagnetically coupled to the verdazyl radical (J = +220 cm-1). The magnetization versus field behavior of 5 is consistent with an S = 5/2 ground-state species