A copper(II) complex of a pentadentate ligand featuring large ferro- and antiferromagnetic interactions

A novel pentadentate ligand based on a 2,2'-bipyridine template and functionalized with two methylene bridged nitroxide arms provides mononuclear Cu(II) and Zn(II) complexes displaying a facial and meridional conformation, respectively; for Cu large intramolecular ferromagnetic (J/kB = +173 K) and antiferromagnetic (J'/kB = -170 K) exchange interactions are evidenced from magnetic measurements.     

Novel square planar copper(II) complexes with imino or nitronyl nitroxide radicals exhibiting large ferro- and antiferromagnetic interactions

This paper reports the synthesis, crystal structures, and magnetic properties of two copper(II) complexes (1, 2) of general formula Cu(tfac)2(radical)2 (tfac = trifluoroacetate; radical = (1) 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITPh) or (2) 2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazoline-1- oxyl (IMPh)). They crystallize in the monoclinic P2(1)/n space group with the following parameters: (1) a = 13.212(2) A, b = 9.136(1) A, c = 15.587(2) A, beta = 114.61(1) degrees, Z = 2; (2) a = 11.059(2) A, b = 15.289(1) A, c = 10.694(2) A, beta = 114.20(1) degrees, Z = 2. In both complexes the copper(II) ion is coordinated to two radicals in a slightly distorted square planar surrounding. The copper(II)-radical exchange couplings are antiferromagnetic for the nitronyl nitroxide (NITPh) complex (1) and ferromagnetic in the case of the imino nitroxide (IMPh) analogue (2). The ground state has been found to be a spin-doublet for 1 and the spin-quartet for 2. No thermal population of the highest states has been observed, indicating copper(II)-radical couplings of magnitude of J > 500 cm-1.     

Competing ferro- and antiferromagnetic interactions in (manganese,sodium)phenylsilsesquioxane with metal oxide fragments

Magnetic measurements of individual (manganese,sodium)phenylsilsesquioxane containing eight Mn^II ions were performed. The magnetic arrangement of manganese ions in this cage-like compound was investigated. The magnetic moment of the cage molecule equals 21 μ_B (at 300 K), that is a consequence of the mutual antiferromagnetic compensation of four Mn^II ions, whereas the other four Mn^II ions have aligned spins. Hence, this metallasiloxane is a promising compound for the chemical design of molecular nanomagnets. At low temperatures, the antiferromagnetic coupling causes a decrease in the effective magnetic moment (to 16 μ_B at 40 K).     

A new oxamido-bridged dinuclear copper(II) complex with large antiferromagnetic exchange interactions

A new oxamido-bridged dinuclear compound [Cu₂(µ-pmox)(DMF)₄]?·?2ClO₄ (1) (H₂pmox?=?N,N′-bis-(2-methylpyridyl)oxalamide, DMF?=?dimethylformamide) was synthesized and structurally characterized. The five-coordinate Cu(II) is bridged by oxamido groups and further cross-linked by C–H···O hydrogen bonds between the uncoordinated oxygen of perchlorate and methyl of DMF. The complex was also characterized by infrared spectroscopy and magnetic measurement. The copper complex exhibits strong antiferromagnetic interactions via the trans oxamido bridge with J of ?414?cm^?1, where J is the exchange parameter in the isotropic Hamiltonian H?=??JS₁S₂.     

The novel azomethine ligands for binuclear copper(II) complexes with ferro- and antiferromagnetic properties

A series of novel binuclear ferro- and antiferromagnetic Cu(2+) chelates of structurally broadly varied Schiff bases (derived from o-tosylamino(hydroxyl)benzaldehydes and monoalkylated o-phenylenediamine, o-aminophenol, o-aminothiophenol, 1,2-diaminobenzimidazole, 1-aminobenzimidazoline-2-thione) and β-diketimines (derived from 2,6-di-i-Pr-aniline) has been prepared. The tautomerism of the ligands and structureof their copper complexes have been studied with the use of IR, ¹H NMR EPR and EXAFS spectroscopy. Molecular and crystal structure of a β-diketimine copper dimer has been determined by X-ray crystallography. The magnetic measurements (2–300?K) performed for all the complexes showed that the ferro- and antiferromagnetic character of the exchange interaction depends both on the structure of the coordination site (origin of the ligating centers) and the structure of the ligands (in particular, on the type of the cycle annelated to the bridging fragment). Whereas S-binding metal chelates 13 (X?=?NTs, Y?=?S, R?=?H) are diamagnetic, the complexes 15 with annelated azole moieties are ferromagnetic.     

Metal-dependent ferro- versus antiferromagnetic interactions in molecular crystals of square Planar (M(II) imino-nitroxide radical) complexes (M = Pt,Pd)

The synthesis and structural, spectral, and magnetic characterizations of two new complexes of formula [Pt(IM(2)Py)Cl(2)] (A) and [Pd(IM(2)Py)Cl(2)] (B) are reported. IM(2)Py stands for the imino-nitroxide radical ligand 2-(ortho-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl. Their crystal structures were solved at room temperature and at 120 K revealing structural phase transitions from pseudo-orthorhombic to monoclinic systems for the two compounds which remain isostructural in the whole temperature range explored. Structural parameters for A: T = 293 K [120 K], monoclinic (P2(1)/n) [P2(1)/c], a = 7.906(2) [7.989(3)] A, b = 17.872(9) [10.168(4)] A, c = 10.357(3) [17.623(6)] A, beta = 90.732(13) degrees [95.940(2)] degrees, Z = 4 [4]. Structural parameters for B: T = 293 K [120 K], monoclinic (P2(1)/n) [P2(1)/c], a = 7.900(3) [7.9730(2)] A, b = 17.907(9) [10.1806(3)] A, c = 10.299(3) [17.7171(4)] A, beta = 90.524(14) degrees [95.747(2)] degrees, Z = 4 [4]. In both complexes, the metal coordination is essentially planar. The average Pt-N, Pt-Cl and Pd-N, Pd-Cl bond lengths are 1.996(6) [1.88], 2.295(2) [2.248(8)] A and 2.015(7) [2.029(8)], 2.287(3) [2.294(3)] A, respectively. The solid state structure is characterized by a pairlike molecular packing stacked in columns parallel to the a axis; this dimer character is reinforced at low temperature. Despite their structural similarity, the investigation of the magnetic properties revealed that dominant ferromagnetic interactions govern the behavior of the Pt derivative A, whereas antiferromagnetic interactions take place for the Pd compound B. A rationalization for this rather intriguing difference is proposed in light of the spin population deduced from density functional theory calculations. The electronic absorption spectra of A and B present structured absorption bands in the visible which are attributed to MLCT transitions. Both compounds are nonluminescent at room temperature. However, a weak emission is detected for A in butyronitrile glasses at 77 K, indicating that the MLCT excited state is strongly quenched at low temperature.     

A new metal-organic open framework consisting of threefold parallel interwoven (6,3) nets

Threefold parallel interwoven (6,3) nets were assembled from Ni(II) cyclam complex and 1,3,5-tris[2-(4-carboxyphenyl)-1-ethynyl]benzene. The network generates triangular voids of effective size ca. 18.4 x 14.7 x 9.5 A. It contains 35% free volume of the crystal volume and is stable up to 300 degrees C.     

Ligand effects on the ferro- to antiferromagnetic exchange ratio in bis(o-semiquinonato)copper(II)

Heterospin complexes [Cu(SQ)2Py].C7H8, Cu(SQ)2DABCO, and [Cu(SQ)2NIT-mPy].C6H6, where Cu(SQ)2 is bis(3,6-di-tert-butyl-o-benzosemiquinonato)copper(II), DABCO is 1,4-diazabicyclo(2,2,2)octane, and NIT-mPy is the nitronyl nitroxide 2-(pyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, have been synthesized. The molecules of these complexes have a specific combination of the intramolecular ferro- and antiferromagnetic exchange interactions between the odd electrons of Cu(II) and SQ ligands, characterized by large exchange coupling parameters |J| approximately 100-300 cm(-1). X-ray and magnetochemical studies of a series of mixed-ligand compounds revealed that an extra ligand (Py, NIT-mPy, or DABCO) coordinated to the metal atom produces a dramatic effect on the magnetic properties of the complex, changing the multiplicity of the ground state. Quantum chemical analysis of magnetostructural correlations showed that the energy of the antiferromagnetic exchange interaction between the odd electrons of the SQ ligands in the Cu(SQ)2 bischelate is extremely sensitive to both the nature of the extra ligand and structural distortions of the coordination unit, arising from extra ligand coordination.     

One-dimensional assembling of diiodo[phthalocyaninato(1-)] chromate(III) molecules through neutral I(2) molecules. Alternating ferro- and antiferromagnetic interactions in the metal-radical system

Crystals of diiodo[phthalocyaninato(1-)] chromate(III) diiodine, CrPcI(2).I(2), were grown directly in the reaction of chromium powder with 1,2-dicyanobenzene under a stream of iodine at about 250 degrees C. The CrPcI(2).I(2) complex crystallizes in the centrosymmetric space group of the triclinic system with one molecule per unit cell, with the cell dimensions a = 7.851(2) A, b = 8.402(2) A, c = 12.668(3) A, alpha = 80.32(3)(o), beta = 74.06(3)(o), gamma = 82.33(3)(o), and V = 788.7(3) A(3). The X-ray single-crystal analysis shows that each of the centrosymmetric CrPcI(2) molecules is bridged by a neutral I(2) molecule (detected also by Raman spectroscopy) and develops a polymeric one-dimensional structure. The magnetic measurements have been carried out in the temperature range 300-2 K. Temperature dependence of the effective magnetic moment, mu(eff), shows the ferro- and antiferromagnetic interactions in the system of the paramagnetic central Cr(3+) ion and surrounding pi-conjugated radical ligand Pc(1-). The conductivity measurement on a polycrystalline sample exhibits weak temperature dependence (dsigma/dT < 0). The UV-vis spectrum exhibits, besides the B- and Q-bands, one additional band assigned to the electronic transition from a deeper level to the half-occupied HOMO level in the one-electron oxidized phthalocyaninato(1-) radical ligand.     

Template synthesis of lanthanide (Pr, Nd, Gd) coordination polymers with 2-hydroxynicotinic acid exhibiting ferro-/antiferromagnetic interaction

Lanthanide coordination polymers were synthesized from Pr(III), Nd(III), and Gd(III) salts; 2-hydroxynicotinic acid (Hnica); and MnSO 4.H 2O under hydrothermal conditions. In the absence of (CH 3) 3CCOONa, 1D polymers with an infinite Ln(III)-O-Ln(III) chain structure, [Pr(Hnica)(H 2O) 2SO 4] n ( 1), [Nd(Hnica)(H 2O) 2SO 4] n ( 2), and [Gd(Hnica)(H 2O) 2SO 4] n ( 3), were generated. When (CH 3) 3CCOONa was added to the synthetic systems, 2D coordination polymers {[Pr 3(Hnica) 6(H 2O) 9].3H 2O.SO 4.NO 3} n ( 4), {[Nd 3(Hnica) 6(H 2O) 9].3H 2O.SO 4.NO 3} n ( 5), and {[Gd(Hnica) 2(H 2O) 2]ClO 4.H 2O} n ( 6) were obtained. Complexes 4 and 5 both exhibit Kagome lattice structure, while 6 displays a rhombic grid structure. All complexes were characterized by elemental analysis, IR spectra, UV-vis spectra, and X-ray single-crystal diffraction. The variable-temperature magnetic susceptibility studies reveal ferromagnetic interactions between gadolinium(III) ions in 3 and 6 and antiferromagnetic interactions in 1, 2, 4, and 5.     

Coexistence of chiral hydrophilic and achiral hydrophobic channels in one multi-helical-array metal-organic framework incorporating helical water cluster chains

A novel host-guest metal-organic compound with both chiral hydrophilic and achiral hydrophobic channels has been obtained through the reaction of Cd(II) ion and a versatile asymmetrical ligand of H(4)bptc (H(4)bptc = 1,1'-biphenyl-2,2',3,3'-tetracarboxylic acid) based on hydro(solvo)thermal reactions.     

Understanding hydrogen sorption in a polar metal-organic framework with constricted channels

A high fidelity molecular model is developed for a metal-organic framework (MOF) with narrow (approximately 7.3 A?) nearly square channels. MOF potential models, both with and neglecting explicit polarization, are constructed. Atomic partial point charges for simulation are derived from both fragment-based and fully periodic electronic structure calculations. The molecular models are designed to accurately predict and retrodict material gas sorption properties while assessing the role of induction for molecular packing in highly restricted spaces. Thus, the MOF is assayed via grand canonical Monte Carlo (GCMC) for its potential in hydrogen storage. The confining channels are found to typically accommodate between two to three hydrogen molecules in close proximity to the MOF framework at or near saturation pressures. Further, the net attractive potential energy interactions are dominated by van der Waals interactions in the highly polar MOF - induction changes the structure of the sorbed hydrogen but not the MOF storage capacity. Thus, narrow channels, while providing reasonably promising isosteric heat values, are not the best choice of topology for gas sorption applications from both a molecular and gravimetric perspective.     

The high possibility of antiferromagnetic exchange interactions in dinuclear Fe(III) dithiocarbamates

In this study, the possible antiferromagnetic interactions in a dinuclear entity have been considered. It has been realized that, if the proposed structure is a real structure, there should be antiferromagnetic interactions between the dinuclear Fe(III) ions. It is also suggested that the predicted theoretical results should be tested with additional experimental measurements     

(6,3)-honeycomb structures of uranium(VI) benzenedicarboxylate derivatives: the use of noncovalent interactions to prevent interpenetration

Four two-dimensional coordination polymers containing the uranyl cation (UO2(2+)), (NH4)UO2(BDC)1.5 . 2.5H2O (1), KUO2(NDC)1.5 . 2H2O (2), [C(NH2)3]UO2(NDC)1.5 . 2H2O (2b), and UO2(HBDC-Br)2 (3) (BDC = 1,4-benzenedicarboxylate, NDC = 1,4-naphthalenedicarboxylate, BDC-Br = 2-bromoterephthalate) have been synthesized by hydrothermal reactions. Compounds 1-2b have the same honeycomb (6,3) net but with two-fold interpenetration in 1 and without interpenetration in 2 and 2b. The use of 2-bromoterephthalate yields compound 3 with a (4,4) net. The structures of 2 and 2b show that the interpenetration can be prevented by the addition of a bulky substituent to the ligand. Maintaining the desired topology, however, requires a careful choice of the substituent group. Compounds 1, 2, and 2b have a similar structural arrangement to that of benzenetricarboxylic acid (trimesic acid, H3BTC). In H3BTC, the six rings are formed by hydrogen bonding and the interpenetration is more complex than that in 1. Crystal data: 1, triclinic, space group P, a = 10.453(8) A, b = 12.316(9) A, c = 13.441(10) A, alpha = 78.49(1) degrees , beta = 82.17(1) degrees , gamma = 85.57(1) degrees , and Z = 4; 2, monoclinic, space group C2/c, a = 12.7795(9) A, b = 19.728(1) A, c = 15.379(1) A, beta = 92.247(1) degrees , and Z = 8; 2b, monoclinic, space group C2/c, a = 12.7214(8) A, b = 19.645(1) A, c = 17.065(1) A, beta = 98.896(1) degrees , and Z = 8; 3, monoclinic, space group P21/c, a = 7.873(5) A, b = 18.358(14) A, c = 6.893(5) A, beta = 115.96(2) degrees , and Z = 2.     

Coexistence of water dimer and hexamer clusters in 3D metal-organic framework structures of Ce(III) and Pr(III) with pyridine-2,6-dicarboxylic acid

Ce(NO3)3.6H2O or Pr(NO3)3.6H2O and pyridine-2,6-dicarboxylic acid form a linear coordination polymeric structure under hydrothermal conditions. Hexameric water clusters join these linear chains through bonding to the metal ions. Other coordinated water and the carboxylate oxygen form an intricate array of hydrogen bonding resulting in a 3D network where each metal ion shows 9-coordination with an approximate D3 symmetry. Dimeric water clusters are also located in the void spaces. In the structure containing Pr(III), the water dimers are hydrogen-bonded to the hexamers, whereas in the Ce(III) structure, the dimers and the hexamers are far apart.     

Mutual induced coordination in halogen-bonded anionic assemblies with (6,3) cation-templated topologies

We describe the use of halogen bonding for the deliberate construction of cation-templated anionic (6,3) networks thanks to a strategy based on the mutual induced fitting of the valences of the building blocks.     

The role of antiferromagnetic exchange interactions in dinuclear Cr(III) dithiocarbamates and a stepwise behavior of magnetic moment

In this study the role of the antiferromagnetic interactions in recently synthesized dinuclear Cr(III) complex has been investigated. Since there was not enough structural information for the characterization of the synthesis, we claim that there should be antiferromagnetic interactions between the dinuclear Fe(III) ions, if the proposed structure is the real structure. A new experiment is proposed to test the predictions of this theoretical investigation.     

Magnetic interactions in p-phenylene-bis(nitronyl nitroxide) biradicals with large torsion angles

We have designed and synthesized new biradicals of p-phenylene-bis(nitronyl nitroxide) substituted with two methoxy groups at 2,3- (2) and 2,5-positions (3). A parent biradical p-phenylene-bis(nitronyl nitroxide) (1) has intramolecular antiferromagnetic exchange interaction of 2J/k_B = −104 K ∼ −106 K with a torsion angle of 28.5° between the phenyl and the imidazole rings of nitronyl nitroxide. X-ray crystal structure analysis shows that the bulky substituents in 2 and 3 give large torsion angles of 65–70°. The larger torsion angles should weaken the magnitude of intramolecular exchange interactions, which is attributed to a decrease in π-conjugation over the p-phenylene and the radical groups. Magnetic susceptibility measurements indicate that the intramolecular exchange interactions in 2 and 3 are severely weakened to about 6% of that in 1, 2J/k_B = −6 K ∼ −8 K. The relation between the torsion angle and the intramolecular exchange interaction is consistent with DFT calculations. The ground-state singlet biradicals with suppressed intramolecular exchange interactions can be a building block for exotic exchange-coupled spin systems as predicted in our theoretical studies.