RESUMEN

The cobalt(II)-mediated self-assembly of the potentially tris(chelating) N,N'-2,2'-(4,4'-bithiazole)bis(oxamate) (dabtzox) ligand gives a new metal-organic supramolecular nanomagnet of formula K6Co3(dabtzox)3·8H2O·MeOH (1) featuring a unique linear triple-stranded trinuclear structure of the helicate type.

RESUMEN

To investigate the herbicidal potential of 2,5-diketopiperazines (2,5-DKPs), we applied a known protocol to produce a series of 2,5-DKPs through intramolecular N-alkylation of Ugi adducts. However, the method was not successful for the cyclization of adducts presenting aromatic rings with some substituents at the ortho position. Results from DFT calculations showed that the presence of voluminous groups at the ortho position of a benzene ring results in destabilization of the transition structure. Lower activation enthalpies for the SN2-type cyclization of Ugi adducts were obtained when bromine, instead of a chlorine anion, is the leaving group, indicating that the activation enthalpy for the cyclization step controls the formation of the 2,5-DKP. Some Ugi adducts and 2,5-DKPs formed crystals with suitable qualities for single-crystal X-ray diffraction data collection. Phytotoxic damage of some 2,5-DKPs on leaves of the weed Euphorbia heterophylla did not differ from those caused by the commercial herbicide diquat.

Asunto(s)

Herbicidas , Alquilación , Teoría Funcional de la Densidad , Dicetopiperazinas , Estructura Molecular , Rayos X

RESUMEN

Three heterobimetallic complexes of formula [Ca2Cu3(mpyba)2(2-apyma)(H2O)7]·8.3H2O (1), [Sr2Cu3(mpyba)2(2-apyma)(H2O)8]·11.6H2O (2) and [Sr4.5Cu4(mpyba)4(ox)(H2O)20]·8.5H2O (3) [H4mpyba = N,N'-2,6-pyridinebis(oxamic acid), 2-apyma = 2-(6-aminopyridinyl)oxamate and ox = oxalate] have been synthesized and structurally characterized. Complexes 1 and 2 are isostructural compounds, with tricopper(ii) units having mpyba and its hydrolytic product (2-apyma) as ligands. They are interlinked through strontium(ii) (1) and calcium(ii) (2) ions to afford neutral two-dimensional networks. Two of the copper(ii) ions are five-coordinate in distorted square pyramidal (Cu3) and trigonal bipyramidal (Cu1) surroundings, whereas the other (Cu2) is six-coordinate in an elongated octahedral environment. The main difference between their structures, apart from the number of water molecules, resides in the nature of the alkaline earth cation coordinated to the oxamate fragments, Sr2+ (1)/Ca2+ (2), which exhibit eight and seven coordination, respectively. The π-π interactions and an extensive network of hydrogen bonds in 1 and 2 lead to supramolecular 3D structures. The relatively small size of their cavities, in the micropore domain, hinders the inclusion of N2 but allows CO2 adsorption (0.45 and 0.52 mmol g-1 for 1 and 2, respectively). The structure of 3 is made up of [3,3] metallacyclophane-type motifs, having the formula [Cu2(mpyba)2(H2O)2]4-. These act as tetrakis(bidentate) ligands towards the strontium(ii) ions (Sr1, Sr2 and Sr3), leading to a sheet-like polymer growing in the bc plane, which extends further along the crystallographic a axis by a bis(chelating) oxalate between the Sr1 atoms. The investigation of the magnetic properties of 1-3 in the temperature range 1.9-300 K shows the occurrence of an overall antiferromagnetic behaviour for 1 and 2 [J12 = J23 = -9.71(2) (1) and -10.81(5) cm-1 (2), with the Hamiltonian being defined as H = -J12S1·S2 - J23S2·S3 + gßH[S1 + S2 + S3], and a ferromagnetic coupling within the dicopper(ii) metallacylophane unit of 3 [J = +1.86(1) cm-1 through the Hamiltonian H = -JS1·S2 + gßH(S1 + S2)]. Simple orbital symmetry considerations (1-3) and the spin polarization mechanism (3) account for the observed magnetic properties.

RESUMEN

The complexing ability of copper(II) in solution by the ligand N,N'-2,6-pyridinebis(oxamic acid) (H4mpyba, H4L) was determined through potentiometric and UV-vis spectroscopy at 25 °C and 0.15 M NaCl. The logarithms of the equilibrium constants for its copper(II) complexes according to the eqs 2H2L + 2Cu ⇆ [Cu2(H2L)2], 2H2L + 2Cu ⇆ [Cu2(H2L) (HL)] + H, 2H2L + 2Cu ⇆ [Cu2(HL)2] + 2H, 2H2L + 2Cu ⇆ [Cu2(HL)(L)] + 3H, and 2H2L + 2Cu ⇆ [Cu2L2] + 4H were 12.02(7), 8.04(5), 1.26(6), -7.51(6), and -16.36(6), respectively. The knowledge of the solution behavior has supported the synthesis of three new compounds bearing the common building block Cu2L2(4-). Their formulas are (Me4N)4[Cu2(mpyba)2(H2O)2]·H2O (1), (Me4N)4[K2Na2Cu4(mpyba)4(H2O)6.8]·1.6H2O (2), and [Na6Cu2(mpyba)2Cl2(H2O)8]·7H2O (3) (Me4N(+) = tetramethylammonium cation). The [Cu2(mpyba)2(H2O)2](4-) tetraanionic unit, which is present in 1, has a [3,3] metallacyclophane-type motif connected by two N-Cu-N bonds. In 2, a heterotrimetallic decanuclear nanocage is formed through front-to-front assembly of two [Cu2(mpyba)2](4-) units, which also coordinate to potassium(I) and sodium(I) cations by means of carboxylate oxygens from oxamate. The structure of 3 consists of heterobimetallic layers of formula [Na6Cu2(mpyba)2Cl2(H2O)8] and crystallization water molecules, which are interlinked by hydrogen bonds leading to a supramolecular three-dimensional network. The investigation of the magnetic properties of 1-3 in the temperature range 1.9-300 K shows the occurrence of ferromagnetic interactions between the dicopper(II) metallacyclophane unit [J = +6.85 (1), +7.40 (2), and +7.90 cm(-1) (3); H = -JSCu1·SCu2, where SCu1 = SCu2 = 1/2]. Theoretical calculations on 1-3 were carried to substantiate the nature and magnitude of the involved magnetic interactions and to support the occurrence of a spin polarization mechanism accounting for them.