Showing papers by "Rita G. Hazell published in 1997"

Improvement of TADDOLate-TiCl(2)-Catalyzed 1,3-Dipolar Nitrone Cycloaddition Reactions by Substitution of the Oxazolidinone Auxiliary of the Alkene with Succinimide.

Abstract: A significant improvement of metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions of acyclic nitrones with α,β-unsaturated carbonyl compounds is described using succinimide as a new auxiliary for the α,β-unsaturated carbonyl moiety. In the absence of a catalyst, N-crotonoylsuccinimide reacts with C,N-diphenylnitrone to give the endo-isoxazolidine, whereas in the presence of 10 mol % TiCl2(i-PrO)2 the exo-product is obtained. Four different TiCl2−TADDOLate complexes have been tested as catalysts for the 1,3-dipolar cycloaddition reactions, and the most successful catalyst was applied (5 mol %) in a series of reactions between two different alkenoylsuccinimides and three different nitrones. The crude products containing an N-acylsuccinimide moiety were converted directly into the corresponding carboxamides upon treatment with hydrazine. The 1,3-dipolar cycloaddition reactions proceed with a high degree of exo-selectivity, often >90% de, which is an improvement compared with previous experiments pe...

Nickel(II) N2X2 Schiff-base complexes incorporating pyrazole (X = NH, O or S): syntheses and characterization†

Abstract: A series of four-co-ordinate nickel(II) N2X2 complexes (X = NH, O or S) with tetradentate Schiff-base ligands incorporating pyrazole has been prepared in order to compare the influence on the nickel(II) centre of the donor atoms in N2(NH)2 and N2S2 complexes versus N2O2 complexes. In each case, two identical parts of the ligand are linked by a two- or three-carbon aliphatic chain (n = 2 or 3). Crystal structures have been determined for a n = 3 N2(NH)2 complex, for two n = 3 N2S2 complexes and for a n = 2 N2O2 complex. All of the complexes have been investigated in solution by spectroscopic methods (UV/VIS/NIR, 1H NMR). The n = 2 and 3 N2(NH)2 complexes and the n = 2 N2S2 complexes are fully diamagnetic. An extension of the chain linking the two halves of the ligand from two to three carbon atoms induces a spin-equilibrium process (S = 0 ⇄ S = 1) for the resulting N2S2 complex (ΔG = 19–26 kJ mol–1 at 50 °C). Ligand-field parameters have been derived from the electronic spectra and the electrochemical properties of the N2S2 complexes have been investigated by cyclic voltammetry. From these results it is clear that low-spin character is emphasized in N2(NH)2 and N2S2 complexes of nickel(II) when compared with the corresponding N2O2 complexes.

Racemization kinetics and spin states of four-co-ordinatenickel(II) N2X2 Schiff-base and azacomplexes with bi- or tetra-dentate ligands incorporating pyrazole(X = NH or S)

Abstract: Bis(bidentate ligand) and tetradentate ligand nickel(II) N 2 X 2 Schiff-base and aza complexes (X = NH or S) have been prepared and their properties investigated by spectroscopic methods. In the bis(bidentate ligand) complexes the aza function stabilizes the low-spin S = 0 state compared with the imine function. The aza complexes are low spin both in the solid state and in solution; the Schiff-base complexes are either low or high spin (S = 1) in the solid state, and are either in spin equilibrium (S = 0 ⇌ S = 1) or high spin in solution. The crystal structure has been solved for the high-spin complex bis(4-isopropyliminomethyl-1,3-diphenylpyrazol-5-ylaminato)nickel( II). The co-ordination of Ni is pseudo-tetrahedral, the angle between the N–Ni–NH and NH′–Ni–N′ planes being 93.8(1)°. The Ni–N (imine) bond lengths are 1.999(2) and 2.003(3) A, significantly longer than the Ni–N (amine) bond lengths of 1.919(2) A. In the tetradentate ligand complexes the two identical halves of the ligands are linked by aliphatic four-carbon chains. When the linkage is CMe 2 (CH 2 ) 2 CMe 2 the complexes are fully paramagnetic in the solid state and in solution, while complexes bridged by unsubstituted (CH 2 ) 4 are low spin in the solid state and in spin equilibrium in solution. The crystal structure of [N,N′-bis(1,3-dimethyl-5-sulfanylpyrazol-4- ylmethylene)butane-1,4-diaminato]nickel(II) reveals an almost planar co-ordination geometry, the angle between the N–Ni–S and S′–Ni–N′ planes being 7.9(3)°. The Schiff-base complexes are chiral and all show evidence of racemization in solution. Thermodynamic parameters for the spin-equilibrium process in CD 2 Cl 2 [ΔG(25 °C) from -4.32 to 0.71 kJ mol -1 for the bis(bidentate ligand) systems, from 3.72 to 11.3 kJ mol -1 for the tetradentate ligand systems] and kinetic parameters for the racemization process in CD 2 Cl 2 or CDCl 2 CDCl 2 [ΔG ‡ (25 °C) from 40.5 to 53.3 kJ mol -1 for the bis(bidentate ligand) complexes, 46.5 to 63.1 kJ mol -1 for the tetradentate ligand complexes] have been derived using variable-temperature 1 H NMR spectroscopy. Sulfur donor atoms and aryl substituents favour the low-spin state. Ligand-field parameters for the Schiff-base complexes have been derived from the electronic spectra.

Metal(II) N2S2 Schiff-base complexesincorporating pyrazole or isoxazole (M = Ni, Cu or Zn). Spinstates, racemization kinetics and electrochemistry

Abstract: Schiff-base complexes incorporating a five-membered aromatic heterocycle (isoxazole or pyrazole) and biologically important 3d metal(II) ions (M = Ni, Cu or Zn) have been synthesized. The crystal structures have been determined for [N,N′-bis(3-phenyl-5-sulfanylisoxazol-4- ylmethylene)butane-1,4-diaminato]copper(II) 1b and for [N,N′-bis(1-methyl-3-phenyl-5-sulfanylpyrazol-4- ylmethylene)butane-1,4-diaminato]nickel(II) 2a. In both structures the metal atom is co-ordinated to two nitrogen and to two sulfur atoms which form a flattened tetrahedron with the dihedral angles between the two N–M–S planes being 48.6(1) and 12.7(1)° respectively. Their physicochemical properties have been studied in solution. The spin-equilibrium process S = 0 S = 1 and the racemization process Δ Λ for the nickel(II) complexes have been investigated by temperature-dependent 1 H NMR spectroscopy. Complex 2a (ΔG = 7.8 kJ mol -1 at 25 °C) is more stabilized in the low-spin state than is 1a (ΔG = 3.2 kJ mol -1 ). The racemization rates are 2.3 × 10 4 and 1.7 × 10 4 s -1 at 25 °C for 1a and 2a, respectively. The stereochemistry of the zinc(II) and copper(II) complexes in solution seems to be independent of the ligand. For the former the configurations were evaluated from the chemical shift differences between diastereotopic protons. For the latter structural similarities are seen in the electronic and ESR spectra. The electrochemical properties have been investigated by cyclic voltammetry. The isoxazole ligand stabilizes a high potential for the M II –M I reduction (E ½ ≈ 0 for Cu) compared with the pyrazole ligand (E ½ = -407 mV for Cu).