- New Polynuclear Manganese(II) Complexes with Orotic Acid and some of its Derivatives: Crystal Structures, Spectroscopic and Magnetic Studies
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Three polynuclear manganese(II) complexes containing orotic acid (2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid, H3L1) or one of its substituted derivatives 2L2) or 5-nitro-2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid (H3L1)> have been synthesized and characterized by X-ray crystallography.UV/VIS and magnetic susceptibility measurements.Complex 1 consists of neutral 1)2(H2O)6> units, which form polymer chains along the z axis with a Mn(1)...Mn(2) distance in the unit cell of 5.628(1) Angstroem while the Mn(2)...Mn(2) distance in the chain is 4.715(1) Angstroem.Each unit cell of complex 2 contains one neutral centrosymmetric dimer 2)2(H2O)6> containing a short Mn...Mn distance and an antiferromagnetic exchange interaction is present.The experimental data were fitted to the susceptibility equations resulting from the Hamiltonian H = -2JS1S2 to give exchange parameter J = -1.3 cm-1 and g = 1.95.From EPR spectra of 2, the hyperfine interaction parameter A = -0.27 GHz and the zero-field splitting parameter D = +/-2.93 GHz have been calculated.Each unit cell of complex 3 consists of one dinuclear anion 3)2(H2O)4Cl2>2- and of one cation 2+.The Mn(1) and Mn(2) atoms and the water molecule of the cation 2+ are situated at inversion sites.The dinuclear anions are associated to form chains but the shortest Mn...Mn distance of 5.642(3) Angstroem is observed within the 3)2(H2O)4Cl2>2- unit between Mn(1) and Mn(2).
- Nepveu, Francoise,Gaultier, Nicolas,Korber, Nikolaus,Jaud, Joel,Castan, Paule
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p. 4005 - 4014
(2007/10/02)
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- Tautomerism of Orotic Acid Dianion. Effect of Calcium and Magnesium Cations on the Tautomeric Constant and on Tautomerization Dynamics
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Orotic acid dianion (HL2-) exists in basic aqueous solutions as a mixture of two isomeric structures (KT = / = 0.4 +/- 0.08, ΔHT = -0.3 +/- 0.05 kcal, ΔST = -2 +/- 0.4 eu).Its tautomeric interconversion as investigated by T-jump relaxation is fast (τ never exceeds 30 μs in D2O) and is catalyzed by OD- OD- = (3.3 +/- 0.2) 108 M-1 s-1>, by D2O acting as an acid D2O = (3 +/- 0.3) 104 s-1> and by orotate monoanion H2L- = (4 +/- 0.7) 106 M-1 s-1>.The orientation of orotic acid methylation is discussed in terms of the change in tautomer populations caused by temperature.The tautomeric equilibrium constant is markedly affected by alkaline earth cations (Ca2+ and Mg2+) owing to a specific complexation of the N(3)H form.Estimates of the complex formation constants, Kcomp, are (1.7 +/- 0.2) 102 and (5 +/- 0.5) 102 M-1 for Ca2+ and Mg2+, respectively.The relaxation time for the complexation process is shorter than 1 μs for Ca2+, whereas it is in the millisecond range for Mg2+ with a second order rate constant equal to (5.5 +/- 0.6) 105 M-1 s-1.
- Dodin, Guy,Dubois, Jacques-Emile
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p. 3049 - 3056
(2007/10/02)
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