Structural and kinetic studies of spin crossover in an Iron(II) complex with a novel tripodal ligand

Configurational and ligand conformational influences on the kinetics of (1)A(1) reversible arrow T-5(2) spin crossover in the Fe(II) complex with the novel tripodal ligand, 1,1,1-tris((N-(2-pyridylmethyl)-N-methylamino)methyl)ethane (tptMetame), have been explored. Despite having six chelate rings and three chiral nitrogen atoms, only one enantiomeric pair of isomers, Delta, SSS, and Lambda, RRR, of the complex ion is observed. The conformation of the three rings forming the upper ''cap'' of the complex structure can be assigned delta or lambda with respect to the 3-fold molecular axis. X-ray data at 300 and 153 K, above and below the critical temperature for the spin transition, show that the conformation of the ligand ''cap'' is the same as the absolute configuration of the complex, with the same Lambda lambda(CAP)(or Delta delta(CAP)) combination prevailing for both the LS ((1)A(1)) and HS (T-5(2)) isomers. Molecular mechanics calculations further show that the ligand energy remains lowest for this Lambda lambda(CAP) (or Delta delta(CAP)) combination at all Fe-N distances over the range spanning the LS and HS isomers. Measurements of the spin crossover relaxation time have been carried out in solution over the temperature range 293-170 K. The observed monophasic relaxation traces are also consistent with the absolute configuration of the complex remaining unaltered during the spin crossover.

Retention of toxic chromium from aqueous phase by H3PO4-activated lignin: Effect of salts and desorption studies

In this study, the feasibility of using H₃PO₄-activated lignin for hexavalent chromium adsorption has been investigated. The composite of activated lignin was characterized using FTIR, XRD and SEM with EDAX analysis. It was observed that the pH had a strong effect on the adsorption capacity; adsorption of Cr(VI) was more favorable at acidic pH with maximum uptake at pH 2. The adsorption equilibrium data were best represented by Koble-Corrigan isotherm. The monolayer sorption capacity obtained from the Langmuir model was found to be 77.85 mg/g. Adsorption showed pseudo-second order rate kinetics and the process involving the rate-controlling step is complex as it involves both film and intraparticle diffusion processes. The NaOH desorbing agent was able to release approximately 84% of metal ions. Thermodynamic parameters showed that the sorption process is exothermic and non-spontaneous. The overall Cr(VI) retention on the activated lignin surface perhaps includes both the physical adsorption of Cr(VI) and the consequent reduction of Cr(VI) to Cr(III). (C) 2011 Elsevier B.V. All rights reserved.