| Abstrakt: |
The formation reaction of chiral racemic and diastereoisomeric title complexes, (Λ,Δ)-[Cr{(±)-Mebdtp}3] and Λ-(–)589-[Cr{(+)-(S)(S)-Mebdtp}3], was monitored by absorption, UV/Vis, and circular dichroism, CD, spectroscopy, in the presence of either an excess of the racemic, (±)-Mebdtp–, or the enantiopure ligand, (+)-(S)(S)-Mebdtp– = O,O′-bis[(+)-(S)-2-methylbutyl] dithiophosphate ion, at 25 °C in various solvents (ethanol, tetrahydrofuran, chloroform/ethanol, 9:1) and at various pH values and reactant concentrations. The kinetics of the formation reaction of the racemic complex, (Λ,Δ)-[Cr{(±)-Mebdtp}3], shows three consecutive reaction steps. The relative pseudo first-order rate constants depend in different ways on the ligand and H+ concentrations. Circular dichroism measurements during the formation reaction using the enantiopure ligand, as well as in the final equilibrium state, show a prevalence of the Λ-(–)589-[Cr{(+)-(S)(S)-Mebdtp}3] over the Δ-(+)589-[Cr{(+)-(S)(S)-Mebdtp}3] diastereoisomer in all solvents used. A mechanism is proposed which involves two parallel pathways each consisting of three consecutive reactions of solvent substitution by a chiral chelated ligand, interconnected by some reversible inversion reactions to obtain in solution the thermodynamic and kinetic Λ-(–)589-[Cr{(+)-(S)(S)-Mebdtp}3] as the major diastereoisomeric compound. From the equilibrated solution the more stable solid (Λ,Δ)-[Cr{(+)-(S)(S)-Mebdtp}3] crystallizes as a ca. 1:1 compound of a pair of the two diastereoisomers, undergoing a crystallization-induced second-order asymmetric transformation. Δ-[Cr{(+)-(S)(S)-Mebdtp}3] is the thermodynamically stable diastereoisomer in the solid state, though in solution this inverts to the more stable Λ-(–)589-[Cr{(+)-(S)(S)-Mebdtp}3].(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) |
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