Chelate Ring-Closure Kinetics of Cr(CO)_5 (DPPM) Studied by ^{31}P-NMR Spectroscopy

Authors: Saim ÖZKAR, Ceyhan KAYRAN, Ayşin TEKKAYA

Abstract: ^{31}P-NMR spectroscopy was used to study the kinetics of the chelate ring-closure reaction in CR(CO)_5 (DPPM) where DPPM is bis (diphenylphosphino) methane. DPPM is potentially a bidentate ligand, but in this complex it is coordinated to the transition metal in a monodentate fashion and therefore can undergo chelation. The thermal chelate ring-closure reaction was carried out in an NMR sample tube at a constant temperature and the course of the reaction was quantitatively followed by taking ^{31}P-NMR spectra in appropriate time intervals. Kinetic results have shown that the chelate ring-closure reaction of Cr(CO)_5 DPPM is of zero-order. Activation parameters were determined by evaluating the kinetic data. The large negative value of the activation entropy implies that the mechanism of the chelate ring-closure reaction is of associative nature in the transition states. The rate determining step might involve both Cr-CO bond breaking and Cr-P bond making. This will generate a seven coordinate transient complex from which the complete detachment of a CO ligand will lead to the formation of Cr(CO)_4(DPPM).


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