Chelate ring closing and opening behavior in cyclopentadienyl cobalt(III) complexes with pendant nitrogen functional group

Kinetic studies were performed for the chelate ring closing and opening process of cyclopentadienyl cobalt(III) complexes having a pendant N-functional group with an amine, piperidine or pyridine moiety in the side chain. The metal-nitrogen bond energy was measured. The rate of chelation by such pendant N-functionalized side chains in diiodomonocarbonyl cobalt(III) reaction intermediates is determined by the electronic density on the donor atom and the strength of the forming chelated bond. The steric factor around the donor atom plays a secondary role. On the basis of the enthalpies and entropies obtained from the kinetic studies, the process of chelate ring closing in diiodomonocarbonyl cobalt(III) reaction intermediates is via an associative pathway involving loss of CO, while the chelate ring opening process in the resulted chelators is via a metal-nitrogen bond cleavage, solvation then metal-phosphorus bond formation pathway during substitution of PPh₃. The chelator with the most rigid arm of picolyl shows a smallest steric hindrance for incoming PPh₃ compared to the other two analogues.