Divalent metal complexes of an amide-appended N3O-donor ligand: synthesis, structural and spectroscopic features, and amide methanolysis reactivity§

Abstract

Divalent zinc, cobalt, and nickel complexes of an amide-appended N3O-donor ligand (mpppa, N-methyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-pyridylethyl)amine) have been prepared and characterized. X-ray crystallographic studies indicate that [(mpppa)Zn(CH3CN)](ClO4)2 (1) exhibits a distorted square pyramidal geometry, whereas [(mpppa)M(CH3OH)(H2O)](ClO4)2 (M=Co (2·H2O), Ni (3)) exhibit distorted octahedral structures. Each complex was also characterized by analytical and spectroscopic methods. Treatment of 1 with Me4NOH·5H2O in methanol-containing solutions results in amide methanolysis. This reaction proceeds via the initial formation of a deprotonated amide intermediate, [(mpppa−)Zn]ClO4 (4), which has been isolated and characterized. Kinetic studies performed as a function of the amount of methanol present revealed saturation behavior with respect to the alcohol concentration. Variable temperature kinetic studies yielded activation parameters (ΔH ‡=19.8kcal/mol; ΔS ‡=−3.8(1) eu) that are interestingly different from those exhibited by a zinc complex of an amide-appended N4O-donor ligand. These results are discussed in the context of the overall coordination number and geometry of the zinc center. Treatment of 2 and 3 with Me4NOH·5H2O in methanol-containing solutions does not result in amide methanolysis. §Contribution for Peter Williams issue.