Influence of the Chelate Ligand Structure on the Amide Methanolysis Reactivity of Mononuclear Zinc Complexes

Zinc complexes of three new amide-appended ligands have been prepared and isolated. These complexes, [(dpppa)Zn](ClO4)2 (4(ClO4)2; dpppa = N-((N,N-diethylamino)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), [(bdppa)Zn](ClO4)2 (6(ClO4)2; bdppa = N,N-bis((N,N-diethylamino)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)amine), and [(epppa)Zn](ClO4)2 (8(ClO4)2; epppa = N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), have been characterized by X-ray crystallography (4(ClO4)2 and 8(ClO4)2), 1H and 13C
NMR, IR, and elemental analysis. Treatment of 4(ClO4)2 or 8(ClO4)2 with 1 equiv of Me4NOH·5H2O in methanol−acetonitrile (5:3) results in amide methanolysis, as determined by the recovery of primary amine-appended forms
of the chelate ligand following removal of the zinc ion. These reactions proceed via the initial formation of a
deprotonated amide intermediate ([(dpppa-)Zn]ClO4 (5) and [(epppa-)Zn]ClO4 (9)) which in each case has been
isolated and characterized (1H and 13C NMR, IR, elemental analysis). Treatment of 6(ClO4)2 with Me4NOH·5H2O
in methanol−acetonitrile results in the formation of a deprotonated amide complex, [(bdppa-)Zn]ClO4 (7), which
was isolated and characterized. This complex does not undergo amide methanolysis after prolonged heating in a
methanol−acetonitrile mixture. Kinetic studies and construction of Eyring plots for the amide methanolysis reactions
of 4(ClO4)2 and 8(ClO4)2 yielded thermodynamic parameters that provide a rationale for the relative rates of the
amide methanolysis reactions. Overall, we propose that the mechanistic pathway for these amide methanolysis
reactions involves reaction of the deprotonated amide complex with methanol to produce a zinc methoxide species,
the reactivity of which depends, at least in part, on the steric hindrance imparted by the supporting chelate ligand.
Amide methanolysis involving a zinc complex supported by a N2S2 donor chelate ligand (3(ClO4)2) is more complicated,
as in addition to the formation of a deprotonated amide intermediate free chelate ligand is present in the reaction
mixture.