Mononuclear Nitrogen/Sulfur-Ligated Zinc Methoxide and Hydroxide Complexes: Investigating Ligand Effects on the Hydrolytic Stability of Zinc Alkoxide Species

The synthesis and properties of mononuclear zinc methoxide ([(ebnpa)Zn−OCH3]ClO4) (1) and hydroxide ([(ebnpa)Zn−OH]ClO4) (2) complexes of a new mixed nitrogen/sulfur ligand (ebnpa = N-2-(ethylthio)ethyl-N,N-bis(6-neopentylamino-2-pyridylmethyl)amine) are reported. The structures of 1 and 2 were determined by X-ray diffraction. Each possesses a single zinc-coordinated anion (methoxide or hydroxide) and exhibits an overall trigonal bipyramidal geometry. Structural and spectroscopic studies indicate the presence of two hydrogen-bonding interactions involving the oxygen atom of the zinc-bound anion in each complex. Treatment of [(ebnpa)Zn−OH]ClO4 with CH3OH results in the formation of an equilibrium mixture of 1 and 2. 1H NMR spectroscopic methods were used to examine the equilibrium as a function of temperature, yielding KMe (304 K) = 0.30(8), ΔHMe = −0.9(1) kcal/mol, and ΔSMe = −5(1) eu. The negative enthalpy indicates that spontaneous zinc alkoxide formation from a hydroxide precursor occurs in this system at low temperature. Using the experimentally determined ΔHMe value, we found the homolytic Zn−O bond dissociation energy (BDE) in the Zn−OCH3 unit to be ∼ −14 kcal/mol relative to the Zn−O BDE in the Zn−OH unit.