Solvent Effects on the Structural and Formyl Substrate Reactivity Properties of a Nitrogen/Sulfur-Ligated Zinc Hydroxide Complex

The solution structural and formyl substrate reactivity properties of a nitrogen/sulfur-ligated zinc hydroxide complex,
[(bmnpaZn)2(μ-OH)2](ClO4)2 (1, bmnpa = N,N-bis-2-(methylthio)ethyl-N-((6-neopentylamino-2-pyridyl)methyl)amine),
in acetonitrile and methanol are reported. In CH3CN, 1 has a binuclear cation [(bmnpaZn)2(μ-OH)2]2+ that is stabilized
by secondary hydrogen bonding and CH/π interactions involving the bmnpa chelate ligand. In CH3OH, 1 undergoes
reaction with solvent to yield a zinc methoxide species, as determined by 1H NMR and electrospray mass spectral
analysis. Treatment of 1 with methyl formate in CH3CN results in stoichiometric hydrolysis of the formyl ester to
produce [(bmnpa)Zn(O2CH)]ClO4 (2) and methanol. The formate complex was identified via independent synthesis
and characterization (X-ray crystallography, 1H and 13C NMR, FTIR, LRFAB-MS, conductance, and elemental analysis).
In the solid state, 2n has a formate-bridged coordination polymer-type structure. However, in CH3CN, 2 behaves as
1:1 electrolyte, indicating cleavage of the polymer structure into mononuclear [(bmnpa)Zn(O2CH)]ClO4 species.
Treatment of 1 with a stoichiometric amount of formanilide in CH3CN for 48 h at 45 °C results in decomposition
of the zinc hydroxide complex to yield the free bmnpa ligand and an inorganic solid, presumably a zinc hydroxide
or oxide species. Treatment of 1 with a stoichiometric amount of ethyl formate in CD3OD results in rapid, quantitative
transesterification of the formyl carboxylate ester. A control reaction indicates that this transesterification reaction
does not occur on the same time scale in the absence of the catalyst. Treatment of 1 with an excess of ethyl
formate in CD3OD results in catalytic formyl carboxylate ester transesterification, with approximately 1000 turnovers
in 60 min at 22(1) °C. Treatment of a CD3OD solution of 1 (0.5 equiv) with formanilide (1 equiv) results in the
formation of aniline, d3-methyl formate, and the zinc formate complex 2. While aniline is produced stoichiometrically,
the yield of d3-methyl formate varied from 30 to 50%, and the yield of 2 varied from 50 to 70% in repetitive
experiments. Formation of both d3-methyl formate and 2 indicates that both methanolysis and hydrolysis reactions
take place.