Synthesis and Characterization of Mononuclear Zinc Aryloxide Complexes Supported by Nitrogen/Sulfur Ligands Possessing an Internal Hydrogen Bond Donor

Treatment of a dinuclear zinc hydroxide complex ([(bmnpaZn)2(μ-OH)2](ClO4)2 (1) or [(benpaZn)2(μ-OH)2](ClO4)2
(2)) with excess equivalents of an aryl alcohol derivative (p-HOC6H4X; X = NO2, CHO, CN, COCH3, Br, H, OCH3)
yielded the nitrogen/sulfur-ligated zinc aryloxide complexes [(bmnpa)Zn(p-OC6H4NO2)](ClO4) (3), [(benpa)Zn(p-OC6H4NO2)](ClO4) (4), [(benpa)Zn(p-OC6H4CHO)](ClO4) (5), [(benpa)Zn(p-OC6H4CN)](ClO4) (6), [(benpa)Zn(p-OC6H4COCH3)](ClO4)·0.5H2O (7), [(benpa)Zn(p-OC6H4Br)](ClO4) (8), [(benpa)Zn(p-OC6H5)](ClO4) (9), and [(benpa)Zn(p-OC6H5OCH3)](ClO4) (10). The solid state structures of 2, 3, 5, and 6 have been determined by X-ray crystallography.
While 3 and 6 exhibit a mononuclear zinc ion possessing a distorted five-coordinate trigonal bipyramidal geometry,
in 5 each zinc center exhibits a distorted six-coordinate octahedral geometry resulting from coordination of the
aldehyde carbonyl oxygen of another zinc-bound aryloxide ligand, yielding a chain-type structure. Zinc coordination
of the aldehyde carbonyl of 5 is indicated by a large shift (>40 cm-1) to lower energy of the carbonyl stretching
vibration (νCO) in solid state FTIR spectra of the complex. In the solid state structures of 3, 5, and 6, a hydrogen-bonding interaction is found between N(3)−H of the supporting bmnpa/benpa ligand and the zinc-bound oxygen
atom of the aryloxide ligand (N(3)···O(1) ∼ 2.78 Å). Solution 1H and 13C NMR spectra of 3−10 in CD3CN and
FTIR spectra in CH3CN are consistent with all of the aryloxide complexes having a similar solution structure, with
retention of the hydrogen-bonding interaction involving N(3)−H and the oxygen atom of the zinc-coordinated aryloxide
ligand. For this family of zinc aryloxide complexes, a correlation was discovered between the chemical shift position
of the N(3)−H proton resonance and the pKa of the parent aryl alcohol. This correlation indicates that the strength
of the hydrogen-bonding interaction involving the zinc-bound aryloxide oxygen is increasing as the aryloxide moiety
increases in basicity.