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

Abstract

Treatment of a dinuclear zinc hydroxide complex ([(bmnpaZn)(2)(mu-OH)(2)](ClO(4))(2) (1) or [(benpaZn)(2)(mu-OH)(2)](ClO(4))(2) (2)) with excess equivalents of an aryl alcohol derivative (p-HOC(6)H(4)X; X = NO(2), CHO, CN, COCH(3), Br, H, OCH(3)) yielded the nitrogen/sulfur-ligated zinc aryloxide complexes [(bmnpa)Zn(p-OC(6)H(4)NO(2))](ClO(4)) (3), [(benpa)Zn(p-OC(6)H(4)NO(2))](ClO(4)) (4), [(benpa)Zn(p-OC(6)H(4)CHO)](ClO(4)) (5), [(benpa)Zn(p-OC(6)H(4)CN)](ClO(4)) (6), [(benpa)Zn(p-OC(6)H(4)COCH(3))](ClO(4)) x 0.5H(2)O (7), [(benpa)Zn(p-OC(6)H(4)Br)](ClO(4)) (8), [(benpa)Zn(p-OC(6)H(5))](ClO(4)) (9), and [(benpa)Zn(p-OC(6)H(5)OCH(3))](ClO(4)) (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 (nu(C[double bond]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) approximately 2.78 A). Solution (1)H and (13)C NMR spectra of 3-10 in CD(3)CN and FTIR spectra in CH(3)CN 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 pK(a) 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.