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)(mu-OH)(2)](ClO4)(2) (1) or [(benpaZn)(2)(mu-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(pOC(6)H(4)NO(2))1(ClO4) (4), [(benpa)Zn(p-OC6H4CHO)](ClO4) (5), [(benpa)Zn(p-OC6H4CN)](ClO4) (6), [(benpa)Zn(p-OC6H4COCH3)](ClO4)-0.5H(2)O (7), [(benpa)Zn (P-OC6H4Br)] (ClO4) (8), [(benpa)Zn(p-OC6H5)](ClO4) (9), and [(benpa)Zn(pOC(6)H(5)OCH(3))](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 (nu(c=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) similar to 2.78 Angstrom). Solution H-1 and C-13 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 aryloxicle 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.