Coordination of 2-hydroxyhippuric acid to the copper(II) ion: A solution and solid state study

The solution and solid state behaviour of the binary system Cu(II)-2-hydroxyhippuric acid (2-OHhipH) and the corresponding ternary system with 2,2'-bipyridine (bpy) were investigated through pH-metric titrations and X-ray crystallography. In solution the species [CuLH(-1)](-) and [CubpyL] were found to prevail in the binary and in the ternary system, respectively, both involving metal coordination of the phenolate oxygen. A spectrophotometric pK(a) of 5.2(2) was determined for the deprotonation of the phenolic hydroxy group in the ternary system The crystals of the complex [Cu(bpy)(2-OHhip)](2) . 8H(2)O were found suitable for X-ray structure determination. The crystals of the compoundC(19)H(23)CuN(3)O(8) are monoclinic, space group P2(1)/c,a=7.717(2),b=11.502(2),c=23.275(4) Angstrom,beta=94.86(3)degrees,Z=4,R=0.033,R(w)=0.035. The structure consists of dimeric [Cu(bpy)(2-OHhip)](2) it units and lattice water molecules. The Cu(II) atom is pentacoordinated by the two nitrogen atoms of the bpy molecule and the carbonyl and phenolate oxygen atoms of the ligand in the equatorial plane, and by a bridged phenolate oxygen in the apical position, giving rise to a distorted square pyramidal geometry. The binding mode of 2-OHhipH is discussed by reference to related ligands.