The effectiveness of carboxyl as primary ligating group in aryl-SO2 N-protected aminoacids-copper(II) systems: solution and structural investigation on the Cu(II)-N-tosyl-L-leucinate system.

The interaction between N-tosyl-L-leucine (tosyl = 4-toluenesulphonyl) and the copper(II) ion in aqueous-methanolic solution was examined. Two compounds of formula Cu(tsleuO)2·4H2O and Na2[Cu(tsleuNO)2(H2O)]·2H2O·CH3OH [tsleuO = N-tosyl-L-leucinate(1−), tsleuNO = N-tosyl-L-leucinate(2−)] were isolated. For the second complex the crystal and molecular structure was also determined. The crystals were found to be monoclinic, space group P21 with Z = 2 in a unit cell of dimensions a = 16.775(2), b = 11.914(2), c = 9.053(2) Å, β = 100.54(1)°. Full matrix least-squares refinement using 2900 independent reflections reached R = 0.037. The structure is built up of one [Cu (tsleuNO)2(H2O)] dianion, two Na+ cations, two lattice water molecules and one methanol solvate molecule. The coordination about the Cu atom is a distorted square-pyramidal arising from ligation of two trans amino acidate dianions acting as N,O ligands in the basal plane, and of one water oxygen in the axial position. The crystal packing is determined by Na+---O interactions. By potentiometric and spectrophotometric measurements, the number and type of complex species and their cumulative formation constants were determined. The treatment of data confirmed, also in solution, the formation of the complexes separated in the solid state (stable, the first up to pH = 5 and the second above pH = 8). In addition, two complexes of formula [Cu(tsleuNO)] and [Cu(tsleuO)(tsleuNO)]− were found to prevail in the 5–8 pH range. The comparison of data with those of other aryl---SO2 N-substituted α-amino acids indicates the same behaviour towards deprotonation of nitrogen and the effectiveness of carboxyl as a primary ligating group in preventing metal hydrolysis.