Synthesis, crystal structure and molecular orbital investigation of the first platinum complex of piroxicam

The first platinum complex [PtCl2(dmso)(HL)] of the widely used anti-inflammatory drug piroxicam (HL = 4-hydroxy-2-methyl-N-(2-pyridyl)-2H- 1,2-benzothiazine-3-carboxamide 1,1-dioxide; dmso = dimethylsulfoxide) was obtained from K-2[PtCl4]. Its crystal structure reveals that the metal atom is linked to the pyridyl nitrogen atom. A Pt...H-N (amide) interaction is present and made possible by the orientation of the HL molecule which brings the N(amide)-H bond above the co-ordination plane and almost parallel to the N (pyridyl)-Pt vector. The amide hydrogen was located from the Fourier-difference-synthesis and its position refined. The Pt...H distance is 2.35 Angstrom (0.25-0.55 Angstrom shorter than the sum of the van der Waals radii), whereas the v(N-H) vibration band is red-shifted by about 90 cm(-1) upon complexation. An extended-Huckel molecular orbital analysis revealed that a mixing d(z2)(Pt) and p(z)[N(amide)] atomic orbitals occurs, whereas electrostatic Pt...H attractive forces dan play a significant role.