Synthesis of platinum(II)–piroxicam compounds. Crystal structure of trans-dichloro(η²-ethene)(piroxicam)platinum(II) ‡

Abstract

The reaction of Zeise’s salt K[PtCl₃(η²-C₂H₄)]·H₂O and piroxicam [4-hydroxy-2-methyl-N-(2-pyridyl)-2H-1,2-benzothiazine-3-carboxamide 1,1-dioxide, Hpir] in ethanol at room temperature produced crystalline trans-[PtCl₂(η²-C₂H₄)(Hpir)]·0.5C₂H₅OH 1·0.5C₂H₅OH. Crystals of 1·0.5C₆H₆ obtained from a benzene solution of 1·0.5C₂H₅OH were studied via X-ray diffraction techniques. The neutral piroxicam ligand co-ordinates through pyridyl N(1′) and has an EZE conformation typical for the neutral form. The pyridyl system of piroxicam is significantly twisted (20°) around the C(2′)–N(16) vector with respect to the rest of the ligand molecule. A comparison with previously investigated metal–piroxicam complexes shows that the C(2′)–N(16) bond is ‘more flexible’ than the N(16)–C(14), C(14)–C(13) and C(3)–C(4) ones. The co-ordination environment has the usual square-planar geometry for platinum(II) complexes if the middle point of the ethene C–C bond (CM) is considered. The Pt–N bond [2.093(6) Å] is longer than most Pt^II–N(sp²) bond distances, because of the high trans influence of ethene. There is an attractive interaction between the H(N) amide atom and the Pt. The ¹H NMR spectrum of 1·0.5C₂H₅OH recorded from CDCl₃ solution at 22 °C shows that the signal of H(N) undergoes a shift to lower field as much as 1.98 ppm upon complexation; no ¹H–¹⁹⁵Pt coupling could be revealed. The ¹H (ethene)–¹⁹⁵Pt coupling constant is 64 Hz and spectra recorded in the temperature range –50 to 40 °C show that a fast rotation of co-ordinated ethene around the Pt–CM vector takes place at 22 °C. The reaction of potassium tetrachloroplatinate(II) with piroxicam in water at pH 8.5 gives a yellow precipitate whose formula is Pt(pir)₂·3H₂O, 2·3H₂O. ¹H NMR data from CDCl₃ solutions at 22 °C are in agreement with the presence of two isomers which have been tentatively assigned to cis- and trans-[Pt(pir)₂] complexes of N(1′)–O(15) chelating piroxicamate.

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