The Impact of Different Chelating Leaving Groups on the Substitution Kinetics of Mononuclear Pt(II)(1,2-trans-R,R-diaminocyclohexane)(X-Y) Complexes

A set of three oxaliplatin derivatives containing 1,2-trans-R,R-diaminocyclohexane (dach) as a spectator ligand and different chelating leaving groups X-Y, viz., [Pt(dach)(O,O-cyclobutane-1,1-dicarboxylate)], or Pt(dach)(CBDCA), [Pt(dach)(N,O-glycine)]+, or Pt(dach)(gly), and [Pt(dach)(N,S-methionine)]+, or Pt(dach)(L-Met), where L-Met is L-methionine, were synthesized and the crystal structure of Pt(dach)(gly) was determined by X-ray diffraction. The effect of the leaving group on the reactivity of the resulting Pt(II) complexes was studied for the nucleophiles thiourea, glutathione (GSH) and L-Met under pseudo-first-order conditions as a function of nucleophile concentration and temperature, using UV-vis spectrophotometric techniques. 1H NMR spectroscopy was used to follow the substitution of the leaving group by guanosine 5'-monophosphate (5'-GMP2-) under second-order conditions. The rate constants indicate for all reactions a direct substitution of the X-Y chelate by the selected nucleophiles, thereby showing that the nature of the chelate, viz., O-O (CBDCA2-), N-O (glycine) or S-N (L-Met), respectively, plays an important role in the kinetic and mechanistic behavior of the Pt(II) complex. The k1 values for the reaction with thiourea, L-Met, GSH and 5'-GMP2- were found to be as follows (10(3) k1, 37.5 degrees C, M(-1) s(-1)): Pt(dach)(CBDCA) 61 +/- 2, 21.6 +/- 0.1, 23 +/- 1, 0.352 +/- 0.002; Pt(dach)(gly) 82 +/- 3, 6.2 +/- 0.2, 37 +/- 1, 1.77 +/- 0.01; Pt(dach)(L-Met) (thiourea, GSH) 62 +/- 2, 24 +/- 1. The activation parameters for all reactions studied suggest an associative substitution mechanism.