Kinetic study of the reaction of tetramethylplatinum(IV) complexes with acids

Kinetics for the reaction of [PtMe₄(NN)] (NN=1,10-phenanthroline or 2,2′-bipyridine) with HA (HA=formic, acetic, benzoic or salicylic acid) in ethanol, [PtMe₄(NN)]+HA→^kf[PtMe₃(A)(NN)]+CH ₄ has been studied at various temperatures and pressures. The rate constants and activation parameters for the reaction are as follows: k_f (M^-1 s^-1) (25 °C)=(54.6±0.5)×10^-2, ΔH^≠_f (kJ mol^-1)=52.7±0.2, ΔS^≠_f (J mol^-1 K^-1) = -73.2±0.8, ΔV^≠_f (cm³mol^-1) = -8.5±0.5 for the reaction of [PtMe₄(phen)] with salicyclic acid; k_f (M^-1 s^-1) (25 °C) = (29.6±0.8) × 10^-2, ΔH^≠_f (kJ mol^-1) = 54.8±2.8, ΔS^≠_f (J mol^-1 K^-1) = -71.8±9.4, ΔV^≠_f (cm³ mol^-1) = -6.6±1.0 for the reaction of [PtMe₄(phen)] with formic acid; k_f (M^-1 s^-1) (25 °C) = (6.56±0.04) × 10^-1, ΔH^≠_f (kJ mol^-1) = 53.9±1.9, ΔS^≠_f (J mol^-1 K^-1) = -87.0±6.3, ΔV^≠_f (cm³ mol^-1) = -8.2±0.3, k_f (M^-1 s^-1) (in methanol, 25 °C) = (9.14±0.04) × 10^-2, ΔV^≠_f (cm₃ mol^-1) (in methanol) = -15.1±1.6 for the reaction of [PtMe₄(phen)] with benzoic acid; k_f (M^-1 s^-1) (25 °C) = (2.49±0.02) × 10^-2, ΔH^≠_f (kJ mol^-1)=52.4±1.8, ΔS^≠_f (J mol^-1 K^-1) = -99.6±5.8, ΔV^≠_f (cm³ mol^-1) = -10.5±0.5 for the reaction of [PtMe₄(phen)] with acetic acid; k_f (M^-1 s^-1) (25 °C) = (25.6±0.4) × 10^-2, ΔH^≠_f (kJ mol^-1 ) = 49.8±5.0, ΔS^≠_f (J mol^-1 K^-1) = -89.4±16.7, ΔV^≠_f (cm³ mol^-1) = -12.2±0.4 for the reaction of [PtMe₄(bipy)] with formic acid; k_f (M^-1 s^-1) (25 °C) = (1.95±0.04) × 10^-2, ΔH^≠_f (kJ mol^-1) = 54.7±3.0, ΔS^≠_f (J mol^-1 K^-1) = -93.7±10.0, ΔV^≠_f (cm³ mol^-1) = -11.6±1.0 for the reaction of [PtMe₄(bipy) ] with acetic acid. A linear free energy relationship holds between log k_f and pK_a of HA. The negative values of volume and entropy of activation and a large deuterium kinetic isotope effect strongly indicate that the reaction proceeds with an S_E2 mechanism.