A phosphorus-31 nuclear magnetic resonance study of triphenylphosphine oxide exchange on magnesium(II) and zinc(II)

Abstract

³¹P n.m.r, studies show that the rate of triphenylphosphine oxide exchange on [Zn(O=PPh₃)₄]²⁺ in CD₂Cl₂ solution is independent of [O=PPh₃]_free. A typical data set is: k_ex(200 K) = 611 ± 37 s^-1, ΔH^‡ = 32.0 ± 0.7 kJ mol^-1 and ΔS^‡ = -28.3 ± 3.4 J K^-1 mol^-1 for a solution in which [Zn(O=PPh₃)₄²⁺] and [O=PPh₃]_free are 0.163 and 0.639 mol dm^-3 respectively, where

k_ex = exchange rate/(4[Zn(O=PPh₃)₄²⁺])

= (k_BT/h)exp(-ΔH^‡/RT)exp(ΔS^‡/R)

Similarly the rate of ligand exchange on [Mg(O=PPh₃)₅]²⁺ is shown to be independent of [O=PPh₃]_free and a typical data set is k_ex'(220 K) = 38 ± 4 s^-1, ΔH^‡ = 73.7 ± 1.8 kJ mol^-1 and ΔS^‡ = 123 ± 8 J K^-1 mol^-1 for a solution in which [Mg(O=PPh₃)₅²⁺] and [O=PPh₃]_free are 0.0813 and 0.276 mol dm^-3 respectively. The ligand exchange process on both species is considered to proceed through a D mechanism. The apparent equilibrium constant K = [Mg(O=PPh₃)₅²⁺]/ ([Mg(O=PPh₃)₄²⁺][O=PPh₃] ≥ 150 dm³ mol^-1 but in contrast [Zn(O=PPh₃)²⁺ appears to be the only stable zinc(II) species in solution. It is concluded that these data probably reflect the hard acid character of magnesium(II) and the intermediate acid characteristics of zinc(II).