Kinetics of protonation of cis-[FeH2(dppe)2]: formation of the dihydrogen complex trans-[FeH(H2)(dppe)2]+ (dppe = Ph2PCH2CH2PPh2)

Abstract

Reaction of cis-[FeH₂(dppe)₂] (dppe = Ph₂PCH₂CH₂PPh₂) with acids in tetrahydrofuran solution led to formation of trans-[FeH(H₂)(dppe)₂]⁺, although co-ordinated H₂ was substituted for N₂ or the anion of the acid in a slower process. The kinetics of the initial protonation reaction with several acids has been studied using an electrochemical procedure. The protonations are first order with respect to both the concentrations of the iron complex and the acid, with second-order rate constants (in dm³ mol^–1 s^–1) at 25 °C of 9.7 × 10^–3 (HBF₄·Et₂O), 1.39 × 10^–2 (CF₃CO₂H), 2.14 × 10^–2 (CF₃SO₃H), 4.8 × 10^–2 (HCl) and 1.48 × 10^–1 (HBr). The ordering of rate constants within this series of acids is similar to that found for the reactions with cis-[FeH₂{P(CH₂CH₂PPh₂)₃}]. There is a good correlation between the values of log k_HX for the two complexes, which suggests a common mechanism for the formation of dihydrogen complexes in these cases. Kinetic data show that reactions of the dppe complex with all acids are faster than those of the analogous P(CH₂CH₂PPh₂)₃ compound. The reaction of cis-[FeH₂(dppe)₂] with deuteriated acids DX is faster than with HX, and the values of the kinetic isotope effect (k.i.e.), expressed as k_HX/k_DX, are 0.21 (CF₃SO₃H), 0.36 (HCl) and 0.55 (HBr). The inverse k.i.e. suggests that protonation occurs through a late transition state, and comparison with cis-[FeH₂{P(CH₂CH₂PPh₂)₃}] and with theoretical values indicates that protonation of cis-[FeH₂(dppe)₂] occurs through a transition state closer to the dihydrogen complex than in the case of the related P(CH₂CH₂PPh₂)₃ compound.