Large Kinetic Isotope Effects for the Protonolysis of Metal–Methyl Complexes Are Not Reliable Mechanistic Indicators

Large Isotope Effects in Organometallic Chemistry

The kinetic isotope effect (KIE) is key to understanding reaction mechanisms in many areas of chemistry and chemical biology, including organometallic chemistry. This ratio of rate constants, k_H /k_D , typically falls between 1-7. However, KIEs up to 105 have been reported, and can even be so large that reactivity with deuterium is unobserved. We collect here examples of large KIEs across organometallic chemistry, in catalytic and stoichiometric reactions, along with their mechanistic interpretations. Large KIEs occur in proton transfer reactions such as protonation of organometallic complexes and clusters, protonolysis of metal-carbon bonds, and dihydrogen reactivity. C-H activation reactions with large KIEs occur with late and early transition metals, photogenerated intermediates, and abstraction by metal-oxo complexes. We categorize the mechanistic interpretations of large KIEs into the following three types: (a) proton tunneling, (b) compound effects from multiple steps, and (c) semi-classical effects on a single step. This comprehensive collection of large KIEs in organometallics provides context for future mechanistic interpretation.

Mild and selective Pd-Ar protonolysis and C-H activation promoted by a ligand aryloxide group

A bidentate nitrogen-donor ligand with an appended phenol group, C₅H₄NCH[double bond, length as m-dash]N-2-C₆H₄OH, H(L1) was treated with a palladium cycloneophyl complex [Pd(CH₂CMe₂C₆H₄)(COD)], with both Pd-aryl and Pd-alkyl bonds, to give a Pd-alkyl complex, [Pd(CH₂CMe₂C₆H₅)(κ³-N,N',O-OC₆H₄N[double bond, length as m-dash]CH(2-C₅H₄N))], 1. The cleavage of the Pd-aryl bond and the deprotonation of the ligand phenol to afford a bound aryloxide, indicates facile Pd-aryl bond protonolysis. Deuterium labelling experiments confirmed that the ligand phenol promotes protonolysis and that the reverse, aryl C-H activation, occurs under very mild reaction conditions (within 10 min at room temperature). An unusual isomerization of the Pd-alkyl complex 1 to a Pd-aryl complex, [Pd(C₆H₄(2-t-Bu))(κ³-N,N',O-OC₆H₄N[double bond, length as m-dash]CH(2-C₅H₄N))], 2, was observed to give an equilibrium with [2]/[1] = 9 after 5 days in methanol. The isomerization requires that both aryl C-H activation and Pd-alkyl protonolysis steps occur. The very large KIE value (k_H/k_D = ca. 40) for isomerization of 1 to 2, suggests a concerted S_E2-type mechanism for the Pd-alkyl protonolysis step.

Kinetic Isotope Effects as a Probe for the Protonolysis Mechanism of Alkylmetal Complexes: VTST/MT Calculations Based on DFT Potential Energy Surfaces

Protonolysis by platinum or palladium complexes has been extensively studied because it is the microscopic reverse of the C-H bond activation reaction. The protonolysis of (COD)Pt^IIMe₂, which exhibits abnormally large kinetic isotope effects (KIEs), is proposed to occur via a concerted pathway (S_E2 mechanism) with large tunneling. However, further investigation of KIEs for the protonolysis of ZnMe₂ and others led to a conclusion that there is no noticeable correlation between the mechanism and magnitude of KIE. In this study, we demonstrated that variational transition state theory including multidimensional tunneling (VTST/MT) could accurately predict KIEs and Arrhenius parameters of the protonolysis of alkylmetal complexes based on the potential energy surfaces generated by density functional theory. The predicted KIEs, E_a^D - E_a^H values, and A_H/A_D ratios for the protonolysis of (COD)Pt^IIMe₂ and Zn^IIMe₂ by TFA agreed very well with experimental values. The protonolysis of ZnMe₂ with the concerted pathway has a very flat potential energy surface, which produces a very small tunneling effect and therefore a small KIE. The predicted KIE for the stepwise protonolysis (S_E(ox) mechanism) of (COD)Pt^IIMe₂ was much smaller than that of the concerted pathway, but greater than the KIE of the concerted protonolysis of ZnMe₂. A large KIE, which entails a significant tunneling effect, could be used as an experimental probe of the concerted pathway. However, a normal or small KIE should not be used as an indicator of the stepwise mechanism, and the interplay between experiments and reliable theory including tunneling would be essential to uncover the mechanism correctly.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.