Kinetico-mechanistic studies of cyclometalating C–H bond activation reactions on Pd(ii) and Rh(ii) centres: The importance of non-innocent acidic solvents in the process

Abstract

The activation of C-H bonds in homogeneous systems has been the subject of study for many years due to its involvement in important industrial catalytic processes. A large number of reviews on the different areas involved have appeared, but those dealing with kinetic studies, including activation parameters, are rather scarce due to the severe difficulties in interpreting experimental data. In this perspective, the information available from kinetico-mechanistic studies of cyclometalation reactions on Pd(II) and Rh(II) centres via C-H bond activation is considered. Experimental results from studies performed on complexes of these metal centres indicate that the historically accepted electrophilic substitution classification is not a satisfactory mechanistic term for the process occurring during the reaction. A definite acid-assisted phenomenon is evident for all the processes studied, which contradicts the expected need for a proton abstractor in the reaction. This is even more surprising when considering the expected hydrolysis of M-C bonds in such acidic media, indicating that metalation prevails under these conditions. Only the presence of coordinated acid molecules in solvolytic carboxylic acid media can explain the observations. The fine tuning between the proton abstraction capacity of a coordinated RCO(2)H molecule and its Lewis basicity results in a unique reactivity trend. DFT calculations carried out for these acid-assisted processes fully agree with the experimental trends observed.

Cyclopalladation and Reactivity of Amino Esters through CH Bond Activation: Experimental, Kinetic, and Density Functional Theory Mechanistic Studies

“…We have already reported that the metalation of 1 d⋅ HCl– 1 f⋅ HCl ( p ‐OMe, p ‐Br and m ‐Br) affords the corresponding orthopalladated complexes 3 d – f with very similar reaction yields (60–66 % in 24 h), which are similar to the yield obtained for unsubstituted 1 a⋅ HCl in the same time (67 %) 8c. This results indicates that the CH bond activation step occurs in practically the same circumstances in these cases, and that the process is not tuned by changing the electronic nature of the aryl substituents, as found in other cyclopalladated systems 21b. An exception is 1 g⋅ HCl, which seems to give a higher yield of the palladated dinuclear complex 3 g ;8c the fact that this complex is not obtained in pure form, and that the yield can only be estimated by the further reactivity of the mixture containing 3 g , could be responsible for the differences reported.…”

“…For the fully unsubstituted amino ester ligand 1 a , the data collected are consistent with the expected highly ordered and entropy‐demanding transition state, which is accompanied by an important volume contraction 13a. 21a,b In this respect, it is noticeable that no significant differences in the activation parameters are observed for the reactions carried out in acetone or toluene solution, which is indicative of the innocent and nonparticipative nature of these solvents in the process. These differences (or similarities), have already been established as very significant for other organometallic systems of diverse nature 28.…”

“…It is clear from the data collected in Table 2 that the “usual” dramatic acceleration of the cyclometalation process in acetic acid observed for previously studied systems involving N‐imine directing groups does not hold in this case 21b. 23a, 24 A similar behavior has also been observed in kinetico‐mechanistic studies carried out for the cyclopalladation of benzyl amines,25 which can be attributed to the effective protonation of the more basic N‐amine directing group, thus decreasing the concentration of the active {Pd II (amine)} cyclometalating unit by the existence of a protonation equilibrium.…”

“…This effect would not be expected for an electrophilic substitution reaction on the aromatic carbon,30 and has to be related to the induced increase in the leaving capability of the acetate ligand attached to the palladium center, which actuates as the proton abstractor for the reaction in a so called ambiphilic mechanism (Figure 5). 21a, b, 22e In this respect, the fact that the kinetic and activation parameters determined for the cyclometalation of hydrochlorides 1 a⋅ HCl, 1 d⋅ HCl, and 1 e⋅ HCl by Pd(OAc) 2 (Table 2, Figure 1), are the same within experimental error is again consistent with the nonelectrophilic nature of the process involved 8c. 21c Surprisingly, the values collected in Table 2 for the reaction of 1 c in acetone are rather different from the expected values; we can speculate that the generation of acetic acid from the transition state indicated in Figure 5, as free acid in acetone, can lead to a partial protonation of the basic tertiary amine, thus disrupting its coordination to form effective cyclometalating {Pd II (amine)} units.…”

Reaction of dimethylplatinum(II) complexes with PhCH2CH2Br: Comparative reactivity with CH3CH2Br and PhCH2Br and synthesis of Pt(IV) complexes

“…[1] Many studies of the reactivity and characterization of organometallic complexes including C-X (X = halide) bond activation have been performed and related kinetic studies have also been conducted in several cases. [8] In the work reported here, oxidative addition reactions of organoplatinum(II) complexes [PtMe 2 (NN)] (NN = bpy or phen) with PhCH 2 CH 2 Br were carried out to synthesize new organoplatinum(IV) complexes. The reactivity of PhCH 2 CH 2 Br in its oxidative addition reactions was theoretically investigated and compared to that of CH 3 CH 2 Br and PhCH 2 Br.…”