Abstract
The reactions of substituted 1-phenylpyrazoles (phpyz-H) at [MCl2Cp*]2 dimers (M = Rh, Ir; Cp* = C5Me5) in the presence of NaOAc to form cyclometalated Cp*M(phpyz)Cl were studied experimentally and with density functional theory (DFT) calculations. At room temperature, time-course and H/D exchange experiments indicate that product formation can be reversible or irreversible depending on the metal, the substituents, and the reaction conditions. Competition experiments with both para- and meta-substituted ligands show that the kinetic selectivity favors electron-donating substituents and correlates well with the Hammett parameter giving a negative slope consistent with a cationic transition state. However, surprisingly, the thermodynamic selectivity is completely opposite, with substrates with electron-withdrawing groups being favored. These trends are reproduced with DFT calculations that show C-H activation proceeds by an AMLA/CMD mechanism. H/D exchange experiments with the meta-substituted ligands show ortho-C-H activation to be surprising facile, although (with the exception of F substituents) this does not generally lead to ortho-cyclometalated products. Calculations suggest that this can be attributed to the difficulty of HOAc loss after the C-H activation step due to steric effects in the 16e intermediate that would be formed. Our study highlights that the use of substituent effects to assign the mechanism of C-H activation in either stoichiometric or catalytic reactions may be misleading, unless the energetics of the C-H cleavage step and any subsequent reactions are properly taken into account. The broader implications of our study for the assignment of C-H activation mechanisms are discussed.
Highlights
Note for 1a-OMe there are two unique molecules in the unit cell, data have been given for just one molecule see cif for full data
Note for 1b-OMe and 1b-Me there are two unique molecules in the unit cell, data have been given for just one molecule see cif for full data
Calibration curves for mixtures of the two complexes were measured to establish their relative sensitivity e: ratio changed over time and reached 8.6:1(1.1) after 24 hours at r.t. f determined by 19F NMR spectroscopy
Summary
Note for 1b-OMe and 1b-Me there are two unique molecules in the unit cell, data have been given for just one molecule see cif for full data. Table S1c: Selected bond distances (Å) and bond angles [o] for 3a-OMe, 3b-OMe, 3b-CF3, and 3b-NO2. Note for 3b-CF3 there are two unique molecules in the unit cell, data have been given for just one molecule see cif for full data. The percentage deuterium incorporation was calculated from the relative integrations of the ortho protons and other signals in the corresponding free ligand. The results are shown in the Table below. For ligands L1-R the two ortho sites are equivalent whilst for L2-R sites A and B are inequivalent with site A corresponding to formation of the ortho isomer and site B the para isomer (see Scheme 4). B For L2-Me the signals for A and B are very close so calculating an accurate ratio is not possible
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
