Computational study of methane activation by TpRe(CO)2 and CpRe(CO)2 with a stereoelectronic comparison of cyclopentadienyl and scorpionate ligands

A computational investigation of metal-based C-H activation with complexes that possess scorpionate (i.e., tris(pyrazolyl)borate) and cyclopentadienyl ligands was reported. The model tris(azo)borate ligand (Tab; [HB(N=NH)(3)](-) was created as an alternative to the parent tris(pyrazolyl)borate (Tp). The Tab ligand was found to closely reproduce the structure and energetics of Tp in Re-mediated C-H activation of methane. Relative to separated reactants oxidative addition of methane to CpRe(CO)(2) was exothermic, while the same transformations with TpRe(CO)(2) and (Tab)Re(CO)(2) were endothermic. The differences between the Tp and Cp systems were attributed primarily to steric effects. The reactivity differences between Tp and Cp drew attention to how each system would respond, sterically and electronically, to ring substitutions. TpRe(CO)(3) complexes consistently had. a broader upsilon(CO) (a measure of electronic properties) and cone angle (a measure of steric bulk) range in comparison to CpRe(CO)(3) congeners. While Cp was more sensitive to chemical modification on a per substituent basis, Tp was overall more stereoelectronically flexible. Taken together, the results of this research indicated that Tp cannot simply be considered a Cp equivalent and that the reactivity differences for Re(I) are primarily due to steric difierences between Tp and Cp.