Catalytic Tuning of a Phosphinoethane Ligand for Enhanced C−H Activation

Hydrogen atom abstraction (HAA) from 1,4-cyclohexadiene (CD-H) by (dtbpe)Ni(NAr) to form a NiI−amide, (dtbpe)Ni(NHAr), and cyclohexadienyl radical is calculated to be thermodynamically reasonable, ΔHHAA(dtbpe) = −1.3 kcal/mol, dtbpe = bis(di-tert-butylphosphino)ethane, Ar = 2,6-diisopropylphenyl. However, radical rebound to form a metal-bound amine is highly endothermic (ΔHreb(dtbpe) = +25.1 kcal/mol). Analysis of bond enthalpies indicates that weakening of the Ni−N bond (Ni−amide → Ni−amine) upon radical rebound is not compensated by the weak C−N bond formed. Hence, a ligand was sought that would enhance the metal−amine bond strength while diminishing the metal−amide bond strength. Reaction of (dfmpe)Ni(NAr) with CD-H was thus analyzed, dfmpe = bis(di(trifluoromethyl)phosphino)ethane. While there is a small change in the thermodynamics of HAA (ΔHHAA(dfmpe) = −5.7 kcal/mol), there is a profound change in the rebound step (ΔHreb(dfmpe) = −7.8 kcal/mol) upon replacing dtbpe by dfmpe. Regeneration of the nit...