Reversible, metal-free hydrogenactivation by frustrated Lewis pairs

Abstract

The Lewis acid cyclohexylbis(pentafluorophenyl)boron 1, which exhibits about 15% lower Lewis acidity in comparison with B(C₆F₅)₃, activates H₂ in the presence of the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP), 1,2,2,6,6-pentamethylpiperidine (PMP), tri-tert-butylphosphine (t-Bu₃P) leading in facile reactions at room temperature to heterolytic splitting of dihydrogen and formation of the salts [TMPH][CyBH(C₆F₅)₂] 2, [PMPH][CyBH(C₆F₅)₂] 3 and [t-Bu₃PH][CyBH(C₆F₅)₂] 4, which could be dehydrogenated at higher temperatures. The related Lewis acid 1-phenyl-2-[bis(pentafluorophenyl)boryl]ethane 5 exhibiting about 10% lower Lewis acidity than B(C₆F₅)₃ is also capable of splitting H₂ in a heterolytic fashion in the presence of TMP, PMP and t-Bu₃P yielding [TMPH][PhC₂H₄BH(C₆F₅)₂] 6, [PMPH][PhC₂H₄BH(C₆F₅)₂] 7 and [t-Bu₃PH][PhC₂H₄BH(C₆F₅)₂] 8. Under comparable conditions as for 2–4, the dehydrogenations of 6–8 were much slower. 4b and 6 were characterized by single crystal X-ray diffraction studies.