Reversible, metal-free hydrogen activation by frustrated Lewis pairs

The Lewis acid cyclohexylbis(pentafluorophenyl) boron 1, which exhibits about 15% lower Lewis acidity in comparison with B(C6F5)(3), activates H-2 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 H-2 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.