Electron-Poor Rhenium Allenylidenes and Their Reactivity toward Phosphines: A Combined Experimental and Theoretical Study

The reaction of 1-(phenyl)-1-(p-nitrophenyl)-2-propyn-1-ol with the Re(I) precursor [(triphos)(CO)(2)Re(OTf)] in dichloromethane at 0 degrees C afforded the cationic allenylidene complex [(triphos)(CO)(2)Re{C=C=C(C6H5)(p-C6H4NO2)}](+) (3) as a dark burgundy red triflate salt after solvent evaporation. The reaction of 3 with 1.2 equiv of the phosphine PMePh2 at -40 degrees C led first to the gamma-phosphonioalkynyl complex [(triphos)(CO)(2)Re{C CCPh(p-C6H4NO2)(PMePh2)}](+) (5) (observed as a pair of distinct rotamers, 5a,b) and then, on slow increase of the temperature to 0 degrees C, to the alpha-phosphonioallenyl complex [(triphos)(CO)(2)]Re{C(PMePh2)=C=CPh(p-C6H4NO2)}](+) (6). On the other hand, the reaction of 3 with the more nucleophific PMe3 at -60 degrees C led to its complete transformation into a compound, suggested to be the a-phosphonioallenyl derivative [(triphos)(CO)(2)Re{C(PMe3)=C=C(C6H5)(p-C6H4NO2)}(+) (7). To study the effect due to the strongly electron withdrawing p-nitrophenyl substituent on the allenylidene geometry, electronic structure, and reactivity with phosphines, we performed theoretical calculations on 3 and other hypothetical p-nitro-substituted allenylidenes as well as on the products and plausible intermediates of its reaction with PMe3 and PMePh2. Finally, theoretical methods were applied to shed light on the nature of the two rotamers observed for the gamma-phosphonioalkynyl complex S.