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

The reaction of 1-(phenyl)-1-(<i>p</i>-nitrophenyl)-2-propyn-1-ol with the Re­(I) precursor [(triphos)­(CO)<sub>2</sub>Re­(OTf)] in dichloromethane at 0 °C afforded the cationic allenylidene complex [(triphos)­(CO)<sub>2</sub>Re­{CCC­(C<sub>6</sub>H<sub>5</sub>)­(<i>p</i>-C<sub>6</sub>H<sub>4</sub>NO<sub>2</sub>)}]<sup>+</sup> (<b>3</b>) as a dark burgundy red triflate salt after solvent evaporation. The reaction of <b>3</b> with 1.2 equiv of the phosphine PMePh<sub>2</sub> at −40 °C led first to the γ-phosphonioalkynyl complex [(triphos)­(CO)<sub>2</sub>Re­{CCCPh­(<i>p-</i>C<sub>6</sub>H<sub>4</sub>NO<sub>2</sub>)­(PMePh<sub>2</sub>)}]<sup>+</sup> (<b>5</b>) (observed as <i>a pair of distinct rotamers</i>, <b>5a</b>,<b>b</b>) and then, on slow increase of the temperature to 0 °C, to the α-phosphonioallenyl complex [(triphos)­(CO)<sub>2</sub>Re­{C­(PMePh<sub>2</sub>)CCPh­(<i>p-</i>C<sub>6</sub>H<sub>4</sub>NO<sub>2</sub>)}]<sup>+</sup> (<b>6</b>). On the other hand, the reaction of <b>3</b> with the more nucleophilic PMe<sub>3</sub> at −60 °C led to its complete transformation into a compound, suggested to be the α-phosphonioallenyl derivative [(triphos)­(CO)<sub>2</sub>Re­{C­(PMe<sub>3</sub>)CC­(C<sub>6</sub>H<sub>5</sub>)­(<i>p</i>-C<sub>6</sub>H<sub>4</sub>NO<sub>2</sub>)}]<sup>+</sup> (<b>7</b>). To study the effect due to the strongly electron withdrawing <i>p-</i>nitrophenyl substituent on the allenylidene geometry, electronic structure, and reactivity with phosphines, we performed theoretical calculations on <b>3</b> and other hypothetical <i>p</i>-nitro-substituted allenylidenes as well as on the products and plausible intermediates of its reaction with PMe<sub>3</sub> and PMePh<sub>2</sub>. Finally, theoretical methods were applied to shed light on the nature of the two rotamers observed for the γ-phosphonioalkynyl complex <b>5</b>.