Activation of Nitriles by Metal Ligand Cooperation. Reversible Formation of Ketimido- and Enamido-Rhenium PNP Pincer Complexes and Relevance to Catalytic Design

The dearomatized complex <i>cis</i>-[Re­(PNP^<i>t</i>Bu*)­(CO)₂] (<b>4</b>) undergoes cooperative activation of CN triple bonds of nitriles via [1,3]-addition. Reversible C–C and Re–N bond formation in <b>4</b> was investigated in a combined experimental and computational study. The reversible formation of the ketimido complexes (<b>5</b>–<b>7</b>) was observed. When nitriles bearing an alpha methylene group are used, reversible formation of the enamido complexes (<b>8</b> and <b>9</b>) takes place. The reversibility of the activation of the nitriles in the resulting ketimido compounds was demonstrated by the displacement of <i>p-</i>CF₃-benzonitrile from <i>cis</i>-[Re­(PNP^<i>t</i>Bu–NCPh^<i>p</i>CF3)­(CO)₂] (<b>6</b>) upon addition of an excess of benzonitrile and by the temperature-dependent [1,3]-addition of pivalonitrile to complex <b>4</b>. The reversible binding of the nitrile in the enamido compound <i>cis</i>-[Re­(PNP^<i>t</i>Bu–HNCCHPh)­(CO)₂] (<b>9</b>) was demonstrated via the displacement of benzyl cyanide from <b>9</b> by CO. Computational studies suggest a stepwise activation of the nitriles by <b>4</b>, with remarkably low activation barriers, involving precoordination of the nitrile group to the Re­(I) center. The enamido complex <b>9</b> reacts via β-carbon methylation to give the primary imino complex <i>cis</i>-[Re­(PNP^<i>t</i>Bu–HNCC­(Me)­Ph)­(CO)₂]­OTf <b>11</b>. Upon deprotonation of <b>11</b> and subsequent addition of benzyl cyanide, complex <b>9</b> is regenerated and the monomethylation product 2-phenylpropanenitrile is released. Complexes <b>4</b> and <b>9</b> were found to catalyze the Michael addition of benzyl cyanide derivatives to α,β-unsaturated esters and carbonyls