N–H Activation by Rh(I) via Metal–Ligand Cooperation

In continuation of our studies on bond activation and catalysis by pincer complexes, based on metal–ligand cooperation, we present here a rare example of amine N–H activation by Rh­(I) complexes. The novel dearomatized pincer complexes [(PNN*)­RhL′] (PNN = 2-(CH₂-P^<i>t</i>Bu₂)-6-(CH₂-NEt₂)­C₅H₃N, PNN* = deprotonated PNN, L′ = N₂ (<b>5</b>), C₂H₄ (<b>6</b>)) and [(^<i>i</i>PrPNP*)­RhL′] (^<i>i</i>PrPNP = 2,6-(CH₂-P^<i>i</i>Pr₂)₂C₅H₃N, ^<i>i</i>PrPNP* = deprotonated ^<i>i</i>PrPNP, L′ = C₂H₄ (<b>7</b>), cyclooctene (<b>9</b>)) were prepared and fully characterized by NMR and X-ray analysis. Complexes <b>5</b>–<b>7</b> and <b>9</b> undergo facile N–H activation of anilines involving aromatization of the pincer ligand without a change in the formal oxidation state of the metal center to form stable anilide complexes [(PNN)­Rh­(NHAr)] and [(^<i>i</i>PrPNP)­Rh­(NHAr)] (Ar = C₆H₅, <i>o</i>-Br-C₆H₄, <i>m</i>-Cl-<i>p</i>-Cl-C₆H₃, <i>p</i>-NO₂-C₆H₄). Anilines possessing electron-withdrawing groups accelerate the N–H activation and yield more stable anilide complexes. The pincer and the ancillary ligands also affect the activation rate, which supports an associative mechanism. Spin saturation transfer experiments show chemical exchange between the pyridylic arm of the pincer ligand and the NH– protons of anilines prior to and after the N–H activation. The reverse N–H formation by metal–ligand cooperation from the anilide complexes was observed to give free anilines and dearomatized Rh­(I) complexes upon addition of CO or PEt₃. Deprotonation of complexes [(PNL)­Rh­(<i>p</i>-NO₂-NH₂C₆H₄)] (<b>13</b>, P = P^<i>t</i>Bu₂, L = NEt₂; <b>15</b>, P = L = P^<i>i</i>Pr₂) yields the dearomatized anionic complexes [(PNL*)­Rh­(<i>p</i>-NO₂-NH₂C₆H₄)]. An associative mechanism, involving N–H activation of an apically coordinated aniline in a pentacoordinated Rh­(I) complex, is suggested