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-PtBu2)-6-(CH2-NEt2)­C5H3N,
PNN* = deprotonated PNN, L′ = N2 (5), C2H4 (6)) and [(iPrPNP*)­RhL′] (iPrPNP = 2,6-(CH2-PiPr2)2C5H3N, iPrPNP* = deprotonated iPrPNP, L′
= C2H4 (7), cyclooctene (9)) were prepared and fully characterized by NMR and X-ray
analysis. Complexes 5–7 and 9 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 [(iPrPNP)­Rh­(NHAr)]
(Ar = C6H5, o-Br-C6H4, m-Cl-p-Cl-C6H3, p-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(p-NO2-NH2C6H4)] (13, P = PtBu2, L = NEt2; 15, P = L = PiPr2) yields the dearomatized anionic complexes [(PNL*)­Rh­(p-NO2-NH2C6H4)]. An associative mechanism, involving N–H activation of
an apically coordinated aniline in a pentacoordinated Rh­(I) complex,
is suggested.