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

Abstract

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-(CH 2 -P t Bu 2 )-6-(CH 2 -NEt 2 )C 5 H 3 N, PNN* = deprotonated PNN, L′ = N 2 (5), C 2 H 4 (6)) and [( i PrPNP*)RhL′] ( i PrPNP = 2,6-(CH 2 -P i Pr 2 ) 2 C 5 H 3 N, i PrPNP* = deprotonated i PrPNP, L′ = C 2 H 4 (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 [( i PrPNP)Rh(NHAr)] (Ar = C 6 H 5 , o-Br-C 6 H 4 , m-Cl-p-Cl-C 6 H 3 , p-NO 2 -C 6 H 4 ). 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 PEt 3 . Deprotonation of complexes [(PNL)Rh(p-NO 2 -NH 2 C 6 H 4 )] (13, P = P t Bu 2 , L = NEt 2 ; 15, P = L = P i Pr 2 ) yields the dearomatized anionic complexes [(PNL*)Rh(p-NO 2 -NH 2 C 6 H 4 )]. An associative mechanism, involving N−H activation of an apically coordinated aniline in a pentacoordinated Rh(I) complex, is suggested.

Probing the Donor Properties of Pincer Ligands Using Rhodium Carbonyl Fragments: An Experimental and Computational Case Study

“…Unfortunately, despite our repeated attempts, we have so far been unable to grow single crystals of 4a suitable for structure elucidation in the solid state. The preparation of 3a as a [BF 4 ] – salt has previously been reported using an alternative synthetic route, and the relevant spectroscopic data are in close agreement …”

Metal–ligand cooperation by aromatization–dearomatization as a tool in single bond activation

“…The activated complex is accessible at room temperature even in the case of isopropylamine [28]. A rare example of amine N-H activation by Rh(I) occurs by MLC involving ligand aromatization [29]. The new dearomatized rhodium pincer complexes 24 undergo facile N-H activation of anilines, with no change in the formal oxidation state of the metal, to form the aromatic complexes 25.…”

Rational Design of Rhodium Complexes Featuring κ4‐N,N,N,N‐ and κ5‐N,N,N,P,P‐Bis(imino)pyridine Ligands

“…[28] Four-coordinate complexes of the type [( Ar PDI)Rh-(THF)][X] (where the outer-sphere anion X = OTf or Al-(OC 4 F 9 ) 4 ) have also been well-investigated. of [(COD)RhCl] 2 , [35] followed by the addition of iPr 2 Figure S3 of the Supporting Information), and although the slow exchange limit was not reached, it is clear that this complex undergoes chelate arm exchange at a faster rate than [1][BF 4 ] in solution at ambient temperature. of [(COD)RhCl] 2 , [35] followed by the addition of iPr 2 Figure S3 of the Supporting Information), and although the slow exchange limit was not reached, it is clear that this complex undergoes chelate arm exchange at a faster rate than [1][BF 4 ] in solution at ambient temperature.…”