Cationic and Neutral PdII and PtII Pincer Complexes of Phosphinamino-Triazolyl-Pyridine [PN(H)N]: Pincer Ligand-Stabilized Palladium Nanoparticles and Their Catalytic Annulation of Internal Alkynes to Indenones.

ABSTRACT

We describe the synthesis of a triazolyl-pyridine-based aminophosphine, N-(diphenylphosphaneyl)-6-(1-phenyl)-1H-(1,2,3-triazol-4-yl)pyridine-2-amine [2,6-{(PPh2)-N(H)(C5H3N)(C2HN3C6H5)}] [1, PN(H)N hereafter], and its palladium and platinum complexes and their catalytic application. The reaction of 1 with [M(COD)Cl2] (M = Pd or Pt) afforded the cationic complex [(MCl){PN(H)N}-κ3-P,N,N]Cl [M = Pd (2) or Pt (3)]. Alternatively, compounds 2 and 3 were also synthesized by treating [2,6-{H2N(C5H3N)(C2HN3C6H5)}] (A) with [M(COD)Cl2] (M = Pd or Pt), followed by the addition of stoichiometric amounts of PPh2Cl and Et3N. The neutral, dearomatized complexes [(MCl){PNN}-κ3-P,N,N] [M = Pd (4) or Pt (5)] were prepared by the deprotonation of the NH of 2 and 3 with 1 equiv of tBuOK. Compounds 4 and 5 were also synthesized stepwise by treating [2,6-{H2N(C5H3N)(C2HN3C6H5)}] (A) with [M(COD)Cl2] (M = Pd or Pt) to give intermediate complexes [{MCl2}2,6-{NH2(C5H3N)(C2HN3C6H5)-κ2-N,N}] [M = Pd (B) or Pt (C)], which were subsequently phosphinated. The in situ-generated PNN ligand-stabilized Pd nanoparticles from compound 2 catalyzed the annulation of o-bromobenzaldehyde with alkynes to yield indenone derivatives. Mechanistic investigations suggested that the reaction was catalyzed by Pd nanoparticles (Pd@2) generated from compound 2 and proceeded through sequential oxidative addition, alkyne insertion, and reductive elimination steps to produce indanone products.