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

We describe the synthesis of a triazolyl-pyridine-based aminophosphine, N -(diphenylphosphaneyl)-6-(1-phenyl)-1 H -(1,2,3-triazol-4-yl)pyridine-2-amine [2,6-{(PPh 2 )-N(H)(C 5 H 3 N)(C 2 HN 3 C 6 H 5 )}] [ 1 , PN(H)N hereafter], and its palladium and platinum complexes and their catalytic application. The reaction of 1 with [M(COD)Cl 2 ] (M = Pd or Pt) afforded the cationic complex [(MCl){PN(H)N}-κ 3 -P,N,N]Cl [M = Pd ( 2 ) or Pt ( 3 )]. Alternatively, compounds 2 and 3 were also synthesized by treating [2,6-{H 2 N(C 5 H 3 N)(C 2 HN 3 C 6 H 5 )}] ( A ) with [M(COD)Cl 2 ] (M = Pd or Pt), followed by the addition of stoichiometric amounts of PPh 2 Cl and Et 3 N. The neutral, dearomatized complexes [(MCl){PNN}-κ 3 -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 t BuOK. Compounds 4 and 5 were also synthesized stepwise by treating [2,6-{H 2 N(C 5 H 3 N)(C 2 HN 3 C 6 H 5 )}] ( A ) with [M(COD)Cl 2 ] (M = Pd or Pt) to give intermediate complexes [{MCl 2 }2,6-{NH 2 (C 5 H 3 N)(C 2 HN 3 C 6 H 5 )-κ 2 -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.