Detailed mechanistic studies on PNN-palladium pincer complex catalyzed Suzuki-Miyaura cross-coupling reaction proceeding through a PdII/PdIII/PdIV catalytic cycle

Abstract

In this paper, synthesis, Pd and Pt complexes and catalytic investigations of a tridentate pincer ligand 2-(diphenylphosphaneyl)-N-(pyridine-2-ylmethyl)benzamide, {(o-PPh₂)C₆H₄C(O)N(H)CH₂(C₆H₅N)} (1), (hereafter referred to as “PN(H)N” and its anionic form as “PNN”) is described. Reaction of 1 with MCl₂(COD) resulted in pincer complexes [MCl(PNN-κ³-P,N,N)] (Pd 2, Pt 3), whereas the same reaction with Pd(OAc)₂ afforded [CH₃C(O)OPd(PNN-κ³-P,N,N)] (4). The structures of all of the compounds are confirmed by single-crystal X-ray analysis. The palladium complex 4 promoted the Suzuki-Miyaura cross-coupling reaction between aryl halides and boronic acids to form the corresponding biphenyls in excellent yields. The catalytic investigation supported by spectroscopic studies and DFT calculations suggested the involvement of Pd^II/Pd^IV catalytic cycle via a short lived Pd^III intermediate, which transforms into Pd^IV species rapidly. The catalytic process proceeded with a very low catalyst loading under relatively mild reaction conditions.