Well-defined bisMETAMORPhos Pd^I-Pd^I complex: Synthesis, structural characterization, and reactivity

The formation of a bisMETAMORPhos PN-bridged dimeric Pd^I complex (3) from ligand 1 and Pd(dba)₂ is described. The addition of 1 to Pd(dba)₂ initially leads to the formation of Pd⁰ complex 2, which has a highly distorted tetrahedral environment and binds two neutral ligands 1. Complex 2 converts to {Pd^I}₂ complex 3 upon heating. Complex 3 consists of a completely flat Pd-Pd core, with a Pd-Pd bond length of 2.6199(4) Å, and the Pd centers display a highly distorted square planar coordination environment. The formation of complex 3 from 2 is suggested to proceed via an in situ comproportionation pathway. Initial decoordination of one ligand from 2 followed by oxidative addition of one neutral coordinated ligand arm leads to a 1-Pd^IIH complex. Insertion of free dba into the Pd-H bond generates a 1-Pd^IIdba complex that releases 1,5-diphenylpent-1-en-3-one via intra- or intermolecular protonolysis. Concomitantly, comproportionation with 1-Pd⁰ yields 3. Complex 3 was found to function as a precatalyst in the Suzuki-Miyaura cross-coupling of p-chloroacetophenone with good conversions.