Stable mononuclear organometallic Pd(III) complexes and their C-C bond formation reactivity

Organometallic Pd(III) complexes have been implicated as intermediates in a number of catalytic and stoichiometric transformations. While a few dinuclear organometallic Pd(III) complexes have been characterized, no mononuclear organometallic Pd(III) complexes have been isolated to date. Reported herein is the synthesis and characterization of a series of Pd(III) complexes supported by the tetradentate ligand N,N′-di-tert-butyl-2,11-diaza[3.3](2,6) pyridinophane (N4). Chemical or electrochemical oxidation of the Pd(II) complexes (N4)Pd^II(R)(X) (R = Me, X = Cl: 1; R = Ph, X = Cl: 2; R = X = Me: 3) generates [(N4)Pd^IIIMeCl]⁺ (1⁺), [(N4)Pd^IIIPhCl]⁺ (2⁺), and [(N4)Pd ^IIIMe2]⁺ (3⁺). These stable Pd(III) complexes were isolated and characterized by X-ray diffraction, cyclic voltammetry, UV-vis, EPR, magnetic moment measurements, and DFT to confirm the presence of paramagnetic d⁷ Pd(III) centers. Moreover, these Pd(III) complexes undergo light-induced C-C bond formation to give the corresponding homocoupled products ethane or biphenyl. Particularly remarkable is the observation for the first time of ethane formation from monomethyl Pd complexes.