Mechanistic Studies for Pd(II)(O₂) Reduction Generating Pd(0) and H₂O: Formation of Pd(OH)₂ as a Key Intermediate

Abstract

Molecular oxygen (O 2 ) remains to be an ideal yet underutilized feedstock for the oxidative transformation of organic substrates and renewable energy systems such as fuel cells. Palladium (Pd) has shown particular promise in enabling these applications. The present study describes a Pd-mediated O 2 reduction to water via C−H activation of 9,10-dihydroanthracene (DHA) by a Pd(II) η 2 -peroxo complex 1O 2 . The reaction yields stoichiometric anthracene and Pd(0) product 1 and is notable in two respects. First, plots of concentrations of the reaction participants over time have distinctly sigmoidal shapes, indicating that conversion accelerates over time and implying autocatalysis. Second, the reaction proceeds via a genuine monometallic Pd(II) dihydroxide 1(OH) 2 directly observed to grow and decay as an intermediate. Confirming its role as an intermediate, the dihydroxide 1(OH) 2 was found to mediate C−H oxidation of DHA on par in activity with the peroxo compound 1O 2 . Mechanistic studies with density functional theory (DFT) calculations suggested that both 1O 2 and 1(OH) 2 react with DHA by hydrogen atom transfer (HAT) and that autocatalysis in the 1O 2 reaction results from oxidative addition of the initial Pd(II) complex 1O 2 to the Pd(0) product 1. This reaction forms a transient bis(μ-oxo) Pd(II) dimer 1O 2 1 that is more active in the HAT oxidation of DHA than the initial 1O 2 .

Organoplatinum Chemistry Related to Alkane Oxidation: The Effect of a Nitro Substituent in Ligands Having an Appended Phenol Group

“…Thus, in catalysis, it is necessary that the oxidation by dioxygen to form a metal-oxygen bond is followed by a reduction step, involving its cleavage, and that both steps should occur rapidly [7][8][9][10][11][12][13][14]. For example, many noble metal complexes, such as those of palladium(II) [15][16][17][18][19][20] and platinum(II) [21][22][23][24][25][26][27][28][29][30][31], need to be activated in order to react with oxygen. For activation of electron-rich organoplatinum(II) complexes [PtR 2 L 2 ], with R = alkyl or aryl and L = nitrogen-donor ligand, towards dioxygen, it is advantageous to use ligands with a third donor atom and/or a substituent containing a hydrogen bond donor group (usually an NH or OH group) [13,[21][22][23][24]29,30,[32][33][34][35][36][37][38][39][40].…”