Redox trends in cyclometalated palladium(ii) complexes

Abstract

A series of diverse binuclear and mononuclear cyclometalated palladium(ii) complexes of different structure was investigated by electrochemical techniques combined with density functional theory (DFT) calculations. The studies including cyclic and differential pulse voltammetry, X-ray structure analysis and quantum chemical calculations revealed a regularity of the complexes oxidation potential on the metal-metal distance in the complexes: the larger Pd-Pd distance, the higher oxidation potentials. The reduction potentials feature unusually high negative values while no correlation depending on the structure could be observed. These results are in a good agreement with the electron density distribution in the complexes. Additionally, ESR data obtained for the complexes upon oxidation is reported.

Redox-Induced Aromatic C–H Bond Functionalization in Metal Complex Catalysis from the Electrochemical Point of View

“…The ability of Pd(II) to activate C-H bonds through chelation-assisted processes has been known since the 1960s. Nitrogen-containing directing groups were found to promote the insertion of palladium into an ortho-C-H bond, which led to the formation of stable and isolable cyclopalladated complexes from Pd(II) starting material (Scheme 6) [25,[31][32][33][34][35][36][37][38][39][40]. The ability of Pd(II) to activate C-H bonds through chelation-assisted processes has been known since the 1960s.…”

“…However, the electrochemical properties of cyclometalated Pd(II) complexes (reduction/oxidation potentials) determine the mechanism of the reactions. Despite the large literature devoted to the synthesis of various palladacycles and studies of their reactivity, their redox properties, which are necessary to describe and understand the mechanisms of catalytic reactions involving them, are elucidated only in few works [11,15,24,25,[41][42][43][44]. However, …”

“…Nitrogen-containing directing groups were found to promote the insertion of palladium into an ortho-C-H bond, which led to the formation of stable and isolable cyclopalladated complexes from Pd(II) starting material (Scheme 6) [25,[31][32][33][34][35][36][37][38][39][40]. Despite the large literature devoted to the synthesis of various palladacycles and studies of their reactivity, their redox properties, which are necessary to describe and understand the mechanisms of catalytic reactions involving them, are elucidated only in few works [11,15,24,25,[41][42][43][44]. However, the electrochemical properties of cyclometalated Pd(II) complexes (reduction/oxidation potentials) determine the mechanism of the reactions.…”

“…Redox properties are not described for all palladacycles and other palladium derivatives with established structures, so only data where this estimate exists is considered for discussion. Electrochemical data for palladacycles, as established intermediates for many ligand-directed C-H-substitution reactions, are given in [25]. The potential range of the second oxidation peak of palladacycles, for the Pd(III)/Pd(IV) stage [23][24][25], is taken from supporting information data [25].…”

“…In most cases, however, oxidative C-H transformations still rely on the presence of super-stoichiometric amounts of oxidants, which can be problematic in terms of functional group tolerance, waste generation, and cost. Electrochemical oxidation can be a cheaper, greener, and more controllable alternative to these oxidative reactions [13][14][15][16][17][18][19][20][21][22][23][24][25].…”