Improved Oxidative C–C Bond Formation Reactivity of High-Valent Pd Complexes Supported by a Pseudo-Tridentate Ligand

Abstract

There is a large interest in developing oxidative transformations catalyzed by palladium complexes that employ environmentally friendly and economical oxidizing reagents such as dioxygen. Recently, we have reported the isolation and characterization of various mononuclear Pd III and Pd IV complexes supported by the tetradentate ligands N,N'-di-alkyl-butyl-2,11diaza[3.3](2,6)pyridinophane (R N4, R = t Bu, i Pr, Me), and the aerobically-induced CC and Cheteroatom bond formation reactivity was investigated in detail. Given that the steric and electronic properties of the multidentate ligands were shown to tune the stability and reactivity of the corresponding high-valent Pd complexes, herein we report the use of an asymmetric N4 ligand, N-mehtyl-N'-tosyl-2,11-diaza[3.3](2,6)pyridinophane (TsMe N4), in which one amine N atom contains a tosyl group. The N-Ts donor atom exhibits a markedly reduced donating ability, which led to the formation of transiently stable Pd III and Pd IV complexes, and consequently the corresponding O2 oxidation reactivity and the subsequent CC bond formation was improved significantly. Metrical Parameters 1 + DFT 4 + DFT (Expt) 2 + DFT 5 + DFT (Expt) Bond Lengths Pd-N1(Ts/Me) 2.498 2.365 (2.310) 2.529 2.379 (2.302) Pd-N2 (Me) 2.318 2.368 (2.311) 2.313 2.377 (2.338) Pd-N3 (Py) 2.063 2.051 (2.029) 2.111 2.104 (2.085) Pd-N4 (Py) 2.067 2.052 (2.002) 2.181 2.148 (2.085) Pd-Cl1 2.416 2.425 (2.303) 2.466 2.486 (2.344) Pd-Cl2/C1 2.417 2.424 (2.322) 2.073 2.070 (2.021

Recent progress in the chemistry of 12-membered pyridine-containing tetraazamacrocycles: from synthesis to catalysis

“…Very recently, they studied the reactivity of various pyridinophane Pd( ii ) complexes Pd II -L41f derived from non-symmetric L41f (Scheme 25). 91 The use of this ligand resulted in lower stabilities of high valence Pd( iii ) and Pd( iv ) complexes with respect to previously reported examples with N -alkyl-substituted Py 2 N 2 . Nevertheless, the formation of pseudo-tridentate Cl 2 Pd III L41f and Me(Cl)Pd III L41f complexes by the 1e − oxidation process was experimentally confirmed by EPR spectroscopy.…”

Structurally-Responsive Ligands for High-Performance Catalysts

“…The pendent amine must be a sufficiently strong donor to raise the HOMO and promote formation of a Pd­(III)-superoxo, but weak enough that the Pd­(IV) trimethyl intermediate is not too stable for reductive elimination. Recently, a complex was reported with a derivative of Me L in which one of the tertiary amine alkyl substituents was replaced with an electron-withdrawing tosyl group . Reaction times for the oxidatively induced reductive elimination were reduced to 2 h as compared to 5 h for 13 .…”

“…Recently, a complex was reported with a derivative of Me L in which one of the tertiary amine alkyl substituents was replaced with an electron-withdrawing tosyl group. 102 Reaction times for the oxidatively induced reductive elimination were reduced to 2 h as compared to 5 h for 13. This is consistent with a destabilization of a Pd(IV) intermediate (analogous to E) that would lead to faster reductive elimination, but further mechanistic studies are needed to confirm this hypothesis.…”

Electrocatalytic O2 Reduction by an Organometallic Pd(III) Complex via a Binuclear Pd(III) Intermediate

“…Thus, evaluation of new transition metal complexes, including organometallic species, for ORR reactivity could uncover new molecular catalysts with improved properties. We have previously reported the synthesis and characterization of various organometallic Pd III complexes stabilized by the tetradentate ligand N , N ′-di- tert -butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane ( t BuN4) and related analogues. − These complexes were shown to react with O 2 and undergo oxidatively induced C–C and C–heteroatom bond formation reactions in the presence of O 2 . These O 2 -induced oxidative transformations prompted us to evaluate the ORR reactivity of such organometallic complexes, which to the best of our knowledge has never been done before for any molecular Pd catalyst.…”