How a Redox‐Innocent Metal Promotes the Formal Reductive Elimination of Biphenyl Using Redox‐Active Ligands

Abstract

One of the most compelling strategies for utilizing redox-active ligands is to perform redox events at the ligands to avoid accessing prohibitively high energy oxidation states at the metal center. This has been demonstrated experimentally in many systems, yet there is little understanding of the fundamental electronic structures involved with these transformations or how to control them. Here, the reductive elimination of biphenyl from [M(isq)2Ph2] (M = Ti, Zr, and Hf and isq = 2,4-di-tert-butyl-6-tert-butyliminosemiquinone) was studied computationally. It was found that the metal remains in the +IV oxidation state and all redox chemistry was mediated by the redox-active ligands. Two types of electron-transfer mechanisms were identified, an asymmetric unpaired electron transfer (UET) and a symmetric pairwise electron transfer (PET), the former always being lower in energy. The energetic differences between these two mechanisms were explained through simple molecular orbital theory arguments. Despite the metal's redox-inactivity, it still has a marked influence on the calculated energetics of the reaction, with the Ti systems being much more reactive than the Zr/Hf systems. This primarily originates from the shorter Ti-Ph bond, which leads to a stronger filled-filled interaction between these ligands at the reactant state. This greater reactant destabilization leads to the lower activation energies.

Ligand-Induced Reductive Elimination of Ethane from Azopyridine Palladium Dimethyl Complexes

“…For other transition metals (Zr, , Co, Fe − ), reductive elimination can be promoted by the presence of redox-active ligands. , We , and others − have demonstrated that azopyridine , ligands exhibit redox-active behavior when coordinated to transition metals. The low-lying NN π* orbital of azopyridine allows ligand reduction by one electron to generate a radical anion or two electrons to form a hydrazido dianion.…”

“…For other transition metals (Zr, 20,21 Co, 22 Fe 23−25 ), reductive elimination can be promoted by the presence of redox-active ligands. 26,27 We 28,29 and others 30−33 have demonstrated that azopyridine 34,35 ligands exhibit redox-active behavior when coordinated to transition metals.…”

Differences between the elimination of early and late transition metals: DFT mechanistic insights into the titanium-catalyzed synthesis of pyrroles from alkynes and diazenes

“…, Ti IV /Ti II ) because of its weak electronegativity, and thus efforts have been devoted to develop redox catalysis with early TMs using redox-active ligands. 3 Nevertheless, Odom et al 1 a prospected that “ this relative lack of diversity is largely due to the need for further development rather than an inherent lack of utility ”. Indeed, Tonks et al 4 recently accomplished pyrrole synthesis from alkynes and diazenes ( e.g.…”

C−N Bond Formation from a Masked High‐Valent Copper Complex Stabilized by Redox Non‐Innocent Ligands

“…In that case, the geometry of the intermediate allows Ph−CF 3 reductive elimination to occur, a step which is not possible in our present system because of the relative trans positions of the phenyl and CF 3 moieties. Also, iminosemiquinone ligands have been implicated in Ph−Ph reductive elimination from a zirconium (Zr IV ) complex …”