σ‐Noninnocence: Masked Phenyl‐Cation Transfer at Formal Ni^IV

Abstract

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of −2 at a transition‐metal center. For a series of formal high‐valent NiIV complexes, aryl–CF3 bond‐forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015, 137, 8034–8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal NiIV centers are better described as approaching a +II oxidation state, originating from highly covalent metal–ligand bonds, a phenomenon attributable to σ‐noninnocence. A direct consequence is that the elimination of aryl–CF3 products occurs in an essentially redox‐neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF3 group is transferred to an electrophilic aryl group. The uncovered role of σ‐noninnocence in metal–ligand bonding, and of an essentially redox‐neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.

The Myth of d8 Copper(III)

“…Beyond examining ground-state electronic structure, we find that, where the oxidation state formalism dictates that two-electron chemistry is operative at Cu, redox neutrality at Cu is maintained. A recent computational examination of reductive elimination at formally Ni IV centers by Klein and co-workers reveals similar behaviorthe system under investigation has an inverted ligand field, and the lower-valent, physically d 8 Ni II electronic structure is maintained throughout the reaction coordinate. We anticipate that this behavior is generally operative in late, first-row transition metals, and the present work represents another step toward experimentally validating this hypothesis.…”

High-Valent NiIII and NiIV Species Relevant to C–C and C–Heteroatom Cross-Coupling Reactions: State of the Art

“…However, the bonding analysis of complex 24a pointed to an inverted ligand field situation [70] and the Ni IV complexes 24a-e are better described as Ni II species. This effect was called the σ-noninnocence of the cationic aryl ring [70] and the release of PhCF 3 through a redox neutral R.E. step via "σ-noninnocence-induced masked aryl-cation transfer", accordingly [70].…”

“…This effect was called the σ-noninnocence of the cationic aryl ring [70] and the release of PhCF3 through a redox neutral R.E. step via "σ-noninnocence-induced masked aryl-cation transfer", accordingly [70].…”

“…This beneficial effect was attributed to the larger trans-effect of electron rich arenes, along with the lower kinetic barriers associated with the nucleophilic attack of the electron rich σ-aryl ligand to the electrophilic CF 3 -group [69]. However, the bonding analysis of complex 24a pointed to an inverted ligand field situation [70] and the Ni IV complexes 24a-e are better described as Ni II species. This effect was called the σ-noninnocence of the cationic aryl ring [70] and the release of PhCF 3 through a redox neutral R.E.…”

“…This effect was called the σ-noninnocence of the cationic aryl ring [70] and the release of PhCF 3 through a redox neutral R.E. step via "σ-noninnocence-induced masked aryl-cation transfer", accordingly [70]. The use of Py3CH or Tp ligands is necessary for accessing the Ni IV species 20 and 24a-e as shown by the reduced stability of analogous platforms bearing less donating bipyridine ligands (bpy or dtbpy; Scheme 8) [58,69].…”