σ-Noninnocence: Masked Phenyl-Cation Transfer at Formal NiIV

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 Ni^IV 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 Ni^IV 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.

Keywords: coinage metals; homolysis; ligand-field inversion; radicals; trifluoromethyl.

Scheme 1

Reported synthesis of the Ni^iV complex I_R=H by Sanford and co‐workers via oxidation of 1 with TDTT (top), or via reaction of 2 with Ph₂IBF₄ (bottom), and its subsequent aryl trifluoromethyl bond‐forming reductive elimination (center).2

Figure 1

Top: Snyder's originally proposed oxidation state assignment for the [Cu(CF₃)₄]⁻ anion (for the original proposal, see Ref. 6a). Bottom: Conceptual depiction of the changes to the metal–ligand bonding due to the presence of an inverted ligand field (Adapted with permission from Ref. 3a, 5a. Copyright 2016 American Chemical Society).

Figure 2

IBO depictions of three occupied d‐orbitals (top) and six Ni–ligand‐bonding IBOs. Numbers in parentheses indicate the partial‐charge distribution of a given IBO at M06‐L/def2‐TZVPPD/SMD//M06‐L/def2‐SVP/SMD. Orbital iso‐surfaces enclose 80 % of the integrated electron density of the orbital. Hydrogen atoms are omitted for clarity. Depicted using IboView.19

Figure 3

vvIBO depictions of I_R=H. Numbers in parentheses indicate the virtual partial charge distribution at Ni of a given vvIBO at M06‐L/def2‐TZVPPD/SMD//M06‐L/def2‐SVP/SMD. Hydrogen atoms are omitted for clarity. Depicted using IboView.19

Figure 4

Depiction of the Ni−C_Ph (purple, top) and Ni‐C${}_{\mathrm{CF}{}_3}$ (green, bottom) IBOs along the IRC (M06‐L/def2‐SVP/SMD). For the position along the IRC, see Figure 5. Hydrogen atoms are omitted for clarity.

Figure 5

RSSD partial charge changes of the Ni−C_Ph (purple) and Ni−C${}_{\mathrm{CF}{}_3}$ (green) IBOs along the IRC (M06‐L/def2‐SVP/SMD).

Figure 6

Top: Comparison of computed (blue triangles) and experimental (black squares) Hammett plots. Experimental data and Hammett parameters were taken from Ref. 2. Inset: IBO overlap of the aromatic π‐system with Ni at the transition state. Bottom: Plot of the IBO overlap of the aromatic π‐system with Ni vs. Hammett σ‐values.