Exploring the Effect of Pincer Rigidity on Oxidative Addition Reactions with Cobalt(I) Complexes

Abstract

Cobalt complexes containing the 2,6-diaminopyridine-substituted PNP pincer (^iPrP^NMeNP = 2,6-(ⁱPr₂PNMe)₂(C₅H₃N)) were synthesized. A combination of solid-state structures and investigation of the cobalt(I)/(II) redox potential established a relatively rigid and electron-donating chelating ligand as compared to ^iPrPNP (^iPrPNP = 2,6-(ⁱPr₂PCH₂)₂(C₅H₃N)). Based on a buried volume analysis, the two pincer ligands are sterically indistinguishable. Nearly planar, diamagnetic, four-coordinate complexes were observed independent of the field strength (chloride, alkyl, aryl) of the fourth ligand completing the coordination sphere of the metal. Computational studies supported a higher barrier for C-H oxidative addition, largely a result of the increased rigidity of the pincer. The increased oxidative addition barrier resulted in stabilization of (^iPrP^NMeNP)Co(I) complexes, enabling the characterization of the cobalt boryl and the cobalt hydride dimer by X-ray crystallography. Moreover, (^iPrP^NMeNP)CoMe served as an efficient precatalyst for alkene hydroboration likely because of the reduced propensity to undergo oxidative addition, demonstrating that reactivity and catalytic performance can be tuned by rigidity of pincer ligands.

Figure 1.

(A) Pincer modifications for control of steric and electronic properties (B) Pincer methylene dynamics found in C–H oxidative addition step for (PNP)Co-catalyzed C–H borylation chemistry by DFT calculations.

Figure 2.

Structures, features, steric maps and percent buried volumes (%V_bur)^, of (A) (^iPrPNP)Co(CH₂SiMe₃), (B) (Me₄^iPrPNP)Co(CH₂SiMe₃), and (C) 1-CH₂SiMe₃. The Co atom defines the center of the xyz coordinate system. The P–N–P plane defines the xz-plane, and the N–Co line defines the z-axis. A sphere of radius, r = 3.5 Å. Bondi radii scaled by 1.17 Å. Mesh spacing for numerical integration set to 0.1 Å.

Figure 3.

Truncated energy profile of oxidative addition of C₆H₆ to 1-BPin. The free energy values are referenced to the initial reactants. The Gibbs free energies of the corresponding transition state and intermediates for [(^iPrPNP)Co] system are reported in the inset.

Figure 4.

Representations of 1-BPin, TS1, and TS2 (left). Top view (middle) and side view (right) of the space-filling models of 1-BPin, TS1, and TS2, illustrating the extent of shielding of the cobalt. The phenyl (C₆H₅) moiety and the H atom are omitted in top and side view of TS1 and TS2 for clarity. Colors: Co–pink; N–blue; P–orange; B–light pink; O–red; C–dark gray; H–light gray.

Scheme 1.. Synthesis and Solid-State Structures of (^iPrP^NMeNP)Co Complexes.

Conditions: a) 1 equiv. NaEt₃BH. b) 1 equiv. MeLi or 1 equiv. TMSCH₂Li. See supporting information for details. Isolated yields given. Solid-state structures at 30% probability ellipsoids. Hydrogen atoms omitted for clarity.

Scheme 2.. Formation of the Cobalt-Aryl Complex 3-o from C–H Activation.

Reaction conducted with 0.33 mmol of 2, 0.43 mmol of B₂Pin₂, 0.05 mmol (15 mol%) of 1-Me in 0.55 mL of THF or THF-d₈ at 80 °C for 20 h. ¹⁹F NMR for the meta isomer of the cobalt(I) aryl complex (3-m; (^iPrP^NMeNP)Co(3-F-5-CF₃C₆H₃)): δ −62.0 (CF₃), −119.1 (F).

Scheme 3.. Investigation into the C–H Activating Ability of [(^iPrP^NMeNP)Co] Complexes.

^aReaction conducted with 0.5 mmol of 2 and 0.025 mmol (5 mol%) of 1-Me in 1 mL of THF under 1 atm D₂ at 50 °C for 16 h. Percent deuterium incorporation determined by ¹H and ¹⁹F NMR spectroscopy. ^bReaction carried out with 25 μmol of 4 and 50 μmol of 1,3-difluorobenzene in 0.5 mL of C₆D₆ at 50 °C for 20 h. Yields determined by ¹H, ¹⁹F, and ³¹P NMR spectroscopy. ^cReaction conducted with 17 μmol of 3-o, 85 μmol of 2, and 85 μmol of DBPin (90% D) in 0.5 mL of C₆D₆ at 50 °C for 20 h. Yields of cobalt complexes determined by ¹H and ³¹P NMR spectroscopy with respect to starting 3-o used. Percent deuterium incorporation determined by ¹H and ¹⁹F NMR spectroscopy.

Scheme 4.. Proposed Pathways for Arene Isomerization and C–B Bond Formation.

[Co] represents (^iPrP^NMeNP)Co.

Scheme 5.. Attempts to Obtain the Cobalt Boryl and Hydride Complexes and Their Solid-State Structures.^a

^aSee Supporting Information for detailed procedures. ^b1-(H)₂BPin containing trace amount (<5%) of 1-H₃. ^cEllipsoids at 30% probability. Hydrogen atoms, except the cobalt–hydrides omitted for clarity.

Scheme 6.. C–H Activation Reactions of 1-BPin with Fluoroarenes

See Supporting Information for detailed procedures. Yields determined by ¹H and ³¹P NMR spectroscopy and regioselectivity determined by ¹⁹F NMR spectroscopy.

Scheme 7.. Hydroboration of 1-Octene.

A. Possible reaction pathways. B. Reaction conducted with 0.9 mmol of 1-octene, 0.9 mmol (1 equiv.) of HBPin, and 9 μmol (1 mol%) of 1-Me or (^iPrPNP)CoMe. Yield and conversion determined by GC versus a mesitylene internal standard. ^a17% of alkene was isomerized to a mixture of internal alkenes. ^bIsomerization not detected.