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. 2018 Nov 2;8(11):10606-10618.
doi: 10.1021/acscatal.8b03146. Epub 2018 Oct 17.

Cobalt Pincer Complexes in Catalytic C-H Borylation: The Pincer Ligand Flips Rather Than Dearomatizes

Haixia Li  1 , Jennifer V Obligacion  2 , Paul J Chirik  2 , Michael B Hall  1
Affiliations

Cobalt Pincer Complexes in Catalytic C-H Borylation: The Pincer Ligand Flips Rather Than Dearomatizes

Haixia Li et al. ACS Catal. .

Abstract

The mechanism for the borylation of an aromatic substrate by a cobalt pincer complex was investigated by density functional theory calculations. Experimental observations identified trans-(iPrPNP)CoH2(BPin) as the resting state in the borylation of five-membered heteroarenes, and 4-BPin-(iPrPNP)Co(N2)BPin as the resting state in the catalytic borylation of arene substrates. The active species, 4-R-(iPrPNP)CoBPin (R=H, BPin), were generated by reductive elimination of H2 in the former, through Berry pseudorotation to the cis isomer, and N2 loss in the latter. The catalytic mechanism of the resulting Co(I) complex was computed to involve three main steps: C-H oxidative addition of the aromatic substrate (C6H6), reductive elimination of PhBPin, and regeneration of the active complex. The oxidative addition product formed through the most favorable pathway, where the breaking C-H bond of C6H6 is parallel to a line between the two phosphine atoms, leaves the complex with a distorted PNP ligand, which rearranges to a more stable complex via dissociation and re-association of HBPin. Alternative pathways, σ-bond metathesis and the oxidative addition in which the breaking C-H bond is parallel to the Co-B bond, are predicted to be unlikely for this Co(I) complex. The thermodynamically favorable formation of the product PhBPin via reductive elimination drives the reaction forward. The active species regenerates through the oxidative addition of B2Pin2 and reductive elimination of HBPin. In the overall reaction, the flipping (refolding) of the five-membered phosphine rings, which connects the species with two phosphine rings folded in the same direction and that with them folded in different directions, is found to play an important role in the catalytic process, as it relieves steric crowding within the PNP ligand and opens Co coordination space. Metal-ligand cooperation based on the ligand's aromatization/dearomatization, a common mechanism for heavy-metal pincer complexes, and the dissociation of one phosphine ligand, do not apply in this system. This study provides guidance for understanding important features of pincer ligands with first-transition-row metals that differ from those in heavier metal complexes.

Keywords: DFT calculations; catalytic C-H borylation; cobalt pincer complex; mechanistic studies; the flipping of the five-membered phosphine rings.

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Figures

Figure 1.
Figure 1.
A proposed mechanism for the catalytic borylation of 2-methylfuran with HBPin from the resting-state complex 2 in ref. .
Figure 2.
Figure 2.
Calculated energy profiles for the release of H2 from trans-(iPrPNP)CoH2(BPin) complex 2, which isomerizes to cis-(iPrPNP)CoH2(BPin) before the rate determining reductive elimination.
Figure 3.
Figure 3.
Optimized geometries of key species involved in the mechanism in Figure 2 (other species are in SI2). Some bond distances in Å, angles in °, and dihedral angles in ° are given in black, red, and green colors, respectively. Values in the parentheses in complex 2 are from its crystal structure. Geometries of 2, TS2–3, and 3 drawn in another perspective are also shown here, where iPr groups on P and BPin on Co are omitted for clarity.
Figure 4.
Figure 4.
Optimized geometries of transition state TS2–12 and complex 12. Some bond distances in Å and angles in ° are given in black and red colors, respectively.
Figure 5.
Figure 5.
Calculated energy profiles for the oxidative addition of C6H6 to 11. Some bond distances in Å, angles in °, and dihedral angles in ° are given in black, red, and green colors, respectively.
Figure 6.
Figure 6.
Calculated energy profiles for the reductive elimination of B-C bond to form PhBPin. The energies for these species are relative to separate C6H6 and 11, and their optimized geometries are shown in SI14.
Figure 7.
Figure 7.
Calculated energy profiles for the regeneration of (iPrPNP)CoBPin (11) from (iPrPNP)CoH (20) and B2Pin2. Optimized geometries of these species are shown in SI16.
Scheme 1.
Scheme 1.
A cobalt pincer complex 1 and its catalyzed borylation reactions.
See this image and copyright information in PMC

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