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 (iPrPNMeNP = 2,6-(iPr2PNMe)2(C5H3N)) were synthesized. A combination of solid-state structures and investigation of the cobalt­(I)/(II) redox potentials 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 (iPrPNMeNP)­Co­(I) complexes, enabling the characterization of the cobalt boryl and the cobalt hydride dimer by X-ray crystallography. Moreover, (iPrPNMeNP)­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.

Cobalt pincer-type complexes demonstrating unique selectivity for the hydroboration reaction of olefins under mild conditions

“…Based on this and our previous studies on pincer cobalt complexes, as well as literature reports, 44–46,57,63–67 we postulate that in the reaction mixture, as a result of activation with pinacolborane or sodium triethylboron hydride, the two forms of the cobalt complex (Co( i ) and Co( iii )) coexist in equilibrium. Either of them can catalyze the process; however, based on recent reports, 66 we assume that the main catalytically active form is Co( i ) (LCo-H). First, the alkene undergoes 1,2-insertion into a Co–H bond, which is followed by transmetalation with the pinacolborane moiety, resulting in the regeneration of the active form (Scheme 11).…”

C(sp)–H, S–H, and Sn–H Bond Activation with a Cobalt(I) Pincer Complex

“…Understanding how well-defined cobalt complexes cleave various C–H bonds would be useful to guide the design of new catalysts. Of particular note are cobalt aryl complexes, which sometimes can be isolated or spectroscopically observed from the reactions of arenes, presumably involving a C­(sp 2 )–H bond activation step with , or without a directing group (DG). The analogous processes with C­(sp 3 )–H bonds are often intramolecular or DG-promoted. , There are also examples of cobalt alkynyl complexes obtained from HCCR and cobalt species bearing a methyl , or hydroxyl group .…”

PNP‐Pincer Ligands Armed With Methyls: New Tools To Control Sterics and Non‐Innocence

“…This was likely the reason for the superior catalytic performance of a PNP NMe cobalt complex in alkene hydroboration, while the PNP CH2 analog was inactive. [19]…”

“…The NMe‐spacers were recently utilized by the Chirik group to elucidate the effect of pincer rigidity on oxidative addition at cobalt, with PNP NMe cobalt(I) complexes exhibiting higher barrier towards oxidative addition compared to PNP CH2 complexes, which was attributed to the greater rigidity of PNP NMe due to delocalization of the lone pair on the NMe‐spacer onto the pyridine backbone. This was likely the reason for the superior catalytic performance of a PNP NMe cobalt complex in alkene hydroboration, while the PNP CH2 analog was inactive [19] …”