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

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.