C-H activation involving three- and four-coordinate iridium(I) pincer complexes

This thesis is organized around two themes: (1) C-H activation catalysis involving in situ generated three-coordinate iridium-pincer complexes as applied toward the dehydrogenation and metathesis of alkanes and (2) understanding and expanding the reactivity of four-coordinate (pincer)Ir(CO) complexes toward the oxidative addition of C-H bonds. In the first half of this thesis, we describe the preparation of a novel class of pincer complex that is comprised of both phosphine and phosphinite moieties, representing a “hybrid” of the previously reported bis-phosphine (PCP) and bis-phosphinite (POCOP) parent species. The catalytic activity of (tBu4PCOP)Ir toward the transfer and acceptorless dehydrogenation of linear and cyclic alkanes was examined and compared to the activities of (tBu4PCP)Ir and (tBu4POCOP)Ir, generally exhibiting intermediate activity. (tBu4PCOP)Ir and other hybrid (PCOP)Ir complexes were applied to alkane metathesis using a molybdenum alkylidene co-catalyst and found to exhibit far greater activity toward n-hexane metathesis compared to either parent catalyst. The resting state of (tBu4PCOP)Ir in alkane metathesis was found to be mixture of the dissimilar resting states exhibited by the parent catalysts, suggesting an ability to effectively catalyze the respective slow steps associated with the (tBu4PCP)Ir and (tBu4POCOP)Ir routes. In the latter half of the thesis, we explore the unprecedented oxidative addition of C-H bonds to four-coordinate (pincer)Ir(CO) complexes. Employing a unique acid-catalyzed route, the net oxidative addition of phenylacetylene to square planar (tBu4PCP)Ir(CO) was observed to yield exclusively the trans six-coordinate C-H addition product, (tBu4PCP)Ir(CO)(H)(CCPh); detailed mechanistic and theoretical studies indicate that this rare transformation occurs by generating a protonated intermediate that undergoes electrophilic attack of PhCCH with subsequent deprotonation of the alkyne C-H bond in the rate-determining step. Although this acid-catalyzed reaction was observed to occur only with alkynes, the reverse reaction (acid-catalyzed reductive elimination) was found to occur for both alkyl and aryl substrates, suggesting that the acid-catalyzed oxidative addition of Csp3-H and Csp2-H bonds might be kinetically possible but thermodynamically unfavored. Preliminary experimental and theoretical explorations of the factors favoring the thermodynamics of addition support a strategy of utilizing less sterically hindered (pincer)Ir(CO) complexes bearing relatively strong σ-donating ligands trans to the carbonyl ligand.Ph. D.Includes bibliographical referencesby Jason D. Hackenber