Combined Computational and Experimental Study of Substituent Effects on the Thermodynamics of H₂, CO, Arene, and Alkane Addition to Iridium

Abstract

The thermodynamics of small-molecule (H(2), arene, alkane, and CO) addition to pincer-ligated iridium complexes of several different configurations (three-coordinate d(8), four-coordinate d(8), and five-coordinate d(6)) have been investigated by computational and experimental means. The substituent para to the iridium (Y) has been varied in complexes containing the (Y-PCP)Ir unit (Y-PCP = eta(3)-1,3,5-C(6)H(2)[CH(2)PR(2)](2)Y; R = methyl for computations; R = tert-butyl for experiments); substituent effects have been studied for the addition of H(2), C-H, and CO to the complexes (Y-PCP)Ir, (Y-PCP)Ir(CO), and (Y-PCP)Ir(H)(2). Para substituents on arenes undergoing C-H bond addition to (PCP)Ir or to (PCP)Ir(CO) have also been varied computationally and experimentally. In general, increasing electron donation by the substituent Y in the 16-electron complexes, (Y-PCP)Ir(CO) or (Y-PCP)Ir(H)(2), disfavors addition of H-H or C-H bonds, in contradiction to the idea of such additions being oxidative. Addition of CO to the same 16-electron complexes is also disfavored by increased electron donation from Y. By contrast, addition of H-H and C-H bonds or CO to the three-coordinate parent species (Y-PCP)Ir is favored by increased electron donation. In general, the effects of varying Y are markedly similar for H(2), C-H, and CO addition. The trends can be fully rationalized in terms of simple molecular orbital interactions but not in terms of concepts related to oxidation, such as charge-transfer or electronegativity differences.

Highly Active and Recyclable Heterogeneous Iridium Pincer Catalysts for Transfer Dehydrogenation of Alkanes

“…[23] The methoxy-substituted complex 1b was previously reported to be a more robust alkane dehydrogenation catalyst than the parent complex 1a, [24] while giving slightly higher rates of acceptorless dehydrogenation (of cyclodecane) but slightly lower rates of n-octane/ NBE transfer-dehydrogenation. As seen in Table 3 (solution phase), turnover frequencies (TOFs) for COA/TBE transfer-dehydrogenation by 1b are also somewhat lower than are found for 1a.…”

Comparative Reactivity of TpRu(L)(NCMe)Ph (L = CO or PMe3): Impact of Ancillary Ligand L on Activation of Carbon−Hydrogen Bonds Including Catalytic Hydroarylation and Hydrovinylation/Oligomerization of Ethylene

“…45 Thus, to increase the rate of catalytic olefin hydroarylation, it is necessary to decrease the activation barrier of metal-mediated aromatic C-H activation. Although this rationale may be overly simplistic, 55 the predicted oxidative character of the calculated transition state (i.e., calculated Ru-H contact; see above) suggests the possibility that increasing metal-based electron density might reduce the barrier to the aromatic C-H activation step. To begin to formulate an understanding of the impact of ancillary ligands on the energetics of benzene C-H activation, we have compared the rates of C 6 D 6 activation by TpRu(L)-(NCMe)R (R ) Me or Ph; L ) CO or PMe 3 ).…”

Combined Experimental and Computational Studies on the Nature of Aromatic C−H Activation by Octahedral Ruthenium(II) Complexes: Evidence for σ-Bond Metathesis from Hammett Studies

“…Krogh-Jespersen, Goldman, et al have used similar electronic effects to study oxidative addition of dihydrogen and C-H bonds to Ir(I) metals. 53,54 Another practical impetus for this computational Hammett study is the experimental difficulty encountered for C-H activation of functionalized arene substrates. A few experimental studies, in addition to the classic study by Bercaw et al, 26 employing Hammett analyses for C-H bond activating com-plexes (including -hydride elimination processes) have been presented.…”