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Combined Computational and Experimental Study of Substituent Effects on the Thermodynamics of H2, CO, Arene, and Alkane Addition to Iridium
journal contribution
posted on 2002-08-17, 00:00 authored by Karsten Krogh-Jespersen, Margaret Czerw, Keming Zhu, Bharat Singh, Mira Kanzelberger, Nitesh Darji, Patrick D. Achord, Kenton B. Renkema, Alan S. GoldmanThe thermodynamics of small-molecule (H2, arene, alkane, and CO) addition to pincer-ligated
iridium complexes of several different configurations (three-coordinate d8, four-coordinate d8, and five-coordinate d6) 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 = η3-1,3,5-C6H2[CH2PR2]2Y; R = methyl for computations; R = tert-butyl for experiments); substituent effects have been studied
for the addition of H2, 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 H2, 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.