Theoretical Studies of Inorganic and Organometallic Reaction Mechanisms. 15. Catalytic Alkane Dehydrogenation by Iridium(III) Complexes

详细信息    查看全文

• 作者：Shuqiang Niu and Michael B. Hall
• 刊名：Journal of the American Chemical Society
• 出版年：1999
• 出版时间：April 28, 1999
• 年：1999
• 卷：121
• 期：16
• 页码：3992 - 3999
• 全文大小：359K
• 年卷期：v.121,no.16(April 28, 1999)
• ISSN：1520-5126

文摘

Alkane dehydrogenation catalyzed by the Ir(III) complexes (PCP')Ir(H)₂ (1) [PCP' = images/gifchars/eta.gif" BORDER=0 >³-C₆H₃(CH₂PH₂)₂-1,3] and CpIr(PH₃)(H)⁺ (10) [Cp = images/gifchars/eta.gif" BORDER=0 >⁵-C₅H₅] is investigated with density functional theory (DFT). Forboth systems the theoretical results show that catalytic alkane dehydrogenation to alkene proceeds through (i)alkane oxidative addition, (ii) dihydride reductive elimination, (iii) images/gifchars/beta2.gif" BORDER=0 ALIGN="middle">-H transfer from alkyl ligand to metal,and finally (iv) elimination of the olefin. Barriers for steps (i), (ii), and (iv) are critical for the catalytic cycle.The (PCP')Ir(H)₂ system is special because these three barriers are balanced (16, 15, and 22 kcal/mol,respectively), whereas in the CpIr(PH₃)(H)⁺ system these three barriers are unbalanced (9, 24, and 41 kcal/mol, respectively). Thus, in the catalytic cycle for alkane dehydrogenation by (PCP')Ir(H)₂ the reactionendothermicity is achieved gradually. The higher stability of the formally Ir(V) complexes and the images/gifchars/eta.gif" BORDER=0 >²-alkenecomplex, which has some Ir(V)-like character, in the CpIr(PH₃)(H)⁺ system is responsible for the larger barriersin these critical steps. In the key role played by the ligand systems, PCP'(H) vs Cp(PH₃), the former increasesthe energy of the metal-ligand fragment's triplet state relative to that of the singlet and thus destabilizes allthe Ir(V)-like species.