Theoretical Studies of Inorganic and Organometallic Reaction Mechanisms. 14. β-Hydrogen Transfer and Alkene/Alkyne Insertion at a Cationic Iridium Center

Abstract

Recent experimental work shows that alkanes can be activated by Cp*Ir(PMe 3 )(CH 3 ) + at room temperature to generate olefin complexes. The reaction begins with alkane activation by oxidative addition (OA) followed by reductive elimination (RE) of methane and then olefin formation by the -H transfer from the bound alkyl. Ab initio calculations and density functional theory (DFT) studies of ethane activation by CpIr(PH 3 )(CH 3 ) + (1) to generate CpIr-(PH 3 )(η 2 -C 2 H 4 )(H) + (7) show that the -H transfer from CpIr(PH 3 )(C 2 H 5 ) + (5) to 7 is exothermic by 12 and 16 kcal/mol with a very low barrier of 0.7 and 0.4 kcal/mol at the DFT and CCSD levels, respectively. Thus, the rate-determining step in alkane dehydrogenation to olefin complexes by Cp*Ir(PMe 3 )(CH 3 ) + is the alkane OA step. These results are in very good agreement with the experimental work of Bergman and co-workers. A strong stabilizing interaction between either ethylene or acetylene and CpIr(PH 3 )(CH 3 ) + leads to high activation barriers (25-36 kcal/mol) for the insertion processes of ethylene or acetylene. In comparison to ethylene, the insertion reaction of acetylene with the CpIr(PH 3 )(CH 3 ) + complex is more favorable. Thus, the dimerization of terminal alkynes catalyzed by cationic iridium complexes is plausible.

Quantum Dynamical Study of β-Hydrogen Transfer in Two Selected Late-Transition-Metal Complexes

“…In combination with earlier work on the analogous Ir complex, , the geometrical data for one transition-metal triad is now established and available. As expected, a two-step process for the insertion for all three transition-metal complexes is obtained.…”

Stoichiometric and Catalytic Behavior of Cationic Silyl and Silylene Complexes

“…For example, once the β-H elimination products [Cp*(PMe 3 )Ir(R)(olefin)][X] are formed from C−H activation, neither olefin dissociation nor insertion are observed as clean reaction pathways . In fact, high barriers for olefin insertion into the iridium−methyl bonds of [Cp(PH 3 )Ir(Me)] + have been calculated. , Since the complexes described here have the potential to behave as 16-electron Ir(III) silyl complexes, it was hoped that an examination of the insertion reactivity of these species would lead to a catalytic hydrosilylation processes involving an Ir(III)/Ir(V) oxidation state couple.…”

“…13 In fact, high barriers for olefin insertion into the iridium-methyl bonds of [Cp(PH 3 )-Ir(Me)] + have been calculated. 49,50 Since the complexes described here have the potential to behave as 16electron Ir(III) silyl complexes, it was hoped that an examination of the insertion reactivity of these species would lead to a catalytic hydrosilylation processes involving an Ir(III)/Ir(V) oxidation state couple.…”

Electrophile (H+, Me+)-Mediated Carbon−Carbon Bond Formation between η3-Allyl and Alkynyl Groups Coordinated to “Cp*Ir”

“…This hydrogen transfer may occur through the β-hydrogen elimination to produce the Cp*Ir(V) intermediate B , which then undergoes reductive cleavage to give 3 (see Scheme ). Cp*M (M = Ir, Rh) complexes with M in their high oxidation state (Ir(V), Rh(V)) have been observed, suggested as the products from reactions of Cp*Ir(III) complexes with HX (X = halogen, H, and Me), and supported by theoretical calculation . Metallacyclopropanes such as B in Scheme have been frequently suggested as the intermediates in the reactions of metal−alkynyls and related compounds .…”