Heterolytic Oxidative Addition of sp² and sp³ C–H Bonds by Metal–Ligand Cooperation with an Electron-Deficient Cyclopentadienone Iridium Complex

Abstract

Oxidative addition reactions of C−H bonds that generate metal−carbon-bond-containing reactive intermediates have played essential roles in the field of organometallic chemistry. Herein, we prepared a cyclopentadienone iridium(I) complex 1 designed for oxidative C−H bond additions. The complex cleaves the various sp 2 and sp 3 C−H bonds including those in hexane and methane as inferred from their H/D exchange reactions. The hydroxycyclopentadienyl(nitromethyl)iridium(III) complex 2 was formed when the complex was treated with nitromethane, which highlights this elementary metal−ligand cooperative C−H bond oxidative addition reaction. Mechanistic investigations suggested the C−H bond cleavage is mediated by polar functional groups in substrates or another iridium complex. We found that ligands that are more electron-deficient lead to more favorable reactions, in sharp contrast to classical metal-centered oxidative additions. This trend is in good agreement with the proposed mechanism, in which C−H bond cleavage is accompanied by two-electron transfer from the metal center to the cyclopentadienone ligand. The complex was further applied to catalytic transfer-dehydrogenation of tetrahydrofuran (THF).

Cleavage of Si–H and Si–C Bonds by Metal–Ligand Cooperation: Formation of Silyl Anion and Silylene Equivalents from Tertiary Silanes

“…The formation of complex 4a , the umpolung product, was stepwise through (a) oxidative addition of the Si–H bond at the iridium center to afford complex 2a , (b) reversible formation of κ 2 - O , O ′ iridium complexes 3a , and (c) isomerization into hydroxy­cyclopentadienyl iridium silyl complex 4a . The observed stepwise mechanism contrasts with the reported cleavage/formation of H–H, B–H, and C–H bonds by cyclopentadienone metal complexes, which proceeded without any observable intermediates. Upon addition of MePh 2 SiH to the solution of complex 1 , oxidative addition of the Si–H bond to the iridium center proceeded quantitatively to afford complex 2a (Figure a).…”

Cyclopentadienone Vanadium(I) Complex: Synthesis, Red/Ox Activity, and Application to Catalytic Transfer Hydrogenation

“…In the course of its heterolytic bond cleavage, the CpO ligand accepts two electrons from the metal center to form a hydroxycyclopentadienyl (=CpOH) anion, resulting in the concomitant oxidation of ruthenium(0) to ruthenium­(II) (Scheme b­(i)) . The reactivities of CpO metal complexes are due to the degree of metal-to-CpO electron transfer; it was clarified that introducing an electron-deficient ligand and low-valent metal increased their activities in heterolytic bond cleavage due to the enhanced electron transfer . Accordingly, many research groups applied various low-valent late transition metals, such as Fe 0 , Ir I , , Ni 0 , Pt II , etc., to the red/ox-active metal–ligand cooperative system, which enabled unique elementary reactions and catalytic reactions.…”

“…Structurally related tetraphenylcyclopentadienone metal complexes, with carbonyl ligands if available, are selected from the literature-reported complexes for comparison. Data were collected from the literature: Fe(0), Ru(0), Os(0), Rh­(I), Ir­(I), and Pt­(II) complexes with CpO ligand ( 2-X ) and Ru­(II) and Ir­(III) complexes with CpOH ligand ( 3-X ). The metalligand bond distance, which is defined as the average of the metalC2/C3/C4/C5 bond length, of 2.30 Å in complex 2-V is the longest among all the CpO metal complexes 2-X , reflecting the large covalent radius of vanadium (1.53 Å) .…”

Metal–Ligand–Anion Cooperation in C–H Bond Formation at Platinum(II)

“…In C–H activation via MLC, a basic site on the ligand formally accepts the proton, with the anionic carbon fragment binding to the Lewis acidic metal. While there is no net oxidation state change at the metal, a higher oxidation state intermediate is sometimes invoked with a stepwise process of an initial OA, followed by proton transfer to the ligand. ,− Mechanisms where the oxidation state of the metal is maintained throughout the reaction are likely more robust activation paths, particularly under aerobic conditions; the electron-rich, low-valent oxidation state species required for OA can be susceptible to undesirable reactivity in preference to C–H bond activation . MLC mechanisms are also attractive in terms of atom efficiency for functionalization as, in contrast to CMD, the proton remains in the coordination sphere of the metal and can potentially be incorporated into the product at a later step.…”