Modes of Activation of Organometallic Iridium Complexes for Catalytic Water and C–H Oxidation

Abstract

Sodium periodate (NaIO4) is added to Cp*Ir(III) (Cp* = C5Me5(-)) or (cod)Ir(I) (cod = cyclooctadiene) complexes, which are water and C-H oxidation catalyst precursors, and the resulting aqueous reaction is investigated from milliseconds to seconds using desorption electrospray ionization, electrosonic spray ionization, and cryogenic ion vibrational predissociation spectroscopy. Extensive oxidation of the Cp* ligand is observed, likely beginning with electrophilic C-H hydroxylation of a Cp* methyl group followed by nonselective pathways of further oxidative degradation. Evidence is presented that the supporting chelate ligand in Cp*Ir(chelate) precursors influences the course of oxidation and is neither eliminated from the coordination sphere nor oxidatively transformed. Isomeric products of initial Cp* oxidation are identified and structurally characterized by vibrational spectroscopy in conjunction with density functional theory (DFT) modeling. Less extensive but more rapid oxidation of the cod ligand is also observed in the (cod)Ir(I) complexes. The observations are consistent with the proposed role of Cp* and cod as sacrificial placeholder ligands that are oxidatively removed from the precursor complexes under catalytic conditions.

Cryogenic Methods for the Spectroscopy of Large, Biomolecular Ions

“…The goal has been to provide an appreciation of the various methods for cooling and spectroscopic detection as well as an idea of how they have been combined in creative ways in different laboratories around the world to measure spectra of cold ions. It should be noted that there is nothing that restricts these techniques to biomolecular ions -in fact, applications to molecules such as cation-crown ether complexes [185][186][187], reaction intermediates [188], and organometallic catalysts [147] have proven to be particularly powerful.…”

“…Through careful control of the trap temperature, several other molecular tags have been employed, such as N 2 and CO 2 , which condense at 25 and 100 K, respectively. This capability is especially useful in the application of ion spectroscopy to study intermediates in the activation of small molecules by organometallic catalysts [147].…”

Molecular Catalysts for Water Oxidation

“…268 Mass spectroscopic injection of species from acetonitrile showed spectral features consistent with Cp*-containing compounds. A later report from Crabtree, Johnson, and Zare in 2014, 269 however, implicated several activation modes of the Cp* ligand. The aqueous reaction mixture obtained upon treatment of the Cp*Ir complex bearing 2-(2′-pyridyl)-2-propanolate with periodate as chemical oxidant was investigated from milliseconds to seconds using desorption electrospray ionization, electrosonic spray ionization, and cryogenic ion vibrational predissociation spectroscopy.…”

Going beyond electrospray: mass spectrometric studies of chemical reactions in and on liquids

“…They determined that nonselective hydroxylation and oxidation of the Cp* ligand by the periodate oxidant was occurring, which leads to complete degradation of the Cp* ring into CO 2 and organic acid byproducts. 35 …”