Paramagnetic (Alkene)Rh and (Alkene)Ir Complexes: Metal or Ligand Radicals?

Abstract

Keywords: Rhodium / Iridium / Alkene ligands / Metallo-radicals / Ligand radicals In this review a detailed overview is given of the (electronic) structure and reactivity of open-shell (paramagnetic) alkene rhodium and iridium complexes M x (alkene) (M = Rh, Ir; x = 0, +II). In most cases these species are highly reactive, but some notable exceptions of stable species for which not much reactivity has been described are also included (section 2). coupling reactions with a series of other radicals, as well as metal-based allylic hydrogen atom abstractions from M II (α-alkene) species. On the other hand, (donor induced) ligand radical character is crucial in a variety of M-C and C-C radical coupling reactions, C-O bond formation by oxygenation of M II (alkene) species and alkene "insertions" into (por)M-H bonds. The review concludes with some speculations about possible roles of paramagnetic M(olefin) (M = Rh, Ir) species in catalytic olefin oxygenation and carbene transfer reactions.

Redox-active ligands in catalysis

“…The electronic structure characterization of the M II compound reveals the existence of metalloradical species both for Ir and in a related Rh ethylene-bound complex. 30 This journal is c The Royal Society of Chemistry 2012 isolable as the final product when ferricinium ion is substituted for O 2 as oxidant. EPR data are consistent with the Ir(II) assignment.…”

The Shrinking World of Innocent Ligands: Conventionaland Non‐Conventional Redox‐Active Ligands

“…The potential of organometallic ligands for non-innocent behavior may be introduced by remembering the principal bonding patterns: (1) π bonding, organic ligands include potentially non-innocent [55] alkenes such as electron-accepting tetracyanoethene, (NC) 2 C=C(CN) 2 = TCNE, which can act as η 2 -TCNE 2-towards platinum metals, [55] or cyclopentadienides C 5 R 5 -(with oxidized C 5 R 5…”

Factors contributing to one-electron metalloradical activation of ethene and carbon monoxide illustrated by reactions of Co(II), Rh(II), and Ir(II) porphyrins

“…One of the initial motivations for our interest in metal-centered radicals (M Å ) was to explore the potential for metallo-radicals to induce one-electron carbon-centered reactions of substrates like ethene and CO in analogy with reactions of hydrogen atom (H Å ) and methyl radical (CH Å 3 ). Formation and reactions of paramagnetic alkene complexes of rhodium and iridium are the subject of a recent microreview that is highly relevant to this area [19]. Organometallic reactivity of carbon monoxide is experiencing resurgent interest resulting from the recognition that CO from synthesis gas is the probable future carbon source for organic molecules to replace petroleum [20,21].…”