Guanidinato Complexes of Iridium: Ligand-Donor Strength, O₂ Reactivity, and (Alkene)peroxoiridium(III) Intermediates

A series of seven [Ir­{ArNC­(NR₂)­NAr}­(cod)] complexes (<b>1a</b>–<b>1g</b>; where R = Me or Et; Ar = Ph, 4-MeC₆H₄, 4-MeOC₆H₄, 2,6-Me₂C₆H₃, or 2,6-^<i>i</i>Pr₂C₆H₃; and cod = 1,5-cyclooctadiene) were synthesized by two different methods from the neutral guanidines, ArNC­(NR₂)­NHAr, using either MeLi and [{Ir­(cod)}₂(<i>μ-</i>Cl)₂] or [{Ir­(cod)}₂(<i>μ-</i>OMe)₂]. Reaction of <b>1a</b>–<b>1g</b> with CO produced the corresponding [Ir­{ArNC­(NR₂)­NAr}­(CO)₂] complexes (<b>2a</b>–<b>2g</b>), which were characterized by NMR and solution- and solid-state IR spectroscopy. Complexes <b>1b</b> (R = Et, Ar = Ph), <b>1d</b> (R = Et, Ar = 4-MeC₆H₄), <b>1f</b> (R = Me, Ar = 2,6-Me₂C₆H₃), and <b>2b</b> (R = Et, Ar = Ph) were characterized by X-ray crystallography as mononuclear complexes with a guanidinato-κ²<i>N</i>,<i>N′</i> ligand and a cod or two CO ligands coordinated to the Ir center in a distorted square-planar environment. On the basis of the CO stretching frequencies of <b>2a</b>–<b>2g</b> [avg. ν_CO (<i>n-</i>pentane) = 2016–2019 cm^–1] and the alkene ¹³C chemical shifts of <b>1a</b>–<b>1g</b> [δ­(¹³C_CC) = 58.7–61.0 ppm], the donor strength of the guanidinato ligands was evaluated and compared to that of related monoanionic ligands. Reaction of <b>1a</b>–<b>1g</b> in solution with O₂ at 20 °C afforded (alkene)­peroxoiridium­(III) intermediates, [Ir­{ArNC­(NR₂)­NAr}­(cod)­(O₂)] (<b>3</b>). The steric properties of the supporting ligand play a decisive role in O₂ binding in that complexes without ortho substituents react largely irreversibly with O₂ (<b>1a</b>–<b>1e</b>; where Ar = Ph, 4-MeC₆H₄ or 4-MeOC₆H₄), whereas complexes with ortho substituents exhibit fully reversible O₂ binding (<b>1f</b> and <b>1g</b>; where Ar = 2,6-Me₂C₆H₃ or 2,6-^<i>i</i>Pr₂C₆H₃). Complexes <b>3a</b>–<b>3f</b> were characterized by ¹H NMR and IR spectroscopy (ν_OO = 857–872 cm^–1). Decay of the new intermediates and subsequent reaction with cod produced 4-cycloocten-1-one and the respective Ir^I precursor