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

A series of seven [Ir­{ArNC­(NR2)­NAr}­(cod)] complexes (1a–1g; where R = Me or Et; Ar = Ph, 4-MeC6H4, 4-MeOC6H4, 2,6-Me2C6H3, or 2,6-iPr2C6H3; 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(μ-Cl)2] or [{Ir­(cod)}2(μ-OMe)2]. Reaction of 1a–1g with CO produced the corresponding [Ir­{ArNC­(NR2)­NAr}­(CO)2] complexes (2a–2g), which were characterized by NMR and solution- and solid-state IR spectroscopy. Complexes 1b (R = Et, Ar = Ph), 1d (R = Et, Ar = 4-MeC6H4), 1f (R = Me, Ar = 2,6-Me2C6H3), and 2b (R = Et, Ar = Ph) were characterized by X-ray crystallography as mononuclear complexes with a guanidinato-κ2N,N′ 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 2a–2g [avg. νCO (n-pentane) = 2016–2019 cm–1] and the alkene 13C chemical shifts of 1a–1g [δ­(13CC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 1a–1g in solution with O2 at 20 °C afforded (alkene)­peroxoiridium­(III) intermediates, [Ir­{ArNC­(NR2)­NAr}­(cod)­(O2)] (3). 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 (1a–1e; where Ar = Ph, 4-MeC6H4 or 4-MeOC6H4), whereas complexes with ortho substituents exhibit fully reversible O2 binding (1f and 1g; where Ar = 2,6-Me2C6H3 or 2,6-iPr2C6H3). Complexes 3a–3f were characterized by 1H NMR and IR spectroscopy (νOO = 857–872 cm–1). Decay of the new intermediates and subsequent reaction with cod produced 4-cycloocten-1-one and the respective IrI precursor.