Metal−Ligand Cooperation in C−H and H₂ Activation by an Electron-Rich PNP Ir(I) System:  Facile Ligand Dearomatization−Aromatization as Key Steps

Abstract

Unusual reactions are reported, in which the aromatic PNP ligand (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) acts in concert with the metal in the activation of H2 and benzene, via facile aromatization/dearomatization processes of the ligand. A new, dearomatized electron-rich (PNP*)Ir(I) complex 2 (PNP* = deprotonated PNP) activates benzene to form the aromatic (PNP)Ir(I)Ph 4, which upon treatment with CO undergoes a surprising oxidation process to form (PNP*)Ir(III)(H)CO 6, involving proton migration from the ligand "arm" to the metal, with concomitant dearomatization. 4 undergoes stereoselective activation of H2 to exclusively form the trans-dihydride 7, rather than the expected cis-dihydride complex. Our evidence, including D-labeling, suggests the possibility that the Ir(I)-Ph complex is transformed to the dearomatized Ir(III)(Ph)(H) (independently prepared at low temperature), which may be the actual intermediate undergoing H2 activation.

Proton–hydride tautomerism in hydrogen evolution catalysis

“…Milstein and coworkers (50) have observed proton transfer to and from a pincer arm in an Ru-H complex, resulting in aromatization-dearomatization of a pyridine core on the ligand. Additionally, in a related example, an Ir complex was suggested to react with D 2 through initial dearomatization of a pincer ligand (51). It follows that the specific organic acid-base used with the complexes described here could affect the energetics and preference of the reaction channel through hydrogen-bonding interactions.…”

Metal–ligand cooperation by aromatization–dearomatization as a tool in single bond activation

“…Unexpectedly, exclusive formation of the Ir(III) trans-dihydride complexes took place, although concerted H 2 oxidative addition was expected to lead to the cisdihydride complex. Surprisingly, when D 2 was used, the D-Ir-H complex 15 was formed with one D atom incorporated in the benzylic position [18]. This suggested the intermediacy of the dearomatized Ir(III) complex 16, formed by proton migration from the side arm.…”

“…Pincer-type complexes of several transition metals exhibit MLC based on aromatizationdearomatization, including complexes of Ru, Fe, Co, Rh, Ir, Ni, Pd, Pt, Mn and Re, leading to facile activation of various chemical bonds. Upon deprotonation of pyridine-based pincer complexes at the methylenic group, dearomatization of the pyridine core takes place [11][12][13][14][15][17][18][19][20], as judged from crystal structures, which exhibit an exo-cyclic double bond. Activation of chemical bonds (Y-H = O-H, N-H, C-H, B-H, Si-H) by cooperation between the metal and the dearomatized ligand results in ligand aromatization, with no formal change in the oxidation state of the metal (scheme 2).…”

Iridium complexes bearing a PNP ligand, favoring facile C(sp3)–H bond cleavage

“…Herein, we report the synthesis and characterization of Ir(PNP)-(Cl) 3 (2) and Ir(PNP)*(I) 2 . The reaction of Ir(Cl) 3 (CH 3 CN) 3 , (1a) with the PNP 10 ligand in N,N-dimethylacetamide afforded complex 2 in good yields as a yellow-orange micro-crystalline solid.…”

“…The reaction of Ir(Cl) 3 (CH 3 CN) 3 , (1a) with the PNP 10 ligand in N,N-dimethylacetamide afforded complex 2 in good yields as a yellow-orange micro-crystalline solid. § Ir(Cl) 3 (CH 3 CN) 3 was synthesized from IrCl 3 .nH 2 O using the procedure of Tessore and coworkers. 11 The resulting trichloride complex was characterized by multinuclear NMR techniques.…”