Effect of Solvent and Ancillary Ligands on the Catalytic H/D Exchange Reactivity of Cp*Ir^III(L) Complexes

Abstract

The reactivity of a series of Cp*IrIII(L) complexes that contain a diverse set of ancillary ligands, L, (L = PMe3, N-heterocyclic carbene, NHC = 1,3-dimethylimidazol-2-ylidene, aqua, 4-t-butylpyridine, and 4-(2,4,6-tris-(4-t-butylphenyl)pyridinium)pyridine tetrafluoroborate) has been examined in catalytic H/D exchange reactions between C6H6 and a series of deuterated solvents (methanol-d 4, acetic acid-d 4, and trifluoroacetic acid-d 1). These studies demonstrate that (1) the mechanism of catalytic H/D exchange is significantly influenced by the nature of the solvent; (2) electron-donating ligands (PMe3, NHC) promote the formation of Ir hydrides in methanol-d 4, and these are critical intermediates in catalytic H/D exchange processes; and (3) weak/poorly donating ligands (4-t-butylpyridine, 4-(2,4,6-tris-(4-t-butylphenyl)pyridinium)pyridine tetrafluoroborate and aqua) can support efficient H/D exchange catalysis in acetic acid-d 4.

Electrophilic RhI catalysts for arene H/D exchange in acidic media: Evidence for an electrophilic aromatic substitution mechanism

“…After 15 minutes, the solutions were analyzed by 1 H NMR spectroscopy to determine o:m:p ratio for proton exchange ( Table 2). All of the Rh catalysts exhibited selectivity for H/D exchange at the ortho and para positions with (ortho + para) to meta ratios > 6:1, which is very similar to the ratio for trifluoroacetic acid electrophilic aromatic substitution ( (2) 12 (1) A recent report by Ison and coworkers disclosed that the mechanism for catalytic H/D exchange between arenes and acids using Cp*Ir(III) complexes varies as a function of acid identity [82]. The study showed that electrophilic aromatic protonation dominates in trifluoroacetic acid whereas an organometallic mechanism, where an Ir−Ph is formed and is subsequently protonated by the deuterated solvent, occurs in methanol and acetic acid.…”

“…Developing reactivity with earlier transition metals (i.e., earlier than the Ni, Pd and Pt triad) could provide more tolerance of Lewis basic groups, but major challenges include avoiding oxidation of the metal to higher valent states that are incapable of C-H activation and developing complexes that possess electrophilic hydrocarbyl ligands (after C-H activation) [42][43][44][45][46][47][48][49][50][51][52][53]. Recently, a number of catalysts based on Ir and Rh have been shown to be active for benzene C−H activation [50][51][52][53][54][55][56][57][58][59][60][61]. Rhodium complexes are particularly attractive due to the possibility of C−H activation by Rh I [62] or Rh III [55].…”

“…The regioselectivity of the reactions was determined using 1 H NMR spectroscopy (eq 2). For reactions with toluene, electrophilic aromatic protonation mechanisms favor the ortho and para positions whereas organometallic complexes often favor activation of the meta and para positions [64,82]. The Rh complexes (0.5 mol % relative to toluene-d 8 ) were heated (120 °C, 150 °C, or 180 °C) in trifluoroacetic acid with toluene-d 8 .…”

H–D exchange in deuterated trifluoroacetic acid via ligand-directed NHC–palladium catalysis: a powerful method for deuteration of aromatic ketones, amides, and amino acids

“…While evidence suggests metal-catalyzed H-D exchange reactions of arenes in strong acids such as CF 3 COOD, especially with additives such as AgTFA, are in many cases predominantly electrophilic, 5a,8 a Lewis-acid type H-D exchange mechanism under the described conditions is unlikely for several reasons. First, Lewis-acid catalyzed H-D exchange is typically restricted to nonpolar arenes 3a without functional groups that the Lewis acid could coordinate to preferentially.…”

“…However, recent studies have called into question whether certain metal-catalyzed H-D exchange reactions in acidic solvents proceed through C-H activation or electrophilic aromatic substitution mechanisms; in CF 3 COOD, the latter mechanism has been suggested to predominate. 5a,8 …”