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Remarkable base effect in the synthesis of mono- and dinuclear iridium(I) NHC complexes

(2009) ORGANOMETALLICS. 28(8). p.2460-2470
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Abstract
Several [(2-R-1-1,3-phenylene)bis(methylene)]bis(1-R-2-imidazolium) salts, of general formula (R2)(CHimid<boolean AND>CR1<boolean AND>CHimid)X-2, have been synthesized by classical imidazole quatemization and halide metathesis reactions. The reactions of (Et)(CHimid<boolean AND>CH<boolean AND>CHimid)(PF6)(2) (1), (Et)(CHimid<boolean AND>CH<boolean AND>CHimid)I-2 (2), (Mes)(CHimid<boolean AND>CBr<boolean AND>CHimid)-(PF6)(2) (3), and (Me)(CHimid<boolean AND>CBr<boolean AND>CHimid)(PF6)(2) (4) with [Ir(mu-Cl)(cod)](2) (0.5 equiv) in the presence of CS2CO3 or NEt3 as a weak base (1.2 equiv) yielded the mono-NHC complexes [IrCI(cod)(Et)(CHimid<boolean AND>CH<boolean AND>C-NHC)]-(PF6) (5), [IrI(cod)(Et)(CHimid<boolean AND>CH<boolean AND>C-NHC)]I (6), [IrCl(cod)(Mes)(CHimid<boolean AND>CBr<boolean AND>C-NHC)]-(PF6) (7), and [IrCl(cod)(Me)(CHimid<boolean AND>CBr<boolean AND>C-NHC)]-(PF6) (8), respectively. Doubling the number of equivalents of Ir and base afforded the dinuclear complexes [{IrCl(cod)}(2)(Et)(mu-C-NHC<boolean AND>CH<boolean AND>C-NHC)] (11), [[IrI(cod)}(2)(Et)(mu- C-NHC<boolean AND>CH<boolean AND>C-NHC)] (12), [{IrCl(cod)}(2)(Mes)(mu-C-NHC<boolean AND>CBr<boolean AND>C-NHC)] (13), and [{IrCl(cod)}(2)(Me)(mu-C-NHC<boolean AND>CBr<boolean AND>C-NHC)] (14), respectively. A remarkable effect of the nature of the weak base used was observed, since CS2CO3 allowed the reaction to be carried out at room temperature and with a higher yield than with NEt3. The base effect was even more dramatic when 1 or 2, [Ir(mu-Cl)(cod)](2), and the weak base were reacted in a 1:0.5:2 ratio, since the dinuclear complexes [Ir(cod)(Et)(mu-C-NHC<boolean AND>CH<boolean AND>C-NHC)](2)X-2 (9, X = PF6; 10, X = I) were formed only when CS2CO3 was used. The dinuclear complexes 9-14 could also be synthesized in an elegant stepwise manner from 1-4 via the mono-NHC complexes intermediates 5-8, and Cs2CO3 proved again to be more efficient than NEt3. On the basis of our experimental results, we propose possible reaction pathways leading to the formation of the Ir-NHC bond. In addition to the nature of the base, the influence of the imidazolium counteranions and of the imidazolium ring N substituents on these reactions is also discussed.
Keywords
N-HETEROCYCLIC-CARBENE, C-H ACTIVATION, TRANSITION-METAL-COMPLEXES, OXIDATIVE ADDITION, CATALYTIC-ACTIVITY, TRANSFER HYDROGENATION, COUPLING REACTIONS, IMIDAZOLIUM SALTS, PINCER COMPLEXES, PALLADIUM(II) COMPLEXES

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Chicago
Raynal, Matthieu, Catherine Cazin, Christophe Vallee, Helene Olivier-Bourbigou, and Pierre Braunstein. 2009. “Remarkable Base Effect in the Synthesis of Mono- and Dinuclear iridium(I) NHC Complexes.” Organometallics 28 (8): 2460–2470.
APA
Raynal, M., Cazin, C., Vallee, C., Olivier-Bourbigou, H., & Braunstein, P. (2009). Remarkable base effect in the synthesis of mono- and dinuclear iridium(I) NHC complexes. ORGANOMETALLICS, 28(8), 2460–2470.
Vancouver
1.
Raynal M, Cazin C, Vallee C, Olivier-Bourbigou H, Braunstein P. Remarkable base effect in the synthesis of mono- and dinuclear iridium(I) NHC complexes. ORGANOMETALLICS. 2009;28(8):2460–70.
MLA
Raynal, Matthieu et al. “Remarkable Base Effect in the Synthesis of Mono- and Dinuclear iridium(I) NHC Complexes.” ORGANOMETALLICS 28.8 (2009): 2460–2470. Print.
@article{8536922,
  abstract     = {Several [(2-R-1-1,3-phenylene)bis(methylene)]bis(1-R-2-imidazolium) salts, of general formula (R2)(CHimid<boolean AND>CR1<boolean AND>CHimid)X-2, have been synthesized by classical imidazole quatemization and halide metathesis reactions. The reactions of (Et)(CHimid<boolean AND>CH<boolean AND>CHimid)(PF6)(2) (1), (Et)(CHimid<boolean AND>CH<boolean AND>CHimid)I-2 (2), (Mes)(CHimid<boolean AND>CBr<boolean AND>CHimid)-(PF6)(2) (3), and (Me)(CHimid<boolean AND>CBr<boolean AND>CHimid)(PF6)(2) (4) with [Ir(mu-Cl)(cod)](2) (0.5 equiv) in the presence of CS2CO3 or NEt3 as a weak base (1.2 equiv) yielded the mono-NHC complexes [IrCI(cod)(Et)(CHimid<boolean AND>CH<boolean AND>C-NHC)]-(PF6) (5), [IrI(cod)(Et)(CHimid<boolean AND>CH<boolean AND>C-NHC)]I (6), [IrCl(cod)(Mes)(CHimid<boolean AND>CBr<boolean AND>C-NHC)]-(PF6) (7), and [IrCl(cod)(Me)(CHimid<boolean AND>CBr<boolean AND>C-NHC)]-(PF6) (8), respectively. Doubling the number of equivalents of Ir and base afforded the dinuclear complexes [{IrCl(cod)}(2)(Et)(mu-C-NHC<boolean AND>CH<boolean AND>C-NHC)] (11), [[IrI(cod)}(2)(Et)(mu- C-NHC<boolean AND>CH<boolean AND>C-NHC)] (12), [{IrCl(cod)}(2)(Mes)(mu-C-NHC<boolean AND>CBr<boolean AND>C-NHC)] (13), and [{IrCl(cod)}(2)(Me)(mu-C-NHC<boolean AND>CBr<boolean AND>C-NHC)] (14), respectively. A remarkable effect of the nature of the weak base used was observed, since CS2CO3 allowed the reaction to be carried out at room temperature and with a higher yield than with NEt3. The base effect was even more dramatic when 1 or 2, [Ir(mu-Cl)(cod)](2), and the weak base were reacted in a 1:0.5:2 ratio, since the dinuclear complexes [Ir(cod)(Et)(mu-C-NHC<boolean AND>CH<boolean AND>C-NHC)](2)X-2 (9, X = PF6; 10, X = I) were formed only when CS2CO3 was used. The dinuclear complexes 9-14 could also be synthesized in an elegant stepwise manner from 1-4 via the mono-NHC complexes intermediates 5-8, and Cs2CO3 proved again to be more efficient than NEt3. On the basis of our experimental results, we propose possible reaction pathways leading to the formation of the Ir-NHC bond. In addition to the nature of the base, the influence of the imidazolium counteranions and of the imidazolium ring N substituents on these reactions is also discussed.},
  author       = {Raynal, Matthieu and Cazin, Catherine and Vallee, Christophe and Olivier-Bourbigou, Helene and Braunstein, Pierre},
  issn         = {0276-7333},
  journal      = {ORGANOMETALLICS},
  keywords     = {N-HETEROCYCLIC-CARBENE,C-H ACTIVATION,TRANSITION-METAL-COMPLEXES,OXIDATIVE ADDITION,CATALYTIC-ACTIVITY,TRANSFER HYDROGENATION,COUPLING REACTIONS,IMIDAZOLIUM SALTS,PINCER COMPLEXES,PALLADIUM(II) COMPLEXES},
  language     = {eng},
  number       = {8},
  pages        = {2460--2470},
  title        = {Remarkable base effect in the synthesis of mono- and dinuclear iridium(I) NHC complexes},
  url          = {http://dx.doi.org/10.1021/om900125j},
  volume       = {28},
  year         = {2009},
}

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