A low valent Pb(II) hydride complex with NacNac ligand (NacNac = [ArNC(Me)CHC (Me)NAr]-, with Ar = 2,6-iPr2C6H3) is predicted to be the best catalyst for CO2 activation compared to its Ge(II) and Sn(II) analogues, which have been experimentally reported (Jana, A., et al. J. Am. Chem. Soc. 2009, 131, 1288;Jana, A. et al. Angew. Chem. Int. Ed. 2009, 48, 1106). The CO2 activation mechanism mediated by Ge(II), Sn(II) and Pb(II) catalysts was studied in toluene using density functional theory calculations. The results show that the activation can be carried out through two reaction pathways, giving rise to two different conformers which have been computationally predicted for the first time. In all the cases, the activation process was thermodynamically favored, in addition, the Pb(II) catalyst exhibited the lowest activation energy, compared with Ge(II), Sn(II), and even Si(II). Thus, it was found that, going down in group 14, the reactivity of the NacNac based complexes toward CO2 activation increases considerably. This result is due to the increase of the acidic character of the metal as the metal size increases causing a more polar δ+M-Hδ- bond, which allows an easier channel for the hydride transfer reaction. Interestingly, a linear dependence between activation energies and the polarity of the M-H bond was found. Additionally, a detailed characterization of possible interconversions between the products of activation helps to explain the X-ray structures obtained to date. Most importantly, it was found that the direct interconversion of the products obtained after the addition of CO2 from each reaction pathway is not possible, and can be reached by means of subsequent C-O and M-O bond torsions. Our calculations suggest that the CO2 activation catalyzed by Pb(II)-NacNac hydride complex would be both thermodynamically and kinetically viable. The reactivity trend found in this work contributes to the growing development of CO2 activation by transition metal-free catalysts.
Áreas temáticas de ASJC Scopus
- Materiales electrónicos, ópticos y magnéticos
- Energía (todo)
- Superficies, recubrimientos y láminas
- Química física y teórica