Chemistry is the experimental and theoretical study of materials on their properties at both the macroscopic and microscopic levels. 626-55-1, Name is 3-Bromopyridine, molecular formula is C5H4BrN. In an article, author is Astakhov, Alexander, V,once mentioned of 626-55-1, Application In Synthesis of 3-Bromopyridine.
Relative stabilities of M/NHC complexes (M = Ni, Pd, Pt) against R-NHC, X-NHC and X-X couplings in M(0)/M(ii) and M(ii)/M(iv) catalytic cycles: a theoretical study
The complexes of Ni, Pd, and Pt with N-heterocyclic carbenes (NHCs) catalyze numerous organic reactions via proposed typical M-0/M-II catalytic cycles comprising intermediates with the metal center in (0) and (II) oxidation states. In addition, M-II/M-IV catalytic cycles have been proposed for a number of reactions. The catalytic intermediates in both cycles can suffer decomposition via R-NHC coupling and the side reductive elimination of the NHC ligand and R groups (R = alkyl, aryl, etc.) to give [NHC-R](+) cations. In this study, the relative stabilities of (NHC)M-II(R)(X)L and (NHC)M-IV(R)(X)(3)L intermediates (X = Cl, Br, I; L = NHC, pyridine) against R-NHC coupling and other decomposition pathways via reductive elimination reactions were evaluated theoretically. The study revealed that the R-NHC coupling represents the most favorable decomposition pathway for both types of intermediates (M-II and M-IV), while it is thermodynamically and kinetically more facile for the M-IV complexes. The relative effects of the metal M (Ni, Pd, Pt) and ligands L and X on the R-NHC coupling for the M-IV complexes were significantly stronger than that for the M-II complexes. In particular, for the (NHC)(2)M-IV(Ph)(Br)(3) complexes, Ph-NHC coupling was facilitated dramatically from Pt (Delta G = -36.9 kcal mol(-1), Delta G(not equal) = 37.5 kcal mol(-1)) to Pd (Delta G = -61.5 kcal mol(-1), Delta G(not equal) = 18.3 kcal mol(-1)) and Ni (Delta G = -80.2 kcal mol(-1), Delta G(not equal) = 4.7 kcal mol(-1)). For the M-II oxidation state of the metal, the bis-NHC complexes (L = NHC) were slightly more kinetically and thermodynamically stable against R-NHC coupling than the mono-NHC complexes (L = pyridine). An inverse relation was observed for the M-IV oxidation state of the metal as the (NHC)(2)M-IV(R)(X)(3) complexes were kinetically (4.3-15.9 kcal mol(-1)) and thermodynamically (8.0-23.2 kcal mol(-1)) significantly less stable than the (NHC)M-IV(R)(X)(3)L (L = pyridine) complexes. For the Ni-IV and Pd-IV complexes, additional decomposition pathways via the reductive elimination of the NHC and X ligands to give the [NHC-X](+) cation (X-NHC coupling) or reductive elimination of the X-X molecule were found to be thermodynamically and kinetically probable. Overall, the obtained results demonstrate significant instability of regular Ni/NHC and Pd/NHC complexes (for example, not additionally stabilized by chelation) and high probability to initiate NHC-free catalysis in the reactions comprising M-IV intermediates.
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Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem