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An in-depth analysis of the synthesis, characterization, and controversy surrounding the bonding and structures of anionic N-Heterocyclic Carbenes (NHCs). research articles by Wang et al. and Aldeco-Perez et al., the background and context of the research, and the credibility of the claims. The authors' laboratory at the University of Georgia is also introduced, focusing on the study of organometallic compounds and metalloaromaticity.
Typology: Lecture notes
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Wang, Y.; Xie, Y.; Abraham, M. Y.; Wei, P.; Schaefer, H. F.; Schleyer, P. v. R.; Robinson, G. H. “A Viable Anionic N-Heterocyclic Dicarbene” J. Am. Chem. Soc. 2010 , 14370-14372. Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.; Bertrand, G. “Isolation of a C5-Deprotonated Imidazolium, a Crystaline “Abnormal” N-Heterocyclic Carbene” Science. 2009 , 556-559.
“Our laboratory has been studying difficult problems relating to fundamental issues of structure and bonding in organometallic compounds. In particular, we are interested in the synthesis, stabilization, and structure of compounds containing multiple bonds between heavier main group elements (such as gallium and lead) and extending the concept of aromaticity beyond carbon ring systems to metallic ring systems-a phenomenon known as metalloaromaticity.”
Cotton: “It should never have been published in Organometallics!”
have comparably short phosphorous-iron bonds that no one has called a “triple bond.”
between iron and gallium is “nil.”
Ga are held by a dative bond where Ga donates an electron pair to Fe. Cotton: “It’s totally unexceptional - nothing could be more open and shut. ”
atoms is only 3 Angstroms or less; indicating there is an attractive force between the 2 sandwich rings that counteracts repulsive steric effects, shortening the Ga-Ga distance
orbital (in addition to a Ga-Ga sigma bonding orbital) which is nonbonding - unambiguously indicating a double bond, not a triple bond.
HOMO and HOMO-2 correspond to the two strongly polarized Li-C bonding orbitals at the C2 and C4 carbon centers. Natural bond orbital (NBO) analysis documents the ionic bonding character of the Li-C bonds in; the Wiberg bond indices of the Li-C bonds are 0.1, and the natural charges of Li+ are +0.84.
(Structure from silicon insertion previously synthesized by Bertrand Group)
(Now merged with Aventis Pharmaceutical)
using various combinations of group 13 to 15 main group elements.
transition metal catalysts
N N O iPr iPr iPr iPr N N H iPr iPr iPr iPr O Br K 2 CO 3 reflux, 24 hours washing with hexanes recrystallization from ethanol N N O iPr iPr iPr iPr HBF 4 *OEt 2 acetic anhydride 0 °C, overnight recrystallization N N iPr iPr iPr iPr BF 4
Calculated two highest-lying occupied orbitals HOMO (left) and HOMO-1 (right) of the a NHC The HOMO (–4.403 eV) is a σ-type lone-pair orbital at C5; the HOMO-1 (–4.879 eV) is a C5–C4 π-bonding orbital, which exhibits antibonding conjugation with the π orbital of the phenyl substituent at C4.
