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Controversial Bonding and Structures of Anionic N-Heterocyclic Carbenes: A Study, Lecture notes of Chemistry

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

2020/2021

Uploaded on 05/24/2021

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Short Literature:
NHCs: Beyond Organocatalysis
1/31/2011
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.
Marianne Lalonde
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Short Literature:

NHCs: Beyond Organocatalysis

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.

Marianne Lalonde

Prof. Gregory Robinson

• Bachelors:^ Jacksonville State (1980)

• PhD:^ University of Alabama (1984)

• Present:^ Professor at University of Georgia

“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.”

Credibility in Question

Cotton: “It should never have been published in Organometallics!”

• Fe-Ga triple bond:

• Related compounds such as^ (C^6 H^5 )^3 PFe(CO)^4 ,

have comparably short phosphorous-iron bonds that no one has called a “triple bond.”

• DFT calculations confirm that pi-orbital overlap

between iron and gallium is “nil.”

• Theory and experimental data suggest Fe and

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.

• Ga-Ga triple bond:

• Distance between Na ion to sandwich carbon

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

• DFT calculations show there is a pi-bonding

orbital (in addition to a Ga-Ga sigma bonding orbital) which is nonbonding - unambiguously indicating a double bond, not a triple bond.

Credibility in Question

Polymeric Crystal Structure

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.

Lewis Acid Insertion and Crystals

(Structure from silicon insertion previously synthesized by Bertrand Group)

Prof. Guy Bertrand

• Bachelors:^ Université de Montpellier, France (1975)

• PhD:^ Université Paul Sabatier, Toulouse, France (1979)

• Post-Doc:^ Sanofi Research Company, France, France (1981)

(Now merged with Aventis Pharmaceutical)

• Present:^ Professor at UC Riverside:

• Synthesis of stable localized singlet and triplet 1,3-diradicals

using various combinations of group 13 to 15 main group elements.

• Synthesis of stable carbenes and their use as ligands for

transition metal catalysts

Synthesis of aNHC imidizolium

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

Crystal Structure of aNHC

Orbital Calculations

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.