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π-Face Donation from the aromatic N-substituent of N-Heterocyclic Carbene Ligands to Metal and Its Role in Catalysis.
MedLine Citation:
PMID:  22524408     Owner:  NLM     Status:  Publisher    
Abstract/OtherAbstract:
In this work we calculate the redox potential in a series of Ir and Ru complexes bearing a N-heterocyclic carbene (NHC) ligand presenting different Y groups in the para position of the aromatic N-substituent. The calculated redox potentials excellently correlate with the experimental E1/2 potentials, offering a handle to rationalize the experimental findings. Analysis of the HOMO of the complexes before oxidation suggests that electron donating Y groups destabilizes the metal centered HOMO. Energy decomposition of the metal-NHC interaction indicates that electron donating Y groups reinforce this interaction in the oxidized complexes. Analysis of the electron density in the reduced and oxidized states of representative complexes indicates a clear donation from the Cipso of the N-substituents to an empty d orbital on the metal. In case of the Ru complexes this mechanism involves the Ru-alkylidene moiety. All these results suggest that electron donating Y groups render the aromatic N-substituent able to donate more density to electron deficient metals through the Cipso atom. This conclusion suggests that electron donating Y groups could stabilize higher oxidation states during catalysis. To test this hypothesis we investigated the effect of differently donating Y groups in model reactions of Ru-catalyzed olefin metathesis and Pd-catalyzed C-C cross-coupling. Consistently with the experimental results, calculations indicate an easier reaction pathway if the N-substituent of the NHC ligand presents an electron donating Y group.
Authors:
Raffaele Credendino; Laura Falivene; Luigi Cavallo
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-4-23
Journal Detail:
Title:  Journal of the American Chemical Society     Volume:  -     ISSN:  1520-5126     ISO Abbreviation:  -     Publication Date:  2012 Apr 
Date Detail:
Created Date:  2012-4-24     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  7503056     Medline TA:  J Am Chem Soc     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
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