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Pyrazolium- and 1,2-Cyclopentadiene-Based Ligands as sigma-Donors: a Theoretical Study of Electronic Structure and Bonding
Tekijät: Peuronen A, Hanninen MM, Tuononen HM
Kustantaja: AMER CHEMICAL SOC
Julkaisuvuosi: 2012
Journal: Inorganic Chemistry
Tietokannassa oleva lehden nimi: INORGANIC CHEMISTRY
Lehden akronyymi: INORG CHEM
Vuosikerta: 51
Numero: 4
Aloitussivu: 2577
Lopetussivu: 2587
Sivujen määrä: 11
ISSN: 0020-1669
DOI: https://doi.org/10.1021/ic202546a
Tiivistelmä
A high-level theoretical investigation of 1,2-cyclopentadiene (4) was performed using density functional theory and wave function methods. The results reveal that, in contrast to earlier assumptions, the ground state of this ephemeral "allene" is carbene-like with a small diradical component. Furthermore, the electronic structure and chemistry of 4 are found to parallel that of 1,2,4,6-cycloheptatetraene: both molecules possess a low-lying excited singlet state with a closed-shell carbenic structure, enabling rich coordination chemistry. Energy decomposition analyses conducted for currently unknown metal complexes of 4 as well as those involving stable carbenes based on the pyrazolium framework (aka "bent allenes" or remote N-heterocyclic carbenes) indicate that all investigated ligands form particularly strong metal-carbon bonds. Most notably, without exocyclic pi-type substituents, 4 and pyrazolin-4-ylidenes are the strongest donor ligands examined, in large part because of the energy and shape of their highest occupied molecular orbital. As a whole, the current work opens a new chapter in the chemistry of 1,2-cyclopentadiene, which is hoped to spark renewed interest among experimentalists. In addition, results from the conducted bonding analyses underline that more emphasis should be placed on purely carbocyclic carbenes as unprecedented sigma-donor strengths can be realized through this route.
A high-level theoretical investigation of 1,2-cyclopentadiene (4) was performed using density functional theory and wave function methods. The results reveal that, in contrast to earlier assumptions, the ground state of this ephemeral "allene" is carbene-like with a small diradical component. Furthermore, the electronic structure and chemistry of 4 are found to parallel that of 1,2,4,6-cycloheptatetraene: both molecules possess a low-lying excited singlet state with a closed-shell carbenic structure, enabling rich coordination chemistry. Energy decomposition analyses conducted for currently unknown metal complexes of 4 as well as those involving stable carbenes based on the pyrazolium framework (aka "bent allenes" or remote N-heterocyclic carbenes) indicate that all investigated ligands form particularly strong metal-carbon bonds. Most notably, without exocyclic pi-type substituents, 4 and pyrazolin-4-ylidenes are the strongest donor ligands examined, in large part because of the energy and shape of their highest occupied molecular orbital. As a whole, the current work opens a new chapter in the chemistry of 1,2-cyclopentadiene, which is hoped to spark renewed interest among experimentalists. In addition, results from the conducted bonding analyses underline that more emphasis should be placed on purely carbocyclic carbenes as unprecedented sigma-donor strengths can be realized through this route.
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