Core Excitation Spectroscopy of Stable Cyclic Diaminocarbenes, -silylenes, and -germylenes

Lehmann, John F.; Urquhart, Stephen G.; Ennis, Laura E.; Hitchcock, Adam P.; Hatano, Ken; Gupta, S. K.; Denk, Michael

doi:10.1021/om980882z

Cited by 69 publications

(57 citation statements)

References 37 publications

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“…Generally these ligands are formally neutral, two-electron donors which, contrary to Fischer-type or Schrock-type carbene complexes, are best described as pure s-donor ligands without significant metal-ligand p back-bonding [132][133][134][135]. This might be due to a rather high occupancy of the formally empty p p orbital of the carbene carbon atom by p delocalization [136].…”

Section: N-heterocyclic Carbene (Nhc) Complexes Silylenes and Germylmentioning

confidence: 99%

Electronic Structure and Reactivity of Metal Carbenes

Straßner

2004

Metal Carbenes in Organic Synthesis

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Section: N-heterocyclic Carbene (Nhc) Complexes Silylenes and Germylmentioning

confidence: 99%

Electronic Structure and Reactivity of Metal Carbenes

Straßner

2004

Metal Carbenes in Organic Synthesis

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“…They are fundamentally important species that for a long time were considered as prototypical reactive intermediates, too unstable to isolate under usual experimental conditions (1). According to structural, thermodynamic, and magnetic criteria, from the populations, ionization potentials, and inner shell electron energy loss spectroscopy, it has been recognized that the first isolated N-heterocyclic carbenes (NHCs), the imidazol-2-ylidenes A (2) benefit from their aromatic character (3)(4)(5) (Fig. 1).…”

mentioning

confidence: 99%

Stable four-π-electron, four-membered heterocyclic cations and carbenes

Ishida

Donnadieu

Bertrand

2006

Proc. Natl. Acad. Sci. U.S.A.

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. Subsequent deprotonation of the B-amino derivative affords the corresponding N-heterocyclic carbene, which has been isolated as yellow crystals. According to singlecrystal x-ray diffraction studies, both the carbene and its conjugate acid precursor have a planar structure, with no pyramidalization of the ring atoms. The structural parameters indicate that the four electrons are mostly distributed over the N-C-N fragment with little involvement of the boron center, and therefore both types of heterocycles escape from antiaromaticity. However, considering the ring strain and the presence of the Lewis acid center, the thermal stability of the carbene (mp 98°C without decomposition) is rather surprising. These results clearly suggest that the backbone of N-heterocyclic carbenes can be modified at will, without preventing their isolation.aromaticity ͉ boron C arbenes are compounds with a neutral, dicoordinate carbon atom with only six electrons in its valence shell. They are fundamentally important species that for a long time were considered as prototypical reactive intermediates, too unstable to isolate under usual experimental conditions (1). According to structural, thermodynamic, and magnetic criteria, from the populations, ionization potentials, and inner shell electron energy loss spectroscopy, it has been recognized that the first isolated N-heterocyclic carbenes (NHCs), the imidazol-2-ylidenes A (2) benefit from their aromatic character (3-5) (Fig. 1). Indeed, in addition to the stabilizing effect of the -donating, -attracting amino substituents, it has been concluded that aromaticity provides an additional stabilization of Ϸ25 kcal͞mol (3). We have shown that six--electron NHCs B (6), isoelectronic with borazenes (7), are highly thermally stable. Even more strikingly, the two--electron cyclopropenylidene C is stable enough to be isolated at room temperature (8), despite the absence of -donor heteroatoms directly bonded to the carbene center, a necessary criterion for previously isolated carbenes (9-14).As recently written by Mo and Schleyer (15): ''Although aromaticity is a virtual (not directly measurable) quantity, its usefulness as a central chemical concept is undiminished after two centuries.'' Clearly, abnormal energies govern the chemical behavior of (4n ϩ 2) and (4n) -electron-containing conjugated ring systems, which are more and less stable thermodynamically, respectively, than ''they ought to be' ' (16-18).At that point, the question was raised: is it possible to isolate formally antiaromatic heterocyclic carbenes? This endeavor requires the preparation of the stable conjugate acid precursors, which would also feature a (4n)--electron system. Following the analogy between borazenes and NHCs of type B, we chose a BNCN skeleton, and here we report the synthesis and single crystal x-ray diffraction study of cationic, planar, cyclic four--electron four-membered heterocycles D and a corresponding NHC E. Results and Discussion Treatment of N,N'-2,6-diisopropylphenyl-N-trimethylsilylfor-mamidine with...

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“…In the presence of platinum as a catalyst, NHCs react with hydrogen to form aminals, which are 1,1-hydrogenation products [39,70]. NHCs display an ambivalent p-bonding character, thus functioning as a -acid as well as a -base [71]. This dual behavior is evident in dimerization, which is considered the simplest carbene reaction.…”

Section: Stability and Reactivitymentioning

confidence: 99%

“…But backdonation constitutes the major contribution to the p-interaction for systems with a high d electron count. Therefore, the NHC ligands in systems with a high d-electron count are best described as -acids [71].…”

Section: Stability and Reactivitymentioning

confidence: 99%