Electronic Structure of Six-Membered N-Heterocyclic Carbenes and Its Heavier Analogues: Reactivity of the Lone Pair versus the Exocyclic Double Bond

摘要

Electronic structure of the six-membered N-heterocyclic carbene,nsilylene, germylene, and stannylene having an exocyclic double bond at the C3ncarbon atom as well as the relative reactivity of the lone-pair on the divalent groupn14 element and the exocyclic double bond have been studied at the BP86 level ofntheory with a TZVPP basis set. The geometrical parameters, NICS values, andnNBO population analysis indicate that these molecules can be best described as thenlocalized structure 1Xa, where a trans-butadiene (C1−C2−C3−C4) unit isnconnected with diaminocarbene (N1−X−N2) via N-atoms having a littlencontribution from the delocalized structure 1Xb. The proton affinity at X is highernthan at C4 for 1C, and a reverse trend is observed for the heavier analogues.nHence, the lone pair on a heavier divalent Group 14 element is less reactive thannthe exocyclic double bond. This is consistent with the argument that, even thoughnthe parent six-membered carbene and its heavier analogues are nonaromatic innnature, the controlled and targeted protonation can lead to either the aromaticnsystem 3X having a lone pair on X or the nonaromatic system 2X with readilynpolarizable C3−C4 π-bond. The energetics for the reaction with BH3 and W(CO)6nfurther suggest that both the lone pair of Group 14 element and the exocyclicndouble bond can act as Lewis basic positions, although the reaction at one of thenLewis basic positions in 1X does not considerably influence the reactivity at thenother. The protonation and adduct formation with BH3 and W(CO)5 at X lead tonnonaromatic systems whereas similar reactions at C4 lead to aromatic systems due to π-bond polarization at C3−C4. The degreenof polarization of the C3−C4 π-bond is maximum in the protonated adduct and reduces in the complexes formed with BH3 andnW(CO)5.