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

Abstract

Electronic structure of the six-membered N-heterocyclic carbene, silylene, germylene, and stannylene having an exocyclic double bond at the C3 carbon atom as well as the relative reactivity of the lone-pair on the divalent group 14 element and the exocyclic double bond have been studied at the BP86 level of theory with a TZVPP basis set. The geometrical parameters, NICS values, and NBO population analysis indicate that these molecules can be best described as the localized structure 1X(a), where a trans-butadiene (C1-C2-C3-C4) unit is connected with diaminocarbene (N1-X-N2) via N-atoms having a little contribution from the delocalized structure 1X(b). The proton affinity at X is higher than at C4 for 1C, and a reverse trend is observed for the heavier analogues. Hence, the lone pair on a heavier divalent Group 14 element is less reactive than the exocyclic double bond. This is consistent with the argument that, even though the parent six-membered carbene and its heavier analogues are nonaromatic in nature, the controlled and targeted protonation can lead to either the aromatic system 3X having a lone pair on X or the nonaromatic system 2X with readily polarizable C3-C4 π-bond. The energetics for the reaction with BH(3) and W(CO)(6) further suggest that both the lone pair of Group 14 element and the exocyclic double bond can act as Lewis basic positions, although the reaction at one of the Lewis basic positions in 1X does not considerably influence the reactivity at the other. The protonation and adduct formation with BH(3) and W(CO)(5) at X lead to nonaromatic systems whereas similar reactions at C4 lead to aromatic systems due to π-bond polarization at C3-C4. The degree of polarization of the C3-C4 π-bond is maximum in the protonated adduct and reduces in the complexes formed with BH(3) and W(CO)(5).

2-Adamantylidene and its heavier analogues: hyperconjugation versus lone pair stability and electrophilicity versus nucleophilicity

“…Note that the corresponding p-orbital occupancy in the 6-membered NHC and heavier analogues is higher (0.57e for C to 0.33e for Sn) than in 2X. 12 In order to understand the factors contributing to the stability of the singlet state, the bending of the carbene and silylene bridges in 2C and 2Si as well as the strength of the hyperconjugative interactions, we have carried out EDA-NOCV analysis on 2X. 4,24 The interactions of the X2 atom (s 2 p x 0 p y 1 p z 1 ; 3 P state)…”

“…33 A similar trend in bond angle is observed for the 5-membered (100.2°-76.2°at the BP86/def2-TZVPP level of theory) 34 and 6-membered N-heterocyclic carbene (111.5°-88.4°at the BP86/ def2-TZVPP level of theory) and its heavier analogues. 12 The carbene bridge C1-C2-C3 is significantly bent towards C4 and C10 atoms and the bending angle, θ b is 18.5°for 2C (Scheme 2). On the other hand, the bending of the silylene bridge Si1-Si2-Si3 in 2Si is less pronounced (θ b = 10.5°).…”

“…The corresponding s-orbital occupancies in the 6-membered NHC and heavier analogues are lower (1.27e for C to 1.78e for Sn) at the same level of theory. 12 The less than 100°bond angle X1-X2-X3 of 2X (X = Si-Sn) is indicative of the lone pair with a higher percentage of s-character and poor sp n mixing on X2. 33 On the other hand, the decrease in the p x -orbital occupancy of the Ge and Sn indicates their higher electrophilic character and less Cieplak-type hyperconjugative interaction (Scheme 1a).…”

“…Several previous studies indicate the unusual behavior of heavier analogues in terms of structure, bonding and reactivity as compared to carbenes. [9][10][11][12] The unusual behavior of the heavier analogues has enabled their exploitation as ligands in transition metal chemistry, in the synthesis of novel heavier group-14 species and in a variety of reactions such as small molecule activation. [9][10][11] Apart from these, heavier group-14 elements are also known to form catenated compounds, clusters of different sizes and shapes with dangling valencies and bicyclo compounds containing divalent atoms in the bridgehead position.…”

Multiple Cycloaddition Reactions of Ketones with a β‐Diketiminate Al Compound

“…The NBO group chargeo fÀ0.13 ef or the exocyclic CH 2 group indicates 13 %p olarization of the C1=C4 bond. [17] Scheme1.Preparation of compound 2. Chem.E ur.J.2015, 21,19041 -19047 www.chemeurj.org…”