Theoretical Study of N-Heterocyclic-Carbene–ZnX2 (X = H, Me, Et) Complexes

Abstract

This article discusses the properties of as many as 30 carbene–ZnX2 (X = H, Me, Et) complexes featuring a zinc bond C⋯Zn. The group of carbenes is represented by imidazol-2-ylidene and its nine derivatives (labeled as IR), in which both hydrogen atoms of N-H bonds have been substituted by R groups with various spatial hindrances, from the smallest Me, iPr, tBu through Ph, Tol, and Xyl to the bulkiest Mes, Dipp, and Ad. The main goal is to study the relationship between type and size of R and X and both the strength of C⋯Zn and the torsional angle of the ZnX2 plane with respect to the plane of the imidazol-2-ylidene ring. Despite the considerable diversity of R and X, the range of dC⋯Zn is quite narrow: 2.12–2.20 Å. On the contrary, D0 is characterized by a fairly wide range of 18.5–27.4 kcal/mol. For the smallest carbenes, the ZnX2 molecule is either in the plane of the carbene or is only slightly twisted with respect to it. The twist angle becomes larger and more varied with the bulkier R. However, the value of this angle is not easy to predict because it results not only from the presence of steric effects but also from the possible presence of various interatomic interactions, such as dihydrogen bonds, tetrel bonds, agostic bonds, and hydrogen bonds. It has been shown that at least some of these interactions may have a non-negligible influence on the structure of the IR–ZnX2 complex. This fact should be taken into account in addition to the commonly discussed R⋯X steric repulsion.

The physical nature of the ultrashort spike–ring interaction in iron maiden molecules

“…This may be an argument for the opinion that HÁ Á ÁH interactions in aliphatic chains can be (very weakly) stabilizing. [79][80][81][82] This interaction pattern has also recently been found e.g., between C-H bonds of the ethylene groups in ZnEt 2 in my earlier studies 83 of 30 carbene-ZnX 2 complexes (X = H, Me, Et) containing a zinc CÁ Á ÁZn bond. 84 However, the presence of such areas was shown much earlier, e.g., in 2010 by Johnson et al in their article on NCI.…”

On the Coexistence of the Carbene⋯H-D Hydrogen Bond and Other Accompanying Interactions in Forty Dimers of N-Heterocyclic-Carbenes (I, IMe2, IiPr2, ItBu2, IMes2, IDipp2, IAd2; I = imidazol-2-ylidene) and Some Fundamental Proton Donors (HF, HCN, H2O, MeOH, NH3)

“…It can also be seen that these carbenes have large substituents (most often in the form of mesityl groups) in the 1 and 3 positions of the imidazol-2-ylidene parent molecule. Therefore, the study of these types of systems using theoretical methods is also very important [ 80 ]. However, the presence of bulky substituents significantly increases the size of the systems and thus significantly increases the computational cost of theoretical research.…”

“…Dimers (complexes) having this bond will be denoted as IR HD for simplicity. Apart from the description of C⋯H-D hydrogen bonds in these systems, an important topic will be the possible presence of other accompanying bonds as they can sometimes significantly influence the structure of a complex [ 80 ]. Values of some selected parameters characterizing the C⋯H-D hydrogen bond or the proton-donor molecule in the IR HD dimers are listed in Table 1 .…”

“…This concerns the important issue of the coexistence of various types of interactions in molecular systems. Recently, the author of the present paper has shown [ 80 ] that various types of secondary interactions have a significant effect on the mutual orientation of the ZnX molecular plane relative to the imidazol-2-ylidene ring in various types of IR carbenes. This result is important because the possible torsion of the planes is usually attributed to the steric effects resulting from the presence of bulky R substituents.…”

Endo‐ and exohedral complexes of superphane with cations

“…Thus, this finding may be in favor of the stabilizing H⋯H interactions between aliphatic chains 86–89 . For example, quite recently a similar NCI region has been found between ethylene groups in some carbene–ZnEt 2 complexes 90 containing a zinc bond 91 . However, probably for the first time such an area was shown in 2010 by Johnson et al in their article on the NCI method 84 …”