Faculty of Chemistry, Nicolaus Copernicus University in Toruń, Gagarina 7, 87-100 Torun, Poland.
Int J Mol Sci. 2023 May 21;24(10):9057. doi: 10.3390/ijms24109057.
The subjects of the article are halogen bonds between either XCN or XCCH (X = Cl, Br, I) and the carbene carbon atom in imidazol-2-ylidene (I) or its derivatives (IR2) with experimentally significant and systematically increased R substituents at both nitrogen atoms: methyl = Me, -propyl = iPr, -butyl = tBu, phenyl = Ph, mesityl = Mes, 2,6-diisopropylphenyl = Dipp, 1-adamantyl = Ad. It is shown that the halogen bond strength increases in the order Cl < Br < I and the XCN molecule forms stronger complexes than XCCH. Of all the carbenes considered, IMes2 forms the strongest and also the shortest halogen bonds with an apogee for complex IMes2⋯ICN for which D0 = 18.71 kcal/mol and dC⋯I = 2.541 Å. In many cases, IDipp2 forms as strong halogen bonds as IMes2. Quite the opposite, although characterized by the greatest nucleophilicity, ItBu2 forms the weakest complexes (and the longest halogen bonds) if X ≠ Cl. While this finding can easily be attributed to the steric hindrance exerted by the highly branched -butyl groups, it appears that the presence of the four C-H⋯X hydrogen bonds may also be of importance here. Similar situation occurs in the case of complexes with IAd2.
本文的研究对象是 XCN 或 XCCH（X = Cl、Br、I）与咪唑-2-亚基（I）或其衍生物（IR2）中碳烯碳原子之间的卤素键，其中氮原子上的两个 R 取代基具有实验意义且系统地增加：甲基 = Me、-丙基 = iPr、-丁基 = tBu、苯基 = Ph、均三甲苯基 = Mes、2,6-二异丙基苯基 = Dipp、1-金刚烷基 = Ad。结果表明，卤素键的强度顺序为 Cl < Br < I，并且 XCN 分子形成的配合物比 XCCH 更强。在所考虑的所有碳烯中，IMes2 与卤素键形成最强且最短的配合物，其最高点为复合物 IMes2⋯ICN，其中 D0 = 18.71 kcal/mol，dC⋯I = 2.541 Å。在许多情况下，IDipp2 与 IMes2 形成同样强的卤素键。相反，尽管 ItBu2 具有最大的亲核性，但如果 X ≠ Cl，它形成的配合物（以及最长的卤素键）则最弱。虽然这种发现很容易归因于高度支化的叔丁基基团所施加的空间位阻，但四个 C-H⋯X 氢键的存在也可能在这里起到重要作用。在与 IAd2 形成的配合物中也存在类似的情况。
