Ali Muhammad Imran, Hussain Javid, Anwar Muhammad Usman, Al-Harrasi Ahmed, Naseer Muhammad Moazzam
Department of Chemistry, Quaid-i-Azam University Islamabad 45320 Pakistan
Department of Biological Sciences & Chemistry, College of Arts and Sciences, University of Nizwa Nizwa Oman.
RSC Adv. 2025 Jan 13;15(2):1152-1162. doi: 10.1039/d4ra08086f. eCollection 2025 Jan 9.
Noncovalent carbon bonding (C-bonding), a recently explored σ-hole interaction, has primarily been characterized through X-ray structural and computational studies. Evidence of C-bonds in solution is scarce, especially in highly polar solvents like DMSO where solvation effects typically overshadow weak non-covalent interactions. In this work, we present three novel spiroisatin-based -acyl hydrazones (1-3) in which C-bonds play a critical role in stabilizing the conformation in solution. Despite the steric preference for the NH-amide bond to adopt the geometry (H-N-C[double bond, length as m-dash]O ≈ 180°), H and C NMR spectra of compounds 1 and 2 in DMSO- reveal a rotameric mixture with a higher percentage of the conformation (82% and 76%, respectively), attributed to the stability provided by intramolecular C-bonding. Compound 3 also predominantly adopts the conformation in DMSO but to a lesser extent (60%) than compounds 1 and 2, due to competing intramolecular hydrogen bonding. Single-crystal X-ray analysis of compounds 1 and 2 confirmed the conformation, consistent with the solution-state preference. In contrast, compound 3 crystallized in the form, likely due to intramolecular hydrogen bonding and solid-state packing effects, which reinforce the steric preference for the geometry. Density functional theory (DFT) calculations corroborated the experimental data, predicting greater stability for the conformations in compounds 1, 2, and 3 in solution. The ability of intramolecular C-bonding to stabilize the conformation, even in highly polar solvents like DMSO, highlights the broader significance of this interaction in supramolecular chemistry and related fields.
非共价碳键合（C键合）是一种最近被探索的σ-空穴相互作用，主要通过X射线结构和计算研究进行表征。溶液中C键的证据很少，特别是在像二甲基亚砜（DMSO）这样的高极性溶剂中，溶剂化效应通常会掩盖弱的非共价相互作用。在这项工作中，我们展示了三种新型的基于螺异吲哚酮的酰腙（1-3），其中C键在稳定溶液中的构象方面起着关键作用。尽管NH-酰胺键在空间上倾向于采用顺式几何结构（H-N-C=O≈180°），但化合物1和2在DMSO-d6中的1H和13C NMR光谱显示出一种旋转异构体混合物，其中顺式构象的百分比更高（分别为82%和76%），这归因于分子内C键合提供的稳定性。化合物3在DMSO中也主要采用顺式构象，但程度比化合物1和2小（60%），这是由于分子内氢键的竞争。化合物1和2的单晶X射线分析证实了顺式构象，与溶液状态的偏好一致。相比之下，化合物3以反式形式结晶，可能是由于分子内氢键和固态堆积效应，这加强了对反式几何结构的空间偏好。密度泛函理论（DFT）计算证实了实验数据，预测化合物1、2和3在溶液中的顺式构象具有更高的稳定性。分子内C键合即使在像DMSO这样的高极性溶剂中也能稳定顺式构象的能力，突出了这种相互作用在超分子化学及相关领域的更广泛意义。
