Department of Chemistry, Georgetown University, Box 51277-1227, Washington, D.C. 20057, United States.
The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States.
J Am Chem Soc. 2021 Oct 6;143(39):15960-15974. doi: 10.1021/jacs.1c04108. Epub 2021 Sep 21.
The life-sustaining reduction of N to NH is thermoneutral yet kinetically challenged by high-energy intermediates such as NH. Exploring intramolecular H-bonding as a potential strategy to stabilize diazene intermediates, we employ a series of TpCu complexes that exhibit H-bonding between pendant aromatic N-heterocycles (Het) such as pyridine and a bridging -NH ligand at copper(I) centers. X-ray crystallography and IR spectroscopy clearly reveal H-bonding in TpCu while low-temperature H NMR studies coupled with DFT analysis reveals a dynamic equilibrium between two closely related, symmetric H-bonded structural motifs. Importantly, the Het pendant negligibly influences the electronic structure of TpCu centers in TpCu(CNAr) complexes that lack H-bonding as judged by nearly indistinguishable ν(CN) frequencies (2113-2117 cm). Nonetheless, H-bonding in the corresponding TpCu complexes results in marked changes in ν(NN) (1398-1419 cm) revealed through resonance Raman studies. Due to the closely matched N-H BDEs of NH and the pyH cation radical, the aromatic N-heterocyclic pendants may encourage partial H-atom transfer (HAT) from NH to Het through redox-non-innocent H-bonding in TpCu. DFT studies reveal modest thermodynamic barriers for concerted transfer of both H-atoms of coordinated NH to the Het pendants to generate tautomeric TpCu complexes, identifying metal-assisted concerted dual HAT as a thermodynamically favorable pathway for N/NH interconversion.
将 N 维持生命的还原为 NH 是热中性的，但由于高能中间体如 NH 的存在，动力学上受到挑战。为了探索分子内氢键作为稳定重氮中间物的潜在策略，我们采用了一系列具有铜（I）中心桥接 -NH 配体的芳香族 N-杂环（Het）如吡啶和悬挂 N-杂环之间氢键的 TpCu 配合物。X 射线晶体学和 IR 光谱清楚地表明 TpCu 中存在氢键，而低温 H NMR 研究与 DFT 分析相结合表明，两种密切相关的、对称的氢键结构基序之间存在动态平衡。重要的是，Het 悬挂基团对缺乏氢键的 TpCu(CNAr) 配合物中 TpCu 中心的电子结构几乎没有影响，这可以从几乎相同的 ν(CN) 频率（2113-2117 cm）来判断。尽管如此，在相应的 TpCu 配合物中氢键的存在导致 ν(NN)（1398-1419 cm）通过共振拉曼研究明显变化。由于 NH 和 pyH 阳离子自由基的 N-H BDE 非常匹配，芳香族 N-杂环悬垂基团可能通过 TpCu 中的氧化还原非惰性氢键促进 NH 上的部分 H 原子转移（HAT）到 Het。DFT 研究表明，协同转移配位 NH 的两个 H 原子到 Het 悬垂基团以生成互变异构的 TpCu 配合物的热力学势垒适中，确定了金属辅助协同双重 HAT 是 N/NH 相互转化的热力学有利途径。
