Reaction of stable N-heterocyclic silylenes and germylenes with muonium.
Reaction of stable N-heterocyclic silylenes and germylenes with muonium.
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DOI：
10.1002/chem.200901281
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发表时间：
2009-08
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通讯作者：
Brett M. McCollum;J. Brodovitch;Jason A. C. Clyburne;A. Mitra;P. W. Percival;Adam C. Tomasik;R. West
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文献类型：
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作者：
Brett M. McCollum;J. Brodovitch;Jason A. C. Clyburne;A. Mitra;P. W. Percival;Adam C. Tomasik;R. West
卡宾、亚硅基和亚甲甲锗烷基 (R2E:, E= C, Si, Ge) 是含有具有六个价电子的中性双配位第 14 族原子的分子。由于其高反应性，这些物质在许多热和光化学反应中作为中间体发挥着关键作用，因此在合成化学中极其重要。 [1]研究卡宾、亚硅基和亚甲锗烷基与氢原子的反应会很有趣，但这会很困难。相反，我们使用μ (Mu=[μ+ eÀ]) 作为氢原子的类似物，并用μ 子自旋光谱来表征μ 化自由基产物。早期的一份报告描述了一些稳定的 N-杂环卡宾（类似于 1）与 μ 的反应性，[2]，最近我们报道了稳定的硅烯 2 与 μ 的反应。 [3]在后一种情况下，超精细耦合常数的惊人低值（235.49 MHz）使我们认为最初形成的自由基已与另一个亚硅基分子偶联，产生二硅烷基自由基。在这里，我们报道了硅烯 2 和 3 以及甲亚甲基 4 与锷的反应。尽管电子顺磁共振（EPR）研究已经报道了稳定的亚甲硅基和甲亚甲亚甲基与自由基的加合物，[4, 5]，目前的工作是第一个以缪子化的锗为中心的自由基的报告。根据已发表的程序合成的化合物2-4，[6]在四氢呋喃中进行了研究，在氮气气氛下制备，并在配有薄钢箔窗的不锈钢容器中无氧密封。光谱实验是在加拿大温哥华 TRIUMF 回旋加速器设施的 M20 μ 子束线上使用其他地方报道的设备和测量程序进行的。 [7]两种光谱技术被用来表征μ子化自由基：横向场μ子自旋旋转（TF-μSR）和μ子水平交叉共振（μLCR）。[8, 9]前一种技术可用于确定μ子化自由基的μ子超精细常数（hfc），后者可用于确定同一自由基中其他自旋活性核的超精细常数。在这两种技术中，光谱探针（正 μ 子）都是从自旋极化 μ 子束注入样品中。停在样品中的一小部分 μ 子可以拾取电子形成 μ 原子，然后与不饱和化合物反应形成 μ 化自由基。其他 μ 子（有机材料中通常占 65%）通过辐射分解结合到抗磁性分子中。这两个分数通过它们的自旋进动频率来区分，如图 1 所示的 TF-μSR 谱所示。与给定的 μ 原子基 Aμ 相关的 μ 子超精细耦合常数 (hfc) 很容易通过一对自由基峰的分离来确定，这对自由基峰出现在抗磁峰的两侧：Aμ= 235.39 (2)、154.87 (8) 和 650.18 (6) MHz分别为由 2、3 和 4 形成的自由基。
Carbenes, silylenes, and germylenes (R2E:, E= C, Si, Ge) are molecules that contain neutral dicoordinate Group14 atoms with six valence electrons. Due to their resulting high reactivity, these species play key roles as intermediates in numerous thermal and photochemical reactions, and hence are extremely important in synthetic chemistry.[1] It would be interesting to study the reactions of carbenes, silylenes, and germylenes with hydrogen atoms, but this would be difficult. Instead, we employ muonium (Mu=[μ+ eÀ]) as an analogue of the hydrogen atom, and characterize muoniated free-radical products with muon spin spectroscopy. An earlier report described the reactivity of some stable N-heterocyclic carbenes (similar to 1) with muonium,[2] and recently we reported the reaction of stable silylene 2 with muonium.[3] In the latter case, the surprisingly low value (235.49 MHz) for the hyperfine coupling constant led us to suggest that the initially formed radical had coupled with another molecule of silylene to yield a disilanyl radical. Here we report the reaction of silylenes 2 and 3 and germylene 4 with muonium. Although the electron paramagnetic resonance (EPR) studies have already been reported for adducts of stable silylenes and germylenes with free radicals,[4, 5] the present work is the first report of a muoniated germanium-centered radical.Compounds 2–4, synthesized according to published procedures,[6] were studied in tetrahydrofuran, prepared under a nitrogen atmosphere and sealed oxygen-free in stainless steel vessels fitted with a thin steel foil window. Spectroscopic experiments were performed at the M20 muon beam line of the TRIUMF cyclotron facility in Vancouver (Canada) using apparatus and measurement procedures reported elsewhere.[7] Two spectroscopic techniques were used to characterize the muoniated radicals: transverse field muon spin rotation (TF-μSR) and muon level-crossing resonance (μLCR).[8, 9] The former technique can be used to determine the muon hyperfine constant (hfc) of a muoniated free radical, and the latter can be used to determine hyperfine constants for other spin-active nuclei in the same free radical. In both techniques the spectroscopic probe (the positive muon) is injected into the sample from a beam of spinpolarized muons. A fraction of the muons stopping in the sample can pick up an electron to form muonium atoms, which can then react with unsaturated compounds to form muoniated radicals. Other muons (typically 65% in organic materials) become incorporated in diamagnetic molecules through radiolysis. The two fractions are distinguished by their spin precession frequencies, as shown in the TF-μSR spectra displayed in Figure 1. The muon hyperfine coupling constant (hfc) associated with a given muoniated radical, Aμ, is readily determined from the separation of the pair of radical peaks, which occur on either side of the diamagnetic peak: Aμ= 235.39 (2), 154.87 (8), and 650.18 (6) MHz for the radicals formed from 2, 3, and 4, respectively.