Hashimoto Shuichi, Saitoh Masashi, Taira Nobuyuki, Schmidt Wolfgang, Hirai Katsuyuki, Tomioka Hideo
Department of Ecosystem Engineering, Graduate School of Engineering, The University of Tokushima, Tokushima-shi, Tokushima 770-8506, Japan.
J Phys Chem B. 2005 Nov 3;109(43):20407-14. doi: 10.1021/jp0548974.
We report an attempt to generate and characterize a triplet carbene, bis(2,4,6-trichlorophenyl)carbene ((3)1), in zeolites Y and L and in a molecular sieve VPI-5 in which a possible dimerization and reaction with the precursor of carbene are significantly retarded, thus making the triplet carbene longer lived than in solution at room temperature. The adsorption of a corresponding diazomethane (1-N(2)), the precursor of 1, was carefully examined by comparing the absorption spectrum after adsorption with that of 1-N(2) in n-pentane, which revealed that 1-N(2) was adsorbed with the diazo group intact only in VPI-5, while in other zeolites 1-N(2) was found to be decomposed upon adsorption. This difference in reactivity of the hosts was ascribed to the absence of Brønsted-acid sites in VPI-5. The photoirradiation of 1-N(2) in VPI-5 at 77 K was monitored by emission spectroscopy, which revealed that bis(2,4,6-trichlorophenyl)methyl radical (1-H) was produced as the only detectable species under these conditions. This is interpreted as indicating that nascent (3)1 may undergo efficient hydrogen abstraction as a result of multiple excitation by repeated refraction and reflection of the light in a light-scattering medium. In accord with this interpretation, the emission due to (3)1 was observed when irradiation was carried out on a translucent glassy sample prepared by submerging VPI-5 incorporating 1-N(2) in a refractive-index-matching fluid such as propylene glycol or glycerol. ESR signals ascribable to (3)1 were also observed under these conditions. Laser photolysis of 1-N(2) in VPI-5 at room temperature with fast detection of both emission and absorption showed that the bands due to 1-H were detected in the nanosecond time regime probably because of the extremely fast H abstraction by (3)1. However, a variable-temperature ESR study showed that the signals due to (3)1 survive up to 220 K in VPI-5 while the signals disappear at 120 K in 2-methyltetrahydrofuran, suggesting that triplet carbene is stabilized in VPI-5. Thus, a triplet carbene was generated and characterized in a zeolite for the first time and shown to be stabilized extrinsically. The present study also proposes a solution to the issues of acidic sites and multiple excitation often observed in zeolites.
我们报道了一项在Y型和L型沸石以及VPI - 5分子筛中生成并表征三重态卡宾双（2,4,6 - 三氯苯基）卡宾（(3)1）的尝试。在这些分子筛中，卡宾的可能二聚化以及与卡宾前体的反应显著延迟，从而使三重态卡宾在室温下比在溶液中寿命更长。通过比较吸附后的吸收光谱与正戊烷中1 - N(2)的吸收光谱，仔细研究了相应重氮甲烷（1 - N(2)），即(3)1的前体的吸附情况。结果表明，仅在VPI - 5中，1 - N(2)以完整的重氮基团被吸附，而在其他沸石中，发现1 - N(2)在吸附时会分解。主体的这种反应性差异归因于VPI - 5中不存在布朗斯台德酸位点。通过发射光谱监测了77 K下VPI - 5中1 - N(2)的光照射情况，结果表明在这些条件下，双（2,4,6 - 三氯苯基）甲基自由基（1 - H）是唯一可检测到的物种。这被解释为表明新生的(3)1可能由于光在光散射介质中的多次折射和反射而经历多次激发，从而有效地进行氢提取。与这种解释一致，当对浸没在诸如丙二醇或甘油等折射率匹配流体中的含有1 - N(2)的VPI - 5制备的半透明玻璃状样品进行照射时，观察到了归因于(3)1的发射。在这些条件下也观察到了归因于(3)1的电子自旋共振（ESR）信号。室温下对VPI - 5中1 - N(2)进行激光光解，并快速检测发射和吸收，结果表明在纳秒时间范围内检测到了归因于1 - H的谱带，这可能是由于(3)1极其快速的氢提取。然而，变温ESR研究表明，归因于(3)1的信号在VPI - 5中在高达220 K时仍然存在，而在2 - 甲基四氢呋喃中在120 K时信号消失，这表明三重态卡宾在VPI - 5中得到了稳定。因此，首次在沸石中生成并表征了三重态卡宾，并表明其在外部得到了稳定。本研究还提出了解决沸石中经常观察到的酸性位点和多次激发问题的方法。
