Emission spectroscopic investigation of triplet diarylcarbene generated in molecular sieve VPI-5.
Emission spectroscopic investigation of triplet diarylcarbene generated in molecular sieve VPI-5.
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DOI：
10.1021/jp0548974
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发表时间：
2005-10
期刊：
影响因子：
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通讯作者：
S. Hashimoto;M. Saitoh;Nobuyuki Taira;W. Schmidt;Katsuyuki Hirai;H. Tomioka
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文献类型：
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作者：
S. Hashimoto;M. Saitoh;Nobuyuki Taira;W. Schmidt;Katsuyuki Hirai;H. Tomioka
我们报道了在Y沸石和L分子筛以及在VPI-5分子筛中合成和表征了三重卡宾，二(2，4，6-三氯苯基)卡宾((3)1)，其中可能的二聚反应和与卡宾前体的反应被显著地抑制，从而使三重卡宾在室温下比在溶液中的寿命更长。通过比较吸附后的吸收光谱和1-N(2)在正戊烷中的吸附，考察了1的前体1-N(2)的吸附情况，发现1-N(2)在VPI-5分子筛上吸附时重氮基完整，而在其他沸石上1-N(2)在吸附后发生分解。这种宿主反应活性的差异归因于VPI-5中没有布朗斯特酸中心。用发射光谱监测了VPI-5中1-N(2)在77K下的光照射，发现在此条件下，只有二(2，4，6-三氯苯基)甲基自由基(1-H)能被检测到。这被解释为表明，由于光在光散射介质中的重复折射和反射的多次激发，新生的(3)1可以进行有效的氢提取。根据这一解释，当将含有1-N(2)的VPI-5浸入折射率匹配的流体如丙二醇或甘油中制备的半透明玻璃样品上进行照射时，观察到了(3)1的发射。在这些条件下也观察到了归属于(3)1的ESR信号。1-N(2)在VPI-5中的室温激光光解和发射和吸收的快速检测表明，可能是由于(3)1的极快的氢吸收，在纳秒范围内检测到了1-H的谱带。然而，变温ESR研究表明，(3)1的信号在VPI-5中存活到220K，而在2-甲基四氢呋喃中，信号在120K消失，这表明三重态卡宾在VPI-5中稳定存在。因此，首次在沸石中生成并表征了三线态卡宾，并证明了它是外在稳定的。本研究还提出了解决沸石中经常观察到的酸性中心和多重激发的问题。
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.