Experimental and computational study of alkane dehydrogenation catalyzed by a carbazolide-based rhodium PNP pincer complex
Experimental and computational study of alkane dehydrogenation catalyzed by a carbazolide-based rhodium PNP pincer complex
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DOI：
10.1039/c5sc04794c
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发表时间：
2016-01-01
期刊：
影响因子：
8.4
通讯作者：
Brookhart, Maurice
中科院分区：
文献类型：
--
作者：
Bezier, David;Guan, Changjian;Brookhart, Maurice
研究了一种双膦咔唑钳形配体的铑配合物在烷烃脱氢反应中的催化性能，并与铱配合物进行了比较。发现（carb-PNP）RhH_2催化环辛烷/叔丁基乙烯（COA/TBE）转移脱氢，其转化频率高达10 min（-1），转化数高达340，与惰性Ir类似物形成鲜明对比. TONS受到催化剂分解的限制。通过机械、实验和计算（DFT）研究的组合，发现Rh和Ir类似物之间的差异可归因于Rh的14-电子（carb-PNP）M（I）片段的更大的可及性。相比之下，Ir更强烈地偏向于M（III）氧化态。因此，（carb-PNP）RhH 2而不是（carb-PNP）IrH 2可以通过牺牲氢受体，特别是TBE脱氢。发现（carb-PNP）Rh催化的COA/TBE转移脱氢循环的限速段是COA的脱氢。在这一段中，速率决定步骤被计算为（carb-PNP）Rh（环辛基）（H）经历β-H激动剂中间体的形成，而相反步骤（β-H激动剂相互作用的丧失）是受体TBE和乙烯的氢化的速率限制。这样的步骤以前没有被提出作为在烷烃脱氢的情况下的速率限制，据我们所知，也没有相反的步骤被提出作为烯烃氢化的速率限制。
A rhodium complex based on the bis-phosphine carbazolide pincer ligand was investigated in the context of alkane dehydrogenation and in comparison with its iridium analogue. (carb-PNP) RhH2 was found to catalyze cyclooctane/t-butylethylene (COA/TBE) transfer dehydrogenation with a turnover frequency up to 10 min(-1) and turnover numbers up to 340, in marked contrast with the inactive Ir analogue. TONs were limited by catalyst decomposition. Through a combination of mechanistic, experimental and computational (DFT) studies the difference between the Rh and Ir analogues was found to be attributable to the much greater accessibility of the 14-electron (carb-PNP) M(I) fragment in the case of Rh. In contrast, Ir is more strongly biased toward the M(III) oxidation state. Thus (carb-PNP)RhH2 but not (carb-PNP)IrH2 can be dehydrogenated by sacrificial hydrogen acceptors, particularly TBE. The rate-limiting segment of the (carb-PNP)Rh-catalyzed COA/TBE transfer dehydrogenation cycle is found to be the dehydrogenation of COA. Within this segment, the rate-determining step is calculated to be (carb-PNP) Rh(cyclooctyl)(H) undergoing formation of a beta-H agostic intermediate, while the reverse step (loss of a beta-H agostic interaction) is rate-limiting for hydrogenation of the acceptors TBE and ethylene. Such a step has not previously been proposed as rate-limiting in the context of alkane dehydrogenation, nor, to our knowledge, has the reverse step been proposed as rate-limiting for olefin hydrogenation.