Fulton John L, Linehan John C, Autrey Tom, Balasubramanian Mahalingam, Chen Yongsheng, Szymczak Nathaniel K
Fundamental Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
J Am Chem Soc. 2007 Oct 3;129(39):11936-49. doi: 10.1021/ja073331l. Epub 2007 Sep 7.
X-ray absorption fine structure (XAFS) is used to determine the structure of the rhodium cluster present during the catalyzed dehydrocoupling of amine boranes under operando conditions. We show how a variety of XAFS strategies can be used in combination with other analytical methods to differentiate homogeneous from heterogeneous systems. Analysis of the in situ XAFS spectra using a series of amine boranes (NH3BH3, R2NHBH3, and RNH2BH3 where R = methyl, isopropyl, tert-butyl, and cyclohexyl) and rhodium catalyst precursor compounds (including chloro-(1,5-cyclooctadiene)rhodium (I) dimer, bis(1,5-cyclooctadiene)rhodium (I) trifluoromethanesulfonate, chlorodicarbonylrhodium (I) dimer, dichloro(pentamethylcylcopentadienyl)rhodium (III) dimer, hexarhodium hexadecacarbonyl, and tetrarhodium dodecacarbonyl) strongly suggest that the active catalyst species for this reaction is a homogeneous rhodium complex. Rhodium clusters containing four or six rhodium atoms (Rh(4-6)) bound to amine boranes are observed as the major (>99%) rhodium containing species during and after the catalyzed anaerobic dehydrocoupling. During the later stages of the reaction a nonmetallic rhodium complex precipitates in which individual Rh(4-6) clusters likely form polymer chains ligated by the reaction products that have two or more ligating sites. The best fits of the XAFS data, using ab initio calculations of FEFF theory, show that the major rhodium species (80%) has each rhodium atom directly bound to three rhodium atoms with an observed bond distance of 2.73 A and to two boron atoms at 2.10 A. A minor (20%) rhodium species has each rhodium atom bound to four rhodium atoms with a bond distance of about 2.73 A and a single rhodium atom at a nonbonding distance of 3.88 A. No metallic rhodium was observed at any time during the anaerobic reaction.
X射线吸收精细结构（XAFS）用于在操作条件下确定胺硼烷催化脱氢偶联过程中存在的铑簇结构。我们展示了如何将多种XAFS策略与其他分析方法结合使用，以区分均相体系和非均相体系。使用一系列胺硼烷（NH3BH3、R2NHBH3和RNH2BH3，其中R = 甲基、异丙基、叔丁基和环己基）和铑催化剂前体化合物（包括氯（1,5 - 环辛二烯）铑（I）二聚体、双（1,5 - 环辛二烯）铑（I）三氟甲磺酸酯、氯二羰基铑（I）二聚体、二氯（五甲基环戊二烯基）铑（III）二聚体、六铑十六羰基化合物和四铑十二羰基化合物）对原位XAFS光谱进行分析，强烈表明该反应的活性催化剂物种是一种均相铑配合物。在催化厌氧脱氢偶联过程中和之后，观察到含有四个或六个与胺硼烷结合的铑原子（Rh(4 - 6)）的铑簇是主要的（>99%）含铑物种。在反应后期，一种非金属铑配合物沉淀出来，其中单个Rh(4 - 6)簇可能形成由具有两个或更多配位点的反应产物连接的聚合物链。使用FEFF理论的从头算计算对XAFS数据进行的最佳拟合表明，主要的铑物种（80%）中每个铑原子直接与三个铑原子键合，观察到的键长为2.73 Å，与两个硼原子键合，键长为2.10 Å。一种次要的（20%）铑物种中每个铑原子与四个铑原子键合，键长约为2.73 Å，与一个铑原子的非键合距离为3.88 Å。在厌氧反应的任何时候都未观察到金属铑。
