Direct observation of a transition metal alkane complex, CpRe(CO)2(cyclopentane), using NMR spectroscopy
Direct observation of a transition metal alkane complex, CpRe(CO)2(cyclopentane), using NMR spectroscopy
复制标题
DOI：
10.1021/ja981702f
复制
发表时间：
1998-09-30
影响因子：
15
通讯作者：
Ball, GE
中科院分区：
文献类型：
--
作者：
Geftakis, S;Ball, GE
实验上，存在的短寿命的烷烃络合物已被推断出CH活化过程与金属，如Rh 3和相关的氢同位素混乱实验与Ir.4烷烃配位后，CH键断裂步骤一般是非常迅速和不可逆的系统中的烷基氢化物是philically青睐的产品。然而，烷烃络合物相对于CH活化产物不稳定的情况并不总是如此。例如，W（CO）5-（烷烃）类型的配合物长期以来在基质分离相、溶液相和气相中就已为人所知。[5]在二氢取代反应的研究中也推测出了铱的烷烃络合物。到目前为止，红外光谱已被证明是这些研究中最有价值的工具。不幸的是，在所有的情况下，这些研究提供的信息很少的性质，这些弱相互作用与烷烃的键合或开/关率的烷烃。最近，乔治等人报道，CpRe（CO）2（正庚烷），光解产生的母体三羰基在庚烷中，具有最长的寿命测量的任何已知的烷烃复合物。7在所采用的条件下，在298 K下，在没有添加配体的情况下，衰变速率仅为40 s-1。这一观察促使我们进行这项调查，因为在180 K，假设合理的活化参数的衰变过程中，我们预计这种和相关物种的化学寿命是足够的NMR研究。首先，我们建立了一个装置，使样品在NMR光谱仪中和较低的温度下连续照射紫外光。使用2米长、单芯、外径1毫米的光纤电缆将广谱紫外/可见光直接传输到5毫米NMR管的内部。这连接到一个100瓦的汞弧灯（Oriel），配备了一个标准的光纤耦合设备，位于距离磁铁1.5米。将CpRe（CO）_3（~（14）× 10 ~（-3）M）的纯环戊烷过饱和溶液冷却到-80 ℃或更低温度，用该装置光解，在~ 1H NMR谱的Cp区δ 4.920处出现一个新的单峰，其代价是
Experimentally, the existence of short-lived alkane complexes has been inferred from detailed studies of CH activation processes with metals such as Rh3 and related hydrogen isotope scrambling experiments with Ir. 4 After alkane coordination, the CH bond-breaking step is generally very rapid and irreversible in systems where the alkyl hydride is the thermodynamically favored product. It is not always the case that the alkane complex is thermodynamically unstable with respect to the CH activation product, however. For example, complexes of the type W (CO) 5-(alkane) have long been known in matrix-isolated, solution, and gas phases. 5 Alkane complexes of iridium have also been inferred in studies of dihydrogen substitution reactions. 6 Until now, infrared spectroscopy has proven the most valuable tool in these investigations. Unfortunately, in all cases, these studies give scant information about the nature of these weak interactions with alkanes with little information about the bonding or the on/off rates of the alkane.Recently, George et al. reported that CpRe (CO) 2 (n-heptane), photolytically produced by irradiation of the parent tricarbonyl in heptane, has the longest lifetime measured of any known alkane complex. 7 Under the conditions employed, at 298 K, the decay rate in the absence of added ligand was only 40 s-1. This observation prompted us into this investigation since at∼ 180 K, assuming reasonable activation parameters for the decay process, we expected the chemical lifetime of this and related species to be sufficient for an NMR study. First, we built apparatus to allow a sample to be continuously irradiated with UV light while in the NMR spectrometer and at lower temperature. Broad-spectrum UV/visible light is delivered directly to the inside of the 5-mm NMR tube using a 2-mlong, single-core, 1-mm-outside diameter, fiber optic cable. This connects to a 100-W mercury arc lamp (Oriel) fitted with a standard fiber optic coupling device located∼ 1 m from the magnet. When a supersaturated solution of CpRe (CO) 3 (∼ 4× 10-3 M) in neat cyclopentane is cooled to-80 C or below and photolyzed using this equipment, a new singlet peak at δ 4.920 in the Cp region of the 1H NMR spectrum appears at the expense