Cowan Alexander J, Portius Peter, Kawanami Hajime K, Jina Omar S, Grills David C, Sun Xue-Zhong, McMaster Jonathan, George Michael W
School of Chemistry, University of Nottingham, University Park, Nottingham, United Kingdom.
Proc Natl Acad Sci U S A. 2007 Apr 24;104(17):6933-8. doi: 10.1073/pnas.0610567104. Epub 2007 Apr 4.
We have used fast time-resolved infrared spectroscopy to characterize a series of organometallic methane and ethane complexes in solution at room temperature: W(CO)5(CH4) and M(eta5-C5R5)(CO)2(L) [where M = Mn or Re, R = H or CH3 (Re only); and L = CH4 or C2H6]. In all cases, the methane complexes are found to be short-lived and significantly more reactive than the analogous n-heptane complexes. Re(Cp)(CO)2(CH4) and Re(Cp*)(CO)2(L) [Cp* = eta5-C5(CH3)(5) and L = CH4, C2H6] were found to be in rapid equilibrium with the alkyl hydride complexes. In the presence of CO, both alkane and alkyl hydride complexes decay at the same rate. We have used picosecond time-resolved infrared spectroscopy to directly monitor the photolysis of Re(Cp*)(CO)3 in scCH4 and demonstrated that the initially generated Re(Cp*)(CO)2(CH4) forms an equilibrium mixture of Re(Cp*)(CO)2(CH4)/Re(Cp*)(CO)2(CH3)H within the first few nanoseconds (tau = 2 ns). The ratio of alkane to alkyl hydride complexes varies in the order Re(Cp)(CO)2(C2H6):Re(Cp)(CO)2(C2H5)H > Re(Cp*)(CO)2(C2H6):Re(Cp*)(CO)2(C2H5)H approximately equal to Re(Cp)(CO)2(CH4):Re(Cp)(CO)2(CH3)H > Re(Cp*)(CO)2(CH4):Re(Cp*)(CO)2(CH3)H. Activation parameters for the reactions of the organometallic methane and ethane complexes with CO have been measured, and the DeltaH++ values represent lower limits for the CH4 binding enthalpies to the metal center of W-CH4 (30 kJ.mol(-1)), Mn-CH4 (39 kJ.mol(-1)), and Re-CH4 (51 kJ.mol(-1)) bonds in W(CO)5(CH4), Mn(Cp)(CO)2(CH4), and Re(Cp)(CO)2(CH4), respectively.
W(CO)5(CH4) 以及 M(η5-C5R5)(CO)2(L) [其中 M = Mn 或 Re，R = H 或 CH3（仅适用于 Re）；L = CH4 或 C2H6]。在所有情况下，发现甲烷配合物寿命较短，且比类似的正庚烷配合物活性显著更高。发现 Re(Cp)(CO)2(CH4) 和 Re(Cp*)(CO)2(L) [Cp* = η5-C5(CH3)(5) 且 L = CH4、C2H6] 与烷基氢化物配合物处于快速平衡状态。在 CO 存在下，烷烃配合物和烷基氢化物配合物均以相同速率衰减。我们利用皮秒时间分辨红外光谱直接监测了 Re(Cp*)(CO)3 在 scCH4 中的光解，并证明最初生成的 Re(Cp*)(CO)2(CH4) 在最初几纳秒内（τ = 2 ns）形成了 Re(Cp*)(CO)2(CH4)/Re(Cp*)(CO)2(CH3)H 的平衡混合物。烷烃配合物与烷基氢化物配合物的比例按以下顺序变化：Re(Cp)(CO)2(C2H6):Re(Cp)(CO)2(C2H5)H > Re(Cp*)(CO)2(C2H6):Re(Cp*)(CO)2(C2H5)H 约等于 Re(Cp)(CO)2(CH4):Re(Cp)(CO)2(CH3)H > Re(Cp*)(CO)2(CH4):Re(Cp*)(CO)2(CH3)H。已测量了有机金属甲烷和乙烷配合物与 CO 反应的活化参数，ΔH++ 值分别代表 W(CO)5(CH4)、Mn(Cp)(CO)2(CH4) 和 Re(Cp)(CO)2(CH4) 中 W-CH4（30 kJ·mol(-1)）、Mn-CH4（39 kJ·mol(-1)）和 Re-CH4（51 kJ·mol(-1)）键中 CH4 与金属中心结合焓的下限。
