Stahl Nicholas G, Salata Michael R, Marks Tobin J
Department of Chemistry, Northwestern University, Evanston, IL 60208-3113, USA.
J Am Chem Soc. 2005 Aug 10;127(31):10898-909. doi: 10.1021/ja0429622.
The thermodynamic and structural characteristics of Al(C6F(5)3-derived vs B(C6F5)3-derived group 4 metallocenium ion pairs are quantified. Reaction of 1.0 equiv of B(C6F5)3 or 1.0 or 2.0 equiv of Al(C6F5)3 with rac-C2H4(eta5-Ind)2Zr(CH3)2 (rac-(EBI)Zr(CH3)2) yields rac-(EBI)Zr(CH3)(+)H3CB(C6)F5)(3)(-) (1a), rac-(EBI)Zr(CH3)+H3CAl(C6F5)(3)(-) (1b), and rac-(EBI)Zr2+H3CAl(C6F5)3(2) (1c), respectively. X-ray crystallographic analysis of 1b indicates the H3CAl(C6F5)(3)(-) anion coordinates to the metal center via a bridging methyl in a manner similar to B(C6F5)3-derived metallocenium ion pairs. However, the Zr-(CH3)(bridging) and Al-(CH3)(bridging) bond lengths of 1b (2.505(4) A and 2.026(4) A, respectively) indicate the methyl group is less completely abstracted in 1b than in typical B(C6F5)3-derived ion pairs. Ion pair formation enthalpies (DeltaH(ipf)) determined by isoperibol solution calorimetry in toluene from the neutral precursors are -21.9(6) kcal mol(-1) (1a), -14.0(15) kcal mol(-1) (1b), and -2.1(1) kcal mol(-1) (1b-->1c), indicating Al(C6F5)3 to have significantly less methide affinity than B(C6F5)3. Analogous experiments with Me2Si(eta5-Me4C5)(t-BuN)Ti(CH3)2 indicate a similar trend. Furthermore, kinetic parameters for ion pair epimerization by cocatalyst exchange (ce) and anion exchange (ae), determined by line-broadening in VT NMR spectra over the range 25-75 degrees C, are DeltaH++(ce) = 22(1) kcal mol(-1), DeltaS++(ce) = 8.2(4) eu, DeltaH++(ae) = 14(2) kcal mol(-1), and DeltaS++(ae) = -15(2) eu for 1a. Line broadening for 1b is not detectable until just below the temperature where decomposition becomes significant ( approximately 75-80 degrees C), but estimation of the activation parameters at 72 degrees C gives DeltaH++(ce) approximately 22 kcal mol(-1)and DeltaH++(ae) approximately 16 kcal mol(-1), consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.
对源自Al(C6F(5)3)和源自B(C6F5)3的第4族金属茂离子对的热力学和结构特征进行了量化。1.0当量的B(C6F5)3或1.0或2.0当量的Al(C6F5)3与rac-C2H4(η5-Ind)2Zr(CH3)2（rac-(EBI)Zr(CH3)2）反应，分别生成rac-(EBI)Zr(CH3)(+)H3CB(C6)F5)(3)(-)（1a）、rac-(EBI)Zr(CH3)+H3CAl(C6F5)(3)(-)（1b）和rac-(EBI)Zr2+H3CAl(C6F5)3(2)（1c）。1b的X射线晶体学分析表明，H3CAl(C6F5)(3)(-)阴离子通过桥连甲基以类似于源自B(C6F5)3的金属茂离子对的方式与金属中心配位。然而，1b的Zr-(CH3)(桥连)和Al-(CH3)(桥连)键长（分别为2.505(4) Å和2.026(4) Å）表明，1b中的甲基比典型的源自B(C6F5)3的离子对中甲基的离去程度更低。通过甲苯中的等压溶液量热法由中性前体测定的离子对形成焓（ΔH(ipf)）分别为-21.9(6) kcal mol(-1)（1a）、-14.0(15) kcal mol(-1)（1b）和-2.1(1) kcal mol(-1)（1b→1c），表明Al(C6F5)3的甲基亲和力明显低于B(C6F5)3。用Me2Si(η5-Me4C5)(t-BuN)Ti(CH3)2进行的类似实验表明了类似的趋势。此外，通过在25 - 75℃范围内的变温核磁共振谱线展宽测定的通过助催化剂交换（ce）和阴离子交换（ae）实现离子对差向异构化的动力学参数，对于1a为ΔH++(ce) = 22(1) kcal mol(-1)，ΔS++(ce) = 8.2(4) eu，ΔH++(ae) = 14(2) kcal mol(-1)，ΔS++(ae) = -15(2) eu。直到温度略低于分解变得显著的温度（约75 - 80℃）之前，1b的谱线展宽都无法检测到，但在72℃下对活化参数的估计给出ΔH++(ce)约为22 kcal mol(-1)和ΔH++(ae)约为16 kcal mol(-1)，这与桥连甲基与锆茂中心的结合比在1a中更强一致。
