Böhrer Hannes, Trapp Nils, Himmel Daniel, Schleep Mario, Krossing Ingo
Albert-Ludwigs-Universität Freiburg, Institut für Anorganische und Analytische Chemie and Freiburger Materialforschungszentrum (FMF), Albertstr. 21, 79104 Freiburg, Germany.
Dalton Trans. 2015 Apr 28;44(16):7489-99. doi: 10.1039/c4dt02822h.
The possibility of obtaining frustrated Lewis pairs (FLPs) suitable for H2-activation based on the Lewis acid B(Ohfip)3 1 (Ohfip = OC(H)(CF3)2) was investigated. In this context, the crystal structure of 1 as well as the crystal structure of the very weak adduct 1·NCMe was determined. When reacting solutions of 1 with H2 (1 bar) and selected phosphanes, amines, pyridines and N-heterocyclic carbenes, dihydrogen activation was never observed. Without H2, adduct formation with 1 was observed to be an equilibrium process, regardless of the Lewis base adduct. Thus, the thermodynamics of H2 activation of 1 in comparison with the well-known B(C6F5)3 was analyzed using DFT calculations in the gas phase and different solvents (CH2Cl2, ortho-difluorobenzene and acetonitrile). These investigations indicated that FLP chemistry based on 1 is considerably less favored than that with B(C6F5)3. This is in agreement with control NMR experiments indicating hydride transfer from H-B(Ohfip)3 upon reaction with B(C6F5)3, giving H-B(C6F5)3 and B(Ohfip)3 in toluene and also MeCN. Induced by these unsuccessful reactions, the Lewis acidity towards HSAB hard and soft ions was investigated for gaining a deeper insight. A unified reference system based on the trimethylsilyl compounds Me3Si-Y (Y = F, Cl, H, Me) and their respective ions Me3Si(+)/Y(-) calculated at the G3 level was chosen as the anchor point. The individual ion affinities were then assessed based on subsequent isodesmic reactions calculated at a much less expensive level (RI-)BP86/SV(P). This method was validated by systematic calculations of smaller reference systems at the frozen core CCSD(T) level with correlation effects extrapolated to a full quadruple-ζ basis. Overall, 33 common and frequently used Lewis acids were ranked with respect to their FIA, CIA, HIA and MIA (fluoride/chloride/hydride/methyl ion affinity).
研究了基于路易斯酸B(Ohfip)3 1（Ohfip = OC(H)(CF3)2）获得适用于H2活化的受阻路易斯对（FLP）的可能性。在此背景下，测定了1的晶体结构以及非常弱的加合物1·NCMe的晶体结构。当1的溶液与H2（1巴）以及选定的膦、胺、吡啶和N-杂环卡宾反应时，从未观察到二氢活化。在没有H2的情况下，观察到与1形成加合物是一个平衡过程，与路易斯碱加合物无关。因此，使用气相和不同溶剂（二氯甲烷、邻二氟苯和乙腈）中的密度泛函理论（DFT）计算分析了1与著名的B(C6F5)3相比的H2活化热力学。这些研究表明，基于1的FLP化学比基于B(C6F5)3的FLP化学受到的青睐要少得多。这与对照核磁共振实验一致，该实验表明在甲苯和乙腈中与B(C6F5)3反应时，H-B(Ohfip)3会发生氢化物转移，生成H-B(C6F5)3和B(Ohfip)3。受这些不成功反应的启发，研究了对HSAB硬离子和软离子的路易斯酸度，以获得更深入的了解。选择基于三甲基硅基化合物Me3Si-Y（Y = F、Cl、H、Me）及其各自离子Me3Si(+)/Y(-)在G3水平计算的统一参考体系作为锚点。然后基于在成本低得多的水平（RI-)BP86/SV(P)计算的后续等键反应评估各个离子亲和力。该方法通过在冻结核心CCSD(T)水平对较小参考体系进行系统计算并将相关效应外推到完整的四重ζ基组进行了验证。总体而言，对33种常见且常用的路易斯酸的氟离子亲和力（FIA）、氯离子亲和力（CIA）、氢离子亲和力（HIA）和甲基离子亲和力（MIA）进行了排名。
