Boosting the Metathesis Activity of Molybdenum Oxo Alkylidenes by Tuning the Anionic Ligand σ Donation

The catalytic performances of molecular
and silica-supported molybdenum
oxo alkylidene species bearing anionic O ligands [ORF9,
OTPP, OHMT – where ORF9 = OC­(CF3)3, OTPP = 2,3,5,6-tetraphenylphenoxy, OHMT = hexamethylterphenoxy]
with different σ-donation abilities and sizes are evaluated
in the metathesis of both internal and terminal olefins. Here, we
show that the presence of the anionic nonafluoro-tert-butoxy X ligand in Mo­(O)­{CH-4-(MeO)­C6H4}­(THF)2{X}2 (1; X = ORF9) significantly increases the catalytic performances in the metathesis
of both terminal and internal olefins. Its silica-supported equivalent
displays slightly lower activity, albeit with improved stability.
In sharp contrast, the molecular complexes with large aryloxy anionic
X ligands show little activity, whereas the activity of the corresponding
silica-supported systems is greatly improved, illustrating that surface
siloxy groups are significantly smaller anionic ligands. Of all of
the systems, compound 1 stands out because of its unique
high activity for both terminal and internal olefins. Density functional
theory modeling indicates that the ORF9 ligand is ideal
in this series because of its weak σ-donating ability, avoiding
overstabilization of the metallacyclobutane intermediates while keeping
low barriers for [2 + 2] cycloaddition and turnstile isomerization.

我们针对带有不同σ给电子能力与空间尺寸的阴离子氧配体[ORF9、OTPP、OHMT——其中ORF9=OC(CF₃)₃，OTPP=2,3,5,6-四苯氧基，OHMT=六甲基三联苯氧基]的分子型与二氧化硅负载氧代亚烷基钼物种，开展了其在内烯烃与端烯烃的复分解反应中催化性能的评价研究。
研究发现，在配合物Mo(O){=CH-4-(MeO)C₆H₄}(四氢呋喃(THF))₂{X}₂（1；X=ORF9）中，阴离子全氟叔丁氧基X配体的存在可显著提升该催化剂在内烯烃与端烯烃的复分解反应中的催化性能。其二氧化硅负载对应体系的催化活性略低，但稳定性有所提升。
与之形成鲜明对比的是，带有大体积芳氧基阴离子X配体的分子型配合物几乎无催化活性；而相应的二氧化硅负载体系的活性则大幅提升，这表明表面硅氧基是尺寸显著更小的阴离子配体。
在所考察的所有体系中，化合物1因对内烯烃与端烯烃均具有独特的高活性而脱颖而出。
密度泛函理论（Density Functional Theory, DFT）建模结果表明，ORF9配体在该系列配体中表现最优：其σ给电子能力较弱，既可避免金属环丁烷（metallacyclobutane）中间体过度稳定，又能维持[2+2]环加成与旋转异构化（turnstile isomerization）的能垒处于较低水平。