Charge Distribution in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: A Combined X-ray, XAS, XES, DFT, Mossbauer, and Catalysis Approach
Charge Distribution in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: A Combined X-ray, XAS, XES, DFT, Mossbauer, and Catalysis Approach
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DOI：
10.1021/acscatal.0c03978
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发表时间：
2020-12-18
期刊：
影响因子：
12.9
通讯作者：
Buchmeiser, Michael R.
中科院分区：
文献类型：
--
作者：
Benedikter, Mathis;Musso, Janis;Buchmeiser, Michael R.
研究了不同NHC的阳离子钼亚氨基亚烷基NHC单（壬基-叔丁氧基）络合物中N-杂环卡宾（NHC）和金属之间的电荷离域，即，1，3-二均三甲苯基咪唑-2-亚基（IMes）、1，3-二均三甲苯基-4，5-二氯咪唑-2-亚基（IMesCl（2））、1，3-二均三甲苯基-4，5-二甲基咪唑-2-亚基（IMesMe（2））和1，3-二均三甲苯基-4，5-二氢咪唑-2-亚基（IMesH（2））。在相应的阳离子配合物Mo中的结合情况（N-2，6-Me2C6H3）（CHCMe2Ph）（NHC）（OC（CF3）（3））（+）B（Ar-F）（4）-（NHC = IMes（1）、IMesCl（2）（2）、IMesMe（2）（3）和IMesH（2）（4））与类似的中性Schrock催化剂Mo（N-2，6-Me2C6H3）（CHCMe2Ph）（（OC（CF3）（3）（2）（5）.单晶X射线数据被用作在PBE 0-D3 BJ/def 2-SVP理论水平上优化催化剂几何形状的起点;将获得的数据与从X射线吸收（XAS）和发射光谱（XES）获得的数据进行比较。催化剂1、2和5的XANES（X射线吸收近边结构）和K β-XES的非常相似的X射线光谱特征表明在所有三种情况下在Mo中心存在相似的氧化态和电荷。然而，与5相比，1和2中的电荷离域更明显。这得到了量子化学（QC）计算的支持，该计算表明，所有NHC对钼上的阳离子电荷的补偿程度非常相似，导致Mo上的电荷模型5（CM 5）部分电荷在+1.292和+1.298之间。因此，NHC中的部分电荷在+0.486至+0.515的范围内。阳离子钼亚氨基亚烷基NHC（壬基-叔丁氧基）络合物中正电荷的这种强离域也通过以下发现来说明：类似的中性Schrock催化剂5尽管是中性14电子络合物，但在钼（+1.435）处具有更多的正电荷。配合物1和2以及含乙腈的衍生物1中心点MeCN和2中心点MeCN的电荷分析显示，在乙腈上发现约+0.1的小部分正电荷，伴随着Mo上正电荷的增加。因此，在亚氨基、醇盐和NHC配体处的部分电荷略微降低。最后，在一组烯烃复分解反应中测定了配合物1-4对许多纯烃基底物的催化活性。发现基于咪唑-2-亚基的配合物1-3的Tolman电子参数（TEP）与催化剂活性（表示为3 min后的转换频率，TOF 3 min）的相关性。Fe ~（57）穆斯堡尔谱测量了Mo（N-2，6-Me_2C_6H_3）（CH-二茂铁基）（NHC）（OTf）（2）和Mo（N-2，6-Me_2C_6H_3）（CH-二茂铁基）（NHC）（OTf）（+）B（Ar-F）（4）（-）（NHC = IMes（6，8）和IMesH（2）（7，9））的四极分裂。这些表明与含有IMesH（2）配体的相同催化剂相比，阳离子钼中心和基于咪唑-2-亚基的NHC之间的电荷分布显著更有效。
The charge delocalization between the N-heterocyclic carbene (NHC) and the metal in cationic molybdenum imido alkylidene NHC mono(nonafluoro-tert-butoxide) complexes has been studied for different NHCs, i.e., 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene (IMesCl(2)), 1,3-dimesityl-4,5-dimethylimidazol-2-ylidene (IMesMe(2)), and 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (IMesH(2)). The binding situation in the corresponding cationic complexes Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(NHC)(OC(CF3)(3))(+) B(Ar-F)(4) - (NHC = IMes (1), IMesCl(2) (2), IMesMe(2) (3), and IMesH(2) (4) was compared to that of the analogous neutral Schrock catalyst Mo(N-2,6-Me2C6H3)(CHCMe2Ph)((OC(CF3)(3)))(2) (5). Single-crystal X-ray data were used as a starting point for the optimization of the geometries of the catalysts at the PBE0-D3BJ/def2-SVP level of theory; the obtained data were compared to those obtained from X-ray absorption (XAS) and emission spectroscopy (XES). The very similar X-ray spectroscopic signatures of the XANES (X-ray absorption near-edge structure) and K beta-XES of catalysts 1, 2, and 5 suggest that a similar oxidation state and charge are present at the Mo center in all three cases. However, charge delocalization is more pronounced in 1 and 2 compared to 5. This is supported by quantum chemical (QC) calculations, which reveal that all NHCs compensate to a very similar extent for the cationic charge at molybdenum, leading to charge model 5 (CM5) partial charges at Mo between +1.292 and +1.298. Accordingly, the partial charge in the NHCs was in the range of +0.486 to +0.515. This strong delocalization of the positive charge in cationic molybdenum imido alkylidene NHC (nonafluoro-tert-butoxide) complexes is also illustrated by the finding that the analogous neutral Schrock catalyst 5 has a more positive charge at molybdenum (+1.435) despite being a neutral 14-electron complex. Complementarily, charge analysis on complexes 1 and 2 and the acetonitrile-containing derivatives 1 center dot MeCN and 2 center dot MeCN revealed that a small partial positive charge of about +0.1 was found on acetonitrile, accompanied by an increase in positive charge on Mo. Accordingly, the partial charges at the imido, the alkoxide, and NHC ligands decreased slightly. Finally, the catalytic activity of complexes 1-4 was determined for a number of purely hydrocarbon-based substrates in a set of olefin metathesis reactions. A correlation of the Tolman electronic parameter (TEP) with catalyst activity, expressed as the turnover frequency after 3 min, TOF3min, was found for complexes 1-3 based on imidazol-2-ylidenes. Fe-57-Mossbauer measurements on Mo(N-2,6-Me2C6H3)(CH-ferrocenyl)(NHC)(OTf)(2) and Mo(N-2,6-Me2C6H3)(CH-ferrocenyl)(NHC)(OTf)(+) B(Ar-F)(4)(-) (NHC = IMes (6, 8) and IMesH(2) (7, 9)) revealed significant changes in the quadrupole splitting of these complexes. These suggest a significantly more efficient charge distribution between the cationic molybdenum center and an imidazol-2-ylidene-based NHC compared to the same catalysts containing the IMesH(2) ligand.