Structural Investigation of the Anion Effect in Ruthenium(II) Arene Complexes Bearing Pyridyl-Triazole Ligands: Implications for Transfer Hydrogenation Catalysis

Cationic ruthenium(II) half-sandwich complexes bearing
1-octyl-4-pyridyl-1H-1,2,3-triazole ligands were
found to display strong ion-pairing,
the strength of which was dependent on the nature of the anionic counterion.
A detailed spectroscopic analysis revealed that the ion-pairing process
is also solvent-dependent. For the complex containing the 1-octyl-4-pyridyl-1H-1,2,3-triazole ligand and a tetraphenylborate counterion
(1a), the spectral data reveal that the complex forms
a collection of four ions (quadruple ions) in CHCl3. In
contrast, one set of contact ion-pairs (CIPs) was observed in MeCN
with the extent of ion-pairing being 88%. Analogous complexes in which
the tetraphenylborate counterion is replaced by a hexafluorophosphate
(2a) and tetrafluoroborate (3a) anion exist
as CIPs (92 and 90%) and solvent-separated ion-pairs (55 and 56%)
in CHCl3 and MeCN, respectively. Conductivity measurement
results indicate that the extent of ion-pairing for 1a, 2a, and 3a in 2-PrOH is greater in comparison
to the CIPs formed in CH2Cl2. Our results provide
a rationale for the reactivity differences observed for 1a and 2a in transfer hydrogenation catalysis.

携带1-辛基-4-吡啶基-1H-1,2,3-三唑配体的阳离子型钌(II)半三明治配合物被发现表现出强离子配对（ion-pairing）现象，其强度取决于阴离子抗衡离子的性质。详细的光谱分析表明，离子配对过程同样受溶剂影响。对于含有1-辛基-4-吡啶基-1H-1,2,3-三唑配体和四苯基硼酸根抗衡离子的配合物（1a），光谱数据显示该配合物在三氯甲烷（CHCl3）中形成四离子聚集体（四重离子）。与之相反，在乙腈（MeCN）中则观察到一组接触离子对（contact ion-pairs, CIPs），其离子配对程度为88%。将四苯基硼酸根抗衡离子替换为六氟磷酸根（2a）和四氟硼酸根（3a）的类似配合物，分别在三氯甲烷和乙腈中以接触离子对（占比92%和90%）以及溶剂分离离子对（占比55%和56%）的形式存在。电导测量结果表明，1a、2a和3a在异丙醇（2-PrOH）中的离子配对程度高于二氯甲烷（CH2Cl2）中形成的接触离子对。本研究结果为1a与2a在转移氢化催化（transfer hydrogenation catalysis）中表现出的反应活性差异提供了合理依据。