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 1octyl-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-4pyridyl-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.