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.