Toward an Understanding of the Anion Effect in CpRu-Based Diels−Alder Catalysts via PGSE-NMR Measurements

The rate dependence of the [Ru(η5-C5H5)(BIPHOP-F)(acetone)][Y]-catalyzed Diels−Alder
reaction of cyclopentadiene with methacrolein on the anion, Y, is shown to be due to selective
ion pairing. Pulsed gradient spin−echo (PGSE) diffusion measurements on the model Cp
and indenyl complexes [Ru(η5-C5H5)(CH2CH-CN)(BIPHOP-F)][Y], Y = BF4 and BArF, and
[Ru(η5-C9H7)(CH2CH-CN)(BIPHOP-F)][Y], Y = BF4 and BArF, respectively, combined with
1H−19F HOESY NMR data can be used to understand how the ion pairing for the BF4 anion
differs relative to that of the BArF anion. Solid-state structures for [Ru(η5-C5H5)(CH2CH-CN)(BIPHOP-F)][BF4] and [Ru(η5-C5H5)(CH2CH-CN)(BIPHOP-F)][BArF] are reported and
support the NMR solution data. The model carbonyl complexes [Ru(η5-C5H5)(BIPHOP-F)(CO)][Y], 8 (Y = BF4, SbF6, and BArF) have been synthesized. The IR CO stretching
frequencies for 8 showed little variation with anion, thereby proving that the anions do not
affect the Lewis acidity of the salts.