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

The rate dependence of the [Ru(η 5 -C 5 H 5 )(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( η 5 -C 5 H 5 )(CH 2 CH-CN)(BIPHOP-F)][Y], Y = BF 4 and BArF, and [Ru( η 5 -C 9 H 7 )(CH 2 CH-CN)(BIPHOP-F)][Y], Y = BF 4 and BArF, respectively, combined with 1 H− 19 F HOESY NMR data can be used to understand how the ion pairing for the BF 4 anion differs relative to that of the BArF anion. Solid-state structures for [Ru( η 5 -C 5 H 5 )(CH 2 CH-CN)(BIPHOP-F)][BF 4 ] and [Ru( η 5 -C 5 H 5 )(CH 2 CH-CN)(BIPHOP-F)][BArF] are reported and support the NMR solution data. The model carbonyl complexes [Ru( η 5 -C 5 H 5 )(BIPHOP-F)(CO)][Y], 8 (Y = BF 4, SbF 6, 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.