Infrared evidence for competitive [Li +] and Et 3B coordination to [Fe(CO) 4CHO −] and thermodynamic evaluation of the ion pair equilibrium

Abstract

The reaction betweem Fe(CO) 5 and [Et 3BH][Li], Super-Hydride, has been examined in THF over the temperature range −80°C to room temperature by variable-temperature FT-IR spectroscopy. FT-IR studies reveal that the anionic formyl complex, [Fe(CO) 4CHO −], exists in up to three chemically distinct contact ion pairs, one of which involves the termolecular formyl oxygen-BEt 3-lithium contact ion pair [Fe(CO) 4CHOBEt 3][Li]. From −80°C to −55°C, [Fe(CO) 4CHOBEt 3][Li] is shown to be in equilibrium with [Fe(CO) 4CHOBEt 3 −], while from −55°C to room temperature, the dominant equilibrium involves the conversion of [Fe(CO) 4CHOBEt 3 −] into the known contact ion pair [Fe(CO) 4CHO][Li]. The temperature-dependent equilibrium constant associated with the different ion pairs has been determined by IR Band-shape analysis and the values for Δ H and Δ S are repoted. Proof for the proposed multiple ion pair equilibria between [Fe(CO) 4CHO − and Et 3B and [Li + is substantiated by the use of the hydride reagent LS-Selectride. Here, ion pairing is selective for [Fe(CO) 4CHO][Li] from −80°C to room temperature as a result of trisiamylborane's inability to coordinate to the formyl oxygen