A sandwich complex of trimesitylborane Mes3B: synthesis, characterization and anion binding properties of Mes2B[(η6-Mes)FeCp]+

Abstract

As part of our continuing interest in the chemistry of Lewis acidic organoboron compounds as anion receptors, we have synthesized the hexafluorophosphate salt of the cationic borane Mes₂B[(η⁶-Mes)FeCp]⁺ ([1]⁺) by reaction of [(η⁶-naphthalene)FeCp][PF₆] with Mes₃B. This compound, which undergoes two quasi-reversible reductions at E_1/2 = −1.56 V and −2.40 V, reacts with both fluoride and cyanide anions in THF or MeOH to afford the corresponding zwitterionic fluoro- (1-F) and cyano-borate (1-CN) species, respectively. Spectrophotometric titrations carried out in THF indicate that the fluoride and cyanide binding constants of [1]⁺ are much larger than those of Mes₃B. This difference points to the favorable inductive and coulombic effects imparted by the cationic [CpFe]⁺ moiety on the Lewis acidity of the boron center.