Neutral and cationic biimidazoledihalogenobis(trimethylphosphine)rhenium(iii) complexes: ion-pairing, acid–base and redox properties

Abstract

Cationic Re(III) complexes [ReX₂(PMe₃)₂(biimH₂)]X (where biimH₂ = 2,2′-biimidazole; X = Cl, Br, I) are prepared by phosphine displacement in the corresponding PPh₃ complexes in the presence of excess PMe₃. The N–H protons of co-ordinated biimidazole are moderately acidic and the monodeprotonated form [ReX₂(PMe₃)₂(biimH)] is isolated after addition of one equivalent of NaOCH₃. Crystal structures determined for two compounds of each series reveal that strong hydrogen bonding takes place between the N–H groups and the halide counter-ion in the cationic complexes, whereas the neutral molecules crystallise as dimeric units tightly associated via two complementary N–H⋯N interactions. The acidities of the two N–H groups of [ReCl₂(PMe₃)₂(biimH₂)]⁺ in CH₂Cl₂ are estimated from UV-visible spectra and found to correspond approximately to those of formic acid and 3-chlorophenol, respectively. Electrochemical data on the series of PPh₃ and PMe₃ complexes with the three halides indicate that the redox potentials of both the metal and the counter-ion are affected by ion pairing in solution.