Neutral and cationic biimidazoledihalogenobis(trimethylphosphine)rhenium(iii) complexes: ion-pairing, acid–base and redox propertiesElectronic supplementary information (ESI) available: details of the structure determination and refinement for compounds 4, 5, 7 and 9·OPPh3. See http://www.rsc.org/suppdata/dt/b1/b103719f/

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.