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

Cationic Re(III) complexes [ReX2(PMe3)(2)(biimH(2))]X (where biimH(2) = 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 H . . .N interactions. The acidities of the two N-H groups of [ReCl2(PMe3)(2)(biimH(2))](+) 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.