Preparations, Characterizations, and Structures of (Biimidazole)dihalobis(triphenylphosphine)rhenium(III) Salts: Strong Ion-Pairing and Acid−Base Properties

Two-electron reduction occurs when the Re(V) precursors ReOX3(PPh3)2 and ReO(OEt)X2(PPh3)2 are reacted
with biimidazole (biimH2) in boiling chloroform, affording rhenium(III) cationic complexes of the type cis,trans-[ReX2(PPh3)2(biimH2)]X with X = Cl, Br, and I. Crystal structures are determined for the compounds with the
three halogens, as well as for the [ReCl2(PPh3)2(biimH2)](benzoate) salt. In all cases, the counterion is attached
to the complex cation via hydrogen bonding with the N−H groups of coordinated biimidazole. Variable-temperature
1H NMR spectroscopy shows that a mixture of [ReCl2(PPh3)2(biimH2)](benzoate) and [ReCl2(PPh3)2(biimH2)]Cl
is in slow exchange below −50 °C in CD2Cl2, indicating that ion pairing is retained in solution. Both N−H
groups can be deprotonated with sodium methoxide, and their acidities are evaluated from UV−visible spectra.
Competition between monodeprotonated [ReCl2(PPh3)2(biimH)] and various carboxylic acids reveals that the acidity
of the first N−H proton corresponds to that of acetic acid (pKa(aq) ∼ 4.8). By a similar competitive reaction
between bis-deprotonated [ReCl2(PPh3)2(biim)]- and phenols, the second acidity is estimated to be close to that
of phenol (pKa(aq) ∼ 9.8).