RHENIUM(V)-OXO COMPLEXES WITH IMIDAZOLES: PRIME CANDIDATES FOR DISORDER, SUPERSTRUCTURE AND COUNTER-ION SUBSTITUTION. Suzanne Bélanger and André L. Beauchamp, Département de chimie, Université de Montréal, C.P. 6128, Succ. centre-ville, Montréal, Québec, H3C 3J7, Canada
Rhenium(V) dioxo complexes show interesting optical properties and a potential for applications as radiopharmaceuticals. To understand these properties, compounds with various imidazoles were prepared. These compounds proved to be a prolific source of challenging crystallographic problems.
A trans octahedral species containing an axial O=Re=O unit and four equatorial imidazoles can possess a 4-fold axis, but symmetry is often lowered by the poorly scattering imidazoles adopting different orientations. In addition, instability generates ReO4- with liberation of protons. As a result, partial anion substitution by ReO4- and mixed [ReO2L4]+/[ReO(OH)L4]2+ phases are not uncommon.
These features lead to space group ambiguity, disorder and extremely unstable
refinement conditions. The correct structure is reached only after the unit
cell, Laue symmetry and systematic absences have been carefully checked.
Refinement in all possible space groups has to be considered and it must be
ascertained that disorder is not an artefact due crystal twinning. The examples
to be discussed include the oxo-bridged dimer
[Re2O3(BiimH2)4]Cl4, a
stoichiometric hemiprotonated phase
[ReO2(BimH)4][ReO(OH)(BimH)4](ReO4)
(BiimH2 = bi-imidazole, BimH = benzimidazole, 1-MeIm =
1-methylimidazole).