Taming 2,2'-biimidazole ligands in trivalent chromium complexes

Complete or partial replacement of well-known five-membered chelating 2,2’-bipyridine (bipy) or 1,10-
phenanthroline (phen) ligands with analogous didentate 2,2’-biimidazole (H2biim) provides novel perspectives
for exploiting the latter pH-tuneable bridging unit for connecting inert trivalent chromium with
cationic partners. The most simple homoleptic complex [Cr(H2biim)3]3+ and its stepwise deprotonated
analogues are only poorly soluble in most solvents and their characterization is limited to some solidstate
structures, in which the pseudo-octahedral [CrN6] units are found to be intermolecularly connected
via peripheral N–H⋯X hydrogen bonds. Moreover, the associated high-energy stretching N–H vibrations
drastically quench the targeted near infrared (NIR) CrIII-based phosphorescence, which makes these
homoleptic building blocks incompatible with the design of molecular-based luminescent assemblies.
Restricting the number of bound 2,2’-biimidazole ligands to a single unit in the challenging heteroleptic
[Cr(phen)2(Hxbiim)](1+x)+ (x = 2–0) complexes overcomes the latter limitations and allows (i) the synthesis
and characterization of these [CrN6] chromophores in the solid state and in solution, (ii) the stepwise and
controlled deprotonation of the bound 2,2’-biimidazole ligand and (iii) the implementation of Cr-centered
phosphorescence with energies, lifetimes and quantum yields adapted for using the latter chromophores
as sensitizers in promising ‘complex-as-ligand’ strategies.