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 solid-state 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) Cr^III-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(Hbiim)]⁽¹⁺ (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.