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

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 53; no. 38; pp. 15801 - 15814
• Main Authors: Chong, Julien, Benchohra, Amina, Besnard, Céline, Guénée, Laure, Rosspeintner, Arnulf, Cruz, Carlos M, Jiménez, Juan-Ramón, Piguet, Claude
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 01.10.2024; The Royal Society of Chemistry
• Subjects: Chelation; Chemistry; Chromophores; Hydrogen bonds; Ligands; Phosphorescence; Solid state; Trivalent chromium
• ISSN: 1477-9226; 1477-9234; 1477-9234
• DOI: 10.1039/d4dt01608d

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 (H biim) 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(H biim) ] 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 [CrN ] units are found to be intermolecularly connected peripheral N-H⋯X hydrogen bonds. Moreover, the associated high-energy stretching N-H vibrations drastically quench the targeted near infrared (NIR) Cr -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) (H biim)] ( = 2-0) complexes overcomes the latter limitations and allows (i) the synthesis and characterization of these [CrN ] 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.