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 (Hbiim) 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(Hbiim)] 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.

Additional Insights into the Design of Cr(III) Phosphorescent Emitters Using 6-Membered Chelate Ring Bis(imidazolyl) Didentate Ligands.

The interest in Cr(III) complexes has been renewed over the past decades for building practical guidelines in the design of efficient earth-abundant phosphorescent near-infrared emitters. In that context, we report the first family of homoleptic tri(didentate) Cr(III) complexes [Cr] based on polyaromatic ligands inducing 6-membered chelate rings, namely, the bis(1-methylimidazol-2-yl)ketone ( = bik), bis(1-methylimidazol-2-yl)methane ( = bim), and bis(1-methylimidazol-2-yl)ethane ( = bie) ligands. The programmed close-to-perfect octahedral microsymmetry of {CrN} chromophores found in [Cr(bik)](OTf) (), [Cr(bim)](OTf) (), and [Cr(bie)](BF) () ensures a ligand-field strength large enough to induce intense and long-lived Cr-based phosphorescence.

Heteroleptic -[Cr(NNN)(CN)] complexes: synthetic challenge, structural characterization and photophysical properties.

The substitution of three water molecules around trivalent chromium in CrBr·6HO with the tridentate 2,2':6',2''-terpyridine (tpy), ,'-dimethyl-,'-di(pyridine-2-yl)pyridine-2,6-diamine (ddpd) or 2,6-di(quinolin-8-yl)pyridine (dqp) ligands gives the heteroleptic -[Cr(L)Br] complexes. Stepwise treatments with Ag(CFSO) and KCN under microwave irradiations provide -[Cr(L)(CN)] in moderate yields. According to their X-ray crystal structures, the associated six-coordinate meridional [CrNC] chromophores increasingly deviate from a pseudo-octahedral arrangement according to L = ddpd ≈ dpq ≪ tpy; a trend in line with the replacement of six-membered with five-membered chelate rings around Cr.