Heteroleptic mer-[Cr(N∩N∩N)(CN)3] complexes: synthetic challenge, structural characterization and photophysical properties

Abstract

The substitution of three water molecules around trivalent chromium in CrBr3·6H2O with the tridentate 2,2′:6′,2′′-terpyridine (tpy), N,N′-dimethyl-N,N′-di(pyridine-2-yl)pyridine-2,6-diamine (ddpd) or 2,6-di(quinolin-8-yl)pyridine (dqp) ligands gives the heteroleptic mer-[Cr(L)Br3] complexes. Stepwise treatments with Ag(CF3SO3) and KCN under microwave irradiations provide mer-[Cr(L)(CN)3] in moderate yields. According to their X-ray crystal structures, the associated six-coordinate meridional [CrN3C3] 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 CrIII. Room-temperature ligand-centered UV-excitation at 18 170 cm−1 (λexc = 350 nm), followed by energy transfer and intersystem crossing eventually yield microsecond metal-centered Cr(2E → 4A2) phosphorescence in the red to near infrared domain 13 150-12 650 cm−1 (760 ≤ λem ≤ 790 nm). Decreasing the temperature to liquid nitrogen (77 K) extends the emission lifetimes to reach the millisecond regime with a record of 4.02 ms for mer-[Cr(dqp)(CN)3] in frozen acetonitrile.