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

The substitution of three water molecules around trivalent chromium in CrBr(3)·6H(2)O 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)Br(3)] complexes. Stepwise treatments with Ag(CF(3)SO(3)) 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 [CrN(3)C(3)] 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(III). Room-temperature ligand-centered UV-excitation at 18 170 cm(−1) (λ(exc) = 350 nm), followed by energy transfer and intersystem crossing eventually yield microsecond metal-centered Cr((2)E → (4)A(2)) phosphorescence in the red to near infrared domain 13 150–12 650 cm(−1) (760 ≤ λ(em) ≤ 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.