Optical, spectroscopic, and electrochemical properties of cyclometalated complexes of platinum metals based on 2-tolylpyridine and benzo[h]quinoline in ethylenediamine

Cyclometalated [M(C∧N)En]PF₆ (M = Pd(II), Pt(II)) and [M(C∧N)₂En]PF₆ (M = Rh(III), Ir(III)) complexes ((C∧N)^? corresponds to the deprotonated forms of 2-tolylpyridine and benzo[h]quinoline, and En is ethylenediamine) are studied by ¹H NMR spectroscopy, electronic absorption and emission spectroscopy, and voltammetry. Metalation of heterocyclic ligands leads to the formation of five-membered {M(C∧N)} cycles in the composition of square planar and octahedral complexes of the cis-C,C structure. Correlation of the energy positions of the long-wavelength metal-to-ligand charge-transfer absorption bands with the difference between the potentials of one-electron waves of metal-centered oxidation and ligand-centered reduction of complexes is shown. The phosphorescence of the complexes in the visible region of 469–524 nm is attributed to the radiative transition from the metal-modified intraligand excited state. The temperature quenching of the phosphorescence of complexes is attributed to the thermally activated population of metal-centered electronically excited states with subsequent nonradiative deactivation.