Molecular Rigidification of Cyclometalated N∧C∧N Pt(II)- and Pd(II)-Based Triplet Emitters

Abstract

In an effort to develop transition metal complexes with efficient phosphorescence, we synthesized cyclometalated complexes [M(N^∧C^∧N)Cl] (M = Pd or Pt) with amine groups NH(C₆H₅) at the two ortho-positions of the pyridyl rings (HL^NHPh) or NH(C₆H₅CH₂) (HL^NHBn) for intramolecular hydrogen bonding with the Cl^– coligand. Single-crystal X-ray diffractometry confirmed the hydrogen bonding and showed a distorted square planar geometry around the metal centers with τ₄ values of around 0.25 due to marked distortion of the Cl^– coligand from the coordination plane. Long-wavelength UV–vis absorption energies are increased for Pt vs Pd. The Pt(II) complexes are intense triplet emitters in solution at 298 K (λ_max = 520 nm) and at 77 K. The Pd(II) derivatives showed vibrationally structured phosphorescence bands in a frozen glassy matrix at 77 K resembling those of the Pt complexes but were nonemissive in liquid solutions at 298 K. The DFT-optimized electronic structures of the complexes show comparable geometries in their singlet ground state (S₀) and in the first triplet (T₁) excited states. Both are distorted by a combination of steric repulsion of the bulky ortho-RNH groups of the tridentate N^∧C^∧N ligands, the Cl^– coligand, and the N–H···Cl···H–N hydrogen bonds.