Photoluminescence of Ni(II), Pd(II), and Pt(II) Complexes [M(Medpb)Cl] Obtained from C‒H Activation of 1,5-Di(2-pyridyl)-2,4-dimethylbenzene (MedpbH)

The three complexes [M(Medpb)Cl] (M = Ni, Pd, Pt) containing the tridentate ,,-cyclometalating 3,5-dimethyl-1,5-dipyridyl-phenide ligand (Medpb) were synthesised using a base-assisted C‒H activation method. Oxidation potentials from cyclic voltammetry increased along the series Pt < Ni < Pd from 0.15 to 0.74 V. DFT calculations confirmed the essentially ligand-centred π*-type character of the lowest unoccupied molecular orbital (LUMO) for all three complexes in agreement with the invariant reduction processes. For the highest occupied molecular orbitals (HOMO), contributions from metal d, phenyl C4, C2, C1, and C6, and Cl p orbitals were found. As expected, the d (HOMO-1 for Ni) is stabilised for the Pd and Pt derivatives, while the antibonding d orbital is de-stabilised for Pt and Pd compared with Ni. The long-wavelength UV-vis absorption band energies increase along the series Ni < Pt < Pd. The lowest-energy TD-DFT-calculated state for the Ni complex has a pronounced d-type contribution to the overall metal-to-ligand charge transfer (MLCT) character. For Pt and Pd, the d orbital is energetically not available and a strongly mixed Cl-to-π*/phenyl-to-π*/M(d)-to-π* (XLCT/ILCT/MLCT) character is found. The complex [Pd(Medpb)Cl] showed a structured emission band in a frozen glassy matrix at 77 K, peaking at 468 nm with a quantum yield of almost unity as observed for the previously reported Pt derivative. No emission was observed from the Ni complex at 77 or 298 K. The TD-DFT-calculated states using the TPSSh functional were in excellent agreement with the observed absorption energies and also clearly assessed the nature of the so-called "dark", i.e., d‒d*, excited configurations to lie low for the Ni complex (≥3.18 eV), promoting rapid radiationless relaxation. For the Pd(II) and Pt(II) derivatives, the "dark" states are markedly higher in energy with ≥4.41 eV (Pd) and ≥4.86 eV (Pt), which is in perfect agreement with the similar photophysical behaviour of the two complexes at low temperatures.