Photoluminescence of Ni(II), Pd(II), and Pt(II) Complexes [M(Me2dpb)Cl] Obtained from C‒H Activation of 1,5-Di(2-pyridyl)-2,4-dimethylbenzene (Me2dpbH)

Lukas Kletsch; Rose Jordan; Alicia S Köcher; Stefan Buss; Cristian A Strassert; Axel Klein

doi:10.3390/molecules26165051

Photoluminescence of Ni(II), Pd(II), and Pt(II) Complexes [M(Me₂dpb)Cl] Obtained from C‒H Activation of 1,5-Di(2-pyridyl)-2,4-dimethylbenzene (Me₂dpbH)

Molecules. 2021 Aug 20;26(16):5051. doi: 10.3390/molecules26165051.

Authors

Lukas Kletsch¹, Rose Jordan¹, Alicia S Köcher¹, Stefan Buss^{2

3}, Cristian A Strassert^{2

3}, Axel Klein¹

Affiliations

¹ Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln, Greinstraße 6, D-50939 Köln, Germany.
² Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstr. 28/30, D-48149 Münster, Germany.
³ CeNTech, CiMIC, SoN, Westfälische Wilhelms-Universität Münster, Heisenbergstr. 11, D-48149 Münster, Germany.

Abstract

The three complexes [M(Me₂dpb)Cl] (M = Ni, Pd, Pt) containing the tridentate N,C,N-cyclometalating 3,5-dimethyl-1,5-dipyridyl-phenide ligand (Me₂dpb^-) 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_yz, phenyl C4, C2, C1, and C6, and Cl p_z orbitals were found. As expected, the d_z² (HOMO-1 for Ni) is stabilised for the Pd and Pt derivatives, while the antibonding d_x²_-y² 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_z²-type contribution to the overall metal-to-ligand charge transfer (MLCT) character. For Pt and Pd, the d_z² orbital is energetically not available and a strongly mixed Cl-to-π*/phenyl-to-π*/M(d_yz)-to-π* (XLCT/ILCT/MLCT) character is found. The complex [Pd(Me₂dpb)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.

Keywords: C‒H activation; cyclometalation; nickel; palladium; photoluminescence; platinum.

Abstract

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