Dinuclear Cyclometalated Pincer Nickel(II) Complexes with Metal-Metal-to-Ligand Charge Transfer Excited States and Near-Infrared Emission.

Facile non-radiative decay of low-lying metal-centered (MC) dd excited states has been well documented to pose a significant obstacle to the development of phosphorescent Ni complexes due to substantial structural distortions between the dd excited state and the ground state. Herein, we prepared a series of dinuclear Ni complexes by using strong σ-donating carbene-phenyl-carbene (C C C) pincer ligands, and prepared their dinuclear Pt and Pd analogues. Dinuclear Ni complexes bridged by formamidinate/α-carbolinato ligand exhibit short Ni-Ni distances of 2.947-3.054 Å and singlet metal-metal-to-ligand charge transfer (MMLCT) transitions at 500-550 nm. Their MMLCT absorption energies are red-shifted relative to the Pt and Pd analogues at ~450 nm and ≤420 nm respectively. One-electron oxidation of these Ni complexes produces valence-trapped dinuclear Ni species, which are characterized by EPR spectroscopy. Upon photoexcitation, these Ni complexes display phosphorescence (τ=2.6-8.6 μs) in the NIR (800-1400 nm) spectral region in 2-MeTHF and in the solid state at 77 K, which is insensitive to π-conjugation of the coordinated [C C C] ligand. Combined with DFT calculations, the NIR emission is assigned to originate from the dd excited state. Studies have found that the dinuclear Ni complex can sensitize the formation of singlet oxygen and catalyze the oxidation of cyclo-dienes under light irradiation.