Brightly Luminescent Platinum Complexes of N Ligands Forming Six-Membered Chelate Rings: Offsetting Deleterious Ring...

Brightly emissive platinum(II) complexes (λ_emission,max = 607-612 nm) of the type LPtCl are reported, where L is a cyclometalated N-coordinating ligand derived from 1,3-di(2-trifluoromethyl-4-phenanthridinyl)benzene (LH) or 1,3-di(2-tert-butyl-4-phenanthridinyl)benzene (LH). Metathesis of the chlorido ligand can be achieved under mild conditions, enabling isolation of ionic compounds with the formula [LPtL']PF₆ where L' = pyridine or (4-dimethylamino)pyridine (DMAP), as well as the charge-neutral species LPt(C≡C─C₆H₄─tBu) (C≡C─C₆H₄─tBu = 4-tert-butylphenylacetylido). Compared with N-ligated Pt(II) complexes that form 5-membered chelates, these compounds all contain 6-membered rings. Expanding the chelate ring size from 5 to 6 has been previously demonstrated to enhance emission in some N-coordinated Pt(II) species─for example, in complexes of 2,6-di(8-quinolinyl)pyridine vs those of 2,2':6',2″-terpyridine─but in related N-coordinated species, luminescence quantum yields are significantly lower for the 6-membered chelate ring complexes. Here, we demonstrate that site-selective benzannulation of the quinolinyl side-arms can offset the deleterious effect of changing the chelate ring-size and boost photophysical properties such as the quantum yield. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations suggest that benzannulation counterintuitively destabilizes the emissive triplet states compared to the smaller π-system, with the "imine-bridged biphenyl" form of the phenanthridinyl arm helping to buffer against larger molecular distortions, enhancing photoluminescence quantum yields up to 0.09 ± 0.02. The spontaneous formation under aerated conditions of a Pt(IV) derivative (LPtCl₃) is also reported, together with its molecular structure in the solid state.