Brightly Luminescent Platinum Complexes of N∧C – ∧N Ligands Forming Six-Membered Chelate Rings: Offsetting Deleterious Ring Size Effects Using Site-Selective Benzannulation

Brightly emissive platinum­(II) complexes (λemission,max = 607–612 nm) of the type R LPtCl are reported, where R L is a cyclometalated N∧C–∧N-coordinating ligand derived from 1,3-di­(2-trifluoromethyl-4-phenanthridinyl)­benzene ( CF3 LH) or 1,3-di­(2-tert-butyl-4-phenanthridinyl)­benzene ( t Bu LH). Metathesis of the chlorido ligand can be achieved under mild conditions, enabling isolation of ionic compounds with the formula [ CF3 LPtL′]­PF6 where L′ = pyridine or (4-dimethylamino)­pyridine (DMAP), as well as the charge-neutral species t Bu LPt­(CCC6H4tBu) (CCC6H4tBu = 4-tert-butylphenyl­acetylido). Compared with N∧N∧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∧N∧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∧C–∧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 ( CF3 LPtCl3) is also reported, together with its molecular structure in the solid state.