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

Abstract

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.

Luminescent cyclometalated alkynylplatinum(ii) complexes with 1,3-di(pyrimidin-2-yl)benzene ligands: synthesis, electrochemistry, photophysics and computational studies

“…Rather similar frontier orbital characteristics were found for related chloro- and alkynyl-platinum( ii ) complexes bearing a N^C^N ligand. 66 The calculated ionization energies (IE; see Computational details) are gathered in Table 2. Their variation across the 1–14 series mainly reflects that of their HOMOs.…”

“…Following previous computational studies on related platinum complexes, 31,49,63,[65][66][67] we performed density functional theory (DFT) calculations at the PBE0/Def2-TZVP level were performed to optimize the geometries of chloro-and alkynyl-platinum complexes 1-14. Solvent (CH 2 Cl 2 ) effect corrections were included (see Computational details).…”

“…81 TD-DFT calculations were carried out at the same level of calculations, which has been shown to provide satisfactory results on related investigations. 31,66 Only singlet-singlet transitions were taken into account in these calculations. Only singlet-singlet transitions were taken into account in the TD-DFT calculations.…”

Donor–Acceptor Boron-Ketoiminate Complexes with Pendent N-Heterocyclic Arms: Switched-on Luminescence through N-Heterocycle Methylation

“…A comparably more minor shift is observed for the 1 H resonance attributed to the C H unit in the 6-position of phenanthridine, with an average upfield shift of ∼0.09 ppm. Our previous studies on coordination complexes supported by phenanthridine-based ligands typically saw larger shifts in the 1 H resonance of the C 6 H (using the IUPAC numbering highlighted in Scheme ) upon binding to Lewis acids such as transition metals, with the shift being dependent on the metal. , Indeed, Pt­(II) complexes of L3 and L4 exhibit a more pronounced downfield shift of the C 6 H resonance of ∼0.7 ppm . The 1 H NMR spectroscopy therefore implies that the phenanthridinyl unit does not associate significantly with the boron center in solution.…”

“…Our previous studies on coordination complexes supported by phenanthridine-based ligands typically saw larger shifts in the 1 H resonance of the C 6 H (using the IUPAC numbering highlighted in Scheme 1) upon binding to Lewis acids such as transition metals, with the shift being dependent on the metal. 27,28 ∼0.7 ppm. 25 The 1 H NMR spectroscopy therefore implies that the phenanthridinyl unit does not associate significantly with the boron center in solution.…”