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

Ortiz, Robert J.; Braun, Jason D.; Williams, J.A. Gareth; Herbert, David E.

Abstract

Brightly emissive platinum(II) complexes (λemission,max = 607–612 nm) of the type RLPtCl are reported, where RL is a cyclometalated N∧C–∧N-coordinating ligand derived from 1,3-di(2-trifluoromethyl-4-phenanthridinyl)benzene (CF3LH) or 1,3-di(2-tert-butyl-4-phenanthridinyl)benzene (tBuLH). Metathesis of the chlorido ligand can be achieved under mild conditions, enabling isolation of ionic compounds with the formula [CF3LPtL′]PF6 where L′ = pyridine or (4-dimethylamino)pyridine (DMAP), as well as the charge-neutral species tBuLPt(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 (CF3LPtCl3) is also reported, together with its molecular structure in the solid state.

Citation

Ortiz, R. J., Braun, J. D., Williams, J. G., & Herbert, D. E. (2021). Brightly Luminescent Platinum Complexes of N∧C–∧N Ligands Forming Six-Membered Chelate Rings: Offsetting Deleterious Ring Size Effects Using Site-Selective Benzannulation. Inorganic Chemistry, 60(22), 16881–16894. https://doi.org/10.1021/acs.inorgchem.1c02551

• Journal Article Type: Article
• Acceptance Date: Oct 15, 2021
• Online Publication Date: Nov 3, 2021
• Publication Date: Nov 15, 2021
• Deposit Date: Nov 4, 2021
• Publicly Available Date: Nov 3, 2022
• Journal: Inorganic Chemistry
• Print ISSN: 0020-1669
• Electronic ISSN: 1520-510X
• Publisher: American Chemical Society
• Peer Reviewed: Peer Reviewed
• Volume: 60
• Issue: 22
• Pages: 16881–16894
• DOI: https://doi.org/10.1021/acs.inorgchem.1c02551
• Public URL: https://durham-repository.worktribe.com/output/1223239