Synthesis, Luminescence, and Electrochemistry of Tris-Chelate Platinum(IV) Complexes with Cyclometalated N-Heterocyclic Carbene Ligands and Aromatic Diimines

Abstract

Dicationic, C₂-symmetrical, tris-chelate Pt(IV) complexes of general formula [Pt(trz)₂(N∧N)](OTf)₂, bearing two cyclometalated 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) ligands and one aromatic diimine [N∧N = 2,2′-bipyridine (bpy, 2), 4,4′-di-tert-butyl-2,2′-bipyridine (dbbpy, 3), 4,4′-dimethoxi-2,2′-bipyridine (dMeO-bpy, 4), 1,10-phenanthroline (phen, 5), 4,7-diphenyl-1,10-phenanthroline (bphen, 6), dipyrido[3,2-a:2′,3′-c]phenazine (dppz, 7), or 2,3-diphenylpyrazino[2,3-f][1,10]phenanthroline (dpprzphen, 8)] are obtained through chloride abstraction from [PtCl₂(trz)₂] (1) using AgOTf in the presence of the corresponding diimine. Complexes 2–4 show long-lived phosphorescence from ³LC excited states involving the diimine ligand, with quantum yields that reach 0.18 in solution and 0.58 in the solid matrix at room temperature for 3. Derivatives with more extended aromatic systems show dual phosphorescent/fluorescent emissions (5, 6) or mainly fluorescence (7, 8) in solution. Comparisons with similar complexes bearing cyclometalated 2-arylpyridines instead of aryl-N-heterocyclic carbenes indicate that the {Pt(trz)₂} subunit is crucial to enable efficient emissions from diimine-centered excited states. It is also shown that the introduction of protective bulky substituents on the diimine, such as the tert-butyl groups in 3, is a key strategy to reach higher emission efficiencies. The new compounds represent rare examples of luminescent Pt(IV) complexes that show quasi-reversible one-electron reductions, indicating an unusually high redox stability.