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

Dicationic, -symmetrical, tris-chelate Pt(IV) complexes of general formula Pt(trz)(N∧N), bearing two cyclometalated 4-butyl-3-methyl-1-phenyl-1-1,2,3-triazol-5-ylidene (trz) ligands and one aromatic diimine [N∧N = 2,2'-bipyridine (bpy, ), 4,4'-di--butyl-2,2'-bipyridine (dbbpy, ), 4,4'-dimethoxi-2,2'-bipyridine (dMeO-bpy, ), 1,10-phenanthroline (phen, ), 4,7-diphenyl-1,10-phenanthroline (bphen, ), dipyrido[3,2-:2',3'-]phenazine (dppz, ), or 2,3-diphenylpyrazino[2,3-][1,10]phenanthroline (dpprzphen, )] are obtained through chloride abstraction from [PtCl(trz)] () using AgOTf in the presence of the corresponding diimine. Complexes show long-lived phosphorescence from LC 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 . Derivatives with more extended aromatic systems show dual phosphorescent/fluorescent emissions (, ) or mainly fluorescence (, ) in solution. Comparisons with similar complexes bearing cyclometalated 2-arylpyridines instead of aryl-N-heterocyclic carbenes indicate that the {Pt(trz)} 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 -butyl groups in , 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.