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

Dicationic, <i>C</i>₂-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-1<i>H</i>-1,2,3-triazol-5-ylidene (trz) ligands and one aromatic diimine [N∧N = 2,2'-bipyridine (bpy, <b>2</b>), 4,4'-di-<i>tert</i>-butyl-2,2'-bipyridine (dbbpy, <b>3</b>), 4,4'-dimethoxi-2,2'-bipyridine (dMeO-bpy, <b>4</b>), 1,10-phenanthroline (phen, <b>5</b>), 4,7-diphenyl-1,10-phenanthroline (bphen, <b>6</b>), dipyrido[3,2-<i>a</i>:2',3'-<i>c</i>]phenazine (dppz, <b>7</b>), or 2,3-diphenylpyrazino[2,3-<i>f</i>][1,10]phenanthroline (dpprzphen, <b>8</b>)] are obtained through chloride abstraction from [PtCl₂(trz)₂] (<b>1</b>) using AgOTf in the presence of the corresponding diimine. Complexes <b>2-4</b> 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 <b>3</b>. Derivatives with more extended aromatic systems show dual phosphorescent/fluorescent emissions (<b>5</b>, <b>6</b>) or mainly fluorescence (<b>7</b>, <b>8</b>) 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 <i>tert</i>-butyl groups in <b>3</b>, 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.