Luminescent Platinum(IV) Complexes Bearing Cyclometalated 1,2,3‐Triazolylidene and Bi‐ or Terdentate 2,6‐Diarylpyridine Ligands

Abstract

The synthesis, structure, and photophysical properties of luminescent PtIV complexes that combine cyclometalated 1,2,3‐triazolylidene and bi‐ or terdentate 2,6‐diarylpyridine ligands are reported. The targeted complexes represent the first examples of PtIV species with a cyclometalated mesoionic aryl‐NHC ligand. They exhibit moderate or weak emissions in fluid solution at 298 K arising from 3LC states, which become very intense in poly(methyl methacrylate) (PMMA) matrices at 298 K. DFT and TD‐DFT calculations confirm that the chromophoric ligand is the cyclometalated 2,6‐diarylpyridine and show that the aryl‐NHC ligand exerts a beneficial effect on the emission efficiencies of these derivatives by increasing the energy of deactivating LMCT excited states with respect to comparable PtIV complexes with cyclometalated 2‐arylpyridine ligands.

Strongly Luminescent Pt(IV) Complexes with a Mesoionic N-Heterocyclic Carbene Ligand: Tuning Their Photophysical Properties

“…Reasonably, the cationic Pt­(IV) intermediate complex isomerizes to avoid the trans -arrangement of the tolyl and triazolylidene groups, which explains the low yield in 5d . A similar result was previously found upon the oxidation of an analogous complex bearing monocyclometalating 2,6-di­( p -tolyl)­pyridine with PhICl 2 …”

“…A similar result was previously found upon the oxidation of an analogous complex bearing monocyclometalating 2,6-di(p-tolyl)pyridine with PhICl 2 . 64 The crystal structures of 5a and 5d (Figures 4 and 5, respectively) confirmed the expected ligand arrangement. In both cases, the Pt−C* bond is nearly 0.1 Å longer than those in complexes 3 because of the high trans-influence exerted by the metalated aryl ring of the C ∧ N ligand.…”

“…Scheme shows the synthetic route to the targeted bis-cyclometalated complexes ( OC -6-54)-[PtCl 2 (C ∧ N)­(trz)] ( 3a – e ), where trz = cyclometalating 4-butyl-3-methyl-1-phenyl-1 H -1,2,3-triazol-5-ylidene and C ∧ N = cyclometalating 1-phenylpyrazole (ppz, a ), 2-(2,4-difluorophenyl)­pyridine (dfppy, b ), 2-phenylpyridine (ppy, c ), 2-( p -tolyl)­pyridine (tpy, d ), or 2-(2-thienyl)­pyridine (thpy, e ), based on the previously described methodology for ( OC -6-54)-[PtCl 2 (dtpyH)­(trz)] (dtpyH = monocyclometalating 2,6-di­( p -tolyl)­pyridine) . The reaction of dichlorido-bridged dimers [Pt 2 (μ-Cl) 2 (C ∧ N) 2 ] ( 1a – e ) with the in situ-generated silver carbene “AgI­(trzH)” led to the selective formation of complexes trans-C,C* -[PtCl­(C ∧ N)­(trzH)] ( trans-C,C* - 2a – e ), where the carbene coordinates in a trans -configuration to the metalated aryl of the C ∧ N ligand.…”

“…We have recently developed a synthetic method that allowed the preparation of the first examples of Pt(IV) complexes bearing a cyclometalated aryl-NHC ligand. 64 The reported complexes combined a mesoionic carbene of the 1,2,3triazolylidene subclass (C ∧ C*) and either a monocyclometalating 2,6-diarylpyridine or a dicyclometalating 2,6-diarylpyridine (C ∧ N ∧ CH or C ∧ N ∧ C, respectively) and were found to display exceptionally intense phosphorescence in poly(methyl methacrylate) (PMMA) matrices at 298 K, which originated from a 3 LC state involving the C ∧ N ∧ CH or C ∧ N ∧ C ligand. In addition, the complex with a monocyclometalating C ∧ N ∧ CH ligand showed an intense luminescence in a fluid solution, which was marginally enhanced compared to that of similar bis-cyclometalated Pt(IV) complexes containing only C ∧ N ligands.…”

The Exchange of Cyclometalated Ligands

“…The overall photochemical 8-5 ⟶ 8-9 transformation in Scheme 24 is identical to the exchange of cyclopalladated ligands listed in Table 1. A carbene complex like 8-7 was also reported for a 1,2,3-triazolylidene derivative [74].…”