Half-Lantern Pt(II) and Pt(III) Complexes. New Cyclometalated Platinum Derivatives

Abstract

The divalent complex [{Pt(bzq)(μ-L)} 2 ] (1) [Hbzq = benzo[h]quinolone, HL = CF 3 C 4 H 2 N 2 SH: 4-(trifluoromethyl)pyrimidine-2-thiol] was obtained from equimolar amounts of [Pt(bzq)(NCMe) 2 ]ClO 4 and 4-(trifluoromethyl)pyrimidine-2-thiol with an excess of NEt 3. The presence of a low intensity absorption band at 486 nm (CH 2 Cl 2), assignable to a metal-metal-to-ligand charge transfer transition (1 MMLCT) [dσ*(Pt) 2 →π*(bzq)], is indicative of the existence of two platinum centers located in close proximity because the rigidity of the half-lantern structure allows the preservation of these interactions in solution. Compound 1 undergoes two-electron oxidation upon treatment with halogens X 2 (X 2 : Cl 2 , Br 2 or I 2) to give the corresponding dihalodiplatinum (III) complexes [{Pt(bzq)(μ-L)X} 2 ] (L = CF 3 C 4 H 2 N 2 S-κN,S; X: Cl 2, Br 3, I 4). Complexes 2-4 were also obtained by reaction of 1 with HX (molar ratio 1:2, 10% excess of HX) in THF with yields of about 80% and compound 2 was also obtained by reaction of [{Pt(bzq)(μ-Cl)} 2 ] with HL (4-(trifluoromethyl)pyrimidine-2-thiol) in molar ratio 1:2 in THF, although in small yield. The X-ray structures of 2 and 3 confirmed the half-lantern structure and the anti configuration of the molecules. Both of them show Pt-Pt distances (2.61188(15) Å 2, 2.61767(16) Å 3) in the low range of those observed in Pt 2 (III,III)X 2 half-lantern complexes.

Highly Emissive Dinuclear Platinum(III) Complexes

“…36 Second, dinuclear Pt(III) complexes are believed to be more stable and could accommodate more intimate Pt•••Pt interactions than the dinuclear Pt(II) complexes because of the invulnerability toward nucleophilic reagents (e.g., halide (X − )) for the Pt(III) complexes. 33,37,38 Third, the octahedral coordination geometry allows the attachment of multidentate chelate ligands with various functionalities so that the synergy among the axial coordination ligand, bridging ligand, and cyclometalated ligand can finely harness the Pt•••Pt distance, endowing the corresponding complexes with unique properties. 23,39−41 Despite the fact that many ligands have been employed (e.g., pyridine deriatives, 11,24,33 2-mercaptobenzothiazole and 2mercaptobenzoxazole 25,42,43 ), the endeavors aimed to synthesize emissive dinuclear Pt(III) complexes were not successful.…”

“…The corresponding stereostructure is beneficial to decreasing quenching by intermolecular interactions such as π-stacking . Second, dinuclear Pt­(III) complexes are believed to be more stable and could accommodate more intimate Pt···Pt interactions than the dinuclear Pt­(II) complexes because of the invulnerability toward nucleophilic reagents (e.g., halide (X – )) for the Pt­(III) complexes. ,, Third, the octahedral coordination geometry allows the attachment of multidentate chelate ligands with various functionalities so that the synergy among the axial coordination ligand, bridging ligand, and cyclometalated ligand can finely harness the Pt···Pt distance, endowing the corresponding complexes with unique properties. ,− …”

“…Despite the fact that many ligands have been employed (e.g., pyridine deriatives, ,, 2-mer­capto­benzo­thia­zole and 2-mer­capto­benzox­azole ,, ), the endeavors aimed to synthesize emissive dinuclear Pt­(III) complexes were not successful. The main reason lies in the short-lived lowest-lying charge-transfer electronic states of the dinuclear Pt­(III) complexes, resulting in inefficient electron-transfer kinetics and a lack of photo­lumin­escence (vide infra). ,,,,− ,− It is worth noting that only two types of dinuclear Pt­(III) complexes have been reported with luminescent properties (i.e., [Pt 2 (μ-pop) 4 X 2 ] 4– (pop = P , P -pyrophosphite, P 2 O 5 H 2 2– , X = Cl, Br, SCN), [Pt 2 (μ-pop) 4 X 2 ] 2– (X = py) and [Pt 2 (μ-C 6 H 3 -5-R-2-AsPh 2 ) 4 X 2 ] (R= methyl or isopropyl; X = Cl, Br, or I)). , Among them, the former [Pt 2 (μ-pop) 4 X 2 ] n − ( n = 2, 4) analogs exhibited red luminescence only in the 77 K ethanol glass, while the latter displayed weak emission in the NIR region at room temperature with no emission yield provided and without applications. To the best of our knowledge, there have been no applications of platinum­(III) complexes to OLEDs so far.…”

“…The crucial electronic transitions of nonemissive phosphorescent Pt­(III) complexes normally possess metal-centered (MC) dσ M –dσ M * character as the lowest-lying electronic excited state. ,− ,− We thus proposed that judiciously designed rigid bridging ligands may reorder the frontier molecular orbitals such that the electron clouds of the highest occupied molecular orbital (HOMO) of the dinuclear platinum­(III) complexes are located on the coordinated ligands rather than on the two platinum atoms. The approach to achieving this goal is the use of donor (D)–acceptor (A)-type bridging ligands to alter the nature of the electronic excited state, which has been employed in designing emissive Au or Ag complexes in previous reports. − Accordingly, the electronic transition of dinuclear Pt­(III) complexes may no longer be dominated by the dσ M –dσ M * metal-centered (MC) d 7 –d 7 electronic transition but by the bridging ligand–metal–metal charge transfer (LMMCT), resulting in emissive dinuclear Pt­(III) complexes.…”

Electron Delocalization in Spectroelectrochemically and Computationally Characterized [Pt{(p-BrC6F4)NCH═C(Cl)NEt2}Cl(py)]+ Formed by Electrochemical Oxidation of [PtII{(p-BrC6F4)NCH═C(Cl)NEt2}Cl(py)]

“…Stable Pt III compounds are normally found as oligomers (Pt II –Pt III ) − or dimers containing a Pt–Pt bond , and/or bridging ligands. ,, However, stable Pt III mononuclear species are known. ,− The first mononuclear Pt III complex was reported in 1984 by Usón et al, who synthesized [(NBu 4 )­Pt III (C 6 Cl 5 ) 4 ] by the oxidation of [(NBu 4 ) 2 Pt II (C 6 Cl 5 ) 4 ] with Cl 2 or Br 2 in CCl 4 solution. Later, an EPR study of [(NBu 4 )­Pt III (C 6 Cl 5 ) 4 ] was reported where charge transfer from the ligand to Pt III was proposed.…”