Platinum(IV) Complexes with Tridentate, NNC-Coordinating Ligands: Synthesis, Structures, and Luminescence

[Image: see text] Platinum(II) complexes of NNC-cyclometalating ligands based on 6-phenyl-2,2′-bipyridine (HL(1)) have been widely investigated for their luminescence properties. We describe how PtL(1)Cl and five analogues with differently substituted aryl rings, PtL(2–6)Cl, can be oxidized with chlorine and/or iodobenzene dichloride to generate Pt(IV) compounds of the form Pt(NNC-L(n))Cl(3) (n = 1–6). The molecular structures of several of them have been determined by X-ray diffraction. These PtL(n)Cl(3) compounds react with 2-arylpyridines to give a new class of Pt(IV) complex of the form [Pt(NNC)(NC)Cl](+). Elevated temperatures are required, and the reaction is accompanied by competitive reduction processes and generation of side-products; however, four examples of such complexes have been isolated and their molecular structures determined. Reaction of PtL(1)Cl(3) with HL(1) similarly generates [Pt(NNC-L(1))(2)](2+), which we believe to be the first example of a bis-tridentate Pt(IV) complex. The lowest-energy bands in the UV–vis absorption spectra of all the PtL(n)Cl(3) compounds are displaced to higher energy relative to the Pt(II) precursors, but they red-shift with the electron richness of the aryl ring, consistent with predominantly (1)[π(Ar) → π*(NN)] character to the pertinent excited state. A similar trend is observed for the [Pt(NNC)(NC)Cl](+) complexes. They display phosphorescence in solution at room temperature, centered around 500 nm for [PtL(1)(ppy)Cl](+) and [Pt(L(1))(2)](2+), and 550 nm for methoxy-substituted derivatives. The lifetimes are in the microsecond range, rising to hundreds of microseconds at 77 K, consistent with triplet excited states of primarily (3)[π(Ar) → π*(NN)] character with relatively little participation of the metal.