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

Platinum(II) complexes of -cyclometalating ligands based on 6-phenyl-2,2'-bipyridine (HL) have been widely investigated for their luminescence properties. We describe how PtLCl and five analogues with differently substituted aryl rings, PtLCl, can be oxidized with chlorine and/or iodobenzene dichloride to generate Pt(IV) compounds of the form Pt(-L)Cl ( = 1-6). The molecular structures of several of them have been determined by X-ray diffraction. These PtLCl compounds react with 2-arylpyridines to give a new class of Pt(IV) complex of the form [Pt()()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 PtLCl with HL similarly generates [Pt(-L)], 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 PtLCl 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 [π → π*] character to the pertinent excited state. A similar trend is observed for the [Pt()()Cl] complexes. They display phosphorescence in solution at room temperature, centered around 500 nm for [PtL(ppy)Cl] and [Pt(L)], 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 [π → π*] character with relatively little participation of the metal.