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

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.