Synthesis, Characterization, and Protonation Reactions of Ar‐BIAN and Ar‐BICAT Diimine Platinum Diphenyl Complexes

Abstract

AbstractPtII diphenyl complexes (N–N)PtPh2 [N–N = diimines Ar–N=C(An)C=N–Ar with Ar = substituted aryl groups] have been prepared and characterized by 1H, 13C, and 195Pt NMR spectroscopy. The 195Pt NMR spectroscopic data establish the electronic influence exerted by substituents at the backbone of the diimine ligand system to the metal center. When compared to diimines Ar–N=CMe–CMe=N–Ar, the electron‐withdrawing ability of the Ar‐BIAN ligand and the electron‐donating ability of the O,O‐heterocyclic Ar‐BICAT systems are demonstrated. Trends in 195Pt NMR chemical shifts suggest that electronic tuning of the metal center is better achieved through variations of the diimine backbone substituents rather than variation of the substituents at the N‐Aryl groups. Protonation of (N–N)PtPh2 in dichloromethane/acetonitrile at –78 °C furnishes the corresponding PtIV hydrides (N–N)PtPh2H(NCMe)+. The PtIV hydrides liberate benzene with the formation of (N–N)PtPh(NCMe)+ when the temperature is raised. A second protonation and rapid benzene elimination produces the dicationic PtII species (N–N)Pt(NCMe)22+ at approximately 50 °C. Protonation of (N–N)PtPh2 in the absence of acetonitrile results in the clean formation of (N–N)PtPh(η2‐C6H6)+ at temperatures that depend on the steric hindrance provided by the alkyl substituents at the diimine N‐aryl groups. These findings support the notion that the metal is the kinetically preferred site of protonation. The results qualitatively agree with a recent mechanistic study of protonation‐induced reactions of (diimine)PtPh2 complexes that bear simple methyl substituents at the diimine backbone. Several compounds have been crystallographically characterized. All complexes have the expected square planar environment at the metal. Modest variations in the metric parameters suggest that the Ar‐BICAT system has a weaker trans influence than the Ar‐BIAN and Ar‐DAB systems.