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

Abstract

PtII 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.

Combined Low Temperature Rapid Scan and1H NMR Mechanistic Study of the Protonation and Subsequent Benzene Elimination from a (Diimine)platinum(II) Diphenyl Complex Relevant to Arene C−H Activation

“…The kinetic measurements were done by monitoring the absorbance vs time evolution over the whole spectral range. The analysis of this experimental observation, not seen previously with related diimine Pt complexes, 173 will be further addressed in the discussion section.…”

“…This is in agreement with previously mentioned observation on closely related diimine (Ar-DAB)PtPh 2 system (Table1). 173 The electronic influence on the kinetics of that first protonation reaction can be discussed and rationalized by the analysis of Table 1 which summarizes the data presented herein for all three Ar-BIAN complexes 1b-d that are sterically protected at the metal by the 2,6-Me 2 substitution pattern of the N-Aryl group, as well as the Ar-DAB diimine system. Entries 2-4 show that the rate increases with increasing donor ability of the N-aryl groups respectively H, Me and Br substituted in the para position.…”

“…The π-benzene complex is thermally unstable and decomposes by liberation of benzene at ca -50 °C. 173 The identity of the product complex has not been resolved but a reasonable candidate is (Ar- …”

“…It was reported that torsion angles between the diimine backbone and the N-aryl groups of the Ar-DAB ligand metal complex spanned between 90° and 101°, 81 whereas in the case of the Ar-BIAN complex 1b they spanned between 88° and 84°. 173 This reveals that the Ar-DAB diimine backbone tends to flip further …”

Cationic Platinum(II) Complexes Bearing Aryl-BIAN Ligands: Synthesis and Structural and Optoelectronic Characterization

“…To the best of our knowledge, cationic Pt(II) Ar-BIAN complexes have not been previously reported though there are a few reports on neutral Pt(II) Ar-BIAN complexes and their applications in catalysis, 15 activation of CO 2 , 16 and C-H bond activation. 17 Notably, Weinstein and Castellano have also studied the photophysical properties of [Pt(Mes-BIAN)L 2 ] complexes, with L an acetylide, a thiolate or a chloride, and found them to be near-infrared emitters in DCM solution at room temperature (Mes-BIAN = bis(mesitylimino)acenaphthene). 18…”