Assembly of Cyclometalated Platinum(II) Complexes via 1,1′-Bis(diphenylphosphino)ferrocene Ligand: Kinetics and Mechanisms

Abstract

The kinetics and mechanism of the reaction of the cyclometalated complexes [PtAr(C−N)(SMe 2 )], 1, in which Ar is Ph, p -MeC 6 H 4, or p -MeOC 6 H 4, and C−N is either ppy (deprotonated 2-phenylpyridine) or bhq (deprotonated benzo[ h ]quinoline), with 1,1′-bis(diphenylphosphino)ferrocene, dppf, were studied using UV−visible and 31 P NMR spectroscopies. When 0.5 equiv of dppf was added, the binuclear Pt(II) complex [Pt 2 Ar 2 (C−N) 2 (μ-dppf)], 2, was formed in a good yield. The complexes were fully characterized using multinuclear ( 1 H, 31 P, and 195 Pt) NMR spectroscopy, and the structure of complex [Pt 2 ( p -MeOC 6 H 4 ) 2 (bhq) 2 (μ-dppf)], 2c′ ·CH 2 Cl 2, was further identified by X-ray crystallography. On the basis of low-temperature 31 P NMR studies involving the starting complex [Pt( p -MeC 6 H 4 )(ppy)(SMe 2 )], 1b, we suggest that dppf displaces the labile ligand SMe 2 to give an uncommon complex, [Pt( p -MeC 6 H 4 )(ppy)(dppf-κ 1 P )], A, in which dppf-κ 1 P is a monodentate dppf ligand, which rapidly forms an equilibrium with the chelating dppf isomer complex [Pt( p -MeC 6 H 4 )(dppf)(ppy-κ 1 C )], B, in which ppy - κ 1 C is the deprotonated ppy ligand that is C-ligated with the dangling N atom. In the second step, A is reacted with the remaining second half of starting complex 1b to give the final Pt(II)−Pt(II) binuclear complex [Pt 2 ( p -MeC 6 H 4 ) 2 (ppy) 2 (μ-dppf)], 2b . A competitive-consecutive second-order reaction mechanism was suggested for the reaction using chemometric studies, and the rate constants at 5 °C for first and second steps were estimated as k 2 = 10.7 ± 0.2 L mol −1 s −1 and k 2 ′ = 0.68 ± 0.05 L mol −1 s −1, respectively. When the starting complex [Pt( p -MeC 6 H 4 )(ppy)(SMe 2 )], 1b, was reacted with 1 equiv of dppf, similarly the complex A, in equilibrium with B, was formed first, with the rate constant at 5 °C being k 2 = 10.5 ± 0.5 L mol −1 s −1, estimated using UV−visible spectroscopy. Subsequently, however, A and B would slowly and reversibly react with each other to form a new species, C, the structure of which, on the basis of 31 P and 195 Pt NMR spectra, was proposed to be [( p -MeC 6 H 4 )(ppy)Pt(μ-dppf)Pt( p -MeC 6 H 4 )(ppy-κ 1 C )(dppf-κ 1 P )]; the same results were obtained when more than 1, e.g., 2, equiv of dppf was used, with a similar rate constant of k 2 = 10.6 ± 0.6 L mol −1 s −1 . The complexes 1b and 2b were shown to have some interesting photophysical properties as investigated by absorption and electroluminescence spectroscopies.