Assembly of cyclometalated platinum(II) complexes via 1,1'-bis(diphenylphosphino)ferrocene ligand: kinetics and mechanisms

The kinetics and mechanism of the reaction of the cyclometalated complexes [PtAr(C−N)(SMe₂)], 1, in which Ar is Ph, p-MeC₆H₄, or p-MeOC₆H₄, 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 ³1P NMR spectroscopies. When 0.5 equiv of dppf was added, the binuclear Pt(II) complex [Pt₂Ar₂(C−N)₂(μ-dppf)], 2, was formed in a good yield. The complexes were fully characterized using multinuclear (¹H, ³¹P, and ¹⁹⁵Pt) NMR spectroscopy, and the structure of complex [Pt₂(p-MeOC₆H₄)₂(bhq)₂(μ-dppf)], 2c′·CH₂Cl₂, was further identified by X-ray crystallography. On the basis of low-temperature ³¹P NMR studies involving the starting complex [Pt(p-MeC₆H₄)(ppy)(SMe₂)], 1b, we suggest that dppf displaces the labile ligand SMe₂ to give an uncommon complex, [Pt(p-MeC₆H₄)(ppy)(dppf-κ¹P)], A, in which dppf-κ¹P is a monodentate dppf ligand, which rapidly forms an equilibrium with the chelating dppf isomer complex [Pt(p-MeC₆H₄)(dppf)(ppy-κ¹C)], B, in which ppy-κ¹C 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 [Pt₂(p-MeC₆H₄)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 k₂ = 10.7 ± 0.2 L mol⁻¹ s⁻¹ and k₂′ = 0.68 ± 0.05 L mol⁻¹ s⁻¹, respectively. When the starting complex [Pt(p-MeC₆H₄)(ppy)(SMe₂)], 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 k₂ = 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 ³¹P and ¹⁹⁵Pt NMR spectra, was proposed to be [(p-MeC₆H₄)(ppy)Pt(μ-dppf)Pt(p-MeC₆H₄)(ppy-κ¹C)(dppf-κ¹P)]; the same results were obtained when more than 1, e.g., 2, equiv of dppf was used, with a similar rate constant of k₂ = 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.