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.