Cyclometalation of Aryl‐Substituted Phosphinines through CH‐Bond Activation: A Mechanistic Investigation

Abstract

A series of 2,4,6-triarylphosphinines were prepared and investigated in the base-assisted cyclometalation reaction using [Cp*IrCl2]2 (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) as the metal precursor. Insight in the mechanism of the C-H bond activation of phosphinines as well as in the regioselectivity of the reaction was obtained by time-dependent (31)P{(1)H} NMR spectroscopy. At room temperature, 2,4,6-triarylphosphinines instantaneously open the Ir-dimer and coordinate in an η(1)-fashion to the metal center. Upon heating, a dissociation step towards free ligand and an Ir-acetate species is observed and proven to be a first-order reaction with an activation energy of ΔEA = 56.6 kJ mol(-1) found for 2,4,6-triphenylphosphinine. Electron-donating substituents on the ortho-phenyl groups of the phosphorus heterocycle facilitate the subsequent cyclometalation reaction, indicating an electrophilic C-H activation mechanism. The cyclometalation reaction turned out to be very sensitive to steric effects as even small substituents can have a large effect on the regioselectivity of the reaction. The cyclometalated products were characterized by means of NMR spectroscopy and in several cases by single-crystal X-ray diffraction. Based on the observed trends during the mechanistic investigation, a concerted base-assisted metalation-deprotonation (CMD) mechanism, which is electrophilic in nature, is proposed.

A π‐Conjugated, Covalent Phosphinine Framework

“…As the phosphinine P atom influences the reactivity of directly attached substituents considerably, we chose to include aryl spacers on which the polymerizable reactive groups are situated. Combining several reports about synthesis of aryl halide‐substituted λ 3 ‐phoshpinines, and the bromophenyl coupling reaction, we obtain 2,4,6‐tri(4‐bromophenyl)‐λ 3 ‐phosphinine ( 3 ) as the principle P‐containing tecton. As previous reports suggest a high reactivity (and hence low stability) of λ 3 ‐phosphinine, we further synthesize the corresponding, protected λ 5 ‐phosphinine compound, 1,1‐dimethoxy‐2,4,6‐tri(4‐bromophenyl)‐λ 5 ‐phosphinine ( 4 ) as another P‐containing tecton .…”

“…The preparation procedure is a modified method based on previous reports for phosphinine compounds . Compound 2 (5.21 g, 8.20 mmol) and anhydrous toluene (40 mL) were added to a 100 mL one‐neck flask under inert atmosphere in a glovebox.…”

Synthesis, Electronic Properties and OLED Devices of Chromophores Based on λ5‐Phosphinines

“…l 3 -Phosphinines 1-3 were synthesized according to knownl iterature procedures from the corresponding pyrylium-salts and P(SiMe 3 ) 3 . [7,15,16] Crystals of 2,6-di(2'-fluorophenyl)-4-phenyl-phosphinine, suitable for X-ray diffraction, were obtained by slow crystallization from acetonitrile.T he molecular structureo f3 in the crystal (Figure 3) shows the expected planar phosphorus heterocycle, while the three aryl groupsa re not in plane with the central hexagon. It should be noted that crystallographicallyc haracterized 2,4,6-triaryl-substituted l 3 -phosphinines are rare.T his is in fact the first observation of the "statistical average"a rrangement of the aryl groups attached to C(1) and C(5) for this class of compounds.…”

“…The NMR spectra were recorded on aJ EOL ECX400 (400 MHz) spectrometer and chemical shifts are reported relative to the residual resonance in the deuterated solvents. Phosphinines 1, [15] 2 [7] and 3 [16] were prepared according to the literature. UV/Vis spectra were recorded at RT with aV ARIAN Cary 5000 spectrophotometer.U V/Vis/NIR emission and excitation spectra measurements were recorded with an FL 920 Edinburgh Instrument equipped with aH amamatsu R5509-73 photomultiplier for the NIR domain (300-1700 nm) and corrected for the response of the photomultiplier.Q uantum yields were calculated relative to quinine sulfate (H 2 SO 4 ,0 .1 m, f ref = 0.55).…”

2‐(Trimethylsilyl)‐λ3‐Phosphinine: Synthesis, Coordination Chemistry, and Reactivity

“…To date, several synthetic routes to phosphinines have been reported in the literature . They individually offer the possibility to realize different substitution patterns and to introduce functionalities into specific positions of the 6‐membered, aromatic ring.…”