Cyclometalation of Aryl-Substituted Phosphinines
FULL PAPER
temperature compared with the non-deuterated one, de-
creasing the ratio at elevated temperatures.
ACHTUNGTRENNUNG
Comparing cyclometalations of symmetrically substituted
phosphinines with electron-donating and -withdrawing sub-
stituents, a shift in the rate-limiting step from the dissocia-
tion step for phosphinines bearing donating substituents to
À
the C H activation step for the phosphinines with electron-
À
withdrawing substituents is observed. The increase in the ac-
cumulation of the free ligand intermediate points to this be-
havior, together with the fact that the monocoordination is
consumed faster than the phosphinine intermediate, which is
most clear for complexes 6 and 8 (Figure 12). So, the with-
tuted phosphinines to the C H activation step for phosphi-
nines bearing electron-withdrawing groups. Further investi-
gation will focus on the reactivity of these new cyclometalat-
ed compounds for future applications in molecular materials
and homogeneous catalysis.
À
drawing substituents seem to slow down the C H activation
step to the extent that it becomes the rate-limiting step,
whereas the donating substituents show only a marginal
effect on the rate-limiting dissociation step. The fact that we
observe any effect at all from the electron-donating substitu-
ents on the overall rate strongly points to the dissociation
step being an equilibrium, which we did not take into ac-
count for kinetic calculations but did not rule it out. The
Experimental Section
General considerations: All manipulations were carried out under an
inert argon atmosphere using standard Schlenk techniques or in an
MBraun glovebox. All glassware was dried prior to use to remove traces
of water. [Cp*IrCl2]2 was obtained from Strem chemicals Inc. All other
chemicals were obtained from Sigma–Aldrich or ABCR. The solvents
were dried and deoxygenated using custom-made solvent purification col-
umns filled with Al2O3. The 1H, 13C{1H}, 19F{1H}, and 31P{1H} NMR spec-
tra were recorded on a Varian Mercury 400 MHz spectrometer and
chemical shifts are reported relative to residual proton resonance of the
deuterated solvents. Elemental analyses were performed by H. Kolbe,
Mikroanalytisches Laboratorium, Mꢀlheim a.d. Ruhr (Germany).
À
effect of the electron-donating substituents on the C H acti-
vation step translates to the overall reaction rate only
through a forward shift in the equilibrium of the dissociation
step.
Conclusion
General procedure for the preparation of symmetrically substituted 2,4,6-
aryl-substituted pyrylium salts:[12] The acetophenone (2 equiv) with the
appropriate substitution pattern and benzaldehyde (1 equiv) were intro-
duced in a dry Schlenk flask under argon. A deep-red reaction mixture
was obtained upon drop-wise addition of HBF4·Et2O (52% w/w;
2 equiv), which became fluorescent shortly after complete addition. The
reaction mixture was heated to reflux (T=808C). 1,2-dichloroethane was
added in case the reaction mixture was too viscous. After approximately
4 h the pyrylium salt was precipitated in a large amount of diethyl ether
under vigorous stirring. The salt was filtered off and thoroughly washed
with diethyl ether. The pyrylium salts were obtained as yellow powders
and if necessary crystallized from hot methanol.
We have synthesized and characterized a series of 2,4,6-triar-
ylphosphinine derivatives bearing either electron-withdraw-
ing or electron-donating substituents in specific positions of
the heterocyclic framework. We could further demonstrate
the effect of the substitution pattern on the rate and the re-
À
gioselectivity of the cyclometalation reaction through C H
bond activation. The general selectivity and reactivity in the
cyclometalation reaction of phosphinines is sensitive to
steric effects, whereas electronic effects have the main con-
tribution. Electron-donating groups accelerate the cyclome-
talation reaction, whereas electron-withdrawing groups in-
hibit C H activation. This enhancing or inhibitory effect is
predominant for symmetrical phosphinines substituted in
the meta position, indicating an electrophilic mechanism for
the C H activation reaction through an arenium complex,
similar to the aromatic substitution reaction in organic
chemistry. Slower cyclometalation reactions have occurred
for 2,4,6-triarylphosphinines bearing ortho-substituents, re-
gardless of their nature. 31P{1H} NMR spectroscopic investi-
gations and X-ray crystal structure determinations of the
corresponding new phosphinine–IrIII complexes gave further
insight in the regioselectivity of the cyclometalation reac-
tion. Ir(PC)’ and Ir(PC)’’ are kinetically favored when
donor-functionalized non-symmetrical phosphinines are cy-
General procedure for the preparation of non-symmetrically substituted
2,4,6-aryl-substituted pyrylium salts:[12] Chalcone (2 equiv) and acetophe-
none (1 equiv) were introduced in a dry Schlenk flask under argon. Upon
drop-wise addition of HBF4·Et2O (52% w/w; 2 equiv), the reaction mix-
ture turned deep-red and became fluorescent shortly after complete addi-
tion. Workup was performed as described above for the symmetrically
substituted pyrylium salts.
À
À
Modified literature procedure[43] for the preparation of 2,4,6-aryl-substi-
tuted phosphinines: PACHTNUTGRNEUNG(SiMe3)3 (1–2.2 equiv) was added drop-wise to a
slurry of the functionalized pyrylium salt (1 equiv) in 1,2-dimethoxy-
ethane (DME) and the resulting mixture was heated at reflux (T=908C)
for 16 h. During that time the pyrylium salt dissolved completely. The
DME was subsequently removed in vacuo and the brown residue was fil-
tered through a pad of neutral alumina using diethyl ether as eluent. The
product was washed three times with diethyl ether and afterwards dried
in vacuo. The remaining orange-brown residue was recrystallized from
hot acetonitrile.
General procedure for the cyclometalation of 2,4,6-aryl-substituted phos-
phinines: [Cp*IrCl2]2 (0.031 mmol, 1.0 equiv), 2,4,6-aryl-substituted phos-
phinine (0.062 mmol, 2.0 equiv), and NaOAc (0.069 mmol, 2.3 equiv)
À
clometalated, but the C H activation of the unsubstituted
were dissolved in dichloromethane (0.6 mL) and transferred into
a
aryl-group was shown to be the thermodynamic product. Ki-
netic investigation of the simplified two-step mechanism has
shown that the dissociation step is first order and rate-limit-
ing for 2,4,6-triphenylphosphinine (1) as well as 2,6-di-
Young NMR-tube, all under inert atmosphere. The reaction mixture was
heated to T=808C in an oil bath until the reaction was completed. Time-
dependent 31P{1H} NMR spectroscopy was used to monitor the reaction.
Afterwards the orange reaction mixture was filtered over silica and was
Chem. Eur. J. 2013, 00, 0 – 0
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!
Broeckx, Leen E. E.
