Mechanistic study on photooxidation of triarylphosphines by time-resolved infrared spectroscopy

Article abstract of DOI:10.1080/10426507.2010.524182

Upon laser flash photolysis on a solution of triarylphosphine Ar ₃P in acetonitrile or dichloromethane using a YAG laser at 266 nm, transient absorption was observed by time-resolved infrared spectroscopy. Rate constants were determined for the formation step of the absorption. Dependency of the rate on [Ar₃P] and atmosphere, along with independent observation in laser flash photolysis/UV-Vis spectroscopy of the Ar₃P solutions, suggeststhat electron transfer occurs from Ar₃P to the singlet photoexcited state of Ar₃P, generating triarylphosphine radical cation Ar₃P .+, which reacts with a trace amount of H2O in the solvent to give the final product Ar₃P O, which has been observed in steady-state photolysis. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426507.2010.524182

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Phosphorus, Sulfur, and Silicon and the
Related Elements
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Mechanistic Study on Photooxidation
of Triarylphosphines by Time-Resolved
Infrared Spectroscopy
Shinro Yasui a & Masaaki Mishima b
a Institute for Human and Environmental Sciences , Tezukayama
University , Nara , Japan
b Institute for Materials Chemistry and Engineering , Kyushu
University , Fukuoka , Japan
Published online: 25 Apr 2011.
To cite this article: Shinro Yasui & Masaaki Mishima (2011) Mechanistic Study on Photooxidation of
Triarylphosphines by Time-Resolved Infrared Spectroscopy, Phosphorus, Sulfur, and Silicon and the
Related Elements, 186:4, 838-840, DOI: 10.1080/10426507.2010.524182
To link to this article: http://dx.doi.org/10.1080/10426507.2010.524182
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Phosphorus, Sulfur, and Silicon, 186:838–840, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2010.524182
MECHANISTIC STUDY ON PHOTOOXIDATION
OF TRIARYLPHOSPHINES BY TIME-RESOLVED
INFRARED SPECTROSCOPY
Shinro Yasui1 and Masaaki Mishima2
1Institute for Human and Environmental Sciences, Tezukayama University,
Nara, Japan
2Institute for Materials Chemistry and Engineering, Kyushu University,
Fukuoka, Japan
Abstract Upon laser flash photolysis on a solution of triarylphosphine Ar3P in acetonitrile
or dichloromethane using a YAG laser at 266 nm, transient absorption was observed by
time-resolved infrared spectroscopy. Rate constants were determined for the formation step of
the absorption. Dependency of the rate on [Ar3P] and atmosphere, along with independent
observation in laser flash photolysis/UV-Vis spectroscopy of the Ar3P solutions, suggests
that electron transfer occurs from Ar3P to the singlet photoexcited state of Ar3P, generating
triarylphosphine radical cation Ar3P ·+, which reacts with a trace amount of H2O in the solvent
to give the final product Ar3P O, which has been observed in steady-state photolysis.
Keywords Laser flash photolysis; radical cation; time-resolved infrared spectroscopy;
triarylphosphine
INTRODUCTION
Trivalent phosphorus compounds Z3P, including phosphines, phosphinites, phos-
phonites, and phosphites, are oxidized by many types of oxidants in the dark to afford the
corresponding pentavalent oxo-compounds Z3P O.2–3 The oxidation reactions are exother-
mic, and the mechanistic aspects of these oxidation processes have been well explored. On
the other hand, photo-oxidation of Z3P has been studied much less often. Thus, although
some publications report the oxidation of phosphines to phosphine oxides initiated by
photo-irradiation, the species that is initially excited under the conditions is not phosphine
itself.4,5
Recently, we observed that upon steady-state irradiation of UV light from a Xe
lamp, triarylphosphines Ar3P (Figure 1) are oxidized quantitatively to the corresponding
phosphine oxides Ar3P O. Quite interestingly, the oxidation took place even under an
argon atmosphere, except for the reaction of Mes3P. In the reaction of Mes3P, O2 is a
prerequisite for formation of Mes3P O. Certainly, Ar3P is photoexcited directly because
Received 27 July 2010; accepted 13 September 2010.
One of the authors (S.Y.) greatly appreciates financial support by Tezukayama research grant 2009.
Address correspondence to Shinro Yasui, Institute for Human and Environmental Sciences, Tezukayama
University, Gakuen-Minami, Nara 631-8585, Japan. E-mail: yasui@tezukayama-u.ac.jp
838

PHOTOOXIDATION OF TRIARYLPHOSPHINES
839
CH3
P
H3C
P
P
CH3O
P
3
3
3
3
CH3
CH3
Mes3P
o-, m-, p-Tol3P
Ph3P
p-An3P
Figure 1 Triarylphosphines used in this study.
there is no species other than Ar3P in the system. In an attempt to reveal the mechanism
of this peculiar oxidation, we investigated the photoreaction of Ar3P by using laser flash
photolysis (LFP) with time-resolved infrared spectroscopy (TRIR). An advantage of IR
spectroscopy is that the spectrum obtained involves much more definite information than
its UV-Vis counterpart.
RESULTS AND DISCUSSION
The photoreaction of Ar3P was followed by laser flash photolysis time-resolved
infrared spectroscopy (LFP-TRIR). Thus, when the acetonitrile or dichloromethane solution
of Ar3P was photolyzed by using a YAG laser at 266 nm, a transient absorption was observed
at the region of 1110 cm−1 ∼1220 cm −1, showing existence of an intermediate. The
appearance of the transient absorption and its decay occurred with microsecond timescale
and with millisecond timescale, respectively. Pseudo-first order rate constants kFobs for the
formation of the intermediate were determined by varying the concentration of Ar3P as
well as the atmosphere (argon, air, or oxygen). kFobs was found to depend linearly on [Ar3P]
(Figure 2). Interestingly, kFobs for the reaction of Mes3P depended linearly on [O2], whereas
those for other Ar3P’s were almost constant irrespective of atmosphere. These observations
finally give a rate Equation (1).
kFobs = k2F[Ar3P] + koF[O2]
(1)
Importantly, kFO exerts meaningful values only for the reaction of Mes3P.
The acetonitrile solution of Ar3P was also photolyzed at 266 nm to be followed
by time-resolved UV-Vis spectroscopy. Within 100 ns, the transient spectrum that was
assignable to triarylphosphine radical cation Ar3P ·+ appeared, in comparison with the
3.0
Mes3P
-
m Tol3P
2.0
1.0
0
p-An3P
p-Tol P
×
3
Ph3P
o-Tol3P
0
2
4
6
8
10
[Ar3P], mM
Figure 2 Dependence of kF on [Ar3P]. In CH2Cl2 under air.
obs

840
S. YASUI AND M. MISHIMA
spectra previously observed in the LFP of the acetonitrile solution of Ar3P in the presence
of dicyanoanthracene1 and also in the pulse radiolysis on the benzonitrile solution of Ar3P.6
The spectrum decayed at a rate approximately identical to that of the formation rate observed
on the TRIR. This observation in LFP-TRUV-Vis spectroscopy strongly suggests that the
intermediate observed in the LFP-TRIR experiment results from Ar3P ·+ 7
.
Since there is no acceptor in the solution of these LFP experiments, electron transfer
may take place from Ar3P to the singlet photoexcited state of Ar3P, generating Ar3P ·+
.
The kinetics represented by Eq. (1) support this mechanism. Radical cation Ar3P ·+, once
formed, reacts with a trace amount of H2O in the solvent to give the final product
Ar3P O.2 In the reaction of Mes3P, three mesityl ligands are bulky enough to prevent
a water molecule from attacking the central phosphorus atom. This may be the reason why
O2 is prerequisite to the photooxidation of Mes3P to Mes3P O.
REFERENCES
1. Yasui, S.; Tojo, S.; Majima, T. Org. Biomol. Chem. 2006, 4, 2696–2973.
2. Yasui, S.; Shioji, K.; Tsujimoto, M.; Ohno, A. J. Chem. Soc., Perkin 2 1999, 855–862.
3. Beaver, B.; Rawlings, D.; Neta, P.; Alfassi, Z. B.; Das, T. N. Heteroatom Chem. 1998, 9, 133–138.
4. Somasundaran, N.; Slinivasan, G. J. Am. Chem. Soc. 1996, 61, 2895–2896.
5. Okubo, K.; Nanjo, T.; Fukuzumi, S. Bull. Chem. Soc. Jpn. 2006, 79, 1489–1500.
6. Tojo, S.; Yasui, S.; Majima, T. J. Org. Chem. 2006, 71, 8227–8232.
7. A brief outline of experiments is given here: Triarylphosphines were purchased (Aldrich) and
recrystallized from ethanol before use. TRIR experiments were conducted with a JASCO TRIR-
1000 dispersive-type IR spectrometer. Sample solutions flowing through a CaF2 cell were excited
by pulses (266 nm) from a Continuum cw Q-sw Nd:YAG laser. Changes in IR intensity were
monitored by an MCT photovoltaic IR detector.