Infrared, UV/Vis, and W-band EPR spectroscopic characterization and photochemistry of triplet mesitylphosphinidene

Article abstract of DOI:10.1002/anie.200462862

(Chemical Equation Presented) Monitoring the cleavage of phosphiranes 1 and 2 by using a variety of low-temperature and time-resolved spectroscopic techniques has shown that triplet mesitylphosphinidene (3) is the principal product of this photolytic reac

Full text of DOI:10.1002/anie.200462862

Angewandte
Chemie
Radicals
Infrared, UV/Vis, and W-band EPR Spectroscopic
Characterization and Photochemistry of Triplet
Mesitylphosphinidene**
Gꢀtz Bucher, Mark L. G. Borst, Andreas W. Ehlers,
Koop Lammertsma,* Stefano Ceola, Martina Huber,
Dirk Grote, and Wolfram Sander
Although many simple carbenes (R2C)[1] and nitrenes
(RN)[2–4] have been detected by various spectroscopic tech-
niques, analogous phosphinidenes (RP) have not.[5–7] In sharp
contrast to the abundant examples of transition-metal com-
plexed phosphinidenes,[8–10] evidence of noncomplexed phos-
phinidines has remained elusive except for a recent matrix-IR
study by Glatthaar and Maier[11] on H3SiP, which was
generated by the reaction of atomic silicon with phosphine,
and an earlier electron paramagnetic resonance (EPR) study
by Gaspar and co-workers[12] on mesitylphosphinidene (2,4,6-
Me3C6H2P; MesP; 3), which was obtained by photolysis of
trans-1-mesityl-2,3-dimethylphosphirane (1) in methylcyclo-
hexane at T= 77 K (Scheme 1). No other direct experimental
confirmation has been obtained for the existence of 3.
Theoretical studies concur with the spectroscopic obser-
vation that a strongly preferred triplet ground state is
predicted for aryl phosphinidene.[13,14] From the X-band
Scheme 1. Photochemical formation of triplet MesP (3).
[*] M. L. G. Borst, Dr. A. W. Ehlers, Prof. Dr. K. Lammertsma
Department of Organic and Inorganic Chemistry
Faculty of Sciences
Vrije Universiteit
De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands)
Fax: (+31)20-598-7488
E-mail: k.lammertsma@few.vu.nl
Priv.-Doz. Dr. G. Bucher, D. Grote, Prof. Dr. W. Sander
Lehrstuhl fꢀr Organische Chemie II
Ruhr-Universitꢁt Bochum
Universitꢁtsstr. 150, 44801 Bochum (Germany)
Dr. S. Ceola, Dr. M. Huber
Department of Molecular Physics
University of Leiden
Huygens Lab
2300 RA Leiden (The Netherlands)
[**] G.B. thanks K. Gomann for his help with the X-band EPR
measurements, and the Dr. O. Rꢂhm-Gedꢁchtnisstiftung for
financial support. This work was supported in part by the Council for
Chemical Sciences of the Netherlands Organization for Scientific
Research (M.B. and K.L.).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200462862
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Communications
EPR spectrum, a zero-field-splitting (ZFS) parameter, D, of
(from 2), whose IR spectra were also recorded independently
by matrix isolation of these alkenes. The remaining bands are
assigned to triplet 3 based on a comparison with the IR
spectrum calculated at the B3LYP/6-31G(d) level (Table 1).
j D/hc j = 3.521 cmꢀ1 was determined,[12] which is much larger
than that for triplet phenylnitrenes ( ꢁ 1 cmꢀ1) and triplet
phenylcarbenes ( ꢁ 0.5 cmꢀ1). The unusually large value of D
was attributed to second-order spin–orbit contributions.
Recent calculations on spin–orbit coupling in alkyl nitrenes,
phosphinidenes, and arsinidenes yielded a D value of j D/hc j
= 3.46 cmꢀ1 for triplet methyl phosphinidene,[15] in line with
the results of Gaspar and co-workers.[12] However, it should
also be noted that on photolysis of more congested phosphir-
anes, Yoshifuji and co-workers[16] were unable to obtain an
EPR signal in the reported region other than a weak signal at
5 K that was believed to be due to triplet oxygen.
Table 1: Calculated and experimental IR data of 3 (range 1700 to
500 cmꢀ1).
No. n˜calcd
n˜calcd
(ꢃ0.97) intensity
[cmꢀ1 [kmmolꢀ1
Calcd
n˜exp.
[cmꢀ1
Exp.
intensity
Description[a]
[cmꢀ1
]
]
]
]
46
45
44
43
42
41
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
1644.7 1595.0 50.2
8A
1575.9 1528.6
1527.6 1481.8
4.7
8.3
1532.4 vw
8B+t p-CH3
das o-CH3
das o-CH3
das p-CH3
das p-CH3
t o-CH3
t o-CH3
19B+t p-CH3
ds CH3
ds CH3
19A+ds CH3
ds CH3
14
–
–
We have investigated the photochemistry of 1 and 2 by
using matrix-isolation spectroscopy and laser flash photolysis;
herein we report EPR, infrared, and UV/Vis data that
unequivocally support the formation of mesitylphosphinidene
3.
1523.0 1477.3 33.0
1516.5 1471.0
1515.9 1470.4
1512.6 1467.2
5.6
9.3
0.2
–
–
–
–
–
–
w
1511.5 1466.2 15.9
1431.1
1457.6 1413.9
1445.8 1402.4
1443.3 1400.0
1443.3 1400.0
1439.2 1396.0
1319.2 1279.6
1309.9 1270.6
1271.5 1233.4
1206.5 1170.3
1074.9 1042.7
1070.8 1038.7
1068.3 1036.3
1.6
3.1
1.4
1.0
1.2
1.6
9.4
0.8
0.3
2.2
0.0
5.0
1407.3 vw
Initially, using an X-band EPR spectrometer, we tried in
vain to reproduce the EPR spectrum reported by Gaspar and
co-workers.[12] Photolysis (248 nm, KrF-excimer, or 254 nm)
of 2, which was matrix-isolated in glassy methylcyclohexane
(T= 5 K), resulted in the formation of yellow matrices but the
corresponding spectra only showed strong doublet signals
around g = 2 (g is the g factor) without EPR transitions
attributable to triplet 3. However, employing a W-band
(95 GHz) EPR spectrometer, we detected a very weak signal
exactly at the place predicted (4926 mT, Figure 1) for a triplet
species with the ZFS parameters reported by Gaspar and co-
workers; a background scan without sample showed no
transitions in this spectral region.
1398.9
w
–
–
–
–
1296.0 vw
1286.2
w
13
3
9A
1237.8 vw
1170.5 vw
–
–
-
–
w
17B+1 CH3
17A+1 CH3
1 CH3
1 CH3
1 CH3
1 CH3
1 CH3
1 CH3
1 CH3
10A
1033.3
1065.3 1033.3 19.2
1059.7 1027.9
1041.9 1010.6
0.7
0.0
1.9
1.1
0.3
0.0
–
–
–
–
1026.8
973.9
941.1
898.0
874.0
725.5
630.5
572.8
996.0
944.7
912.9
871.1
993.0 vw
–
–
–
–
–
–
847.8 12.5
849.8
709.7
625.8
m
w
w
5
4
6A
12
703.7
611.6
555.6
0.9
4.8
0.03
542.9 vw
[a] Descriptions are analogous to those of the vibrations of benzene as
proposed by Pitzer and Scott.[17]
When the sample was subjected to further irradiation
(lexc = 385–420 nm or lexc > 455 nm, 90 min), the IR bands
associated with both triplet 3 and trans-2-butene or ethylene
disappeared, while those of the starting materials 1 or 2
partially re-emerged, together with a new set of IR bands.[18a]
The new photoproduct showed a broad IR band at n˜ =
Figure 1. W-band EPR spectra of a sample of 2 in polycrystalline cyclo-
hexane (T=5 K), irradiated for 60 min with l=254 nm. a) Simulated
spectrum. Simulation parameters: line width=10 mT, D=ꢀ3770.6 mT
(jD/hcj=3.521 cmꢀ1). b) Experimental spectrum obtained by sweep-
ing the magnetic field from lower to higher values and c) from higher
to lower values.
ꢀ1
ꢀ
2261.2 cm , thus indicating the presence of a P H bond. By
comparison with
a
calculated IR spectrum (B3LYP/6-
31G(d)), the product could be identified as 1H,2H-dihydro-
benzophosphete 4 (Figure 2; see also the Supporting Infor-
mation).[18b] Intramolecular C H insertion of the phosphorus
ꢀ
center of supermesitylphosphinidene (2,4,6-tBu3(C6H2)P;
Mes*P) into the ortho-tBu group to give a phosphaindane is
common[6–7] and l5-benzophosphetes have been reported
previously.[19] The new dihydro-l3-benzophosphete 4, which
is energetically favored over triplet 3, may be formed similarly
or via phosphaquinone methide 5 (Scheme 2).
Monitoring by IR spectroscopy of the photolysis (lexc
=
254 nm, 48 h) of 1 or 2, which were matrix-isolated in Ar at
10 K, showed slow disappearance of the phosphirane IR
bands and formation of very weak new bands. Some of these
bands are attributable to trans-2-butene (from 1) or ethylene
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Figure 2. Top: Difference IR spectrum (Ar, 10 K) of the spectra
obtained after photolysis of 2 at lexc =254 nm (48 h) and subsequently
at lexc >385 nm (90 min). Bands pointing down disappear on photoly-
sis at lexc >385 nm and are attributed to a mixture of ethylene (E) and
triplet 3. The splitting of phosphinidene bands at n˜ =1028.6 and
1061.3 cmꢀ1 (n26) is likely due to Fermi resonance.[18b] Several very
weak bands remain unassigned. Bands pointing up are attributed to
the secondary photoproduct 4 and traces of 2. The highest intensity of
the difference spectrum amounts to ca. 0.1. The assignment of a
number of selected bands has been indicated by giving the transition
numbers (see Tables 1 and S1). Bottom: calculated IR difference spec-
trum (0.97ꢃscaling, (U)B3LYP/6-31G(d)) of 4 minus triplet 3.
Figure 3. UV/Vis spectra (Ar, 10 K) obtained after photolysis of 1 after
photolysis at lexc =254 nm for 80 min (c), after subsequent photoly-
sis at lexc >455 nm for 10 min (a), and after extended photolysis at
lexc >455 nm for 100 min (g).
These observations are rationalized by assigning the
absorptions l = 278, 292, 350, and 420–470 nm to triplet
mesitylphosphinidene 3, and those at l = 370 and 390 nm to a
secondary photoproduct other than 4. The UV/Vis spectrum
of 3, with a weak, structured broad band extending to l =
470 nm, strongly resembles the UV/Vis spectra of known
triplet aryl nitrenes.[4a] The intense excitations at l = 285, 310,
340, 415, and 440 nm calculated with time-dependent B3LYP/
6-311 + G(d,p)[20] for triplet mesitylphosphinidene provide
convincing evidence that triplet 3 is indeed formed upon
photolysis of matrix-isolated mesitylphosphiranes 1 and 2,[21]
albeit in small yield.[22] The origin of the absorptions at 370
and 390 nm cannot be attributed to secondary photoproduct 4
because its calculated UV/Vis spectrum (TD-B3LYP/6-311 +
G(d,p)) gives a longest-wavelength absorption at lmax
=
255 nm. Among the compounds considered, the calculated
spectrum of didehydrophosphepine 6 (lmax = 360 and 385 nm)
matches best the experimental data.[23,24] Formation of 6 from
3 may occur by ring enlargement, either directly,[25] or via
bicyclic phosphirene 7.[26] Borden and co-workers showed by
using multiconfigurational second-order perturbation theory
(CASPT2) that the ring expansion of singlet phenylphosphi-
nidene 3A to 6A (via 7A) is endothermic with sizeable
barriers (Scheme 2);[27] our calculations at the B3LYP/6-
31G(d) level give similar results, except for the state 1A2 of the
parent molecule.[28] The tentative assignment of didehydro-
phosphepine 6 as a minor photoproduct may appear surpris-
ing as it is the least-stable isomer of those considered, though
bulky linear phospha-allenes are known.[29] However, steady-
state irradiation under conditions of matrix-isolation spec-
troscopy frequently allows for the observation of processes
that are thermodynamically unfavorable and whose outcome
mostly depends on the concentration of photostationary
equilibriums.
Scheme 2. B3LYP/6-31G(d) energies [kcalmolꢀ1] for the C9H11P and
unsubstituted C6H5P (A) isomers relative to triplet 3 and 3A. CASPT2-
(8/8)/6-31G(d) energies for A are given in parentheses.
The UV/Vis spectrum further supports the photochemical
formation of 3 from matrix-isolated phosphiranes 1 (and 2).
Photolysis (lexc = 254 nm, 80 min, Ar, 10 K) gave rise to two
new strong absorptions at l = 278 and 292 nm, three weaker
absorptions around 350, 370, and 390 nm, and a weak, broad
absorption with fine structure between 420 and 470 nm.
Subsequent long-wavelength photolysis (lexc > 455 nm)
resulted in depletion of the absorptions at l = 278, 292, 350,
and > 420 nm, while those at l = 370 and 390 nm initially
increased in intensity (Figure 3). Extended near-UV photol-
ysis (lexc = 360–400 nm, 2 h) resulted in bleaching of all
absorptions with l > 300 nm.
Finally, we explored the laser flash photolysis (LFP) of an
argon-purged cyclohexane solution of 1 at ambient temper-
ature with an excitation of 266 nm and nanosecond time
resolution. The resulting transient spectrum (Figure 4) con-
tained a single species with absorptions at lmax = 285 nm (vs)
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Compex 110 excimer laser operated with Kr/F2 (248 nm, 200 mJ per
pulse) or externally (T= 77 K) by using l = 254 nm (Hg low-pressure
lamp). W-band EPR-spectroscopic measurements employed a Bruker
Elexsys E680 spectrometer. Prior to measurement, a degassed
solution of 2 in cyclohexane was irradiated (l = 254 nm, 60 min, T=
77 K) in a quartz capillary cooled to 77 K in a quartz dewar and
transferred frozen into the precooled cryostat. Experimental param-
eters: frequency = 94.185061 GHz, microwave power= 15 dB, modu-
lation amplitude = 0.1 mT, modulation frequency = 100 kHz. Simula-
tions were performed by using the Easyspin routine[33] and all the
parameters entering the simulation are given in the figure caption.
The setup used for laser flash photolysis has been described
before.[34] Solutions (ca. 0.1 mm) of
1 in cyclohexane (Baker,
spectroscopic grade) were purged with Ar for 20 min prior to the
experiment. Tetramethylallene (Aldrich) and ethyl propiolate
(Fluka) were of the highest purity commercially available and were
used as received. 1-Hexene (Aldrich) was freshly distilled immedi-
ately prior to use.
Figure 4. Transient spectra, recorded after LFP (lexc =266 nm) of phos-
phirane 1 in cyclohexane under Ar atmosphere. Black circles: 470 ns
after LFP, white circles: 1.7 ms after LFP, black triangles: 75 ms after
LFP. Inset: transient trace, monitored at l=285 nm.
All calculations were performed with Gaussian98.[35] All geom-
etry optimizations and frequency calculations (scaled by 0.97)[36] were
performed at the (U)B3LYP/6-31G(d) level of theory. UV spectra
were calculated using time-dependent density functional theory
(TDDFT; B3LYP/6-311 + G(d,p)).
and 400–475 nm (broad, weak) and a lifetime t = 13 ms, which
we assign to phosphinidene 3 based on the similarity with the
UV/Vis spectrum of the matrix-isolated species at 10 K
(Figure 3). The transient species could be quenched with
oxygen,[30] and it reacted with the p-systems ethyl propiolate
Received: December 9, 2004
Published online: April 21, 2005
HC CCOOEt (kETP = (7.7 ꢃ 1.1) ꢀ 106 Lmolꢀ1 sꢀ1) and tetra-
ꢂ
methylallene (kTMA = (5.0 ꢃ 0.8) ꢀ 106 Lmolꢀ1 sꢀ1, see the Sup-
porting Information) but not with 1-hexene. This reactivity
supports the formation of triplet 3, which is expected to give
an allylic triplet diradical with tetramethylallene, whereas
such stabilization cannot occur with hexene as a reaction
partner.
Keywords: EPR spectroscopy · laser spectroscopy ·
.
matrix isolation · photolysis · radicals
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In conclusion, we have unequivocally identified triplet
mesitylphosphinidene 3 as the primary reaction product in the
photolytic cleavage of phosphiranes 1 and 2 by using a variety
of low-temperature and time-resolved spectroscopic techni-
ques. Under conditions of matrix isolation, 3 is formed in very
low yield only, which is likely to be due to the efficient
photoinduced addition of 3 to the alkene still present in the
matrix cage. Upon further irradiation, 3 rearranges into
1H,2H-dihydrobenzophosphete 4. Results obtained by using
laser flash photolysis provide further support for the forma-
tion of 3, and show that 3 will readily be quenched by reactive
p-systems. Further studies on the photochemistry of 1, 2, and
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The matrix isolation setup used in this work has been described
before.[31] As light sources, a Hg low-pressure lamp (l = 254 nm;
Grꢁntzel, Germany) and Hg high-pressure lamps (Osram, 500 W,
Oriel housing) in combination with cut-off filters were used. Argon
(Messer-Griesheim, 99.999%) was used as the matrix material.
Compounds
1
and
2
were synthesized according to reported
[14] A. W. Ehlers, K. Lammertsma, E. J. Baerends, Organometallics
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procedures.[32] Deposition was performed by using the slow-spray-
on technique at a sample temperature of 208C. Trans-2-butene (99%)
was supplied by Matheson and deposited as 0.1%/99.9% mixture
with Ar. X-band EPR-spectroscopic measurements were performed
by using a Bruker Elexsys E500 EPR spectrometer with an ER077R
magnet (75 mm pole cap distance) and an ER047 XG-T microwave
bridge. Frozen solutions of 2 in methylcyclohexane were irradiated
both within the resonator cavity (T= 5 K) by using a Lambda-Physik
´
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presence of the alkene in the matrix cage decreases with longer
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irradiation times. b) The observed spectrum of 3 has two
medium intensity bands at n˜ = 1040–1070 cmꢀ1 but only one is
calculated. This may be due to Fermi resonance between the
bands at n˜calcd = 1065.3 cmꢀ1 (A’) and n˜calcd = 572.8 cmꢀ1 (A’,
n˜exp. = 542.9 cmꢀ1), which would split the band at n˜calcd
=
1065.3 cmꢀ1 into the two components observed at 1028.6 and
1061.3 cmꢀ1
.
[19] U. Heim, H. Pritzkow, U. Fleischer, H. Grꢂtzmacher, M.
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9613; b) P. W. Thulstrup, L. Broge, E. Larsen, J. Springborg, J.
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[21] The formation of a triplet diradical by cleavage of only one P C
bond is considered unlikely. Not only is the triplet diradical
5.2 kcalmolꢀ1 less stable than triplet 3, its calculated IR spectrum
and UV spectrum do not correlate well with the experimental
spectra. See the Supporting Information for details.
[22] The reversibility of the phosphirane cleavage indicates that the
low yield of
3 is due to an unfavorable photostationary
equilibrium rather than a very low quantum yield or pronounced
filter effects.
[23] The observation of an extremely weak IR band at n˜ =
= =
1723.7 cmꢀ1 supports the formation of 6 (C C P stretching
mode: n˜calcd = 1712.9 cmꢀ1 (scaled by 0.97, weak).
[24] Phosphaquinone methide 5 (anti-5: lmax
lmax calcd = 495 nm) and bicyclic phosphirene 7 (lmax
and 494 nm) are less likely candidates.
,
calcd = 482 nm; syn-5:
,
,
calcd = 334
[25] J. C. Hayes, R. S. Sheridan, J. Am. Chem. Soc. 1990, 112, 5879 –
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[28] A spin-projection correction was applied, see E. Goldstein, B.
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[30] When the sample was briefly brought into contact with air, the
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[34] G. Bucher, Eur. J. Org. Chem. 2001, 2463 – 2475.
[35] Gaussian98 (RevisionA.3): M. J. Frisch et al., see Supporting
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[36] In the spectral region of interest, a scaling factor of 0.97 for the
fundamental frequencies was used because of its better corre-
lation with observed values than the 0.9614 recommended in
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Angew. Chem. Int. Ed. 2005, 44, 3289 –3293
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3293