Photochemistry of matrix-isolated cyclopentadienylidene revisited

Article abstract of DOI:10.1016/S0022-2860(00)00631-1

Irradiation of diazocyclopentadiene (1) in a matrix at 10 K with the wavelength λ = 366 nm leads - as already known - to rapid loss of nitrogen and the formation of cyclopentadienylidene (T-2). Upon photoexcitation of cyclopentadienylidene (2) with light of the wavelength λ = 313 nm the (s-E)-(E)-conformer of 2-penten-4-yn-1-ylidene (T-7a) can be detected, which is effectively converted into 3-ethynylcyclopropene (6) by using light with a wavelength of λ = 436 nm. The structural elucidation of T-7a and 6 is based on the comparison of the experimental and calculated IR spectra. Irradiation of cyclopentadienylidene (2), isolated together with the eliminated nitrogen in the same matrix cage, with light of wavelengths longer than 570 nm initiates a partial back-reaction to the starting material, diazocyclopentadiene (1) by recapturing the evolved nitrogen molecule.

Full text of DOI:10.1016/S0022-2860(00)00631-1

Journal of Molecular Structure 556 (2000) 179–187
www.elsevier.nl/locate/molstruc
Photochemistry of matrix-isolated cyclopentadienylidene revisited
*
G. Maier , J. Endres
Institut fu¨r Organische Chemie der Justus-Liebig-Universita¨t, Heinrich-Buff-Ring 58, 35392 Giessen, Germany
Dedicated to Professor Lou Allinger in recognition of his significant contributions to the field of molecular mechanics
Received 6 December 1999; accepted 15 February 2000
Abstract
Irradiation of diazocyclopentadiene (1) in a matrix at 10 K with the wavelength l ˆ 366 nm leads—as already known—to
rapid loss of nitrogen and the formation of cyclopentadienylidene (T-2). Upon photoexcitation of cyclopentadienylidene (2)
with light of the wavelength l ˆ 313 nm the (s-E)-(E)-conformer of 2-penten-4-yn-1-ylidene (T-7a) can be detected, which is
effectively converted into 3-ethynylcyclopropene (6) by using light with a wavelength of l ˆ 436 nm. The structural elucida-
tion of T-7a and 6 is based on the comparison of the experimental and calculated IR spectra. Irradiation of cyclopentadieny-
lidene (2), isolated together with the eliminated nitrogen in the same matrix cage, with light of wavelengths longer than 570 nm
initiates a partial back-reaction to the starting material, diazocyclopentadiene (1) by recapturing the evolved nitrogen molecule.
᭧ 2000 Elsevier Science B.V. All rights reserved.
Keywords: Photochemistry; Carbenes; Matrix isolation; Quantum mechanical calculations
1. Introduction
co-workers led to the conclusion that cyclopentadie-
nylidene (2) can exist as the monomer and has a triplet
ground-state with a C2v-symmetrical structure [4].
One of the two unpaired electrons should be located
at the carbenic center while the other electron is delo-
calized over the five-membered carbocyclic ring.
Poliakoff and co-workers in 1981 reported for the
first time the spectroscopic (IR and UV/Vis) identifi-
cation of cyclopentadienylidene (2) in various matrix
materials (e.g. nitrogen and carbon monoxide) [5].
Cyclopentadienylidene has a long history. Shortly
after the first application of the matrix isolation tech-
nique for the study of highly reactive molecules [1,2]
DeMore and co-workers photolyzed diazocyclopenta-
diene (1) in a nitrogen matrix at 20 K in order to
generate cyclopentadienylidene (2) [3]. These authors
were only able to detect the dimer of 2, fulvalene (3).
An EPR-experiment carried out by Wasserman and
∆
ν
2
* Corresponding author. Tel.: ϩ49-641-99-34300/301; fax; ϩ49-641-99-34309.
0022-2860/00/$ - see front matter ᭧ 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(00)00631-1

180
G. Maier, J. Endres / Journal of Molecular Structure 556 (2000) 179–187
structures. The C2v-symmetrical singlet cyclopenta-
1
dienylidene (in its A1 electronic state) is reported
to have two imaginary frequencies and to lie
15.3 kcal molϪ1 above bicyclo[2.1.0]penta-2,4-
diene.
After reading this report we decided to study cyclo-
pentadienylidene (2) once more. Since in our goal to
identify the photobehavior of 2 we have to rely upon
the comparison of the calculated with the experimental
matrix IR spectra, we have performed additional calcu-
lations at the B3LYP level of theory using a standard 6-
311 ϩ G(d,p) basis set of the ground state multiplicity,
the geometry and the vibrational spectrum of 2 and its
isomers [7]. In these calculations cyclopentadienyli-
dene (2) has a triplet ground state (3B1, T-2) and is
4.3 kcal molϪ1 below the first excited singlet state
(1A2, S-2, Scheme 1) (Calculated (B3LYP/6-
311 ϩ G(d,p)) relative energies of the singlet state
(1A2) and the triplet state (3B1) of cyclopentadienyli-
dene (2).). This result is in good agreement with the
calculations published by Schaefer and co-workers
ꢀDES–T ˆ ϩ6:3 kcal molϪ1† [8].
Scheme 1.
The photochemistry of 2 has not been widely
studied yet. Poliakoff and co-workers report that 2
forms a terminal alkyne, probably 3-ethynylcyclopro-
pene (6), upon irradiation with wavelengths longer
than 200 nm [5].
2. Results and discussion
2.1. Calculations
Neither the 1,2,4-cyclopentatriene nor bicyclo-
[2.1.0]penta-2,4-diene structures nor the C2v-symme-
trical cyclopentadienylidene (in its 1A1 electronic
state) were found to be stationary points on the singlet
energy hypersurface. A closer examination of the wave
functions used by Lewars (see above) shows clearly that
Lewars performed numerous calculations on the
C5H4 energy hypersurface [6]. According to these
calculations the minima of the singlet energy
hypersurface are best represented by the 1,2,4-
cyclopentatriene and bicyclo[2.1.0]penta-2,4-diene
1.457
-1.115
111.2
138.4
142.6
107.8
112.9
108.1
104.5
0.569
-0.100
148.3
106.2
108.2
109.0
137.3
108.2
0.371
0.039
148.2
135.7
108.1
S-2
T-2
Fig. 1. Calculated (B3LYP/6-311ϩG(d,p)) bond lengths (pm), angles (Њ) and Mulliken spin densities (italics) for singlet (S-2) and triplet
cyclopentadienylidene (T-2).

G. Maier, J. Endres / Journal of Molecular Structure 556 (2000) 179–187
181
Scheme 2.
3.0
2.5
..
H
T-7a
2.0
1.5
A
1.0
0.5
0.0
T-2
250
300
350
450
500
400
λ /nm
Fig. 2. Experimental difference UV spectrum after 10 min irradiation of cyclopentadienylidene (T-2) with light of the wavelength l ˆ 313 nm.
Absorptions with negative values decreased upon irradiation.

182
G. Maier, J. Endres / Journal of Molecular Structure 556 (2000) 179–187
they are not stable. There exists a wave function that is
energetically more favorable [6]. This stable function,
used in our calculations, leads to the open shell cyclo-
pentadienylidene (S-2) in its 1A2 electronic state.
lated relative energies (kcal molϪ1) of selected
isomers of the elemental composition C5H4 (B3LYP/
6-311ϩG(d,p))).
On the basis of a B3LYP/6-311ϩG(d,p) calculation
S-2 has a planar C2v-symmetrical structure, in agree-
ment by calculations previously published by
Schaefer and co-workers [8]. The structure of S-2 is
rather astonishing. The calculated bond distance of the
C3–C4 bond (C1 is the carbenic center) is in the range
of a typical CyC double bond, while the C2–C3 bond
length is comparable with a C–C single bond between
two sp2-carbon atoms. The calculated Mulliken spin
2.3. Photolysis of diazocyclopentadiene (1) in argon
matrices
The matrix UV spectrum of diazocyclopentadiene
(1) shows only one intense absorption at lmax ˆ 293
nm [5]. The concentration is too low for detection
of the long tailing of this band into the visible region
(1 forms yellow crystals, mp, 22ЊC [9])
densities show clearly that one of the two (paired)
electrons is localized at the carbenic carbon atom
(C1), while the other electron is delocalized over
C5, C1 and C2 (Fig. 1).
Photolysis of 1 in an argon matrix with light of the
wavelength l ˆ 313 nm leads after 2 min to several
new absorptions in the UV spectrum (Fig. 2), which
can be assigned to cyclopentadienylidene (2) and its
dimer 3 by comparison with the absorptions observed
by Poliakoff et al. [5] and DeMore et al. [7] (only
spectrum of 3). Conspicuous is the appearance of a
double band at lmax ˆ 238/245 nm which has not been
described by these authors. Although the character-
istic UV absorptions of cyclopentadienylidene (2)
and fulvalene (3) can be observed in the matrix UV
spectrum, the matrix IR spectrum shows after photo-
lysis of diazocyclopentadiene (1) only the absorptions
of 2, but not of 3. Obviously the intensities of the
absorptions of 3 are too weak to be detected in the
matrix IR spectrum. An additonal absorption at
approx. 3330 cmϪ1, that is typical of an acetylenic
C–H stretching vibration, can be observed under
these conditions. This can be assigned, as is discussed
below, to carbene T-7a.
The calculated structure of the triplet state of cyclo-
3
pentadienylidene (T-2, B1, Fig. 1) is also in good
agreement with the calculations of Schaefer and co-
workers [8].
The calculated Mulliken spin densities indicate that
one of the two unpaired electrons is localized at the
carbenic center (C1) while the other electron is delo-
calized over the five-membered carbocyclic ring, as
has been predicted by Wasserman and co-workers
from their EPR-experiments [4].
2.2. C5H4 energy hypersurface
Numerous isomers of the elemental composition
C5H4 are conceivable. A few of them have been calcu-
lated within the scope of this work (Scheme 2) (Calcu-

G. Maier, J. Endres / Journal of Molecular Structure 556 (2000) 179–187
183
H
:
T-7d
..
H
T-7c
H
:
T-7b
..
H
T-7a
0.1
0.0
A
?
4
*
-0.1
6
4000 3500 3000 2500 2000
1500
v/cm
1000
500
-1
~
Fig. 3. Comparison of the experimental difference IR spectrum (absorptions with negative values increased upon irradiation) after irradiation of
the photoproducts of T-2 (generated by irradiation with l ˆ 313 nm) with light of the wavelength l ˆ 436 nm with the calculated (B3LYP/6-
311ϩG(d,p)) IR spectra of the four possible conformers of 2-penten-4-yn-1-ylidene (T-7a–d) and 3-ethynylcyclopropene (6). The asterisk
marks a combination band of 6 (n10 ϩ n13).

184
G. Maier, J. Endres / Journal of Molecular Structure 556 (2000) 179–187
However, a more selective photolysis of 1 is
absorption at 3328.6 cmϪ1 diminishes in the matrix IR
spectrum, while a new band at 3334.1 cmϪ1 arises.
Both absorptions are in the typical region of C–H
stretching vibrations of a C–H bond. The experi-
mental difference spectrum of this end product of
the photolytic conversion with l ˆ 313 nm is in
very good agreement with a sum of the calculated
IR spectra of the ring-opened isomer, 2-penten-4-yn-
1-ylidene in the triplet ground state (T-7a; photopro-
duct of T-2) and 3-ethynylcyclopropene (6; photopro-
duct of T-7a; Fig. 3, Tables 1 and 2).
possible by use of light of wavelength l ˆ 366 nm.
The only photoproduct of such an irradiation is cyclo-
pentadienylidene (T-2). Other than a broad absorption
at lmax ˆ 296 nm no double band at lmax ˆ 238/
245 nm can be observed in the experimental matrix
UV spectrum. The observed IR absorptions of T-2 in
an argon matrix are in good agreement with the
absorptions observed in a nitrogen matrix by Poliakoff
and co-workers [5].
Subsequent irradiation of T-2 with light of wave-
lengths longer than 570 nm (20 h) leads back to a
small amount of diazocyclopentadiene (1). In this
context it is no surprise that irradiation of 1 in a 15N2
matrix creates, after short irradiation time, a new diazo
absorption at 2038.3 cmϪ1. The experimental isotopic
shift ꢀDn~exp ˆ Ϫ65 cmϪ1† for the diazo vibration of 1
(2103 cmϪ1 for 1, 2038 cmϪ1 for [15N2]-1) is in very
good agreement with the calculated isotopic shift
ꢀDn~calc ˆ Ϫ73 cmϪ1; 2205 cmϪ1 for 1, 2132 cmϪ1
for [15N2]-1). Apparently T-2 formed upon irradiation
reacts with the the nitrogen molecules of the matrix
under back-formation of diazocyclopentadiene (1).
Secondary photolysis of cyclopentadienylidene (T-
2) with light of wavelength l ˆ 313 nm initiates the
rapid decomposition of T-2 and several new IR
absorptions between 540 and 640 and a band at
3328.6 cmϪ1 appear. These data are of relevance to
the mechanism of the known isomerization T-2 ! 3-
ethynylcyclopropene (6) [5]. However, the calculated
spectrum of 3-ethynylcyclopropene (6) is only in poor
agreement with the experimental difference spectrum
after longer (10 min) irradiation of T-2 with
l ˆ 313 nm. Based on the IR absorptions of 6 and
T-7a (see below) the difference spectrum contains
the bands of T-7a and a small amount of 6. The matrix
UV spectrum after the secondary photolysis of T-2
with l ˆ 313 nm shows an increase of the double
band at lmax ˆ 238/245 nm and of a new absorption
between 380 and 458 nm, while the absorption of T-2
decreases. So it is tempting to assume the UV absorp-
tions appearing upon irradiation of T-2 belong to the
ring-opened product T-7a (Fig. 2).
The (s-E)-(E)-conformer of 2-penten-4-yn-1-
ylidene (T-7a) rearranges upon further irradiation
with light of the wavelength l ˆ 436 nm to 3-
ethynylcyclopropene (6). To a very small extent
T-7a also undergoes a 1,2-hydrogen-shift gener-
ating ethynylallene (4) as indicated by a tiny
band at 1954.6 cmϪ1
.
In an argon matrix 3-ethynylcyclopropene (6) is
stable against irradiation with light of wavelength
l ˆ 254 nm. In contrast, if 6 is irradiated in a xenon
matrix for 22 h with l ˆ 254 nm it is completely
converted into 1,4-pentadiyne (5).
3. Conclusion
Diazocyclopentadiene (1) can be selectively photo-
lyzed with light of the wavelength l ˆ 366 nm with
formation of cyclopentadienylidene (T-2). Irradiation
of T-2 with light of wavelengths longer than 570 nm
initiates a partial back-reaction to 1 as T-2 recaptures
the evolved nitrogen molecule. Subsequent photolysis
of T-2 with light of the wavelength l ˆ 313 nm
generates the (s-E)-(E)-conformer of 2-penten-4-yn-
1-ylidene (T-7a), which can be photochemically
converted into 3-ethynylcyclopropene (6) upon irra-
diation with light of the wavelength l ˆ 436 nm. 6 is
stable against irradiation with l ˆ 254 nm in an argon
matrix, but in a xenon matrix 6 rearranges to 1,4-
pentadiyne (5).
4. Experimental section
Further irradiation of the photoproducts of T-2
(formed by irradiation with l ˆ 313 nm) with light
of the wavelength l ˆ 436 nm finally results in a
decrease of the newly formed UV absorptions. Simul-
taneously it can be observed that the intensity of the
4.1. Cryostat
Displex closed cycle refrigeration system HC-2
from APD.

G. Maier, J. Endres / Journal of Molecular Structure 556 (2000) 179–187
185
Table 1
Comparison of the calculated (B3LYP/6-311ϩG(d,p)) IR spectrum of the (s-E)-(E)-conformer of 2-penten-4-yn-1-ylidene (T-7a) with the
experimental (argon, xenon, nitrogen, 10 K) IR absorptions of T-7a (derived from the disappearance of T-7a upon irradiation with l ˆ 436 nm)
Sym.
A0
n
(cmϪ1) (Irel)a
n
(cmϪ1) (Irel)
Ärel
Äexp
n1
nxC–H
Ar: 3328.6 (m)
N2: 3322.2 (m)
Xe: 3305.6 (m)
–
–
–
3471.1 (1.00)b
n2
n3
n4
n5
n6
A0
A0
A0
A0
A0
n:C–H
nCH
3248.2 (0.03)
3156.9 (0.02)
3047.3 (0.04)
2091.2 (0.00)
1463.1 (0.11)
nCH
nCxC
–
d
d
d
yCH ϩ nCyC (i.p.)
Ar: 1431.1 (w)
N2:–
Xe: 1425.5 (w)
Ar:–
N2: 1362.3 (w)
Xe:–
Ar:–
N2: 1208.0 (w)
Xe:–
Ar: 1137.5 (w)
N2: 1161.2 (w)
Xe: 1078.7 (w)
–
Ar: 840.6 (m)
N2: 850.3 (w)
Xe: 841.6 (w)
Ar: 774.0 (w)
N2: 764.9 (vw)
Xe:–
n7
n8
n9
A0
A0
A0
yCH ϩ nC–C (i.p.)
1363.2 (0.04)
1236.2 (0.04)
1186.9 (0.01)
yC–H (i.p.)
nCyC
n10
n16
A0
dCCC (i.p.)
1057.0 (0.04)
870.5 (0.14)
A0 0
d
yCH (o.o.p.)
n11
n17
n12
n18
n19
n20
A0
dCH (i.p.)
795.7 (0.25)
772.1 (0.15)
676.8 (0.62)
593.5 (0.77)
561.1 (0.60)
559.2 (0.08)
A0 0
A0
d
d
yCH (o.o.p.)
yCH (i.p.)
Ar: 745.5 (w)
N2:–
Xe:–
Ar: 629.6 (s)
N2: 641.4 (s)
Xe: 652.2 (s)
Ar: 563.6 (s)
N2: 580.1 (s)
Xe: 572.3 (m)
Ar: 550.6 (s)
N2: 563.6 (s)
Xe: 556.1 (s)
Ar: 542.7 (w)
N2:–
A0 0
A0 0
A0 0
dCH (o.o.p.)
d
xC–H (o.o.p.)
dCCC (o.o.p.)
Xe: 553.0 (w)
–
–
–
–
n13
n21
n14
n15
A0
dC–CxC (i.p.)
dC–CxC (o.o.p.)
dCCC (i.p.)
433.0 (0.04)
397.7 (0.10)
174.5 (0.01)
166.6 (0.00)
A0 0
A0
A0 0
dC–CxC (o.o.p.)
a
vs ˆ very strong, s ˆ strong, m ˆ medium, w ˆ weak.
Calc. abs. intensity: 81.9 km molϪ1
b
.

186
G. Maier, J. Endres / Journal of Molecular Structure 556 (2000) 179–187
Table 2
4.2. Spectrometers
Comparison of the calculated (B3LYP/6-311ϩG(d,p)) IR spectrum
of 3-ethynylcyclopropene (6) with the experimental (argon, xenon,
nitrogen, 10 K) IR absorptions of 6 (derived from the appearance of
6 upon irradiation of T-7a with l ˆ 436 nm)
IR: FT-IR spectrometer IFS 55 from Bruker, reso-
lution 0.7 cmϪ1; UV: Diode Array Spectrometer HP
8453 from Hewlett–Packard (190–1100 nm, 1 diode
nmϪ1).
Sym.
A0
n
(cmϪ1) (Irel)
n
(cmϪ1) (Irel)
Ärel
Äexp
n1
n2
nxC–H
Ar: 3334.1 (m)
N2: 3320.0 (m)
Xe: 3313.3 (m)
–
3478.0 (0.85)
4.3. Light sources
Mercury high-pressure lamp HBO 200 from Osram
with monochromator or cut-off filters and mercury
low-pressure spiral lamp from Grantzel.
A0
nyCH
nyCH
nCH
3300.8 (0.00)
3251.6 (0.02)
3075.5 (0.32)
n14 A0 0
–
A0
A0
A0
A0
A0
Ar: 2954.4 (w)
N2: 2978.4 (w)
Xe: 2967.0 (w)
Ar: 2125. 8 (w) 2200.5 (0.13)
N2: 2121.3 (w)
¨
n3
n4
n5
n6
n7
4.4. Diazocyclopentadiene (1)
nCxC
Prepared according to the literature [10], using
cyclopentadiene and tosylazide in the presence of
piperidine as a base. 1 was used for the matrix-isola-
tion experiments without further purification. Matrix
isolated samples were prepared by sublimation of 1 at
Ϫ50ЊC at approx. 10Ϫ6 mbar and deposition on a CsI
window at 10 K together with a large excess of inert
gas.
Xe: 2116.4 (w)
nCyC
Ar: 1662.6 (m)
N2: 1653.2 (m)
Xe: 1653.2 (m)
Ar: 1284.9 (w)
N2: 1340.6 (w)
Xe: 1277.3 (w)
–
1727.4 (0.45)
1365.8 (0.10)
1151.1 (0.02)
dyCH (i.p.)
dCCC (i.p.)
dCCC (i.p.)
n15 A0 0
Ar: 1017.7 (vw) 1045.1 (0.03)
N2: 1024.6 (vw)
Xe: 1017.8 (vw)
n16 A0 0
dCH (o.o.p.)
Ar: 982.5 (s)
N2: 987.6 (s)
Xe: 981.4 (s)
Ar: 944.7 (m)
N2: 945.7 (m)
Xe: 940.3 (m)
Ar: 889.4 (m)
N2: 889.3 (w)
Xe: 882.6 (w)
–
Ar: 793.5 (m)
N2: 797.8 (m)
Xe: 790.1 (m)
Ar: 643.4 (s)
N2: 657.7 (s)
Xe: 639.2 (s)
1001.3 (0.48)
957.8 (0.12)
905.6 (0.11)
Acknowledgements
n8
n9
A0
A0
dys CH (i.p.)
This work was supported by the Fonds der
Chemischen
Industrie
and
the
Deutsche
as
yCH
d
(i.p.)
Forschungsgemeinschaft. We wish to thank Dr
H.P. Reisenauer for hints and most helpful
discussions.
n17 A0 0
n18 A0 0
dCH (o.o.p.)
dCCC (o.o.p.)
857.8 (0.01)
805.4 (0.07)
References
n10 A0
n19 A0 0
n11 A0
dCCC (i.p.)
693.0 (0.58)
645.3 (0.65)
637.6 (1.00)a
[1] I. Norman, G. Porter, Nature 174 (1954) 508.
[2] E. Whittle, D.A. Dows, G.C. Pimentel, J. Chem. Phys. 22
(1954) 1943.
[3] W.B. DeMore, H.O. Pritchard, N.J. Davidson, J. Am. Chem.
Soc. 81 (1959) 5874.
dxC–H (o.o.p.) Ar: 611.1 (s)
N2: 617.9 (s)
Xe: 607.3 (s)
[4] E. Wasserman, L. Barash, A.M. Trozzolo, R.W. Murray, W.A.
Yager, J. Am. Chem. Soc. 86 (1964) 2304.
dyCH (i.p.)
Ar: 606.2 (s)
N2: 611.3 (s)
Xe: 598.6 (s)
–
Ar: 513.6 (m)
N2: 515.6 (m)
Xe: 511.6 (m)
–
[5] M.S. Baird, I.R. Dunkin, N. Hacker, M. Poliakoff, J.J. Turner,
J. Am. Chem. Soc. 103 (1981) 5190.
n20 A0 0
n12 A0
dCCC (o.o.p.)
dCCC (i.p.)
541.3 (0.00)
530.6 (0.19)
[6] E. Lewars, J. Mol. Struct. (Theochem) 423 (1998) 173.
[7] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill,
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M.A.Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B.
Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara,
n21 A0 0
n13 A0
dRing (o.o.p.)
dCCC (i.p.)
221.1 (0.05)
207.2 (0.05)
–
a
Calc. abs. intensity: 86.2 km molϪ1
.

G. Maier, J. Endres / Journal of Molecular Structure 556 (2000) 179–187
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