Triplet 1,3-diphenylpropynylidene (Ph-C-C-C-Ph)

Article abstract of DOI:10.1021/ja066300j

Photolysis (λ > 571 nm) of 1,3-diphenyldiazopropyne (9) affords triplet 1,3-diphenylpropynylidene (3), as characterized by IR, UV/vis, and EPR spectroscopy in low-temperature matrices. Two conformational isomers of triplet 3 are spectroscopically distingu

Full text of DOI:10.1021/ja066300j

Published on Web 02/06/2007
Triplet 1,3-Diphenylpropynylidene (Ph-C-C-C-Ph)
Jeffrey T. DePinto, Wendy A. deProphetis, Jessica L. Menke,
and Robert J. McMahon*
Contribution from the Department of Chemistry, UniVersity of Wisconsin-Madison,
1101 UniVersity AVenue, Madison, Wisconsin 53706-1396
Received August 30, 2006; E-mail: mcmahon@chem.wisc.edu
Abstract: Photolysis (λ > 571 nm) of 1,3-diphenyldiazopropyne (9) affords triplet 1,3-diphenylpropynylidene
(3), as characterized by IR, UV/vis, and EPR spectroscopy in low-temperature matrices. Two conformational
isomers of triplet 3 are spectroscopically distinguishable. The initially formed, non-relaxed conformer is
believed to reflect the geometry of the diazo precursor, as enforced by the rigid matrix. Annealing the
matrix permits the structure to relax to the equilibrium D2d geometry. The highly symmetric equilibrium
structure of 3 is best envisioned as a 1,3-allenic diradical. Density functional theory calculations suggest
that the equilibrium structure does not exhibit a bond-localized structure that would be characteristic of an
acetylenic carbene. Chemical trapping with O2, however, affords products that are familiar as carbene
trapping products: carbonyl oxide 10, ketone 11, and dioxirane 12. Irradiation (λ > 261 nm) of triplet 1,3-
diphenylpropynylidene (3) results in cyclization to singlet diphenylcyclopropenylidene (6), a process that is
photochemically reversible at λ ) 232 nm. Diphenyl-1,2-propadienylidene (7) was not observed under any
irradiation conditions.
1. Introduction
(Chart 1). Calculations predict that the nature of a defect in a
carbon chain displays an alternating dependence on chain
length: the defect in a chain of formula HC4n+1H (HCH, HC5H,
HC9H, etc.) is a carbene, while the defect in a chain of formula
HC4n+3H (HC3H, HC7H, HC11H, etc.) is an allenic 1,3-
diradical.30,31 Detailed calculations of triplet HC3H and HC5H
did not find multiple energy minima that correspond to localized,
bond-shift isomers on the potential energy surface.20,32 Never-
theless, all of these systems appear to exhibit reactivity that is
An understanding of the structure, properties, and reactivity
of acetylenic carbenes and conjugated carbon chains underpins
diverse topics in contemporary chemistry. Propynylidene (1),
the simplest acetylenic carbene, is an important intermediate in
astrochemistry1,2 and combustion processes,3,4 while organo-
metallic complexes of alkynyl carbenes are utilized in organic
synthesis.5,6 Fundamental knowledge of electronic delocalization
in carbon chains is central to the understanding of conducting
polymers and other advanced electronic and optical materials.7-12
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12574.
Key issues associated with acetylenic carbenes involve the
nature of the electronic delocalization and the possible involve-
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and five-carbon derivatives (1-5) have been investigated
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9
2308
J. AM. CHEM. SOC. 2007, 129, 2308-2315
10.1021/ja066300j CCC: $37.00 © 2007 American Chemical Society

Triplet 1,3-Diphenylpropynylidene
A R T I C L E S
Chart 1. Odd-Member Carbon Chains: Delocalized versus
Localized
Table 1. Computed (B3LYP/6-31G*) Energies for C15H10 Isomers
number of
a
energy
ZPVE
Erel
imaginary
isomer
(hartree)
(kcal/mol)
(kcal/mol)
frequencies
Figure 1. Computed relative energies (kcal/mol; B3LYP/6-31G*, including
ZPVE) of C15H10 isomers. (Structure 8 is a transition state.)
1
33a
13a
36
-577.4855212
-577.4671197
-577.4205414
-577.4921476
-577.412360
-577.4466075
-577.3645203
-577.3858107
-577.4266455
-577.4561440
121.37231
122.42340
121.32226
123.47655
121.66524
123.29231
121.90478
121.54504
121.13859
122.43292
2.1
14.7
42.8
0.0
48.3
16.2
78.5
64.8
38.8
21.5
0
0
0
0
0
0
0
1
0
0
of this size.) The computed relative energies for the C3Ph2
isomers are in reasonable accord with those of the parent C3H2
isomers.20 Singlet diphenylcyclopropenylidene (6) is the lowest
energy isomer in the series. Triplet 1,3-diphenylpropynylidene
(3) lies 2-3 kcal/mol higher, although this energy difference
is likely to be underestimated because of the tendency of density
functional theory to overemphasize delocalization in carbon
chain molecules and other conjugated systems.32,37,38 The
singlet-triplet energy gap (closed-shell singlet state) of 1,3-
diphenylpropynylidene (3) is approximately 12 kcal/mol, which
places the singlet state of 3 (Erel ) 14.6 kcal/mol) close in energy
to singlet diphenyl-1,2-propadienylidene (7) (Erel ) 16.2 kcal/
mol). Our earlier studies of photoinduced isotope label scram-
bling in the parent 1,2-propadienylidene led us to consider the
possible involvement of singlet cyclopropyne.20 We therefore
computed the cyclopropyne isomer in the diphenyl series. The
singlet state of diphenylcyclopropyne (8) is significantly higher
in energy than the other isomers (Erel ) 64.8 kcal/mol) and is
computed to be a transition state for the automerization of singlet
diphenyl-1,2-propadienylidene (7). The triplet state, however,
is computed to be a minimum on the potential energy surface.
Unlike the parent cyclopropyne, we could not locate a structure
for singlet diphenylcyclopropyne in which the tetracoordinate
carbon is planar, a result that is not surprising in view of the
steric requirements of the phenyl substituents.
16
37
17
38
18
316
116
a Includes ZPVE.
typical of a carbene.13,25,26,33 Transition metal complexes of these
carbenes (as singlets) do exhibit bond-shift isomerism and
carbenic reactivity at the remote position,34,35 which has been
exploited in synthetic methodology and in enyne metathesis
chemistry.5,6
In the current investigation, we describe experimental and
computational studies of triplet 1,3-diphenylpropynylidene (3).18
Bond-shift isomers, were they to exist for this Ph-C-C-C-
Ph species, would be degenerate by virtue of the symmetrical
disubstitution. EPR spectra reveal a type of isomerism that
cannot be attributed to bond-shift isomerism. We attribute these
isomers to non-relaxed and relaxed conformations of triplet 3.
The structure, spectroscopy, and photochemistry of triplet Ph-
C-C-C-Ph (3) will be described in detail.
2. Results and Discussion
2.1. Computational Studies. 2.1.1. C15H10 Isomers. Opti-
mized structures, harmonic vibrational frequencies, and infrared
intensities for singlet and triplet states of C15H10 isomers 3, 6,
7, 8, and 16 were computed using density functional theory
(B3LYP/6-31G*; Table 1, Figure 1).36 (Few viable alternatives
to DFT methods exist for computational studies of molecules
1
3
The ring-expanded structures 16 and 16 are analogous to
those observed in the rearrangements of arylcarbenes:39-43 116
(37) Plattner, D. A.; Houk, K. N. J. Am. Chem. Soc. 1995, 117, 4405-4406.
(38) Woodcock, H. L.; Schaefer, H. F., III; Schreiner, P. R. J. Phys. Chem. A
2002, 106, 11923-11931.
(39) Wentrup, C. ReactiVe Molecules; John Wiley: New York, 1984; p 333.
(40) Wentrup, C. In Methoden der Organischen Chemie (Houben-Weyl); Regitz,
M., Ed.; G. Thieme: Stuttgart, 1989; Vol. E19b, pp 824-1021.
(41) Matzinger, S.; Bally, T.; Patterson, E. V.; McMahon, R. J. J. Am. Chem.
Soc. 1996, 118, 1535-1542.
(42) Wong, M. W.; Wentrup, C. J. Org. Chem. 1996, 61, 7022-7029.
(43) Schreiner, P. R.; Karney, W. L.; Schleyer, P. v. R.; Borden, W. T.; Hamilton,
T. P.; Schaefer, H. F. J. Org. Chem. 1996, 61, 7030-7039.
(33) Wierlacher, S.; Sander, W.; Marquardt, C.; Kraka, E.; Cremer, D. Chem.
Phys. Lett. 1994, 222, 319-324.
(34) Casey, C. P.; Kraft, S.; Powell, D. R. J. Am. Chem. Soc. 2002, 124, 2584-
2594.
(35) Casey, C. P.; Dzwiniel, T. L.; Kraft, S.; Guzei, I. A. Organometallics 2003,
22, 3915-3920.
(36) Details are available as Supporting Information.
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J. AM. CHEM. SOC. VOL. 129, NO. 8, 2007 2309

A R T I C L E S
DePinto et al.
Table 2. Computed (B3LYP/6-31G*) Energies for Conformers of Triplet Ph-C-C-C-Ph (3)
number of
imaginary
frequencies
a
C
−
C
−
θ
Ph
dihedral
energy
ZPVE
Erel
isomer
angle (
,deg)
angle (
φ
,deg)b
(hartree)
(kcal/mol)
(kcal/mol)
33b
33b
33b
33b
33b
33a
33a
125
0
0
2
55
88
90
0
-577.472466
-577.479233
-577.482621
-577.484232
-577.485198
-577.4855212
-577.4846747
122.05664
121.95810
121.80839
121.63951
121.57039
121.37231
121.56935
8.9
4.5
2.3
1.1
0.4
0.0
0.7
0
0
0
0
0
0
1
135
145
155
165
180
180
a Includes ZPVE. b Dihedral angle between the planes defined by the phenyl rings.
Scheme 1
is a 1,2,4,6-cycloheptatetraene derivative,44 while 316 is a 2,4,6-
cycloheptatrienylidene derivative.45 Although the structures are
analogous, the energetics appear to be rather different. In the
parent case, 1,2,4,6-cycloheptatetraene is approximately 13 kcal/
mol lower in energy than triplet phenyl carbene;41-43 in the
1
current system, cycloheptatetraene derivative 16 is approxi-
mately 20 kcal/mol higher in energy than triplet 3a. We surmise
that these differences arise from a significantly greater thermo-
dynamic stability of triplet 3a versus triplet phenyl carbene. The
increased delocalization that is inherent in the “propargylene”
moiety of triplet 3a, along with the increased delocalization
afforded by the two phenyl substituents, represents a structure
that is considerably more delocalized, and greatly stabilized,
relative to triplet phenyl carbene. Moreover, the delocalization
in triplet 3a is likely to be overestimated by the computational
method employed (see above), which will accentuate the
computed energy difference between singlet 16 and triplet 3a.
It seems unlikely that a phenylethynyl substituent imparts a
significant perturbation to the relative thermodynamic stabilities
of the 1,2,4,6-cycloheptatetraene moiety.
2.1.2. Ph-C-C-C-Ph (33). In the current investigation, a
key point of interest centers on structural details of the triplet
ground state of 1,3-diphenylpropynylidene (33). We studied
conformational isomers that exhibit variation with respect to
both the C-C-Cipso angle (θ) of the carbon chain and the
dihedral angle (φ) between the phenyl rings (Table 2). DFT
calculations predict a D2d structure for triplet 3 in which the
three-carbon chain is linear and the phenyl rings lie in
perpendicular planes (33a, φ ) 90°; Figure 1). A structure in
which the phenyl rings are coplanar (33a, φ ) 0°) lies <1 kcal/
mol higher in energy; this structure is a transition state with a
low-frequency imaginary vibration (18i cm-1) involving tor-
sional motion of the phenyl rings. Calculation of the coplanar
structure (33a, φ ) 0°), using a larger basis set, affords a
transition state with an imaginary vibration of even lower
surface for phenyl ring rotation in triplet 3 is very flat. At this
level of theory, the coplanar structure (33a, φ ) 0°) is a transition
state for aryl ring rotation in the perpendicular triplet 33a (φ )
90°).
Spurred by the experimental observation of significant
annealing effects on the EPR spectra of triplet 3, we computed
a series of structures for triplet 3 in which the C-C-Cipso angle
(θ) of the carbon chain was varied from that of the diazo
compound precursor (θ ) 125°) to that of the fully optimized
33a (θ ) 180°) (Table 2). It is interesting to note that bending
the linear triplet 3 by 35° (from θ ) 180° to θ ) 145°), which
is accompanied by a significant change in the dihedral angle
between the phenyl rings (from φ ) 90° to φ ) 2°), costs only
2 kcal/mol in energy. This behavior underscores the flatness of
the potential energy surface. Even at θ ) 125°, the structure
optimizes to a minimum on the potential energy surface with
an energy that is only 9 kcal/mol higher than that of the fully
optimized triplet 3a. The closed-shell singlet state of 3 is
computed to exhibit a C2h structure that appears to be best
depicted as an allenic 1,3-diradical (13a).36
2.2. Generation and Characterization of Ph-C-C-C-
Ph (3). Photolysis (λ > 571 nm) of 1,3-diphenyl-diazopropyne
(9) (Ar, 10 K) produces spectroscopic features readily attribut-
able to triplet 1,3-diphenylpropynylidene (3) (Scheme 1), as
monitored by EPR (Figure 2), UV/vis (Figure 3), and IR
spectroscopy (Figure 4). As described previously, the triplet EPR
spectrum obtained following the initial photolysis changes rather
frequency than the earlier calculation (6i cm-1 vs 18i cm-1
;
B3LYP/6-311G* vs B3LYP/6-31G*). Attempts to identify a
stationary point (either a transition state or an energy minimum)
with a dihedral angle between the phenyl rings of approximately
45° invariably led to the perpendicular triplet (33a, φ ) 90°).
The identification of a specific conformer of triplet 3 as an
energy minimum or a transition state represents a subtle issue
that may be beyond the capabilities of the theoretical methods
employed in the current investigation. The potential energy
(44) McMahon, R. J.; Abelt, C. J.; Chapman, O. L.; Johnson, J. W.; Kreil,
C. L.; LeRoux, J.-P.; Mooring, A. M.; West, P. R. J. Am. Chem. Soc. 1987,
109, 2456-2469.
(45) Kuzaj, M.; Lu¨erssen, H.; Wentrup, C. Angew. Chem., Int. Ed. Engl. 1986,
25, 480-482.
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Triplet 1,3-Diphenylpropynylidene
A R T I C L E S
Figure 3. UV/vis spectra: (a) diphenyldiazopropyne (9) prior to irradiation
(Ar, 10 K); (b) triplet 1,3-diphenylpropynylidene (3) obtained upon
photolysis (λ > 571 nm) of 9 (Ar, 10 K); (c) spectrum obtained upon
warming the matrix to 39 K and recooling to 10 K; (d) diphenylcyclopro-
penylidene (6) obtained upon irradiation of triplet 3 (λ > 261 nm; Ar, 10
K); and (e) triplet 1,3-diphenylpropynylidene (3) regenerated upon short-
wavelength irradiation of 6 (λ ) 232 ( 10 nm; Ar, 10 K).
Figure 2. EPR spectra of triplet 1,3-diphenylpropynylidene (3) obtained
upon irradiation (λ > 571 nm) of 1,3-diphenyldiazopropyne (9). (a,c,e)
Initially formed triplet prior to annealing. (b,d,f) Relaxed triplet after
annealing.
splitting parameter, D, reveals increasing delocalization upon
phenyl substitution in the series HCCCH (1a), 0.628 cm-1 14,15,19
,
significantly upon annealing the matrix.18 The effects of
annealing on the UV/vis and IR spectra are more subtle.
HCCCPh (2a), ca. 0.53 cm-1 23,49
and PhCCCPh (3a), ca. 0.48
,
cm-1, which is consistent with the proposed structural assign-
ment. Comparison of D values for the three-carbon and five-
carbon chain systems is also instructive. The D values of the
non-relaxed triplets PhC3Ph (3b) and PhC5Ph (5c) are quite
similar (0.48 cm-1). Upon annealing, however, the D values
change in opposite directions. The D value for the relaxed triplet
2.2.1. EPR and UV/Vis Spectroscopy. The changes in the
EPR spectra that are observed upon annealingsthe collapse of
distinct X2 and Y2 transitions to give a single XY2 transition
(Figure 2)sreflect a transformation from a structure of lower
symmetry to a structure of higher symmetry. The structure of
higher symmetry is consistent with that predicted by DFT
calculations for triplet 1,3-diphenylpropynylidene (33a). The D2d
structure computed for triplet 3 would be expected to display a
single XY2 transition (i.e., a small E value), which is charac-
teristic of an axially symmetric triplet.46-48 The zero-field
(46) Weltner, W., Jr. Magnetic Atoms and Molecules; Van Nostrand Reinhold:
New York, 1983.
(47) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance Spectroscopy; Chapman
and Hall: New York, 1986.
(48) Sander, W.; Bucher, G.; Wierlacher, S. Chem. ReV. 1993, 93, 1583-1621.
(49) DePinto, J. T. Ph.D. Thesis; University of Wisconsin, Madison, WI, 1993.
9
J. AM. CHEM. SOC. VOL. 129, NO. 8, 2007 2311

A R T I C L E S
DePinto et al.
triplet alkynyl carbenes.18,22,23,49 The non-relaxed triplet is likely
to be constrained with respect to both C-C-Cipso angle (θ)
and dihedral angle (φ) (structure 3b; Scheme 1). Calculations
reveal that such structures may exist as minima on the potential
energy surface and that the energetic penalty for imposing
geometric constraints on these structures is modest (Table 2).
Such a non-relaxed triplet may be expected to resemble a
“localized” aryl carbene structure, in terms of both the EPR
spectrum (distinct X2 and Y2 transitions) and the UV/vis
spectrum (characteristic π-π* absorption with extensive vi-
bronic coupling in the region 450-550 nm) (Figure 3).48 The
observed data are in accord with these expectations. Annealing
the matrix leads to a significant diminution and broadening of
the visible absorption features. Good models for the electronic
absorption spectrum of the chromophore in the relaxed, D2d
structure of triplet 3a are not available in the literature.
2.2.2. IR Spectroscopy and Chemical Trapping. The
infrared spectrum of triplet Ph-C-C-C-Ph (3), as generated
upon photolysis (λ > 571 nm) of diazo compound 9 (Ar, 10
K), is shown in Figure 4a. The IR spectrum exhibits subtle, but
reproducible, changes upon annealing the matrix (Figure 4b).
These changes are consistent with the interpretation in terms
of the conformational relaxation of the initially formed triplet
3b to the D2d structure 3a. The experimental IR spectra of the
two species are very similar. Both exhibit strong deformation
modes that are characteristic of a monosubstituted benzene ring
(3b, 748, 680 cm-1; 3a, 745, 678 cm-1).44,54 Calculations
establish that the vibrational frequency of these two deformation
modes varies with the bond angle at the carbene center (θ); the
computed frequency for each mode decreases as θ increases
from 125° to 180°.36 Thus, the shift to lower frequency upon
annealing is consistent with an increase in carbene angle. The
computed spectra depicted in Figure 4b (θ ) 155° and θ )
180°) represent our best assessment of the limiting cases
regarding the variation in bond angle. Structures that are
significantly bent (θ ) 125°-135°; i.e., θ similar to that in the
diazo compound precursor) are inconsistent with the experi-
mental IR spectra, both before and after annealing. For these
structures, calculations predict an alkynyl stretching vibration
at ca. 1930 cm-1, with moderate intensity, which is not observed
in the experimental spectra. The structure with θ ) 145° also
appears to be inconsistent with the experimental IR spectra
because of the relatively poor agreement between computed and
experimental values of a low-frequency deformation (calc. 551
cm-1; expt. ca. 500 cm-1). To the best of our knowledge, this
is the first instance in which annealing effects on EPR spectra
of triplet carbenes have been corroborated by detailed modeling
of the corresponding IR spectra.
Figure 4. (a) Bottom: Unscaled computed (B3LYP/6-31G*) IR spectrum
of diazo compound 9. Middle: IR subtraction spectrum obtained upon
photolysis (λ > 571 nm) of diphenyldiazopropyne (9) showing the growth
of 1,3-diphenylpropynylidene (3) with the concomitant disappearance of 9
(Ar, 10 K). Top: Computed IR spectrum of triplet 3b (θ ) 155°, φ )
55°). (b) Bottom: Computed IR spectrum of triplet 3b (θ ) 155°, φ )
55°). Middle: IR subtraction spectrum showing the spectral changes
associated with annealing the sample (warming to 50 K followed by
recooling to 10 K). Top: Computed IR spectrum of triplet 3a (θ ) 180°,
φ ) 90°).
PhC3Ph (3a) is slightly smaller than that for the non-relaxed
3b (0.468 vs 0.484 cm-1), while the D value for the relaxed
triplet PhC5Ph (5a) is slightly larger than that for the non-relaxed
5c (0.497 vs 0.482 cm-1).22 On one hand, it seems counterin-
tuitive that the shorter conjugated system would exhibit greater
delocalization (i.e., smaller D value). On the other hand, the
annealing behavior appears to be consistent with other experi-
mental and theoretical evidence ascribing predominant 1,3-
diradical character for triplet PhC3Ph (3a; D ) 0.468 cm-1
)
versus 1,1-diradical (carbenic) character for triplet PhC5Ph (5b;
D ) 0.497 cm-1).24,30,31
The identity of triplet 1,3-diphenylpropynylidene (3) is further
confirmed through oxygen trapping studies (Scheme 2).24,36,48
Photolysis (λ > 571 nm) of 1,3-diphenyldiazopropyne (9) in
an argon matrix doped with 5% O2 produces a mixture of triplet
3 and trapping product, 1,3-diphenylpropynone (11) (major
product), along with small amounts of carbonyl oxide 10,
dioxirane 12, and esters 13 and 14. Subsequent warming of the
matrix to 35 K and recooling to 10 K results in trapping of
residual triplet 1,3-diphenylpropynylidene (3) by O2 to produce
The initially formed triplet of lower symmetry likely arises
from a non-relaxed conformation of triplet 3. The differing EPR
spectra (before and after annealing) are not artifacts of any
particular matrix medium, because EPR spectra obtained in
different matrixessargon, 2-methyltetrahydrofuran (MeTHF),
and methylcyclohexane (MCH)sall exhibit similar features.
Similar annealing effects have been observed on the EPR spectra
of hindered triplet diarylcarbenes,50-53 as well as substituted
(50) Wasserman, E.; Kuck, V. J.; Yager, W. A.; Hutton, R. S.; Greene, F. D.;
Abegg, V. P.; Weinshenker, N. M. J. Am. Chem. Soc. 1971, 93, 6335-
6337.
(51) Nazran, A. S.; Gabe, E. J.; LePage, Y.; Northcott, D. J.; Park, J. M.; Griller,
D. J. Am. Chem. Soc. 1983, 105, 2912-2913.
(52) Hu, Y.; Hirai, K.; Tomioka, H. J. Am. Chem. Soc. 1999, 103, 9280-9284.
(53) Makarov, B. P.; Tomioka, H. Org. Biomol. Chem. 2004, 2, 1834-1837.
(54) McMahon, R. J.; Chapman, O. L. J. Am. Chem. Soc. 1987, 109, 683-692.
9
2312 J. AM. CHEM. SOC. VOL. 129, NO. 8, 2007

Triplet 1,3-Diphenylpropynylidene
A R T I C L E S
Scheme 2
Scheme 3
raenes, upon visible irradiation,44,55,56 we considered the pos-
sibility that triplet 3b might exhibit photochemical ring expan-
sion to yield 1-(phenylethynyl)-1,2,4,6-cycloheptatetraene (16)
(Scheme 3). Irradiation into the visible absorption of triplet 3b
(λ ) 540 ( 7 nm or λ > 440 nm) does not, however, result in
observable photochemistry.
Photolysis (λ > 261 nm) of triplet 1,3-diphenylpropynylidene
(3) in an argon matrix at 10 K, either before or after the matrix
is annealed at ca. 40 K, results in a decrease in intensity of the
IR, UV/vis, and EPR signals of triplet 3 along with the growth
of diphenylcyclopropenylidene (6) (Figures 3d and 5b). The
absence of new EPR signals is consistent with the expectation
of a singlet ground electronic state for carbene 6. The strong
C-C stretch at 1332 cm-1 is characteristic of a cycloprope-
nylidene moiety.20,57 The experimental spectrum matches well
with the computed spectrum of singlet diphenylcycloprope-
nylidene (6) (Figure 5b). The photoisomerization of triplet 3 to
singlet 6 is reversible. Photolysis (λ ) 232 ( 10 nm) of
diphenylcyclopropenylidene (6) leads to the increase of the IR
(Figure 3d), UV/vis (Figure 5c), and EPR signals of 3 and the
decrease of the IR and UV/vis signals of 6.
carbonyl oxide (10), which exhibits strong IR absorptions
characteristic of an O-O stretch at 969 and 929 cm-1 and an
acetylenic CtC stretch at 2194 cm-1. Further long-wavelength
photolysis (λ > 613 nm) results in the growth of dioxirane 12
and ketone 11 and the disappearance of carbonyl oxide 10.
Dioxirane 12 undergoes further photochemistry at
λ > 444 nm to lead to the growth of esters 13 and 14. In these
experiments, the identity of ketone 11 was confirmed by
comparison of the infrared spectrum with that of an authentic
sample (Ar, 10 K). The identities of carbonyl oxide 10, dioxirane
12, and esters 13 and 14 were assigned on the basis of their
characteristic photochemistry and by comparison to computed
infrared spectra.
The photochemistry exhibited by the C3Ph2 isomers differs
from that of the C3H2 and C3HPh isomers. In the latter cases,
1,2-propadienylidene isomers (H2CdCdC: or H(Ph)CdCdC:)
are formed under a variety of irradiation conditions.20,23,49 In
the C3Ph2 system, singlet Ph2CdCdC: (7) was not detected
under any photolysis conditions, as indicated by the absence of
an intense IR band predicted (B3LYP/6-31G*) to lie near 2035
cm-1 and the absence of the characteristic 1,2-propadienylidene
chromophore in the UV/vis spectrum.
2.4. Bond-Shift Isomerism. Acetylenic carbenes have at-
tracted interest, over many years, because of the possibility that
they may exhibit bond-shift isomerism (Chart 1). It is now well-
established that triplet ethynyl carbene (1a, HC3H; propy-
nylidene, propargylene) and triplet diethynyl carbene (4b, HC5H)
For further characterization of 1,3-diphenylpropynylidene (3),
we sought to trap the carbene with carbon monoxide (Scheme
2). Carbon monoxide reacts with many triplet carbenes to form
ketenes.48 Photolysis (λ > 571 nm) of 1,3-diphenyldiazopropyne
(9) in an argon matrix doped with 0.33% carbon monoxide
produces triplet 1,3-diphenylpropynylidene (3) as the only
product. Warming the matrix to 40 K results in the conforma-
tional isomerization from the non-relaxed to the relaxed carbene
(3), but not in ketene formation, as demonstrated by the absence
of a strong infrared absorption in the vicinity of 2100 cm-1
.
Thus, PhC3Ph (3) and the parent HC3H (1) exhibit similar
reactivity patterns, in the sense that both species are trapped by
O2, but not by CO, at low temperatures.20
2.3. Photochemistry. The electronic absorption spectrum of
the non-relaxed triplet 3b exhibits a weak visible absorption,
with vibrational structure, that is characteristic of triplet aryl
carbenes (Figure 3b). Because matrix-isolated aryl carbenes
often undergo ring expansion, affording 1,2,4,6-cycloheptatet-
(55) Chapman, O. L.; Johnson, J. W.; McMahon, R. J.; West, P. R. J. Am. Chem.
Soc. 1988, 110, 501-509.
(56) Matzinger, S.; Bally, T. J. Phys. Chem. A 2000, 104, 3544-3552.
(57) Maier, G.; Reisenauer, H. P.; Schwab, W.; Ca´rsky, P.; Hess, B. A.; Schaad,
L. J. J. Am. Chem. Soc. 1987, 109, 5183-5188.
9
J. AM. CHEM. SOC. VOL. 129, NO. 8, 2007 2313

A R T I C L E S
DePinto et al.
equilibrium structure of the triplet carbene achieved. The same
hypothesis rationalizes the EPR data observed for triplet
phenylpropynylidene (2b, 2c)21,23 and triplet 1,5-diphenylpen-
tadiynylidene (5) systems,22 without the need to invoke bond-
shift isomerism.
In our opinion, the use of the term “bond-shift isomers” in
the description of alkynyl carbenes is inappropriate when the
initially formed carbene isomer is obtained only through the
external influence of the matrix environment. Although our
studies do not support the existence of bond-shift isomers for
the triplet “alkynyl carbenes” HC3H (1a),20 PhC3Ph (3a) (this
work), and HC5H (4b),24 the data provide an interesting point
of comparison with the organometallic complexes of these
species. As ligands in transition-metal complexes, singlet
“alkynyl carbenes” exhibit localized carbene structures, bond-
shift isomerism, and interesting reactivity patterns that have been
exploited in synthetic chemistry.5,6,34,35 An interesting question
that remains unresolved, concerning the “free” alkynyl carbenes,
is why the reactivity of these symmetrical, highly conjugated
species seems to be so characteristic of a classical, localized
carbene.13,24-26
3. Summary
Triplet 1,3-diphenylpropynylidene (3) has been characterized
by IR, UV/vis, and EPR spectroscopy in low-temperature
matrices. Two conformational isomers of 1,3-diphenylpropy-
nylidene (3b and 3a) are spectroscopically distinguishable
(Scheme 1). The initially formed conformer 3b reflects the
geometry of the diazo precursor, as enforced by the rigid matrix.
Annealing the matrix permits the structure to relax to the
equilibrium D2d geometry 3a. The highly symmetric equilibrium
structure is best envisioned as a 1,3-allenic diradical. Density
functional theory calculations suggest that the equilibrium
structure does not exhibit a bond-localized structure that would
be characteristic of an acetylenic carbene. Thus, the structure
of the delocalized triplet carbene contrasts with that of the
localized carbene structure observed in organometallic com-
plexes of (singlet) alkynyl carbenes. Irradiation (λ > 261 nm)
of triplet 1,3-diphenylpropynylidene (3) results in cyclization
to singlet diphenylcyclopropenylidene (6), a process that is
photochemically reversible at λ ) 232 nm.
Figure 5. (a) Bottom: Unscaled computed (B3LYP/6-31G*) IR spectrum
of diazo compound 9. Middle: IR subtraction spectrum obtained upon
photolysis (λ > 571 nm, λ > 534 nm) of 1,3-diphenyldiazopropyne (9)
showing the growth of 1,3-diphenylpropynylidene (3) with the concomitant
disappearance of 9 (Ar, 10 K). Top: Computed IR spectrum of triplet 3a.
(b) Bottom: IR subtraction spectrum showing the growth of diphenylcy-
clopropenylidene (6) along with the disappearance of 3 upon photolysis (λ
> 261 nm) of 3 (Ar, 10 K). Top: Computed IR spectrum of singlet 6. (c)
IR subtraction spectrum obtained upon photolysis (λ ) 232 ( 10 nm) of 6
showing a decrease in the IR bands attributed to 6 with the regrowth of 3
(Ar, 10 K).
4. Methods
4.1. Computational Methods. DFT computations were performed
with the Gaussian 9858 and Gaussian 0359 packages using the Becke
three-parameter gradient-corrected exchange functional60 with the
correlation functional of Lee, Yang, and Parr61 (B3LYP). The 6-31G*
basis set was used for geometry optimizations, relative energies, and
harmonic vibrational frequency computations. Vibrational frequencies
were used to determine the nature of the stationary points as an energy
minimum, transition state, or saddle point.
4.2. Matrix Isolation Spectroscopy. The apparatus and experimental
techniques for low-temperature matrix isolation spectroscopy are
described in the Supporting Information. In warming experiments, argon
matrixes were capped with xenon to better maintain the integrity of
the matrix at elevated temperatures.62
both exhibit symmetrical structures (C2 and D∞h, respectively)
and that localized bond-shift isomers are not minima on the
respective potential energy surfaces.20,24,29,32 It is conceivable
that substituents might perturb the situation, such that the bond-
shift isomers become energy minima. The symmetrical substitu-
tion pattern of triplet 1,3-diphenylpropynylidene (3), however,
would render localized bond-shift isomers to be degenerate.
Thus, the experimental observation of conformational isomerism
for triplet 1,3-diphenylpropynylidene (3) requires an explanation
other than traditional bond-shift isomerism. Our current obser-
vations are straightforwardly rationalized by the hypothesis that
the initially formed triplet carbene is not able to achieve its
equilibrium structure because of the influence of the matrix host
in enforcing a geometry that resembles that of the precursor
diazo compound. Only after annealing the matrix is the
(58) Frisch, M. J.; et al. Gaussian 98, revision A.6; Gaussian, Inc.: Pittsburgh,
PA, 1998.
(59) Frisch, M. J.; et al. Gaussian 03, revision B.05; Gaussian, Inc.: Wallingford,
CT, 2004.
(60) Becke, A. D. Phys. ReV. A: At. Mol. Opt. Phys. 1988, 38, 3098-3100.
(61) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B: Condens. Matter 1988,
37, 785-789.
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Triplet 1,3-Diphenylpropynylidene
A R T I C L E S
4.3. 1,3-Diphenyldiazopropyne (9). A solution of 1,3-diphenyl-2-
propyn-1-one trisylhydrazone (0.15 g, 0.309 mmol) in CH2Cl2 (1.7 mL)
was added all at once to a suspension of 60% NaH/mineral oil (0.0124
g, 0.309 mmol; Aldrich) in CH2Cl2 (5.5 mL) at room temperature in
the dark. Within 5 min of stirring at room temperature, the red color
of the diazo compound began to appear. After being stirred for 35 min
at room temperature, the cloudy red reaction mixture was poured into
22 mL of pentane, and the white precipitate was removed by suction
filtration. The solvent was removed from the filtrate in vacuo, allowing
the evaporation of solvent to keep the solution cold. Once the solvent
was removed, the crude diazo compound was immediately purified by
rapid elution (low N2 pressure) column chromatography (3 cm × 25
cm) in the dark using neutral alumina (activity grade V) and 5:1 pentane/
CH2Cl2 as the eluent. The diazo compound can be visually observed
moving down the column as a red band (Rf ) 0.7). Once the diazo
band was collected, the solvent was removed in vacuo, allowing the
evaporation of the solvent to keep the solution cold. The diazo
compound was obtained as a deep red oil (0.0460 g, 0.211 mmol, 68%
yield). In most instances, the purified diazo compound was used
immediately for matrix isolation. However, the diazo compound can
be stored at -78 °C under N2 in the dark for several days without
noticeable decomposition. 1,3-Diphenyldiazopropyne (9): 1H NMR
(CDCl3) δ 7.42 (m, 7H), 7.15 (m, 3H); 1H NMR (CD2Cl2) δ 7.32 (m,
10H); IR (CD2Cl2, 298 K) 3083 w, 3064 w, 3036 w, 2206 w, 2050 vs,
1596 m, 1491 m, 1452 w, 1444 w, 1366 w, 1288 w, 1275 w, 1182 w,
1172 w, 1071 w, 1027 w, 754 m, 612 w, 527 w cm-1; IR (Ar, 10 K)
3114 w, 3095 w, 3090 w, 3081 w, 3073 w, 3065 w, 3042 w, 3017 w,
2218 m, 2050 vs, 1959 w, 1953 w, 1941 w, 1935 w, 1886 w, 1868 w,
1854 w, 1812 w, 1601 m, 1581 w, 1506 w, 1497 m, 1490 m, 1457 w,
1445 w, 1370 w, 1601 m, 1581 w, 1506 w, 1497 m, 1490 m, 1457 w,
1445 w, 1370 w, 1325 w, 1303 w, 1289 w, 1280 w, 1274 w, 1184 w,
1173 w, 1096 w, 1077 w, 1070 w, 1031 w, 1016 w, 998 w, 907 w,
895 w, 751 s, 690 s, 611 w, 541 w, 526w, 515 w, 463 w cm-1; UV/vis
(Ar, 10 K) 227.2, 232.4, 240.0, 244.4, 252.0, 283.2, 290.8, 299.6,
319.2 nm.
solvent (2-methyltetrahydrofuran, MeTHF, or methylcyclohexane,
MCH, spectroscopic grade) to give a solution of concentration ca. 5
mM. A small volume of this solution (ca. 0.4-0.5 mL) was placed in
a 3 mm quartz EPR tube (Wilmad Glass Co., Inc.; model 727-SQ-
250M) with a Pyrex 14/20 ground glass joint attached to the top. The
sample was subjected to five freeze-pump-thaw cycles at 77 K in
the dark, and the tube was sealed under vacuum with an O2/gas torch.
In most instances, the sample was used immediately. However, the
solution can be stored at -78 °C in the dark for several days without
noticeable decomposition. To prevent crystallization and ensure that a
glass was formed for the liquid helium experiments, the sample was
first frozen to a glass by immersing in liquid nitrogen and rapidly
transferring (ca. 1-2 s) the tube from the liquid nitrogen dewar to the
pre-cooled liquid helium cryostat at 4 K.
4.4. Non-relaxed Triplet 1,3-Diphenylpropynylidene (3b). IR (Ar,
10 K) 3119 w, 3085 w, 3076 w, 3065 w, 3057 w, 3024 w, 3007 w,
1954 w, 1939 w, 1924 w, 1878 w, 1864 w, 1804 w, 1790 w, 1649 w,
1576 w, 1556 m, 1510 w, 1478 m, 1461 m, 1438 m, 1270 w, 1241 w,
1231 w, 1221 w, 1173 w, 1163 w, 1158 w, 1092 w, 1065 w, 1023 w,
903 w, 748 s, 680 s, 497 w, 489 w, 461 w cm-1 (Figures 4a and 5a);
UV/vis (Ar, 10 K) λmax 217.2, 266.4, 279.2, 287.2, 297.2, 307.6, 314.4,
324.8, 332.4, 335.6, 343.2, 456.4, 459.6, 463.3, 472.3, 494.9, 498.1,
498.7, 507.6, 508.5, 510.9, 511.8, 512.8, 513.9, 520.8, 522.1, 539.5,
540.8 nm (Figure 3); EPR (Ar, 10 K) |D/hc| ) 0.484, |E/hc| ) 0.00423
cm-1; Z1 1778, X2 5316, Y2 5490, Z2 8571 G; microwave frequency
9.5304 GHz (Figure 2).
4.5. Relaxed Triplet 1,3-Diphenylpropynylidene (3a). IR (Ar, 10
K) 3114 w, 3108 w, 3090 w, 3081 w, 3071 w, 3060 w, 3044 w, 1950
w, 1934 w, 1920 w, 1873 w, 1859 w, 1797 w, 1785 w, 1645 w, 1572
w, 1553 m, 1503 w, 1494 w, 472 m, 1458 m, 1435 m, 1267 w, 1236
w, 1170 w, 1154 w, 1088 w, 1062 w, 1021 w, 898 w, 756 w, 745 s,
695 w, 690 w, 678 s, 499 w, 485 w, 461 w cm-1 (Figure 4b); UV/vis
(Ar, 10 K) 273.2, 278.4, 294.4 nm (Figure 3); EPR (Ar, 10 K) |D/hc|
) 0.468, |E/hc| < 0.0002 cm-1; Z1 1612, XY2 5306, Z2 8404 G;
microwave frequency 9.5313 GHz (Figure 1).18,49
Once obtained, the diazo compound was dissolved in a minimal
amount of CH2Cl2, and transferred to a small glass tube (1 cm × 7
cm) used for subliming the compound onto the matrix isolation window.
The solvent was removed in vacuo at -41 °C, and the tube was vented
to N2. After the sample tube was transferred to the matrix isolation
apparatus, the sample was subjected to three freeze-pump-thaw cycles
at 77 K, and the sample tube was evacuated to 2 × 10-6 Torr at room
temperature. Once the pressure in the matrix isolation vacuum system
had fallen below to 2 × 10-6 Torr, the sample tube was cooled to
-78 °C with powdered dry ice, and the cold window was cooled to 30
K. The diazo compound was sublimed from the sample tube at room
temperature and co-deposited with argon onto a cold window (CsI for
IR spectroscopy; sapphire for UV/vis spectroscopy) maintained at 30
K. The small glass sample tube (1 cm × 7 cm), which places the diazo
compound ca. 7-9 cm away from the cold window, was essential for
efficient sublimation of the diazo compound at room temperature. For
EPR experiments in argon, the diazo compound was co-deposited with
argon on a copper rod maintained at 10 K.
4.6. Diphenylcyclopropenylidene (6). IR (AR, 10 K) 3110 w, 3093
w, 3086 w, 3074 w, 3061 w, 3042 w, 3034 w, 3008 w, 1978 w, 1959
w, 1916 w, 1899 w, 1814 w, 1805 w, 1606 w, 1603 w, 1454 w, 1332
s, 1313 w, 1304 w, 1259 w, 1254 w, 1170 w, 1075 w, 1051 m, 1024
w, 1021 w, 962 w, 929 w, 762 s, 690 s, 623 w, 583 m, 518 m cm-1
(Figure 5); UV/vis (Ar, 10 K) λmax 220.0, 225.2, 244.8, 255.2, 266.8,
272.8, 280.0, 286.4, 295.6 nm (Figure 3).
Acknowledgment. We gratefully acknowledge the National
Science Foundation for financial support of this project, as well
as support for the Departmental EPR spectrometer and comput-
ing facilities.
Supporting Information Available: Synthetic procedures,
experimental IR spectra from oxygen trapping studies, plots of
computed IR spectra for conformers of triplet 1,3-diphenylpro-
pynylidene (3), Cartesian coordinates for computed (B3LYP/
6-31G*) structures, harmonic vibrational frequencies and IR
intensities, and complete literature citations for refs 58 and 59.
This material is available free of charge via the Internet at
http://pubs.acs.org.
For EPR experiments involving frozen organic glasses, a solution
of 1,3-diphenyldiazopropyne (9) was prepared by rapidly weighing the
purified diazo compound and dissolving in the appropriate amount of
(62) Swanson, B. I.; Jones, L. H. J. Mol. Spectrosc. 1981, 89, 566-568.
JA066300J
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J. AM. CHEM. SOC. VOL. 129, NO. 8, 2007 2315