A carbene-carbene complex equilibrium

Article abstract of DOI:10.1021/ja104514h

Phenylchlorocarbene, generated by laser flash photolysis of phenylchlorodiazirine, formed highly stable φ-type complexes with 1,3,5-trimethoxybenzene in pentane. The carbene and carbene complexes were in equilibrium. We measured the equilibrium constant (

Full text of DOI:10.1021/ja104514h

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Published on Web 07/20/2010
A Carbene-Carbene Complex Equilibrium
Robert A. Moss,* Lei Wang, Christina M. Odorisio, and Karsten Krogh-Jespersen*
Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of New Jersey,
New Brunswick, New Jersey 08903
Received May 25, 2010; E-mail: moss@rutchem.rutgers.edu; krogh@rutchem.rutgers.edu
Abstract: Phenylchlorocarbene, generated by laser flash pho-
tolysis of phenylchlorodiazirine, formed highly stable π-type
complexes with 1,3,5-trimethoxybenzene in pentane. The carbene
and carbene complexes were in equilibrium. We measured the
equilibrium constant (K ) 1264 M-1 at 294 K) and, from its
temperature dependence, extracted the associated thermody-
namic parameters: ∆H° ) -7.1 kcal/mol, ∆S° ) -10.2 eu, and
∆G° ) -4.1 kcal/mol. The carbene complexes were characterized
by UV-vis spectroscopy and computational analysis.
The search for transient carbene complexes has a venerable
history. Indirect evidence in early reports highlights the modulation
of carbenic reactivity and selectivity by electron-donating solvents
such as dioxane, tetrahydrofuran (THF), benzene, or anisole.1
Singlet carbenes deploy a formally vacant p orbital so that electron
donation to this receptor by solvent heteroatoms or π-bonds provides
a rationale for the observed solvent effects on reactivity, while
computational studies support the formation of carbene-solvent
complexes between, e.g., methylchlorocarbene and benzene or
anisole.2
Figure 1. Calibrated12 UV-vis spectra acquired 50 ns after LFP generation
of PhCCl in TMB/pentane solution; blue spectrum at 0.26 mM TMB, red
spectrum at 0.75 mM TMB.
Direct observation of the complex provides stronger evidence
for specific carbene solvation. Using photoacoustic spectroscopy,
Kahn and Goodman detected heat deposition ascribed to the
formation of a methylene-benzene π-complex.3 More recently,
Platz et al. employed time-resolved IR spectroscopy to demonstrate
formation of an O-ylide from an amidochlorocarbene and dioxane,
suggesting the existence of a precursor carbene-dioxane complex.4
We used laser flash photolysis (LFP), coupled with UV-visible
spectroscopy, to identify the spectroscopic signatures of transient
carbene complexes formed between electron-rich aromatic ethers,
such as anisole or 1,3,5-trimethoxybenzene, and halocarbenes, such
as methylchlorocarbene,5 benzylchlorocarbene,5 p-nitrophenylchlo-
rocarbene,6 and dichlorocarbene.7,8 Computational studies provided
analyses of the absorption spectra and likely structures of the
observed complexes.5–8 Here, we offer the first direct spectroscopic
evidence for a carbene-carbene complex equilibrium, and we
extract the associated thermodynamic parameters. Computational
studies afford quantitative support for our analysis.
LFP of phenylchlorodiazirine9 in pentane affords phenylchlo-
rocarbene (PhCCl), whose UV-vis spectrum features a strong π
f p absorption at 308 nm and a weak σ f p absorption at 588
nm10,11 (cf. Figures S-2 and S-3 in the Supporting Information).
LFP of the diazirine in the presence of 0.26 mM 1,3,5-trimethoxy-
benzene (TMB) gives the calibrated12 UV-vis spectrum shown in
blue in Figure 1, with a strong absorption at 324 nm and weaker
absorptions at 484 and 596 nm (τ ≈ 0.15 ms). Computational
analysis (see below) of the PhCCl-TMB reaction enables us to
assign the 484 nm absorption to specific PhCCl/TMB π-complexes
A and B (see Figure 2). These carbene π-complexes also contribute
Figure 2. Four computed complexes formed between PhCCl and TMB
(in perspective, carbene on top; green, chlorine; red, oxygen).
to the 324 nm absorption in Figure 1, although that signal is mainly
due to PhCCl. The 596 nm absorption can be attributed mainly to
these π-complexes, with a minor contribution from PhCCl.
The simultaneous appearance in Figure 1 of absorptions due to
both PhCCl and PhCCl/TMB complexes suggests the establishment
of a carbene-carbene complex equilibrium, eq 1. In accord with
this idea, repetition of the PhCCl-TMB LFP experiment at 0.75
9
10.1021/ja104514h  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 10677–10679 10677

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
C O M M U N I C A T I O N S
Figure 4. Plot of ln K (M-1) vs 1/T (K-1) for the equilibrium of eq 1. The
slope (3570) affords ∆H° ) -7.1 ( 0.6 kcal/mol, and the intercept (-5.13)
gives ∆S° ) -10.2 ( 2.2 eu. The correlation coefficient is r ) 0.988.
Figure 3. Relative absorption intensities at 324 nm/484 nm vs 1/[TMB]
(M-1) for the LFP generation of PhCCl in TMB/pentane solution at 294 K.
The slope of the correlation line is 2.84 × 10-3 M (r ) 0.997), leading to
K ) 1264 M-1 for eq 1.13
Table 1. Thermodynamic Parameters for PhCCl-TMB Complexes
Computed at the B97D/6-311+G(d) Level
a
a
a,b
a,c
species
∆H°
∆S°
∆G°
corr ∆H°
mM TMB affords the red spectrum in Figure 1. Comparison of the
blue spectrum (0.26 mM TMB) with the red spectrum (0.75 mM
TMB) reveals significant increases in the latter case of the 484 and
596 nm PhCCl-TMB π-complex absorptions versus the (mainly)
PhCCl absorption at 324 nm.
A
B
C
D
-10.91
-9.97
-8.59
-7.79
-35.8
-34.7
-32.2
-34.2
-0.23
0.37
1.02
-8.16
-7.44
-6.43
-5.47
2.42
a ∆H° and ∆G° in kcal/mol, ∆S° in eu, relative to the separated
reactants; T ) 298.15 K. b The free energy differences were computed
K
using
a reference state of 1 M concentration for each species
PhCCl + TMB y
\
z PhCCl/TMB
(1)
participating in the reaction. c Counterpoise corrected enthalpies.
Information as Tables S-1-S-4). The order of complex stability
(D < C < B < A) was independent of which functionals were
employed, but the enthalpies for carbene complex formation were
much more favorable when one of the dispersion-corrected func-
tionals was applied (B97D, M06-2X, or wB97XD). Moreover, the
thermodynamic parameters produced by these latter functionals were
very similar, despite the different approaches taken to include long-
range, weak interactions (cf. Tables S-2-S-4). Three of the
identified PhCCl-TMB complexes (A, B, and D) show the carbene
carbon interacting with an unsubstituted carbon of TMB (C2); the
fourth complex (C) features an ylidic-type C(PhCCl)-O(Me)
interaction. The two most stable conformers (A and B) are both
sandwich-type π-complexes involving substantial overlap between
the aromatic ring moieties. Complexes A and B are nearly
isoenergetic and structurally very similar; e.g., the computed
C(PhCCl)-C2(DMB) distance is 2.77 Å in A and 2.96 Å in B.
The more spatially extended complexes (C and D) are found at
slightly higher energies (2-3 kcal/mol above A), and we consider
these complexes to be noncompetitive at ambient temperature.
The binding enthalpies for complexes A and B computed at the
B97D/6-311+G(d) level (∆H° ≈ -10 to -11 kcal/mol) are in fair
agreement with the measured value (∆H° ) -7.1 kcal/mol). Even
better agreement with experiment is obtained when basis set
superposition errors17a are approximately accounted for via the
counterpoise correction17b (Table 1): ∆H° ) -8.2 and -7.4 kcal/
mol for A and B, respectively.
Despite “contamination” of the 324 nm PhCCl absorption by
the PhCCl-TMB complexes, it is possible to evaluate the contribu-
tion of the carbene alone by application of the Beer-Lambert law
to the mixture of absorbers. The analysis, which ultimately employs
the computed oscillator strengths (f) in place of the unknown
extinction coefficients of PhCCl and PhCCl-TMB, is detailed in
the Supporting Information. Accordingly, a plot of the quotient of
the calibrated intensities of the 324 and 484 nm absorptions vs
1/[TMB] at 294 K gives the linear correlation of Figure 3, where
the slope (2.84 × 10-3 M) leads to K ) 1264 M-1 for eq 1.13
Formation of the PhCCl-TMB complexes from PhCCl and TMB
is thus thermodynamically favorable (K . 1) at room temperature.
We similarly determined K at four additional temperatures (see
Figures S-7-S-10 in the Supporting Information), yielding the
following values of K (M-1): 4711 at 262 K, 2409 at 275 K, 1962
at 283 K, and 647 at 304 K. A plot of ln K vs 1/T appears in Figure
4, from which the slope and intercept give ∆H° ) -7.1 ( 0.6
kcal/mol and ∆S° ) -10.2 ( 2.2 eu, respectively, leading to ∆G°
) -4.1 ( 0.9 kcal/mol (at 298 K). The thermodynamic parameters
show that the equilibrium of eq 1 is enthalpy-driven in the direction
of carbene complex formation.
Electronic structure calculations based on density functional
theory provide structures and energetics of potential PhCCl-TMB
complexes and rationalize their electronic absorption spectra. We
have applied four density functional combinations (PBE,14a
B97D,14b M06-2X,14c and wB97XD14d) and 6-311+G(d)15 basis
sets in search of stable PhCCl-TMB complexes.16 Four minima
(A-D, Figure 2; see Figure S-11 in the Supporting Information
for additional views of complexes A-D) were located on the
potential energy surface, and computed thermodynamic parameters
obtained with the B97D functionals are shown in Table 1 (additional
tables of thermodynamic data are available in the Supporting
However, the computed entropies for complex formation are far
more negative (∼ -35 eu, Table 1) than observed (-10 eu). This
may be largely ascribed to the different physical phases serving as
references for the calculations (idealized gas phase) vs experiment
(condensed phase). Due to the substantial overestimation of reaction
entropies, the computed free energies of complexation (∆G° ≈ 0
9
10678 J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
C O M M U N I C A T I O N S
Table 2. Electronic Transition Wavelengths (λ, nm) and Oscillator
Strengths (f) (TD-B3LYP/6-311+G(d)//B97D/6-311+G(d) with
CPCM18c Solvent Correction (Heptane)) for PhCCl and PhCCl/
TMB Complexes A and B
Acknowledgment. We are grateful to the National Science
Foundation and to the Petroleum Research Fund for financial
support.
λ(f)
PhCCl
A
B
Supporting Information Available: Figures S-1-S-11; Tables
S-1-S-4; spectra calibration; equilibrium constant calculations; opti-
mized geometries, absolute energies, electronic excitation energies, and
oscillator strengths for all relevant computed species; and complete
ref 16. This material is available free of charge via the Internet at http://
pubs.acs.org.
λ1(f1)
λ2(f2)
λ3(f3)
λ4(f4)
λ5(f5)
λ6(f6)
710(0.003)
336(0.037)
298(0.475)
278(0.000)
259(0.001)
237(0.010)
666(0.023)
503(0.004)
430(0.154)
322(0.155)
317(0.063)
289(0.005)
661(0.022)
507(0.001)
467(0.110)
336(0.040)
313(0.143)
291(0.127)
References
(1) (a) Tomioka, H.; Ozaki, Y.; Izawa, Y. Tetrahedron 1985, 41, 4987. (b)
Ruck, R. T.; Jones, M., Jr. Tetrahedron Lett. 1998, 39, 2277. (c) Moss,
R. A.; Yan, S.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1998, 120, 1088.
(2) Krogh-Jespersen, K.; Yan, S.; Moss, R. A. J. Am. Chem. Soc. 1999, 121,
6269.
kcal/mol) become strongly dominated by the entropy term, and the
computed equilibrium constant (K ≈ 1) is consequently too small.
On the basis of the thermodynamic parameters presented in Table
1, we consider the significant absorption features in Figure 1 to
arise exclusively from absorption by PhCCl and π-complexes of
types A and B (Figure 2). The electronic transitions of lowest
energy computed for these species are listed in Table 2
(TD-B3LYP/6-311+G(d)//B97D/6-311+G(d)).18a,b PhCCl has a
weak, broad σ f p absorption near 600 nm and a very intense
π(Ph) f p charge-transfer absorption around 310 nm; TMB is
spectroscopically silent above ca. 300 nm. Strong absorptions are
computed for π-complexes A and B at 430 (f ) 0.15) and 467 nm
(f ) 0.11), respectively, and we assign the absorption observed
around 480 nm (Figure 1) to these species with confidence.
Absorption from complexes A and B is mixed in with PhCCl
absorption at longer wavelengths (>600 nm), as well as in the strong
absorption peaking at 324 nm. The UV-vis excitations in com-
plexes A and B originate in the π-system of DMB and terminate
in the predominantly empty carbenic p orbital LUMO of PhCCl,
and therefore they exhibit significant charge-transfer character. We
note that the average of the f values for the A and B bands around
450 nm ((0.154 + 0.110)/2 ) 0.132) and also the f value for PhCCl
absorption computed at 298 nm (f ) 0.475) are used in the
numerical evaluation of the equilibrium constant for carbene
complex formation (see the Supporting Information for details).
(3) Kahn, M. I.; Goodman, J. L. J. Am. Chem. Soc. 1995, 117, 6635.
(4) Tippman, E. M.; Platz, M. S.; Svir, I. B.; Klymenko, O. V. J. Am. Chem.
Soc. 2004, 126, 5750.
(5) Moss, R. A.; Tian, J.; Sauers, R. R.; Krogh-Jespersen, K. J. Am. Chem.
Soc. 2007, 129, 10019.
(6) Moss, R. A.; Wang, L.; Weintraub, E.; Krogh-Jespersen, K. J. Phys. Chem.
A 2008, 112, 4651.
(7) Moss, R. A.; Wang, L.; Odorisio, C. M.; Krogh-Jespersen, K. J. Phys.
Chem. A 2010, 114, 209.
(8) Moss, R. A.; Wang, L.; Odorisio, C. M.; Krogh-Jespersen, K. Tetrahedron
Lett. 2010, 51, 1467.
(9) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396. Phenylchlorodiazirine
has maxima at 369, 384, and 389 nm; it was used with A369 ≈ 0.5.
(10) Turro, N. J.; Butcher, J. A., Jr.; Moss, R. A.; Guo, W.; Munjal, R. C.;
Fedorynski, M. J. Am. Chem. Soc. 1980, 102, 7576.
(11) Pliego, J. R., Jr.; De Almeida, W. B.; Celebi, S.; Zhu, Z.; Platz, M. S. J.
Phys. Chem. A 1999, 103, 7481.
(12) The intensities of the “raw” UV-vis absorptions are corrected for
wavelength-dependent variations in sample absorptivity, xenon monitoring
lamp emission, and detector sensitivity from 244 to 804 nm. For details,
see Figure S-1 and the discussion in the Supporting Information.
(13) K ) (1/slope)(f3/f1), where f3 (0.475) is the computed oscillator strength of
PhCCl at 324 nm and f1 (0.132) is the (average) computed oscillator strength
of PhCCl-TMB π-complexes A and B at 484 nm. See the Supporting
Information for details.
(14) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865.
(b) Grimme, S. J. Comput. Chem. 2006, 27, 1787. (c) Zhao, Y.; Truhlar,
D. G. J. Phys. Chem. A 2006, 110, 5121. (d) Chai, J.-D.; Head-Gordon,
M. Phys. Chem. Chem. Phys. 2008, 10, 6615.
(15) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 721.
(b) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (c) Krishnan,
R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650.
(d) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (e)
Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R.
J. Comput. Chem. 1983, 4, 294.
In conclusion, experimental and computational studies indicate
that PhCCl, generated by LFP of phenylchlorodiazirine, forms
highly stable complexes with TMB in pentane. We have measured
the equilibrium constant, extracted the associated thermodynamic
parameters for this carbene-carbene complex system, and char-
acterized the complexes in detail by spectroscopic and computa-
tional analysis.
(16) All calculations made use of the Gaussian 09 program package: Frisch,
M. J.; et al. Gaussian 09, Revision A02; Gaussian, Inc.: Wallingford, CT,
2009.
(17) (a) Liu, B.; McLean, A. D. J. Chem. Phys. 1973, 59, 4557. (b) Boys, S. F.;
Bernardi, F. Mol. Phys. 1970, 19, 553.
(18) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5468. (b) Lee, C.; Yang, W.;
Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Barone, V.; Cossi, M. J. Phys.
Chem. A 1998, 102, 1995.
JA104514H
9
J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010 10679