Hammett analysis of a family of carbene-carbene complex equilibria

Article abstract of DOI:10.1021/ol200083a

p-X-substituted phenylchlorocarbenes (X = NO₂, CF₃, Cl, H, Me, and MeO) form π-type complexes with trimethoxybenzene in pentane. The carbenes and complexes are in equilibrium, and logarithms of the measured equilibrium constants are well correlated by Hammett σ_p constants with ρ = 2.48. The carbene complexes are characterized by UV-vis spectroscopy, and computational analysis is afforded by DFT calculations.(Figure Presented)

Full text of DOI:10.1021/ol200083a

ORGANIC
LETTERS
2011
Vol. 13, No. 5
1198–1201
Hammett Analysis of a Family of
Carbene-Carbene Complex Equilibria†
Lei Wang, Robert A. Moss,* Jack Thompson,‡ and Karsten Krogh-Jespersen*
Department of Chemistry and Chemical Biology, Rutgers, The State University of
New Jersey, New Brunswick, New Jersey 08903, United States
moss@rutchem.rutgers.edu; krogh@rutchem.rutgers.edu
Received January 11, 2011
ABSTRACT
p-X-substituted phenylchlorocarbenes (X = NO2, CF3, Cl, H, Me, and MeO) form π-type complexes with trimethoxybenzene in pentane. The
carbenes and complexes are in equilibrium, and logarithms of the measured equilibrium constants are well correlated by Hammett σp constants
with F = 2.48. The carbene complexes are characterized by UV-vis spectroscopy, and computational analysis is afforded by DFT calculations.
We recently provided the first direct spectroscopic evi-
dence for a carbene-carbene complex equilibrium, eq 1.1,2
Phenylchlorocarbene (1d), generated by laser flash photo-
lysis (LFP) of phenylchlorodiazirine, formed π-type com-
plexes (τ ∼ 500 ns) with1,3,5-trimethoxybenzene (TMB) in
pentane. From the UV-vis absorptions of carbene 1d and
complex(es) 2d as a function of TMB concentration, we
determined K = 1264 M-1 at 294 K.
respective complexes with TMB, 2a-2f, where the para
substituents are NO2, CF3, Cl, H, Me, or MeO. The
measured equilibrium constants (K at 294 K) range from
a high of 1.34 ꢀ 105 M-1 for 1a (X = NO2) to a low of 164
M-1 for 1f (X = MeO). The values of K are well correlated
by the Hammett equation (log K vs σp) with F = 2.48.
p-X-Substituted phenylchlorodiazirines 3b-3f were pre-
pared by Graham oxidation3 of benzamidine hydrochlor-
ides 4b-4f, which are described in the literature.4 Detailed
procedures for the preparation of diazirine 3a were pub-
lished more recently.5 UV spectra of diazirines 3a-3f
appear in the Supporting Information.
LFP6 of diazirines 3a-3f in pentane afforded carbenes
1a-1f, which displayed strong π(Ph) f p(carbene) absorp-
tions between 308 and 324 nm and weak σ(carbene) f
p(carbene) absorptions between 588 and 676 nm.7,8 In the
Here, we document equilibria within a family of
p-substituted phenylchlorocarbenes, 1a-1f, and their
† Dedicated to Professor Ronald Breslow on the occasion of his 80th
birthday.
(3) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
(4) Moss, R. A.; Terpinski, J.; Cox, D. P.; Denney, D. Z.; Krogh-
Jespersen, K. J. Am. Chem. Soc. 1985, 107, 2743. Full experimental
details for the benzamidine preparations appear in this publication.
(5) Moss, R. A.; Wang, L.; Weintraub, E.; Krogh-Jespersen, K. J.
Phys. Chem. A 2008, 112, 4651.
‡ Undergraduate exchange student from The University of Manchester,
Manchester, England.
(1) Moss, R. A.; Wang, L.; Odorisio, C. M.; Krogh-Jespersen, K. J.
Am. Chem. Soc. 2010, 132, 10677.
(2) For other examples of carbene complexes, see: (a) Tippmann,
E. M.; Platz, M. S.; Svir, I. B.; Klymenko, O. V. J. Am. Chem. Soc. 2004,
126, 5750. (b) Wang, J.; Kubicki, J.; Peng, H.; Platz, M. S. J. Am. Chem.
Soc. 2008, 130, 6604.
(6) We used a XeF2 excimer laser emitting 14 ns light pulses at 351 nm
with 55-65 mJ power. For more details, see: Moss, R. A.; Tian, J.;
Sauers, R. R.; Ess, D. H.; Houk, K. N.; Krogh-Jespersen, K. J. Am.
Chem. Soc. 2007, 129, 5167.
r
10.1021/ol200083a
Published on Web 02/10/2011
2011 American Chemical Society

Figure 2. Two computed complexes formed between carbene 1e
(X = Me) and TMB (in perspective and with depth fading,
carbene on top; green chlorine; red oxygen). Computed ab-
sorptions are (A) 437 nm and (B) 465 nm; the corresponding
observed absorptions are at 404 and 484 nm.
Figure 1. UV-vis spectra of carbene 1a and its TMB complex-
(es) (2a) in 0.011 mM TMB in pentane: 1a absorptions at 316
and 628 nm; 2a absorptions at 468 and 516 nm.
presence of TMB, new absorptions appeared due to the
formation of carbene complexes 2a-2f. The intensities
of the carbene complex absorptions grew as the con-
centration of TMB was increased, with concomitant
diminution in the intensities of the associated carbene
absorptions. An example of the calibrated9 carbene
plus carbene complex spectra for the p-NO2 (1a/2a)
system appears in Figure 1; complete sets of calibrated
spectra for all carbenes and their complexes at various
TMB concentrations are collected in the Supporting
Information.10
formation of complexes 2a-2f is calculated to be en-
thalpically favorable in all cases; further discussion of the
carbene complex structures and energetics appears below.
Although the 2a-2f complexes are computed to also
absorb in the 308-324 nm region where the carbenes have
their principal absorptions (Table 1), it is possible to
quantitatively evaluate the contribution of the carbenes
alone in the UV-vis spectra of the mixtures of carbenes
and carbene complexes.1 Theanalysissubstitutescomputed
oscillator strengths (f) for the requisite but experimentally
unknown extinction coefficients of the carbenes and their
complexes; see the Supporting Information for details. We
are thus able to evaluate K for the equilibrium of eq 1 for
each of the six carbene-carbene complex systems.
As an example, consider 1c/2c (X = Cl). A plot of the
quotient of the calibratedintensitiesof the 312 nm (1cþ 2c)
and 500 nm (2c) absorptions versus 1/[TMB] affords the
linear correlation of Figure 3, where the slope (8.62 ꢀ 10-4
M) leads to K = 5440 M-1 for eq 1.11
In a similar manner, K was determined for the other
carbene-carbene complex systems. Details of these equili-
brium constant measurements appear in the Supporting
Information. Table 2 collects the K values, their loga-
rithms, and the appropriate Hammett σp constants.12 Note
that these K values are associated with carbene complexes
of type B (see Figure 2), because the complex absorption of
lowest energy was monitored.13
Table 1. Absorption Maxima of Carbenes and Complexes
carbene
λ
max (nm)a
complex
λ
max (nm)b
1a
1b
1c
1dc
1e
1f
316, 628
308, 628
324, 650
308, 588
316, 644
324, 676
2a
2b
2c
2dc
2e
2f
468, 516
404, 500
436, 500
484, 596
404, 484
436, 492
a In pentane. b In pentane and TMB. c From ref 1.
Table 1 records the observed absorption maxima of
carbenes 1a-1f and complexes 2a-2f; computational
analysis (see below) helps us assign the TMB-induced
absorptions to specific ArCCl/TMB complexes. Figure 2
illustrates the two dominant π-complexes computed for
the 1e/TMB complexes (X = Me); similar structures are
computed for the other carbene/TMB complexes.1 The
A good Hammett correlation of log K versus σp appears
in Figure 4, where F = 2.48 and r = 0.990. The positive
sign associated with F means that the complexation in eq 1
(11) K = (1/slope)(f3/f1), where f3 (0.493) is the computed oscillator
strength of carbene 1c at 310 nm and f1 (0.105) is the computed oscillator
strength for complex 2c at 483 nm (assigned to the observed absorption
at 500 nm). See the Supporting Information for details.
(12) Smith, M. B.; March, J. March’s Advanced Organic Chemistry,
6th ed.; Wiley Interscience: Hoboken, NJ, 2007; p 404.
(13) The absorptions used to calculate K are those corresponding to
the computed absorptions for the B-type complexes of Figure 2. The
observed absorptions assigned to the A-type complexes overlap signifi-
cantly with the “B” absorptions and cannot be separately evaluated; cf.
the absorption spectra in the Supporting Information.
(7) The σ f p absorption of 1d appears at 588 nm.1,8
(8) Pliego, J. R., Jr.; De Almeida, W. B.; Celebi, S.; Zhu, Z.; Platz,
M. S. J. Phys. Chem. A 1999, 103, 7481.
(9) Calibration corrects the raw UV-vis absorptions for wavelength-
dependent variations in sample absorptivity, xenon monitoring lamp
emission, and detector sensitivity.1 Calibration spectra are collected in
the Supporting Information.
(10) The spectra for carbene 1d (X = H) and its TMB complexes
appear in ref 1 and its Supporting Information.
Org. Lett., Vol. 13, No. 5, 2011
1199

Figure 3. Relative absorption intensities at 312 nm/500 nm
versus 1/[TMB] for carbene 1c in pentane/TMB solution at 294
K. The slope of the correlation line is 8.62 ꢀ 10-4 M (r = 0.997),
leading to K = 5440 M-1 for eq 1.11
Figure 4. Hammett correlation of log K versus σp for the
experimental data of Table 2; F = 2.48, r = 0.990.
(B3LYP/6-311þG(d)//B97D/6-311þG(d)).16 We have
previously shown that density functionals such as B97D,
which explicitly include dispersion corrections, provide
appropriate binding energies of PhCCl/TMB complexes.1
We also found that sandwich-type structures, in which the
π-systems of the aromatic ring moieties overlap signifi-
cantly, formed the most stable complexes (viz. A and B in
Figure 2). In both types of complexes, the binding enthal-
pies are substantial, ΔHo ranges from -15.5 kcal/mol (2a,
A) to -9.5 kcal/mol (2f, B) (see Table 3), and the carbene
carbon interacts strongly with the unsubstitutedC2 carbon
of TMB. Computed C(carbene)-C2(TMB) distances (r)
Table 2. Equilibrium Constants for Equation 1
b
carbene (X)
K (M-1 a
)
log Ka
σp
1a (NO2)
1b (CF3)
1c (Cl)
134,000
25,800
5,440
1,450c
1,080
164
5.13
4.41
3.74
3.16
3.03
2.21
0.81
0.53
0.24
1d (H)
0.00
1e (Me)
1f (OMe)
-0.14
-0.28
˚
span the range 2.7-3.0 A in type A complexes and 2.9-3.1
a Shown to 3 significant figures. b From ref 12. c This value differs
from K given in ref 1 (1260 M-1) because here only the computed f value
for the B-type complex is used to calculate K. In ref 1 we used an average
of the f values computed for the A- and B-type complexes.
˚
A in type B complexes (Table 3). Within each type of
Table 3. Thermodynamic Parameters for p-X-PhCCl/TMB
Complexes 2a-2f: Enthalpy (ΔH°), Entropy (ΔS°), and Gibbs
Free Energy (ΔG°) Changes and Equilibrium Constants (K)
Computed at the B97D/6-311þG(d) Level
is favored by substitution of electron-withdrawing substi-
tuents on carbenes 1a-1f.12 The magnitude of F obtained
here is similar to that established for the dissociation
equilibria of ArOH in water (2.11), ArSH in 49% ethanol
(2.24), ArNH3þ in water (2.77), and ArCOOH in ethanol
(2.28).14
complex
ra
ΔH°b ΔS°b,c ΔG°b ΔΔG°
Kc
Krel
A-type
Qualitatively, we can interpret the positive F in two
complementary ways: (1) electron-withdrawing groups
destabilize carbenes 1a-1f,15 shifting the equilibrium to-
ward complexes 2a-2f where TMB donates electron den-
sity to the vacant carbenic p orbital (viz. Figure 2); (2)
stronger π-type charge transfer complexes 2a-2f will be
formed when the electron-rich TMB interacts with the
more electron-deficient carbene center/aromatic ring in-
duced by an electron-withdrawing substituent.
Electronic structure calculations based on density func-
tional theory provide structures and energetics of potential
2a-2f complexes (B97D/6-311þG(d), data in Table 3) and
predict electronic absorption frequencies and intensities
2a (NO2) 2.66 -15.5 -38.5 -4.0
2b (CF3) 2.71 -14.1 -38.0 -2.8
-3.1 877
181
23
-1.9
-0.9
0.0
113
2c (Cl)
2d (H)
2e (Me)
2.82 -12.3 -35.3 -1.8
2.81 -11.0 -33.7 -0.9
2.87 -11.1 -36.2 -0.3
20
4.2
4.8
1.0
0.3
0.6
0.7
1.6
2.8
2f (OMe) 2.99 -10.8 -34.1 -0.6
0.3
B-type
2a (NO2) 2.88 -13.4 -37.8 -2.2
2b (CF3) 2.93 -12.8 -38.3 -1.3
-2.6
-1.7
-0.9
0.0
40
80
19
4.6
9.5
2.3
0.5
0.4
0.4
2c (Cl)
2d (H)
2e (Me)
3.00 -11.0 -35.1 -0.5
2.96 -10.0 -35.0
0.4
0.5
0.5
1.0
0.9
0.8
3.02
-9.8 -34.6
-9.5 -33.5
0.1
2f (OMe) 3.11
0.1
a
C(carbene)-C2(TMB) distance, in A. b ΔH° and ΔG° in kcal/mol,
ΔS° in eu, relative to the separated reactants; T = 298.15 K. ΔG° and
ΔS° computed using a reference state of 1 M concentration for each
˚
(14) (a) Jaffe, H. H. Chem. Rev. 1953, 53, 191. (b) Wells, P. R. Chem.
Rev. 1963, 63, 171.
(15) Rondan, N. G.; Houk, K. N.; Moss, R. A. J. Am. Chem. Soc.
species participating in the reaction. c Units are M-1
.
1980, 102, 1770.
1200
Org. Lett., Vol. 13, No. 5, 2011

complex we generally find that as the carbene-TMB
interaction distance decreases, the binding enthalpy in-
creases. In type A complexes, the carbenic Cl atom points
toward the open back-side of an adjacent MeO group and
does not appear to be sterically encumbered. In type B
complexes, the Cl atom is situated above an adjacent
oxygen atom and is sterically encumbered by the MeO
group. As illustrated in Figure 2, the Cl-MeO steric
interaction forces the aromatic rings in Type B complexes
to rotate relative to each other, and consequently, there is
more stabilizing face-to-face ring overlap in complexes of
type A. For a particular para substituent (X), the A - B
energy difference is 1-2 kcal/mol (Table 3).
The entropy changes associated withcomplex formation
(ΔSo ≈ -35 eu, Table 3) are overestimated by the electro-
nic structure calculations, which refer to idealized gas
phase rather than solution phase conditions. Thus, despite
the very favorable complexation enthalpies obtained at the
B97D/6-311þG(d) level, the computed Gibbs free energies
of complexation are too large, and the computed equili-
brium constants are consequently smaller than the experi-
mentally derived quantities. Unquestionably, electron-
withdrawing groups (NO2, CF3, and Cl) in the p-position
lead to morefavorable free energiesof complexation (larger
K), and electron-donating groups (Me and MeO) disfavor
complex formation. The relative magnitudes of the sub-
stituent effects appear to be reproduced much better for the
electron-withdrawing groups than for the electron-donat-
ing groups; in particular, the destabilizing effect of MeO is
seriously underestimated by the calculations.17 Neverthe-
less, a plot of the Gibbs free energy changes, or equiva-
lently, a logarithmic plot of the equilibrium constants
versus the Hammett substituent parameters, displays a re-
asonable straight line. For the B complexes, the slope F =
1.95 (r = 0.982) is in satisfactory agreement with the
experimentally defined value of 2.48 (Figure 4); for the A
complexes F = 2.49 (r = 0.979).13 These Hammett correla-
tion diagrams appear in the Supporting Information.
p-X-PhCCl/TMB complex formation is accompanied
by the appearance of an electronic absorption band in
the 400-500 nm region. The new band embodies consider-
able π(HOMO, TMB) f p(LUMO, carbene) charge trans-
fer character, although neither experimental (Table 1) or
computed (Table S-1 in Supporting Information) band
maxima shift in a systematic manner as the para sub-
stituent (X) changes. For a particular substituent, the B
complex absorbs at lower energy with a smaller oscilla-
tor strength than the A complex (Table S-1), qualita-
tively in accord with the lesser overlap and interaction
between the aryl rings present in the B complexes
(Figure 2). In the experimental spectra, we therefore
associate the higher-energy band with the A-type com-
plexes and the lower-energy band with the B-type com-
plexes. Flanking these signature bands are mixed
absorption bands reflecting the coexistence of free car-
benes and carbene complexes in solution.
The experimental spectra appear to show larger in-
tensities in the low-energy bands, which we ascribe to B-
type complex formation, than in the high-energy bands
(A-type complexes). In contrast, the calculations predict
that a carbene/TMB solution mixture at equilibrium
should contain more complexes of type A than of type
B (Table 3); furthermore, the signature transition of
complex formation is predicted to carry larger intensity
in the A-type complexes (Table S-1). The electronic
structure calculations locate the single most stable car-
bene/TMB configuration on the potential energy surface
in an idealized gas phase. It is plausible that on the
“solution free energy surface” complexes of the less
geometrically restricted B-type actually dominate, so
that the absorption toward the red appears more intense
in the experimental spectra as a result of a larger
equilibrium concentration of complexes.
In summary, using a combination of LFP and DFT
techniques, we have determined equilibrium constants for
a family of p-substituted phenylchlorocarbene/TMB com-
plexes. The substituent effects on the experimental and
computational equilibrium constants are well correlated
by the Hammett equation.
Acknowledgment. We are grateful to the National
Science Foundation for financial support.
(16) All calculations made use of the Gaussian 09, Revision A02
program package: Frisch, M. J. et al. . Gaussian, Inc.: Wallingford, CT,
2009; see Supporting Information for additional computational details and
the complete reference for Gaussian 09.
(17) It was necessary to tighten the geometry optimization criteria
and to increase the size of the integration grid relative to their default
values to reach even this level of agreement and consistency. Test
calculations with larger basis sets and/or different dispersion-corrected
functionals did not show any improvement for X = MeO.
Supporting Information Available. Figures S-1-S-51,
Table S-1, computational details, and optimized geometries,
absolute energies, electronic excitation energies and oscilla-
tor strengths of all relevant species. This material is available
free of charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 5, 2011
1201