Arene-mercury complexes stabilized by aluminum and gallium chloride: Catalysts for H/D exchange of aromatic compounds

Article abstract of DOI:10.1021/ja012029o

Dissolution of Hg(arene)₂(MCI₄)₂ [arene = C₆H₅Me, C₆H₅Et, o-C₆H₄Me₂, C₆H₃-1,2,3-Me₃; M = Al, Ga] in C₆D₆ results in a rapid H/D exchange and the formation of the appropriate d_n-arene and C₆D₅H. H/D exchange is also observed between C₆D₆ and the liquid clathrate ionic complexes, [Hg(arene)₂(MCl₄)]-[MCl₄], formed by dissolution of HgCl₂ and MCl₃ in C₆H₆, m-C₆H₄Me₂, or p-C₆H₄Me₂. The H/D exchange reaction is found to be catalytic with respect to Hg(arene)₂(MCl₄)₂ and independent of the initial arene ligand. Reaction of a 1:1 ratio Of C₆H₅Me and C₆D₆ with <0.1 mol % Hg(C₆H₅Me)₂(MCl₄)₂ results in an equilibrium mixture of all isotopic isomers: C₆H_5-xD_xMe and C₆D_6-xH_x (x = 0-5). DFT calculations on the model system, Hg(C₆H₆)₂(AlCl₄)₂ and [Hg(C₆H₆)₂(AlCl₄)⁺, show that the charge on the carbon and proton associated with the shortest Hg···C interactions is significantly higher than that on uncomplexed benzene or HgCl₂(C₆H₆)₂. The protonation of benzene by either Hg(C₆H₆)₂(AlCl₄)₂ or [Hg(C₆H₆)₂(AlCl₄)]⁺ was calculated to be thermodynamically favored in comparison to protonation of benzene by HO₂CCF₃, a known catalyst for arene H/D exchange. Arene exchange and intramolecular hydrogen transfer reactions are also investigated by DFT calculations.

Full text of DOI:10.1021/ja012029o

Published on Web 03/16/2002
Arene-Mercury Complexes Stabilized by Aluminum and
Gallium Chloride: Catalysts for H/D Exchange of Aromatic
Compounds
Alexander S. Borovik and Andrew R. Barron*
Contribution from the Department of Chemistry and Center for Nanoscale Science and
Technology, Rice UniVersity, Houston, Texas 77005
Received August 22, 2001
Abstract: Dissolution of Hg(arene)2(MCl4)2 [arene ) C6H5Me, C6H5Et, o-C6H4Me2, C6H3-1,2,3-Me3; M )
Al, Ga] in C6D6 results in a rapid H/D exchange and the formation of the appropriate dn-arene and C6D5H.
H/D exchange is also observed between C6D6 and the liquid clathrate ionic complexes, [Hg(arene)2(MCl4)]-
[MCl4], formed by dissolution of HgCl2 and MCl3 in C6H6, m-C6H4Me2, or p-C6H4Me2. The H/D exchange
reaction is found to be catalytic with respect to Hg(arene)2(MCl4)2 and independent of the initial arene
ligand. Reaction of a 1:1 ratio of C6H5Me and C6D6 with <0.1 mol % Hg(C6H5Me)2(MCl4)2 results in an
equilibrium mixture of all isotopic isomers: C6H5-xDxMe and C6D6-xHx (x ) 0-5). DFT calculations on the
model system, Hg(C6H6)2(AlCl4)2 and [Hg(C6H6)2(AlCl4)]+, show that the charge on the carbon and proton
associated with the shortest Hg‚‚‚C interactions is significantly higher than that on uncomplexed benzene
or HgCl2(C6H6)2. The protonation of benzene by either Hg(C6H6)2(AlCl4)2 or [Hg(C6H6)2(AlCl4)]+ was calculated
to be thermodynamically favored in comparison to protonation of benzene by HO2CCF3, a known catalyst
for arene H/D exchange. Arene exchange and intramolecular hydrogen transfer reactions are also
investigated by DFT calculations.
Introduction
titanium(IV)-arene complexes. Gaseous deuterium can be
exchanged with C6H6 if tantalum (Cp2TaH3) or iridium [(PhEt2P)2-
Electrophilic aromatic substitution is a well-understood
reaction in organic chemistry, and hydrogen exchange is perhaps
the simplest of this type of reaction.1 In general, the reaction
involves the addition of a strong protic acid (HA) to an aromatic
(ArH). For a quantitative study of the reaction, dedeuteriation
(i.e., eq 1) has a number of advantages including (a) the
determination of the rate of reactivity of specific positions and
(b) the susceptibility of the rate of hydrogen exchange to
substitution effects but not steric effects. The majority of H/D
IrH5] hydrides are used as catalysts.6
As was reported previously,7,8 the 1H and 13C NMR for Hg-
(arene)2(MCl4)2 (M ) Al, Ga) could not be obtained in C6D6
solution due to a facile H/D exchange reaction of the aromatic
protons with the deuterium of the solvent. The rates of these
exchange reactions are faster than would be expected given the
conditions of the reaction: aprotic solVent and without a strong
protic acid. In this regard the reaction of Hg(arene)2(MCl4)2
with C6D6 is unusual. This observation prompted the following
question: Do the coordinated arenes behave as strong protic
acids?
ArD + HA h ArH + DA
(1)
exchange reactions are carried out under acid-catalyzed condi-
tions. The addition of water or a mineral acid can significantly
enhance the rate of exchange between C6D6 and HO2CCF3,2
whereas the addition of an aprotic Lewis acid reduces the rate
of exchange.3 H/D exchange reactions for aromatic compounds
are also known to occur in the presence of transition metal
catalysts. For example, H/D exchange between heavy water and
aromatic hydrocarbons is homogeneously catalyzed by [PtCl4]2-
salts.4 Recently Calderazzo et al.5 have reported H/D exchange
of the ring protons between C6D6 and C6H6-nMen promoted by
We report, herein, the catalytic activity of Hg(arene)2(MCl4)2
toward H/D exchange between C6D6 and a variety of aromatic
substrates, as well as a DFT study on the possible reaction
intermediates.
Results and Discussion
Dissolution of Hg(arene)2(MCl4)2 (M ) Al, Ga) in C6D6
results in the rapid (<5 min at 25 °C) quantitative formation of
(4) Hodges, R. J.; Garnett, J. L. J. Catal. 1969, 13, 83.
* To whom correspondence should be addressed. E-mail: arb@rice.edu.
URL: www.rice.edu/barron.
(5) Calderazzo, F.; Pampaloni, G.; Vallieri, A. Inorg. Chim. Acta 1995, 229,
179.
(1) Taylor, R. Electrophilic Aromatic Substitution; Wiley: New York, 1990,
Chapter 3.
(6) Barefield, E. K.; Parshall, G. W.; Tebbe, F. N. J. Am. Chem. Soc. 1970,
92, 5234.
(2) Mackor, E. L.; Smit, P. J.; van der Waals, J. H. Trans. Faraday Soc. 1957,
53, 1309.
(7) Borovik, A. S.; Bott, S. G.; Barron, A. R. Angew. Chem., Int. Ed. 2000,
39, 4117.
(3) Taylor, R. ComprehensiVe Chemical Kinetics, Elsevier: Amsterdam, The
(8) Borovik, A. S.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 2001, 123,
Netherlands, 1972; Vol. 13, pp.194-277.
11219.
9
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J. AM. CHEM. SOC. 2002, 124, 3743-3748
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A R T I C L E S
Borovik and Barron
dn-arene and C6D5H (eq 2, where M ) Al, Ga and arene )
C6H5Me, C6H5Et, o-C6H4Me2, C6H3-1,2,3-Me3). The identity
exchange.11 These rates are increased with temperature such
that at 80 °C the total turnover rate approaches 4000 min-1
,
while the “H/D-only” turnover rate under these conditions is
400 min-1
.
Hg(arene)2(MCl4)2 + nC6D6 f
No degradation of the catalyst is observed with time. The
addition of further quantities of either C6D6 or arene results in
continued H/D exchange demonstrating that the Hg(arene)2-
(MCl4)2 is a living catalyst. Furthermore, the catalyst solutions
may be stored (in the absence of light12) indefinitely and
maintain their catalytic activity.
Hg(dn-arene)2(MCl4)2 + nC6D5H (2)
1
of the deuterated products is confirmed by H and 13C NMR
spectroscopy as well as GC/MS. Deuteration is limited to the
aromatic CH groups, with no exchange occurring for the alkyl
substituents (i.e., Me and Et groups). Exchange is not limited
to the use of C6D6 as a deuterium source, but is also observed
with C6D5CD3, in which case C6D5-xHxCD3 is formed along
with the appropriate dn-arene. If a 1:1 ratio of C6H5Me and C6D6
is mixed with Hg(C6H5Me)2(MCl4)2 as a catalyst, then an
equilibrium mixture of all isotopic isomers is observed. For
example, the exchange between C6H5Me with 1 equiv of C6D6
yields C6H5-xDxMe and C6D6-xHx (x ) 0-5).
We have reported that liquid clathrates are formed by
dissolution of HgCl2 and MCl3 in C6H6, m-C6H4Me2, or p-C6H4-
Me2.8 The upper (neutral) layer contains Hg(arene)2(MCl4)2
while the lower (ionic) layer contains [Hg(arene)2(MCl4)][MCl4].
The layers may be physically separated and H/D exchange is
observed independently for both layers with the addition of
C6D6. The rate of exchange for the upper (neutral) layer is slower
than that for the lower (ionic) layer. Although the difference in
reactivity could be a function of the relative concentrations of
mercury/Group 13 complex in each layer, DFT calculations
suggest that the rationale for this difference is more complex
(see below).9
There appears to be a slightly greater rate of H/D exchange
when using aluminum as opposed to gallium as may be expected
from the greater Lewis acidity of the former.13 This small
difference is consistent with the greater effect of AlCl3 that
GaCl3 on the arene as measured from the UV-visible spectra.8
The Group 13 anion (i.e., [MCl4]-) clearly affects the activation
of the mercury since Damude et al.14 reported that the mercury‚
‚‚arene complexes formed with the hexafluoride anions of
1
arsenic and antimony showed well-characterized H and 13C
NMR with no evidence for H/D exchange. Furthermore, we
note that the copper(I) complex, Cu(C6H6)(AlCl4), has been
structurally characterized,15 but no H/D exchange was reported.
Garnett et al. have reported16 that AlCl3 will catalyze the H/D
exchange between C6D6 and toluene. They suggested that traces
of dissolved water were likely involved in the proton transfer.
This proposal was recently supported by Gaines and Beall with
regard to the AlCl3-catalyzed H/D exchange between B10H14
1
and C6D6.17 Using H NMR, we have determined that the rate
of H/D exchange for Hg(C6H5Me)2(AlCl4)2 is approximately
1.5 times faster (per Al) than that observed for AlCl3.
DFT Calculations. Hydrogen/deuterium exchange reactions
for aromatic compounds are known to occur in the presence of
strong acids. For example, addition of H2SO4 to a mixture of
D2O and C6H6 allows for the synthesis of C6D6.1 Other suitable
acid catalysts include HO2CCF3 and HBr. In all cases an acid
with a suitable pKa (<0.2) is required.18
Quantitative exchange, for example, formation of C6D5Me
and C6D5H, from C6H5Me and 5 molar equiv of C6D6, requires
at least 10 H/D exchange reactions per mercury center,
indicating that the reaction is catalytic. If C6H5Me and C6D6
(excess) are mixed in the presence of <0.1 mol % of Hg(C6H5-
Me)2(MCl4)2 (per C6H5Me) complete scrambling of the aromatic
hydrogen/deuteriums occurs, confirming the catalytic nature of
the H/D exchange reaction. Each of the isolable arene com-
plexes7,8 will catalyze the exchange between C6D6 and the
appropriate arene. Furthermore, the mercury complex does not
have to contain the particular arene for it to catalyze that arene’s
H/D exchange. Thus, H/D exchange is catalyzed for C6D6 and
xylenes in the presence of <0.1 mol % of Hg(C6H5Me)2(MCl4)2.
This would suggest there must be an exchange between
coordinated and “free” arenes, and that the identity of the
aromatic initially bonded to mercury does not affect the overall
reaction.10
The acid-catalyzed reaction takes place through an A-SE2
mechanism (eq 3), consisting of a bimolecular reaction between
an acid HA and the aromatic ring, resulting from the transfer
of a hydrogen cation from the former to the latter.13 The
Wheland intermediate (I) then loses a hydrogen cation to A-.
The reaction is reversible, and the energy profile must be
The observation of separate resonances for each of the
potential isomers in the 1H NMR of an equilibrium mixture of
C6H5Me and C6D5Me catalyzed by Hg(C6H5Me)2(MCl4)2 (i.e.,
C6H5-xDxMe) indicates that the lifetime of each species is
greater than that of the NMR experiment (10-5 s). Thus, the
rate of H/D exchange is e10-5 s-1 at 25 °C. By using 0.065
mol % of Hg(C6H5Me)2(MCl4)2 in a 1:10 solution of C6H5Me
and C6D6, an H/D turnover rate is determined to be ca. 3.3 min-1
at room temperature; however, a rate of ca. 33 min-1 is
calculated for all exchanges including degenerate D/D and H/H
(11) It should be noted that precautions were taken to ensure that no hydrolysis
of the catalyst occurred when measuring H/D exchange reactions.
(12) Solutions of Hg(arene)2(MCl4)2 are light sensitive, see refs 7 and 8.
(13) We are presently undertaking detailed kinetic studies of the H/D exchange
reaction in concert with ligand exchange. The results of these investigations
will be published elsewhere: Branch, C. S.; Bott, S. G.; Barron, A. R.,
57th Southwest Region ACS Meeting, San Antonio Texas, 2001.
(14) (a) Damude, L. C.; Dean, P. A. W. J. Organomet. Chem. 1979, 181, 1. (b)
Damude, L. C.; Dean, P. A. W.; Sefcik, M. D.; Schaefer, J. J. Organomet.
Chem. 1982, 226, 105.
(15) Turner, R. W.; Amma, E. L. J. Am. Chem. Soc. 1966, 88, 1877.
(16) Garrett, J. L.; Long, M. A.; Vining, R. F. W. J. Chem. Soc., Chem. Commun.
1972, 1172.
(17) Gaines, D. F.; Beall, H. Inorg. Chem. 2000, 39, 1812.
(18) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic
Chemistry, 3rd ed.; Harpers Collins: New York, 1987.
(9) Further kinetic studies and site selectivity studies will be reported elsewhere.
(10) The observation of a single set of resonances in the 1H NMR spectrum of
Hg(C6H5Me)2(AlCl4)2 dissolved in C6H5Me suggests that the rate of arene
exchange is greater than 10-5 s-1, see ref 8.
9
3744 J. AM. CHEM. SOC. VOL. 124, NO. 14, 2002

Arene−Mercury Complexes
A R T I C L E S
Figure 2. Calculated structure for the [HgPh(C6H6)(AlCl4)2]- anion
showing both λ1-phenyl and η1-benzene ligands.
suggesting that concentration is not the only factor in determin-
ing the relative rate of the lower and upper clathrate layers.
The magnitude of the increase in relative charges on the C
and H atoms of the coordinated arene in Hg(C6H6)2(AlCl4)2 is
significantly larger than that calculated for HgCl2(C6H6)2
(-0.188 on C and 0.145 on H).8 This is in accord with the
experimental observation that no H/D exchange is observed by
the addition of HgCl2 to a mixture of C6D6 and C6H5Me. Thus,
it would appear that the influence of the Group 13 halides is to
activate the mercury by dramatically increasing the Lewis acidity
of the latter.
The increased “acidity” of the arene proton associated with
the Hg‚‚‚C interaction suggests that it may be possible for the
coordinated arene to “protonate” a second arene molecule in a
manner similar to that observed for H/D exchange with mineral
acids. To investigate the potential of such a reaction we have
determined the structure and stability of the species formed from
the deprotonation of one of the arene rings in Hg(C6H6)2(AlCl4)2,
i.e., eq 4.
Figure 1. Calculated charge distribution on the aromatic C and H atoms
in (a) Hg(C6H6)2(AlCl4)2 and (b) [Hg(C6H6)2(AlCl4)]+.
symmetrical about the intermediate I, except for a small isotope
effect. An intramolecular exchange of the proton between
positions of equal proton affinity on the same carbonium ion
(I) has also been proposed. Brichall and Gillespie concluded,
however, that the reaction shown in eq 3 was the most
important.19
If such a mechanism is valid in the present system, the acidity
of the aromatic protons (deuteriums) of the coordinated arene
would be expected to be increased from their uncomplexed
values [pKa (benzene) ) 43]20 through activation by the
mercury/Group 13 chloride complex. To investigate this pos-
sibility we have performed DFT calculations at the B3LYP level
using the 6-31G** basis set for C and H and the Stuttgart RLC
ECP basis set for Hg, Cl, and Al. We have previously calculated
the structures for Hg(C6H6)2(AlCl4)2 and [Hg(C6H6)2(AlCl4)]+.8
Hg(C6H6)2(AlCl4)2 + C6H6 f
[HgPh(C6H6)(AlCl4)2]- + [C6H7]+ (4)
The optimized geometry for the [HgPh(C6H6)(AlCl4)2]- anion
is shown in Figure 2; selected structural features are given in
Table 1 along with those of Hg(C6H6)2(AlCl4)2 for comparison.
The most notable feature of the calculated structure for the
[HgPh(C6H6)(AlCl4)2]- anion is that the phenyl group, formed
from deprotonation of one benzene, is σ-bound to mercury. The
calculated Hg-C σ-bond distance (2.079 Å) is comparable to
that in HgPh2 [2.085(7) Å] as determined by X-ray crystal-
lography.21 A second feature of note is that the closest Hg‚‚‚
C(p) distance in the [HgPh(C6H6)(AlCl4)2]- anion (2.873 Å) is
significantly longer than the comparable interaction calculated
for the neutral complex, Hg(C6H6)2(AlCl4)2 (2.402 Å). A
consequence of the weaker Hg‚‚‚arene interaction is the
distortion of the mercury away from tetrahedral to trigonal
planar. This partial dissociation of the second arene ligand
suggests a possible route for arene exchange to occur during
the H/D exchange reaction.
The charge distribution on the aromatic C and H atoms in
Hg(C6H6)2(AlCl4)2 is shown in Figure 1a; the equivalent values
for uncomplexed C6H6 are C (-0.0804) and H (+0.084). It may
be clearly seen that the charge on the proton associated with
the shortest Hg‚‚‚C interaction (+0.194) is significantly higher
than that on free benzene. Similarly, the carbon associated with
the short Hg‚‚‚C contact has over a 3-fold increase in negative
charge (-0.304) as compared to free benzene. It should also
be noted that this effect is present, albeit reduced, on the C and
H atoms ortho to the short Hg‚‚‚C interaction.
Similar effects are observed for the cationic complexes, e.g.,
[Hg(C6H6)2(AlCl4)]+, see Figure 1b. The greater charge effects
on the cationic complexes are consistent with the H/D exchange
activity of the lower 1:1 electrolyte layers of the clathrates,7,8
The total energies of the model compounds, along with C6H6
and [C6H7]+, were determined at the B3LYP/Stuttgart RLCECP
for Hg, Cl, and Al, and the 6-31G** for C and H. From these
values the enthalpy of reaction (∆H ) 427 kJ‚mol-1) may be
(19) Birchall, T.; Gillespie, R. J. Can. J. Chem. 1964, 42, 502.
(20) Streitwieser, A., Jr.; Scannon, P. J.; Niemeyer, H. M. J. Am. Chem. Soc.
1972, 94, 7936.
(21) Grdenic, D.; Kamenar, B.; Nagl, A. Acta Crystallogr. 1977, B33, 587.
9
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A R T I C L E S
Borovik and Barron
Table 1. Calculated Bond Lengths (Å) and Angles (deg) in Mercury-Arene Complexes
Hg(C H )
[HgPh(C H )
[Hg(C H )
HgPh(C H )
(AlCl4)
[HgPh
(AlCl4)2]-
[HgPh2H
(AlCl4)]-
6
6 2
6
6
6
6 2
6
6
(AlCl4)2a
(AlCl4)2]-
(AlCl4)]+ a
Hg-C (p)
Hg-C (s)
Hg-Cl
2.402
2.873
2.079
2.509, 2.599
2.324, 2.339
2.222-2.242
2.371
2.816
2.069
2.519, 2.646
2.341, 2.314
2.195, 2.203
2.267
2.068
2.508
2.352
2.217, 2.230
2.463
2.393
2.200-2.245
2.466
2.364
2.181
2.522
2.343
2.193
1.476
80.79
84.37
91.52
Al-Clbr
Al-Clter
C-Hbr
C-Hg-C
Cl-Hg-Cl
Hg-Cl-Al
109.37
95.94
119.43
105.01
90.95
121.78, 124.56
110.06
85.24
92.49
105.78
81.67
91.30, 93.92
134.60
90.79
116.19
a Borovik, A. S.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 2001, 123, 11219.
for the ionic layer versus neutral layer of the liquid clathrates
formed with benzene and m- and p-xylene and (b) the greater
charge effects on the benzene ligands in [Hg(C6H6)2(AlCl4)]+
in comparison to those in Hg(C6H6)2(AlCl4)2, see Figure 1.
In comparison to a reaction known to occur during acid-
catalyzed H/D exchange (i.e., eq 6), a mechanism by which
the Hg‚‚‚arene complex protonates free C6H6 (eqs 4 and 7) is
not thermodynamically unreasonable.14 Furthermore, our pro-
posal that complexation of the arene to the “activated” HgCl2
results in a dramatic increase in the acidity of the aromatic
protons is plausible. Based upon the foregoing, we may propose
that the H/D exchange between Hg(C6H5Me)2(AlCl4)2 and C6D6,
for example, occurs in a manner similar to that observed for
protic acids, i.e., eq 8.
Figure 3. Calculated structure for the [HgPh(AlCl4)2]- anion showing the
trigonal planar coordination of the mercury.
calculated for the model reaction shown in eq 4. While this
number is open to interpretation due to the lack of solvation
effects being taken into account, it is ca. 44% of the value
calculated for the self-ionization of benzene (∆H ) 970
kJ‚mol-1), eq 5. More importantly, the calculated ∆H for eq 4
2C6H6 f [C6H5]- + [C6H7]+
(5)
should be compared to the calculated value for the protonation
of benzene by trifluoroacetic acid (∆H ) 634 kJ‚mol-1), an
acid that is known to catalyze H/D exchange (eq 6).22 It is
HO2CCF3 + C6H6 f [O2CCF3]- + [C6H7]+
(6)
important to note that these calculated values do not take into
account the activation barrier for these reactions or solvation
effects; however, the trend is consistent with the higher rate
observed for Hg(C6H5Me)2(AlCl4)2 as compared to HO2CCF3
in the H/D exchange between C6H5Me and C6D6.
The structure of the product that would be formed from
deprotonation of the cation [Hg(C6H6)2(AlCl4)]+, i.e., the neutral
complex HgPh(C6H6)(AlCl4), was also optimized and its total
energy determined. The energy (∆H ) 164 kJ‚mol-1) for
protonation of benzene by [Hg(C6H6)2(AlCl4)]+ (eq 7) is
significantly lower than that for the reaction shown in eq 4. It
Arene Exchange and Intramolecular H-Transfer. As noted
above, the benzene ligand in the anionic model complex
[HgPh(C6H6)(AlCl4)2]- is only weakly associated. On the basis
of the optimized structure of [HgPh(AlCl4)2]- (Figure 3 and
Table 1) and its total energy, the bond dissociation energy for
the neutral benzene (eq 9) is calculated to be 7.7 kJ‚mol-1. A
dissociation energy of 12.7 kJ‚mol-1 was similarly obtained for
HgPh(C6H6)(AlCl4), eq 10. The related arene dissociation for
[HgPh(C6H6)(AlCl4)2]- f [HgPh(AlCl4)2]- + C6H6 (9)
HgPh(C6H6)(AlCl4) f HgPh(AlCl4) + C6H6 (10)
[Hg(C6H6)2(AlCl4)]+ + C6H6 f
HgPh(C6H6)(AlCl4)2 + [C6H7]+ (7)
the neutral complex (eq 11) is calculated to be an exothermic
process (-7.6 kJ‚mol-1), in the absence of solvation affects.
Clearly the stability of mercury-arene complexes in solution
is determined by the excess of arene solvent present. An
is important to note that this relative trend is in good agreement
with both (a) the higher exchange rate experimentally observed
(22) We note that solvation effects are likely to be important in the reactions
shown in eqs 4 and 6.
Hg(C6H6)2(AlCl4)2 f Hg(C6H6)(AlCl4)2 + C6H6 (11)
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Arene−Mercury Complexes
A R T I C L E S
Intermolecular exchange is based on the protonation of C6D6
by the mercury-arene complex followed by the recombination
of the resulting [C6D6H]+ and mercury-arene anion with 50%
probability for the successful H/D exchange (eq 8). This would
be expected to follow the same kinetic and site selectivity trends
as a traditional mechanism involving a Wheland intermediate.14
In contrast, the intramolecular exchange would require that the
deprotonated mercury-phenyl species be sufficiently long-lived
to allow for an intramolecular deuterium shift. Although
plausible, the latter mechanism would be more susceptible to
steric limitations and as such we propose that such a mechanism
is less likely.
Figure 4. Calculated structure for the Hg(C6H6)(AlCl4)2 showing the λ2-
coordination of the “AlCl4” ligands to mercury.
Conclusions
Group 13 chloride-stabilized arene-mercury complexes are
highly active catalysts for aromatic H/D exchange. The rate of
these exchange reactions is unexpectedly fast, given that they
are carried out in an aprotic solvent and without a Bro¨nsted
acid. Most importantly, it should be noted that ordinarily the
addition of aprotic Lewis acid reduces the rate of H/D
exchange.1,3 Thus, the catalytic activity of Hg(arene)2(AlCl4)2
and [Hg(arene)2(AlCl4)]+ for H/D exchange is highly unusual.
We have previously shown23 that Group 13 Lewis acids, such
as AlR3, can increase the proton acidity, as measured by a
decrease in the pKa, by at least 7 units. A maximum decrease
of 30 units is estimated from the activation of amines. If we
compare the pKa of benzene (43) with those of H2SO4 (ca. -2)
and HO2CCF3 (>2),13 then a significant decrease in the pKa of
an aromatic hydrogen would be required for H/D exchange to
occur. On the basis of DFT calculations, it appears that the
coordination of the arene to the mercury/Group 13 complex
results in a significant increase in the acidity of the aromatic
hydrogens. It is particularly important to note the activation from
the mercury/Group 13 complex is significantly greater than that
for HgCl2 itself. Thus, the Lewis acidity of the normally weakly
acidic mercury is enhanced by the presence of the two strong
Lewis acids. This indirect activation is sufficient to cause the
coordinated arene to act as the acid in a typical electrophilic
aromatic substitution reaction. Using DFT calculations we have
shown that the protonation of “free” benzene by the acidic
coordinated arene is thermodynamically favored in comparison
with the known reaction of benzene with HO2CCF3.
Figure 5. Calculated structure for the [HgPh2H(AlCl4)]- anion; a possible
intramolecular hydrogen transfer transition state.
interesting feature of mono-benzene complex Hg(C6H6)(AlCl4)2
(Figure 4 and Table 1) is that two AlCl4 moieties are bound λ2
to the mercury, rather than λ1 as observed for both anionic and
nondissociated complexes (cf., Figure 1), and thus account for
the exothermic nature of the benzene dissociation. Thus, the
dissociation of benzene from Hg(C6H6)(AlCl4)2 can be consid-
ered as an intramolecular SN2 substitution of benzene by a
terminal chlorine atom of the [AlCl4]- anionic ligand. Con-
straining angles of “AlCl4” groups in their approach to mercury
center, the approximate activation energy for the dissociation
of benzene from neutral complex can be calculated (60
kJ‚mol-1).
If arene exchange (eq 12) is a highly facile reaction10 then
the possibility of an intramolecular H/D exchange reaction may
be considered. Once the phenyl derivative is formed (eq 4), arene
[HgPh(C6H6)(AlCl4)2]- + C6D6 h
[HgPh(C6D6)(AlCl4)2]- + C6H6 (12)
Experimental Section
NMR spectra were obtained on Bruker AC-250 and Avance 200,
400, and 500 spectrometers. Chemical shifts are reported relative to
internal solvent resonances. NMR tubes are cleaned in basic solution,
followed by acetone wash. The tubes are dried and stored in an oven
prior to use, from which they are taken directly to the port on the drybox
which is immediately evacuated. The C6D6 is predried and stored in
the drybox over molecular sieves. GC/MS analyses were carried out
with use of a Finnigan MAT 95 mass spectrometer operating with an
electron beam energy of 70 eV for EI mass spectra and equipped with
a Hewlett-Packard 5890 series II gas chromatograph using a DB5 30
m × 0.25 mm i.d. column with a 0.25 mm coating of DB-5 stationary
phase and injector and transfer line temperatures of 180 and 250 °C,
respectively. The column was started at 35 °C for 2 min, then heated
at 25 °C‚min-1 for 6 min and maintained at 185 °C for 1 min. Isotope
patterns for all deuterium containing species were matched with
exchange may occur more rapidly than intermolecular proton
transfer (eq 8), allowing for H/D exchange to occur by way of
a intramolecular H/D exchange reaction in which hydrogen
transfer from the neutral arene to the phenyl group would be
necessary (eq 13). We have calculated the possible transition
[Hg(C6D6)(C6H5)(AlCl4)2]- h
[Hg(C6D5)(C6H5D)(AlCl4)2]- (13)
state with the corresponding activation energy (233.7 kJ‚mol-1
)
for such a reaction, i.e., [HgPh2H(AlCl4)2]-. The molecular
symmetry turned out to be C2V as shown in Figure 5 and
Table 1.
Two possible mechanisms for H/D exchange may be pro-
posed: intermolecular (eq 8) and intramolecular (eq 13).
(23) McMahon, C. N.; Bott, S. G.; Barron, A. R. J. Chem. Soc., Dalton Trans.
1997, 3129.
9
J. AM. CHEM. SOC. VOL. 124, NO. 14, 2002 3747

A R T I C L E S
Borovik and Barron
calculated distributions. The synthesis of Hg(arene)2(MCl4)2 (arene )
C6H5Me, C6H5Et, o-C6H4Me2, and C6H3-1,2,3-Me3; M ) Al, Ga) was
as reported previously.7,8 Solvents and all arenes were distilled and
degassed prior to use.
at elevated temperature. The NMR tube was heated at 80 °C for 30 s.
The H NMR spectrum was recorded shortly afterward showing that
1
30% of toluene have exchanged, given the average turnover rate of
400 min-1 per Hg. Using a 1:1 ratio of toluene and C6D6 results in a
mixture of all isotopic isomers, for example, three well-resolved peaks
for CH3, as well as 3 peaks for C-CH3, are observed in 13C NMR. All
other resonances are hard to describe and consist of overlapping triplets
and singlets.
Reaction of Hg(arene)2(MCl4)2 with C6D6. In a 5 mm NMR tube
containing Hg(arene)2(MCl4)2 (0.10 mmol) was added C6D6 (0.5 mL).
If necessary, the resulting solution can be heated to allow all the catalyst
to dissolve. Usually the formation of a benzene clathrate layer is
1
observed on the bottom of the NMR tube. H and 13C NMR spectra
In a 5 mm NMR tube containing Hg(C6H5Me)2(AlCl4)2 (20 mg,
0.028 mmol) were added C6D6 (0.500 g, 8.452 mmol) and C6H4Me2
(80 mg, 0.755 mmol). The NMR tube was heated at 80 °C for 1 min.
were obtained immediately. Then, the NMR tube solution was quenched
with water and analyzed by GC/mass spectroscopy.
C6D5Me: GS/MS (EI, %) m/z 97 (M+, 100). 1H NMR (C6D6)
δ 2.10 (3H, s, CH3). 13C NMR (C6D6) δ 139.3 (s, C-CH3), 129.5 [t,
J(C-D) ) 23.8 Hz, o-CD], 128.5 [t, J(C-D) ) ca. 24 Hz, m-CD],
124.7 [t, J(C-D) ) 24.4 Hz, p-CD], 21.5 (s, CH3).
The H NMR spectrum was recorded shortly afterward showing the
1
complete H/D exchange between C6H4Me2 and C6D6.
In a 5 mm NMR tube containing either Hg(C6H5Me)2(AlCl4)2 (6.9
× 10-6 mol) or AlCl3 (1.4 × 10-5 mol) were added C6D6 (0.500 g,
8.452 mmol) and C6H5Me (0.500 mg, 0.543 mmol). The NMR tube
was heated at 30 °C for 100 min. The 1H NMR spectrum was recorded
to determine the relative extent of the H/D exchange between C6H5Me
and C6D6.
Computational Methods. All density functional calculations were
carried out using a Gaussian-98 suite.24 Complete geometry optimiza-
tions were performed at the B3LYP25 level using the 6-31G** basis
set for C and H only and Stuttgart RLC ECP basis set for Hg, Cl, and
Al. C2 and Cs symmetries were imposed on neutral and cationic
molecules, respectively. Vibrational frequencies were then evaluated
for benzene complexes to verify the existence of the true potential
minimum and to determine zero-point energies.
C6D5Et: GS/MS (EI, %) m/z 111 (M+, 45), 96 (M+ - Me, 100).
1H NMR (C6D6) δ 7.16 (5H, s, C6D5H), 2.44 [2H, q, J(H-H) )
7.6 Hz, CH2], 1.07 [3H, t, J(H-H) ) 7.6 Hz, CH3]. 13C NMR (C6D6)
δ 145.5 (s, C-CH3), 128.5 [t, J(C-D) ) 24 Hz, o,m-CD], 124.9 [t,
J(C-D) ) 25.0 Hz, p-CD], 29.4 (s, CH2), 16.1 (s, CH3).
o-C6D4Me2: GS/MS (EI, %) m/z 110 (M+, 100), 95 (M+ - Me,
1
90). H NMR (C6D6) δ 7.16 (4H, s, C6D5H), 1.98 (6H, s, CH3). 13C
NMR (C6D6) δ 141.3 (s, C-CH3), 131.3 [t, J(C-D) ) 24.2 Hz, o-CD],
124.5 [t, J(C-D) ) 24.1 Hz, m-CD], 20.2 (s, CH3).
1,2,3-C6D3Me3: GS/MS (EI, %) m/z 123 (M+, 80), 108 (M+ - Me,
100). 1H NMR (C6D6) δ 7.16 (3H, s, C6D5H), 2.09 (6H, s, CH3), 1.91
(3H, s, CH3). 13C NMR (C6D6) δ 139.3 (s, 1,3-C-CH3), 137.6 (s, 2-C-
CH3), 127.1 [t, J(C-D) ) 23.8 Hz, 4,6-CD], 125.4 [t, J(C-D) ) 24.6
Hz, 5-CD], 21.0 (s, 1,3-CH3), 15.7 (s, 2-CH3).
Acknowledgment. Financial support for this work is provided
by the Robert A. Welch Foundation. The Bruker Avance 200
and 500 NMR spectrometers were purchased with funds from
ONR Grant N00014-96-1-1146 and NSF Grant CHE-9708978,
respectively.
Reaction of C6D6 with Clathrates. In a 5 mm NMR tube containing
1
a clathrate bottom layer (50 mg) was added C6D6 (0.5 mL). H and
13C NMR spectra were obtained immediately. Then, the NMR tube
solution was run through silica gel to kill the catalyst. The resulting
colorless mixture of dn-arenes was analyzed by GC/mass spectroscopy.
If the top clathrate layer is used the heating of the sample is necessary
to complete the exchange reaction.
Supporting Information Available: Structural parameters and
energies for optimized structures from DFT calculations (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
m-C6D4Me2: GS/MS (EI, %) m/z 110 (M+, 90), 95 (M+ - Me,
1
100). H NMR (C6D6) δ 7.16 (4H, s, C6D5H), 2.14 (6H, s, CH3). 13C
NMR (C6D6) δ 152.0 (s, C-CH3), 134.9 [t, J(C-D) ) 24.5 Hz, 2-CD],
132.9 [t, J(C-D) ) 24.9 Hz, 5-CD], 119.9 [t, J(C-D) ) 24.7 Hz,
4,6-CD], 22.4 (s, CH3).
JA012029O
(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.9;
Gaussian, Inc.: Pittsburgh, PA, 1998.
p-C6D4Me2: GS/MS (EI, %) m/z 110 (M+, 100), 95 (M+ - Me,
1
85). H NMR (C6D6) δ 7.16 (4H, s, C6D5H), 2.10 (6H, s, CH3). 13C
NMR (C6D6) δ 136.9 (s, C-CH3), 129.4 [t, J(C-D) ) 24.1 Hz, CD],
21.2 (s, CH3).
Catalytic H/D Exchange. In a 5 mm NMR tube containing
Hg(C6H5Me)2(AlCl4)2 (5 mg, 0.007 mmol) were added C6D6 (0.710 g,
8.452 mmol) and C6H5Me (85 mg, 0.924 mmol). When the catalyst
was completely dissolved the 1H NMR spectra were recorded at 1.5 h
intervals. After the first 1.5 h, the integration of C6D5H and C6D5-xHx
reveals that 40% of toluene protons have exchanged, given the average
turnover rate of 3 min-1 per Hg. The same experiment was performed
(25) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Stephens, P. J.; Devlin,
C. F.; Chabalowski, C. F.; Frisch M. J. J. Phys. Chem. 1994, 98, 11623.
(c) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785.
9
3748 J. AM. CHEM. SOC. VOL. 124, NO. 14, 2002