Arene-mercury complexes stabilized by gallium chloride: Relative rates of H/D and arene exchange

Article abstract of DOI:10.1021/ja0206590

We have previously proposed that the Hg(arene)₂(GaCl₄)₂ catalyzed H/D exchange reaction of C₆D₆ with arenes occurs via an electrophilic aromatic substitution reaction in which the coordinated arene protonates the C₆D₆. To investigate this mechanism, the kinetics of the Hg(C₆H₅Me)₂(GaCl₄)₂ catalyzed H/D exchange reaction of C₆D₆ with naphthalene has been studied. Separate second-order rate constants were determined for the 1- and 2-positions on naphthalene; that is, the initial rate of H/D exchange = k_1i[Hg][C-H₁] - k_2i[Hg][C-H₂]. The ratio of k_1i/k₂ ranges from 11 to 2.5 over the temperature range studied, commensurate with the proposed electrophilic aromatic substitution reaction. Observation of the reactions over an extended time period shows that the rates change with time, until they again reach a new and constant second-order kinetics regime. The overall form of the rate equation is unchanged: final rate = k_1f[Hg][C-H₁] + k_2f[Hg][C-H₂]. This change in the H/D exchange is accompanied by ligand exchange between Hg(C₆D₆)₂(GaCl₄₎₂ and naphthalene to give Hg(C₁₀H₈)₂(GaCl₄)₂, that has been characterized by ¹³C CPMAS NMR and UV-visible spectroscopy. The activation parameters for the ligand exchange may be determined and are indicative of a dissociative reaction and are consistent with our previously calculated bond dissociation for Hg(C₆H₆)₂(AlCl₄)₂. The initial Hg(arene)₂(GaCl₄)₂ catalyzed reaction of naphthalene with C₆D₆ involves the deuteration of naphthalene by coordinated C₆D₆; however, as ligand exchange progresses, the pathway for H/D exchange changes to where the protonation of C₆D₆ by coordinated naphthalene dominates. The site selectivity for the H/D exchange is initially due to the electrophilic aromatic substitution of naphthalene. As ligand exchange occurs, this selectivity is controlled by the activation of the naphthalene C - H bonds by mercury.

Full text of DOI:10.1021/ja0206590

Published on Web 10/30/2002
Arene-Mercury Complexes Stabilized by Gallium Chloride:
Relative Rates of H/D and Arene Exchange
Catherine S. Branch and Andrew R. Barron*
Contribution from the Department of Chemistry and Center for Nanoscale Science and
Technology, Rice UniVersity, Houston, Texas 77005
Received May 6, 2002
Abstract: We have previously proposed that the Hg(arene)2(GaCl4)2 catalyzed H/D exchange reaction of
C6D6 with arenes occurs via an electrophilic aromatic substitution reaction in which the coordinated arene
protonates the C6D6. To investigate this mechanism, the kinetics of the Hg(C6H5Me)2(GaCl4)2 catalyzed
H/D exchange reaction of C6D6 with naphthalene has been studied. Separate second-order rate constants
were determined for the 1- and 2-positions on naphthalene; that is, the initial rate of H/D exchange )
k1i[Hg][C-H1] + k2i[Hg][C-H2]. The ratio of k1i/k2i ranges from 11 to 2.5 over the temperature range studied,
commensurate with the proposed electrophilic aromatic substitution reaction. Observation of the reactions
over an extended time period shows that the rates change with time, until they again reach a new and
constant second-order kinetics regime. The overall form of the rate equation is unchanged: final rate )
k1f[Hg][C-H1] + k2f[Hg][C-H2]. This change in the H/D exchange is accompanied by ligand exchange
between Hg(C6D6)2(GaCl4)2 and naphthalene to give Hg(C10H8)2(GaCl4)2, that has been characterized by
13C CPMAS NMR and UV-visible spectroscopy. The activation parameters for the ligand exchange may
be determined and are indicative of a dissociative reaction and are consistent with our previously calculated
bond dissociation for Hg(C6H6)2(AlCl4)2. The initial Hg(arene)2(GaCl4)2 catalyzed reaction of naphthalene
with C6D6 involves the deuteration of naphthalene by coordinated C6D6; however, as ligand exchange
progresses, the pathway for H/D exchange changes to where the protonation of C6D6 by coordinated
naphthalene dominates. The site selectivity for the H/D exchange is initially due to the electrophilic aromatic
substitution of naphthalene. As ligand exchange occurs, this selectivity is controlled by the activation of
the naphthalene C-H bonds by mercury.
and C6D5H.3 The H/D exchange reaction was found to be
catalytic with respect to Hg(arene)2(MCl4)2 and independent of
the initial arene ligand. 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.4
The majority of H/D exchange reactions are carried out under
acid catalyzed conditions. The addition of water or a mineral
acid can significantly enhance the rate of exchange between
C6D6 and HO2CCF3,5 whereas the addition of an aprotic Lewis
acid reduces the rate of exchange.6 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 homoge-
neously catalyzed by [PtCl4]2- salts.7 Calderazzo et al.8 have
Introduction
We have recently reported that the reaction of HgCl2 with
2 equiv. of MCl3 (M ) Al, Ga) in an aromatic solvent
yields Hg(arene)2(MCl4)2, where arene ) C6H5Me, C6H5Et,
o-C6H4Me2, and C6H3-1,2,3-Me3.1,2 In the solid state, these
compounds exist as either neutral complexes in which two
arenes are bound to the mercury and the MCl3 groups are bound
through bridging chlorides to the mercury (I) or as a cation-
anion pair (II). In solution, however, all the complexes exist in
their neutral forms.2
(1) Borovik, A. S.; Bott, S. G.; Barron, A. R. Angew. Chem., Int. Ed. 2000,
39, 4117.
(2) Borovik, A. S.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 2001, 123,
11219.
(3) Borovik, A. S.; Barron, A. R. J. Am. Chem. Soc. 2002, 124, 3743.
(4) Although AlCl3 will catalyze the H/D exchange between C6D6 and toluene,
traces of dissolved water are involved in the proton transfer; see (a) Garrett,
J. L.; Long, M. A.; Vining, R. F. W. J. Chem. Soc., Chem. Commun. 1972,
1172. (b) Gaines, D. F.; Beall, H. Inorg. Chem. 2000, 39, 1812.
(5) Mackor, E. L.; Smit, P. J.; van der Waals, J. H. Trans. Faraday Soc. 1957,
53, 1309.
Dissolution of Hg(arene)2(MCl4)2 in C6D6 results in a rapid
H/D exchange and the formation of the appropriate dn-arene
(6) Taylor, R. ComprehensiVe Chemical Kinetics; Elsevier: Amsterdam, 1972;
Vol 13, pp 194-277.
(7) Hodges, R. J.; Garnett, J. L. J. Catal. 1969, 13, 83.
* To whom correspondence should be addressed: www.rice.edu/barron.
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J. AM. CHEM. SOC. 2002, 124, 14156-14161
10.1021/ja0206590 CCC: $22.00 © 2002 American Chemical Society

Arene−Mercury Complex H/D and Arene Exchange
A R T I C L E S
reported H/D exchange of the ring protons between C6D6 and
C6H6-nMen promoted by titanium(IV)-arene complexes. Gas-
eous deuterium can be exchanged with C6H6 if tantalum
(Cp2TaH3) or iridium [(PhEt2P)2IrH5] hydrides are used as
catalysts.9
On the basis of DFT calculations,3 we have proposed that
the reaction pathway for arene H/D exchange involves the
protonation of benzene by Hg(C6H6)2(MCl4)2 and the formation
of a Wheland intermediate, eq 1. Furthermore, we proposed that
the coordination of the arene to the mercury/group 13 complex
results in a significant increase in the acidity of the aromatic
hydrogens,10 sufficient to cause the coordinated arene to act as
the acid in a typical electrophilic aromatic substitution reaction.
Figure 1. Representative plot determining the first-order rate constant for
the initial H/D exchange between C6D6 and the 1-position on C10H8 (R )
0.991).
Hg(C6H6)2(MCl4)2 + C6D6 f
[HgPh(C6H6)(MCl4)2]- + [C6D6H]+ (1)
than Hg(C6H6)2(GaCl4)2 directly. This is because the toluene
complex may be isolated as a solid while the benzene complex
forms a liquid clathrate.2 We have previously shown that the
identity of the initial arene complex is unimportant with regard
to the product distribution with respect to the H/D exchange.2
We proposed that this was due to the facile ligand exchange
between toluene and benzene, resulting in the formation of
Hg(C6H6)2(GaCl4)2 in situ. This assumption is confirmed by the
present study.
To confirm that the mechanism of H/D exchange catalyzed
by Hg(arene)2(MCl4)2 involves an electrophilic aromatic sub-
stitution reaction, we have investigated the kinetics of the H/D
exchange reaction. In particular, we are interested in comparing
the site preference of the reaction with that of a well-
characterized electrophilic aromatic substitution reaction.11
Naphthalene undergoes a number of usual electrophilic
aromatic substitution reactions such as nitration, halogenation,
sulfonation, and Friedel-Crafts acylation. Although the exact
ratio of products is dependent on the reactants and the reaction
conditions, the 1-position is the more reactive (e.g., eq 2).12,13
In a typical experiment, 0.78 mmol of naphthalene is
dissolved in 1 mL of C6D6, to which is added ∼3 µmol of
Hg(C6H5Me)2(GaCl4)2. These quantities enable 1H NMR mea-
surements to be made over ∼3-12 h for runs between 20 and
65 °C and allow us to overcome the errors that occur when
initial rates are measured at temperatures different from ambient.
We note that only H/D exchange reactions may be followed by
1H NMR spectroscopy; degenerate H/H and D/D exchanges are
not observed. Retrogressive H/D exchanges (i.e., between two
naphthalene molecules) should not be significant during the early
stages of the reaction and in the presence of a large excess of
C6D6. The rate of loss of naphthalene C-H groups (and, thus,
concurrent formation of naphthalene C-D groups) may be
If arene H/D exchange catalyzed by Hg(C6H6)2(MCl4)2 involves
an electrophilic aromatic substitution reaction, a similar site
preference should be observed for the H/D exchange between
C6D6 and naphthalene. The results of this study are presented
herein.
1
followed by H NMR.
Separate first-order observed rate constants, kobs (eq 3), were
calculated from the corresponding plot of -ln[C-Hn] versus
time (e.g., Figure 1) for the 1- and 2-positions on naphthalene,
where [C-Hn] is the concentration of the hydrogen atoms at
the n-position on naphthalene (III). A plot of kobs versus the
Results and Discussion
The reaction of naphthalene (C10H8) with excess C6D6 in the
presence of a catalytic quantity of Hg(C6H5Me)2(GaCl4)2 results
in the formation of C10D8 and C6D5H. The presence of octa-
deuterionaphthalene (C10D8) is confirmed by 13C NMR spec-
troscopy and MS (see Experimental Section).
We have studied the kinetics of the Hg(C6H5Me)2(GaCl4)2
catalyzed reaction of C6D6 with naphthalene. Even though the
catalytic exchange involves C6D6 and C10D8, we employed
Hg(C6H5Me)2(GaCl4)2 as a source of Hg(C6H6)2(GaCl4)2, rather
(8) Calderazzo, F.; Pampaloni, G.; Vallieri, A. Inorg. Chim. Acta 1995, 229,
179.
concentration of the catalyst, [Hg(arene)2(GaCl4)2], shows a
linear dependence (Figure 2), indicating the rate of H/D
exchange is first order with respect to both the substrate and
catalyst. An overall rate equation may be described for the initial
phase of the H/D exchanges, eq 4.14,15Measurement of the
temperature dependence (20-65 °C) for k1i and k2i (e.g., Figure
(9) Barefield, E. K.; Parshall, G. W.; Tebbe, F. N. J. Am. Chem. Soc. 1970,
92, 5234.
(10) We have previously shown that group 13 Lewis acids, such as AlR3, can
increase the proton acidity of coordinated alcohols, as measured by a
decrease in the pKa, by at least 7 units. McMahon, C. N.; Bott, S. G.;
Barron, A. R. J. Chem. Soc., Dalton Trans. 1997, 3129.
(11) Taylor, R. Electrophilic Aromatic Substitution; Wiley: New York, 1990;
Chapter 3.
(12) Makor, E. L.; Hofstra, A.; van der Waals, J. H. Trans. Faraday Soc. 1958,
54, 66.
(13) Fischer, P.; Taylor, R. J. Chem. Soc., Perkin Trans. 1980, 781.
(14) We have defined k1i and k2i as the rate constants for the initial H/D exchange
reaction.
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A R T I C L E S
Branch and Barron
The ratio of k1i/k2i ranges from 11 to 2.5 over the temperature
range studied (35-65 °C). This agrees with previous proton
affinity studies12,13 and suggests that protonation of the naph-
thalene is occurring during the H/D exchange. The site selectiv-
ity for the Hg(arene)2(GaCl4)2 catalyzed H/D exchange reaction
is commensurate with an electrophilic aromatic substitution
reaction. In addition, the rate equation is consistent with the
protonation of naphthalene by the coordinated benzene in
Hg(C6D6)2(GaCl4)2 (eq 6).
Figure 2. Plot of kobs as a function of catalyst concentration for the initial
H/D exchange between C6D6 and the 1-position on C10H8 (R ) 0.992).
Hg(C6D6)2(GaCl4)2 + C10H8 f
[Hg(C6D5)(C6D6)(GaCl4)2]- + [C10H8D]+ (6)
Although the rates for H/D exchange follow first-order
kinetics during the first hour, observation of the reactions over
an extended time period shows that the rates change with time.
After an initial period, the reaction rates increase until they once
again reach a new first-order kinetics regime. The final rates
do not change throughout the rest of the observed reaction (e.g.,
Figure 4). The overall form of the rate equation is unchanged
(eq 7), although the rate constants k1f and k2f are both increased.
As may be seen from Table 1, the value for k2f is closer to that
of k1f than during the initial reaction period (i.e., k1i > k2i, while
k1f ≈ k2f). The ∆Hq and ∆Sq for these reactions may be
determined from the temperature dependence for k1f and k2f (see
Table 1). As with the initial reactions, the positive values of
∆Sq are indicative of a dissociative reaction.16
Figure 3. Representative Eyring plot for the initial H/D exchange between
C6D6 and the 1-position (9, R ) 0.993) and 2-position (0, R ) 0.993) on
C10H8.
Table 1. Summary of Activation Enthalpy and Entropy for the
Hg(C6H5Me)2(GaCl4)2 Catalyzed H/D Exchange Reactiona
q
q
naphthalene
substituent
knb at 338 K
∆H
(kJ mol-1
∆S
(J K-1 mol-1
)
final rate of H/D exchange )
3
(102 mol-1 dm s-1
)
)
k1f[Hg][C-H1] + k2f[Hg][C-H2] (7)
initial reaction
final reaction
1
2
1
2
7.5(3)
2.9(1)
9.5(4)
8.7(3)
77(1)
110(10)
61(2)
158(5)
250(20)
114(6)
120(10)
The ∆Hq for the H/D exchange at the 1-position is only
slightly decreased between the two reaction regimes. In contrast,
the ∆Hq for the H/D exchange at the 2-position decreases
significantly such that it becomes comparable to that for
exchange at the 1-position. This is in direct contrast to the trend
expected for an electrophilic aromatic substitution reaction on
naphthalene. It would, therefore, appear that the reaction
pathway changes with time. Two questions are raised from these
observations. What is the change in reaction pathway, and why
does the site selectiVity of the H/D exchange change?
63(5)
a Esd’s given in parentheses. b Value calculated from thermodynamic
data.
3) allows for the determination of ∆Hq and ∆Sq (see Table 1).
The large positive value of ∆Sq indicates a dissociative
reaction.16
-d[C-Hn]/dt ) kobs[C-Hn]
(3)
We have observed that, during the NMR experiments, a small
quantity of a dark orange solid precipitate is observed once the
H/D exchange has concluded. MS of this material shows that
naphthalene is the only arene there, while 13C CPMAS NMR
spectroscopy and analysis indicates the composition to be Hg-
(C10H8)2(GaCl4)2 (see Figure 5). 13C CPMAS NMR spectro-
scopic peak assignments were obtained by a combination of
dipolar dephasing experiments17 and comparison with calculated
(DFT) chemical shifts (see Experimental Section). Figure 6
shows the calculated structure for Hg(C10H8)2(GaCl4)2. Table
rate H/D exchange ) k1i[Hg][C-H1] + k2i[Hg][C-H2] (4)
The initial H/D exchange with the 2-position is sufficiently
slow at 25 °C under the conditions studied that it is not possible
to measure the rate; however, above 35 °C, the relative rates
may be determined and show a preferential reaction with the
1-position (e.g., eq 5).
(15) It should be noted that the major source of error in the calculation of the
individual second-order rate constants involves the measurement of the mass
of catalyst.
(16) Atwood, J. D. Inorganic and Organometallic Reaction Mechanisms;
VCH: New York, 1997; p 14.
(17) Alemany, L. B.; Grant, D. M.; Alger, T. D.; Pugmire, R. J. J. Am. Chem.
Soc. 1983, 105, 6697.
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Arene−Mercury Complex H/D and Arene Exchange
A R T I C L E S
Table 2. Experimental and Calculated (DFT) 13C NMR Spectra for
Hg(C10H8)2(GaCl4)2
a
atom δ (ppm)
calculated shift δ (ppm)
experimental shift
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
138.6
108.0
126.1
134.6
127.0
131.7
128.0
128.4
131.3
136.2
145
119
127
136
134
136
130
130
136
140
a See Figure 6 for atom numbering scheme.
Table 3. Calculated Bond Lengths (Å) and Angles (°) in
Hg(arene)2(GaCl4)2
Figure 4. Representative plot of -ln[C-H1] (9) and -ln[C-H2] (0) versus
time (s) for the Hg(arene)2(GaCl4)2 catalyzed H/D exchange between C6D6
and C10H8 at 308 K.
Calcd
Expa
C H R(R) Me, Et)
C H
C H
10
8
6
6
6 5
Hg-C
2.410
2.690
2.470
2.429
2.411
2.830
2.450
2.448
2.33(1)-2.249(9)
2.71(1)-2.75(1)
Hg‚‚‚C
Hg-Cl
Ga-Clbr
Ga-Clter
2.634(4)-2.652(2)
2.239(2)-2.230(4)
2.220-2.260
2.225-2.272 2.128(4)-2.166(3)
Cl-Hg-Cl
C-Hg-C
103.63
101.89
Hg-Cl-Ga 116.63
102.86
108.54
119.33
84.5(6)-85.6(2)
126(1)-131.3(6)
110.6(1)-111.4(2)
a Borovik, A.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 2001, 123,
11219.
(including the dipolar dephasing experiment), we propose that
Hg(C10H8)2(GaCl4)2 crystallizes with C2 symmetry. Second,
there is a strong correlation between Hg‚‚‚C distance and the
13C chemical shift.2 Thus, on the basis of the observed 13C
chemical shifts for C(1) and C(2) (see Table 2) and using the
previously reported relationship,2 the Hg-C(2) and Hg‚‚‚C(1)
distances are predicted to be 2.38 and 2.86 Å, respectively. Table
3 gives the calculated (DFT) bond lengths and angles for
Hg(C10H8)2(GaCl4)2 and Hg(C6H6)2(GaCl4)2 along with X-ray
crystallographic data for Hg(C6H5R)2(GaCl4)2 (R ) Me, Et).
As may be seen from Table 3, the calculated Hg-C(2) and Hg‚
‚‚C(1) distances are slightly shorter (∼2%) than those predicted
from 13C NMR spectroscopy; however, we have previously
observed a similar underestimation of the Hg-C distance by
DFT calculations.2
Figure 5. 13C CPMAS NMR spectra of Hg(C10H8)2(GaCl4)2.
The UV-visible spectrum for Hg(C10H8)2(GaCl4)2 (λ ) 313
nm) is distinct from that of the toluene (λ ) 305 nm) or benzene
(λ ) 279 nm) complexes. The UV-visible spectrum for a
solution of Hg(C6H5Me)2(GaCl4)2 in benzene shows a change
with the addition of naphthalene consistent with the formation
of Hg(C10H8)2(GaCl4)2 (i.e., eq 8).
Figure 6. Calculated structure for Hg(C10H8)2(GaCl4)2 showing atom
numbering scheme.
Hg(C6H6)2(GaCl4)2 + 2 C10H8 f
Hg(C10H8)2(GaCl4)2 + 2 C6H6 (8)
2 gives a comparison of the calculated and experimental 13C
NMR shifts for Hg(C10H8)2(GaCl4)2.
The time at which Hg(C10H8)2(GaCl4)2 is dominant coincides
with the point at which the H/D exchange kinetics alter from
the initial to final rates. We propose, therefore, that the change
in the reaction rates observed by 1H NMR accompanies ligand
exchange. Since the naphthalene complex is the dominant
species during the later H/D exchange reaction, we propose that
the rate equation is consistent with the protonation of d6-benzene
by the coordinated naphthalene in Hg(C10H8)2(GaCl4)2 (eq 9).
We have previously noted that, even without X-ray crystal-
lographic data, the 13C CPMAS NMR spectrum of Hg(arene)2-
(MCl4)2 (M ) Al, Ga) is able to provide limited structural
information. First, the number of aromatic resonances observed
in the 13C CPMAS NMR spectrum correlates well with the
crystallographic symmetry. Thus, on the basis of the observation
of a single set of resonances in the 13C CPMAS NMR spectrum
9
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A R T I C L E S
Branch and Barron
Figure 8. Schematic representation of the relative reactivities of the
hydrogens in the 1- and 2-positions of naphthalene toward H/D exchange.
Figure 7. Representative plot of -ln[Hg(C6D6)2(GaCl4)2] versus time (s)
at 298 K (R ) 0.966) for the ligand exchange between Hg(C6D6)2(GaCl4)2
and C10H8.
This is supported by the study of the H/D exchange between
isolated Hg(C10H8)2(GaCl4)2 and C6D6.
Hg(C10H8)2(GaCl4)2 + C6D6 f
[Hg(C10H7)(C10H8)(GaCl4)2]- + [C6D6H]+ (9)
We have assumed that the initial phase of the reaction is
dominated by eq 6 and the final reaction is dominated by eq 9
and that the rate for H/D exchange at the n-position of the
naphthalene is defined as a combined rate equation (eq 10).
When the above is given and the 1H NMR data over the whole
reaction is used, the concentration of Hg(C6H6)2(GaCl4)2 at time
t may be determined. The rate of ligand exchange (eq 8) is
determined to be first order with respect to the concentration
of Hg(C6H6)2(GaCl4)2 (eq 11). First-order rate constants, kex,
were calculated from the corresponding plot of -ln[Hg(C6H6)2-
(GaCl4)2] versus time. Measurement of the temperature depen-
dence for kex (Figure 7) allows for the determination of ∆Hq
Figure 9. Calculated charge distribution on the aromatic C and H atoms
in Hg(C10H8)2(GaCl4)2.
crystallographic analysis of Hg(C6H6-nMen)2(MCl4)2 that there
is a similar preference for the coordination of the mercury to
the carbon para to the arene’s substituents.2 On the basis of
13C CPMAS NMR chemical shifts2 and DFT calculations,3 the
charge on the proton and carbon associated with the shortest
Hg‚‚‚C interaction is significantly higher than that on free arene.
The effect is also present, albeit reduced, on the C and H atoms
ortho to the short Hg‚‚‚C interaction. Thus, the C-H bond on
the carbon associated with mercury is the most highly activated
(Figure 8). This is confirmed by observation of the calculated
charge distribution on the aromatic C and H atoms for
Hg(C10H8)2(GaCl4)2 (Figure 9).
and ∆Sq to be 63(1) kJ mol-1 and 86(7) J K-1 mol-1
,
respectively. The positive values of ∆Sq are indicative of a
dissociative reaction (eq 12),16 while the value for ∆Hq [63(1)
kJ mol-1] is consistent with our calculated bond dissociation
energy (∆H ) 60 kJ mol-1) for Hg(C6H6)2(AlCl4)2.3
The relative rate for the H/D exchange between naphthalene
and Hg(C6D6)2(GaCl4)2 (eq 9) is controlled in the same way as
that for any related electrophilic aromatic substitution reaction
of naphthalene. In contrast, the relative rate for the H/D
exchange between benzene and Hg(C10H8)2(GaCl4)2 (eq 6) is
controlled by the relative activation of the naphthalene C-H
bonds. Since the mercury will preferentially bind to the carbon
at the 2-position, this C-H bond will be activated significantly
more that the C-H bond associated with the 1-position (see
Figures 8 and 9). The change in site selectivity upon ligand
exchange is unfortunate in that it precludes the easy site specific
H/D exchange for many arenes; however, the presence of a
ligand exchange in favor of the most substituted arene does
allow for the isolation of complexes previously unobtainable
by direct synthesis.
H/D exchange for n-position )
kni[Hg][C-Hn] + knf[Hg][C-Hn] (10)
rate of ligand exchange ) kex[Hg(C6H6)2(GaCl4)2] (11)
Hg(C6H6)2(GaCl4)2 f Hg(C6H6)(GaCl4)2 + C6H6 (12)
On the basis of the foregoing, the initial Hg(arene)2(GaCl4)2
catalyzed reaction of naphthalene with d6-benzene involves the
deuteration of naphthalene by coordinated C6D6 (cf., eq 6). As
ligand exchange progresses, the pathway for H/D exchange
changes to where the protonation of C6D6 by coordinated
naphthalene dominates (eq 9). We propose it is this change in
the reaction pathway that is responsible for the apparent loss in
site selectivity for the H/D exchange.
On the basis of the 13C CPMAS NMR spectrum for
Hg(C10H8)2(GaCl4)2 and the calculated structure (Figure 8), it
appears that there is a preference for coordination of the mercury
to the 2-position of the naphthalene. Such a preferential
coordination to the 2-position is predicted on steric and
electronic grounds. We have previously also observed by X-ray
The presence of ligand exchange to the more stable substituted
arene also helps explain why the use of Hg(C6H5Me)2(GaCl4)2
as a Friedel-Crafts catalyst (e.g., alkylation of toluene by
ethylene) does not allow for complete conversion of all the
toluene.18 Presumably, preferential coordination of the poly-
(18) Borovik, A.; Branch, C. S.; Barron, A. R., submitted for publication.
9
14160 J. AM. CHEM. SOC. VOL. 124, NO. 47, 2002

Arene−Mercury Complex H/D and Arene Exchange
A R T I C L E S
With regard to the rate of exchange of the arene ligands, we provided
an explanation in the text. The concentration of Hg(C6H6)2(GaCl4)2 at
any time (t) may be determined from the relative rate of H/D exchange,
assuming it is a combination of two independent reactions (eqs 6 and
9). If the concentration of Hg(C6H6)2(GaCl4)2 as a function of time is
known, then a standard first-order plot may be drawn and the kex
obtained (Figure 7). Measurement of the temperature dependence for
alkylated toluene occurs such that ligand exchange with toluene
to provide fresh substrate cannot occur.
Experimental Section
Solution NMR spectra were obtained on Bruker Avance 200 and
500 spectrometers. Chemical shifts are reported relative to internal
solvent resonances. NMR tubes were cleaned in basic solution, followed
by an acetone wash. The tubes were 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 was predried and stored in
the drybox over molecular sieves. 13C MAS spectra were obtained on
the Bruker Avance 200 spectrometer. A 7-mm zirconium dioxide rotor
was used for all spectra, with the spin rates up to 7 kHz. UV-visible
spectral data were recorded on a Varian Cary 4 spectrometer. GC/MS
analyses were carried out using 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 DB-5 30 m × 0.25 mm id 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 calculated distributions. The synthesis of Hg(C6H5Me)2-
(GaCl4)2 was as reported previously.1,2 Solvents and all arenes were
distilled and degassed prior to use.
Reaction of Hg(C6H5Me)2(MCl4)2 with C10H8 in C6D6. To a yellow
solution of Hg(C6H5Me)2(GaCl4)2 (0.128 g, 0.158 mmol) in C6D6 (35
g, 416 mmol) was added naphthalene (2.50 g, 19.5 mmol). The resulting
orange solution was allowed to stir for 12 h. After the reaction was
complete, water (5 mL) was added to deactivate the catalyst. The
aromatic and aqueous layers were allowed to separate, and the aromatic
layer was isolated by decanting. The solid was isolated by removing
the volatiles in the aromatic layer, and its identity was confirmed by
MS and 13C NMR.
k
ex allows for the determination of ∆Hq and ∆Sq in the normal manner.
Errors were calculated by standard methods.
Hg(C10H8)2(GaCl4)2. Hg(C6H5Me)2(GaCl4)2 (0.50 g, 0.062 mmol)
was dissolved in toluene (20 mL), and the solution was heated
slightly to ensure complete dissolution. A large excess of naphthalene
(7.93 g, 0.062 mol) was added to the warm yellow solution, resulting
in a red solution. The reaction flask was covered in aluminum foil to
prevent decomposition of the light-sensitive mercury complexes and
was allowed to cool to room temperature. The solution was stirred for
12 h to ensure that the ligand exchange between C6H5Me and
naphthalene went to completion. Upon cooling to -24 °C, orange
precipitate formed. Yield: 80%. MS (EI, %): m/z (M+, 100). 13C
CPMAS NMR (50.32 MHz): δ 145 (1C, 1-C), 140 (1C, 10-C), 136
(3C, 4-C, 6-C, 9-C), 134 (1C, 5-C), 130 (2C, 7-C, 8-C), 127 (1C, 3-C),
119 (1C, Hg‚‚‚CH).
Computational Methods. All density functional calculations were
carried out using a Gaussian-98 suite.19 Complete geometry optimiza-
tions were performed at the B3LYP20 level using the 6-31G** basis
set for C and H only and the Stuttgart RLC ECP basis set for Hg, Cl,
and Ga. C2 symmetry was imposed. Vibrational frequencies were then
evaluated for naphthalene complexes to verify the existence of the true
potential minimum and to determine zero-point energies. 13C NMR
chemical shifts for Hg(C10H8)2(GaCl4)2 were calculated at the same
level of theory.
Acknowledgment. Financial support for this work is provided
by the Robert A. Welch Foundation and the Petroleum Research
Fund. 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.
C10D8. MS (EI, %): m/z 136 (M+, 100), 134 (M+ - H, 24), 132
(M+ - 2H, 5). 13C NMR (C6D6): δ 134.2 (s, 9-C), 128.2 [t, J(C-D)
) 24 Hz, 1-CD], 125.8 [t, J(C-D) ) 24 Hz, 2-CD].
Catalytic H/D Exchange. In a typical experiment, naphthalene
(0.100 g, 0.78 mmol) was dissolved in C6D6 (∼1 mL) in a 5-mm NMR
tube. To this was added Hg(C6H5Me)2(GaCl4)2 (3.00 mg, 3.71 µmol),
turning the clear solution light yellow. The sample was protected from
light by a sleeve of aluminum foil because Hg(C6H5Me)2(GaCl4)2 is
light sensitive. At temperatures other than 294 K, the NMR was
equilibrated before the sample was inserted. Each NMR measurement
yielded a spectrum consisting of two sets of naphthalene peaks and a
peak due to C6D5H. The peaks were integrated over consistent ranges.
From these integrations, the concentrations of C-H at both the 1- and
2-positions, as well as the growing C6D5H concentration, were
determined.
The methods by which kobs and kn were determined are standard.16
Because the rate of ligand exchange is sufficiently slow, under the
conditions studied, the initial reaction is essentially that as described
in eq 6. Thus, kobs may be determined in the ordinary manner (see Figure
1). A plot of kobs versus [catalyst] allows for the determination of the
second-order rate constants, ki1 and ki2 (see Figure 2). Once ligand
exchange has completed (although presumably an equilibrium, eq 8 is
sufficiently shifted to the right to be an irreversible reaction), the rate
constants are obtained by a similar process.
Supporting Information Available: Structural parameters and
energies for optimized structures from DFT calculations. Order-
ing information is given on any current masthead page. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA0206590
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R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
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