Synthesis of oxorhenium acetyl and benzoyl complexes incorporating diamidopyridine ligands: Implications for the mechanism of CO insertion

Article abstract of DOI:10.1021/om3002872

A series of oxorhenium alkyl, phenyl, and vinyl complexes of the form [(DAP)Re(O)(R)] (R = aryl, vinyl, alkyl) was reported, and their reactivity with CO was examined. The methyl complex 5a reacts with CO at a significantly faster rate (2.5 h) than the phenyl complex 7a (24 h). Computational (B3PW91) studies reveal that although the acyl complex is the least stable (ΔG ₃₅₃ = -11.2 kcal/mol) with respect to CO insertion compared to the benzoyl complex (ΔG₃₅₃ = -14.5 kcal/mol), the activation energy for CO insertion is lower for the methyl complex (ΔG ^?₃₅₃ = 14.6 kcal/mol) than for the phenyl complex (ΔG^?₃₅₃ = 17.4 kcal/mol). This is consistent with the previously proposed mechanism, where CO inserts directly into the Re-R bond without prior formation of a CO adduct. The X-ray crystal structures of complexes 6, 7a, 8a, and 9a are reported.

Full text of DOI:10.1021/om3002872

Article
pubs.acs.org/Organometallics
Synthesis of Oxorhenium Acetyl and Benzoyl Complexes
Incorporating Diamidopyridine Ligands: Implications for the
Mechanism of CO Insertion
Cassandra P. Lilly, Paul D. Boyle, and Elon A. Ison*
Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United
States
S
* Supporting Information
ABSTRACT: A series of oxorhenium alkyl, phenyl, and vinyl complexes of the
form [(DAP)Re(O)(R)] (R = aryl, vinyl, alkyl) was reported, and their reactivity
with CO was examined. The methyl complex 5a reacts with CO at a significantly
faster rate (2.5 h) than the phenyl complex 7a (24 h). Computational (B3PW91)
studies reveal that although the acyl complex is the least stable (ΔG353 = −11.2
kcal/mol) with respect to CO insertion compared to the benzoyl complex (ΔG353
= −14.5 kcal/mol), the activation energy for CO insertion is lower for the methyl
complex (ΔG⧧ = 14.6 kcal/mol) than for the phenyl complex (ΔG⧧ = 17.4
353
353
kcal/mol). This is consistent with the previously proposed mechanism, where CO
inserts directly into the Re−R bond without prior formation of a CO adduct. The
X-ray crystal structures of complexes 6, 7a, 8a, and 9a are reported.
study of the synthesis and reactivity of these complexes is
warranted.
INTRODUCTION
■
Acyl complexes have been proposed as intermediates in many
important catalytic reactions, including acetic acid synthesis,1
hydroformylation,2 and hydroacylation reactions.3 However, for
these systems, the acyl ligands are often incorporated into
complexes where the metal is in a low oxidation state.4 The
reaction between [(DAAm)Re(O)(CH3)] (1; DAAm = N,N-
bis(2-arylaminoethyl)methylamine; aryl = C6F5, Mes) and CO,
to form the rhenium(III) acetate complex [(DAAm)Re-
(O2CCH3)(CO)] (2) was previously reported (Scheme 1).5
In addition to the previous work with the DAAm ligands, we
have also reported the synthesis of related complexes
incorporating diamidopyridine (DAP = 2,6-bis((mesitylamino)-
methyl)pyridine) pincer ligands.8 These complexes exhibit
reactivity similar to that of the DAAm complexes, but their rigid
structure leads to greater stability and increased reactivity in
catalytic reactions. The pyridine backbone of the DAP pincer
ligands also allows for the ability to tune the electronics at the
metal center by incorporating substituents in the para position
of the pyridine ring, as outlined in Scheme 2.
Scheme 1
Scheme 2
It was shown the reaction proceeds by direct insertion of CO
into the Re−Me bond in 1, without prior formation of a CO
adduct, to produce an oxorhenium acyl intermediate,
[(DAAm)Re(Ac)] (3). This is followed by 1,2-migration of
the acyl ligand to the terminal oxo, in the presence of CO, to
generate 2.
In this paper, the syntheses of several new oxorhenium alkyl,
phenyl, and vinyl complexes of the form [(DAP)Re(O)(R)] (R
= aryl, vinyl, alkyl) are reported, and their reactivity with CO is
examined. Insights into the mechanism of CO insertion into
Re−R bonds were attained from these studies. In addition,
computational (DFT) studies were utilized in order to
Complex 3 is a rare example of a high-oxidation-state (ReV)
acyl complex, incorporating an oxo ligand,6 and unlike acyl
complexes incorporating low-valent metals, very little is known
about the reactivity of high-valent metal acyls. The incorpo-
ration of strongly π donating oxo ligands and π accepting acyl
ligands may lead to enhanced reactivity.7 Thus, a systematic
Received: April 9, 2012
Published: May 25, 2012
© 2012 American Chemical Society
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understand the differences in reactivity with CO for alkyl, vinyl,
and aryl complexes incorporating DAP ligands.
RESULTS AND DISCUSSION
■
Synthesis of [(DAP)Re(O)(R)] Complexes. The chloride
complexes 4 were synthesized by treatment of the DAP ligand
with Re(O)Cl3(PPh3)2, as previously reported.8 In the case of
complex 4b, the ligand 2,6-bis((mesitylamino)methyl)-4-
methoxypyridine, which features a methoxy group in the para
position of the pyridine backbone, was utilized. This ligand was
synthesized by the reaction of 2,6-dibromomethyl-4-methox-
ypyridine9 with 2 equiv of LiNHMes.
Oxorhenium alkyl, aryl, and vinyl complexes (5−7) were
synthesized by transmetalation from the corresponding
Grignard reagents according to Scheme 3. The original
Figure 1. X-ray crystal structure of 6. Ellipsoids are shown at the 50%
probability level. Hydrogen atoms are omitted, and the mesityl
substituents on the diamido ligand are depicted in wireframe for
clarity. Selected bond lengths (Å) and bond angles (deg): Re2−O1B,
1.7010(19); Re2−N1B, 1.968(2); Re2−N3B, 1.973(2); Re2−N2B,
2.057(3); Re2−C1B, 2.094(3); N1B−Re2−N2B, 76.10(8); N3B−
Re2−N2B, 76.11(8); O1B−Re2−C1B, 108.02(10); N1B−Re2−C1B,
88.30(9); N3B−Re2−C1B, 87.52(9); N2B−Re2−C1B, 136.35(11);
O1B−Re2−N3B, 111.73(9); O1B−Re2−N1B, 112.37(9); N3B−
Re2−N1B, 134.83(9); O1B−Re2−N2B, 116.28(9).
Scheme 3
synthesis of the oxorhenium methyl complex 5a involved
treatment of the DAP ligand with PPh3 and methyltrioxo-
rhenium (MTO).8 However, this synthesis results in the
generation of 1 equiv of OPPh3 as a byproduct, which is
difficult to separate from 5a. Thus, the transmetalation strategy
represents an improved synthesis, as it avoids the generation of
this byproduct. Complex 5b, which features a methoxy group in
the para position of the pyridine backbone of the DAP ligand,
was also synthesized as a pink powder in 72% yield by this
method.
Figure 2. X-ray crystal structure of 7a. Ellipsoids are shown at the 50%
probability level. Hydrogen atoms are omitted, and the mesityl
substituents on the diamido ligand are depicted in wireframe for
clarity. Selected bond lengths (Å) and bond angles (deg): Re1−O1A,
1.6956(16); Re1−N1A, 1.9677(17); Re1−N3A, 1.9726(18); Re1−
N2A, 2.0678(18); Re1−C1A, 2.077(2); O1A−Re1−N1A, 113.24(7);
O1A−Re1-N3A, 113.13(7); N1A−Re1−N3A, 132.44(7); O1A−Re1−
N2A, 114.83(7); N1A−Re1−N2A, 75.81(7); N3A−Re1−N2A,
75.90(7); O1A−Re1−C1A, 103.13(8); N1A−Re1−C1A, 90.25(7);
N3A−Re1−C1A, 89.23(7); N2A−Re1−C1A, 142.04(8).
The oxorhenium vinyl (6) and aryl complexes (7) were also
1
synthesized by this method. The H NMR spectrum of 6
displays a resonance at 6.67 ppm for Re−C−Hα, which is
similar to the case for the rhenium vinyl complex, CpRe(NO)-
(PPh3)(CHC(CH3)2), isolated by Gladysz and co-workers,
in which Hα resonates at 7.14 ppm.10 Complex 6 is not very
stable in solution. When 6 is monitored by 1H NMR
spectroscopy in CD2Cl2, complex decomposition is observed
after 1 h. Decomposition was also observed when 6 was heated
in C6D6 at 80 °C for 1 h. However, 6 is stable for up to 10 days
in benzene at room temperature.
X-ray Crystal Structures. X-ray-quality crystals of 6 and 7a
were obtained by slow diffusion of pentane into a concentrated
solution of the corresponding complex in dichloromethane at
room temperature (Figures 1 and 2). The vinyl ligand in 6 is
bound in an η1 fashion (Figure 1). The Re−C bond lengths in
6 (2.094(3) Å), and 7a (2.077(2) Å) are shorter than the sp3
carbon−Re bond in 5a (2.1278(17) Å).8 For both structures
the coordination sphere around the rhenium atom can best be
described as distorted square pyramidal, with the oxo ligand
occupying an apical position. The Re−oxo bond length for both
structures is typical for triply bonded rhenium oxos.
Synthesis of Acetyl and Benzoyl Complexes. DFT
calculations for the migratory insertion of CO into 1 suggest
that the rate-determining step for the insertion of CO into the
Re−Me bond is the addition of CO to the Re complex.5b Thus,
the rate of insertion may be influenced in these systems by
changing the electron density at the metal center. This may be
achieved by altering the ancillary diamido ligand. Since CO
addition is rate-determining, the electronic character of the
migrating R group should have a minimal effect on the rate of
insertion of CO. In order to investigate the factors that affect
CO insertion in the corresponding DAP complexes, the
reaction of complexes 5 and 7 with CO was examined.
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The reaction of 5 and 7 with CO (60 psi, C6D6, 80 °C) was
1
monitored over time by H NMR spectroscopy (see Figure S1
in the Supporting Information). These experiments allowed for
a comparison of the rates of insertion into the Re−CH3 bond
versus the Re−Ar bond. Unfortunately, similar comparisons
with the vinyl complex 6 were not possible, because complex 6
decomposes upon heating in C6D6. As shown in Scheme 4, the
Scheme 4
Figure 3. X-ray crystal structure of 8a. Ellipsoids are shown at the 50%
probability level. Hydrogen atoms are omitted, and the mesityl
substituents on the diamido ligand are depicted in wireframe for
clarity. Selected bond lengths (Å) and bond angles (deg): Re1−O1,
1.700(2); Re1−N1, 1.971(3); Re1−N2, 2.059(3); Re−N3, 1.967(3);
Re1−C27, 2.039(4); O1−Re1−N3, 110.78(12); O1−Re1−N1,
109.97(11); N3−Re1−N1, 137.92(11); O1−Re1−N2, 106.45(13);
N3−Re1−N2, 76.25(12); N1−Re1−N2, 76.48(11); O1−Re1−C27,
106.45(13); N3−Re1−C27, 88.78(12); N1−Re1−C27, 89.54(13);
N2−Re1−C27, 136.62(12).
reaction of 5 with CO results in the formation of 8 in 2.5 h.
Interestingly, altering the electronics in the para position of the
pyridine backbone as in 5b apparently has no effect on the rate
of insertion, as the formation of 8b also occurs in 2.5 h. In
contrast, the formation of 9 from aryl complexes 7 requires 24
h. These data suggest that CO inserts more readily into alkyl
bonds than aryl bonds.
Scheme 5
The migratory aptitude of the aryl ligand was also examined
by changing the electronics of the aryl ligand by incorporating
substituents in the para position of the phenyl ring. As shown in
Scheme 4, changing the electronic nature of the aryl ligand has
no effect on the rate of insertion of CO.
Thus, it appears that insertion of CO is governed by the
nature of the Re−R bond. In order to investigate the difference
in reactivity further, DFT calculations were performed (vide
infra).
X-ray Structures of Acyl and Benzoyl Complexes. X-
ray-quality crystals of 8a were obtained by slow diffusion of
pentane into a concentrated solution of 8a in dichloromethane
(Figure 3). The geometry about the metal center can best be
described as distorted square pyramidal with the oxo in the
apical position. The acyl oxo is positioned anti to the Re−O
bond, similar to that of its DAAm analogue;5a the Re−C bond
length decreases from 2.1278(17) Å in 3 to 2.039(4) Å. This
can be attributed to the change from an sp3 to an sp2 carbon
center along with π back-bonding from the metal center into
the acyl π* orbital. The IR stretching frequency of the C(O)−
Me bond in 8a is 1599 cm−1. Bergman and co-workers report
an IR stretching frequency of 1630 cm−1 for the acyl complex
[CpRe(CO)2(COCH3)(CH3)],11 while Gladysz and co-work-
ers report an IR stretching frequency of 1545 cm−1 for the
complex [CpRe(NO)(PPh3)(COCH3)].4b These differences
can be attributed to contributions from the two resonance
forms depicted in Scheme 5. The alkylidene resonance form B
is the major contributor to the bonding in complex 8a and
[CpRe(NO)(PPh3)(COCH3)], while the acyl resonance form
A is the major resonance contributor in [CpRe-
(CO)2(COCH3)(CH3)].10
Figure 4. X-ray crystal structure of 9a. Ellipsoids are shown at the 50%
probability level. Hydrogen atoms are omitted, and the mesityl
substituents on the diamido ligand are depicted in wireframe for
clarity. Selected bond lengths (Å) and bond angles (deg): Re1−O1A,
1.692(4); Re1−N1A, 1.965(5); Re1−N3A, 1.983(4); Re1−N2A,
2.076(4); Re1−C1A, 2.059(6); C1A−O2A, 1.231(7); O1A−Re1−
N1A, 111.43(18); O1A−Re1−N3A, 110.17(18); N1A−Re1−N3A,
137.18(18); O1A−Re1−N2A, 118.62(18); N1A−Re1−N2A,
75.63(18); N3A−Re1−N2A, 76.10(17); O1A−Re1−C1A, 104.1(2);
N1A−Re1−C1A, 92.1(2); N3A−Re1−C1A, 87.3(2); N2A−Re1−
C1A, 137.2(2).
about the metal center is best described as distorted square
pyramidal, with the oxo ligand occupying the apical position.
The benzoyl oxo is anti with respect to the Re−O bond. The
Re−Cbenzoyl bond length is 2.059(6) Å, which is comparable to
the Re−C bond length in 7a (2.077(2) Å). The C1A−O2A
bond length is 1.231(7) Å and is analogous to the C27−O2
bond length (1.224(4) Å) in 8a.
X-ray-quality crystals of 9a were also obtained by slow
diffusion of pentane into a concentrated solution of 9a in
dichloromethane (Figure 4). To our knowledge, this is the first
example of an oxobenzoyl complex. As in 8a, the geometry
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The IR stretching frequency of the C(O)−Ph bond in 9a is
1557 cm−1. This IR stretch is within the range reported for
other Re benzoyl complexes. For example, Sironi and co-
workers reported a ν(C(O)−Ph) stretch of 1608 cm−1 for the
dicarbonyl complex [Cp(CO)2Re(Me)(COPh)].4a Similarly,
Djukic and co-workers reported a ν(C(O)−Ph) stretch of 1589
cm−1 for the rhenium benzoyl complex (benzoyl)tricarbonyl[3-
methyl-2-{(η6-phenyl)tricarbonylchromium(0)-κC2′}pyridine-
κN]rhenate(I),12 while Gladysz and co-workers reported a
ν(C(O)−Ph) stretch of 1514 cm−1 for the benzoyl complex
[CpRe(NO)(PPh3)(COC6H5)].4b The phenyl ring in the
benzoyl group rotates −45.4(7)° out of the plane in order to
decrease the steric interactions between Re−O and the benzoyl
group.
Scheme 6. Geometries for Transition States for the Insertion
of CO into the Re−R Bonds in 5 and 7
DFT Calculations. In order to acquire a deeper under-
standing of the energetics of the CO insertion reaction, DFT
(B3PW91)13 calculations were performed. All calculations were
performed with the 6-31G(d,p) basis set14 on the C, H, N, and
O atoms and the SDD15 pseudopotential and basis set
augmented with an f polarization function16 on the Re atom.
As described previously, this functional and basis set accurately
reproduces the geometries of related complexes.5b Solvation
energies were calculated by applying the SMD17 solvation
model as implemented in Gaussian 09,18 to structures
optimized in the gas phase.
The results of the computational analysis are summarized in
Table 1. Formations of the benzoyl complexes are the most
are consistent with the mechanism proposed earlier where CO
inserts directly into the Re−R bond without prior formation of
a CO adduct. The difference in the rate of the reactions reflects
the differences in Re−R bond strengths. Recall from the X-ray
crystal structures that the Re−Me bond in 5a is 2.1278(17) Å
while the Re−phenyl bond in 7a is 2.077(2) Å. Thus, the aryl
complexes react more slowly because they form stronger bonds
to Re than the corresponding alkyl counterparts, and as a result,
insertion of CO into these bonds is more difficult.
Table 1. Thermodynamic and Kinetic Data for the Insertion
Reaction of CO with [(DAP)Re(O)(R)] Complexes
Conclusions. A series of oxorhenium alkyl, phenyl, and
vinyl complexes of the form [(DAP)Re(O)(R)] (R = aryl,
vinyl, alkyl) was reported, and their reactivity with CO was
examined. The methyl complexes 5a,b react with CO at a
significantly faster rate (2.5 h) than the phenyl complexes 7 (24
h). In addition, changing the electronics of the migrating aryl
ligand had no effect on the rate of insertion. Thus, it appears
that the insertion of CO into the Re−R bonds in these
complexes depends on the nature of the R group: i.e., aryl
versus alkyl. Computational (DFT) studies reveal that although
the acyl complex is the least stable (ΔG353 = −11.2 kcal/mol)
with respect to CO insertion in comparison to the benzoyl
complexes (ΔG353 = −16.3 and −14.5 kcal/mol), the activation
energy for CO insertion is lower for the methyl complex
a
R
ΔG353
ΔG⧧
353
Me
−11.2 (−18.5)
−16.3 (−23.5)
−14.5 (−24.0)
14.6 (8.33)
16.3 (21.9)
17.4 (9.22)
p-methoxyphenyl
phenyl
a
Solvation energies were computed geometries optimized in the gas
phase (given in parentheses) using the SMD17 method, with benzene
as the solvent, as implemented in Gaussian 09.
exergonic (ΔG353 = −16.3 kcal/mol, R = p-methoxyphenyl;
ΔG353 = −14.5 kcal/mol, R = phenyl), while formation of the
acetyl complex is the least favorable (ΔG353 = −11.2 kcal/mol).
In spite of this, formation of the acetyl complex occurs most
readily (2.5 h), in comparison to 24 h for the benzoyl
complexes. This suggests that the origin for the difference in
reactivities is kinetic. This is also evident when the energies of
activation are compared, as the formation of the acetyl complex
(ΔG⧧
= 14.6 kcal/mol) than for the phenyl complexes
(ΔG⧧353 = 16.3 and 17.4 kcal/mol). Interestingly, unlike the
353
case for the previously reported DAAm complexes, 1,2-
migration of the acyl ligand to the oxo ligand is not observed
in this system. Studies are currently being undertaken in our
laboratories in order to understand the differences in reactivity
between the DAAm and DAP complexes.
proceeds with the lowest activation energy (ΔG⧧ = 14.6
353
kcal/mol), while the formation of the benzoyl complexes
proceed with highesr activation energies (ΔG⧧ = 16.3 and
353
17.4 kcal/mol). As shown in Scheme 6, in the transition state
for the insertion reaction there is a greater degree of Re−R
bond cleavage in the alkyl complex (Re−CH3 = 2.40 Å) than in
the phenyl complexes (Re−Ph = 2.31 Å; Re−Ph-p-OMe = 2.29
Å). In addition, animation of the imaginary frequency
associated with each transition state led to complete
dissociation of CO and the formation of 8 or 9. These data
EXPERIMENTAL SECTION
■
General Considerations. 2,6-Dibromomethyl-4-methoxypyri-
dine9 and Re(O)Cl(DAP)8 were prepared according to previously
published procedures. All other reagents were purchased from
commercial sources and used as received unless otherwise noted.
THF was distilled from Na/K alloy benzophenone ketyl. H and 13C
1
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NMR spectra were recorded on a Varian Mercury 400 MHz or a
Varian Mercury 300 MHz spectrometer at room temperature. H and
(DAP)Re(O)(CHC(CH3)2) (6). 6 was obtained as a brown solid in
37.8% yield. 1H NMR (CD2Cl2, δ): 8.05 (t, J = 7.8 Hz, 1H,
NC2H2CH), 7.66 (d, J = 7.8 Hz, 2H, NC2H2CH), 6.80 (s, 4H, Mes m-
H), 6.67 (s, 1H, CH(CH3)2), 5.64 (d, J = 20.7 Hz, 2H, MesNCH2),
5.37 (d, J = 20.7 Hz, 2H, MesNCH2), 2.33 (s, 6H, Mes CH3), 2.24 (s,
6H, Mes CH3), 1.58 (s, 6H, Mes CH3), 1.44 (s, 3H, CH(CH3)2), 1.35
(s, 3H, CH(CH3)2). 13C NMR (CD2Cl2, δ): 168.39, 155.19, 154.81,
141.44, 136.73, 134.40, 133.85, 128.95, 128.57, 127.70, 116.91, 78.45,
27.84, 24.95, 21.03, 18.77, 18.11. Anal. Calcd for C29.5H37ClN3ORe: C,
52.78; N: 6.26; H, 5.56. Found: C, 52.26; N, 6.11; H, 5.40.
1
13C NMR chemical shifts are listed in parts per million (ppm) and are
referenced to residual protons and carbons of the deuterated solvents,
respectively. High-pressure reactions were performed in a stainless
steel Parr 4590 Micro Bench Top Reactor. FTIR spectra were
obtained in KBr thin films on a JASCO FT/IR-4100 instrument.
Elemental analyses were performed by Atlantic Microlabs, Inc. X-ray
crystallography was performed at the X-ray Structural Facility of North
Carolina State University by Dr. Paul Boyle.
(DAP)Re(O)(C6H5) (7a). 7a was obtained as a brown solid in 67.8%
yield. 1H NMR (CD2Cl2, δ): 8.13 (t, J = 8.0 Hz, 1H, NC2H2CH), 7.74
(d, J = 8.0 Hz, 2H, NC2H2CH), 6.61 (s, 2H, Mes m-H), 6.57 (s, 2H,
Mes m-H), 6.38 (dd, J = 6.8 Hz, 2H, Ph m-H), 6.18 (br, 2H, Ph o-H),
6.03 (t, J = 7.2 Hz, 1H, Ph p-H), 5.72 (d, J = 20.4 Hz, 2H, MesNCH2),
5.54 (d, J = 20.4 Hz, 2H, MesNCH2), 2.38 (s, 6H, Mes CH3), 2.10 (s,
6H, Mes CH3), 1.72 (s, 6H, Mes CH3). 13C NMR (CD2Cl2, δ):
175.49, 167.32, 153.68, 141.66, 135.88, 134.07, 133.48, 131.27, 128.48,
128.10, 125.46, 121.26, 116.88, 78.76, 20.55, 18.87, 18.08. Anal. Calcd:
C, 57.21; N, 6.46; H, 5.27. Found: C, 57.02; N, 6.24; H, 5.32.
(DAP)Re(O)(C6H4-p-OCH3) (7b). 7b was obtained as a red solid in
59.1% yield. 1H NMR (CD2Cl2, δ): 8.12 (t, J = 8.0 Hz, 1H,
NC2H2CH), 7.73 (d, J = 7.6 Hz, 2H, NC2H2CH), 6.62 (s, 4H, Mes m-
H), 6.05 (s, 4H, Ph), 5.71 (d, J = 20.4 Hz, 2H, MesNCH2), 5.54 (d, J =
20.0 Hz, 2H, MesNCH2), 3.45 (s, 3H, Ph OCH3), 2.37 (s, 6H, Mes
CH3), 2.12 (s, 6H, Mes CH3), 1.68 (s, 6H, Mes CH3). 13C NMR
(CD2Cl2, δ): 167.44, 162.99, 156.02, 153.91, 141.63, 135.89, 134.08,
133.54, 132.17, 128.55, 128.24, 116.80, 111.76, 78.98, 55.20, 20.59,
18.86, 18.08. Anal. Calcd for C32.75H37.5Cl1.5N3O2Re: C, 52.83; N,
5.64; H, 5.08. Found: C, 52.67; N, 5.27; H, 5.69.
2,6-Bis((mesitylamino)methyl)-4-methoxypyridine. n-Butyllithium
(6.8 mmol, 4.25 mL) was added dropwise to a stirred solution of 2,4,6-
trimethylaniline (6.8 mmol, 0.95 mL) in dry THF at −78 °C using
standard Schlenk line techniques. The mixture was warmed to ambient
temperature and stirred for 1 h. The reaction mixture was cooled to
−78 °C, and a solution of 2,6-dibromomethyl-4-methoxypyridine (3.4
mmol, 1.0 g) in THF was added slowly. The solution was warmed to
room temperature and stirred for 18 h. The mixture was quenched
with a saturated NaHCO3 solution (50 mL) and extracted with diethyl
ether (50 mL). The organic extracts were dried over sodium sulfate
and filtered. The solvent was removed under reduced pressure,
resulting in a pale yellow solid (1.17 g, 42.6% yield). 1H NMR
(CDCl3, δ): 6.86 (s, 4H, Mes m-H), 6.75 (s, 2H, Pyr m-H), 4.22 (s,
4H, MesNHCH2), 3.82 (s, 2H, OCH3), 2.34 (s, 12H, Mes o-CH3),
2.26 (s, 6H, Mes p-CH3). 13C NMR (CDCl3, δ): 166.95, 160.70,
143.88, 131.58, 129.99, 129.72, 106.59, 55.43, 54.29, 20.85, 18.79.
HRMS (ESI): calcd for C23H15F12N3 404.2696 [M + H+], found
404.2697. Anal. Calcd: C, 51.99; N, 5.77; H, 4.71. Found: C, 51.57; N,
5.57; H, 5.03.
(4-OMeDAP)Re(O)Cl (4b). ReOCl3(PPh3)2 (500 mg, 0.60 mmol),
(MesNHCH2)2NC5H2OMe (242 mg, 0.60 mmol), and 2,6-lutidine (1
mL, 8.83 mmol) were added to a 100 mL round-bottom flask with 60
mL of EtOH. The mixture was stirred at room temperature for 4 days
to yield a green precipitate. The precipitate was filtered and washed
with diethyl ether to give 2 (293 mg, 76.4% yield). 1H NMR (CD2Cl2,
δ): 7.27 (s, 2H, pyridine m-H), 6.92 (s, 2H, Mes m-H), 6.84 (s, 2H,
Mes m-H), 5.52 (d, J = 18.0 Hz, 2H, MesNCH2), 5.31 (d, J = 18.0 Hz,
2H, MesNCH2), 4.06 (s, 3H, Pyr 4-OCH3), 2.43 (s, 6H, Mes CH3),
2.28 (s, 6H, Mes CH3), 1.60 (s, 6H, Mes CH3). 13C NMR (CD2Cl2,
δ): 171.79, 169.63, 154.57, 136.19, 134.98, 134.55, 128.79, 128.64,
103.15, 79.05, 57.21, 20.79, 18.19, 18.08. Anal. Calcd for
C26.25H31.5Cl1.5N3O2Re: C, 47.74; N: 6.63; H, 4.81. Found: C, 47.88;
N, 6.45; H, 4.87.
General Procedure for Rhenium Alkyls and Phenyls. The
respective Grignard reagent (0.982 mmol) was added dropwise to a
solution of the respective rhenium chloride complex (0.492 mmol) in
CH2Cl2 (15 mL) under an inert atmosphere. Water was added to the
solution (25 mL), and the organic layer was extracted and dried over
NaSO4. The mixture was filtered, and solvent was removed under
reduced pressure. The resulting residue was dissolved in a minimal
amount of CH2Cl2, and the respective product was obtained by
precipitation from excess hexanes.
(DAP)Re(O)(CH3) (5a). 5a was obtained as a red solid in 56.9% yield.
1H NMR (CD2Cl2, δ): 8.04 (t, J = 7.9 Hz, 1H, NC2H2CH), 7.67 (d, J
= 7.6 Hz, 2H, NC2H2CH), 6.92 (s, 2H, Mes m-H), 6.86 (s, 2H, Mes
m-H), 5.63 (d, J = 22.1 Hz, 2H, MesNCH2), 5.42 (d, J = 20.3 Hz, 2H,
MesNCH2), 2.39 (s, 6H, Mes CH3), 2.28 (s, 6H, Mes CH3), 1.76 (s,
3H, Re−CH3), 1.53 (s, 6H, Mes CH3). 13C NMR (CD2Cl2, δ): 168.59,
153.59, 141.42, 136.97, 135.22, 132.41, 129.16, 129.00, 116.71, 79.47,
21.06, 18.60, 17.93, 14.60. Anal. Calcd: C, 53.04; N: 7.14; H, 5.48.
Found: C, 52.86; N, 6.96; H, 5.39.
(DAP)Re(O)(C6H4-p-Cl) (7c). 7c was obtained as a reddish brown
1
solid in 41.2% yield. H NMR (CD2Cl2, δ): 8.15 (t, J = 7.6 Hz, 1H,
NC2H2CH), 7.75 (d, J = 8.0 Hz, 2H, NC2H2CH), 6.63 (s, 2H, Mes m-
H), 6.61 (s, 2H, Mes m-H), 6.35 (d, J = 8.0 Hz, 2H, Ph m-H), 6.10
(br, 2H, Ph o-H), 5.73 (d, J = 20.4 Hz, 2H, MesNCH2), 5.55 (d, J =
20.0 Hz, 2H, MesNCH2), 2.37 (s, 6H, Mes CH3), 2.13 (s, 6H, Mes
CH3), 1.70 (s, 6H, Mes CH3). 13C NMR (CD2Cl2, δ): 173.52, 167.49,
153.66, 142.20, 136.03, 134.20, 132.62, 128.95, 128.47, 127.68, 125.36,
117.36, 78.85, 20.90, 19.15, 18.35. Anal. Calcd for C31.5H34Cl2N3ORe:
C, 51.99; N, 5.77; H, 4.71. Found: C, 51.57; N, 5.57; H, 5.03.
General Procedure for CO Insertions. The respective rhenium
complex (0.308 mmol) was placed in a 50 mL glass-lined Parr reactor
and dissolved in benzene (10 mL). The reactor was purged and
pressurized with CO (200 psi). The reaction mixture was heated to 80
°C and stirred for either 2.5 h (methyl complexes) or 24 h (phenyl
complexes). The mixture was cooled to room temperature, and the
solvent was removed under reduced pressure. The resulting residue
was dissolved in a minimal amount of CH2Cl2, and the respective
product was precipitated out by the addition of excess hexanes.
(DAP)Re(O)(COCH3) (8a). 8a was obtained as an orange solid in
57.8% yield. 1H NMR (CD2Cl2, δ): 8.08 (t, J = 7.6 Hz, 1H,
NC2H2CH), 7.67 (d, J = 7.6 Hz, 2H, NC2H2CH), 6.85 (s, 2H, Mes m-
H), 6.80 (s, 2H, Mes m-H), 5.65 (d, J = 21.0 Hz, 2H, MesNCH2), 5.44
(d, J = 21.0 Hz, 2H, MesNCH2), 2.48 (s, 6H, Mes CH3), 2.26 (s, 6H,
Mes CH3), 1.88 (s, 6H, Mes CH3), 1.78 (s, 3H, Re−COCH3). 13C
NMR (CD2Cl2, δ): 259.1, 167.61, 154.08, 141.52, 136.38, 135.96,
134.71, 129.00, 128.61, 117.68, 75.69, 47.99, 21.05, 18.49, 18.42. Anal.
Calcd for C28H34Cl2N3O2Re: C, 47.93; N, 5.99; H, 4.88. Found: C,
48.21; N, 5.92; H, 4.78. IR (FTIR, cm−1): ν(C−O) 1599 cm−1.
(4-OMeDAP)Re(O)(COCH3) (8b). In a J. Young tube 5b (4.1 mg,
0.0066 mmol) was dissolved in CD2Cl2, and CO (60 psi) was added
via three freeze−pump−thaw cycles. Product formation was observed
by 1H NMR spectroscopy. 1H NMR (CD2Cl2, δ): 7.09 (s, 2H,
pyridine m-H), 6.82 (s, 2H, Mes m-H), 6.78 (s, 2H, Mes m-H), 5.54
(d, J = 19.9 Hz, 2H, MesNCH2), 5.29 (d, J = 21.0 Hz, 2H, MesNCH2),
4.00 (s, 3H, Pyr 4-OCH3), 2.44 (s, 6H, Mes CH3), 2.25 (s, 6H, Mes
CH3), 1.86 (s, 6H, Mes CH3), 1.73 (s, 3H, Re−COCH3).
(4-OMeDAP)Re(O)CH3 (5b). 5b was obtained in 71.7% yield as a
pink solid. 1H NMR (CD2Cl2, δ): 7.14 (s, 2H, Pyr m-H), 6.93 (s, 2H,
Mes m-H), 6.87 (s, 2H, Mes m-H), 5.56 (d, J = 21.0 Hz, 2H,
MesNCH2), 5.31 (d, J = 21.0 Hz, 2H, MesNCH2), 4.02 (s, 3H, Pyr 4-
OCH3), 2.39 (s, 6H, Mes CH3), 2.29 (s, 6H, Mes CH3), 1.68 (s, 3H,
Re−CH3), 1.55 (s, 6H, Mes CH3). 13C NMR (CD2Cl2, δ): 169.91,
169.79, 153.42, 136.75, 134.93, 133.81, 128.81, 128.63, 102.45, 78.69,
56.78, 20.73, 18.25, 17.61, 13.44. Anal. Calcd: C, 52.41; N: 6.79; H,
5.54. Found: C, 52.00; N, 6.68; H, 5.54.
(DAP)Re(O)(COC6H5) (9a). 9a was obtained in 55.6% yield as an
orange solid. 1H NMR (CD2Cl2, δ): 8.14 (t, J = 7.8 Hz, 1H,
NC2H2CH), 7.70 (d, J = 7.8 Hz, 2H, NC2H2CH), 7.16 (t, J = 8.1 Hz,
4299
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Table 2. Selected Crystallographic Data and Collection Parameters for (DAP)Re(O)(CHC(CH3)2) (6), (DAP)Re(O)(C6H5)
(7a), (DAP)Re(O)(COCH3) (8a), and (DAP)Re(O)(COC6H5) (9a)
6
7a
8a
9a
empirical formula
formula wt
cryst dimens/mm
cryst syst
space group
a/Å
C28.85H35.74Br0.04N3ORe
629.75
C31H34N3ORe
650.81
C28H34C12N3O2Re
701.68
C32H34N3O2Re
678.82
0.41 × 0.10 × 0.03
monoclinic
P21/c
0.19 × 0.17 × 0.13
monoclinic
P21/c
0.16 × 0.14 × 0.05
monoclinic
P21/c
0.08 × 0.08 × 0.01
triclinic
P1
̅
16.998(5)
18.682(5)
20.185(7)
90.00
17.067(6)
16.635(7)
18.941(7)
90.00
7.9735(6)
13.8238(10)
25.7538(18)
90.00
13.536(5)
13.995(4)
15.660(6)
78.110(18)
79.25(2)
75.795(18)
2785.0(17)
4
b/Å
c/Å
α (deg)
β (deg)
124.049(7)
90.00
92.891(12)
90.00
95.107(4)
90.00
γ (deg)
V/Å3
5311(3)
5371(3)
8
2827.4(4)
4
Z
8
ρ/g cm−3
R1, wR2(I > 2σ(I))
GOF
1.575
1.610
1.648
1.619
0.0356, 0.0677
1.024
0.0331, 0.0571
1.036
0.0390, 0.0750
1.028
0.0429, 0.724
1.003
1H, Ph p-H), 7.04 (dd, J = 8.1 Hz 2H, Ph m-H), 6.82 (d, J = 7.2 Hz,
2H, Ph o-H), 6.75 (s, 2H, Mes m-H), 6.53 (s, 2H, Mes m-H), 5.70 (d,
J = 20.7 Hz, 2H, MesNCH2), 5.49 (d, J = 20.4 Hz, 2H, MesNCH2),
2.18 (s, 6H, Mes CH3), 2.17 (s, 6H, Mes CH3), 1.96 (s, 6H, Mes
CH3). 13C NMR (CD2Cl2, δ): 262.02, 167.63, 154.16, 147.57, 141.74,
136.44, 136.10, 134.57, 130.28, 128.79, 128.31, 127.56, 126.72, 117.74,
75.85, 20.91, 18.62, 18.28. Anal. Calcd: C, 56.62; N: 6.19; H, 5.05.
Found: C, 56.40; N, 6.19; H, 5.08. IR (FTIR, cm−1): ν(C−O) 1557
cm−1.
X-ray Structural Determination of (DAP)Re(O)(CHC(CH3)2) (6).
The unit cell dimensions were determined from a symmetry-
constrained fit of 9300 reflections with 5.0° < 2θ < 64.76°. The data
collection strategy was a number of ω and φ scans which collected
data up to 79.48° (2θ). The structure was solved by direct methods
using the XS program.22 All non-hydrogen atoms were obtained from
the initial solution. The hydrogen atoms were introduced at idealized
positions and were allowed to ride on the parent atom. The structure
contains two molecules in the asymmetric unit, designated A and B in
this report. The structure also exhibits a compositional disorder with
the bromo derivative of the complex cocrystallizing at the same site as
molecule A. The normalized occupancy of the bromine atom refined
to a value of 0.075(2). The assignment of the Br atom was inferred by
the Re−X bond length of 2.509(6) Å in comparison to the Re−Br
bond lengths of other Re−oxo bromide compounds recovered from a
CSD search. Note: the final difference Fourirer map showed a number
of large peaks. However, these peaks were in chemically unreasonable
positions or could not be refined reasonably. It cannot be said for sure
what the origin of these peaks are. See Table 2.
X-ray Structural Determination of (DAP)Re(O)(C6H5) (7a). The
unit cell dimensions were determined from a symmetry-constrained fit
of 9482 reflections with 4.9° < 2θ < 65.28°. The data collection
strategy was a number of ω and φ scans which collected data up to
75.66° (2θ). The structure was solved by direct methods using the XS
program.22 All non-hydrogen atoms were obtained from the initial
solution. The hydrogen atoms were introduced at idealized positions
and were allowed to ride on the parent atom. The final difference
Fouirer map contains a region of large positive residual region of
electron density (3.17 e/Å3). An attempt was made to model this site
both as an oxygen and as a lithium, but in both cases the displacement
parameter refined to a large value which was incongruent with the rest
of the structure. The behavior of the refinement as well as the fact that
there is no analogous peak close to the B molecule and the peak is not
at a chemically sensible position led to the conclusion that this peak is
an artifact rather than an indicator of actual atomic electron density.
See Table 2.
(DAP)Re(O)(COC6H4-p-OCH3) (9b). 9b was obtained in 63.6% yield
1
as an orange solid. H NMR (CD2Cl2, δ): 8.13 (t, J = 8.0 Hz, 1H,
NC2H2CH), 7.69 (d, J = 8.0 Hz, 2H, NC2H2CH), 6.86 (d, J = 9.2 Hz,
2H, Ph H), 6.74 (s, 2H, Mes m-H), 6.56 (d, J = 8.8 Hz, 2H, Ph H),
6.52 (s, 2H, Mes m-H), 5.69 (d, J = 20.4 Hz, 2H, MesNCH2), 5.49 (d,
J = 20.8 Hz, 2H, MesNCH2), 3.74 (s, 3H, Ph OCH3), 2.18 (s, 6H, Mes
CH3), 2.16 (s, 6H, Mes CH3), 1.94 (s, 6H, Mes CH3). 13C NMR
(CD2Cl2, δ): 260.55, 167.70, 161.76, 154.19, 141.68, 139.97, 136.45,
136.05, 134.41, 128.71, 128.67, 128.28, 117.70, 112.71, 75.94, 55.69,
20.91, 18.64, 18.34. Anal. Calcd: C, 55.91; N, 5.93; H, 5.12. Found: C,
55.10; N, 6.32; H, 5.31. IR (FTIR, cm−1): ν(C−O) 1552 cm−1.
(DAP)Re(O)(COC6H4-p-Cl) (9c). 9c was obtained in 52.7% yield as
an orange solid. 1H NMR (CD2Cl2, δ): 8.14 (t, J = 7.6 Hz, 1H,
NC2H2CH), 7.70 (d, J = 8.0 Hz, 2H, NC2H2CH), 7.02 (d, J = 8.4 Hz,
2H, Ph H), 6.80 (d, J = 8.4 Hz, 2H, Ph H), 6.74 (s, 2H, Mes m-H),
6.54 (s, 2H, Mes m-H), 5.71 (d, J = 21.2 Hz, 2H, MesNCH2), 5.50 (d,
J = 20.4 Hz, 2H, MesNCH2), 2.18 (s, 6H, Mes CH3), 2.17 (s, 6H, Mes
CH3), 1.94 (s, 6H, Mes CH3). 13C NMR (CD2Cl2, δ): 260.62, 167.58,
154.07, 145.93, 141.88, 136.40, 136.11, 135.99, 134.70, 128.84, 128.30,
128.15, 127.72, 117.85, 75.77, 20.91, 18.60, 18.28. Anal. Calcd for
C32.5H34Cl2N3O2Re: C, 51.65; N, 5.56; H, 4.53. Found: C, 51.08; N,
5.55; H, 4.68. IR (FTIR, cm−1): ν(C−O) 1554 cm−1.
General Procedure for X-ray Determination. The sample was
mounted on a Mitegen polyimide micromount with a small amount of
Paratone N oil. All X-ray measurements were carried out on a Bruker-
Nonius Kappa Axis X8 Apex2 diffractometer at a temperature of 110
K. The frame integration was performed using SAINT.19 Unless
otherwise noted, the resulting raw data were scaled and absorption-
corrected using a multiscan averaging of symmetry equivalent data
using SADABS.20 The structural model was fit to the data using full-
matrix least squares based on F2. The calculated structure factors
included corrections for anomalous dispersion from the usual
tabulation. The structure was refined using the XL program from
SHELXTL;21 graphic plots were produced using the NRCVAX
crystallographic program suite. Additional information and other
relevant literature references can be found in the reference section of
the Facility’s Web page (http://www.xray.ncsu.edu).
X-ray Structural Determination of (DAP)Re(O)(COCH3) (8a). The
unit cell dimensions were determined from a symmetry-constrained fit
of 9918 reflections with 5.6° < 2θ < 58.58°. The data collection
strategy was a number of ω and φ scans which collected data up to
65.22° (2θ). The structure was solved by direct methods using the
SIR92 program.23 Most non-hydrogen atoms were obtained from the
initial solution. The remaining non-hydrogen atom positions were
obtained from a subsequent difference Fourier map. The hydrogen
atoms were introduced at idealized positions and were allowed to ride
on the parent atom. See Table 2.
4300
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Organometallics
Article
X-ray Structural Determination of (DAP)Re(O)(COC6H5) (9a). The
unit cell dimensions were determined from a symmetry-constrained fit
of 9995 reflections with 5.8° < 2θ < 63.48°. The data collection
strategy was a number of ω and φ scans which collected data up to
64.62° (2θ). The structure was solved by direct methods using the XS
program.22 All non-hydrogen atoms were obtained from the initial
solution. The hydrogen atoms were introduced at idealized positions
and were allowed to ride on the parent atom. See Table 2.
(2) Ojima, I.; Tsai, C.-Y.; Tzamarioudaki, M.; Bonafoux, D. The
Hydroformylation Reaction; Wiley: New York, 2004.
(3) (a) Chaplin, A. B.; Hooper, J. F.; Weller, A. S.; Willis, M. C. J.
Am. Chem. Soc. 2012, 134, 4885−4897. (b) Meng, Q.; Shen, W.; Li, M.
J Mol Model 2012, 18, 1229−39. (c) Willis, M. C. Chem Rev 2010, 110,
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(4) (a) Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Maggioni, D.;
Mercandelli, P.; Sironi, A. Inorg. Chim. Acta 2003, 350, 475−485.
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Organometallics 2012, DOI: 10.1021/om3003366.
Computational Methods. Theoretical calculations were carried
out using the Gaussian0918 implementation of B3PW91 (the B3
exchange functional13a and PW91 correlation functional13b) density
functional theory.13c All geometry optimizations were carried out using
tight convergence criteria (“opt=tight”) and pruned ultrafine grids
(“Int=ultrafine”). All calculations were conducted with the same basis
set combination. The basis set for rhenium was the small-core
(311111,22111,411) → [6s5p3d] Stuttgart−Dresden basis set and
relativistic effective core potential (RECP) combination (SDD)15 with
an additional f polarization function.16 The 6-31G(d,p)14 basis sets
were used for all other atoms. Cartesian d functions were used
throughout; i.e., there are six angular basis functions per d function. All
structures were fully optimized, and analytical frequency calculations
were performed on all structures to ensure either a zeroth-order saddle
point (a local minimum) or a first-order saddle point (transition state
TS) was achieved. The minimum associated with each transition state
was determined by animation of the imaginary frequency and, if
necessary, with intrinsic reaction coordinate (IRC) calculations.
Solvation energies were computed geometries optimized in the gas
phase using the SMD17 method, with benzene as the solvent, as
implemented in Gaussian 09. In this method an IEFPCM24 calculation
is performed with radii and electrostatic terms from Truhlar and co-
workers’ SMD17 solvation model. Thermochemical data were
calculated using unscaled vibrational frequencies and default
parameters at 353.15 K and 1 atm. In this paper energies are reported
in kcal/mol with gas-phase energies in parentheses and solvation
energies without parentheses.
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598, 167−173. (c) Cai, S.; Hoffman, D. M.; Lappas, D.; Woo, H. G.;
Huffman, J. C. Organometallics 1987, 6, 2273−2278. (d) Hoffman, D.
M.; Lappas, D.; Wierda, D. A. Polyhedron 1996, 15, 3309−3314.
(7) (a) Feng, S. G.; Luan, L.; White, P.; Brookhart, M. S.; Templeton,
J. L.; Young, C. G. Inorg. Chem. 1991, 30, 2582−2584. (b) Malarek, M.
S.; Evans, D. J.; Smith, P. D.; Bleeker, A. R.; White, J. M.; Young, C. G.
Inorg. Chem. 2006, 45, 2209−2216. (c) Su, F. M.; Cooper, C.; Geib, S.
J.; Rheingold, A. L.; Mayer, J. M. J. Am. Chem. Soc. 1986, 108, 3545−
3547. (d) Cross, J. L.; Crane, T. W.; White, P. S.; Templeton, J. L.
Organometallics 2003, 22, 548−554.
(8) Lilly, C. P.; Boyle, P. D.; Ison, E. A. Dalton Trans. 2011, 40,
11815−11821.
(9) Pellegatti, L.; Zhang, J.; Drahos, B.; Villette, S.; Suzenet, F.;
Guillaumet, G.; Petoud, S.; Toth, E. Chem. Commun. 2008, 6591−3.
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ASSOCIATED CONTENT
■
(12) (a) Djukic, J.-P.; Maisse, A.; Pfeffer, M.; Dotz, K. H.; Nieger, M.
̈
S
* Supporting Information
Organometallics 1999, 18, 2786−2790. (b) Djukic, J.-P.; Doetz, K. H.;
Pfeffer, M.; De, C. A.; Fischer, J. Inorg. Chem. 1998, 37, 3649−3651.
(13) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−52. (b) Perdew,
J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.;
Singh, D. J.; Fiolhais, C. Phys. Rev. B: Condens. Matter 1992, 46, 6671−
87. (c) Parr, R. G., Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(14) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257−61.
Tables giving Cartesian coordinates and figures giving the
structures for all optimized complexes, text giving the full
Gaussian 09 reference, a figure giving the time profile for CO
insertion into 5 and 7, and CIF files giving crystal data for 6, 7a,
8a, and 9a. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
(15) Dolg, M.; Stoll, H.; Preuss, H.; Pitzer, R. M. J. Phys. Chem. 1993,
97, 5852−9.
Corresponding Author
(16) Ehlers, A. W.; Boehme, M.; Dapprich, S.; Gobbi, A.; Hoellwarth,
A.; Jonas, V.; Koehler, K. F.; Stegmann, R.; Veldkamp, A.; et al. Chem.
Phys. Lett. 1993, 208, 111−14.
*E-mail: eaison@ncsu.edu.
Notes
The authors declare no competing financial interest.
(17) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B
2009, 113, 6378−6396.
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B. et al. Gaussian 09,
Revision A.02; Gaussian, Inc., Wallingford, CT, 2009.
(19) SAINT version 2009.9; Bruker-Nonius, Madison, WI 53711,
2009.
(20) SADABS version 2009.9; Bruker-Nonius, Madison, WI 53711,
2009.
ACKNOWLEDGMENTS
■
We acknowledge North Carolina State University and the
National Science Foundation via the CAREER Award (CHE-
0955636) for funding.
(21) XL version 2009.9; Bruker-Nonius, Madison, WI 53711, 2009.
(22) XS version 2009.9; Bruker-Nonius, Madison, WI 53711, 2009.
(23) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435−436.
(24) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105,
2999−3093.
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