Reactions of group III biheterocyclic complexes

Article abstract of DOI:10.1021/ja902794w

Group III alkyl complexes supported by a ferrocene diamide ligand (1,1′-fc(NSitBuMe₂)₂) have been found to be reactive toward aromatic N-heterocycles such as 1-methylimidazole and pyridines. These reactions were investigated experime

Full text of DOI:10.1021/ja902794w

Published on Web 07/01/2009
Reactions of Group III Biheterocyclic Complexes
Colin T. Carver,‡ Diego Benitez,§ Kevin L. Miller,‡ Bryan N. Williams,‡
Ekaterina Tkatchouk,§ William A. Goddard III,§ and Paula L. Diaconescu*,‡
Department of Chemistry & Biochemistry, UniVersity of California, Los Angeles,
California 90095, and Materials and Process Simulation Center, California Institute of
Technology, Pasadena, California 91125
Received April 7, 2009; E-mail: pld@chem.ucla.edu
Abstract: Group III alkyl complexes supported by a ferrocene diamide ligand (1,1′-fc(NSitBuMe2)2) have
been found to be reactive toward aromatic N-heterocycles such as 1-methylimidazole and pyridines. These
reactions were investigated experimentally and computationally. An initial C-H activation event is followed
by a coupling reaction to form biheterocyclic complexes, in which one of the rings is dearomatized. In the
case of 1-methylimidazole, the biheterocyclic compound could not be isolated and further led to an imidazole
ring-opened product; in the case of pyridines, it transformed into an isomer with extended conjugation of
double bonds. Mechanisms for both reactions are proposed on the basis of experimental and computational
results. DFT calculations were also used to show that an energetically accessible pathway for the ring-
opening of pyridines exists.
transition metals such as tantalum,8,10-15 niobium,16-19 and
titanium,20,21 with the exception of a rhenium system22,23 that
Introduction
Hydrodenitrogenation (HDN),1-6 the decomposition of ni-
trogen-containing impurities from fuels, is becoming increas-
ingly important as limited oil supplies dictate a shift toward
hydrocarbon sources containing larger amounts of N-heterocy-
cles.6 The breaking of C-N bonds in heterocycles is the most
difficult step in HDN processes, requiring reduction of the
aromatic ring with H2 prior to the cleavage of the C-N bond.1–6
Current industrial heterogeneous catalysts for this process
operate at 300-500 °C and 200 atm H2, making them rather
energy intensive.1,2 The development of soluble organometallic
catalysts7,8 for C-N bond cleavage may lead to novel processes,
which would operate under mild conditions and do not require
the reduction of the aromatic rings with hydrogen. In addition,
studies of the activation of C-N bonds in heterocycles by
organometallic complexes might elucidate the mechanism
underlying the heterogeneous HDN processes.
employs external bases and electrophilic reagents. The strong
thorium-oxygen bond can drive the ring-opening of pyridine-
N-oxides by thorium bis-cyclopentadienyl complexes.24 In a
previous communication, the ring-opening of imidazole rings
was reported to be mediated by scandium and yttrium alkyl
complexes25 supported by a ferrocene diamide ligand (NNfc )
(10) Gray, S. D.; Weller, K. J.; Bruck, M. A.; Briggs, P. M.; Wigley, D. E.
J. Am. Chem. Soc. 1995, 117, 10678–10693.
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Chem. 1997, 528, 225–228.
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D. P.; Weller, K. J.; Wigley, D. E. Polyhedron 1995, 14, 3315–3333.
(16) Kleckley, T. S.; Bennett, J. L.; Wolczanski, P. T.; Lobkovsky, E. B.
J. Am. Chem. Soc. 1997, 119, 247–248.
Examples of homogeneous systems9 that mediate the ring-
opening of N-heterocycles are exclusively based on early
(17) Bonanno, J. B.; Veige, A. S.; Wolczanski, P. T.; Lobkovsky, E. B.
Inorg. Chim. Acta 2003, 345, 173–184.
‡ University of California, Los Angeles.
§ California Institute of Technology.
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Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1991, 30, 2494–2508.
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Chem. Soc. 1988, 110, 4421–4423.
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155–208.
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Chem. Soc. 2007, 129, 12640–12641.
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7559.
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16, 3139–3163, and references therein.
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A R T I C L E S
Carver et al.
Scheme 1. Proposed Mechanism for the Ring-Opening of 1-Methylimidazole by the Scandium Alkyl Complex 1Sc-THF
1,1′-fc(NSitBuMe2)2).26-37 At the time, the mechanism proposed
for this transformation (Scheme 1) involved the C-H activation
of one imidazole ring by 1Sc-THF27 to give 2Sc-imidazole,
nucleophilic attack of the imidazolyl carbon of 2Sc-imidazole
on one of the coordinated imidazoles to lead to coupling of
two rings in 3Sc-(imidazole)x (x ) 0, 1, see below),38 and,
finally, ring-opening of the dearomatized ring in the new
biheterocyclic structure formed, 3Sc-(imidazole)x, to give the
final product 4Sc. The intermediate 3Sc-(imidazole)x could not
be isolated, but an analogous coupling product of two pyridine
rings was characterized.25 We had also observed a subsequent
reaction of this pyridine-coupled complex, but the reaction gave
an intractable mixture of products. In the present paper we report
on group III (Sc, Y, Lu) biheterocyclic complexes, based on
imidazole and pyridine, that, although share similar structures,
lead to different transformations. The ring-opening of imidazole
reported previously was not mirrored by these analogous
pyridine complexes, which, instead, underwent an isomerization
process. In order to determine the mechanistic origin of this
difference in reactivity, DFT calculations were carried out on
various plausible pathways. The theoretical and experimental
results are consistent, establishing the nature of intermediates
and transition states and explaining the two distinct outcomes.
structures, in which one of the rings is dearomatized, was
expanded to other six-membered aromatic N-heterocycles such
as quinoline and isoquinoline (Scheme 2). The presence of a
substituent ortho to the heteroatom aids both the C-H activation
and the coupling reactions. Similar reactions were also found
for the analogous yttrium and lutetium complexes (Scheme 2).
The reaction (in C6D6 or toluene) times vary from 15 min for
6Sc-pyPh-iqn to 2 h at room temperature for 6M-pyPh-iqn (M )
Y, Lu) and likely depend on (a) the probability of the second
pyridine to coordinate to the metal center and (b) the release of
the steric hindrance that takes place after the coupling event.
The displacement of THF and coordination of the second
pyridine to 5M-pyridine were not observed, but they must occur
before the coupling process takes place. Also, the formation of
5Y-qn or 5Lu-qn (qn ) 8-methylquinoline) was not ac-
complished, although a reaction between 1Y-THF or 1Lu-THF
and 8-methylquinoline occurred (those results will be reported
elsewhere).
Complexes 6M were characterized by elemental analysis, 1H
and 13C NMR spectroscopy, and, in the case of 6Sc-pyPh-iqn
(iqn ) 3-methylisoquinoline), X-ray crystallography (Figure 1;
the structural parameters are discussed below). The dearoma-
tization of the second pyridine ring was consistent with peaks
observed in the olefinic region [see the Supporting Information
for a detailed NMR analysis using 2D heteronuclear multiple
bond coherence-gradient pulse (HMBC-gp), 2D heteronuclear
Results and Discussion
Synthesis and Characterization of Complexes. The scandium-
mediated coupling of pyridine rings to form biheterocyclic
Scheme 2. Coupling of 2-Phenylpyridine or 8-Methylquinoline with
3-Methylisoquinoline Mediated by Group III Metal Centers
(27) Carver, C. T.; Monreal, M. J.; Diaconescu, P. L. Organometallics 2008,
27, 363–370.
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46, 7226–7228.
(29) Shafir, A.; Arnold, J. Organometallics 2003, 22, 567–575.
(30) Shafir, A.; Arnold, J. Inorg. Chim. Acta 2003, 345, 216–220.
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2001, 20, 1365–1369.
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(33) Ulrich, S. Z. Anorg. Chem. 2005, 631, 2957–2966.
(34) Siemeling, U.; Auch, T.-C. Chem. Soc. ReV. 2005, 34, 584.
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Bildstein, B. Z. Anorg. Allg. Chem. 2003, 629, 1334–1336.
(36) Eppinger, J. r.; Nikolaides, K. R.; Zhang-Presse, M.; Riederer, F. A.;
Rabe, G. W.; Rheingold, A. L. Organometallics 2008, 27, 736–740.
(37) Siemeling, U.; Auch, T.-C.; Tomm, S.; Fink, H.; Bruhn, C.; Neumann,
B.; Stammler, H.-G. Organometallics 2007, 26, 1112–1115.
(38) Deelman, B.-J.; Stevels, W. M.; Teuben, J. H.; Lakin, M. T.; Spek,
A. L. Organometallics 1994, 13, 3881–3891.
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Reactions of Group III Biheterocyclic Complexes
A R T I C L E S
Figure 1. Extreme left and right: Relevant metrical parameters (in Å) from the X-ray crystal structures of 6Sc-pyPh-iqn and 7Sc-pyPh-iqn and, in parentheses,
bond orders calculated by ADF2008.01 from geometry optimizations on the full molecules. Center: ORTEP representations of 6Sc-pyPh-iqn and 7Sc-pyPh-iqn
with ellipsoids drawn at 50% probability (tBu methyl, pyridine phenyl groups, and irrelevant hydrogen atoms were removed for clarity).
Scheme 3. Isomerization of Biheterocyclic Ligands in 6M
Complexes (M ) Sc, Y, Lu)
the basis of the information obtained from the DEPT-135
spectrum. By contrast, all carbon atoms in 6Sc-qn-iqn were
shown by DEPT-135 to be quaternary, methyne, or methyl
atoms (see the Supporting Information for a detailed NMR
analysis).
The comparison of metrical parameters for the structures of
6Sc-pyPh-iqn and 7Sc-pyPh-iqn also agrees with the structural
assignment for 7Sc-pyPh-iqn (Figure 1): the C1-C2 distance
between the two heterocyclic rings decreased from 1.505(4) Å
in 6Sc-pyPh-iqn to 1.398(3) Å in 7Sc-pyPh-iqn; the Sc-N1
distance increased from 2.086(2) Å in 6Sc-pyPh-iqn to 2.178(2)
Å in 7Sc-pyPh-iqn, while the Sc-N2 distance decreased from
2.303(2) Å in 6Sc-pyPh-iqn to 2.224(2) Å in 7Sc-pyPh-iqn. All
other distances (Figure 1) and the sum of the angles around C2
of 360.3° are in accord with the delocalization of an electron
pair in 7Sc-pyPh-iqn over the N1-C1-C2-N2-C3 atoms.
The assigned electronic structure of 7Sc-pyPh-iqn is also
supportedbybondordercalculationsbasedonNalewajski-Mrozek
valence indices41 obtained from geometry optimizations on the
full molecules (6Sc-pyPh-iqn and 7Sc-pyPh-iqn) using ADF2008.01.
multiple quantum coherence-gradient pulse (HMQC-gp), and
distortionless enhancement by polarization transfer (DEPT-135)
These calculations show that the C1-C2 bond order increases
from 1.00 in 6Sc-pyPh-iqn to 1.33 in 7Sc-pyPh-iqn, and the
Sc-N1 bond order increases from 0.39 in 6Sc-pyPh-iqn to 0.52
in 7Sc-pyPh-iqn, while the Sc-N2 bond order decreases from
0.74 in 6Sc-pyPh-iqn to 0.48 in 7Sc-pyPh-iqn. The bonds over
which the electron pair is delocalized in 7Sc-pyPh-iqn feature
bond orders consistent with aromatic character: 1.25 for N1-C1,
1.18 for C2-N2, and 1.67 for N2-C3 (Figure 1).
The isomerization reaction described here is not limited
to the complexes 6M-pyPh-iqn (M ) Sc, Y, Lu). The
analogous coupled products of 2-phenylpyridine with 2-pi-
coline or 8-methylquinoline with 3-methylisoquinoline, 6Sc-
qn-pic or 6Sc-qn-iqn, respectively, also show similar trans-
formations upon heating to form the complexes 7Sc-qn-pic
and 7Sc-qn-iqn, respectively. In the case of 7Sc-qn-pic, the
coupling product 6Sc-qn-pic could not be isolated because
its formation was competitive with its isomerization. Al-
though 7Sc-qn-pic could not be characterized by single-crystal
X-ray diffraction, we were able to obtain a structure for 7Sc-
qn-iqn (see the Supporting Information for details). These
additional reactions were limited to scandium complexes
because intractable mixtures of products were observed for
the analogous yttrium and lutetium cases. Other biheterocyclic
complexes, analogous to 6M, could be obtained, but the
heating of their solutions also gave intractable mixtures of
products. In some of those mixtures, complexes of the type
experiments].39,40
In order to test the reactivity of the newly obtained pyridine-
coupled products, C6D6 solutions of the complexes 6M-pyPh-
iqn (M ) Sc, Y, Lu) were heated at 70 or 85 °C for several
days, during which time their color changed from red to green
(Scheme 3). All 6M-pyPh-iqn (M ) Sc, Y, Lu) compounds
transformed to 7M-pyPh-iqn (M ) Sc, Y, Lu) in at least 60%
conversion. However, only in the case of scandium, 7Sc-pyPh-
iqn was isolated and purified, owing to solubility differences
between 6Sc-pyPh-iqn and 7Sc-pyPh-iqn. The identity of the
products 7M-pyPh-iqn (M ) Y, Lu) was established by
comparison of their NMR spectra with those of 7Sc-pyPh-iqn
(see the Supporting Information for details). Crystals of the
product 7Sc-pyPh-iqn were obtained from a concentrated hex-
anes/toluene solution; the X-ray crystal structure obtained
revealed that the hydrogen atom from the isoquinoline C2 atom
1
migrated to C4 (Figure 1). H NMR spectroscopy supported
this interpretation since a new peak at 3.20 ppm was found for
7Sc-pyPh-iqn, consistent with its benzylic character. A thorough
1
investigation of the H and 13C NMR spectra of 7Sc-qn-iqn
using 2D HMQC-gp and 2D HMBC-gp39,40 showed that the
analogous peak at 3.14 ppm in its 1H NMR spectrum
corresponds to a peak at 30.0 ppm in its 13C NMR spectrum.
This 13C NMR peak was assigned to a methylene carbon on
(39) Derome, A. E. Modern NMR techniques for chemistry research;
Pergamon: Cambridge, 1988.
(40) Rahman, A.-U. One and two dimensional NMR spectroscopy Elsevier:
Amsterdam, 1989.
(41) Michalak, A.; DeKock, R. L.; Ziegler, T. J. Phys. Chem. A 2008, 112,
7256–7263.
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A R T I C L E S
Carver et al.
Figure 2. Pathways studied computationally for the isomerization of 6Sc-pyPh-iqn to 7Sc-pyPh-iqn; all energies given in kcal/mol.
1
7M could be identified by H NMR spectroscopy, but their
separation and isolation were not successful.
Mechanistic Studies. Because this previously unreported
isomerization process is general for the complexes described
here, it became imperative to understand its mechanism. For
this purpose, 3-methylisoquinoline was deuterated at the 1-posi-
tion (deuterium incorporation at the 3-methyl position was also
observed),42 and the compound 6Sc-qn-iqn-d4 was prepared.
Compound 6Scqn-iqn-d4 was heated at 70 °C, and the formation
of 7Sc-qn-iqn-d4 was monitored by both 1H and 2H NMR
spectroscopy. A kinetic isotope effect of 4 ( 1 was estimated
from the monitoring of the reaction mixtures.
In order to establish whether the isomerization process is intra-
or intermolecular, a mixture of 6Sc-qn-iqn-d4 and 6Sc-pyPh-iqn
1
2
was heated at 70 °C (eq 1). Inspection by H and H NMR
spectroscopy showed that there was no deuterium incorporation
in 7Sc-pyPh-iqn, indicating that the isomerization reaction is an
intramolecular process. An intermolecular process would involve
the deprotonation of C2 followed by the protonation of C4, and
incorporation of deuterium into 7Sc-pyPh-iqn would have been
observed. The intramolecular nature of the transformation is
also supported by two other observations: (1) reactions run in
the presence of excess 3-methylisoquinoline did not proceed at
a different rate than reactions run in its absence, and (2) the
addition of Et3N to 6Sc-qn-iqn did not influence the rate of the
isomerization process.
reported for the DBU-catalyzed deconjugation of 7-substi-
tuted 3,4-didehydro-2-oxepanones.43 That study concluded
that DBU is a special base and its effect on the reaction was
dependent on the presence of the imine NdC bond since
similar strength bases did not produce the same results. On
the basis of these observations, we conclude that DBU acts
as a catalyst, but it may change the mechanism of the
isomerization reaction as compared to that of reactions run
in its absence (see the Supporting Information for the
corresponding NMR spectra).
At the suggestion of a reviewer, the isomerization reactions
were also run in the presence of 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU); the complex 6Sc-qn-iqn transforms to 7Sc-qn-
iqn at room temperature within 30 min, while for 6Sc-pyPh-
iqn an equilibrium mixture of 75% 7Sc-pyPh-iqn and 25%
6Sc-pyPh-iqn was observed at room temperature after 8 h,
regardless of whether DBU was used in catalytic (20 mol
%) or stoichiometric amount. A crossover experiment
between 6Sc-pyPh-iqn-d4 and 6Sc-qn-iqn, run in the presence
of DBU, showed, again, that there is no deuterium incorpora-
tion into 7Sc-pyPh-iqn. However, the deuterium label was
found in various positions of DBU, so the crossover
experiment was inconclusive with respect to the nature of
the DBU-mediated process. A similar observation has been
Computational Studies. The mechanism for the isomerization
of 6Sc-pyPh-iqn to 7Sc-pyPh-iqn was explored using density
functional theory (DFT). Geometry optimizations were carried
out on the full molecules using Jaguar 7.5, and energies were
computed at the B3LYP/LACV3P++**(2f)//B3LYP/LACVP**
level of theory.44 Three different intramolecular pathways were
examined for the migration of the proton from the isoquinoline
C2 to C4: (1) nitrogen-assisted, (2) metal-assisted, and (3) direct
transfer (Figure 2). All pathways led to the product 7Sc-pyPh-
(42) Rubottom, G. M.; Evain, E. J. Tetrahedron 1990, 46, 5055–5064.
(43) Jeyaraj, D. A.; Kapoor, K. K.; Yadav, V. K.; Gauniyal, H. M.; Parvez,
M. J. Org. Chem. 1998, 63, 287–294.
(44) Jaguar, v.7.5;Schro¨dinger, LLC: New York, 2008.
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Reactions of Group III Biheterocyclic Complexes
A R T I C L E S
iqn, which was found to be more stable than the coupled product
6Sc-pyPh-iqn by 1.0 kcal/mol, in agreement with the experimental
results. In order to evaluate the operating pathway, the corre-
sponding transition structures and intermediates involved in each
process were sought out.
that the reaction involving a total of 3 equiv of 1-methylimi-
dazole is faster (5 h) than the one involving only 2 equiv
(the reaction was 95% complete in 7 days). Starting from
the compound 2Sc-imidazole, the B3LYP45-49 method of DFT
predicts that the binding of a third equivalent of 1-meth-
ylimidazole is favored by 0.8 kcal/mol. The coupling of the
1-methylimidazolyl and of one of the coordinated 1-meth-
ylimidazole ligands to give 3Sc (21.6 kcal/mol) or 3Sc-
imidazole (4.8 kcal/mol) proceeds via transition structures
at 32.9 and 21.1 kcal/mol, respectively.
In order to establish the pathway involving ring-opening
to form the product 4Sc, a relaxed coordinate scan was carried
out starting from it (4Sc). In addition to the ring-opening step,
the scan revealed that the dearomatized 1-methylimidazole
fragment has to undergo a rotation to allow coordination of
the nitrogen bearing the methyl group. Two cases were
investigated:
(1) A rotation along the carbon-carbon bond between the
coupled rings of 3Sc-imidazole via a transition state at 33.6
kcal/mol allows the coordination of the nitrogen bearing the
methyl group to form the intermediate ISc3-4-imidazole at
22.5 kcal/mol. The ring-opening process then occurs via a
transition structure at 40.3 kcal/mol that rearranges to the
product 4Sc-imidazole (-2.3 kcal/mol).
The first pathway involved the migration of the proton to
the amide nitrogen atom. On the basis of Mulliken charge
comparison (-0.65 for N2, -0.94, and -0.96 for Nfc) and given
its proximity to C2, this nitrogen atom could be basic enough
to participate in or mediate the H transfer. However, this process
involves an activation barrier ∆Gq of 60.0 kcal/mol, leading to
an intermediate, IN6-7, at 27.9 kcal/mol on the potential free
energy surface (PES) relative to 6Sc-pyPh-iqn. The transfer of
the proton from N2 to C4 involves an even more improbable
transition structure, which was estimated to be ca. 52 kcal/mol
higher in energy than the intermediate IN6-7. In addition, a
pathway involving the ferrocene diamide ligand was explored.
We found that this process also involves proton transfer via
IN6-7, as a consequence of the steric hindrance caused by the
tert-butyldimethylsilyl groups, which prevents the biheterocyclic
ligand from rotating and approaching the amidic N (or Sc)
facially. On the basis of the high energies found for the two
transition states, these mechanistic proposals were considered
unrealistic.
Next, a metal-mediated isomerization was considered, in
which ꢀ-hydride elimination to scandium occurs. A stationary
point for the structure IH6-7 was found at 24.7 kcal/mol on the
PES. However, the ligand appears to be distorted (see the
Supporting Information for details), and the hydride lies away
from the source heterocycle. If such an intermediate were part
of the isomerization, it is likely that reinsertion would occur
into other aromatic groups, which was not observed experi-
mentally. In addition, a direct transfer of H from C2 to Sc was
(2) A similar rotation takes place for 3Sc via a transition
state at 37.0 kcal/mol to allow the formation of the
intermediate ISc3-4 at 27.4 kcal/mol. The ring-opening process
then occurs via a transition structure at 40.3 kcal/mol that
rearranges to the product 4Sc (-11.2 kcal/mol).
The results described above support the experimental
observation that the reaction involving a total of 3 equiv of
1-methylimidazole is faster than the one involving only 2
equiv. The calculations led to the complex 2Sc-imidazole as
the resting state, as observed experimentally. All transition
states and intermediates for the pathway involving 2 equiv
of 1-methylimidazole are at least 5 kcal/mol higher in energy
than those corresponding to the pathway involving 3 equiv,
explaining the slower reaction rate observed when only 2
equiv of 1-methylimidazole was used. The final product, 4Sc,
forms after the ring-opened intermediate 4Sc-imidazole
loses a 1-methylimidazole ligand (a process favored by 8.9
kcal/mol).
Because the ring-opening of pyridine derivatives is relevant
to HDN processes, the difference between the actual ring-
opening of 1-methylimidazole and a hypothesized one for
pyridine substrates was investigated computationally. The
ring-opening of pyridine was proposed to start from the model
complex 6Sc-py-py (Figure 4), which represents the product
of coupling between two pyridine rings. Two pathways were
explored: (1) a rotation along C1-C2 and a disconnection
at the newly formed C-C single bond which is directly
analogous to the ring-opening of 1-methylimidazole (dashed
lines in Figure 4), and (2) a breaking of the bond N2-C2
(Figure 1) between the amide nitrogen and the saturated
carbon (solid lines in Figure 4).
not found computationally. All attempts to locate such a
6-7
transition structure led to the intermediate IN
via the same
60.0 kcal/mol free energy activation barrier. Thus, it was
concluded that the pathway is inoperative.
Finally, a direct pathway was considered. The proposed
avenue involves a transition structure in which the heterocycle
adopts a boat-like conformation. Such a conformation
facilitates the direct proton transfer, leading to a free
activation energy of ∆Gq ) 29.9 kcal/mol in benzene as a
solvent. Thus, it was concluded that the operating pathway
is a direct transfer from C2 to C4 and that the driving force
for the isomerization reactions of 6M-pyPh-iqn to 7M-pyPh-
iqn is the increased stability (-1.0 kcal/mol for scandium)
of the products 7M-pyPh-iqn. It is also proposed that the small
difference in stability between 6M-pyPh-iqn and 7M-pyPh-iqn
explains why some reactions could not be driven to comple-
tion. To support this hypothesis, the gas-phase electronic
energy differences between 6M-pyPh-iqn and 7M-pyPh-iqn
were calculated from full geometry optimizations (using
ADF2008.01), which led to -1.2 kcal/mol for Sc, -1.5 kcal/
mol for Y, and -1.7 kcal/mol for Lu. By contrast, the
compound 7Sc-qn-iqn is more stable by 8.5 kcal/mol than
6Sc-qn-iqn, reflected by the full conversion of 6Sc-qn-iqn to
7Sc-qn-iqn.
The first pathway involves an activation barrier of 28.4
kcal/mol and leads to the intermediate ISc6-8 (24.0 kcal/mol).
The same DFT methodology (B3LYP/LACV3P++**(2f)//
B3LYP/LACVP**) was applied to the study of the ring-
opening reaction of 1-methylimidazole described in Scheme
1 (Figure 3). Two pathways were investigated: one involving
two and the other involving three 1-methylimidazole ligands.
These pathways were based on the experimental observation
(45) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(46) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
(47) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200–
1211.
(48) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(49) Hertwig, R. H.; Koch, W. Chem. Phys. Lett. 1997, 268, 345–351.
9
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A R T I C L E S
Carver et al.
Figure 3. Proposed mechanisms for the ring-opening of 1-methylimidazole by 1Sc-THF; the dashed pathway involves only 2 equiv of 1-methylimidazole.
Values are free energies (∆G343K) (not drawn to scale) in kcal/mol and referenced to 2Sc-imidazole.
Breaking the C-C single bond leads to the product 8Sc (24.7
kcal/mol) via a relative activation barrier of 38.1 kcal/mol.
Such a high activation barrier makes it unlikely that this
mechanism would take place as proposed. Alternatively, the
second pathway, with the elongation of the N2-C2 bond,
leads to the product 9Sc (22.3 kcal/mol) via an activation
barrier of only 27.8 kcal/mol. The accessibility of the
transition state for this pathway suggests that there may be
reaction conditions that would lead to this process, provided
that the ring-opened product can be stabilized versus the
starting material.
tramolecularly, while DFT calculations established that the most
likely pathway for the isomerization is a direct proton transfer
between two carbons of the dearomatized pyridine ring. Given
the importance of C-N bond cleavage in aromatic N-hetero-
cycles, the ring-opening of 1-methylimidazole was also inves-
tigated computationally to establish similarities and differences
with the reaction of pyridine substrates. It was concluded that
the stability of the products is the most important factor in
determining the different outcomes. DFT calculations carried
out on mechanisms analogous to that for the ring-opening of
1-methylimidazole suggest that a pathway leading to the ring-
opening of pyridines is energetically accessible, and, thus,
experimental conditions may be found for the ring-opening of
pyridines.
Conclusions
The reactions of two types of aromatic N-heterocycles
(pyridines and imidazoles) with group III alkyl complexes were
investigated experimentally and computationally. The two
substrates form analogous biheterocyclic structures after the
initial C-H activation event. However, in the case of 1-meth-
ylimidazole, the biheterocyclic compound could not be isolated
and led to an imidazole ring-opened product. In the case of
pyridines, a biheterocyclic product could be isolated and, by
contrast, transformed into an isomer with extended conjugation
of double bonds. The mechanism for the isomerization reaction
was investigated both experimentally and computationally:
experimentally, it was shown that the reaction proceeds in-
Experimental Section
All experiments were performed under a dry nitrogen atmo-
sphere using standard Schlenk techniques or an MBraun inert-
gas glovebox. Solvents were purified using a two-column solid-
state purification system by the method of Grubbs50 and
transferred to the glovebox without exposure to air. NMR
solvents were obtained from Cambridge Isotope Laboratories,
(50) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
9
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Reactions of Group III Biheterocyclic Complexes
A R T I C L E S
Figure 4. Proposed mechanisms for the ring-opening of pyridine by 1Sc-THF. All energies (not drawn to scale) are given in kcal/mol and referenced to
6Sc-py-py.
degassed, and stored over activated molecular sieves prior to
use. Scandium, yttrium, and lutetium oxides were purchased from
Stanford Materials Corp. (Aliso Viejo, CA) and used as received.
K(CH2Xy-3,5), KCH2Ph, 1Sc-THF, 1Y-THF, and 5Sc-pyPh were
prepared following published procedures.27 The aromatic het-
erocycles were distilled or recrystallized before use; all other
materials were used as received. 1H NMR spectra were recorded
on Bruker300, Bruker500, or Bruker600 spectrometers (work
supported by the NSF grant CHE-9974928) at room temperature
in C6D6 unless otherwise specified. Chemical shifts are reported
with respect to internal solvent, 7.16 ppm (C6D6). CHN analyses
were performed at the UC Berkeley Micro-Mass Facility (College
of Chemistry, University of California, Berkeley).
1H NMR (500 MHz, C6D6), δ (ppm): 8.02-7.97 (m, 3H,
NC5H3), 7.36-7.26 (m, 5H, py-C6H5), 4.14 (s, 2H, fc-CH), 4.06
(s, 4H, OCH2CH2), 3.50 (s, 2H, fc-CH), 3.43 (s, 2H, fc-CH),
1.41 (t, 4H, OCH2CH2), 0.86 (s, 18H SiC(CH3)3), -0.07 (s, 12H,
Si(CH3)2). 13C NMR (126 MHz, C6D6), δ (ppm): 220.4, 153.4,
139.3, 134.2, 129.8, 128.7, 128.3, 126.9, 118.7, 104.8, 71.5, 66.2,
34.1, 32.1, 27.1, 25.0, 22.4, 19.8, 13.9, -3.2. Anal. Calcd for
C37H54FeN3OYSi2: C, 58.65; H, 7.18; N, 5.54. Found: C, 59.04;
H, 7.48; N, 5.21.
Synthesis of 5Lu-pyPh. The compounds 1Lu-THF (0.1663 g, 0.206
mmol) and 2-phenylpyridine (0.0317 g, 0.204 mmol) were com-
bined in a Schlenk tube with 5 mL of uene. The resultant bright
orange solution was heated at 70 °C for ∼2 h until the solution
had a dark brown color. The solution was then dried under
vacuum,and the resultant solid was taken up in n-pentane and
filtered through Celite. The filtrate was concentrated under vacuum
and stored at -35 °C overnight, giving brown-orange crystals.
Yield: 0.1193 g, 62.9%.
Synthesis of 5Sc-qn. The compound 1Sc-THF (300 mg, 0.442
mmol) was combined with 1.05 equiv of 8-methylquinoline (66.4
mg, 0.464 mmol) in toluene (8 mL) and stirred for 4 h at room
temperature. The solvent was removed, the resulting red-orange
solid extracted in hexanes, the solution filtered through Celite,
and the filtrate concentrated and placed in a -35 °C freezer.
Yield: 253.2 mg, 81.5% as two crops of crystals from hexanes.
1H NMR (500 MHz, C6D6), δ (ppm): 7.97 (d, 1H, quinoline),
7.72 (d, 1H, quinoline), 7.42 (t, 1H, quinoline), 7.33 (t, 1H,
quinoline), 7.18 (t, 1H, quinoline), 4.25 (s, 2H, fc-CH), 4.21 (s,
4H, OCH2CH2), 4.11 (s, 2H, fc-CH), 3.73 (s, 2H, fc-CH), 4.11 (s,
2H, fc-CH), 3.28 (s, 2H, fc-CH), 3.00 (s, 3H, quinoline-CH3), 1.52
(s, 4H, OCH2CH2), 0.82 (s, 18H, SiC(CH3)3), -0.14 and -0.42 (s,
6H, SiCH3). 13C NMR (126 MHz, C6D6), δ (ppm): 144.2, 133.6,
133.4, 130.0, 129.0, 127.0, 126.6, 125.9, 101.4, 72.8, 68.2, 68.1,
67.5, 66.3, 27.7, 25.4, 20.2, 19.1, -2.3, -3.1. Anal. Calcd for
C36H55FeN3OScSi2: C, 61.52; H, 7.89; N, 5.98. Found: C, 61.72;
H, 7.68; N, 5.63.
1H NMR (500 MHz, C6D6), δ (ppm): 8.10 and 8.09 (d, 2H,
pyridyl), 7.98 and 7.96 (d, 1H, pyridyl), 7.38-7.18 (m, 5H, py-
C6H5), 4.13 (br, 8H, both OCH2CH2 and fc-CH), 3.68 and 3.35
(br, 4H, fc-CH), 1.43 (br, 4H, OCH2CH2), 0.85 (s, 18H, SiC(CH3)3),
-0.10 (s, 12H, (Si(CH3)2)2. 13C NMR (126 MHz, C6D6), δ (ppm):
229.5, 154.1, 139.3, 135.2, 130.7, 129.3, 128.8, 128.6, 127.4, 119.2,
104.1, 72.4, 66.5, 27.5, 25.4, 20.3, 14.4, -2.8. Anal. Calcd for
C37H54N3OSi2FeLu: C, 52.66; H, 6.45; N, 4.98. Found: C, 52.11;
H, 6.67; N, 4.80.
Synthesis of 6Sc-pyPh-iqn. The compound 5Sc-pyPh (0.3000 g,
0.420 mmol) was combined with 1.1 equiv of 3-methylisoquinoline
(0.0662 g, 0.4263 mmol) in toluene (10 mL). The reaction mixture
was stirred for 3 h at room temperature. The solvent was removed,
and the resulting blood red solid was washed with hexanes and
extracted in toluene. Yield: 0.2633 g, 79.8% as crystals from
toluene:pentane at -35 °C.
Synthesis of 5Y-pyPh. The compound 1Y-THF (0.2204 g, 0.323
mmol) was combined with 0.95 equiv of 2-phenylpyridine (0.0475
g, 0.306 mmol) in toluene (10 mL) and stirred for 1 h at 50 °C.
The solvent was removed under reduced pressure, and the resulting
dark green solid was dissolved in minimal pentane (ca. 2 mL) and
left overnight to precipitate a dark yellow-green solid. Yield: 0.1394
mg, 60.1%.
1H NMR (500 MHz, C6D6), δ (ppm): 7.66 (d, 2H, aryl),
7.30-7.19 (m, 4H, aryl), 7.16-7.05 (m, 4H, aryl), 6.94 (d, 1H,
aryl), 6.78 (d, 1H, aryl), 5.80 (s, 1H, isoquinoline-N-CH), 5.75
(s, 1H, olefinic), 4.23 (s, 1H, fc-CH), 4.06 (s, 1H, fc-CH), 3.92
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A R T I C L E S
Carver et al.
(s, 1H, fc-CH), 3.89 (s, 1H, fc-CH), 3.36 and 3.36 (s, 1H, fc-
CH), 3.28 (s, 1H, fc-CH), 3.27 (s, 1H, fc-CH), 2.18 (s, 1H, fc-
CH), 2.18 (s, 3H, isoquin-CH3), 0.83 and 0.77 (s, 9H, SiC(CH3)3),
0.00 (s, 6H, SiCH3), -0.12 and -0.36 (s, 3H, SiCH3). 13C NMR
(126 MHz, C6D6), δ (ppm): 164.6, 158.3, 152.5, 139.5, 139.1,
138.2, 130.6, 130.1, 127.6, 127.2, 126.1, 125.5, 122.8, 122.5,
122.4, 121.4 99.7, 99.3, 97.2, 69.7, 68.9, 68.4, 68.0, 67.5, 68.0,
67.4, 67.1, 66.8, 27.7, 27.5, 22.7, 20.0, 19.9, -2.6, -2.8, -4.3.
λ
max/nm (ε/M-1 cm-1) ) 286 (15 800), 332 (11 700). Anal. Calcd
for C43H55FeN4ScSi2 · toluene: C, 68.47; H, 7.23; N, 6.38. Found:
C, 68.16; H, 7.41; N, 6.70.
Synthesis of 6Sc-qn-iqn. The compound 5Sc-qn (0.2000 g, 0.285
mmol) was combined with 1.1 equiv of 3-methylisoquinoline
(0.0448 g, 0.313 mmol) in toluene (10 mL) and stirred for 2 h
at room temperature. The solvent was removed, and the blood
red solid was washed with hexanes and extracted in toluene.
Yield: 0.1476 g, 67.12% as the first crop of crystals from toluene:
pentane. Crystals suitable for X-ray diffraction were obtained
from toluene:pentane at -35 °C; however, upon cooling to 110
K the crystals shattered, preventing full crystallographic analysis.
126.0, 125.7, 125.3, 122.2, 122.0, 121.0, 120.7, 103.1, 103.0,
96.7, 68.6, 68.3, 68.2, 67.9, 67.7, 66.9, 66.8, 66.5, 65.2, 31.8,
27.4, 27.3, 26.5, 22.9, 22.2, 19.85, 19.82, 14.2, -2.9, -3.0, -3.1,
-4.5. Anal. Calcd for C42H55FeN4YSi2: C, 62.31; H, 6.69; N,
6.76. Found: C, 62.15; H, 6.79; N, 6.38.
Synthesis of 6Lu-pyPh-iqn. The compound 5Lu-pyPh (0.1193 g,
0.128 mmol) was combined with 3-methylisoquinoline (0.0546
g, 0.381 mmol) in ∼3 mL of toluene. The solution was stirred
at room temperature for 2 h. The red solution was then dried
under vacuum, leaving a red powder. The solid was taken up in
hexanes and filtered through Celite. The filtrate was concentrated
under vacuum and cooled to -35 °C to afford red crystals of
6
Lu-pyPh-iqn. Yield: 0.0720 g, 62.0%. Average yield for this
reaction: 70%. Note: The percent yield was larger when the
reaction was done on a smaller scale.
1H NMR (500 MHz, C6D6), δ (ppm): 7.46 (d, 1H, s), 7.26 (tr,
1H, e), 7.25 (tr, 1H, d), 7.21 (d, 1H, r), 7.13 (d, 1H, c), 7.11
(d, 1H, m), 7.06 (tr, 1H, o), 6.85 (d, 1H, f), 5.86 (s, 1H, j), 5.84 (s,
1H, a), 4.22 (s, 1H, fc-CH), 4.18 (s, 1H, fc-CH), 4.06 (s, 1H, fc-
CH), 4.05 (s, 1H, fc-CH), 3.41 (s, 1H, fc-CH), 3.37 (s, 1H,
fc-CH), 3.27 (s, 1H, fc-CH), 3.21 (s, 1H, fc-CH), 3.01 (s, 3H,
quin-CH3), 2.26 (s, 3H, isoquin-CH3), 0.76 and 0.62 (s, 9H,
SiC(CH3)3), -0.14, -0.15, -0.21, and -0.43 (s, 3H, SiCH3).
13C NMR (126 MHz, C6D6), δ (ppm): 167.4 (t), 152.8 (h), 145.8
(l), 139.7 (b or g), 139.3 (s), 132.0 (q or n), 127.4 (r), 127.1 (e),
127.0, 126.4 (b or g), 124.3 (c), 123.1 (f), 122.5 (o), 121.6 (d),
99.0 and 98.5 (fc-C-N), 97.4 (a), 69.3, 69.0, 68.8, 68.5, 68.2,
68.0, 67.9, and 67.7 (fc-CH), 27.5 and 27.3 ([SiC(CH3)3]2), 22.8
(i), 19.9 and 19.8 ([SiC(CH3)3]2), 18.5 (k), -2.7, -3.0, -3.8,
and -4.0 ([Si(CH3)2]2). λmax/nm (ε/M-1 cm-1) ) 236 (81 400),
320 (34 900), 362 (22 300), 526 (1810). Anal. Calcd for
[C42H55FeN4ScSi2]2 · toluene: C, 66.73; H, 7.26; N, 6.84. Found:
C, 66.90; H, 7.30; N, 6.53.
1H NMR (500 MHz, C6D6), δ (ppm): 7.67 and 7.65 (d, 2H, r
and t), 7.31 (t, 2H, q and u), 7.27, 7.26 (d, 1H, l), 7.24 (t, 1H,
m), 7.08 (s, 1H, n), 7.07 (s, 1H, h), 7.04 (d, 1H, s), 7.03 (d, 1H,
e), 6.83, 6.81, 6.78, 6.77 (d of d, 2H, f and g), 5.88 (s, 1H, j),
5.63 (s, 1H, c), 4.14, 4.06, 3.89, 3.80, 3.39, 3.36, 3.32, 2.07 (s,
1H each, fc-H), 2.26 (s, 3H, a), 0.87 (s, 9H, SiC(CH3)3), 0.75
(s, 9H, SiC(CH3)3), -0.00 (s, 3H, Si(CH3)), -0.01 (s, 3H,
Si(CH3)), -0.11 (s, 3H, Si(CH3)), -0.43 (s, 3H, Si(CH3)). 13C
NMR (126 MHz, C6D6), δ (ppm): 165.5 (d,i), 157.3 (o), 153.0.
(b), 139.6 (h), 138.2 (p), 132.5 (q,u), 130.5 (s), 127.1 (l) 126.02
(k), 125.97 (r and t), 125.7 (e), 122.42 (n), 122.38, 121.5 (f and
g), 121.3 (m), 102.3 (fc-C), 97.9 (j), 68.41, 68.26, 68.00, 67.41,
67.33, 66.36, 65.82, 65.37 (fc-CH), 67.5 (c), 36.3 and 34.8 (tBu-
C(CH3)3), 27.6 and 27.3 (tBu-C(CH3)3), 22.6 (a), -2.80, -2.89,
-2.97, -4.50 ([Si(CH3)2]2). Anal. Calcd for C43H55N4Si2FeLu:
C, 56.45; H, 6.06; N, 6.12. Found: C, 56.33; H, 6.29; N, 5.78.
Synthesis of 7Sc-qn-iqn. The compound 6Sc-qn-iqn (0.2600 g,
0.336 mmol) was dissolved in toluene (10 mL) and stirred at 70
°C for 96 h. After 12 h the solution turned dark purple. The solvent
was removed, and the resulting purple solid was filtered in hexanes.
Yield: 0.1953 g, 75.12%. Crystals suitable for X-ray diffraction
were grown from a solution of Et2O:pentane at -35 °C.
Synthesis of 6Y-pyPh-iqn. The compound 5Y-pyPh (0.0873 g,
0.115 mmol) was combined with 3 equiv of 3-methylisoquinoline
(0.0545 g, 0.381 mmol) in C6D6 (4 mL) and stirred for 2 h at
room temperature. The solvent was removed by vacuum, and
the resulting dark red oil was dissolved in minimal hexanes (ca.
3 mL) and left overnight at -35 °C to precipitate dark red
crystals. Yield: 0.0693 g, 72.7%.
1H NMR (300 MHz, C6D6), δ (ppm): 7.53 (m, 2H, q and u),
7.28 (m, 4H, m,r,s,t), 7.08 (m, 2H, f,g), 6.99 (m, 2H, e,h), 6.83
(d, 1H, n), 6.78 (d, 1H, l), 5.88 (s, 1H, j), 5.65 (s, 1H, c), 4.16
(s, 1H, fc-CH), 4.10 (s, 1H, fc-CH), 3.91 (s, 1H, fc-CH), 3.81
(s, 1H, fc-CH), 3.30 (s, 1H, fc-CH), 3.27 (s,1H, fc-CH), 3.22
(s, 1H, fc-CH), 1.97 (s, 1H, fc-CH), 2.19 (s, 3H, a), 0.89 (s,
9H, SiC(CH3)3), 0.75 (s, 9H, SiC(CH3)3), 0.01 (s, 3H, Si(CH3)2),
-0.02 (s, 3H, Si(CH3)2), -0.06 (s, 3H, Si(CH3)2), -0.39 (s, 3H,
Si(CH3)2). 13C NMR (126 MHz, C6D6), δ (ppm): 165.9, 165.9,
156.9, 152.9, 139.5, 139.3, 138.2, 132.4, 130.3, 128.2, 126.9,
1H NMR (500 MHz, C6D6), δ (ppm): 7.94 (d, 1H, c), 7.48 (d, 1H,
r), 7.17 (tr, 1H, e), 7.07 (tr, 1H, d), 6.92 (d, 2H, m,p), 6.89 (d, 1H, f),
6.76 (d, 1H, s), 6.73 (tr, 1H, o), 4.17 (s, 2H, fc-CH), 4.08 (s, 2H,
fc-CH), 3.24 (s, 2H, fc-CH), 3.19 (s, 2H, fc-CH), 3.14 (s, 2H, a), 2.71
(s, 3H, k), 1.96 (s, 3H, i), 0.81 (s, 18H, SiC(CH3)3), 0.09 and 0.06 (s,
6H, SiCH3). 13C NMR (126 MHz, C6D6), δ (ppm): 150.7 (t), 149.6
(h), 148.8 (l), 133.8 (s), 132.3 (g), 130.7 (m), 127.2 (f), 127.0 (e),
126.8 (p), 125.7 (b), 124.9 (d), 123.6 (l), 123.3 (q), 122.7 (r), 120.8
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Reactions of Group III Biheterocyclic Complexes
A R T I C L E S
121.4 (i, k, l, or m), 119.9 (i, k, l, or m), 118.3 (f), 117.8 (h), 102.2
(b), 98.1 (fc-C-N), 69.2, 68.7, 68.4 and 66.9 (fc-CH), 30.0 (a), 27.5
(SiC(CH3)3), 23.2 (o), 21.1 (e), 19.9 (SiC(CH3)3), -3.6 and -3.8
(Si(CH3)2. λmax/nm (ε/M-1 cm-1) ) 250 (37 300), 294 (28 200),
390 (9740), 446 (8290), 740 (1410), 822 (1620), 926 (1420). Anal.
Calcd for C38H53FeN4ScSi2: C, 63.14; H, 7.39; N, 7.75. Found: C,
63.80; H, 7.67; N, 7.76.
DBU-Catalyzed Isomerization. The compound 6Sc-qn-iqn (60
mg, 0.078 mmol) was combined with 1.3 equiv of DBU (15.4 mg,
0.100 mmol) in C6D6 (0.5 mL); 95% conversion to 7Sc-qn-iqn was
observed within 20 min.
The compound 6Sc-pyPh-iqn (30 mg, 0.039 mmol) was combined
with DBU (1.2 mg, 0.0078 mmol, 20 mol %) in C6D6; 75%
conversion to 7Sc-pyPh-iqn was observed after 8 h. Addition of
excess DBU did not change the final ratio of 6Sc-pyPh-iqn to 7Sc-
pyPh-iqn.
(c), 119.6 (o), 116.4 (j), 97.8 (fc-C-N), 69.3 (fc-C-H), 68.7 (fc-C-H),
68.3 (fc-C-H), 67.8 (fc-C-H), 38.5 (a), 27.4 (SiC(CH3)3), 25.4 (i), 19.9
(SiC(CH3)3), 19.2, (k), -3.4 (SiCH3), -3.8 (SiCH3). λmax/nm (ε/M-1
cm-1) ) 250 (37 200), 292 (28 500), 758 (790), 832 (1210), 938
(1190). Anal. Calcd for C42H55FeN4ScSi2: C, 65.26; H, 7.17; N, 7.24.
Found: C, 64.97; H, 7.22; N, 7.17.
Deuterium Labeling Experiment. The compound 5Sc-qn (0.1000
g, 0.1422 mmol) was combined with 1 equiv of deuterated
3-methylisoquinoline (0.0207 g, 0.1422 mmol) in C6D6 (1.5 mL)
and allowed to stand overnight at room temperature. The next day,
the C6D6 solution was divided into equal portions, the solvent was
removed from one of them, and the obtained solid was dissolved
in C6H6. The two reactions were heated simultaneously at 70 °C
and monitored by 1H and 2H NMR spectroscopy, respectively. After
6 days, the solvents were switched, and the complementary NMR
spectra were collected.
Synthesis of 7Sc-pyPh-iqn. The compound 5Sc-pyPh (0.2000 g,
0.281 mmol) was combined with 1.1 equiv of 3-methylisoquino-
line (0.0441 mg, 0.308 mmol) in toluene (10 mL) and stirred at
70 °C for 10 days. After 12 h the solution turned dark green.
The solvent was removed, and the resulting green solid was
extracted in hexanes. Monitoring the reaction in a J-Young tube
by 1H NMR spectroscopy revealed that too much decomposition
of 7Sc-pyPh-iqn occurred by the time all of the 6Sc-pyPh-iqn was
converted. Therefore, the reaction was stopped when the ratio
of 7Sc-pyPh-iqn:6Sc-pyPh-iqn was ca. 5:1. Yield: 0.0959 g, 43.6%
as crystals from hexanes.
Crossover Experiment. A mixture of 6Sc-pyPh-iqn (0.0250 g,
0.032 mmol) and 6Sc-qn-iqn -d4 (0.0250 g, 0.032 mmol) was
dissolved in C6D6 (1 mL); a second portion of the same mixture
was dissolved in C6H6. These two samples were heated to 70 °C
(eq 1) while monitoring by 1H and 2H NMR spectroscopy,
respectively. No deuterium incorporation into 7Sc-pyPh-iqn was
observed over the course of the isomerization of 6Sc-qn-iqn-d4 to
7Sc-qn-iqn-d4.
1H NMR (600 MHz, C6D6), δ (ppm): 8.06 (d, 1H, aryl), 7.71
(d, 2H, aryl), 7.40 (d, 1H, aryl), 7.25-7.16 (m, 2H, aryl), 7.10-6.92
(m, 4H, aryl), 6.65 (d, d 1H, aryl), 5.68 (d, 1H, aryl), 4.08 (s, 2H,
fc-CH), 4.00 (s, 2H, fc-CH), 3.20 (s, 2H, CH2), 3.13 (s, 2H, fc-
CH), 2.81 (s, 2H, fc-CH), 1.88 (s, 3H, C-CH3), 0.83 (s, 18H,
SiC(CH3)3), 0.24 and 0.14 (s, 6H, SiCH3). 13C NMR (151 mHz,
C6D6), δ (ppm): 155.7, 153.8, 142.0, 135.9, 132.9, 131.4, 127.2,
127.3, 126.9,125.6, 123.9, 123.7, 118.5, 117.8, 112.4, 106.2, 98.8,
69.5, 68.8, 68.5, 67.6, 38.5, 27.6, 25.2, 20.1, -3.1, -3.9. λmax/nm
(ε/M-1 cm-1) ) 280 (10 100), 342 (5060), 412 (9910), 458 (5960),
600 (3880). Anal. Calcd for C43H55FeN4ScSi2: C, 65.80; H, 7.06;
N, 7.14. Found: C, 65.57; H, 7.28; N, 7.26.
X-ray Crystal Structure of 6Sc-pyPh-iqn. X-ray-quality crystals
were obtained from a concentrated toluene:pentane solution placed
in a -35 °C freezer in the glovebox. A total of 11 280 reflections
(-21 e h e 21, -13 e k e 13, -36 e l e 36) were collected at
T ) 100(2) K with 2θmax ) 56.57°, of which 6454 were unique
(Rint ) 0.1123). The residual peak and hole electron density were
0.44 and -0.43 e A-3, respectively. The least-squares refinement
converged normally with residuals of R1 ) 0.0544 and GOF )
0.992. One molecule of toluene solvent was found in the unit cell.
Crystal and refinement data for 3Sc-pyPh-iqn: formula
C50H63N4Si2FeSc, space group P2(1)/c, a ) 16.1271(19), b )
10.5039(13), and c ) 27.189(3) Å, ꢀ ) 95.680(1)°, V ) 4583.2(10)
Å3, Z ) 4, µ ) 0.555 mm-1, F(000) ) 1864, R1 ) 0.1187 and wR2
) 0.1237 (based on all 11280 data, I > 2σ(I)).
Synthesis of 7Sc-qn-pic. The compound 5Sc-qn (0.2000 g, 0.285
mmol) was combined with 1.1 equiv of 2-picoline (0.0292 mg,
0.313 mmol) in toluene (10 mL). The reaction was heated at 70
°C for 4 days. The solvent was removed, and the resulting brown
solid was extracted in hexanes. Yield: 0.1523 g, 74.0%. Attempts
to isolate the 6Sc-qn-pic product were unsuccessful because the
coupling reaction is competitive with the isomerization one.
X-ray Crystal Structure of 7Sc-pyPh-iqn. X-ray-quality crystals
were obtained from a concentrated Et2O:pentane solution placed
in a -35 °C freezer in the glovebox. A total of 9528 reflections
(-22 e h e 22, -18 e k e 18, -24 e l e 24) were collected at
T ) 100(2) K with 2θmax ) 56.07°, of which 6056 were unique
(Rint ) 0.0828). The residual peak and hole electron density were
0.42 and -0.45 e A-3. The least-squares refinement converged
normally with residuals of R1 ) 0.0447 and GOF ) 1.012. Crystal
and refinement data for 7Sc-pyPh-iqn: formula C43H55N4Si2FeSc,
space group P2(1)/c, a ) 16.782(3), b ) 13.647(2), and c )
18.368(3) Å, ꢀ ) 109.108(2)°, V ) 3975.0(11) Å3, Z ) 4, µ )
0.632 mm-1, F(000) ) 1664, R1 ) 0.0902 and wR2 ) 0.1197 (based
on all 9528 data, I > 2σ(I)).
Computational Details. Calculations were performed using
density functional theory (DFT). For the mechanistic investigations,
the B3LYP functional (as implemented in Jaguar 7.5, release 207)
with the 6-311++G** basis set51 for the main group atoms was
employed. Small core effective potentials (angular momentum
1H NMR (500 MHz, C6D6), δ (ppm): 7.33 (d, 1H, h), 6.93 (m,
2H, two of i, k, l, or m), 6.91 (m, 1H, c), 6.81 (tr, 1H, i, k, l, or m),
6.33 (d, 1H, i, k, l, or m), 5.50 (tr, 1H, b), 4.23 (s, 2H, fc-CH),
4.05 (s, 4H, fc-CH and a), 3.30 (s, 4H, fc-CH), 2.75 (s, 3H, e),
2.36 (s, 3H, o), 0.83 (s, 18H, SiC(CH3)3), -0.02 and -0.11 (s,
6H, SiCH3). 13C NMR (126 MHz, C6D6), δ (ppm): 159.4 (g), 154.4
(n), 148.6 (d), 144.0 (p or j), 138.5 (i, k, l, or m), 123.3 (p or j),
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A R T I C L E S
Carver et al.
projected pseudopotentials) were used for Fe and Sc.52-54 The basis
set for Fe and Sc was LACV3P++**(2f), which includes a
double-ꢁ f-shell into the LACV3P++** basis set.55
All structures were optimized in the gas phase (starting with
geometries obtained from the X-ray crystal structures) using the
LACVP** and 6-31G** basis sets.52,56 For stationary points and
transition structures, the analytic Hessian was calculated to obtain
vibrational frequencies, which in turn were used to obtain zero-
point and vibrational thermodynamic corrections (ZPE, H(T), and
S) to 343 K.
The calculations using the Amsterdam Density Functional (ADF)
package (version ADF2008.01)59-61 used the BLYP functional for
full geometry optimizations and the B3LYP functional for single-
point calculations on the geometries calculated with the BLYP
functional. For all atoms, standard triple-ꢁ STA basis sets from
the ADF database ZORA/TZP were employed.
Acknowledgment. P.L.D. thanks Prof. Jennifer A. Love,
University of British Columbia, for a useful suggestion, Dr. Saeed
Khan, UCLA, for help with some crystallography details, and Dr.
Robert Taylor for help with NMR experiments. D.B. and E.T. thank
Dr. Robert J. Nielsen for helpful suggestions. The experimental
work was supported by UCLA. The MSC computational facilities
were funded by grants from ARO-DURIP and ONR-DURIP.
Solvent corrections were based on single-point self-consistent
Poisson-Boltzmann continuum solvation calculations for benzene
(ε ) 2.284 and R0 ) 2.60 Å) using the PBF module in Jaguar.57,58
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Supporting Information Available: Experimental details for
compound characterizations, DFT calculation details, and full
crystallographic descriptions (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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