Synthesis and solid-state characterization of self-assembled macrocyclic molecular rotors of bis(dithiocarbamate) ligands with diorganotin(IV)

Article abstract of DOI:10.1021/om401094d

Two bis(dithiocarbamate) (bdtc) metallamacrocyclic compounds, 1 and 2, and the deuterated analogues 1-d₈ and 1-d₂₀ were readily prepared through self-assembly processes involving the corresponding secondary bis(diamines), with two equivalents each of CS₂ and dimethyltin(IV) dichloride. Solid-state characterization using FTIR, PXRD, and TGA indicated that the solid phases of both macrocycles were amorphous solids. For compound 1, a crystalline phase could only be obtained in the form of a dichloromethane solvate; however, the corresponding crystal lattice was unstable and collapsed rapidly under ambient conditions. The bdtc ligands containing para-disubstituted phenylene (compound 1) and bicyclo[2.2.2]octane groups (compound 2) showed rotational motion within the macrocyclic assemblies in the solid state. For compound 1, the internal rotation of the phenylene groups was examined first by ¹³C NMR CPMAS spectroscopy using the 1-d₂₀ derivative in which the hydrogen atoms of the pendant phenyl groups had been substituted with deuterium atoms and also by ²H NMR spin echo experiments using the 1-d₈ derivative in which the rotating phenylene groups have been deuterated. Line shape analysis using a log-Gaussian distribution model indicated that the central phenylene rings experience fast 2-fold flip reorientations over the sp²-sp³ carbon atom axes, overcoming an activation energy of E_a = 10 kcal/mol with a preexponential factor A = 3.9 × 10¹⁴ s^-1. For compound 2, the ¹³C CPMAS experiments suggested that the bicyclo[2.2.2]octane moieties also undergo fast internal dynamics, which is in agreement with the higher symmetry of these fragments when compared to the phenylene spacers.

Full text of DOI:10.1021/om401094d

Article
pubs.acs.org/Organometallics
Synthesis and Solid-State Characterization of Self-Assembled
Macrocyclic Molecular Rotors of Bis(dithiocarbamate) Ligands with
Diorganotin(IV)
Aaron Torres-Huerta,†,‡ Braulio Rodríguez-Molina,‡ Herbert Hopfl,*,† and Miguel A. Garcia-Garibay*,‡
́
̈
†Centro de Investigaciones Químicas, Universidad Auton
Cuernavaca, Mexico
́
oma del Estado de Morelos, Avenida Universidad 1001, C.P. 62209,
́
‡Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States
S
* Supporting Information
ABSTRACT: Two bis(dithiocarbamate) (bdtc) metallamacrocy-
clic compounds, 1 and 2, and the deuterated analogues 1-d8 and 1-
d20 were readily prepared through self-assembly processes
involving the corresponding secondary bis(diamines), with two
equivalents each of CS2 and dimethyltin(IV) dichloride. Solid-state
characterization using FTIR, PXRD, and TGA indicated that the
solid phases of both macrocycles were amorphous solids. For
compound 1, a crystalline phase could only be obtained in the
form of a dichloromethane solvate; however, the corresponding
crystal lattice was unstable and collapsed rapidly under ambient
conditions. The bdtc ligands containing para-disubstituted phenyl-
ene (compound 1) and bicyclo[2.2.2]octane groups (compound
2) showed rotational motion within the macrocyclic assemblies in
the solid state. For compound 1, the internal rotation of the
phenylene groups was examined first by 13C NMR CPMAS spectroscopy using the 1-d20 derivative in which the hydrogen atoms
of the pendant phenyl groups had been substituted with deuterium atoms and also by 2H NMR spin echo experiments using the
1-d8 derivative in which the rotating phenylene groups have been deuterated. Line shape analysis using a log-Gaussian
distribution model indicated that the central phenylene rings experience fast 2-fold flip reorientations over the sp2−sp3 carbon
atom axes, overcoming an activation energy of Ea = 10 kcal/mol with a preexponential factor A = 3.9 × 1014 s−1. For compound
2, the 13C CPMAS experiments suggested that the bicyclo[2.2.2]octane moieties also undergo fast internal dynamics, which is in
agreement with the higher symmetry of these fragments when compared to the phenylene spacers.
Molecular rotation in the solid state can be described as a
Brownian process where the rotator undergoes random changes
in orientation between crystallographically defined energy
minima. Previous studies on molecular rotors have shown
that site exchange in the solid state may be readily measured
using various NMR methods when it occurs with frequencies
spanning from the kHz (103 s−1) to MHz (106 s−1) and GHz
(109 s−1) regimes.4 These values depend on the nature of the
stator, the axial symmetry order of the rotator, and the free
volume available in the crystal.4a,6,7 For the particular case of
rotors containing phenylene and bicyclo[2.2.2]octane moieties,
which are also employed herein, their rotation within
triphenylsilyl stators occurs in the kHz and MHz regimes,
respectively, at ambient temperature.7 Although the field of
molecular machinery has evolved significantly during the past
few years, there are still many challenges that need to be solved,
including the synthesis of molecular rotors featuring two or
INTRODUCTION
■
Artificial molecular machines can be defined as structurally
engineered chemical structures with multiple components
hierarchically assembled to perform integrated functions,
seeking to imitate the movement and functions of biomolecular
machines.1 Due to the challenge that represents the
construction of such large biological structures; current
approaches are based on the synthesis of compounds that
emulate the shape and function of simpler macroscopic
devices.2 In this field, the synthesis of molecular rotors
designed to show internal rotation in the solid state has
received considerable attention due to the potential techno-
logical applications based on controllable dynamics.3
The conjunction of rigid and mobile parts within the same
molecule that are capable of forming an ordered array and act
in a collective manner gives rise to materials that we have
termed amphidynamic crystals.4 Among them, we have focused
our efforts on the synthesis of molecular rotors with a rotating
component, or rotator, linked by means of a suitable axle to a
crystal-forming bulky framework known as the stator.5
Received: November 11, 2013
Published: December 18, 2013
© 2013 American Chemical Society
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more rotating units.8 This subject is particularly interesting,
because it offers a platform to explore possible rotator−rotator
interactions where the corresponding units can rotate
independently, in a random fashion, or are correlated in a
disrotatory manner when their motion is determined by strong
dipole−dipole interactions or by steric gearing.9
The inclusion of several rotating units within the same
structure may be achieved via catenation of the rotating units
into oligomeric or polymeric molecular chains or by
construction of discrete macrocycles. Of these, the latter
approach has been scarcely explored since their synthesis
requires high dilution conditions and often occurs with low
yields. In order to circumvent this limitation, we envision the
use of self-assembly processes as an efficient route to the
synthesis of multiple rotors (Figure 1a). One of the options in
aromatic hydrogen atoms of the benzyl amine were replaced by
deuterium. We also synthesized the isotopologue 1-d8, where
the protons of the phenylene rotators rings were replaced by
deuterons, thus enabling the analysis of the internal rotation
2
using line shape analysis of variable-temperature H NMR spin
echo experiments. Considering that the shape of the rotators in
close-packed crystals will be matched as close as possible by the
surfaces of next-neighboring molecules, the use of rotators that
are more cylindrical, with a higher symmetry order, gives rise to
a more symmetric environment around the mobile component7
that is expected to have a smaller rotational barrier. To test this
idea, we also analyzed compound 2, which features a C3 axially
symmetric bicyclo[2.2.2]octane rotator, which, with other
things being equal, would be expected to have a lower
activation energy and rotate faster than the phenylene ring with
a lower C2 axial symmetry.
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization. The
synthesis of macrocyclic rotor 1 is illustrated in Scheme 1. Self-
assembling reactions were carried out through one-pot
syntheses, in which first the corresponding secondary amine
was treated in ethanol with KOH and an excess of carbon
disulfide to generate a reactive dianionic intermediate (Scheme
1). After addition of dimethyltin dichloride dissolved in EtOH,
the macrocyclic compounds precipitated immediately. In this
way, fresh samples of compound 1 were prepared in good
yields (87%) starting from compound 4, which was synthesized
by refluxing p-xylylenediamine and benzaldehyde 3 in EtOH
and NaBH4. The deuterated isotopologue 1-d20 was synthe-
sized in 87% yield, following the above-stated procedure,
starting from deuterated benzaldehyde.18
Figure 1. (a) Schematic representation showing the composition of a
macrocyclic molecular dirotor formed by a metallosupramolecular self-
assembly process: two independent molecular rotators (in red) are
linked through benzylamine (blue) and CS2 (yellow) that formed
dithiocarbamates with Sn(IV) metal centers (cyan). (b) Line diagram
of the macrocyclic complexes 1 and 2, which have been prepared
through a self-assembling reaction between bis(dithiocarbamate)
ligands and dimethyltin(IV) chloride.
The second analogue, 1-d8, was synthesized similarly in 86%
yield starting from deuterated terephthaldehyde, which was
obtained in a stepwise manner. Reaction of 1,4-dibromoben-
zene-d4 with one equivalent of n-BuLi followed by reaction with
dimethylformamide gave 4-bromobenzaldehyde-d4 11, which
was successively protected as a ketal with ethylene glycol (see
Supporting Information). Lithiation of the resulting 2-(4-
bromophenyl)-1,3-dioxolane-d4 12 and its reaction with DMF
afforded terephthalaldehyde-d4 5, which after reaction with
benzylamine in EtOH and subsequent reduction with NaBH4
yielded the required diamine 4-d4 in 90% yield (Scheme 1).
For the synthesis of the macrocycle with a bicyclo[2.2.2]-
octane fragment as rotator, we prepared first N,N-
dibenzylbicyclo[2.2.2]octane-1,4-dicarboxamide (7) starting
from dimethylbicyclo[2.2.2]octane-1,4-dicarboxylate (6),19 as
shown in Scheme 2. Compound 7 was then reduced with
LiAlH4 to afford compound 8 in 96% over two steps. However,
compound 8 did not react with KOH/carbon disulfide and
dimethyltin(IV) dichloride. To evaluate whether the steric
hindrance prevented the introduction of the dithiocarbamate
fragment, we tried unsuccessfully to isolate the intermediate
potassium dithiocarbamate salt using different bases, solvents,
and reaction times. At this stage, considering that the size of the
aromatic rings in the diamine 8 prevented the formation of the
bdtc, we converted compound 6 to the aliphatic amide 9 in
73% yield and subsequently reduced it with LiAlH4 to give
compound 10 in good yield (95%). With the less voluminous
n-butyl group, compound 10 could be transformed to
compound 2, although in a relatively low yield (35%).
this direction consists in the preparation of metallamacrocycles
via self-assembly between metal ions and organic ligands. Self-
assembling reactions are thermodynamically controlled and
reversible, thus allowing for the generation of a single product
in good yields.10 This approach has been successfully employed
in the synthesis of metallosupramolecular compounds with
large cavities having applications in host−guest chemistry,11
catalysis,12 biomedicine,13 and ion sensing.14
We report herein on the synthesis and characterization of
two metallamacrocyclic complexes, 1 and 2, and their
evaluation as molecular dirotors by using solid-state NMR.
The dinuclear complexes shown in Figure 1b were obtained by
self-assembling reactions using two different bis-
(dithiocarbamate) ligands (bdtc) and dimethyltin(IV) chloride.
The bdtc binder and tin(IV) metal centers have been chosen
due to the structural variety of macrocyclic and cage-like
compounds that can be prepared from this type of building
blocks.15 The ligands used for the preparation of 1 and 2
contain 1,4-disubstituted phenylene and bicyclo[2.2.2]octane
(BCO) spacers as potential solid-state rotators. Compound 1
was reported previously and prepared as described.16
Considering that a large number of aromatic hydrogen atoms
from the benzyl groups in the stator may interfere with 13C
NMR CPMAS spectroscopic analyses of the site exchange
process experienced by the aromatic phenylene rotator,17 we
synthesized also the deuterated analogue 1-d20, in which all
The FT-IR spectra of all tin(IV) complexes showed two
bands characteristic of the dithiocarbamate group in the regions
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a
Scheme 1. Synthesis of Macrocyclic Dirotor 1 and Its Deuterated Analogues 1-d20 and 1-d8
a
Reagents and conditions: (a) i. p-xylylenediamine/EtOH, reflux 12 h; ii. NaBH4/EtOH, rt 12 h. (b) i. KOH, CS2/EtOH, rt 2 h; ii. (CH3)2SnCl2/
EtOH, rt 12 h. (c) i. Benzylamine/EtOH, reflux 12 h, ii. NaBH4/EtOH, rt 12 h. The yields of each step are included within the scheme.
1475−1464 and 989−968 cm−1, which correspond to the
vibration of the N−CS2 and CS2 units, respectively (Table 1).
shift of the corresponding signals has been reported between
−310 and −340 ppm when R is aliphatic and between −500
and −520 ppm when R is aromatic. Conversely, in
pentacoordinated compounds, with the formula R2Sn(dtc)Cl,
the chemical shifts are reported between −80 and −100 ppm
when R is aliphatic and −300 to −320 ppm when R is
aromatic.21
Further evidence of the formation of the desired compounds
was provided by mass spectrometry, where compounds 1-d8, 1-
d20, and 2 gave peaks corresponding to the expected weight of
the [M + H]+ species: 1239.0980, 1251.1764, and 1159.2277,
respectively. Analysis of the isotopic patterns showed excellent
agreement with the simulated spectra, thus adding evidence to
the formation of dinuclear macrocyclic structures. This was also
confirmed by 2D DOSY NMR experiments in solution for
compounds 1, 1-d8, and 1-d20. In all cases, only one signal was
observed with a logarithmic diffusion coefficient of −9.15 for
1
In the H NMR spectra, the signals corresponding to the two
NCH2 methylene groups appeared at ca. δ 5.07 for complexes
1, 1-d20, and 1-d8 and at δ 3.65 and 3.77 for macrocycle 2.
Their corresponding 13C carbon signals gave chemical shifts in
the range δ 53.3 to 55.8 for macrocycles 1, 1-d8, and 1-d20 and
δ 56.1 and δ 62.8 for compound 2. The carbon signals for the
dithiocarbamate group were observed in the region δ 201.7−
202.7, which is typical for the bdtc groups coordinated to
R2Sn(IV) moieties.20 Complementarily, 119Sn NMR confirmed
the presence of the metal nodes in the macrocyclic compounds
with single signals at ca. δ −337 for compound 1 and its
analogues and at δ −346 for compound 2. It is important to
note that 119Sn NMR can be used as additional evidence of the
coordination geometry around the metal. In the case of
hexacoordinated tin, with the formula R2Sn(dtc)2, the chemical
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b
Scheme 2. Synthesis of Macrocyclic Dirotor 2
b
Reagents and conditions: (a) Me3Al, BnNH2, toluene, 0 °C. (b) LiAlH4, THF, reflux. (c) Me3Al, BnNH2, toluene, 0 °C. (d) CS2, KOH, EtOH, rt 2
h, then Me2SnCl2, EtOH, rt, 12 h.
Table 1. Selected Spectroscopic Data for Compounds 1, 1-d20, 1-d8, and 2 (IR, cm−1; NMR in CDCl3, ppm)
NCH2
−CH2NCH2−
13C
CS2
N−CS2
1
compound
δ
119Sn
δ H
δ
δ
13C
ν (cm−1
)
ν (cm−1
)
1
−337.3
−337.3
−337.1
−346.6
5.07
55.8, 55.3
53.7, 53.3
53.7, 53.3
62.8, 56.1
202.7
202.7
202.7
201.5
973
969
968
989
1466
1464
1466
1475
1-d20
1-d8
2
5.08
5.06
3.77, 3.65
compounds 1 and 1-d20 and −9.35 for compound 1-d8. The
rather narrow signal observed in the 2D DOSY along with the
mass spectrometry experiments ruled out the existence of other
oligocyclic, linear oligomeric, or polymeric structures.
Solid-State Characterization. Single-Crystal and Powder
X-ray Diffraction Analysis. The molecular structure of
compound 1 had been established previously by single-crystal
X-ray diffraction at 293 K.16 Compound 1 crystallizes in the
space group P1 with dichloromethane molecules in the lattice.
̅
Preliminary analysis of the molecular structure showed a
centroid···centroid distance of 9.1 Å between opposite
phenylene rings that would allow the rotation in an isolated
molecule without close contacts between rotating entities
(Figure 2a). A closer inspection of the structure revealed that
the sulfur atoms and one of the Sn-Me groups generate a
relatively crowded environment around the phenylene rotator
(highlighted in red, Figure 2b), which may prevent its rotation
to a large extent since angular displacements of the ring must
overcome the restraint of the stator (in blue). Additionally,
potential rotation of the phenylene ring would take place over a
sp2−sp3 axis, which is expected to impose some hindrance
between the NCH2 methylenes and ortho-C6H4 hydrogen
atoms. Despite these structural restraints, the solid-state NMR
experiments demonstrated that the phenylene group can rotate
in the MHz regime, as discussed in the following sections.
Initial characterization of different batches of compound 1-
d20 showed some inconsistencies regarding the melting point:
freshly grown crystals obtained from dichloromethane solution
Figure 2. (a) Molecular structure of compound 1 obtained at 100(2)
K. Thermal ellipsoids are drawn at the 50% probability level. (b)
Space-filling diagram of the macrocyclic dirotor 1, showing a crowded
environment between the central phenylene in red and the stator in
blue. Disordered dichloromethane molecules were removed for clarity.
melted at 240 °C, while solid samples showed no clear melting
point after being exposed to air several minutes and displayed
only a small shrinking between 132 and 134 °C. The latter
value agrees with the melting point reported for the deuterium-
free analogue 1 (128−130 °C).16 To find the source of these
discrepancies, we collected X-ray data for 1-d20 at 100(2) K,
taking freshly grown crystals from a dichloromethane solution.
In this single-crystal X-ray analysis, we were able to corroborate
that the dinuclear macrocycle crystallizes in parallel layers with
two disordered dichloromethane molecules next to the dtc
moiety within the crystal lattice (Figures S1−S3, Supporting
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Information). A comparison between the two collected
structures did not show significant changes in the molecular
structure, crystallographic parameters, or packing structure, as
shown in the Supporting Information (Tables S1, S2).
Simultaneously, powder X-ray diffraction experiments con-
firmed that crystalline macrocycle 1 and its deuterated
analogues become amorphous within minutes upon air
exposure at room temperature (Supporting Information Figure
S5, b). This can be attributed to the evaporation of the
disordered dichloromethane molecules, which are expected to
be only loosely bound.22
The irreversible desolvation process prompted us to screen
for a more stable crystalline form of 1; however, recrystalliza-
tion using pure solvents or mixtures gave only solids that were
amorphous or lose crystallinity rapidly. Regarding compound 2
containing the bicyclo[2.2.2]octane rotator, the low solubility
of the freshly synthesized compound limited the crystallization
attempts, which were carried out in dichloromethane, chloro-
form, carbon tetrachloride, benzene, acetonitrile, or DMF.
Unfortunately, we could not obtain single crystals or crystalline
solids of 2 from the solvents employed, as evidenced by optical
microscopy and PXRD (Supporting Information Figure S5, c).
Thermal Stability. The thermal stability of macrocycles 1, 1-
d20, and 1-d8 was examined also by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) under
argon flow. DSC experiments from the desolvated samples only
revealed changes just before 300 °C attributed to the
decomposition of the solids (see Supporting Information for
the DSC trace of compound 1). By using TGA, it was shown
that the amorphous samples display no additional loss of weight
after the initial desolvation, with a decomposition process that
initiates above 250 °C, as indicated by an abrupt change in the
trace. The shrinking observed at 132−134 °C in the visual
determination of the melting point was then correlated to the
softening of the solid. Amorphous samples of compound 2
presented a similar behavior to that of compound 1, with
decomposition after 250 °C.
Figure 3. 13C NMR CPMAS spectra of compound 1-d20. (a)
Experiment acquired at 295 K with a contact time of 5 ms. (b) NQS
experiment acquired at 295 K showing only quaternary and highly
mobile carbon atoms. (c) Experiment with a contact time of 0.1 ms at
295 K showing only protonated carbon atoms. Note: The signal
corresponding to the central phenylene (C4) appears in all the
experiments.
the aromatic C−H signal at ca. 128 ppm assigned to phenylene
C4 remains even after a long dephasing time (60 μs),
suggesting that the phenylene ring undergoes an internal
motion with a frequency higher than that of the heteronuclear
1H−13C coupling (Figure 3b). The assignment of the C4 signal,
which arises from a protonated aromatic carbon, was made
possible also by a spectrum acquired with a very short cross-
polarization time (0.1 ms), as depicted in Figure 3c. While
short contact times are able to transfer magnetization to
protonated carbon atoms, they do not allow for signals
corresponding to quaternary carbons (C1, C3, C6) to be
observed.
Solid-State Cross-Polarization Magic Angle Spinning
(CPMAS) 13C NMR of 1-d20. Having established the amorphous
nature of samples of macrocycles 1 and 2, we carried out a
series of 13C NMR CPMAS experiments to obtain information
about the dynamic behavior of their rotators in the solid state.
With 13C signals arising from magnetization originally
1
Low-temperature 13C NMR CPMAS experiments were also
carried out to explore the possibility of observing the splitting
of the rotator signals due to a potential slowing-down process
between two magnetically nonequivalent sites.17 However, the
spectrum obtained at 155 K, the lowest temperature achievable
in our spectrometer, showed no changes with respect to the
one obtained at 295 K. This result indicates that the broad
linewidth of the rotator signal and the limited signal dispersion
in our magnetic field make it unfavorable to determine a more
narrow range of rotational dynamics for compound 1.
established in the H nuclei, we were able to analyze signals
assigned to the phenylene rotator in the case of compound 1-
d20. In the spectrum, the signals corresponding to the
macrocyclic structure of 1-d20 have a chemical shift comparable
to that found in solution NMR, but with a broad linewidth due
to the amorphous nature of the sample (Figure 3a). Signals at
ca. 55 ppm correspond to the methylenes attached to the
nitrogen atom, and the small signal at ca. 16 ppm corresponds
to the methyl groups. The dithiocarbamate carbon appears at
ca. 201 ppm, and the two signals observed at ca. 128 and 134
ppm correspond to the protonated and quaternary aromatic
carbons, respectively. The assignment of these two signals was
also possible by using the nonquaternary suppression (NQS)
experiment and by using a very short contact time (Figure 3b
and c, respectively). While the NQS sequence is generally used
to detect quaternary carbons by suppressing signals of C−H
and CH2 groups, it also helps identify protonated carbons that
experience motion in a range that is similar or greater than the
Solid-State 13C NMR CPMAS of 2. Experiments carried out
in compound 2 showed similar results to those observed with 1.
The normal 13C NMR CPMAS spectrum in Figure 4a consists
of the dithiocarbamate signal by 200 ppm and a series of
aliphatic signals between 65 and 10 ppm that can be assigned
by analogy to the solution spectra. Several signals remained
after a long dephasing delay in the NQS spectrum, as shown in
Figure 5b. These included signals from the quaternary carbon
atoms and the mobile methyl groups C8 and C9 (at ca. δ 13.9)
and a signal assigned to C4 (−CH2−) from the bicyclo[2.2.2]-
1
magnitude of the H−13C dipolar coupling, typically in the
range of 25−30 kHz. Notably, NQS experiments showed that
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powder sample gives rise to a broad symmetric spectrum with
two maxima and two shoulders known as a Pake pattern.
Variations in the powder pattern of a solid sample occur when
the C−2H bonds experience reorientations between different
sites with exchange rates in a range of ca. 103−108 s−1.24 Line
shape analysis of the deuterium NMR spectra allows for a
dynamic characterization in terms of the frequency and type of
motion that consist of discrete jumps between different sites or
in terms of a diffusive process with no rotational barrier.25
Figure 4. 13C NMR CPMAS spectra of compound 2. (a) Contact time
of 5 ms at 295 K. (b) 13C NMR CPMAS NQS at 295 K, showing only
quaternary or highly mobile carbon atoms. (c) Optimized contact time
of 0.1 ms at 295 K, showing only the protonated carbon atoms. The
signal C4 remains constant.
octane rotator at δ 30.6. This observation is consistent with the
BCO experiencing a rotational dynamic process in the 25−30
1
Figure 5. Left: Variable-temperature 2H NMR quadrupolar echo
experiments for macrocyclic 1-d8 (T = 247−362 K). Right: Calculated
spectra based on 180° jumps of the phenylene-d4 group taking into
account a log-Gauss distribution of activation energies (σ = 2) with the
corresponding mean rotation frequencies.
kHz regime or faster. As described in 1, the use of short H to
13C polarization transfer made it possible to identify only
protonated carbon atoms in 2 (Figure 4c).
2
Rotational Dynamics by Line Shape Analysis of H NMR
Spin Echo. Encouraged by the 13C NMR CPMAS results, we
2
carried out variable-temperature H NMR spin echo experi-
2
ments with compound 1-d8 in order to analyze the rotational
dynamics of the H-labeled phenylene rotators. Quadrupolar
Variable-temperature H NMR analyses of compound 1-d8
2
were carried out from 247 to 395 K, using a 90° pulse of 2.5 μs
and a recycle delay of 20 s. The line shape obtained at 247 K
has features that suggest the coexistence of static deuterons
with others that experience rotational exchange in the kilohertz
range, as suggested by peaks at the extreme of the spectrum
separated by 126 kHz and peaks arising near the center with a
distance of 28 kHz. The line shape evolved as the temperature
was increased, changing at 362 K into a form analogous to the
one that results from 2-fold flips in the fast exchange regime
(Figure 5a). Given the amorphous nature of the sample, we
considered that a Gaussian distribution of activation energies
would describe the internal dynamics of 1-d8 in a reasonable
manner. The spectra simulated in this manner contain
contributions of a log-Gaussian distribution of slow and fast
components. We carried out the corresponding line shape
simulations using the program Express26 assuming a symmetric
rotational potential with two minima related by 180° jumps,
using a QCC = 180 kHz and an asymmetry parameter η = 0.03
with 6 kHz of line broadening. Two models with different
widths of the Gaussian distribution (σ = 2 and 3) were explored
in this manner. Those widths characterize either a considerable
(σ = 2) or severe (σ = 3) heterogeneity of activation energies. A
relatively good fit could be obtained for each model, with the
calculated line shapes obtained with the intermediate width
value (σ = 2) depicted in Figure 5b. An Arrhenius plot of the
mean rotational rates (ln krot) versus 1/T allowed us to extract
an activation energy Ea = 10 kcal mol−1 with a preexponential
echo 2H NMR spectroscopy can be used to determine internal
molecular dynamics by studying changes in the spectral line
shape of powdered samples as a function of temperature. The
method relies on the magnetic interactions that occur as a
result of a dynamic process involving changes in the orientation
of the C−2H bond vectors with respect to the orientation of the
external magnetic field. A single crystal with only one type of
C−2H bond would give a doublet with a quadrupolar splitting
Δν that depends on the orientation angle β that the bond
makes with respect to the external field,
3
Δν = (e2qzzQ /h)(3 cos2 β − 1)
4
3
4
=
QCC(3 cos2 β − 1)
(1)
where Q represents the electric quadrupole moment of the
deuterium, e and h are the electric charge and Planck constant,
and qzz is the magnitude of the principal component of the
electric field gradient tensor, which lies along the C−2H bond
(eq 1 assumes an axially symmetric electric field gradient (qxx =
qyy) with an asymmetry parameter η = 0). The frequency
difference (Δν) for signals of phenylene C−2H bonds with a
quadruple coupling constant (QCC) = 180 kHz23 changes
from 270 kHz when β = 0 to 135 kHz for β = 90° and is 0 kHz
when the C−2H bond is oriented at the magic angle of β =
54.74°. A collection of doublets in all possible orientations in a
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factor A = 3.9 × 1014 s−1 (Supporting Information). Compared
to more characteristic A values (10−12−1013 s−1), the high
preexponential factor obtained was attributed to the increasing
local fluidity in amorphous materials as they approach the glass
transition temperature. Conversely, the model considering a
broader distribution (σ = 3) rendered a higher activation energy
along with an unreasonable high A value. This alternative
model was included in the Supporting Information for
comparison purposes.
Compound 4-d10: 1,4-Bis(benzylaminemethyl)benzene-d10.
The product was obtained from reaction of p-xylylenediamine (306
mg, 2.25 mmol) with benzaldehyde-d5 (500 mg, 4.5 mmol) in a 2:1
ratio in refluxing ethanol for 12 h. Then, 90% of the solvent was
removed using a Dean−Stark trap. The reaction mixture was then
allowed to reach room temperature, and sodium borohydride dissolved
in 20 mL of ethanol was added. The mixture was stirred for 12 h to
afford the N,N-disubstituted 1,4-bis(benzylaminomethyl)benzene-d10.
To purify compound 4-d10, 0.1 M HCl (10 mL) was added to the
mixture dissolved in ether (20 mL), whereupon the salt formed was
extracted with water (30 mL). Subsequently, NaOH was added until a
pH = 14 was reached to regenerate the desired diamine 4-d10. The
final product was extracted with ether (30 mL). Yield: 609 mg (83%).
CONCLUSIONS
■
̃
IR: ν 2820 (m), 2273 (m), 1570 (w), 1512 (m), 1448 (s), 1418 (m),
We have readily synthesized macrocyclic complexes 1 and 2
(along with deuterated analogues 1-d20 and 1-d8) in good yields
through the self-assembly of dithiocarbamate ligands with
Sn(IV) for their evaluation as molecular rotors. We identified
that macrocycle 1 crystallizes as a solvate with two disordered
dichloromethane molecules as previously reported, but they
escape upon air exposure, yielding an amorphous solid. By
contrast, complex 2 was obtained only as an amorphous
powder from all crystallization attempts. Solid-state 13C NMR
was used to initially assess the internal dynamics in both
complexes. The 13C spectrum of amorphous isotopologue 1-d20
suggested that the phenylene ring undergoes rotations with
1356 (m), 1152 (m), 1098 (s), 1019 (m), 961 (w), 840 (s), 681 (s)
cm−1. 1H NMR (300 MHz, CDCl3) δ: 1.65 (s, 2H, N-H), 3.82 (s, 2H,
−CH2-N), 3.80 (s, 2H, −CH2-N), 7.31 (s, 4H, Haromatic). 13C NMR
(75 MHz, CDCl3) δ: 52.8 (−CH2-N), 128.1, 138.9, 140.1 (Caromatic).
High-resolution MS ESI(+)-TOF: calculated for C22H14D10N2 ([M +
H]+) 327.2645, found 327.2631, error 4.3 ppm.
Compound 1-d20: [{Me2Sn(1,4-Bn-ambdtc)}2]-d20. 1,4-Bis-
(benzylaminemethyl)benzene-d10 2 (0.100 g, 3.06 mmol), potassium
hydroxide (0.034 g, 6.13 mmol), and carbon disulfide (1 mL) were
dissolved in ethanol (20 mL) and stirred for 2 h. Then, dimethyltin
dichloride (0.067 g, 3.06 mmol) dissolved in 10 mL of ethanol was
added dropwise, and the mixture was stirred for 12 h. The solvent was
evaporated under vacuum, and the product was extracted with
dichloromethane; after evaporation a yellowish powder was obtained.
1
frequencies higher than the H−13C dipolar coupling (25−30
kHz). Similarly, the 13C spectrum of 2 indicated a fast rotating
̃
Yield: 167 mg (87%). IR: ν 1512 (w), 1464 (s), 1406 (s), 1355 (s),
bicyclo[2.2.2]octane moiety resulting from its higher symmetry
1314 (w), 1260 (w), 1203 (s), 1152 (s), 1052 (m), 969 (s), 907 (s),
842 (m), 783 (s), 727 (s) cm−1. 1H NMR (500 MHz, CDCl3) δ: 1.74
(s, 3H, Sn-CH3), 5.08 (s, 4H, −CH2-N), 7.29 (s, 2H, Haromatic). 13C
NMR (125 MHz, CDCl3) δ: 16.0 (Sn-CH3) 53.3, 53.7 (−CH2-N)
128.2 (Cipso), 134.7, 134.8 (Cm,p), 202.7 (Cdtc). 119Sn NMR (186
MHz, CDCl3) δ: −337.3. High-resolution MS ESI(+)-TOF: calculated
for C52H40D20N4S8Sn2 ([M + H]+) 1251.1711, found 1251.1764, error
4.23 ppm.
2
as compared to flat aromatic rings. Variable-temperature H
NMR experiments of compound 1-d8 confirmed the high-
rotation frequency of the phenylene ring, and line shape
simulations were successfully performed by considering a
Gaussian distribution of activation energies with random
rotation trajectories of 180° jumps. From our analysis, we
conclude that the aromatic rotators in 1 overcome an Ea = 10
kcal mol−1, which is relatively low considering the structural
restraints imposed by the macrocycle. This Ea value is similar to
those found for this moiety in other molecular rotors with
higher crystallinity. The self-assembly approach proved to be
useful for the rapid construction of molecular rotors with two
rotary components. Further optimization based on the
screening of different ligands to yield highly crystalline
materials is under way.
Compound 11: 4-Bromobenzaldehyde-d4. 1,4-Dibromoben-
zene-d4 (2.000g, 8.33 mmol) was dissolved in dry THF at −78 °C
under an argon atmosphere. Then, n-BuLi 1.6 M in hexanes (6.25 mL,
0.01 mol) was added dropwise, and the mixture was stirred for 2 h at
−78 °C. Afterward, dry DMF (1.29 mL, 16 mmol) was added
dropwise, and the mixture was stirred at room temperature for 12 h.
The mixture was treated with 0.5 M HCl (10 mL) and diluted with
brine (10 mL) for 30 min. After evaporation of the solvent under
vacuum, the product was extracted with ethyl ether. Yield: 1.073 g
̃
(68%). IR: ν 2959 (m), 2872 (w), 1701 (s), 1686 (s), 1530 (s), 1465
(w), 1310 (m), 1287 (w), 1175 (m), 1156 (s), 1015 (s), 871 (w), 850
(w), 817 (w), 782 (s), 684 (w) cm−1. 1H NMR (500 MHz, CDCl3) δ:
9.97 (s, 1H). 13C NMR (125 MHz, CDCl3) δ: 129.4, 134.8 (Cipso),
130.1, 131.7 (t, JCD = 24.9 Hz), 191.1 (CO). High-resolution MS
LIFDI(+)-TOF: calculated for C7H2D4OBr ([M + H]+) 187.9769,
found 187.9774, error: 2.5 ppm.
Compound 12: 2-(4-Bromophenyl)-1,3-dioxolane-d4. A mix-
ture of 4-Dibromobenzaldehyde-d4 (1.050 g, 5.67 mmol), excess
ethylene glycol (5 mL), and p-toluenesulfonic acid (9.7 mg, 0.057
mmol) in dry toluene (30 mL) was refluxed 12 h. The mixture was
allowed to reach room temperature, diluted with 20 mL of brine, and
EXPERIMENTAL SECTION
■
Dimethyltin(IV) chloride, terephthalaldehyde, benzaldehyde, benzyl-
amine, p-xylylenediamine 1,4-dibromobenzene-d4, butylamine, NaBH4,
and LiAlH4 are commercially available and were used without further
purification. Benzaldehyde-d5 was prepared using a reported method-
ology.27
Compound 1: [{Me2Sn(1,4-Bn-ambdtc)}2]. The synthesis of
compound 1 was accomplished as previously described.16 Spectro-
scopic data are reported here for comparison purposes with the
deuterated complexes, particularly the 13C NMR data. Yield: 87%. IR
̃
̃
extracted with 30 mL of ethyl ether. Yield: 1.220 g (94%). IR (KBr): ν
̃
(amorphous system): ν
3023 (w), 2919 (w), 1603 (w), 1512 (w),
2968 (w), 2884 (m), 1569 (m), 1474 (w), 1415 (w), 1376 (s), 1304
(w), 1280 (w), 1179 (s), 1076 (s), 1013 (s), 964 (s), 941 (s), 818 (w),
697 (w), cm-1. 1H NMR (500 MHz, CDCl3) δ: 4.11 (m, 4H,
−CH2−), 5.77 (s, 1H, C−O). 13C NMR (125 MHz, CDCl3) δ: 65.2
1466 (s), 1450 (s) 1405 (s), 1351 (m), 1300 (w), 1268 (w), 1213 (s),
1145 (s), 1111 (m), 1077 (m), 1044 (w), 973 (s), 933 (m), 887 (w),
785 (s), 730 (s), 695 (s). Single crystal 3027 (w), 2916 (w), 1603 (w),
1514 (w), 1492 (w), 1467 (s), 1450 (m), 1406 (s), 1353 (m), 1310
(w), 1265 (m), 1214 (s), 1193 (m), 1149 (s), 1111 (w), 1078 (w),
1008 (s), 977 (s), 949 (m), 913 (m), 889 (m), 782 (s), 731 (s), 696
(s) cm−1. 1H NMR (500 MHz, CDCl3) δ: 1.73 (s, 3H, Sn-CH3), 5.07
(s, 4H, −CH2-N), 7.36−7.29 (m, 5H, Haromatic). 13C NMR (75 MHz,
CDCl3) δ: 15.9 (Sn-Me), 55.3, 55.8 (−CH2-N), 127.7, 127.9, 128.8,
134.8, 134.9, (Caromatic), 202.7 (Cdtc). 119Sn NMR (186 MHz, CDCl3)
δ: −337.3.
(−CH2−), 102.9 (C−O), 122.9, 136.7 (Cipso), 127.6, 130.9 (t, JCD
=
24.7 Hz). High-resolution MS LIFDI(+)-TOF: calculated for
C9H5D4O2Br ([M + H]+) 232.0031, found 232.0038, error 2.8 ppm.
Compound 5: Terephthalaldehyde-d4. 2-(4-Bromophenyl)-1,3-
dioxolane-d4 12 (1.220 g, 5.32 mmol) was dissolved in dry THF under
an argon atmosphere; then n-BuLi 1.6 M in hexanes (3.99 mL 6.39
mmol) was added dropwise at −78 °C, and the mixture was stirred for
360
dx.doi.org/10.1021/om401094d | Organometallics 2014, 33, 354−362

Organometallics
Article
2 h. Afterward, dry DMF (0.82 mg, 10 mmol) was added dropwise,
and the reaction mixture was stirred at room temperature for 12 h.
Then, the mixture was treated with HCl (0.5 M, 30 mL), diluted with
brine (10 mL), and stirred for 2 h. After evaporation of the solvent
under vacuum, the product was extracted with ethyl ether. Yield: 105
1.47 (q, J = 7.3 Hz, 2H, −CH2−), 1.39 (s, 6H, BCO), 1.31 (sext, J =
7.3 Hz, 2H, −CH2−), 0.89 (t, J = 7.3 Hz, 3H, −CH3). 13C NMR (75
MHz, CDCl3) δ: 60.3 (NCH2−), 50.73 (NCH2−), 32.0, 31.8, 20.3
(CBCO), 20.3 (−CH2−), 13.92 (−CH3) ppm. High-resolution MS
ESI(+)-TOF: calculated for C18H36N2 ([M + H]+) 281.2957, found
281.2951, error 2.1 ppm.
Compound 2. Complex 2 was obtained using the procedure
described previously for compound 1-d20. Yield: 65 mg (34%). IR
̃
(KBr): ν 2929 (m), 2857 (m), 1475 (s), 1454 (m), 1413 (s), 1367
(m), 1329 (m), 1294 (m), 1252 (m), 1216 (s), 1187 (m), 1167 (m),
1105 (m), 1062 (m), 989 (s), 941 (m), 911 (w), 864 (w), 786 (s)
cm−1. 1H NMR (500 MHz, CDCl3) δ: 3.77 (s, 2H, NCH2−), 3.65 (s,
2H, NCH2−), 1.72 (s, broad, 2H, −CH2−), 1.58 (s, 6H, BCO), 1.49
(s, 3H, Sn-Me), 1.29 (s, broad, 2H, −CH2−), 0.92 (t, J = 7.4 Hz, 3H,
−CH3). 13C NMR (125 MHz, CDCl3): δ 201.5 (Cdtc), 62.8, 56.1
(NCH2−), 35.2, 30.5 (CBCO), 28.0, 20.1, 15.8 (Sn-Me), 13.92
(−CH3). 119Sn NMR (186 MHz, CDCl3): δ −346.6. High-resolution
MS LIFDI(+)-TOF: calculated for C44H80N4S8Sn2 ([M + H]+)
1159.2260, found 1159.2277, error 1.5 ppm.
̃
mg (90%). IR: ν 2919 (w), 2865 (w), 2776 (w), 1682 (s), 1574 (w),
1463 (w), 1419 (m), 1403 (s), 1316 (s), 1304 (s), 1181 (w), 1111(s),
857 (s), 816 (m), 807 (w) 748 (s), 681 (s) cm−1. 1H NMR (300 MHz,
CDCl3) δ: 10.14 (s, 1H). 13C NMR (75 MHz, CDCl3) δ: 129.7 (t, JCD
= 24.8 Hz) C-D), 139.9 (Cipso), 191.4 (CO). High-resolution MS
LIFDI(+)-TOF: calculated for C8H2D4O2 ([M + H]+) 138.0629,
found 138.0622, error 5.0 ppm.
Compound 4-d4: 1,4-Bis(benzylaminomethyl)benzene-d4.
The formation of 4-d4 was carried out by following the procedure
described for compound 4-d10 by reacting compound 5 and
̃
benzylamine. Yield: 420 mg (90.5%). IR: ν 3515 (w), 3317 (w),
3026 (w), 2832 (m), 2480 (w), 2265 (w), 1605 (w), 1495 (m), 1452
(s), 1356 (w), 1314 (w), 1331 (w), 1098 (w), 1028 (w), 973 (w), 817
1
(w), 736 (s), 699 (s) cm−1. H NMR (300 MHz, CDCl3) δ: 1.58 (s,
2H, N-H), 3.81 (d, 4H, −CH2−), 7.31 (s, 5H, Haromatic). 13C NMR (75
MHz, CDCl3) δ: 52.8, 53.2 (−CH2−), 128.1−127.6 (Caromatic), 138.8,
140.3 (Cipso). High-resolution MS ESI(+)-TOF: calculated for
C22H20D4N2 ([M + H]+) 321.2269, found 321.2271, error 0.6 ppm.
Compound 1-d8: [{Me2Sn(1,4-Bn-ambdtc)}2]-d8. Complex 1-d8
was obtained following the procedure described previously for
Compound 7: N,N-Dibenzylbicyclo[2.2.2]octane-1,4-dicar-
boxamide. Compound 7 was obtained using the procedure described
̃
previously for compound 9. Yield: 325 mg (98%). IR: ν 3320 (m),
2916 (w), 2866 (w), 1631 (s), 1523 (s), 1496 (s), 1453 (s), 1421 (m),
1355 (w), 1298 (s), 1236 (m), 1157 (w), 1123 (w), 1080 (w), 1027
1
(w), 963 (w), 848 (w), 723 (s), 694 (s) cm−1. H NMR (300 MHz,
CDCl3) δ: 7.28 (m, 10H, Haromatic), 5.83 (t, J = 5.6 Hz, 2H, NH), 4.42
(d, J = 5.6 Hz, 4H, NCH2−), 1.82 (s, 12H, BCO). 13C NMR (75
MHz, CDCl3) δ: 176.7 (CO), 138.2 (Cipso), 128.6, 127.5 (Co,m),
127.3 (Cp), 43.3 (NCH2−), 38.7, 28.4 (CBCO). High-resolution MS
LIFDI(+)-TOF: calculated for C24H28N2O2 ([M + H]+) 376.2145,
found 376.2143, error 0.6 ppm.
Compound 8: N-((4-((Benzylamino)methyl)bicyclo[2.2.2]-
octan-1-yl)methyl)(phenyl)methanamine. Compound 8 was
obtained using the procedure described previously for compound
̃
compound 1. Yield: 165 mg (86%). IR: ν 3059 (w), 3027 (w),
2920 (w), 1603 (w), 1583 (w), 1494 (m), 1466 (s), 1450 (s), 1407
(s), 1365 (m), 1264 (w), 1209 (s), 1147 (s), 1078 (w), 1046 (w),
1028 (w), 968 (s), 935 (w), 886 (w), 786 (s), 731 (s) cm−1. 1H NMR
(500 MHz, CDCl3) δ: 1.75 (s, 3H, Sn-CH3), 5.06 (s, 4H, −CH2−),
7.37−7.26 (s, 10H, Haromatic). 13C NMR (500 MHz, CDCl3) δ: 16.0
(Sn-CH3) 53.3, 53.7 (−CH2−) 127.8−128.9 (Caromatic), 134.7, 135.3
(Cipso), 202.7 (Cdtc). 119Sn NMR (186 MHz, CDCl3) δ: −337.1. High-
resolution MS LIFDI(+)-TOF: calculated for C52H52D8N4S8Sn2 ([M
+ H]+) 1239.0884, found 1239.0908, error 2.0 ppm.
̃
10. Yield: 185 mg (95%). IR: ν 3063 (w), 3019 (w), 2920 (s), 2855
(s), 1581 (w), 1453 (s), 1359 (w), 1302 (w), 1252 (w), 1199 (w),
Compound 9: N,N-Dibutylbicyclo[2.2.2]octane-1,4-dicarbox-
amide. A mixture of trimethylaluminum (2 M, 3.7 mL, 7.5 mmol) in
dry toluene (20 mL) was cooled using an ice/salt bath; then
benzylamine (1.5 mL, 0.015 mol) was added. The resulting mixture
was stirred for 30 min, and the reaction was warmed to room
temperature. Subsequently, dimethyl bicyclo[2.2.2]octane-1,4-dicar-
boxylate 6 (0.56 g, 2.5 mmol) previously dissolved in dry toluene (20
mL) was transferred over the initial flask, and the resulting mixture was
refluxed for 4 h, then allowed to reach room temperature, treated with
0.5 M HCl (10 mL), and diluted with brine (10 mL). The product was
1
1109 (w), 1028 (w), 905 (w), 822 (w), 735 (s), 697 (s) cm−1. H
NMR (500 MHz, CDCl3) δ: 7.31 (m, 5H, Haromatic), 3.78 (s, 2H,
NCH2−), 2.29 (s, 2H, NCH2−), 1.41 (s, 6H, BCO). 13C NMR (125
MHz, CDCl3) δ: 140.1 (Cipso), 128.2, 127.9 (Co,m), 126.7 (Cp), 59.1
(NCH2−), 54.3 (NCH2−), 31.7, 29.4 (CBCO). High-resolution MS
ESI(+)-TOF: calculated for C24H32N2 ([M + H]+) 349.2644, found
349.2652, error 2.3 ppm.
ASSOCIATED CONTENT
* Supporting Information
■
then extracted with ethyl ether. Yield: 560 mg (73%). IR: ν
̃
3331 (s),
S
3067 (w), 2930 (m), 2868 (m), 1631 (s), 1536 (s), 1456 (m), 1433
(w), 1361 (w), 1301 (m), 1224 (w), 1145 (w), 1129 (w), 944 (w),
Spectroscopic data for all compounds, DSC trace of compound
1, and thermogravimetric traces of 1, 1-d8, 1-d20, and 2. PXRD
patterns of compounds 1 and 2. Arrhenius plot and alternative
simulated line shapes using σ = 3. Crystallographic parameters
and crystallographic information file (CIF) of 1 (100 K). This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
863 (w), 828 (w), 738 (w), 662 (w) cm−1. H NMR (300 MHz,
CDCl3) δ: 5.52 (s, 2H, NH), 3.20 (q, J = 6.8 Hz, 4H, NCH2−), 1.77
(s, 12H, BCO), 1.45 (quint, J = 6.8 Hz, 4H, −CH2−), 1.35 (sext, J =
6.8 Hz, 4H, −CH2−), 0.90 (t, J = 6.8, Hz 6H, −CH3). 13C NMR (75
MHz, CDCl3) δ: 177.0 (CO), 39.1 (NCH2−), 38.7, 31.7 (−CH2−),
28.3 (CBCO), 20.1 (−CH2−), 13.77 (−CH3). High-resolution MS
ESI(+)-TOF: calculated for C18H32N2O2 ([M + H]+) 309.2542, found
309.2536, error 1.9 ppm.
Compound 10: N-((4-((Butylamino)methyl)bicyclo[2.2.2]-
octan-1-yl)methyl)butan-1-amine. A mixture of compound 9
(0.30 g, 0.97 mmol) and LiAlH4 (0.44 g, 12 mmol) in dry THF (30
mL) was refluxed for 24 h. The reaction was allowed to reach room
temperature; then 0.1 M NaOH (15 mL) was added, and the product
was extracted with ethyl ether. To purify the compound, 0.1 M HCl
(10 mL) was added to the diamine 10 dissolved in ether (20 mL), and
the salt formed was extracted with water (30 mL). Subsequently,
NaOH was added until pH = 14 was reached to regenerate the
diamine, and the product was extracted with diethyl ether (30 mL).
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail (H. Hopfl): hhopfl@ciq.uaem.mx.
̈
*E-mail (M. A. Garcia-Garibay): mgg@chem.ucla.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from Consejo Nacional de Ciencia y
■
́
Tecnologıa (CONACYT) (Project No. CB2010-158098) is
̃
Yield: 260 mg (95%). IR (KBr): ν 2956 (s), 2920 (s), 2856 (s), 2805
gratefully acknowledged. A.T.-H. thanks CONACYT for a
Ph.D. scholarship and a research stay at UCLA. We also
acknowledge support by the National Science Foundation grant
(m), 1662 (w), 1458 (s), 1375 (w), 1256 (w), 1118 (m), 1070 (w),
1
1012 (w), 891 (w), 804 (w), 746 (w), 703 (w) cm−1. H NMR (300
MHz, CDCl3) δ: 2.54 (t, J = 7.3 Hz, 2H, NCH2−), 2.26 (s, NCH2−),
361
dx.doi.org/10.1021/om401094d | Organometallics 2014, 33, 354−362

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Article
M. D.; Beer, P. D. Tetrahedron 2004, 60, 11227. (d) Arens, V.; Dietz,
C.; Schollmeyer, D.; Jurkschat, K. Organometallics 2013, 32, 2775.
DMR1101934. Solution NMR experiments carried out at 500
MHz were obtained in the Bruker AV500 acquired with
support by the National Science Foundation equipment grant
CHE1048804.
(15) (a) Cruz-Huerta, J.; Carillo-Morales, M.; Santacruz-Juar
Hernandez-Ahuactzi, I. F.; Escalante-García, J.; Godoy-Alcantar, C.;
Guerrero-Alvarez, J.; Hopfl, H.; Morales-Rojas, H.; Sanchez, M. Inorg.
Chem. 2008, 47, 9874. (b) Reyes-Martínez, R.; García y García, P.;
Lopez-Cardoso, M.; Hopfl, H.; Tlahuext, H. Dalton Trans. 2008, 6624.
(c) Tlahuext, H.; Reyes-Martínez, R.; Vargas-Pineda, G.; Lop
́
ez, E.;
́
́
̈
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