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Synthesis and Solid-State Characterization of Self-Assembled Macrocyclic Molecular Rotors of Bis(dithiocarbamate) Ligands with Diorganotin(IV) Aarón Torres-Huerta,†,‡ Braulio Rodríguez-Molina,‡ Herbert Höpfl,*,† and Miguel A. Garcia-Garibay*,‡ †

Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Avenida Universidad 1001, C.P. 62209, Cuernavaca, México ‡ Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: Two bis(dithiocarbamate) (bdtc) metallamacrocyclic compounds, 1 and 2, and the deuterated analogues 1-d8 and 1d20 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 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.

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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 technological 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 © 2013 American Chemical Society

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 Received: November 11, 2013 Published: December 18, 2013 354

dx.doi.org/10.1021/om401094d | Organometallics 2014, 33, 354−362

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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 using line shape analysis of variable-temperature 2H 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.

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RESULTS AND DISCUSSION Synthesis and Spectroscopic Characterization. The synthesis of macrocyclic rotor 1 is illustrated in Scheme 1. Selfassembling 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 synthesized in 87% yield, following the above-stated procedure, starting from deuterated benzaldehyde.18 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-dibromobenzene-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-(4bromophenyl)-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,Ndibenzylbicyclo[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%). The FT-IR spectra of all tin(IV) complexes showed two bands characteristic of the dithiocarbamate group in the regions

Figure 1. (a) Schematic representation showing the composition of a macrocyclic molecular dirotor formed by a metallosupramolecular selfassembly 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.

this direction consists in the preparation of metallamacrocycles via self-assembly between metal ions and organic ligands. Selfassembling 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 355


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Scheme 1. Synthesis of Macrocyclic Dirotor 1 and Its Deuterated Analogues 1-d20 and 1-d8a

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). In the 1H 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

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, 1d20, 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 356


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Scheme 2. Synthesis of Macrocyclic Dirotor 2b

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) compound 1 1-d20 1-d8 2

δ

119

Sn

−337.3 −337.3 −337.1 −346.6

NCH2

−CH2NCH2−

CS2

δ H

δ C

1

5.07 5.08 5.06 3.77, 3.65

13

55.8, 53.7, 53.7, 62.8,

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 P1̅ 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 1d20 showed some inconsistencies regarding the melting point: freshly grown crystals obtained from dichloromethane solution

55.3 53.3 53.3 56.1

N−CS2

13

−1

δ C

ν (cm )

ν (cm−1)

202.7 202.7 202.7 201.5

973 969 968 989

1466 1464 1466 1475

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 deuteriumfree 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 357


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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 confirmed 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, recrystallization 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, chloroform, 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, 1d20, 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. 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 established in the 1H nuclei, we were able to analyze signals assigned to the phenylene rotator in the case of compound 1d20. 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 magnitude of the 1H−13C dipolar coupling, typically in the range of 25−30 kHz. Notably, NQS experiments showed that

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 1 H−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 crosspolarization 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. 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. 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]358


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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 kHz regime or faster. As described in 1, the use of short 1H to 13 C polarization transfer made it possible to identify only protonated carbon atoms in 2 (Figure 4c). Rotational Dynamics by Line Shape Analysis of 2H NMR Spin Echo. Encouraged by the 13C NMR CPMAS results, we carried out variable-temperature 2H NMR spin echo experiments with compound 1-d8 in order to analyze the rotational dynamics of the 2H-labeled phenylene rotators. Quadrupolar 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 = (e 2qzzQ /h)(3 cos2 β − 1) 4 3 = QCC(3 cos2 β − 1) (1) 4

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.

Variable-temperature 2H NMR analyses of compound 1-d8 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

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 359


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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%). IR: ν̃ 2820 (m), 2273 (m), 1570 (w), 1512 (m), 1448 (s), 1418 (m), 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. Yield: 167 mg (87%). IR: ν̃ 1512 (w), 1464 (s), 1406 (s), 1355 (s), 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. Compound 11: 4-Bromobenzaldehyde-d4. 1,4-Dibromobenzene-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: ν̃ 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 mixture 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 extracted with 30 mL of ethyl ether. Yield: 1.220 g (94%). IR (KBr): ν̃ 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 (−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)-1,3dioxolane-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

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CONCLUSIONS 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 frequencies higher than the 1H−13C dipolar coupling (25−30 kHz). Similarly, the 13C spectrum of 2 indicated a fast rotating bicyclo[2.2.2]octane moiety resulting from its higher symmetry as compared to flat aromatic rings. Variable-temperature 2H NMR experiments of compound 1-d8 confirmed the highrotation 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.

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EXPERIMENTAL SECTION

Dimethyltin(IV) chloride, terephthalaldehyde, benzaldehyde, benzylamine, 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 methodology.27 Compound 1: [{Me2Sn(1,4-Bn-ambdtc)}2]. The synthesis of compound 1 was accomplished as previously described.16 Spectroscopic data are reported here for comparison purposes with the deuterated complexes, particularly the 13C NMR data. Yield: 87%. IR (amorphous system): ν̃̃ 3023 (w), 2919 (w), 1603 (w), 1512 (w), 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. 360


Organometallics

Article

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): ν̃ 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. Compound 7: N,N-Dibenzylbicyclo[2.2.2]octane-1,4-dicarboxamide. Compound 7 was obtained using the procedure described previously for compound 9. Yield: 325 mg (98%). IR: ν̃ 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 (w), 963 (w), 848 (w), 723 (s), 694 (s) cm−1. 1H 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 10. Yield: 185 mg (95%). IR: ν̃ 3063 (w), 3019 (w), 2920 (s), 2855 (s), 1581 (w), 1453 (s), 1359 (w), 1302 (w), 1252 (w), 1199 (w), 1109 (w), 1028 (w), 905 (w), 822 (w), 735 (s), 697 (s) cm−1. 1H 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.

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 mg (90%). IR: ν̃ 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: ν̃ 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 (w), 736 (s), 699 (s) cm−1. 1H 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 1. Yield: 165 mg (86%). IR: ν̃ 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. Highresolution MS LIFDI(+)-TOF: calculated for C52H52D8N4S8Sn2 ([M + H]+) 1239.0884, found 1239.0908, error 2.0 ppm. Compound 9: N,N-Dibutylbicyclo[2.2.2]octane-1,4-dicarboxamide. 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-dicarboxylate 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 then extracted with ethyl ether. Yield: 560 mg (73%). IR: ν̃ 3331 (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), 863 (w), 828 (w), 738 (w), 662 (w) cm−1. 1H 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). Yield: 260 mg (95%). IR (KBr): ν̃ 2956 (s), 2920 (s), 2856 (s), 2805 (m), 1662 (w), 1458 (s), 1375 (w), 1256 (w), 1118 (m), 1070 (w), 1012 (w), 891 (w), 804 (w), 746 (w), 703 (w) cm−1. 1H NMR (300 MHz, CDCl3) δ: 2.54 (t, J = 7.3 Hz, 2H, NCH2−), 2.26 (s, NCH2−),

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ASSOCIATED CONTENT

S Supporting Information *

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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (H. Höpfl): hhopfl@ciq.uaem.mx. *E-mail (M. A. Garcia-Garibay): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Consejo Nacional de Ciencia y Tecnologı ́a (CONACYT) (Project No. CB2010-158098) is 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 361


Organometallics

Article

M. D.; Beer, P. D. Tetrahedron 2004, 60, 11227. (d) Arens, V.; Dietz, C.; Schollmeyer, D.; Jurkschat, K. Organometallics 2013, 32, 2775. (15) (a) Cruz-Huerta, J.; Carillo-Morales, M.; Santacruz-Juárez, E.; Hernández-Ahuactzi, I. F.; Escalante-García, J.; Godoy-Alcantar, C.; Guerrero-Alvarez, J.; Höpfl, H.; Morales-Rojas, H.; Sánchez, M. Inorg. Chem. 2008, 47, 9874. (b) Reyes-Martínez, R.; García y García, P.; López-Cardoso, M.; Höpfl, H.; Tlahuext, H. Dalton Trans. 2008, 6624. (c) Tlahuext, H.; Reyes-Martínez, R.; Vargas-Pineda, G.; LópezCardoso, M.; Höpfl, H. J. Organomet. Chem. 2011, 696, 693. (d) Macgregor, M. J.; Hogarth, G.; Thompson, A. L.; Wilton-Ely, J. D. E. T. Organometallics 2009, 28, 197. (e) Konarev, D. V.; Kovalevsky, A. Y.; Khasanov, S. S.; Saito, G.; Lopatin, D. V.; Umrikhin, A. V.; Otsuka, A.; Lyubovskaya, R. N. Eur. J. Inorg. Chem. 2006, 1881. (f) Vickers, M. S.; Cookson, J.; Beer, P. D.; Bishop, P. T.; Thiebaut, B. J. Mater. Chem. 2006, 16, 209. (16) Santacruz-Juárez, E.; Cruz-Huerta, J.; Hernández-Ahuactzi, I.; Reyes-Martínez, R. F.; Tlahuext, H.; Morales-Rojas, H.; Höpfl, H. Inorg. Chem. 2008, 47, 9804. (17) Karlen, S. D.; Garcia-Garibay, M. A. Chem. Commun. 2005, 189. (18) Deuterated benzaldehyde 3-d5 was obtained from bromobenzene-d5 and DMF, to afford 1,4-bis(benzylaminemethyl)benzene-d10 (4-d10) in 83% yield. (19) Kumar, K.; Wang, S. S.; Sukenik, C. N. J. Org. Chem. 1984, 49, 665. (20) (a) Smith, P. J.; Tupciauskas, A. P. Annu. Rep. NMR Spectrosc. 1978, 8, 291−370. (b) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 1985, 16, 73. (c) Lockhart, T. P.; Manders, W. F.; Schlemper, E. O. J. Am. Chem. Soc. 1985, 107, 7451. (d) Dakternieks, D.; Zhu, H.; Masi, D.; Mealli, C. Inorg. Chem. 1992, 31, 3601. (e) Seth, N.; Gupta, V. D.; Nöth, H.; Thomann, M. Chem. Ber. 1992, 125, 1523. (f) Sharma, J.; Singh, Y.; Bohra, R.; Rai, A. K. Polyhedron 1996, 15, 1097. (21) (a) Celis, N. A.; Villamil-Ramos, R.; Höpfl, H.; HernándezAhuactzi, I. F.; Sánchez, M.; Zamudio-Rivera, L. S.; Barba, V. Eur. J. Inorg. Chem. 2013, 16, 2912. (b) Zia-ur, R.; Niaz, M.; Shaukat, S.; Saqib, A.; Ian, S. B.; Auke, M.; Momin, K. Polyhedron 2009, 28, 3439. (22) Rojas-León, I.; Guerrero-Alvarez, J. A.; Hernández-Paredes, J.; Höpfl, H. Chem Commun. 2012, 48, 401. (23) (a) Gall, C. M.; DiVerdi, J. A.; Opella, S. J. J. Am. Chem. Soc. 1981, 103, 5039. (b) Hirschinger, J.; Miura, H.; Gardner, K. H.; English, A. D. Macromolecules 1990, 23, 2153. (24) (a) Reichert, D. NMR Studies of Dynamic Processes in Organic Solids. In Annual Reports on NMR Spectroscopy; Elsevier, 2005; p 55. (b) Ratcliffe, C. I. Rotational & Translational Dynamics. In NMR Crystallography; Harris, R. K., Wasylishen, R. E., Duer, M. J., Eds.; Wiley, 2009. (25) Woessner, D. E. J. Chem. Phys. 1962, 36, 1. (26) Vold, R. L.; Hoatson, G. L. J. Magn. Reson. 2009, 198, 57. (27) Beinhoff, M.; Weigel, W.; Jurczok, M.; Rettig, W.; Modrakowski, C.; Brüdgam, I.; Hartl, H.; Schlüter, A. D. Eur. J. Org. Chem. 2001, 20, 3819.

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.

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REFERENCES

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