Reversible Interconversion of a Static Metallosupramolecular Cage

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Reversible Interconversion of a Static Metallosupramolecular Cage Assembly into a High-Speed Rotor: Stepless Adjustment of Rotational Exchange by Nucleophile Addition Suchismita Saha, Pronay Kumar Biswas, and Michael Schmittel* Center of Micro- and Nanochemistry and Engineering, Organische Chemie I, Adolf-Reichwein-Str. 2, D-57068 Siegen, Germany

Inorg. Chem. Downloaded from pubs.acs.org by AUBURN UNIV on 02/24/19. For personal use only.

S Supporting Information *

ABSTRACT: The self-assembled cage ROT-1 was prepared from the pyridine-terminated rotator 1, the phenanthroline-appended stator 2, DABCO, and copper(I) ions in a ratio of 1:1:1:4. This four-component assembly is held together by two pyridine→[Cu(phenAr2)]+ as well as two DABCO→zinc porphyrin interactions (phenAr2 = 2,9-diarylphenanthroline) and does not show any motion on the NMR time scale (k < 0.1 s−1, 298 K). However, it is converted to the fast nanorotor ROT-1xCD3CN by addition of CD3CN [x = (v/v)% of acetonitrile in dichloromethane] due to acceleration of both pyridine→copper(I) dissociation steps. Now the rotator is able to visit all four copper(I)loaded phenanthroline stations of the stator. Depending on the amount of CD3CN, the exchange frequency of the nanorotor varies from 0.7 s−1 (CD3CN:CD2Cl2 = 1:29) to 8000 s−1 (CD3CN:CD2Cl2 = 1:5) at 25 °C. When iodide (I−) is added to the static assembly ROT-1, the rotational speed increases even more drastically (k = 20 000 s−1), again due to accelerating the rate-determining pyridine→copper(I) dissociation step. In both cases, a sigmoidal relationship is established between exchange frequency and the concentration of added nucleophile (CD3CN or iodide) that suggests the presence of a cooperative effect. Reversible switching between the static assembly and fast rotor was performed several times without any decomposition of the system. In contrast, addition of the common nucleophile PPh3 to ROT-1 does not increase the rotational speed, a finding that is explained on thermodynamic grounds.

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of nanorotors with more than three components,23−25 less than a handful of examples are known where the modulation of the rotational frequency26,27 has been accomplished by external inputs. For instance, we recently demonstrated that the rotational frequency of four-component nanorotors may be influenced by addition of external brake stones26,27 and/or by exchange of a machine component, such as a rotator.10,28 Obviously, change of metals ions,25 steric shielding at the binding site(s),29 distance of translocation in the machine,30 and even addition of nucleophiles31 will also affect the dynamics of the machinery to a great extent. In particular, kinetics of ligand substitution by nucleophiles32,33 should be a premium strategy to influence the rotor speed if the rate-determining step depends on ligand−metal dissociation. For the present case, kinetic data of solventfacilitated ligand exchange reactions in copper(I), silver(I) diimine complexes may serve as orientation.34−38 Actually, very few examples have been reported where the rate of a machine increases upon the addition of external nucleophiles.31 For instance, Sauvage et al. showed that the addition of Cl− can

INTRODUCTION In recent years, synthetic molecular machines1−4 have gained enormous interest because of their ability to mimic powerful and fascinating biological systems.5−7 As most machines in nature (like ATP synthase,6 bacterial flagella, kinesin motor proteins, etc.) are mainly multicomponent aggregates, the preparation of multicomponent artificial machineries8−12 by self-assembly is a topic of paramount interest. However, due to their tendency to dissociate, clean nanomechanical motion in multicomponent devices is more challenging to conceive than that in covalent or topologically linked molecular machines.13 Rotation is one of the key nanomechanical phenomena in biological machines with the rotational speed being highly regulated by “chemical” stimuli. For example, the rotary motion present in biological motors like bacterial flagella14 is tuned by the intensity of the proton motive force15 and is completely stopped by the inhibitory protein epsE that acts as a mechanical clutch preventing further bacterial movement.16 In first attempts to implement speed regulation in artificial machinery, scientists have established stop and go motion using switchable brakes in motors and rotors.17−22 However, it would be a big step to continuously adjust the rotational frequency and even more if such mode were realized in multicomponent machines. Although there are first examples © XXXX American Chemical Society

Received: December 21, 2018

A

DOI: 10.1021/acs.inorgchem.8b03567 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (a) The four components of the nanorotor along with their cartoon representations. (b) Preparation of preROT-1 and ROT-1.

Figure 2. (a) Comparison of partial 1H NMR (CD2Cl2, 400 MHz, 298 K) of preROT-1 and ROT-1. Mesityl protons 9-H are denoted as 9c-H, 9uH, and 9Cu-H for HETPYP-I complexed, uncomplexed, and Cu+-loaded phenanthroline protons, respectively. (b) Partial 1H NMR (CD3CN, 400 MHz, 298 K) of ROT-1100CD3CN. (c) Partial VT 1H NMR (CD3CN, 600 MHz) of ROT-1100CD3CN.

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acceleratethe very slow gliding motion in copper-diimine based machines.31 Here we reveal how the rotational frequency of a fourcomponent nanorotor is continuously adjusted either by changing the composition of the solvent or by addition of iodide as nucleophile.39 Still, to the best of our knowledge, the dependence of rotational frequency on the solvent composition is yet to be explored for nanomechanical rotors. In detail, we demonstrate the transformation of a static cage assembly into a high-speed rotor by varying the solvent from a noncoordinating (CD2Cl2) to a coordinating (CD3CN) solvent and the even more drastic acceleration by far more than 2 × 105 after adding iodide. In contrast, the prototypical nucleophile PPh3 is unable to increase the rotational frequency of the nanorotor.

RESULTS AND DISCUSSION

For the present work, we chose cage assembly ROT-1, which is composed of four components: (a) the tetrakis(2,9-diarylphenanthroline)-appended zinc porphyrin as stator 2, (b) the 5,10-disubstituted zinc porphyrin as rotator 1 with two flexible pyridine-terminated rotator arms, (c) copper(I) ions as a “glue” to link the pyridine terminals of the rotator10 via heteroleptic complexation40,41 to the phenanthroline stations of the stator, and (d) DABCO as a dynamic hinge10,42 setting up a hetero-bis(zinc porphyrin) sandwich complex via axial coordination (Figure 1a). The sterically bulky aryl groups (mesityl and duryl) at the 2,9-position of the phenanthroline in 2 are essential to prevent formation of the homoleptic phenanthroline complex.43 B

DOI: 10.1021/acs.inorgchem.8b03567 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 3. (a) Partial 1H NMR (400 MHz, 298 K) of ROT-1xCD3CN (x is the (v/v)% of CD3CN present in the solvent mixture of CD3CN and CD2Cl2) at different ratios of CD3CN and CD2Cl2. (b) Cartoon representation for the interconversion of ROT-1 to ROT-1xCD3CN and vice versa. (c) Sigmoidal relationship of rotational frequency and concentration of added nucleophile (CD3CN). (d) Partial VT 1H NMR (600 MHz, CD2Cl2:CD3CN = 5:1) of ROT-116.7CD3CN.

protons at the HETPYP-I complexation site remain more or less in the same position as in preROT-1, while the protons at the formerly unloaded phenanthrolines experience a downfield shift upon loading of the diimine stations with Cu+ (Figure 2a and SI, Figures S6 and S8). As the protons of the HETPYP-I complexed (Npy→[Cu(phenAr2)]+) and of exclusively Cu+loaded phenanthroline stations do not merge in a single set in the 1H NMR of ROT-1, either its rotational frequency is slower than the NMR time scale or there is no rotation at all. Results of a 2D ROESY experiment with 300 ms mixing time demonstrate that there are no exchange signals between any HETPYP-I complexed and Cu+-loaded phenanthroline stations. Interestingly, there are exchange signals between the pairwise protons (10-H, 10′-H), (11-H, 11′-H), (a-H, a′-H), (b-H, b′-H), (e-H, e′-H), and (f-H, f′-H), which reflect slow rotation (∼1.1 s−1 at 25 °C) about the porphyrin−aryl axes (SI, Figures S23 and S24). Therefore, ROT-1 behaves as a static four-component hetero-porphyrin cage in CD2Cl2. Another spectroscopic result in the reaction of preROT-1 → ROT-1 is the ∼0.01 ppm downfield shift of protons CH3CN (SI, Figure S8). This signal shift is likely due to coordination of CH3CN (from [Cu(CH3CN)4]PF6) to the copper(I) ion sitting in the phenanthroline binding site. Since the ratedetermining step of rotation requires the detachment of both pyridine arms of 1 at both Npy→[Cu(phenAr2)]+ sites of ROT-1 that could be supported by CH3CN, the question is whether added CH3CN will stabilize preferably the ground or transition state. As a result of these considerations, we investigated the exchange frequency when the solvent was changed from CD2Cl2 (noncoordinating) to CD3CN (coordinating). The solvent was fully removed from ROT-1 so that clearly defined mixed-solvent conditions can be studied. In pure CD3CN, we obtained quantitatively ROT-1100CD3CN (the suffix 100 indicates that ROT-1 is dissolved in 100% CD3CN. The same rotor assembly will be denoted as ROT-1 xCD 3 CN depending on the share of acetonitrile-d3, defined by (v/v)% of CD3CN, in the mixture with dichloromethane-d2). The 1H

When 1, 2, [Cu(CH3CN)4]PF6, and DABCO were mixed in a 1:1:2:1 ratio in an NMR tube and dissolved in CD2Cl2, the cage assembly preROT-1 formed quantitatively irrespective of the sequence of addition (Figure 1b).10 It was characterized unambiguously by 1H NMR, 1H−1H COSY, ESI-MS, and elemental analysis. In presence of Cu+, two Npy→[Cu(phenAr2)]+ complexes (known as HETPYP-I complexation: HETeroleptic PYridine and Phenanthroline)40,41 form between the rotator’s pyridine arms and two stator’s phenanthroline stations. Though the log K of the Npy→[Cu(phenAr2)]+ (3.2 ± 0.6)42 complexation is less than that of the axial Npy→ zinc porphyrin (ZnPor) coordination (3.78 ± 0.02),44 the stronger NDABCO→ZnPor binding replaces the Npy→ZnPor interaction setting up a clean heteroleptic DABCO-ZnPor2 sandwich complex. The two sets of 1H NMR multiplets at −[4.96−4.92] ppm and −[4.86−4.82] ppm in a 1:1 ratio attest the symmetry breaking at the hetero-sandwiched DABCO (between 1 and 2) (Figure 2a).45 The highfield NMR shifts of protons d-H and c-H at 6.62 and 7.36 ppm, respectively, support formation of the HETPYP-I complex (Figure 2a).42 All phenanthroline, mesityl, and duryl protons appear in two sets (1:1 ratio), one set referring to HETPYP-I complexed phenanthroline protons and the other set denoting unloaded phenanthroline stations (Figure 2a; SI, Figure S4). Upon complexation, the pairwise protons (10-H, 10′-H), (11-H, 11′H), (a-H, a′-H), (b-H, b′-H), (e-H, e′-H), and (f-H, f′-H) are no longer chemically equivalent (Figure 2a; SI, Figure S4). One set of these protons resides inside, the other outside of the sandwich. Interconversion on the NMR time scale is not seen because C−C bond rotation about the porphyrin−aryl axes is too slow. When two more equiv of [Cu(CH3CN)4]PF6 was added to preROT-1, the assembly ROT-1, which is a cage in the absence of nucleophiles, formed quantitatively.10 Two multiplets of DABCO protons in the negative region at δ = −[4.95−4.91] ppm and −[4.85−4.81] ppm and the characteristic shifts of protons d-H (δ = 6.64 ppm) and c-H (δ = 7.37 ppm) for the HETPYP-I complexation confirm the formation of ROT-1 (Figure 2a). All phenanthroline, mesityl, and duryl C

DOI: 10.1021/acs.inorgchem.8b03567 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry NMR of ROT-1100CD3CN is highly diagnostic. The DABCO protons appear as two broad peaks at −5.01 and −4.91 ppm in a 1:1 ratio, confirming the sandwich structure about the two different porphyrins (Figure 2b). Protons d-H (7.47 ppm) emerge upfield relative to those of the free rotator (8.47 ppm, in THF-d8, as 1 is insoluble in CD3CN and in CD2Cl2), but in downfield position to those in ROT-1 (6.64 ppm in CD2Cl2) (Figure 2b). In ROT-1100CD3CN, all phenanthroline protons show up as one set of signals which implies fast exchange (SI, Figure S9). For quantitative assessment of the exchange frequency of ROT-1100CD3CN, variable temperature 1H NMR (VT 1H NMR) was performed. Even at −35 °C, no splitting of the phenanthroline protons but signal broadening of protons 9-H, 5+6-H, 14-H, 15-H, 12-H, and 13-H is observed (Figure 2c; SI, Figure S20). As the freezing point of CD3CN is −45 °C, we could not lower the temperature further. Two DABCO signals at negative range prove that the assembly remains intact throughout the whole temperature range (Figure 2c). A titration was performed from ROT-1100CD3CN to ROT-1 which shows that, in the course of decreasing the percentage (v/v)% of CD3CN in CD2Cl2, the rotational exchange speed changes dramatically (Figure 3a; SI, Figures S11 and S12). At amounts ≥ 16.7% of CD3CN, e.g., with ROT-116.7CD3CN, rotation is faster than the NMR time scale and only one set of proton signals is observed. At amounts below ∼8.3% of CD3CN, the rotation becomes decisively slower than the NMR time scale, visible from the fact that all the phenanthroline protons split into two sets in a 1:1 ratio (one for the HETPYPI complexed and the other for the Cu+-loaded phenanthroline station). By further decreasing the amount of CD3CN, signals of protons d-H and c-H move upfield due to the ring current effect from the aryl groups at the stator’s stations, suggesting a stronger HETPYP-I complexation. When no extra CD3CN is added to the system (the only nucleophile is CH3CN from [Cu(CH3CN)4]PF6, as in case of ROT-1), equally there is no rotation observable (SI, Figure S23). In summary, the present four-component supramolecular assembly can be reversibly transformed into a four-component nanorotor and back just by addition/removal of CD3CN as a nucleophilic solvent (Figure 3b). To investigate the rotational motion over a wide range, the kinetics of exchange was measured at various (v/v)% of CD3CN in CD2Cl2 using either VT 1H NMR (fast exchange) or 2D-ROESY (slow exchange). While at 25 °C, the nanorotor ROT-116.7CD3CN shows a single signal for 9-H, the VT 1H NMR exhibits coalescence at 0 °C and a crisp splitting at −10 °C. Splitting continues down to −65 °C (Figure 3d). Equally protons (5+6)-H, 14-H, 15-H, 12-H, and 13-H split up in two sets (SI, Figure S21). The individual rotational rates were determined from simulation of the VT-1H NMR signals using WinDNMR (SI, Figure S22). The exchange frequency of the rotor ROT-116.7CD3CN turned out to be 8000 s−1 at 25 °C and the activation barrier ΔG⧧25 as 50.9 kJ mol−1 (SI, Table S1). Apparently, the nucleophilic solvent CD3CN assists the departure of the rotator’s pyridine terminals by coordinating to the HETPYP-I phenanthroline stations. In the case of ROT116.7CD3CN, the amount of CD3CN is thus sufficient to make rotation fast enough so that only one set of proton signals shows up for all stations. In contrast, when the quantity of CD3CN was less, such as at 8.3%, 5.0%, and 3.3% of CD3CN in CD2Cl2, the displacement of the pyridine arms by CD3CN is slower than the NMR time scale which results in two sets of station’s signals in the 1H NMR. To quantify the rotational

speed in the slow exchange regime, 2D ROESY experiments were performed at 25 °C (SI, Figures S25, S27, and S29). For 8.3%, 5.0%, and 3.3% of CD3CN in CD2Cl2, the corresponding rotational frequency of the rotors turned out to be 1.4, 1.1, and 0.7 s−1 (SI, Figures S26, S28, and S30) with the free energy of activation being 72.2, 73.0, and 74.3 kJ mol−1, respectively. Importantly, the plot of rotational frequency vs concentration of added CD3CN furnishes a sigmoidal relationship (Figure 3c). Such type of sigmoidal curve suggests cooperative effects in the detachment of rotator 1 thus facilitating rotation. To get further insights regarding the effect of external nucleophiles on the kinetics of rotor ROT-1, the effect of iodide (I−) was investigated.39 Each time, 0.5 equiv of tetra-nbutylammonium iodide was added to ROT-1 (1.0 equiv) to − furnish ROT-1yI , where y is the equivalence of added iodide. Up to four equiv of added I−, all phenanthroline protons in the 1 H NMR appear in two sets, one set referring to HETPYP-I complexed phenanthroline stations and the other belonging to the copper(I)-loaded phenanthroline stations. Upon further addition of I−, the two sets of split phenanthroline protons start to coalesce and eventually turn into a single set (Figure 4a; SI, Figure S15).

−

Figure 4. (a) Partial 1H NMR (400 MHz, 298 K) of ROT-1yI (y is the equiv of I− present in the solution). (b) Sigmoidal plot of exchange frequency of rotor (1 mM in CD2Cl2) vs equiv of added nucleophile (I−).

The 1H NMR line width simulation suggests that the rotational frequency of ROT-1 responds more in the case of iodide than toward CD3CN (SI, Figure S16). The initially moderate rotational exchange rate increases drastically after addition of six equiv of I−, furnishing again a sigmoidal curve that indicates a cooperative effect (Figure 4b). Already at eight equiv of I−, the maximum speed is reached. Our explanation is the following. The first two equiv of I− binds to the rotator-free copper(I) complexes, while the next two equiv of I− associates at the HETPYP-I sites to set up the forth coordination. Since the next two equiv of I− does not have a large effect on the rate, it is likely that they partly coordinate to the only trigonal copper centers left, furnishing [Cu(phenAr2)]I2−. Any further addition will now increase the free iodide and thus drastically the system’s dynamics by detaching the pyridine arm from HETPYP-I center. At this point, the exchange rate shoots up! Relying on this model, the acceleration of rotational exchange is due to influencing the rate-determining Npy→[Cu(phenAr2)]I bond cleavage via ligand substitution (iodide replaces pyridine). Beyond eight equiv of I−, further increase in the rotational exchange rate becomes negligible as now the displacement of copper-bound iodide by the rotator arm becomes rate-determining. Dissociation of the whole assembly is not observed even at ten equiv of iodide added. D

DOI: 10.1021/acs.inorgchem.8b03567 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Rotational Frequency and Activation Barrier of ROT-1 with Different Amounts of Nucleophile added equiv of CD3CN 16 (no CD3CN, CH3CN coming from [Cu(CH3CN)4]PF6 1590 3190 15310 19150

ΔG⧧298 (kJ mol−1)

rotational rate of ROT-1xCD3CN 79

1.4 8000 ca. 9500a ca. 9500a

72.2 50.9

added equiv of I−

rotational rate of − ROT-1yI

0

79 67.9 50.2 48.7 48.3

a

Numbers are only approximate. Analysis only by line shape analysis.

105 (k < 0.1 s−1 for static cage and k = 20 000 s−1 for highspeed rotor). The quantity of CD3CN or I− dictates the rotational frequency of the rotor and allows for fine-tuning of the speed without any destruction of the multicomponent assembly even after several cycles. Toggling between different rotational frequencies is possible by changing the amount and nature of nucleophile. Thus, we have in some aspects mimicked control of biological rotor motion. Future advances in this field could emerge from mimicking the proton gradient in ATP synthase by an iodide gradient driving artificial rotors. Moreover, because these types of rotors may act as rotating catalytic machineries28 for chemical reactions, it would be of interest to monitor the effect of added nucleophiles on the rate of catalysis.

Interestingly the initially static cage ROT-1 can be recovered − from any fast rotating state ROT-1yI by addition of y equiv of silver(I) ions. Silver(I) removes the iodide by producing AgI, which precipitates from the mixture. Thus, any rotational speed (k = 0.1 s−1 to k ∼ 20 000 s−1) of ROT-1 can be reached with proper amounts of nucleophile and can be reversed back to the original state by removal of the nucleophile. Clearly, the effect of I− on the kinetics of rotation is much more pronounced compared to that of CD3CN (Table 1). A similar rotational frequency is achieved by adding either 6.5 equiv of iodide or 3190 equiv of CD3CN, a drastic difference that is attributed to the better nucleophilic nature of I−, which can liberate the pyridine arm of the rotor more effectively than CD3CN. Being a stronger nucleophile than CD3CN, I− is more efficient in displacing the rotator head from the copper(I) loaded phenanthroline, thus reducing the activation barrier. Is nucleophilicity the only parameter for accelerating the rotational exchange? According to the suggested mechanism, the rotator heads need to find a stop at [Cu(phenAr2)]I2− sites requiring displacement of iodide by the rotator head, i.e., the formation of a py→[Cu(phenAr2)]I site from attack of the py head onto [Cu(phenAr2)]I2−. At full loading of all copper centers with iodide, the copper−iodide dissociation becomes rate-determining. As a consequence, the rotational acceleration is only highly effective with ligands that are both good nucleophiles and nucleofuges. Interestingly, addition of the equally strong nucleophile PPh3 to ROT-1 does not increase the rotational frequency even up to 10 equiv of PPh3 addition (SI, Figure S18). At higher amount of PPh3, the NMR becomes very broad, preventing kinetic analysis. However, even after addition of 50 equiv of PPh3, the DABCO peaks remain intact, which implies that the assembly is undamaged. As before, we expect that the first 6 equiv of PPh3 is bound to the free coordination sites of the four copper(I) centers. Addition of iodide to ROT-16PPh3 (ROT-1 + 6 equiv PPh3) did not affect the rotational frequency (SI, Figure S19). Presumably, as the binding strength of PPh3 to the Cu+loaded station is much higher (log K = 6.28 ± 0.40), both the rotator arm and iodide are unable to replace the PPh3 ligand (SI, Figure S34).

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EXPERIMENTAL SECTION

All solvents were dried by distillation prior to use while commercial reagents were utilized without any further purification. Bruker Advance (400 MHz) and Varian (600 MHz) spectrometers were used to measure 1H and 13C NMR spectra using a deuterated solvent as the lock and residual protiated solvent as internal reference (CDCl3: δH 7.26 ppm, δC 77.0 ppm; CD2Cl2: δH 5.32 ppm, δC 53.8 ppm; THF-d8: δH 1.72 ppm, 3.58 ppm, δC 25.3 ppm, 67.2 ppm; CD3CN: δH 1.32 ppm, δC 118.26 ppm). The following abbreviations were used to define NMR peak patterns: s = singlet, d = doublet, t = triplet, dd = doublet of doublets, ddd = doublet of doublets of doublets, bs = broad signal, m = multiplet. The coupling constant values are given in hertz (Hz) and, wherever possible, assignment of protons is given. The numbering of carbons in different molecular skeletons does not necessarily follow IUPAC nomenclature rules; it is exclusively done for assigning NMR signals. ROESY spectra were obtained with 1.5 s relaxation delay and 300 ms mixing time. All electrospray ionization (ESI-MS) spectra were recorded on a Thermo-Quest LCQ deca, and the theoretical isotopic distributions of the mass signals were calculated (https://omics.pnl.gov/software/ molecular-weight-calculator) using molecular weight calculator software. Melting points of compounds were measured on a BÜ CHI 510 instrument and are not corrected. Infrared spectra were recorded on a Varian 1000 FT-IR instrument. Elemental analysis was performed using the EA-3000 CHNS analyzer. Column chromatography was performed either on silica gel (60−400 mesh) or on neutral alumina (Fluka, 0.05−0.15 mm, Brockmann Activity 1). Merck silica gel (60 F254) or neutral alumina (150 F254) sheets were used for thin layer chromatography (TLC). All rotor preparations were performed directly in the NMR tube using CD2Cl2, CD3CN, or a mixture of CD2Cl2 and CD3CN as solvent. Pre-Rotor Complex PreROT-1. In an NMR tube, stator 2 (1.50 mg, 0.603 μmol), rotator 1 (0.584 mg, 0.603 μmol), DABCO (67.6 μg, 0.603 μmol), and 2 equiv of [Cu(CH3CN)4]PF6 (0.449 mg, 1.21 μmol) were dissolved in 600 μL of CD2Cl2. Subsequent sonication for 10 min furnished the complex PreROT-1 in quantitative yield. IR (KBr): ν = 558, 720, 796, 846, 995, 1204, 1339, 1383, 1492, 1609, 2920, 2952, 3436 cm−1. 1H NMR (CD2Cl2, 400 MHz): δ = −[4.96− 4.92] (m, 6H), −[4.86−4.82] (m, 6H), 1.45 (s, 6H), 1.52 (s, 6H), 1.97 (s, CH3, CH3CN), 2.02 (s, 6H), 2.05 (s, 18H), 2.09 (s, 12H),

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CONCLUSIONS Both, the nucleophilic solvent CD3CN and iodide speed up rotational exchange in the multicomponent rotor, whereas ligand PPh3 does not increase the rotational frequency. CD3CN and I− lower the activation barrier for departure of the pyridine-terminated arm from the copper(I)-loaded phenanthroline station. By increasing the amount of CD3CN or I−, the four-component static assembly can be reversibly turned into a four-component high-speed nanorotor. With iodide, the effect of maximum acceleration is larger than 2 × E

DOI: 10.1021/acs.inorgchem.8b03567 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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2.15 (s, 6H), 2.17 (s, 6H), 2.38 (s, 6H), 2.48 (s, 6H), 2.58 (s, 6H), 2.60 (s, 6H), 2.64 (s, 6H), 2.74 (s, 12H), 6.62 (d, 3J = 6.4 Hz, 4H), 6.97 (d, 3J = 8.0 Hz, 2H), 7.01 (s, 4H), 7.12 (s, 2H), 7.15 (s, 2H), 7.17 (s, 2H), 7.30 (s, 2H), 7.36 (d, 3J = 6.4 Hz, 4H), 7.38 (d, 3J = 16.4 Hz, 2H), 7.47 (d, 3J = 8.0 Hz, 2H), 7.60 (d, 3J = 8.2 Hz, 2H), 7.63 (d, 3 J = 8.2 Hz, 2H), 7.70 (d, 3J = 8.0 Hz, 2H), 7.80 (d, 3J = 16.4 Hz, 2H), 7.90 (d, 3J = 8.0 Hz, 2H), 7.91 (d, 3J = 8.0 Hz, 2H), 7.92 (d, 3J = 8.0 Hz, 2H), 7.96 (s, 4H), 7.97 (d, 3J = 8.0 Hz, 2H), 7.99 (d, 3J = 8.0 Hz, 2H), 8.03 (d, 3J = 8.2 Hz, 4H), 8.04 (d, 3J = 8.0 Hz, 2H), 8.08 (d, 3 J = 8.0 Hz, 2H), 8.24 (s, 4H), 8.27 (d, 3J = 8.0 Hz, 2H), 8.35 (s, 2H), 8.38 (d, 3J = 8.2 Hz, 2H), 8.41 (d, 3J = 4.6 Hz, 2H), 8.42 (d, 3J = 8.2 Hz, 2H), 8.49 (d, 3J = 8.0 Hz, 2H), 8.53 (d, 3J = 4.6 Hz, 2H), 8.57 (s, 2H), 8.61 (s, 2H), 8.66 (d, 3J = 4.6 Hz, 2H), 8.69 (d, 3J = 4.6 Hz), 8.72 (s, 2H), 8.78 (d, 3J = 8.2 Hz, 2H), 8.79 (d, 3J = 8.2 Hz, 2H) ppm. Elemental analysis: Anal. Calcd for C246H202Cu2F12N20P2Zn2· 3CH2Cl2: C, 70.52; H, 4.94; N, 6.61. Found: C, 70.96; H, 4.54; N, 6.67. ESI-MS: m/z (%) 1792.1 (100) [Cu2(1)(2)]2+, 1844.6 (10) [Cu2(1)(2)(DABCO)]2+. Nanorotor ROT-1. Two equivalents of [Cu(CH3CN)4]PF6 (0.449 mg, 1.21 μmol) was added to complex PreROT-1. Sonication for 10 min quantitatively afforded ROT-1. IR (KBr): ν = 552, 632, 659, 721, 796, 827, 850, 995, 1038, 1064, 1108, 1203, 1336, 1382, 1417, 1480, 1522, 1596, 1632, 2236, 2852, 2916, 3025, 3436 cm−1. 1 H NMR (CD2Cl2, 400 MHz): δ = −[4.95−4.91] (m, 6H), −[4.85− 4.81] (m, 6H), 1.45 (s, 6H), 1.52 (s, 6H), 1.98 (s, CH3, from [Cu(CH3CN)4]PF6), 2.02 (s, 6H), 2.05 (s, 18H), 2.08 (s, 12H), 2.16 (s, 6H), 2.17 (s, 6H), 2.40 (s, 6H), 2.48 (s, 6H), 2.57 (s, 6H), 2.60 (s, 6H), 2.64 (s, 6H), 2.77 (s, 12H), 6.64 (d, 3J = 6.2 Hz, 4H), 6.97 (d, 3J = 8.0 Hz, 2H), 7.07 (s, 4H), 7.12 (s, 2H), 7.15 (s, 2H), 7.18 (s, 2H), 7.28 (s, 2H), 7.37 (d, 3J = 6.2 Hz, 4H), 7.39 (d, 3J = 16.4 Hz, 2H), 7.54 (d, 3J = 8.0 Hz, 2H), 7.71 (d, 3J = 8.0 Hz, 2H), 7.80 (d, 3J = 16.4 Hz, 2H), 7.90 (d, 3J = 8.0 Hz, 2H), 7.91 (d, 3J = 8.0 Hz, 2H), 7.92 (d, 3 J = 8.0 Hz, 2H), 7.94 (d, 3J = 8.2 Hz, 2H), 7.98 (d, 3J = 8.0 Hz, 2H), 7.99 (d, 3J = 8.2 Hz, 2H), 8.00 (d, 3J = 8.0 Hz, 2H), 8.01 (d, 3J = 8.2 Hz, 2H), 8.02 (d, 3J = 8.2 Hz, 2H), 8.03 (d, 3J = 8.0 Hz, 2H), 8.04 (d, 3 J = 8.0 Hz, 2H), 8.18 (s, 2H), 8.19 (s, 2H), 8.24 (s, 4H), 8.27 (d, 3J = 8.0 Hz, 2H), 8.34 (s, 2H), 8.40 (d, 3J = 4.6 Hz, 2H), 8.49 (d, 3J = 8.0 Hz, 2H), 8.54 (d, 3J = 4.6 Hz, 2H), 8.58 (s, 2H), 8.59 (s, 2H), 8.65 (d, 3J = 4.6 Hz, 2H), 8.70 (d, 3J = 8.2 Hz, 2H), 8.71 (d, 3J = 4.6 Hz, 2H), 8.74 (s, 2H), 8.75 (d, 3J = 8.2 Hz, 2H), 8.77 (d, 3J = 8.2 Hz, 2H), 8.78 (d, 3J = 8.2 Hz, 2H) ppm. ESI-MS: m/z (%) 927.2 (100) [Cu4(1)(2)]4+.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03567.

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Article

Synthesis and characterization of ligands and model complexes, proton assignments of the rotor, rate constant determination, titrations, and binding constant measurements (PDF)

AUTHOR INFORMATION

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*E-mail: [email protected]. ORCID

Michael Schmittel: 0000-0001-8622-2883 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are indebted to the Deutsche Forschungsgemeinschaft for generous financial support (DFG Schm 647/20-2). F

DOI: 10.1021/acs.inorgchem.8b03567 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03567 Inorg. Chem. XXXX, XXX, XXX−XXX