Introduction

Transmission of rotary motion is a key process of molecular machines1,2,3. To correlate two or more movable elements in a controllable manner, a stator, which can bring them appropriately close to each other, is a key part of motion. A significant number of excellent examples have been reported on synthetic molecular gearing systems with intramolecularly correlated rotators4,5,6,7,8,9. However, the control of rotary transmission between molecular rotators is still in an early phase10,11, and therefore, in particular, switchable motion transmission is a challenge. Dynamic engagement between rotators is a typical rotary transmission in molecular machines. Triptycene is a well-known part of gear molecules with a rigid, highly symmetrical paddlewheel structure. More than one triptycene rotators can be covalently connected with an organic stator designed so as to have a proper positional relationship between the connected rotators12,13,14,15,16,17,18,19,20,21. Another unique example of covalently linked systems is a silicon-centred bistriptycene system19, which undergoes switchable gearing triggered by a chemical stimulus, fluorination/defluorination. We focused on metal ions as a control element of molecular motions due to their essential features such as reversible bonding natures with ligands, dynamic ligand exchange and external stimuli responsiveness22,23,24. A certain number of excellent examples of metal-mediated molecular machines capable of motion switching via metallation/demetallation or coordination number variation on the metal centre have been reported25,26,27.

Herein, we report a PtII-centred molecular gear that demonstrates control of rotor engagement and disengagement based on photo- and thermally driven cis–trans isomerization at the PtII centre. This molecular rotary motion transmitter has been constructed from two coordinating azaphosphatriptycene rotators and one PtII ion as a stator. Isomerization between an engaged cis-form and a disengaged trans-form is reversibly driven by ultraviolet irradiation and heating. Such a photo- and thermally triggered motional interconversion between engaged/disengaged states on a metal ion would provide a selector switch for more complex interlocking systems.

Results

Design of metal-centred molecular gear

In this study, we have developed a metal-centred molecular gear PtCl212, in which two ligands as rotators, 2-methoxy-9-aza-10-phosphatriptycene (1), directly bind to the PtII stator28,29. One striking feature of this system is a clutch-like function that allows switching of the engagement of the two rotators based on photo- and thermally driven cis–trans isomerization on the PtII centre in a traceless manner with no chemical by-products (Fig. 1)30,31.

Figure 1: Schematic representation of a PtII-centred molecular gear PtCl212.
figure 1

This molecular gear has two azaphosphatriptycene rotators coordinating to the central PtII ion as a stator. Isomerization between an engaged cis-form and a disengaged trans-form are reversibly driven by ultraviolet irradiation at 360 nm and heating.

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Synthesis of azaphosphatriptycene ligand

2-Methoxy-9-aza-10-phosphatriptycene (1), in which the bridgehead positions of triptycene are replaced by a nitrogen and a phosphorus atoms, was chosen as a ligand-type rotator32. The rotator 1 was synthesized from 3-anisidine in four steps in 9% overall yield, and was characterized by 1H, 13C and 31P nuclear magnetic resonance (NMR) spectroscopy, electrospray ionization-mass spectrometry and elemental analysis (Supplementary Figs 1–10). We expected that when two rotators 1 as monodentate phosphine ligands bind to a compatible PtII ion in a cis position, they should gear with each other.

Preparation and characterization of PtII complexes

The reaction of rotator 1 and 0.5 eq of K2PtCl4 in EtOH/H2O (1/1, v v−1) at room temperature for 21 h afforded a mixture of square-planar PtII–phosphine complexes, cis-PtCl212 and a small amount of trans-PtCl212 (Figs 1, 2a). After recrystallization from CHCl3/diethyl ether, pure cis-PtCl212 was successfully obtained as colourless crystals in 59% yield (Supplementary Figs 11 and 13). The coordinating donor atom and the geometry around the PtII centre in solution were determined by NMR spectroscopy. In a 31P NMR spectrum of the cis isomer in C6D6, a 31P-195Pt coupling was observed (J1=3,625 Hz; Supplementary Fig. 12) due to the binding of phosphine ligand to the central PtII ion. Then, a 1H NMR spectrum of the cis isomer showed that the proton signals for the 3-positions of azaphosphatriptycene shifted upfield from the free ligand, indicating that two azaphosphatriptycene ligands were close to each other. Moreover, a lineshape analysis of the spectral pattern of the proton signals of the 4-positions suggests that there are two rotational isomers, meso and dl forms, in a 1:2 ratio in solution (Fig. 2b, Supplementary Fig. 14). We analysed the kinetics of the gear slippage during interconversion between meso and dl isomers of cis-PtCl212. Variable temperature 1H NMR measurement of cis-PtCl212 was described by the two-signal overlap model of the meso and dl isomers. An Eyring plot of the exchange rates of the isomers at every temperature gave activation parameters of the interconversion, ΔH‡=16.4±0.2 kcal·mol−1 and ΔS‡=−0.9±0.7 cal·K−1 (Fig. 2c, Supplementary Fig. 15). The enthalpy value is significantly lower than the reported value for covalently connected triptycene gears15,19.

Figure 2: 1H NMR spectra of rotator 1 and cis- and trans-PtCl212.
figure 2

The signals of a methoxy group of rotator 1 (∼3.8 p.p.m.) are omitted for clarity. For the whole NMR spectra, see the Supplementary Figures 7, 11 and 16. (a) 1H NMR spectrum of (i) 1, (ii) cis-PtCl212 and (iii) trans-PtCl212 (500 MHz, CDCl3, 300 K). The 1H NMR spectra of cis-PtCl212 include meso and dl isomers in a ∼1:2 ratio. (b) Isomerism based on rotational conformation. (c) Observed and simulated spectra of 3-positions' proton at varied temperatures (500 MHz, TCE-d2/toluene-d8=1:1). Left: observed spectra in the range from 380 to 300 K. Right: simulated spectra based on the two-state exchange model. Green and orange circles denote meso and dl isomers, respectively.

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It is noteworthy in connection with motion transmission that a certain number of PtII complexes display photo-driven cis/trans isomerization30,31. Under photoirradiation at 360 nm, cis-PtCl212 was found to be isomerized to trans-PtCl212 in C6H6 at room temperature (cis/trans=50:50 by 1H NMR in CDCl3 at 300 K). As a result of slow evaporation, trans-PtCl212 was isolated as yellow crystals in 29% yield (Supplementary Figs 16 and 17). In a 31P NMR spectrum of the trans isomer in CDCl3, a 31P-195Pt coupling (J1=2,937 Hz, in Supplementary Fig. 18) indicated that the two phosphine ligands bind to the central PtII ion. On the other hand, its 1H NMR spectrum in CDCl3 showed neither splitting nor upfield shift of the proton signals for the 4-positions, suggesting the absence of significant intramolecular interactions between the two rotators.

X-ray crystallographic analyses

Crystals of cis-PtCl212·(ether) suitable for single-crystal X-ray structure analysis were obtained by liquid–liquid diffusion of Et2O into a solution of PtCl212 in toluene. One molecule of diethyl ether was included into the unit structure (Fig. 3, Supplementary Fig. 19). The X-ray diffraction data demonstrated that the two rotators adopt an ‘engaged’ cis form in the solid state. A unit cell consists of a meso isomer of PtCl212, one rotational isomer comes from tight meshing of the two rotators. Notably, intramolecular CH–π interactions were observed between the two rotators. Each PtII ion is in a distorted square planar geometry, in which the P–Pt–P angle is over 90° (99.95(5)°) due to the bulkiness of rotator 1. The two rotators 1 are thus suitably engaged with each other on the PtII stator. In contrast, single-crystal X-ray structure analysis of trans-PtCl212·(C6H6)2 revealed that the two phosphine ligands as rotators are across from one another in the square planar PtII complex (Supplementary Fig. 20). This photoisomerized trans form can be regarded as a ‘disengaged’ state of the metal-centred molecular gear.

Figure 3: X-ray crystal structures of cis- and trans-PtCl212.
figure 3

(a) cis-PtCl212·(ether). (b) trans-PtCl212·(C6H6)2. In both cases, the structures are indicated as ORTEP (Oak Ridge Thermal Ellipsoid Plot) diagram with 50% thermal ellipsoid (upper) and space-filling model (bottom). Solvents are omitted for clarity, and colours are coded according to CPK (Corey, Pauling, Koltun) colouring.

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Photo- and thermally driven isomerization of PtCl212

We then envisioned that this gear system could be applied to a stimuli-responsive molecular switch based on the photo- and thermally driven cis–trans isomerization in an appropriate solvent. It is well known that photo or thermal isomerization of diphosphine PtII complexes highly depends on the solvent polarity8. Polar solvents generally prefer cis form rather than trans form because the cis complex has a dipole moment that interacts better with the solvent polarity. Photo-driven isomerization from cis to trans form was then examined in a solvent with low polarity. Ultraviolet light at 360 nm was irradiated to a solution of pure cis-PtCl212 in toluene-d8 at room temperature (Fig. 4a,b). A photo stationary state was reached after 30 min, where the cis/trans ratio was changed to 15:85 (Supplementary Fig. 21). On the other hand, in more polar 1,1,2,2,-tetrachloroethane-d2 (TCE-d2), thermal isomerization from trans to cis was so fast in the dark at room temperature that it was difficult to obtain a 1H NMR spectrum of pure trans complex in the TCE-d2 solution because of the rapid isomerization to the cis form. After 10 h, the trans complex was transformed into cis form nearly quantitatively (cis/trans=98:2; Supplementary Fig. 22). This structural interconversion was repeatable in a mixed solvent of TCE-d2/toluene-d8=1:1. When a solution of pure cis-PtCl212 was irradiated by ultraviolet light at 360 nm, the cis to trans conversion proceeded smoothly at room temperature, and the reaction achieved its equilibrium (cis/trans=19:81) in 30 min. In this mixed solvent system, the interconversion from trans to cis was slow enough to determine the ratio of the complex by NMR at 300 K. When the trans-based solution was heated at 100 °C for 10 h, the trans-based solution was reversed to the cis-based solution with the cis/trans=78:22. This cis–trans isomerization process was thus repeatable at least three times by the repetition of stimuli (Fig. 4c and Supplementary Figs 23 and 24).

Figure 4: Photo and thermal switching of molecular gear PtCl212.
figure 4

(a) Photo- and thermally induced isomerization between cis- and trans-PtCl212. (b) 1H NMR spectra for photo- and thermally induced isomerization from cis- to trans-PtCl212 (500 MHz, 300 K). (i) A solution of single crystals of cis-PtCl212 in toluene-d8 (cis/trans=99:1); (ii) a solution of (i) after photoirradiation at 360 nm (after 30 min, cis/trans=15:85); (iii) a solution of single crystals of trans-PtCl212 in TCE-d2 (after 5 min, cis/trans=12:88); (iv) a solution of (iii) after 10 h at room temperature (cis/trans=2:98). (c) Reversible switching of the molecular gearing system, PtCl212, in TCE-d2/toluene-d8=1:1 (v v−1).

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Discussion

In conclusion, we have developed a molecular gear, PtCl212, composed of two azaphosphatriptycene rotators 1 with a PtII ion acting as a stator. The repeatable mechanical switching function based on the cis–trans isomerization at the PtII centre was achieved by photoirradiation and heating. Traceless external stimuli-responsive configurational changes of metal ions show promise as a movement element of molecular machines with a motion transmission function.

Methods

General information

Unless otherwise noted, solvents and reagents were purchased and used without further purification. 2-Methoxy-9-aza-10-phosphatriptycene (1) was synthesized from 3-anisidine (Supplementary Figs 1–10 and Supplementary Note 1).

1H, 13C, 31P NMR and other two-dimensional NMR spectra were recorded on a Bruker AVANCE III-500 (500 MHz) spectrometer. Tetramethylsilane was used as an internal standard (δ 0 p.p.m.) for 1H and 13C NMR measurements when CDCl3 was used as solvent. A residual solvent signal was used for calibration of 1H NMR measurements when other deuterated solvents (C6HD5: 7.16 p.p.m.; toluene-d7: 6.97 p.p.m.; 1,1,2,2-tetrachloroethane-d: 5.99 p.p.m.) was used as a solvent. ESI-TOF mass data were recorded on a Micromass LCT Premier XE mass spectrometer. Unless otherwise noted, experimental conditions were as follows: ion mode, positive; capillary voltage, 3,000 V; sample cone voltage, 30 V; desolvation temperature, 150 °C; source temperature, 80 °C). Melting point was measured by Yanaco Micro Melting Point Apparatus MP-500D and uncorrected. Elemental analysis was conducted in the Microanalytical Laboratory, Department Chemistry, Graduate School of Science, the University of Tokyo (Tokyo, Japan). Infrared spectra were recorded on a Jasco FT/IR 4,200 with an ATR equipment.

Synthesis of cis-PtCl212

To a 5.0 mM solution of K2PtCl4/H2O (40 ml, 0.20 mmol, 1.0 eq) was added a 10 mM solution of 2-methoxy-9-aza-10-phosphatriptycene (1) in EtOH (40 ml, 0.40 mmol, 2.0 eq). The suspended solution was then stirred at room temperature for 21 h in the dark. The resulting precipitate was collected by filtration, washed with H2O and EtOH, and dried under vacuum to give a colourless solid (144 mg). The crude product was purified by recrystallization from chloroform/diethyl ether to give cis-PtCl212 (112 mg, 0.118 mmol, 59%) as a colourless solid. 1H NMR (C6D6, 500 MHz, 300 K): δ 7.81–7.78 (m, 4H), 7.71–7.66 (m, 2H), 7.05 (d, J=7.6 Hz, 4H), 6.83 (s, 2H), 6.31–6.27 (m, 4H), 6.04–6.00 (m, 4H), 5.62–5.68 (m, 2H), 2.57 (s, 2H), 2.55 (s, 4H); 31P NMR (C6D6, 202 MHz, 300 K): δ −31.0 (JP–Pt=3,625 Hz); HRMS (CHCl3/CH3CN/HCO2H, positive): [PtCl12]+ (C38H28ClN2O2P2Pt) m/z 836.0958 (required, 836.0957).

Synthesis of trans-PtCl212

In a 50 ml three-necked flask, a solution of cis-PtCl212 (20.0 mg, 21 μmol) in benzene was irradiated at 360 nm for 1 h at room temperature. The solvent was removed by evaporation to give a yellow solid (22.5 mg). The crude product was recrystallized from benzene (2 ml) to obtain yellow crystals of trans-PtCl212·(C6H6)2. After dryness, 5.8 mg of desired complex was obtained, which contains 0.5 eq of benzene (confirmed by NMR, 6.1 μmol, 29%). 1H NMR (CDCl3, 500 MHz, 300 K): δ 8.78–8.75 (m, 4H), 8.70 (dt, J=8.4, 5.4 Hz, 2H), 7.66 (dd, J=7.7, 1.1 Hz, 4H), 7.34 (td, J=7.6, 1.2 Hz, 4H), 7.28–7.25 (m, 2H), 7.22 (td, J=7.5, 1.3 Hz, 4H), 6.72 (dt, J=8.4, 1.2 Hz, 2H), 3.82 (s, 6H); 31P NMR (CDCl3, 202 MHz, 300 K): δ −39.0 (JP–Pt=2,937 Hz); HRMS (CHCl3/CH3CN/HCO2H, positive): [PtCl12]+ (C38H28ClN2O2P2Pt) m/z 836.0958 (required, 836.0957).

Photo- and thermally driven isomerization of PtCl212

A 1.0 mM solution of cis-PtCl212 in TCE-d2/toluene-d8=1:1 (600 μl, 0.60 μmol) and a 100 mM solution of 1,4-dioxane in TCE-d2/toluene-d8=1:1 (6.0 μl, 0.60 μmol) were placed in an NMR tube, which was sealed by a septum rubber and degassed by freeze–pump–thaw three times. The reaction mixture was irradiated with ultraviolet lamp (ASAHI, MAX-303) using 360 nm filter (bandwidth=10 nm) at room temperature, and was heated at 100 °C in the dark (Supplementary Figs 19–22).

X-ray diffraction analysis

Single-crystal X-ray crystallographic analyses were performed using a Rigaku Saturn724+ diffractometer with MoKα radiation (for cis-PtCl212) or Rigaku RAXIS-RAPID imaging plate diffractometer with MoKα radiation (for trans-PtCl2l2), and obtained data were calculated using the Crystal Structure crystallographic software package except for refinement, which was performed using SHELXL-2014 (ref. 33). All hydrogen atoms were placed geometrically and refined using a riding model. Details for the synthesis and X-ray diffraction mesurements of both cis- and trans-complex are given in CIF files and Supplementary Figs 19 and 20 and Supplementary Note 2.

Data availability

Crystallographic data in this paper can be obtained free of charge from the Cambridge Crystallographic Data Centre (http://www.ccdc.cam.ac.uk/data_request/cif). The Deposit numbers are 1404948 (cis-PtCl212) and 1404949 (trans-PtCl212), respectively. All other data are available on this article and its Supplementary Information file.

Additional information

How to cite this article: Ube, H. et al. Metal-centred azaphosphatriptycene gear with a photo- and thermally driven mechanical switching function based on coordination isomerism. Nat. Commun. 8, 14296 doi: 10.1038/ncomms14296 (2017).

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