metal centred azaphosphatriptycene gear with a photo and thermally driven mechanical switching function based on coordination isomerism

Metal-centred azaphosphatriptycene gear with a photo- and thermally driven mechanical switching function based on coordination isomerism Hitoshi Ube 1 , Yoshihiro Yasuda 1 , Hiroyasu Sat

Metal-centred azaphosphatriptycene gear with a photo- and thermally driven mechanical switching function based on coordination isomerism

Hitoshi Ube 1 , Yoshihiro Yasuda 1 , Hiroyasu Sato 2 & Mitsuhiko Shionoya 1

Metal ions can serve as a centre of molecular motions due to their coordination geometry,

reversible bonding nature and external stimuli responsiveness Such essential features of

metal ions have been utilized for metal-mediated molecular machines with the ability to

motion switch via metallation/demetallation or coordination number variation at the metal

centre; however, motion switching based on the change in coordination geometry remain

largely unexplored 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 PtIIion 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.

1Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.2Rigaku Corporation, 3-9-12 Matubara-cho, Akishima, Tokyo 196-8666, Japan Correspondence and requests for materials should be addressed to M.S

(email: shionoya@chem.s.u-tokyo.ac.jp)

T ransmission of rotary motion is a key process of molecular

machines1–3 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

intramole-cularly correlated rotators4–9 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 rotators12–21 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

responsiveness22–24 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 reported25–27.

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

azaphosphatripty-cene 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 PtIIstator28,29 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.

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 PtIIion in a cis position, they should

gear with each other.

Preparation and characterization of PtIIcomplexes The

reac-tion of rotator 1 and 0.5 eq of K2PtCl4in 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 col-ourless crystals in 59% yield (Supplementary Figs 11 and 13) The coordinating donor atom and the geometry around the

PtIIcentre in solution were determined by NMR spectroscopy.

In a 31P NMR spectrum of the cis isomer in C6D6, a31P-195Pt coupling was observed (J1¼ 3,625 Hz; Supplementary Fig 12) due

to the binding of phosphine ligand to the central PtIIion 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,

DHz¼ 16.4±0.2 kcal  mol 1 and DSz¼  0.9±0.7 cal  K 1 (Fig 2c, Supplementary Fig 15) The enthalpy value is sig-nificantly lower than the reported value for covalently connected triptycene gears15,19.

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 ¼ 50:50

by1H NMR in CDCl3at 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, a31P-195Pt coupling (J1¼ 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–p interactions were observed between the two rotators Each PtIIion 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 PtIIstator.

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.

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

OMe

OMe Cl

Cl Pt P P N

N P P N OMe

trans-PtCl212

cis-PtCl212

Cl Pt Cl N

P N

MeO

PtII Stator

hν Δ

Rotator

K2PtCl4 –2KCl

Figure 1 | Schematic representation of a PtII-centred molecular gear PtCl212 This molecular gear has two azaphosphatriptycene rotators coordinating to the central PtIIion 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

Rotator 1 a

OMe

3 ′ 1 ′

4 ′

4 ′

4 ′

4 ′

1 ′

3 ′

4

4

1

1 ′

3 ′

3 ′

1 ′,2 3

2 3

4 1

N P 1 2

3 4

p.p.m

meso

dl

380 K k = 1.0 × 102 s –1

k = 20 s–1

k = 4.0 s–1

k = 0.80 s–1

k = 0.10 s–1

360 K

340 K

320 K

300 K

7.00 6.95 6.90 6.85 7.00 6.95 6.90 6.85

cis Gear (cis-PtCl212)

trans Gear (trans-PtCl212)

Figure 2 |1H NMR spectra of rotator 1 and cis- and trans-PtCl212 The signals of a methoxy group of rotator 1 (B3.8 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-PtCl212and (iii) trans-PtCl212(500 MHz, CDCl3,

300 K) The1H NMR spectra of cis-PtCl212include meso and dl isomers in aB1:2 ratio (b) Isomerism based on rotational conformation (c) Observed and simulated spectra of 3-positions’ proton at varied temperatures (500 MHz, TCE-d2/toluene-d8¼ 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

temperature that it was difficult to obtain a 1H NMR spectrum

of pure trans complex in the TCE-d2solution because of the rapid

isomerization to the cis form After 10 h, the trans complex was

transformed into cis form nearly quantitatively (cis/trans ¼ 98:2;

Supplementary Fig 22) This structural interconversion was

repeatable in a mixed solvent of TCE-d2/toluene-d8¼ 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 ¼ 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

Figure 3 | X-ray crystal structures of cis- and trans-PtCl212 (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

hv (360 nm)

i

a

ii

iii

iv

1.00

) 0.75 0.50 0.25 0.00

Time (h)

UV Heat

Δ

Figure 4 | Photo and thermal switching of molecular gear PtCl212 (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-PtCl212in toluene-d8(cis/trans¼ 99:1); (ii) a solution of (i) after photoirradiation at 360 nm (after 30 min, cis/trans ¼ 15:85); (iii) a solution of single crystals of trans-PtCl212in TCE-d2(after 5 min, cis/trans¼ 12:88); (iv) a solution of (iii) after 10 h at room temperature (cis/trans ¼ 2:98) (c) Reversible switching of the molecular gearing system, PtCl212, in TCE-d2/toluene-d8¼ 1:1 (v v 1)

solution was heated at 100 °C for 10 h, the trans-based solution

was reversed to the cis-based solution with the cis/trans ¼ 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).

Discussion

In conclusion, we have developed a molecular gear, PtCl212,

composed of two azaphosphatriptycene rotators 1 with a PtIIion

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-phospha-triptycene (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 (d 0 p.p.m.) for1H and13C NMR measurements when

CDCl3was 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-phospha-triptycene (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): d 7.81–7.78 (m, 4H),

7.71–7.66 (m, 2H), 7.05 (d, J ¼ 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): d  31.0 (JP–Pt¼ 3,625 Hz); HRMS (CHCl3/CH3CN/

HCO2H, positive): [PtCl12]þ(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 mmol) 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 mmol,

29%).1H NMR (CDCl3, 500 MHz, 300 K): d 8.78–8.75 (m, 4H), 8.70 (dt, J ¼ 8.4,

5.4 Hz, 2H), 7.66 (dd, J ¼ 7.7, 1.1 Hz, 4H), 7.34 (td, J ¼ 7.6, 1.2 Hz, 4H), 7.28–7.25

(m, 2H), 7.22 (td, J ¼ 7.5, 1.3 Hz, 4H), 6.72 (dt, J ¼ 8.4, 1.2 Hz, 2H), 3.82 (s, 6H);

31P NMR (CDCl3, 202 MHz, 300 K): d  39.0 (JP–Pt¼ 2,937 Hz); HRMS

(CHCl3/CH3CN/HCO2H, positive): [PtCl12]þ(C38H28ClN2O2P2Pt) m/z 836.0958

(required, 836.0957)

Photo- and thermally driven isomerization of PtCl212.A 1.0 mM solution of

cis-PtCl212in TCE-d2/toluene-d8¼ 1:1 (600 ml, 0.60 mmol) and a 100 mM solution

of 1,4-dioxane in TCE-d2/toluene-d8¼ 1:1 (6.0 ml, 0.60 mmol) 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 ¼ 10 nm) at room

tempera-ture, 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 þ diffractometer with MoKa radiation

(for cis-PtCl212) or Rigaku RAXIS-RAPID imaging plate diffractometer with

MoKa 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.ca-m.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

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Acknowledgements

This study was supported by JSPS KAKENHI Grant Numbers JP26248016 and

JP16H06509 (Coordination Asymmetry)

Author contributions

H.U., Y.Y and M.S conceived and designed the experiments and analysed the data Y.Y performed the experiments H.U and H.S measured and solved the X-ray crystallographic analyses H.U and M.S prepared the manuscript

Additional information

Supplementary Informationaccompanies this paper at http://www.nature.com/ naturecommunications

Competing financial interests:The authors declare no competing financial interests Reprints and permissioninformation is available online at http://npg.nature.com/ reprintsandpermissions/

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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