Phosphorus(V) Porphyrin-Based Molecular Turnstiles - Inorganic

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Phosphorus(V) Porphyrin-Based Molecular Turnstiles Ivan N. Meshkov,†,‡ Véronique Bulach,*,† Yulia G. Gorbunova,*,‡,§ Nathalie Kyritsakas,† Mikhail S. Grigoriev,‡ Aslan Yu. Tsivadze,‡,§ and Mir Wais Hosseini*,† †

Molecular Tectonics Laboratory, UMR UDS-CNRS, 7140 & icFRC, Université de Strasbourg, F-67000, Strasbourg, France Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky pr. 31-4, Moscow, 119071 Russia § Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky pr. 31, Moscow, 119991 Russia ‡

S Supporting Information *

ABSTRACT: A new cationic molecular turnstile based on a P(V) porphyrin backbone bearing two pyridyl interaction sites, one at the meso position of the porphyrin and the other on the handle connected to the porphyrin through P−O bonds, was designed and synthesized. The dynamic behavior of the turnstile 2, investigated by 1D and 2D 1H NMR techniques, showed that in the absence of an effector, the turnstile is in its open state and undergoes a free rotation of the rotor (the handle) around the stator (the porphyrin backbone). In the presence of an external effector such as Ag+ cation or H+, the turnstile is switched to its closed states 2-Ag+ and 2-H+, respectively. The locking/unlocking process is reversible and may be achieved by precipitation of AgBr upon addition of Et4NBr in the case of the silver-locked turnstile or by addition of Et3N in the case of the proton-locked turnstile.

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INTRODUCTION Control of movement has been a subject of interest since very ancient times. Over the past two decades, chemists designed and explored dynamic systems for which the intramolecular movements are controlled by external stimuli. On the basis of the pioneer investigations by the Sauvage1−3 and by the Stoddart−Balzani4−7 groups, several dynamic systems based on translational or rotatory motions have been designed.8−28 Among the many mobile systems such as, for example, motors, rotors, switches, elevators, wheelbarrows, nanocars, and caterpillars reported to date, molecular turnstiles29−32 have attracted our attention. The design of this category of dynamic entities is based on covalently linked rotors and stators, and their mobility results from the free rotation of the rotor around the stator which defines the open state of the turnstile. By addition of an external effector, the intramolecular movement may be blocked, affording thus the closed state of the turnstile. Over the past decade, we have reported a series of molecular turnstiles based on the porphyrin backbone33−40 or on organometallic Pt(II) complexes.41−44 Recently, we described a purely organic turnstile with optical reading between its open and closed states.45,46 Dealing with Sn(IV) porphyrin-based turnstiles, although the switching between the open and the closed states could be efficiently achieved using complexation processes of either Ag+ or Pd2+, owing to the solution reactivity of the Sn−O bond in the simultaneous presence of proton and even a weak nucleophile, acid−base switching between the two states could not be achieved. We thought that this issue could be solved by replacing Sn(IV) by P(V). Here, we report on the design, synthesis, and characterization of a series of new phosphorus(V) porphyrins-based molecular © XXXX American Chemical Society

turnstiles as well as the investigation of their switching between their open and closed states using either Ag+ or H+ as effector.

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RESULTS AND DISCUSSION Design of P(V) Porphyrin-Based Turnstile. Both the model compound 1 and the turnstile 2 are P(V) porphyrin derivatives (Scheme 1). The porphyrin backbone is considered as the stator, whereas the handle may be seen as the rotor. This assignment is arbitrary, and the two terms may be commuted. The two parts are covalently interconnected using the P(V) atom, located within the tetraaza core of the porphyrin, as a hinge. The interconnection of the handle to the stator is achieved by P−O bonds. Owing to the oxidation state of the phosphorus center, both compounds 1 and 2 are cationic in nature. Whereas for compound 1 all four meso positions on the porphyrin moiety are occupied by phenyl groups, for the turnstile 2 three out of the four meso positions are occupied by phenyl groups and the remaining position bears a pyridyl unit as an interaction site (coordinating or basic center). The handle, symmetric in nature, is composed of a 2,6pyridyl unit, as an interaction site, bearing two resorcinol moieties. The latter is connected to the pyridyl unit, using two ether junctions, by two triethylene glycol spacers. The choice of the latter was based on CPK (Corey−Pauling−Koltun) spacefilling models which revealed that the overall length of the handle should allow its free rotation around the stator. It is worth noting that compound 1 was designed as a model. Indeed, owing to the absence of interaction site on the porphyrin backbone, compound 1 may only be in an open Received: August 16, 2016

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DOI: 10.1021/acs.inorgchem.6b01989 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

noting that, to the best of our knowledge, no example of P(V) meso-pyridylporphyrin derivatives has been described. By slight modifications of the reported procedure,53 compound 4a (Scheme 2) was obtained in 82% yield upon treatment of 3a by a large excess of POCl3 and PCl5 in boiling pyridine during 24 h. Starting with meso-monopyridyltriphenylporphyrin 3b, the same reaction afforded the phosphorylated compound 4b in 40% yield. However, the process required 72 h. The same procedure, using compound 3c or 3d as precursor, failed to prepare the bis- and tetrakis-pyridyl P(V) derivatives, respectively. For the preparation of phosphoruscontaining phthalocyanines, the use of POBr3 in excess was reported to be efficient.60 Starting with 3b, replacement of the POCl3/PCl5 by POBr3 (25 equiv) afforded the P(V) porphyrin 4b in 85% yield in three steps, i.e., formation of the bromo compound, its hydrolysis, and subsequent generation of the chloro complex. It is worth noting that introduction of P(V) required 24 h when POCl3/PCl5 was used, whereas use of POBr3 required only 80 min. Starting with the bis- (3c) and tetrakis-pyridylporphyrin (3d) precursors, this procedure afforded the desired P(V) porphyrins 6c and 6d in 69% and 13% yields, respectively. However, for each transformation, the amount of POBr3 and the reaction time were required to be optimized (see experimental section in ESI). The P(V) porphyrin bearing two axial bromides 5a was found to be highly reactive and could not be isolated. It was hydrolyzed to its dihydroxy analogue 6a in 95% yield upon treatment with water. Purification of 6a was found to be tricky and tedious and required two chromatographic steps: (i) silica gel and (ii) gel permeation (see experimental section in ESI). The same procedure was used for preparation of the mono-, bis-, and tetrakis-pyridyl dihydroxyl derivatives 6b, 6c, and 6d. Axial Ligand Exchange. In order to find the best conditions to prepare the targeted compounds 1 and 2, i.e., introduction of the handle on the P(V) porphyrin precursors, the axial ligand exchange was investigated. Owing to the reactivity of the P−Cl bond toward phenol and alcohol derivatives,61−64 using model resorcinol-type ligands, the exchange was studied on the axial bis-chloro derivatives 4a and 4b. The latter were prepared by treating the dihydroxo complexes 6a and 6b, respectively, with SOCl2. Upon refluxing 4a or 4b with an excess (3 equiv) of 3-methoxyphenol in pyridine, both compounds 7a and 7b were obtained in 50% yield. It should be noted that the same procedure using the dihydroxy complexes 6a and 6b failed, and only decomposition of the porphyrin derivatives was observed. As mentioned above, although the dibromo complexes 5a−d could not be isolated owing to their reactivity, nevertheless, direct treatment of the mixture containing the monopyridyl derivative 5b with a large excess (450 equiv) of resorcinol afforded complex 8b in 55% yield. All isolated P(V) complexes were characterized in solution by 1H and 31P NMR (see experimental section in ESI). Furthermore, complexes 6b and 8b were also investigated in the solid state by X-ray diffraction on single crystals (see crystallographic part in ESI, Table S1, and Figures S34 and S35; for relevant bond distances and angles see Table S2). Single crystals of 6b were obtained at 25 °C upon vapor diffusion of n-pentane into a chloroform solution of the complex. The latter crystallizes (monoclinic, P21/c space group) with 1.6 chloroform molecules. Within the crystal composed of the cationic compound 6b and counterions, a

Scheme 1. Structures of the Phosphorus(V) Porphyrin Model Compound 1, the Turnstile 2, and Its Ag+ (1-Ag+) and H+ (1-H+) Complexes, and Assignments of Protons

state. Compound 2, bearing two pyridyl-type interaction sites, one on the stator and the other on the rotor, was designed to behave as a switchable turnstile. Indeed, compound 2 may be switched between its open and closed states in the absence and presence of an effector, respectively, through simultaneous interaction of the two pyridyl moieties with the effector. Synthesis of P(V) Porphyrins. P(V) porphyrin derivatives for which the phosphorus atom is bound to the tetraaza core of the porphyrin backbone have been reported.47 Depending on the substituents on the porphyrin moiety (β or meso), the reported synthetic strategy for incorporation of P(V) may be divided into two categories. P(V) β-octaethylporphyrin (H2OEP) was first obtained in 1977 by Gouterman48 using PCl3 in boiling pyridine. The replacement of PCl3 by PBr3 has been also documented.49−52 This approach was found to be unsuited for meso-arylporphyrins. However, Carrano and Tsutsui reported in 1977 the phosphorylation of tetraphenylporphyrin using POCl3 in boiling pyridine.53 This method has been further used to prepare P(V) meso-arylporphyrins derivatives.54−58 A series of meso-alkylporphyrins has been prepared using a mixture of PCl3 and lutidine.59 It is worth B


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Inorganic Chemistry Scheme 2. Structure of Precursor Porphyrins 3 and P(V) Porphyrin Derivatives 4−8

partial exchange (38%) of Br− by Cl− anions, arising most likely from the decomposition of CHCl3, was observed. The structure of 6b is given in Figure 1. The phosphorus atom in the oxidation state V adopts a distorted octahedral

Owing to the presence of H-bond donor (OH groups) and acceptor (pyridyl unit), compound 6b, behaving as a selfcomplementary unit, forms a 1D zigzag-type H-bonded network through hydrogen bonds (dO−N = 2.761 Å, dH−N = 1.951 Å) between one of the two axial OH moiety and the N atom of the pyridyl unit of the adjacent porphyrin molecule (Figure 2). It corresponds to a moderate type of hydrogen bonds in the solid phase65 (values for this type of bond distance between donor and acceptor is usually 2.5−3.2 Å) For the bis-resorcinol compound 8b, red-brown single crystals were obtained upon slow diffusion of n-hexane into a chloroform solution of the complex in the presence of traces of

Figure 1. Crystal structure of 6b, (C in gray, P in orange, N in blue and O in red) hydrogen atoms, solvent molecules and counterions are omitted for clarity.

geometry. Its coordination sphere is composed of four nitrogen atoms of the porphyrin core (P−N distances in the 1.859(4)− 1.870(4) Å range) and two oxygen atoms belonging to the two axial hydroxyl groups (P−O distances of 1.605(3) and 1.627(3) Å). The meso substituents are tilted with respect to the porphyrin mean plane with CCC dihedral angles of 118.43° for the pyridine moiety and in the 120.75−121.69° range for phenyl groups. In contrast with Sn(IV)-based complexes,34 owing to the smaller size of the P atom in the oxidation state V, the porphyrin ring is strongly distorted and adopts a “ruffled” deformation. The degree of deformation as well as structural metrics are close to those reported for reported P(V) porphyrin complexes.58

Figure 2. Fragments of the crystal structure of 6b showing the interconnection of consecutive porphyrin molecules leading to a 1-D zigzag-type H-bonded network. Phenyl meso substituents, hydrogen atoms, solvent molecules, and anions are omitted for clarity. C


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Inorganic Chemistry methanol and toluene. The complex crystallizes (triclinic, P-1 space group) with one CHCl3 molecule. The structure of the complex is shown in Figure 3. Again, a mixture of Cl− and Br− anions with a 0.504/0.496 ratio is

Scheme 3. Structure of the Handle 9 and the Intermediate 10

dissolved in freshly distilled pyridine and mixed with the handle 9 (1.1 equiv). This procedure, owing to the reactivity of the compound 5b, is rather tricky and should be precisely conducted as described in the experimental section in order to avoid decomposition. Stirring the mixture under argon for 24 h at 50 °C leads to formation of the intermediate 10 (Scheme 3) as the major component resulting from replacement of one of the two axial Br atoms on the P(V) porphyrin moiety by the handle behaving as a monodentate ligand. During isolation processes, the second Br axial ligands is replaced by an ethoxy group (intermediate 10a, ESI, Figures S6 and S7). Further reflux of the mixture for 2 h affords the desired turnstile 2 in 10% isolated yield. Again, purification was found to be tedious and required two chromatographic steps: (i) SiO2 and (ii) gel permeation on the Bio-Beads S-X1 column (ESI, Figures S3 and S4). Solution Dynamic Behavior of the Model Compound 1 and the Turnstile 2. The dynamic behavior of both model compound 1 and the turnstile 2 was investigated in CD3OD and CD3CN by NMR techniques. For model compound 1, the 1 H NMR spectrum is similar to reported P(V) tetraphenylporphyrin complexes (ESI, Figure S1). As expected, due to 1H−31P coupling, the β-pyrrolic protons Ha (see Scheme 1 for H atoms assignment) appears as a doublet. Moreover, signals corresponding to protons Hj and Hk of the resorcinol moieties are strongly upfield shifted due to the shielding effect of the porphyrin backbone. Signals Hn‑s of the handle appear as a broad multiplet between 3.5 and 4 ppm. The same observations hold for the turnstile 2. Interestingly, as for the model porphyrin 7b, for 2, signals corresponding to the β-pyrrolic protons Ha‑d appeared as a multiplet. This indicates that, on the NMR time scale, the average local environment of the porphyrin 2 is similar to that of 7b, implying thus the free rotation of the handle around the stator. This behavior was further confirmed by a 2D NOESY NMR experiment (Figure 4). As expected, the β-pyrrolic

by slight modification of previously reported procedures.34,66 Using conditions developed for the preparation of methoxyphenol-based compounds 7a and 7b, condensation of the handle 9 with 4a afforded compound 1 in 30% yield (Figures S1 and S2) only in a microwave oven and in dry pyridine. Unfortunately, under the same conditions, condensation of 9 with 4b failed. Indeed, only decomposition products were obtained. However, turnstile 2 could be successfully prepared using a two-step procedure starting from the hydroxy complex 6b. In a first step, compound 6b was treated with SOBr2, affording the bis-bromo derivative 5b that was directly

Figure 4. 1H−1H NOESY spectrum (CD3CN, 25 °C) for the turnstile 2. For signal assignment see Scheme 1.

Figure 3. Crystal structure of 8b (hydrogen atoms, solvent molecules, and anion are omitted for clarity).

present in the crystal. As for 6b discussed above, the phosphorus atom is hexacoordinated and surrounded by four nitrogen atoms of the porphyrin core (P−N distances in the 1.822(5)−1.840(5) Å range) and two oxygen atoms of axial hydroxyl ligands (P−O distances of 1.658(4) and 1.664(4) Å). The meso substituents are tilted with respect to the porphyrin mean plane with CCC dihedral angles in the 116.7−119.8° range. The meso pyridyl unit is disordered over four positions and cannot be identified. No hydrogen bond between the OH moiety of the resorcinol and the N atom of the pyridyl unit is spotted. Again, as for 6b, the P(V) porphyrin resorcinol complex adopts a pronounced “ruffled” deformation of the porphyrin ring. The two axial phenyl rings are not parallel but tilted by ca. 78°. Synthesis of Turnstile. The synthetic strategy adopted for preparation of the model compound 1 and the turnstile 2 was based on condensation of the dichloro porphyrin derivatives 4a and 4b with the handle 9 (Scheme 3). The latter was prepared

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Inorganic Chemistry

respectively, and for Hu and Hv protons of the pyridyl of the handle, signals are shifted by 0.44 and 0.78 ppm, respectively. The β-pyrrolic protons are also affected by the binding process. A splitting of protons Ha−Hd signals into two groups is observed. Owing to rather close chemical shifts of protons Ha and Hb, their signals appeared as a multiplet. In contrast, for protons Hc and Hd, two doublets of doublet (3J = 5.3 Hz, 4JP−H = 3.1) are observed. Signals corresponding to the triethylene glycol portions Hn− Hs of the handle appearing between 3.0 and 5.0 ppm (Figure 5, bottom) are also significantly affected by the presence of Ag+ cation. Indeed, upon addition of 2 equiv of AgOTf, the signal corresponding to protons Ht is downfield shifted by 0.42 ppm. The rather undefined and broad multiplet observed for 2 is split into five distinct signals. Although one would expect triplets for protons Hn−Hq, they appear as multiplets, probably resulting from the superposition of two triplets. It is worth noting that further addition of AgOTf (3 equiv) caused no spectral changes. A stability constant of log K= 2.83 ± 0.8 for the binding of silver cation was estimated using ChemEqui software.67,68 He, Hu, Hr, and Hs proton signals were used for the calculation. The binding constant for 2-Ag+ is an order of magnitude lower than the one obtained for the analogous turnstile based on Sn(IV) porphyrin.33 This observation may be explained by the positively charged nature of 2, the difference in the geometry between the Sn(IV) porphyrin (planar for the porphyrin backbone) and 2 (strongly distorted), and the solvent used. The locking of the rotary movement of the turnstile 2 by Ag+ cation leading to the closed state 2-Ag+ was unambiguously demonstrated by 2D 1H−1H NOESY NMR experiments in CD3CN in the presence of 3 equiv of AgOTf. In addition to similar correlations between the handle and the porphyrin backbone protons observed for the open state of the turnstile 2 (see Figure 4), as expected, other correlations between the stator and the rotor were detected for 2-Ag+ (Figure 6). The simultaneous binding of Ag+ cation by both pyridyl units imposes the close location of the pyridyl group at the meso position of porphyrin to both triethylene glycol fragments of the handle. Consequently, 1H−1H through-space

protons Ha‑d showed cross-peaks with both phenyl and pyridyl protons. As expected, cross-peaks between signals of Ht and Hu, Hn, and Hm protons were observed due to through-space interactions within the handle. Interestingly, no specific correlations between the handle and the porphyrin were detected, confirming the free rotation of the handle around the stator. Both the 1D- and the 2D-NMR observations discussed above imply that in the absence of an effector the turnstile 2 is in its open state. Control of the Turnstile 2 Motion. The turnstile 2 was designed to be switched between its open and closed states using an external effector. Indeed, compound 2 is equipped with two pyridyl units, one on the stator and the other on the rotor, as interaction sites. The rationale behind the choice of pyridyl moiety was, on one hand, its propensity to bind metal cations and, on the other hand, its ability to behave as a base and thus to undergo protonation. Thus, as external effectors, silver cation or proton was chosen (Scheme 1). The locking, through simultaneous binding of the effector by both pyridyl units, of the rotary movement (open state of the turnstile 2) leading to its closed states (2-M+, M = Ag or H) was investigated in solution (methanol-d4 or acetonitrile-d3) by 1Dand 2D-1H NMR experiments. Ag+ cation has been previously used as an effector for Sn(IV) porphyrin-based turnstiles.33,34 Silver Lock. Comparison of the 1H NMR spectra of the open (2) and closed (2-Ag+) states of the turnstile shows the simultaneous binding of silver cation by the pyridyl site on the handle and by the one located at the meso position of the porphyrin (Figure 5). Owing to the presence of Br− anion as

Figure 5. Portions of 1H NMR spectra (CD3OD, 400 MHz, 25 °C) of the turnstile 2 (c = 2.6 × 10−3 M) in the presence of increasing amounts of AgOTf (aromatic region of the spectra (top) and aliphatic region (bottom). For proton assignment see Scheme 1.

the counterion of the cationic turnstile 2, as expected, addition of 1 equiv of AgOTf leads to precipitation of AgBr, and thus, only small changes in 1H NMR spectra are observed. However, addition of a second equivalent of AgOTf leads to substantial changes. Signals belonging to both pyridine moieties are downfield shifted (Figure 5 top). Protons He and Hf belonging to the pyridyl unit located at the meso position of the porphyrin backbone are shifted by 0.30 and 0.43 ppm,

Figure 6. 1H−1H NOESY spectrum (CD3CN, 25 °C) of the closed state of the turnstile 2-Ag+. E


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Inorganic Chemistry correlations between the handle and the meso pyridyl moiety are expected. This was indeed the case. Correlations between meso pyridyl protons Hf and Hs, Hr, and Hq protons of the handle are observed. The same holds for He protons and the Hn proton of the handle. 2-Ag+ was also studied by HR electrospray ionization mass spectrometry. Two molecular ions with m/z values of 615.2309 and 668.1781 were observed. They correspond to the monoprotonated turnstile 2 ([M + H]2+, calcd 615.2323) and to the closed state of turnstile ([M + Ag]2+, calcd 668.1809). The latter peak indicates a 1:1 2/Ag+ stoichiometry. The choice of Ag+ cation as locking agent was motivated by its propensity to form insoluble complexes with halides (AgX, X = Cl, Br) and thus offering the possibility of switching the turnstile from its closed state 2-Ag+ to its open state 2 upon addition of a halide source. Starting from 2-Ag+, tetraethylammonium bromide (Et4NBr) was used as the opening agent and the process was monitored by 1H NMR. Addition of 1 equiv of Et4NBr to the solution containing 2-Ag+ caused precipitation of AgBr and generation of the open state 2. The reversibility of the process was established by the return to the closed state 2-Ag+ upon addition of 1 equiv of AgOTf (ESI, Figure S11). Four and one-half opening/closing cycles were performed (Figure 7).

Figure 8. Portions of 1H NMR spectra (CD3OD, 400 MHz, 25 °C) of the turnstile 2 (c = 9.8 × 10−3 M) in the presence of increasing amounts of TfOH (aromatic (top) and aliphatic (bottom) regions). For hydrogen atom assignment see Scheme 1.

As in the case of silver complexation by 2, the proton binding constant (log K = 3.69 ± 0.06) was also calculated using ChemEqui software using He, Ht, Hr, Hs, and protons signals. As in the case of 2-Ag+, the closed state of 2-H+ turnstile was investigated in CD3CN solution by 1H−1H NOESY NMR experiments (Figure 9). As expected, the m-pyridyl proton Hf, located in close proximity of the handle, showed correlations with the handle protons Hs, Hr, and Hq. Several other correlations between protons of the triethylene glycol spacers and those of the meso phenyl groups were also detected. This

Figure 7. Reversible opening and closing cycles for the turnstile 2. Starting from 2-Ag+, the opening process was achieved by addition of 1 equiv of NEt4Br and the open state 2 thus reached was reclosed upon addition of 1 equiv of AgOTf. For H atoms labeling see Scheme 1.

Proton Lock. As stated above, the closed state of the turnstile 2-H+ may be reached upon monoprotonation of the turnstile 2. The locking process was investigated in CD3OD by titration of a solution of 2 using triflic acid (TfOH) (Figure 8). Owing to the rather low basicity of the pyridyl moiety, 3 equiv of TfOH was required to reach the fully closed state. Upon addition of 3 equiv of TfOH to a solution of the turnstile 2, meso pyridyl protons Hf and He signals were downfield shifted by 0.33 and 0.70 ppm. Signals corresponding to pyridyl protons of the handle were also downfield shifted. However, whereas Hv proton signal was shifted by 0.88 ppm, the signal corresponding to proton Hu, owing to its overlap with the signals of the phenyl protons, could not be identified. β-Pyrrolic proton signals were found to be similar to those observed for the silver complex discussed above. Whereas Ha and Hb protons appeared as multiplets, Hc and Hd protons gave rise to two doublets of doublets (3J = 5.1 Hz, 4JP−H = 3.3). Signals for Ha and Hb protons overlap with the signal of proton Hf. In the presence of 3 equiv of TfOH, substantial changes for the triethylene glycol spacer of the handle protons signals were observed in the 3.0−5.0 ppm range (Figure 8).

Figure 9. 1H−1H NOESY spectrum (CD3CN, 25 °C) of the protonlocked state of the turnstile 2-H+ reached upon addition of 3 equiv of TfOH. F


Inorganic Chemistry

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may be due to the smaller size of the proton (ionic radius of 0.31 Å) when compared to silver cation (ionic radius of 1.45 Å).69 The acid/base reversible locking and unlocking of the turnstile 2 was also investigated by 1H NMR in CD3CN at 25 °C (ESI, Figure S14). Whereas as the proton source TfOH was used, as base Et3N was employed (Figure 10). Starting with the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01989. (CIF) (CIF) Full experimental details (materials and techniques, synthesis, characterization), NMR investigations, HRESI spectra, X-ray tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This contribution was financially supported by the University of Strasbourg, the International Centre for Frontier Research in Chemistry (icFRC), Strasbourg, the Labex CSC (ANR-10LABX- 0026 CSC) within the Investissement d’Avenir program ANR-10-IDEX-0002-02, the Institut Universitaire de France (IUF), the CNRS, and the Ministry of Education and Research and by the Russian Science Foundation (14-13-01373). I.N.M. is grateful to the French government for Joint Supervised Ph.D. Grants.

Figure 10. Acid and base reversible locking and unlocking cycles for the turnstile 2 using proton (triflic acid, TfOH) as closing agent and Et3N as opening reagent monitored by 1H NMR in CD3OD. For hydrogen atom assignment see Scheme 1.

closed state of the turnstile 2-H+ generated upon addition of 3 equiv of TfOH, the opening process was achieved upon addition of 4 equiv of Et3N. The open state of the turnstile 2 thus obtained was reclosed upon addition of 2 equiv of triflic acid. Four and one-half opening/closing cycles were achieved showing the reversibility of the process.

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CONCLUSIONS The unprecedented cationic molecular turnstile 2 based on a P(V) porphyrin backbone bearing a pyridyl interaction site at the meso position of the porphyrin and a handle, also bearing a pyridyl unit, connected to the porphyrin through P−O bonds was designed. The rationale behind the choice of the pyridyl moieties as interaction sites is based on its propensity to reversibly bind metal cations and to undergo protonation. Although P(V) porphyrin derivatives bearing aryl groups at the meso positions have been reported, to the best of our knowledge, no example of pyridyl-functionalized molecule has been documented to date. The synthesis of the turnstile 2 was found to be very challenging and required a proper strategy and suitable synthetic conditions. The dynamic behavior of the turnstile 2 was investigated by 1D and 2D 1H NMR techniques. In the absence of an effector, the turnstile 2 undergoes a free rotation of the rotor (the handle) around the stator (the porphyrin backbone). In the presence of an external effector such as Ag+ cation or H+, the turnstile is switched from its open state 2 to its closed states 2-Ag+ and 2-H+, respectively. The locking of the rotary movement results from simultaneous binding of the effector by both pyridyl groups, one located on the porphyrin backbone and the other on the handle. The locking/unlocking process is reversible. For the turnstile locked by Ag+ cation (2-Ag+), the return to the open state 2 is achieved by precipitation of AgBr upon addition of Et4NBr. For the turnstile locked by H+ cation (2-H+), the locking/unlocking process was based on sequential use of acid (TfOH) and base (Et3N). Again, several opening/closing cycles based on addition of Et4NBr (unlocking agent) and AgOTf (locking agent) were achieved without alteration of the turnstile.

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Phosphorus(V) Porphyrin-Based Molecular Turnstiles - Inorganic

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