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Networking Nanoswitches for ON/OFF Control of Catalysis Nikita Mittal, Susnata Pramanik, Indrajit Paul, Soumen De, and Michael Schmittel J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12951 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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Journal of the American Chemical Society

Networking Nanoswitches for ON/OFF Control of Catalysis Nikita Mittal,‡ Susnata Pramanik,‡ Indrajit Paul, Soumen De, Michael Schmittel* Center of Micro and Nanochemistry and Engineering, Organische Chemie I, Universität Siegen, Adolf-Reichwein Str. 2, 57068 Siegen, Germany KEYWORDS Molecular Switch, Catalysis, Metal Input, Allosteric Control, Network. ABSTRACT: The two nanoswitches 1 and 2 are interdependently linked together in so-called network states (NetStates). In NetState I, defined by the presence of [Cu(1)]+ and 2, the organocatalyst N-methylpyrrolidine catalyzes a conjugate addition. Addition of iron(II) ions as an external chemical trigger to NetState I discharges Cu+ from [Cu(1)]+. The liberated copper(I) ion acts as a second messenger and changes the toggling state at nanoswitch 2. The resulting nanoswitch [Cu(2)]+ captures the catalytically active species from solution and the conjugate addition is turned OFF. Removal of the original trigger reverses the whole sequence and turns catalysis ON. The ON/OFF catalytic cycle was run three times in situ.

While the quest for stand-alone molecular devices and artificial machines1 is a thrilling and demanding activity,2,3 networking them in homo- or hetero-oligomeric arrangements with unusual emergent properties is a supreme task for the upcoming years.4,5 Such endeavor is in line with Breslow’s recent inventory of the upcoming grand challenges of chemistry:6 one needs to move the focus of chemistry from individual substances to interacting chemical systems,7,8 e.g. as shown in biological cybernetic networks.9,10 Research in recent years has revealed an astounding variety of molecular switches11 that allow UP/DOWN and even ON/OFF regulation of catalytic processes.12-16 Since lately the first molecular switches have emerged that demonstrate reliable uni-17,18 or bidirectional19 interswitch communication, the time has come to network two or more of them in an effort to test the faultless information exchange in multi-switch systems with coupled function. As a proof of concept of rudimentary cybernetic regulation in solution, we will demonstrate how to link two distinct triangular nanoswitches in a network that is designed to administrate a catalytic process. In this network the switching states of both nanoswitches are connected via chemical signaling. In detail, when iron(II) ions are added as external trigger to nanoswitch [Cu(1)]+ the latter will command nanoswitch 2 to stop catalysis by sending out copper(I) ions as second messenger. Using a strong binder for iron(II), the full command sequence is inversed and catalysis is turned on. It is notable that Cu+ is involved in dynamic cell signaling pathways of various biological processes.20 The design of the present multi-functional two-nanoswitch system was guided by recent insights gained from some metallo-dependent nanoswitches21 used to trigger catalytic processes, such as click,22 Knoevenagel addition,22,23 and cyclopropanation24 reactions, which recommended using metal ions as chemical signals and coordinating ligands as receptor sites. Because in such setting, the two nanoswitches have individual switching states, but need to be considered as an entity in the process, we have defined the socalled network states (NetStates, s. Figure 1). In a network state both switches have to be in the correct switching states to warrant the required functionality. The NetStates I-II were conceived along the following considerations: (1) the initial network state should be composed of two

switches one of which harbors the projected second messenger. In our example: copper(I) ions; (2) the external input (in our example: iron(II) ions) should trigger the release of the second messenger; (3) the second messenger should act as an input signal for the second nanoswitch to change its toggling state; (4) toggling of the second nanoswitch should release or bind a catalytically active molecule, and (5) the catalyst should catalyze an organic reaction

Figure 1. Reversible bidirectional communication between nanoswitches 1 and 2. The different network states (NetState) encompass both nanoswitches in various switching states. 1 is present in three switching states: free 1, [Cu(1)]+ and the dimeric complex [Fe(1)2]2+. Nanoswitch 2 is present in two switching states: free 2 and [Cu(2)]+.

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where reactants and products do not interfere with the communication, switching and the release/binding protocol. In summary, the five requirements demand in both NetStates I and II a defined self-sorting25-27 of the metal ions on both switches and an interference-free catalytic reaction protocol. Taking into account all these points we chose the nanoswitches 124 and 219 as our two candidates (Figure 1) for the communication/catalysis cascade and established a two-step metal-dependent self-sorting protocol. The choice of 1 and 2 is recommended based on the following considerations: (a) The first network state (NetState I) involves a self-sorting event using 1, 2 and Cu+ in a 1:1:1 ratio. We expected a strong preference of copper(I) ion to bind to switch 1 with its HETTAP (= HETeroleptic Terpyridine And Phenanthroline complex)21 complexation site, because formation of the HETPHEN (= HETeroleptic bis-PHENanthroline complex)21 complex in [Cu(2)]+ requires the rupture of the pyrimidine→zinc(II) porphyrin (Npym→ZnPor) binding interaction in 2, that is worth ca. log K = 3.6.23 (b) The second self-sorting event (NetState I → NetState II) was conjectured based on earlier results,24 where addition of iron(II) to [Cu(1)]+ resulted in a dimeric iron bisterpyridine complex [Fe(Cu(1))2]4+. While in the earlier case, the Cu+ ions remained in the concave binding site of the shielded phenanthroline (log K ≈ 5.1) of [Fe(Cu(1))2]4+,24 in NetState II the copper(I) ions need to be translocated to the HETPHEN coordination site of nanoswitch 2 (log K ≈ 6.1).28 Despite this favorable thermodynamic driving force, undesired interferences are nevertheless possible, e.g. addition of Fe2+ could afford the trishomoleptic complex [Fe(2)3]2+ by coordinating to the pyridyl-pyrimidine swing arm of 2, which equally would cause the toggling arm to detach from the zinc porphyrin. (c) From our previous work22 it was known that switch 2 can modulate the binding of a catalytically active guest molecule, such as piperidine, at its zinc porphyrin site by addition and removal of Cu+ metal ion. Because the intramolecular Npym→ZnPor interaction is intact in switch 2, as present in NetState I (Figure 1), the catalytically active guest molecule would be free in solution and therefore able to catalyze an appropriate reaction. In NetState II, though, the dimeric switch [Fe(1)2]2+ will coexist with nanoswitch [Cu(2)]+, which has its zinc(II) porphyrin site liberated to capture the guest molecule so that the catalytic activity should stop. Synthesis, characterization and switching properties of nanoswitches 124 and 219 have already been reported in our previous publications. To investigate the first self-sorting event (1 + 2 → NetState I), a solution of [Cu(1)]+ was prepared in d2-dichloromethane, then 1 equiv. of switch 2 was added. The self-sorting occurred as anticipated, though, not in a quantitative manner (Figures 2d & S5, SI): the 1H NMR demonstrates that 90% of the copper(I) binds to switch 1 yielding [Cu(1)]+. Reciprocally, 10% of the copper(I) ions is bound as [Cu(2)]+. Thus, 90% of switch 2 remains free in the solution (Figure 2d). We will denote this equilibrium situation as NetState I. The 1H NMR signature of the mesityl protons m-/m'-H at 4.48, 5.89, 6.28, 6.43 ppm (Figure 2b and d) is highly characteristic for the formation of [Cu(1)]+. Presence of the free switch 2 was confirmed by its distinctive signals at 2.87 and 3.31 ppm corresponding to the protons b- and a-H, respectively, that are shifted upfield due to the immersion of the pyrimidine terminal into the π current of the porphyrin ring. The same equilibrium was also obtained by reacting [Cu(2)]+ with switch 1. The time dependence of the copper(I) ion translocation from [Cu(2)]+ to 1 was followed by UVvis spectroscopy. When 1 equiv. of 1 was added to the solution of [Cu(2)]+ (both at c = 1 × 10–4 M) the absorbance of the ZnPor unit at 550 nm fully shifted to 561 nm (Q-band of the self-locked switch 2) within 16 min at room temperature (SI, Figure S23). A kinetic analysis showed a pseudo-first-order kinetics (t1/2 = 105 s at 25 °C) with respect to [Cu(2)]+ (see SI, Figures S24 and S25),

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suggesting that the dissociation of the copper complex is rate-determining.

7.0

6.4

5.8

5.2

1

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Figure 2. Comparison of partial H NMR spectra (400 MHz, 298 K, CD2Cl2) of a) [Fe(1)2]2+; b) [Cu(1)]+; c) [Cu(2)]+; d) switch 1, switch 2 and Cu+ in 1:1:1 ratio (NetState I); e) addition of 0.5 eq. of Fe2+ to solution (d) leading to complete translocation of Cu+ from switch 1 to switch 2 (NetState II) and f) addition of 1 eq. of 4′-N,N-dimethylamino-2,2′:6′,2′′-terpyridine to solution (e) to remove Fe2+ (requires heating for 30 min at 40 °C) with concomitant reappearance of NetState I in the mixture. The forward communication (NetState I → NetState II) was accomplished by treating a solution of [Cu(1)]+ and 2 with 0.5 equiv. of Fe2+ to afford 0.5  [Fe(1)2]2+ and [Cu(2)]+. Formation of the iron bisterpyridine complex [Fe(1)2]2+ was expected from the higher binding constant of the [Fe(terpy)2]2+ (log K ≈ 16.9)29 complexation motif than that for a typical HETTAP complex (log K ≈ 9.1)24. We followed the cascade switching by characteristic 1 H NMR shifts at both nanoswitches (Figures 2e). The diagnostic signals for the a'-H and b'-H protons in [Fe(1)2]2+ show up at 6.97 and 7.05 ppm, respectively, quite different from those in [Cu(1)]+ (6.67 and 6.78 ppm). Moreover, characteristic signals at 5.93, 6.16, 6.26 and 6.37 ppm for the mesityl protons clearly attest the formation of complex [Cu(2)]+. The 1H NMR spectrum (Figure 2e) undoubtedly indicates complete translocation of Cu+ so that the clean formation of NetState II is corroborated. The switching from NetState I  II was also followed by UV-vis spectroscopy monitoring the Q band of the zinc porphyrin (SI, Figure S27). When a solution of NetState I (10–4 M) was treated with 0.5 equiv. of Fe2+ the absorption at 561 nm was fully shifted to 550 nm within a minute, indicating rapid relocation of the toggling arm in 2. However, even after 1 min there are further spectral changes observable at 585 nm. The finding that the final absorbance of the iron(II) bisterpyridine complex with its diagnostic MLCT band at 585 nm is only reached after a total of 3 min suggests a more complex reaction scenario. To elucidate details of this multi-step mechanism, we studied the reaction [Cu(1)]+ + 0.5 Fe2+ → [Fe(Cu(1))2]4+ at c = 10–4 M in dichloromethane separately. A UV-vis band at 585 nm (SI, Figure S28), characteristic for [Fe(terpy)2]2+ complexes,30 appeared whose maximum absorbance was reached within ca. 3-4 min, thus on a similar timescale as in the full cascade switching process (= [Cu(1)]+ + 2 + 0.5 Fe2+ → 0.5 [Fe(1)2]2+ + [Cu(2)]+). This finding suggests, not unexpectedly, that in the initial phase of the cascade switching Fe2+ does cleave complex [Cu(1)]+ (thus sending Cu+ to switch 2) and at the same time bind to the bipyridine site of switch 2 (Figures 3 & S30, SI). Both processes lead to a detachment of the toggling arm from the ZnPor site explaining the rapid initial spectral shift from 561 to 550 nm. Thereafter, on a little bit slower timescale, the


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iron(II) bisterpyridine complex [Fe(1)2]2+ is formed (see UV-vis increase at 585 nm) completing the translocation of the Cu+ ions to [Cu(2)]+. 31 In summary, the UV-vis experiments suggest that the copper translocation from [Cu(1)]+ to switch 2 is finished within 3 min in presence of Fe2+, but follows a complex mechanism. The sum of results demonstrate a fast signaling cascade at c = 10–4 M in dichloromethane.

rin π-ring current (SI, Figure S9). Now the control experiment was performed in presence of 5 (9 mol%) and [Cu(2)]PF6 (10 mol%). Again no product 6 was afforded under standard conditions (SI, Figure S16). Because [Cu(2)]+ is present in NetState II (Figure 1), NetState II should be an OFF state for catalysis. A titration indicated that 5 is strongly bound to the ZnPor unit in [Cu(2)]+ at log K = 4.20 ± 0.68 (SI, Figure S33), thus explaining the lack of product formation.

8%

Figure 4. The model reaction and evolution of 6 with time.

Figure 3. The mechanism of the communication step (grey: present as minor constituents in the fast step). NetState I, i.e. [Cu(1)]+ and free switch 2, was easily regenerated by removing the Fe2+. Treating the mixture of 0.5  [Fe(1)2]2+ and [Cu(2)]+ (= NetState II) with 1 equiv. of 4′-N,N-dimethylamino2,2′:6′,2′′-terpyridine afforded the homoleptic complex [Fe(4′N,N-dimethylamino-2,2′:6′,2′′-terpyridine)2]2+ and to our satisfaction regenerated NetState I. However, unlike the transformation NetState I → II, this communication step was much slower than the forward process and the solution needed to be heated at 40 °C for 30 min or alternatively sonicated for ca. 10 min at 40 °C (Figures 2f & S7f, SI). The much slower back-reaction is not surprising since a kinetically rather stable [Fe(terpy)2]2+ complex has to be cleaved in the rate-determining step. 1H NMR confirmed the regeneration of NetState I (Figure 2f) in solution. Later we will see that the slow regeneration of NetState I reduces the yield in the catalytic cycles 2 & 3. After successfully establishing a reversible NetState I / II transformation via copper(I) ion translocation as a messenger, we evaluated whether this pair of communicating switches could be harnessed for regulating a catalytic process. In earlier work, we had shown that switch 2 is able to modulate the catalytic activity of piperidine by releasing and capturing it at the ZnPor unit in an ON and OFF manner.22 For the present system, though, the catalytic reaction should not interfere with the communication process as would a Knoevenagel addition product22 due to its chelate binding site. After some screening, we finally identified the N-methylpyrrolidine (5)-catalyzed conjugate addition of substrates 3 and 4. The reaction of thiophenol (3) with 2-cyclopentenone (4) in presence of 9 mol% of N-methylpyrrolidine (5) under standardized conditions (DCM, 2 h, 40 °C) furnished product 6 in 332% yield (Figure 4). Before turning to the final multi-component communication/catalysis system we first assessed the ability of zinc(II) 5,10,15,20tetraphenylporphyrin (ZnTPP), which is a mimic of the free ZnPor site in [Cu(2)]+, to stop the conjugate addition reaction by inhibiting the catalytically active N-methylpyrrolidine (5). Indeed, the reaction 3+4 → 6 (both 3,4 used as 100 mol%) did not occur when both 5 (9 mol%) and ZnTPP (9 mol%) were present because the strong (5→ZnTPP) binding does not release any notable amount of 5 into solution. The 1H NMR analysis confirms the binding between N-methylpyrrolidine and ZnTPP as the pyrrolidine protons are diagnostically shifted upfield due to the porphy-

In order to evaluate NetState II as OFF state, we mixed substrates 3, 4, catalyst 5, [Fe(1)2]2+ and [Cu(2)]+ in a 100:100:9:5:10 ratio in CD2Cl2 in the NMR tube and heated it for 2 h at 40 °C. Because no product formation was detected in the 1H NMR, we confirmed herewith NetState II as an OFF state for the reaction (SI, Figure S18). Next, we decided to test the ON state, i.e. NetState I. For that purpose (for yield calculations, tetrachloroethane was used as external standard), we took NetState I, i.e [Cu(1)]+ and switch 2, in a ratio of 10:10 and added the components 3, 4 and 5 in 100:100:9 ratio and heated the mixture in CD2Cl2 at 40 °C for 2 h in the NMR tube. 1H NMR analysis confirmed product formation with 30% yield indicating that catalyst 5 is available to catalyze the reaction (SI, Figure S17). The lower yield (33%  30%) in NetState I with regard to the model reaction (Figure 4) is expected due to the presence of ca. 10% of [Cu(2)]+ in NetState I that inhibits 10% of the organocatalyst. After establishing that NetState I (Figure 5) triggers catalysis (ON state) and NetState II inhibits the catalysis (OFF state), the in situ reversible switching of the catalytic process between ON and OFF was investigated. For this, an NMR tube loaded with complex [Cu(1)]+ and switch 2 (=NetState I) as well as compounds 3, 4, 5 in a 10:10:100:100:9 ratio was heated at 40 °C for

Figure 5. Representation of the ON/OFF regulation of the conjugate addition reaction in an interdependent network of two nanoswitches. 4′-N,N-Dimethylamino-2,2′:6′,2′′-terpyridine is used to remove Fe2+ from NetState II.



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2 h. 1H NMR analysis showed 30% of product 6 (SI, Figure S19a). After the measurement, we added 0.5 eq. of Fe2+ to the same sample and heated it for 2 h again at 40 °C. As anticipated from the control experiments, there was no further product 6 formed as the 1H NMR still showed a total of 30% yield (SI, Figure S19b). As expected, NetState II, formed after addition of Fe2+, firmly immobilizes 5 at the zinc porphyrin site of [Cu(2)]+. To trap Fe2+ ions and regenerate NetState I, 4′-N,N-dimethylamino2,2′:6′,2′′-terpyridine (1 eq.) and consumed amounts of compounds 3 and 4 were added. Heating for 2 h at 40 °C restarted the reaction as seen in the 1H NMR (SI, Figure S19). When the full cycle was repeated 2 times, each cycle provided 6 in 24% yield (Figure 5). The yields obtained in cycles 1-3 reveal that the network states operate close to an ideal communication/catalysis system. Whereas catalyst 5 is available in cycle 1 of NetState I over 120 min, the slow regeneration of Netstate I from II in cycles 2 and 3 over 30 min should lead to a delayed release of 5 and diminished yield of 6. The time-dependent formation of 6 in the model reaction (Figure 4) indicates that a reaction time reduced by 30 min should cause the yield to drop by ca. 8%. Thus, the decline of the yield from 30% (cycle 1) to 24% (in cycles 2 & 3) is well rationalized. In conclusion, we demonstrate how two diverse nanoswitches cooperate via chemical signaling as an interdependent ensemble (network states) in the ON/OFF regulation of an organocatalytic reaction. Addition and removal of iron(II) ions as an external chemical trigger causes NetState I to convert to NetState II and back. Within this conversion Cu+ acts as second messenger that is reversibly translocated from nanoswitch [Cu(1)]+ to nanoswitch 2. In NetState I the organocatalytic process is ON and in NetState II it is OFF. The process was shown to work over three cycles. Because copper(I) ion translocation provides the signal at the second nanoswitch for switching catalysis ON/OFF, the situation is reminiscent of biological systems that use metal ions in dyna-

(1) Richards, V. Nat. Chem. 2016, 8, 1090. (2) Erbas-Cakmak, S.; Leigh, D. A.; McTernan, C. T.; Nussbaumer, A. L. Chem. Rev. 2015, 115, 10081–10206. (3) McConnell, A. J.; Wood, C. S.; Neelakandan, P. P.; Nitschke, J. R. Chem. Rev. 2015, 115, 7729–7793. (4) Coskun, A.; Banaszak, M.; Astumian, R. D.; Stoddart, J. F.; Grzybowski, B. A. Chem. Soc. Rev. 2012, 41, 19–30. (5) Ariga, K.; Li, J.; Fei, J.; Ji, Q.; Hill, J. P. Adv. Mater. 2016, 28, 1251–1286. (6) Breslow, R. Chem. Eng. News 2016, 94, 28–29. (7) Ludlow, R. F.; Otto, S. Chem. Soc. Rev. 2008, 37, 101–108. (8) Grzybowski, B. A.; Otto, S.; Philp, D. Chem. Commun. 2014, 50, 14924–14925. (9) Korzeniewski, B. J. Theor. Biol. 2001, 209, 275–286. (10) Bielecki, A. Biol. Cybern. 2015, 109, 401–419. (11) Blanco, V.; Leigh, D. A.; Marcos, V. Chem. Soc. Rev. 2015, 44, 5341–5370. (12) Marcos, V.; Stephens, A. J.; Jaramillo-Garcia, J.; Nussbaumer, A. L.; Woltering, S. L.; Valero, A.; Lemonnier, J.-F.; Vitorica-Yrezabal, I. J.; Leigh, D. A. Science 2016, 352, 1555−1559. (13) Gaikwad, S.; Goswami, A.; De, S.; Schmittel, M. Angew. Chem. Int. Ed. 2016, 55, 10512–10517. (14) Berrocal, J. A.; Biagini, C.; Mandolini, L.; Stefano, S. D. Angew. Chem. Int. Ed. 2016, 55, 6997 –7001. (15) Kita, M. R.; Miller, A. J. M. J. Am. Chem. Soc. 2014, 136, 14519–14529. (16) Lifschitz, A. M.; Rosen, M. S.; McGuirk, C. M.; Mirkin, C. A. J. Am. Chem. Soc. 2015, 137, 7252–7261. (17) Ray, D.; Foy, J. T.; Hughes, R. P.; Aprahamian, I. Nat. Chem. 2012, 4, 757–762.

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mic cell signaling pathways.20 Considering the fact that only few of the known nanoswitch systems have been switched in situ for turning on/off catalysis,2 the complexity and reproducibility of the current networked system under in situ conditions are remarkable. Moreover, we consider this example as an entry into the arena of molecular cybernetics, which, alike molecular logic gates,32 is a subarea of molecular information processing. According to Korzeniewski9 cybernetic regulation of entangled processes is required for any living organism to prevent decay into equilibrium and death. In this respect, much more research is needed in the study of mutual communication between molecules to achieve regulation of complex functions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at. Synthetic procedures, characterization of metal complexes, NMR spectra, ESI-MS spectra, UV-vis studies, measurements of binding constants and catalytic studies (PDF).

AUTHOR INFORMATION Corresponding Author * [email protected]

Funding Sources We are grateful to the DFG (Deutsche Forschungsgemeinschaft: Schm-647/19-1) and the Universität Siegen for financial support. ‡ Both authors contributed equally to the manuscript.

REFERENCES

(18) Ren, Y.; You, L. J. Am. Chem. Soc. 2015, 137, 14220−14228. (19) Pramanik, S.; De, S.; Schmittel, M. Angew. Chem. Int. Ed. 2014, 53, 4709–4713. (20) Cotruvo, J. A., Jr.; Aron, A. T.; Ramos-Torres, K. M.; Chang, C. J. Chem. Soc. Rev. 2015, 44, 4400–4414. (21) Schmittel, M. Chem. Commun. 2015, 51, 14956–14968. (22) De, S.; Pramanik, S.; Schmittel, M. Angew. Chem. Int. Ed. 2014, 53, 14255–14259. (23) Schmittel, M.; De, S.; Pramanik, S. Angew Chem. Int. Ed. 2012, 51, 3832–3836. (24) De, S.; Pramanik, S.; Schmittel, M. Dalton Trans. 2014, 43, 10977–10982. (25) He, Z.; Jiang, W.; Schalley, C. A. Chem. Soc. Rev., 2015, 44, 779–789. (26) Saha, M. L; Schmittel, M. Org. Biomol. Chem. 2012, 10, 4651– 4684. (27) Safont-Sempere, M. M.; Fernández, G.; Würthner, F. Chem. Rev. 2011, 111, 5784-5814. (28) Schmittel, M.; Pramanik, S.; De, S. Chem. Commun. 2012, 48, 11730–11732. (29) Saha, M. L.; De, S.; Pramanik, S.; Schmittel, M. Chem. Soc. Rev. 2013, 42, 6860–6909. (30) Schubert, U. S.; Hofmeier, H.; Newkome, G. R. Chemistry and Properties of Terpyridine Metal Complexes, in Modern Terpyridine Chemistry, Wiley-VCH, Weinheim, 2006. (31) In detail, the reaction is: 2  [Cu(1)]+ + [Fe(2)]2+ + 2 → [Fe(1)2]2+ + 2  [Cu(2)]+. A further control experiment indicated that copper(I) translocation in the reaction [Fe(Cu(1))2]4+ + 2  2 → [Fe(1)2]2+ + 2  [Cu(2)]+ is not rate limiting (SI, Figure S29). (32) Andreasson, J.; Pischel, U. Chem. Soc. Rev. 2015, 44, 1053– 1069.


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Graphical Abstract:

Switch 1

Switch 1

Catalyst Catalyst

Switch 2

Switch 2 Switch 2

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