Tuning-up and driving a redox-active rotor

Abstract

The dynamic bistable rotational behaviour of a copper(I) coordination environment can be rationally tuned with balancing the substituent size on the rotor. Such rotors are sensitive to weak interactions, and the rotation was driven by a redox reaction on a ferrocenyl moiety via reconstruction of the charge interaction between redox centres.

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The drive toward miniaturizing devices has promoted the need for molecular devices assembled from individual molecular units.¹ This field includes not only static molecular conduction,² but also construction of desired molecular functions by the integration of responses from individual molecular units to transform electrons or photons into a detectable signal.³ Construction of molecular functionalities requires a mechanism that can be repetitively driven by external stimuli and is not encumbered by thermal fluctuations. Such molecular systems may comprise molecular parts constructed from photochromic⁴ or redox-active⁵ molecules, and representative systems can be regarded as molecular machines.⁶ Systematic methods for constructing integrated functionalities from individual molecular processes have been limited, and applications for molecular machines are still under discussion.

Biosystems consist of a series of molecular parts that process energy and sustain living organisms. The molecular components are finely interconnected on energetic and geometric levels to achieve integrated function with high efficiency.⁷ In an effort to construct an artificial molecular processing system, we have designed electron-driven mechanical devices that operate using the rotational motions of coordinated pyrimidine ligands on copper centers.⁸ Two asymmetrically coordinated forms of pyrimidine afford a bistable system with two oxidation potentials at the copper centre. The bistability can induce electron transfervia rotational interconversion. Our system has several advantages for inclusion in functionally integrated systems because it is structurally simple, with a predictable rotational mechanism.

A redox active ferrocene moiety was used as the rotative unit. Rotation was accomplished by precession of the ferrocene unit around the pyrimidine ring. Along with the geometric rearrangement, the electronic interactions differed between the rotational isomers (both through bonds and through space) to afford correlation between the redox behaviour and the rotational dynamics. This system was constructed by tuning the rotational dynamics by balancing the sizes of the two convertible moieties upon rotation.

A series of pyridylpyrimidine ligands that included various hydrocarbons and ferrocene, RFcpmpy, was obtained by cycloaddition of α,β-unsaturated ketones to 2-pyridylamidine. Copper(I) complexes, [Cu(RFcpmpy)(L_Anth2)]BF₄, were obtained immediately by adding RFcpmpy to the solution containing [Cu(CH₃CN)₄]BF₄. L_Anth2 (= 2,9-dianthryl-1,10-phenanthroline) was selected as an auxiliary ligand to prevent formation of a homoleptic complex and to control the rotational interconversion kinetics.

The auxiliary ligand formed a box-like space delimited by the anthracene and phenanthroline walls, and the relative stability of the isomers appeared to be determined by the steric properties of the rotor within the space (the isomer in which the ferrocene moiety was directed toward the copper centre is denoted i-Fc, and the other isomer is denoted o-Fc). The ratios between the isomers formed using each hydrocarbon substituent were estimated from the ¹H NMR spectra (Fig. 1a and ESI‡). Larger hydrocarbon substituents afforded a higher ratio of i-Fc states, as expected. The anomalous [Cu(1-NaphFcpmpy)(L_Anth2)]⁺ isomer ratio seemed to result from attractive π-stacking effects between the naphthyl and phenanthroline moieties and was clearly observed in the crystallographic characterization (Fig. S3, ESI‡). A comparison of the crystal structures of the H and 9-Anth complexes (Fig. 2b and c) elucidated the rotational steric effects in detail. [Cu(9-AnthFcpmpy)(L_Anth2)]⁺ presented a ferrocenyl moiety that was pressed into the inner space, and the bulky ferrocenyl group was accommodated by displacement of the anthryl walls in the auxiliary ligand. The structure of [Cu(9-AnthFcpmpy)(L_Anth2)]⁺ still displayed a bidentate 9-Anth-Fcpmpy structure, although the Cu–N(pm) bond (2.08 Å) was elongated relative to that in [Cu(HFcpmpy)(L_Anth2)]⁺ (2.03 Å). The stability of the bidentate coordination complex was confirmed by the absence of ¹H NMR signals corresponding to the dissociated species; all compounds presented only signals that corresponded to the intact isomers.

Fig. 1 (a) Molar ratios of i-Fc in [Cu(RFcpmpy)(L_Anth2)]BF₄, estimated from the ¹H NMR spectra in acetone-d₆ at 293 K. (b) Crystal structure of [Cu(HFcpmpy)(L_Anth2)]⁺ and (c) [Cu(9-AnthFcpmpy)(L_Anth2)]⁺. Only the cationic part is shown for clarity.

Taken in conjunction with previous results,⁸ these measurements determined the relative preference of the substituents at the i-position. The i-form was preferred in the order Me > H > 1-Naph > 4-tolyl > ferrocenyl > 9-Anth > ^tBu. The size of the substituent was a primary factor for rotors larger than benzene; on the other hand, the Me group achieved particular stability due to the dominant induction effect of the adjacent nitrogen atom in the small substituent. This simple rule may guide the preparation of a variety of ferrocene rotors with rotational balance, ranging from all-o-Fc (R = H) to all-i-Fc (R = ^tBu). Additionally, the rotational direction was easily influenced by weak interactions at the rotor, such as π–π* stacking, as observed in [Cu(1-NaphFcpmpy)(L_Anth2)]⁺.

Among these complexes, [Cu(9-AnthFcpmpy)(L_Anth2)]⁺ was the most suitable for making dynamic bistable states, because the size balance was comparable in ferrocenyl and 9-Anth substituents, with the i-Fc state dominating (oxidation of the compound tended to form the o-Fc⁺ state, as described later). The variable-temperature ¹H NMR spectra of [Cu(9-AnthFcpmpy)(L_Anth2)]⁺ showed an initial drop of o-Fc ratio upon heating, then it smoothly increased above 233 K (Fig. 2a). At low temperature, the dynamic interconversion of [Cu(9-AnthFcpmpy)(L_Anth2)]⁺ isomers ceased due to conformational freezing (the interconversion barrier was high relative to k_BT). The cyclic voltammogram of [Cu(9-AnthFcpmpy)(L_Anth2)]⁺ exhibited two coupled redox waves assigned to the Fc⁺/Fc and Cu(II)/Cu(I) centres (Fig. 2b). The assignment was justified by the characteristically sluggish electron transfer in the second wave at 228 K (Fig. S4, ESI‡). The redox potential difference corresponding to the Cu(II)/Cu(I) reaction in the two isomers was not large enough to induce potential crossing between the copper and the ferrocene.⁸ Conversion between both the bulky ferrocene and anthracene moieties upon rotation could not reduce the Cu(II)/Cu(I) oxidation potential relative to the ferrocenyl oxidation.⁹ As the relative positions and bonding of the redox centres within each isomer differed significantly, we performed a detailed examination of the first redox wave of the ferrocenyl moiety (Fig. 3b). At room temperature, the redox wave appeared to be a single reversible couple. When the sample was cooled to 228 K (slightly above the conformational freezing temperature), the redox wave split into two couples, that could not be observed in the ligand-only redox wave (Fig. S5, ESI‡). The anodic wave was dominated by the current at the positive region, in contrast, the cathodic wave showed a substantial increment of the current at the negative region. This type of asymmetric wave indicates the structural hysteresis¹⁰ accompanied with the redox reaction in the time scale of the potential scan. The structural conversion is likely to be a rotating motion, which affects the ferrocene redox process through a close packing geometric factor. On this assumption, the ferrocenium moiety twisted to the o-position from the initially preferred i-position upon oxidation. Quantification of the electrochemical and rotational dynamics parameters associated with rotation was performed by simulating the voltammogram, according to the redox cycle scheme shown in Fig. 3c (Table S2, Fig. 3b).

Fig. 2 (a) van't Hoff plot of the rotational equilibrium of [Cu(9-AnthFcpmpy)(L_Anth2)]⁺, obtained from the ¹H NMR spectra in acetone-d₆. K_io is defined as [o-Fc]/[i-Fc]. The sample was rapidly cooled to 213 K and data were collected during the heating process. (b) Cyclic voltammogram of [Cu(9-AnthFcpmpy)(L_Anth2)]⁺ in 0.1 M ⁿBu₄NBF₄–acetone recorded in the anodic region at 298 K. Scan rate: 25 mV s⁻¹.

Fig. 3 (a) Schematic representation of rotation induced by a redox reaction at the ferrocenyl rotor. (b) Cyclic voltammograms of [Cu(9-AnthFcpmpy)(L_Anth2)]⁺ in 0.1 M ⁿBu₄NBF₄–acetone for the Fc⁺/Fc process at room temperature (red), at 228 K (blue), and the simulated wave (blue dot). Scan rate: 25 mV s⁻¹. (c) Square scheme composed of the redox reaction (vertical) and the rotation (horizontal) of the ferrocene rotor. Selected parameters obtained from the simulation curve are also presented in (b).

The difference between the redox potentials of the ferrocenyl moieties in each isomer directly indicated that the i-Fc⁺ state was destabilized upon oxidation compared with the o-Fc⁺ state. We considered two factors: (i) the electrostatic repulsion between cations (ferrocenium and copper(I), and (ii) the decreased induction effect of the adjacent nitrogen atom upon oxidation of the ferrocene moiety. A simple estimation of the electrostatic repulsion (e²/4πεr) was made using the two Cu–Fe distances from the crystal structures (5.09 and 7.62 Å) shown in Fig. 1. The estimation afforded a value of 13 kJ mol⁻¹ destabilization of the i-Fc compared with o-Fc. The estimation agreed with the experimental value (5.8 kJ mol⁻¹) determined from the redox potential difference, considering that the value was overestimated by neglecting anionic neutralization and polarization effects.

In conclusion, we developed a novel copper complex family containing a redox-active rotor, the rotational dynamics of which could be systematically tuned. Rotational motion could be induced upon redox reaction at the rotor via charge interactions between the rotor and the copper centre. These results suggest an integrated transformation system in which stimulation at the rotator may drive the movement and induce an electron transfer event. Further extensions of the rotator using an optically and redox-active moiety are currently under investigation.

This work was supported by Grants-in-Aid from MEXT of Japan (20750044, 20245013, and 21108002), and from JST (Research Seeds Quest Program).

Notes and references

1. R. L. Carroll and C. B. Gorman, Angew. Chem., Int. Ed., 2002, 41, 4378; J. Park, A. N. Pasupathy, J. I. Goldsmith, C. Chang, Y. Yaish, J. R. Petta, M. Rinkoski, J. P. Sethna, H. D. Abruña, P. L. McEuen and De. C. Ralph, Nature, 2002, 417, 722; W. Liang, M. P. Shores, M. Bockrath, J. R. Long and H. Park, Nature, 2002, 417, 725; H. Song, Y. Kim, Y. H. Jang, H. Jeong, M. A. Reed and T. Lee, Nature, 2009, 462, 1039.
2. R. L. McCreery and A. J. Bergren, Adv. Mater., 2009, 21, 4303; K. Moth-Poulsen and T. Bjørnholm, Nat. Nanotechnol., 2009, 4, 551; F. Chen and N. J. Tao, Acc. Chem. Res., 2009, 42, 429; A. Nitzan and M. A. Ratner, Science, 2003, 300, 1384.
3. K. Szaciłowski, Chem. Rev., 2008, 108, 3481; K. Morgenstern, Acc. Chem. Res., 2009, 42, 213; T. Kudernac, N. Katsonis, W. R. Browne and B. L. Feringa, J. Mater. Chem., 2009, 19, 7168; W. Y. Kim and K. S. Kim, Acc. Chem. Res., 2010, 43, 111; U. Pischel, Angew. Chem., Int. Ed., 2007, 46, 4026.
4. M. S. Wang, G. Xu, Z. J. Zhang and G. C. Guo, Chem. Commun., 2010, 46, 361; S. Kume and H. Nishihara, Dalton Trans., 2008, 3260.
5. A. H. Flood, J. F. Stoddart, D. W. Steuerman and J. R. Heath, Science, 2004, 306, 2055; S. Kubatkin, A. Danilov, M. Hjort, J. Cornil, J.-L. Brédas, N. Stuhr-Hansen, P. Hedegård and T. Bjørnholm, Nature, 2003, 425, 698.
6. J. J. Davis, G. A. Orlowski, H. Rahman and P. D. Beer, Chem. Commun., 2010, 46, 54; E. R. Kay, D. A. Leigh and F. Zerbetto, Angew. Chem., Int. Ed., 2007, 46, 72; V. Balzani, A. Credi and M. Venturi, Molecular Devices and Machines, Wiley-VCH, 2nd edn, 2008.
7. H. Dau and I. Zaharieva, Acc. Chem. Res., 2009, 42, 1861; R. D. Vale and R. A. Milligan, Science, 2000, 288, 88; W. Junge, H. Sielaff and S. Engelbrecht, Nature, 2009, 459, 364.
8. K. Nomoto, S. Kume and H. Nishihara, J. Am. Chem. Soc., 2009, 131, 3830; S. Kume, K. Nomoto, T. Kusamoto and H. Nishihara, J. Am. Chem. Soc., 2009, 131, 14198.
9. P. Federlin, J. M. Kern, A. Rastegar, C. Dietrich-Buchecker, P. A. Marnot and J. P. Sauvage, New J. Chem., 1990, 14, 9.
10. M. Sano and H. Taube, J. Am. Chem. Soc., 1991, 113, 2327; M. Sano, Molecular Machines and Motors, Springer, Berlin, 2001, pp. 117.

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Footnotes

† This manuscript is part of the ‘Emerging Investigators’ themed issue for ChemComm.

‡ Electronic supplementary information (ESI) available: Synthetic procedures and characterization, crystallography, and details of electrochemistry and simulation analysis. CCDC 788142–788144. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cc02193h

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