Dynamic chemical devices: Modulation of contraction/extension molecular motion by coupled-ion binding/pH change-induced structural switching

Complex [1-Pb (OTf)₄].

Rack-type complexes of ligands 1 and 2 with Ru(II) have been described (34, 35). For the purposes of this work, the related complexes with the much larger Pb(II) ions were investigated in view of the lesser pinching of the tridentate terpy-Pb(II) coordination site (37), compared with the corresponding complexes with transition metal ions (34, 35), and with the expected far greater reversibility caused by easier complexation-decomplexation.

Treatment of 1 with 2 equivalents (eq) of Pb(OTf)₂ (OTf = triflate, CF₃SO₃⁻) (37) in acetonitrile afforded a compound whose ¹H-NMR and electrospray (ES) mass spectral properties (not shown) agree with the formation of a dinuclear [2]-rack type complex [1-Pb₂ (OTf)₄]. Confirmation was obtained by determination of its crystal structure by X-ray crystallography. The crystal structure showed the complex to be composed of one ligand, two lead cations coordinating each of two triflate anions, and one acetonitrile bound to one lead cation (Fig. 2). The ligand adopts a linear shape 1-L with cisoid conformation around all inter-heterocyclic bonds. The pinching angle provides information about the shape of the complexes (35), smaller values being correlated with an increased linearity of the ligand in the complex. It is much smaller for 1-Pb₂ (10.1°) than for the analogous Ru(II) complex (25.6°; ref. 35). Thus, the binding of the large lead(II) ions generates a more linear shape, a feature of much interest in the context of this work. The representations of the extended polymetallic racks 3-Pb₅ and 5-Pb₉ were modeled on the basis of the present crystal structure of the [2]-rack type complex 1-L (see below).

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Figure 2

Representation of the dinuclear Pb(II) rack, 1-L. (a) Structure of the complex showing the coordination environment of the Pb(II) cation. (b) Space-filling representation of the crystal structure showing only ligand 1 and the two lead(II) cations with indication of the end-to-end length of the ligand in the complex. Solvent molecules included in the crystals are omitted for clarity. Carbon, sulfur, light gray; hydrogen, white; nitrogen, black; lead, gray.

Complex [3-Pb₅(OTf)₁₀].

A mixture of ligand 3 and 5 eq Pb(OTf)₂ in acetonitrile was stirred overnight at room temperature. The ¹H NMR spectrum of the resulting complex indicated a high symmetry, consistent with the [5]-rack complex [3-Pb₅(OTf)₁₀]. The linear conformation is clearly indicated by the ROESY spectrum which shows distinct NOE effects between the terminal pyridine H5 and the neighboring SPr-pyridine H3 protons, as well as between the H5 protons of the pyrimidines and the nearby H3 protons of the neighboring two SPr-pyridines (not shown). The ²⁰⁷Pb NMR spectrum displayed three signals in a 1:2:2 ratio, corresponding to Pb(II) cations in three different chemical environments (Fig. 3a). On the basis of the 2D ²⁰⁷Pb-¹H correlation spectrum (not shown), the lead signals from right to left may be assigned as follows [Pb_position (number of vicinal Pb ions, type and number of nitrogen sites)]: Pb_central (2, Py, Pym₂), Pb_interior (2, Py, Pym₂), and Pb_terminal (1, Py₂, Pym). Thus, the strong NOE effects, the chemical shifts and relative intensity of ²⁰⁷Pb signals, as well as the 2D Pb-proton correlations agree with the linear [5]-rack structure 3-L of the complex (Fig. 4a). Furthermore, the ES mass spectrum of this complex showed peaks corresponding to multiply charged species with the correct composition [3-Pb₅(OTf)_n]¹⁰⁻ⁿ, with n = 4–8 (see Materials and Methods).

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Figure 3

83 MHz ²⁰⁷Pb NMR spectra of complexes [3-Pb₅(OTf)₁₀] and [5-Pb₉(OTf)₁₈] in CD₃CN at 25°C.

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Figure 4

(a and b) Modelization of the extended rack-type complexes 3-L (a) and 5-L (b) on the basis of the crystal structure of complex 1-L (Fig. 2) with indication of the end-to-end length of the ligand in the complexes. (c) Acid-base processes modulating coiling/uncoiling motions by interconversion of the helical strand 3-H and the extended linear complex 3-L. The gray spherical objects represent Pb(II) ions; AH⩵CF₃SO₃H, B⩵Et₃N. The SnPr substituents on the ligands have been omitted for clarity.

Complex [5-Pb₉,(OTf)₁₈].

A mixture of ligand 5 and 9 eq of Pb(OTf)₂ in acetonitrile was stirred overnight at room temperature. The ¹H NMR spectrum of the resulting complex indicated high symmetry, which is consistent with complex [5-Pb₉(OTf)₁₈]. The linear conformation of the ligand in the complex is clearly indicated in the ROESY spectrum; distinct NOE effects are observed between the terminal pyridine H5 and the vicinal SPr-pyridine H3 protons as well as between the H5 pyrimidine protons and the nearby H3 protons of the neighboring two SPr-pyridines. The ²⁰⁷Pb NMR spectrum shows three signals in a 2:1:6 ratio, corresponding to Pb(II) cations in three different chemical environments (Fig. 3b). On the basis of the 2D ²⁰⁷Pb-¹H correlation spectrum, the lead signals at 1,134.0 ppm (intensity 6), 1,141.1 ppm (intensity 1), and 1,650.9 (intensity 2) may be assigned as follows [Pb_position (number of vicinal Pb ions, type and number of nitrogen sites)]: Pb_interior (2, Py, Pym₂), Pb_central (2, Py, Pym₂), and Pb_terminal (1, Py₂, Pym). Again, the strong NOE effects, the chemical shifts and relative intensity of the ²⁰⁷Pb signals, as well as the 2D Pb-proton correlations agree with a linear [9]-rack structure 5-L of the complex (Fig. 4b). In addition, the ES mass spectrum of this complex showed peaks that correspond to multiply charged species possessing the correct composition [5-Pb₉(OTf)_n]¹⁸⁻ⁿ, with n = 12–16 (see Materials and Methods).

Structural features of the complexes.

The radiocrystallographic and spectroscopic data indicate that all three complexes—1-Pb₂, 3-Pb₅, and 5-Pb₉—present a rack-type structure. Binding of the Pb(II) ions by imposing terpy-type coordination sites, converts all transoid inter-heterocyclic connections in the ligands into cisoid connections (Fig. 1b). It results in a linearization of the ligand from the helical strands 1-H, 3-H, and 5-H into the extended unfolded forms present in the complexes 1-L, 3-L, and 5-L (Figs. 2 and 4).

Modelization of the structures of these complexes on the basis of the geometry of 1-L in the crystal structure of [1-Pb₂(OTf)₄] obtained above (Fig. 2), yields curved extended entities of about 38-Å and 60-Å lengths for 3-L and 5-L, as represented in Fig. 4. Whereas the helical free ligands H may be expected to behave like a molecular spring, the corresponding complex L should be a rigid, bent molecular stick.

Ionic modulation of extension/contraction molecular motion.

The transformation of the helical-free ligands 1-H, 3-H, and 5-H into the corresponding rigid linear complexes 1-L, 3-L, and 5-L by the binding of several Pb(II) ions offers the unique opportunity to set up a reversible motional process in which a molecular entity undergoes sequential extension and contraction phases triggered by external chemical signals. Stretching/contracting actions occur in the folding/unfolding of proteins (41, 42).

A system allowing to produce reversible pulses of Pb(II) ions was achieved by taking advantage of the properties of the macrobicyclic ligand, cryptand [2.2.2] (43), which forms cryptate inclusion complexes [Mⁿ⁺⊂2.2.2] with numerous metal ions (43, 44). In particular Pb(II) yields a very stable cryptate [Pb(II)⊂2.2.2] (Scheme S1c), with stability constants log K_s of about 12.5 and 12.9 in aqueous solution (44, 45) and in CH₃OH (46) respectively. Even higher stability may be expected in the CDCl₃/CD₃CN 2/1 solvent used here (see below). Furthermore, protonation of the bridgehead nitrogen sites leads to the deprotonated species [2H⁺⊂2.2.2] (Fig. 4c) with release of the included Pb(II) cation from the cryptate (44).

Thus, one may consider to set up a multicomponent/multitrigger system involving a helical ligand strand 1-5, Pb(II) ions, cryptand [2.2.2], acid, and base. Such a system should undergo the following reactions:

• complexation: H + n Pb(II) → L.

• cryptate formation: L + n[2.2.2] → H + n[Pb(II)⊂2.2.2]

• ion release from cryptate by protonation: [Pb(II)⊂2.2.2] + 2H⁺ → [2H⁺⊂2.2.2] + Pb(II)

• deprotonation: [2H⁺⊂2.2.2] + 2(Base) → [2.2.2] + 2(Base, H⁺)

As a result, sequential protonation/deprotonation should lead to alternate [Pb(II) release]/[Pb(II) cryptation] and thus to L ⇌ H interconversion. That this is indeed the case is clearly indicated by the changes occurring in the proton NMR spectrum of the system. Fig. 5 shows the evolution of the aromatic and aliphatic part of the ¹H NMR spectra as a function of the triggering agent, for the system involving ligand 3, Pb(II), [2.2.2], CF₃SO₃H, and Et₃N.

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Figure 5

400 MHz ¹H spectral changes on structural interconversion in the system [3, 5 eq Pb(OTf)₂,[2.2.2], CF₃CO₂H, Et₃N] on successive addition of the different triggering reagents, in CDCl₃/CD₃CN 2/1 solution (see text). The arrows indicate signals characteristic of the species shown. The aromatic domain (Left) of the spectra is amplified by a factor of four with respect to the aliphatic region (Right).

The addition of 5 eq of Pb(OTf)₂ to a solution [CDCl₃/CD₃CN = 2/1 (vol/vol)] of helical ligand 3-H leads to the linear [5]-rack 3-L. This transformation is characterized by the disappearance of the signals of the terminal pyridine hydrogens H2 and H3 (at 6.73 and 7.08 ppm) and by the appearance of the signals of the Pb(II) complexed-pyrimidine hydrogens H8 and H12 (at 10.31 and 10.43 ppm). On addition of 5 eq of the strong Pb(II) complexing agent, the [2.2.2] cryptand, the spectrum of the helical form 3-H is restored, and the aliphatic part of the spectrum shows peaks that correspond to the cryptate complex [Pb(II)⊂2.2.2]. Subsequent addition of 10 eq of CF₃SO₃H induces protonation of the [2.2.2] nitrogen sites, thus releasing the Pb(II) ions, which then complex the helical form 3-H, regenerating the linear 3-L. The aliphatic part of the spectrum displays the peaks characteristic of the protonated cryptate [2H⁺⊂2.2.2]. Deprotonation of the latter by addition of Et₃N results in the reformation of the cryptate [Pb(II)⊂2.2.2] and of helical 3-H. Alternating contraction/extension motions between the helical strand 3-H and the linear [5]-rack 3-L thus may be repetitively triggered by successive additions of acid and base, leading to the sequential exchange of Pb(II) between the ligand 3 and the [2.2.2] cryptand.

These data demonstrate the modulation of a coiling/uncoiling process by a coupled-ion/pH-induced structural switching. Thus, extension/contraction is realized as a controlled motion in an oscillatory periodic sequence (Fig. 6). It amounts to a two-stroke, linear motor-type action with a very large stroke amplitude from 7.5 Å to 38 Å, and from 11 Å to 60 Å in the case of strands 3 and 5, respectively (Fig. 4). This mechano-chemical process is fueled by protonation/deprotonation i.e., by acid/base neutralization energy.

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Figure 6

Modulation of the coiling/uncoiling, contraction/extension process 3-H ⇌ 3-L by coupled-ion/pH-induced structural switching.