Ruthenium(IV) N-confused porphyrin μ-oxo-bridged dimers: acid-responsive molecular rotors

Abstract

Ruthenium(IV) N-confused porphyrin μ-oxo-bridged complexes were synthesized via oxidative dimerization of a ruthenium(II) N-confused porphyrin complex using 2,2,6,6-tetramethylpiperidine 1-oxyl. The deformed core planes in the dimers conferred a relatively high ring rotational barrier of ca. 16 kcal mol⁻¹. Rotation of the complexes was controlled by protonating the peripheral nitrogen.

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The design/creation/operation of molecular rotors, gears, and switches is an interesting topic in machinery nanotechnology, allowing the development of adaptive materials, switchable chiral sensors, stereo-controlled catalysts, etc.¹ The molecular approach to control nanoscale motions (e.g., rotation) is advantageous for realizing well-defined molecular devices or sets of practical supramolecular assembled systems.² As active components of molecular rotors, metal double-decker bisporphyrins and related macrocycles (e.g., phthalocyanines) are representative candidates that afford specific features, such as long-term stability, ease of fabrication, and visual responses that are detectable by microscopy (Fig. 1a).³ Bis-metal μ-oxo (or nitride)-bridged porphyrin complexes are also considered alternatives for modulating complexes' rotational mode. However, there are few investigations concerning molecular rotation because of the limitation in synthesizing molecular rotamers comprising unsymmetrical core skeletons with diamagnetic and oxophilic metal ions (for which the rotational dynamics can be probed by NMR spectroscopy) (Fig. 1b).⁴

Fig. 1 Chemical structures of (a) double-decker metal porphyrin complexes and (b) bis-metal μ-oxo-bridged porphyrin complex as representative motifs of molecular rotors (R¹ = R³ = p-methoxyphenyl, R² = R⁴ = 4-pyridyl). (c) N-confused porphyrin as a key rotor ligand and its (d) bis-iron μ-hydroxo-bridged porphyrin complex. (e) Schematic of the rotational mode of unsymmetrical N-confused porphyrin μ-oxo dimer complex. Sky-blue moiety in the planes is a confused pyrrole ring.

To develop new rotors based on metal μ-oxo-bridged porphyrin dimers, we envisioned that a C_s-symmetric N-confused porphyrin (NCP) isomer possessing an inverted pyrrole ring in the macrocyclic scaffold could act as an alternative ring motif for the complexes (Fig. 1c).⁵ The NCPs bind various metal ions at the NNNC core, forming unique organometallic complexes.^5,6 For example, redox-active metal species such as iron facilitate the generation of metal–oxo linkages to form μ-oxo-bridged dimer complexes.⁷ In the case of iron-coordination of NCP, a unique bis-iron μ-hydroxo-bridged NCP dimer was developed, in which the structure was restricted by the coordination of peripheral sodium, as evident from X-ray crystallography (Fig. 1d).⁶

We herein report the synthesis of novel ruthenium(IV) NCP μ-oxo-bridged dimers (Ru₂-2 and Ru₂-3) as a porphyrin-based molecular rotor. The ruthenium(II) NCP complex (Ru-1) with an axial carbonyl and pyridine (=py) exhibits catalytic reactivity for alkene epoxidation (Scheme 1).⁸ The simultaneous formation of isomers can be anticipated owing to the two stereogenic centers in the NCP planes of Ru₂-2 and Ru₂-3, which differ in the relative position of the nitrogen atoms of the confused pyrrole rings in the scaffold. The Ru–μ-oxo dimers showed distinct ring rotation in solution when probed via NMR spectroscopy (Fig. 1e). Moreover, symmetry-dependent restriction of the rotation was achieved through protonation at the peripheral imino-nitrogen moieties in Ru₂-2 and Ru₂-3 using trifluoroacetic acid (TFA).

Scheme 1 Synthesis of ruthenium(IV) NCP μ-oxo dimers, Ru₂-2 and Ru₂-3.

Treatment of monomer Ru-1 with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) in refluxing o-dichlorobenzene solution under argon atmosphere afforded two products with approximately the same yields (40% each) (Scheme 1). The fast atom bombardment mass spectra of the two products revealed identical parent ion peaks at 1474.2407 (M⁺) and 1474.2400 (M⁺), indicating dimeric ruthenium NCP structures (i.e., Ru₂-2 and Ru₂-3) bridged by an oxygen atom and two axial oxygen atoms (Fig. S1 in ESI†). A characteristic broad feature tailing up to 900 nm was observed in the absorption spectra of both Ru₂-2 and Ru₂-3 (Fig. S2†).

The structures of Ru₂-2 and Ru₂-3 were unambiguously characterized by single-crystal X-ray diffraction analysis (Fig. 2 and Tables S1 and S2 in ESI†). In both complexes, two ruthenium atoms coordinated at the NNNC core in the NCPs are bridged by an oxygen atom to form a sandwich-like structure. The characteristic metalaoxirane structure was found at the inner carbons of the confused pyrrole moieties in the scaffolds.^6,9 Such oxidative reactivity is a characteristic of Ru–NCP, which is in sharp contrast to the reactivity of the Ru^II(CO) azuliporphyrin with an NNNC core.¹⁰ Notably, the confused pyrrole rings in Ru₂-2 and Ru₂-3 are positioned opposite to the C17–Ru1–Ru2–C62(C61) torsion angle of 149.9(2)°, which is anticipated to induce molecular dipole moments. The strict structural difference between Ru₂-2 and Ru₂-3 is found in the position of the nitrogen atoms of the confused pyrrole rings; the former product (Ru₂-2) was eluted in the less polar fraction (R_f = 0.125) via silica gel chromatography using toluene/ethyl acetate (v/v = 19/1) and can be regarded as a syn conformer, whereas the polar fraction (R_f = 0.014) obtained under the same chromatographic conditions was eventually determined to be an anti-conformer (Ru₂-3).¹¹ The Ru1/2–O3 lengths for Ru₂-2 and Ru₂-3 are 1.792(3)/1.804(3) Å and 1.796(2)/1.800(2) Å, respectively, which are typical of μ-oxo-bridged ruthenium(IV) porphyrin complexes (e.g., [Ru^IV(TPP)(p-OC₆H₄CH₃)]₂–O: 1.789(11) Å).¹² The bond angles of Ru1–O3–Ru2 for Ru₂-2 and Ru₂-3 are almost linear 175.9(2)° and 176.1(2)°, respectively, similar to those of other reported ruthenium porphyrin μ-oxo dimer complexes (e.g., 177.8(7)°).¹² Due to the internal metal oxirane moieties, the confused pyrrole rings are inclined inward. Therefore, the overall geometries of the NCP planes are nonplanar with estimated mean plane deviation values (defined by 24 atoms of NCP skeleton) of 0.276 Å and 0.294 Å for Ru₂-2 and 0.286 Å and 0.292 Å for Ru₂-3.

Fig. 2 Top (left) and side (right) views of crystal structures of (a) Ru₂-2 and (b) Ru₂-3 with thermal ellipsoids set at 50% probability level. Hydrogen atoms and solvent molecules excepting the H-atoms hydrogen-bonded to the peripheral nitrogen atoms were omitted for clarity.

As anticipated above, the formal oxidation state of both ruthenium centers is expected to be +4 (d⁴) for the complexes. The X-ray photoelectron spectra of Ru₂-2 and Ru₂-3 revealed higher Ru (3p_3/2) binding energies of 462.2 and 462.1 eV, respectively, than that of the divalent ruthenium (d⁶) complex, Ru-1 (460.9 eV, Fig. S3 in ESI†).

The structures of Ru₂-2 and Ru₂-3 were further characterized by ¹H NMR spectroscopy in 1,1,2,2-tetrachloroethane-d₂ (TCE-d₂) (Fig. 3a and b). At 298 K, complicatedly overlapped signals were observed in the aromatic region (7–10 ppm). To distinguish the signals from the individual groups (β-pyrrole and meso-phenyl) in the complexes, we prepared the deuterated-phenyl derivatives (i.e., Ru₂-2D and Ru₂-3D) as shown in Scheme 1 (see, ESI†). The ¹H NMR spectra of Ru₂-2D and Ru₂-3D (Fig. S4†) show two singlets at 9.38 and 8.98 ppm for Ru₂-2 and 9.32 and 9.05 for Ru₂-3. The remaining six β-CHs are expected to have signals around 8 ppm for both complexes. This spectral feature suggests the existence of two possible rotational isomers in solution with an almost 1 : 1 molecular ratio. The 1D exchange nuclear Overhauser effect spectra revealed chemical exchange of the rotamers at 298 K (Fig. S5 and S6†), which indicated that the NCP rings of Ru₂-2 and Ru₂-3 rotate slowly at 298 K. The rotational mode of the NCP rings was seemingly restricted at 243 K (Fig. S7 and S8†). With increasing temperature, the signal of the set of α-pyrrole protons gradually disappeared and coalesced into a single peak at 9.15 ppm for Ru₂-2 and 9.27 ppm for Ru₂-3 at 408 K (Fig. 3c and d). Signals of the meso-phenyl protons also appeared with an averaged spectral feature in the region of 7.5–8.0 ppm (Fig. S9 and S10†). These results suggest that the rotation rate of the NCP rings is faster than the NMR timescale above 353 K. Furthermore, for the ruthenium(IV) tetrakis(phenyl)-substituted porphyrin μ-oxo dimer (e.g., Ru(TPP)₂O) used as the reference complex, the inward and outward CH signals (i.e., o- and m-CHs) of the meso-phenyl rings did not coalesce even at 408 K in the variable temperature (VT) NMR spectra (Fig. S11†). In this regard, the rotation of the bulky phenyl ring could be restricted by the spatially locked dimeric structure. The rotation mode of the NCP planes could be thus mutually un-synchronized with the rotational motion of the individual meso-phenyl rings.

Fig. 3 ¹H NMR spectra of (a) Ru₂-2 and (b) Ru₂-3 recorded in TCE-d₂ at 298 K. Partial ¹H NMR spectra (495 MHz, black line) of (c) Ru₂-2 and (d) Ru₂-3 in TCE-d₂ as a function of temperature and the line-shape simulated spectra for the compounds (red line).

To gain further insight into the rotational barrier of the complexes, a line-shape analysis of the VT ¹H NMR spectra was conducted to estimate the rate constant for interconversion (k) in the specific temperature range. The free Gibbs's energy (ΔG^‡ = ΔH^‡ − TΔS^‡) of Ru₂-2 and Ru₂-3 was determined to be (16.0 ± 0.8) kcal mol⁻¹ and (16.1 ± 0.4) kcal mol⁻¹, respectively, at 298 K, from the Eyring plots (Fig. S12†). The rate constant (k < 200 s⁻¹) at 333 K indicates that the energy barrier of the ring rotations of Ru₂-2 and Ru₂-3 is more significant than that of the double-decker metal complexes (k = 760 s⁻¹ at 333 K).^3d Considering the structure of the complexes, the bent confused pyrrole moieties in the planes may play an essential role in the rotational motion beyond the barrier.

Upon lowering the temperature, the ratio of the integrated α-CH signals for the rotamer mixture of Ru₂-2 changed to 0.85 (at 9.33 ppm) and 0.15 (at 8.93 ppm) (Fig. S13†). The ratio of the two-rotamers of Ru₂-3 also changed when the temperature was further lowered (∼223 K), affording a thermodynamically stable conformer (Fig. S14†). To understand the structure-dependent features of Ru₂-2 and Ru₂-3, we analyzed the thermodynamic energy profiles of the possible rotational isomers (e.g., Ru₂-2a–f and Ru₂-3a–c) by theoretical calculation with the B3LYP method (Tables S3 and S4†). The most stable isomers (i.e., Ru₂-2c and Ru₂-3c) obtained in the optimization process are highly consistent with the crystal structures. Notably, the torsion angles of C17–Ru1–Ru2–C61(C62) in the structures of Ru₂-2e and Ru₂-3b are −102.8° and 98.5°, respectively, indicating stability (ΔE = +1.19 and +1.45 kcal mol⁻¹, respectively) relative to Ru₂-2c/-3c. Therefore, two energetically close conformers are present in the solution under the experimental conditions.

The effect of protonation on the rotation of the rotamers was evaluated.¹³ Upon addition of trifluoroacetic acid (TFA), the absorption spectra of Ru₂-2 and Ru₂-3 gradually changed with isosbestic points (Fig. S15 and S16†), reflecting protonation at the peripheral imino-nitrogen of the confused pyrrole moieties (Fig. 4a and b).¹⁴ In both complexes, a relatively large amount of TFA (at least 8 equiv) was necessary until spectral saturation (Fig. S15a and S16a). Strikingly, in the case of Ru₂-3, the relatively crowded imino-nitrogen site surrounded by meso-phenyl rings interfered with double protonation, as seen in the acid-spectral titration change after the addition of 8 equiv. of TFA (Fig. 4d, e, S15b and S16b†). In the ¹H NMR spectrum, the resulting dication species of Ru₂-2 (i.e., Ru₂-2H₂²⁺) was well-resolved, demonstrating the interlocked single isomer at 298 K, as evident from the NMR spectral features (Fig. S17a†). The VT NMR spectra revealed that ring rotation becomes feasible with increasing temperature due to the dissociation of the TFA molecule from the complex at equilibrium (Fig. S18†). In sharp contrast, the ¹H NMR spectrum of the protonated species of the anti-symmetrical complex (i.e., Ru₂-3H⁺) exhibited a significantly broadened spectral feature at 298 K in TCE-d₂. The unsymmetrical spectral feature was observed at 243 K (Fig. S17b†), which may be due to the above steric issue in the conformation of Ru₂-3. Therefore, the predominant formation of mono-protonated species was anticipated in Ru₂-3. Eventually, upon the addition of triethylamine as a base, the original spectral signal of the neutral species was reproduced for both complexes (Fig. S15c and S16c†).

Fig. 4 Symmetry-dependent acid-binding behaviors of Ru₂-2 and Ru₂-3. Schematic of acid-induced rotational control for (a) Ru₂-2 and (b) Ru₂-3. (c)–(e) Side-views of the protonation site.

In summary, novel cofacial ruthenium(IV) NCP μ-oxo dimers, Ru₂-2 and Ru₂-3, were synthesized as motifs for molecular rotors. The resulting complexes, Ru₂-2 and Ru₂-3, are syn/anti-stereo-isomers having the nitrogen at different positions of the confused pyrrole rings. The symmetrical difference between the complexes leads to distinct rotational behavior of the Ru–NCP planes through the oxo-axis of the complex, as proven by VT ¹H NMR analysis. Additionally, considering the intrinsic basicity of the peripheral nitrogen sites, the rotational profile can be controlled by protonation. Further study on the cofacial metal–μ-oxo NCP dimers can enable applications in molecular machinery.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Grants-in-Aid (JP19K05439, JP19H04586, and JP20H00406) from the Japan Society for the Promotion of Science (JSPS) and the Konica Minolta Science and Technology Foundation. We acknowledge support from the Cooperative Research Program of the “Network Joint Research Centre for Materials and Devices.” We also acknowledge Prof. M. Watanabe for his kind advice on structural characterization.

Notes and references

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14. We have obtained the preliminary crystal structure of the protonated species of Ru₂-2 from the CHCl₃/hexane solution in the presence of TFA. The discrete syn conformer possessing the C17–Ru1–Ru2–C62 torsion angle of −150.9(4)° was speculated in the crystal structure. See Fig. S19 in ESI.†.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2076068 and 2076069. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1ra05063j

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