Complexes of a [2]rotaxane ligand with terminal terpyridine groups

Abstract

Permanently interlocked [2]rotaxane ligands can be created by capping a pyridine terminated [2]pseudorotaxane with terpyridine containing stoppers. The robust nature of the resulting [2]rotaxane ligand allows coordination to inert metals such as Ru(II) not possible under standard self-assembly conditions.

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Combining the physical properties of transition metals (electronic, magnetic and catalytic) with the dynamic properties of mechanically interlocked molecules (MIMs) has the potential to create chemical systems with a variety of unique applications, the scope of which, are just beginning to be explored.¹ For example, a number of MIM systems have been reported in which a transition metal acts as 1) a templating ion to facilitate interpenetration,² 2) a reporter group to sense binding of a guest,³ 3) an additive that elicits molecular motion⁴ or 4) a building block to create metal organic frameworks (coordination polymers).⁵ However, the synthesis of such sophisticated ligands and their transition metal complexes still present a major challenge for synthetic inorganic chemists.

Probably, the most common and facile method of preparing an interlocked coordination compound is by self-assembly. In such cases, the axle, the wheel and a labile metal fragment are mixed under mild conditions leading to both threading of the axle through the wheel and stoppering of the axle via metal coordination to create an interlocked [2]rotaxane. We originally⁶ stoppered the [2]pseudorotaxane [1⊂DB24C8]²⁺ with the Pd(II) pincer complex [Pd(C₆H₃(CH₂SPh)₂)]⁺ and later reported this could easily be accomplished using simple anionic fragments such [MnBr₃]⁻ and [CoBr₃]⁻ yielding neutral, zwitterionic complexes.⁷ Since the preparation of these metal stoppered rotaxanes required employing experimental conditions that favoured both[2]pseudorotaxane formation and metal ligand coordination, the types of complexes that could be prepared were restricted to those readily formed at room temperature in non-competitive solvents. These self-assembly conditions produced good yields of complexes with labile transition metal ions⁸ but were not sufficient for preparing robust complexes of inert transition metal ions that required much more forcing conditions and/or were not reversible under the milder conditions used.

A practical method of incorporating inert metal ions directly into an interlocked species would be to construct a ligand that was itself a permanently interlocked rotaxane.⁹ This would allow for the adoption of forcing conditions such as elevated temperature and highly polar solvents since unthreading of the rotaxane unit would not be possible. As an example of this approach, we have prepared a bis(terpy) (terpy = 2,2′,6′,2′′-terpyridine) rotaxane ligand by stoppering the [2]pseudorotaxane¹⁰ [1⊂DB24C8]²⁺ with 4′-(4-tolyl)-2,2′,6′,2′′-terpyridine groups to produce the rotaxane ligand [2⊂DB24C8]⁴⁺ as outlined in Scheme 1. 4′-(4-Bromobenzyl)-2,2′,6′,2′′-terpyridine¹¹ was prepared in 35% yield by bromination of 4′-(4-tolyl)-2,2′,6′,2′′-terpyridine¹¹ using N-bromosuccinamde in CCl₄ solution and two equivalents of the resulting benzylbromide derivative used to alkylate the terminal pyridine groups of the [2]pseudorotaxane [1⊂DB24C8]²⁺ producing [2]rotaxane [2⊂DB24C8]⁴⁺ in 31% yield.

Scheme 1 Preparation of the [2]rotaxane ligand: i) 3 equiv. of 4′-(4-bromobenzyl)-2,2′,6′,2′′-terpyridine, MeNO₂, room temperature, 7 days followed by anion exchange by stirring the MeNO₂ layered with NaOTf(aq) for 10 h; isolated yield 31%.

In order to explore the coordination of both labile and inert metal ions to [2⊂DB24C8]⁴⁺, complexes of Zn(II) and Ru(II) were prepared. For Zn(II), [2⊂DB24C8][OTf]₄ was stirred with [Zn(H₂O)₆][OTf]₂ in MeCN solution for 12 h at room temperature yielding an orange solution. The ¹H NMR spectrum of the resulting complex showed only broadened resonances for [2⊂DB24C8]⁴⁺ due to rapid metal ligand exchange.

Diffusion of iso-propyl ether into a MeNO₂ solution of the complex produced essentially quantitative yield of an orange crystalline material. A single crystal X-ray diffraction study‡ showed this material to be binuclear with a Zn(II) ion coordinated to each terpyridine group. Fig. 1 shows a ball-and-stick representation of one of the [(Zn(H₂O)₃)₂(2⊂DB24C8)]⁸⁺ cations. The dumbbell component adopts a zig-zag shaped conformation that is linear through the central interlocked component but bent at the benzylic methylene units that link the terpyridine groups to the rest of the molecule. The DB24C8 wheel is in the S-shaped conformation most commonly observed for this templating motif.¹⁰ The complex is over 4 nm in length with a Zn(II)⋯Zn(II) distance of 37.1 Å.

Fig. 1 A ball-and-stick representation of the cationic portion of the X-ray crystal structure of [(Zn(H₂O)₃)₂(2⊂DB24C8)]⁸⁺. The complex occupies a crystallographic centre of symmetry. All hydrogen atoms, except those on coordinated water molecules, all anions and all solvent molecules have been omitted for clarity. (Zn = blue-gray, O = red, N = blue, C = black, H = white; wheel bonds = silver, axle bonds = gold).

For Ru(II), [2⊂DB24C8][OTf]₄ was refluxed with [RuCl₃(terpy)]¹² in a 1 : 1 EtOH/H₂O solution for 24 h yielding a deep red solution. These are the standard conditions for the preparation of heteroleptic terpyridine complexes such as [Ru(terpy)(terpy′)]²⁺.¹² The ¹H NMR spectrum of the resulting complex (Fig. 2) clearly shows formation of the binuclear product [(Ru(terpy))₂(2⊂DB24C8)]⁸⁺; NMR assignments were confirmed by 2D COSY and NOESY spectra. The axle 2²⁺ exhibits significant chemical shift changes due to both interpenetration through the DB24C8 wheel as well as coordination to Ru(II). The interpenetration effects methylene protonsa and aromatic protonsb which are involved in hydrogen bonding to oxygen atoms of the crown ether resulting in Δδ of 0.33 and 0.30 ppm respectively¹⁰ while π-stacking between the electron rich aromatic rings of DB24C8 and the electron poor pyridinium groups on the axle results in significant shielding and Δδ of −0.21 and −0.18 ppm for protonsc and d respectively.¹⁰ Rotation of the pyridine groups of (2⊂DB24C8)²⁺ into favourable conformations for coordination and bonding to the Ru(II) centre results in coordination shifts for i–m ranging from a Δδ of −1.31 ppm for m to +0.26 ppm for i.

Fig. 2 The ¹H NMR spectrum of the binuclear [2]rotaxane complex [(Ru(terpy))₂(2⊂DB24C8)][OTf]₈ in MeCN-d₃ solution at 298 K. The labelling scheme is shown in Scheme 1.

The UV-visible absorption spectrum of [(Ru(terpy))₂(2⊂DB24C8)]⁸⁺ in MeCN solution showed a single MLCT absorption peak at λ_max = 485 nm. This is comparable to λ_max = 483 nm observed for [Ru(terpy)(4′-(4-tolyl)terpy)]²⁺.¹³ The red shifts from λ_max = 474 nm observed for [Ru(terpy)₂]²⁺ can be attributed to the presence of the electron donating tolyl group.¹⁴

Diffusion of iso-propyl ether into a MeNO₂ solution of [(Ru(terpy))₂(2⊂DB24C8)][OTf]₈ yielded X-ray quality crystals.^‡Fig. 3 shows a ball-and-stick-representation of the complex cation, [(Ru(terpy))₂(2⊂DB24C8)]⁸⁺. As was observed for the Zn(II) complex, the dumbbell adopts a zig-zag shaped conformation that is essentially linear throughout the interlocked component but bent at the benzylic methylene units linking the terpyridine and pyridinium groups. The two different terpyridine groups chelate to Ru(II) in a perpendicular fashion with bond distances and angles typical for Ru(II) bis(terpyridine) complexes. Again, the total length of the complex is over 4 nm with a Ru(II)⋯Ru(II) distance of 36.5 Å.

Fig. 3 A ball-and-stick representation of the cationic portion of the X-ray crystal structure of [(Ru(terpy))₂(2⊂DB24C8)]⁸⁺. The complex occupies a crystallographic centre of symmetry. All hydrogen atoms and anions have been omitted for clarity. (Ru = blue-gray, O = red, N = blue, C = black, H = white; wheel bonds = silver, axle bonds = gold).

Conclusions

Preparation of a binuclear Zn(II) complex of [2⊂DB24C8]⁴⁺ with labile co-ligands (H₂O) shows that this ligand has the potential to be used in self-assembly reactions to create 1-periodic terpy-linked coordination polymers. More importantly, the synthesis of a mixed ligand complex of an inert metal ion, such as Ru(II), clearly demonstrates that the “rotaxane as a ligand” approach provides a methodology to prepare robust complexes that require harsh reaction conditions and/or to facilitate stepwise coordination of different ancillary ligands; in this case terpy followed by rotaxane.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Details of syntheses, spectroscopy and X-ray solutions. CCDC reference numbers 819991 and 819992. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1dt10569h

‡ Crystal data¹⁵ for [(Zn(H₂O)₃)₂(2⊂DB24C8)][(Zn(H₂O)(BF₄))₂ (2⊂DB24C8)][OTf]₇(MeNO₂)₃: C₂₀₀H₂₁₀B₂F₅₀N₂₆O₈₂S₁₄Zn₄, M = 5971.88, T = 173(2)K, monoclinic, space groupP2₁/c, a = 24.665(14), b = 26.270(15), c = 21.104(12) Å, β = 108.612(7), V = 12959(13) ų, ρ_c = 1.530 g cm⁻³, μ = 0.603 mm⁻¹, Z = 2, reflections collected = 121682 (R_int = 0.7726), final R indices [I > 2σI]: R₁ = 0.1241, wR₂ = 0.2584, R indices (all data): R₁ = 0.4350, wR₂ = 0.4726, GoF = 0.934 with data/variables/restraints = 22775/1531/476. Crystal data for [(Ru(terpy))₂(2⊂DB24C8)][OTf]₆[Cl]₂.(MeNO₂)₄: C₁₃₀H₁₁₈Cl₂F₁₈N₂₀O₃₄Ru₂S₆, M = 3311.84, T = 173(2)K, triclinic, space groupP1̄, a = 14.056(3), b = 14.073(3), c = 22.616(5) Å, α = 78.454(4), β = 83.369(4), γ = 62.894(3), V = 3900.2(16) ų, ρ_c = 1.401 g cm⁻³, μ = 0.406 mm⁻¹, Z = 1, reflections collected = 37004 (R_int = 0.1452). Before SQUEEZE:¹⁶R indices [I > 2σI]: R₁ = 0.1627, wR₂ = 0.3859, R indices (all data): R₁ = 0.3037. wR₂ = 0.4588, GoF = 1.297 with data/variables/restraints = 11259/928/355. After SQUEEZE: R indices [I > 2σI]: R₁ = 0.1212, wR₂ = 0.2830, R indices (all data): R₁ = 0.2567. wR₂ = 0.3350, GoF = 0.890 with data/variables/restraints = 13674/928/502.

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