Chemically and Electrochemically Induced Expansion and Contraction of a Ferrocene Rotor

A 2,2'-bipyridine-appended ferrocene rotor, 1,1'-di(5-yl-ethynyl-2,2'-bipyridine)ferrocene, can be switched from a folded/stacked (syn) conformation to an extended/unstacked (anti) conformation by the addition of [Cu(CH3CN)4](PF6) and 6,6'-dimesityl-2,2'-bipyridine. This extension and contraction process was completely reversible and could be triggered either chemically or electrochemically.

Introduction of charge to the ferrocene ''arms'', either by protonation or metallation with Pd 2+ ions, induced rotation to the anticonformation to minimise the electrostatic repulsion between the cationic substituents of the ferrocene rotors.13c Deprotonation or addition of chloride ions removed the electrostatic repulsion restoring the initial stacked syn conformation.13c Other workers 13a,b have generated electrochemical switchable systems by incorporating redox-active organic arms into the ferrocene rotors.
Herein we build on our previous work 13c and develop ferrocene rotor systems that can be switched from the syn to anti conformation by the complexation/decomplexation of a copper 6,6 0 -dimesityl-2,2 0bipyridine fragment.A combination of electrostatic and steric repulsion destabilises the syn conformation upon complexation generating the anti rotamer.This extension and contraction process was completely reversible and could be triggered either chemically or electrochemically.
The ethynylferrocene rotors (1a-b) and model (2a-b) systems were readily synthesised using modified literature procedures (ESI †). 14 S38).This was further supported by computational modelling of the systems.DFT calculations (CAM-B3LYP, DMF solvent field) 13b,15 were used to determine the relative energies of a series of different rotation conformers of 1a-b (the angles between the ferrocene ''arms'' of 1a-b was varied from À1801 to 1801 and the compound energy minimised).These calculations showed, as expected, that the stacked, fully eclipsed (syn) rotamers (angle = 01) were the lowest energy conformations for both 1a and 1b by 3 and 6 kJ mol À1 , respectively (ESI, † Fig. S82 and S86).Additional evidence for the stacked (syn) conformation was obtained from X-ray crystallography (Fig. 3a and ESI †).The molecular structure of 1a clearly showed that the bipyridine substituents are stacked (centroid-centroid distances 3.636 and 3.716 Å) and the ferrocene cyclopentadiene (Cp) rings are eclipsed, with a dihedral angle of 1.581 (Fig. 3a) between the substituents consistent with the 1 H NMR spectral and computational data.
on complexation.As discussed above, the proton resonances of the methylpyridinum unit of 1b are shifted upfield relative to those of the model compound 2b, indicating that the di-armed ferrocene rotor adopts a stacked (syn) conformation in solution (ESI, † Fig. S38).Upon complexation to the copper(I) 3 complex the proton signals of the methylpyridinum unit shift downfield to values that are almost identical to those observed from the model compound 2b indicating that the pyridinum arm of the copper(I) complex is no longer p-stacked (ESI, † Fig. S41).DFT calculations (CAM-B3LYP, DMF solvent field) 13b, 15 were used to examine the relative energies of different rotational conformers of the copper(I) complexes [Cu 2 (1a)(3) 2 ](PF 6 ) 2 , and [Cu(1b)(3)](PF 6 ) 2 (ESI, † Fig. S84 and S88).The resulting plot revealed that the minimum angle between the two arms could not be less than 601, with the energy difference between higher angle conformations being negligible.Thus it can be surmised that the arms of the rotor must have extended to at least 601 in solution.Consistently, the molecular structure of [Cu 2 (1a)(3) 2 ] 2+ as the perchlorate (ClO 4 À ) salt (Fig. 3b and ESI †) showed that the [Cu(3)] + units had coordinated to both bipyridine arms of 1a in the expected ''pacman-like'' arrangement.More importantly it confirmed that the rotor molecule was almost completely extended, with the angle between the two arms ranging from 142-1451 (Fig. 3b and ESI †).
The second step at E appl = 1.1 V passed only 1.0 F mol À1 , thereby confirming the transfer of two and one electrons, respectively.These results confirmed that the first process is the Cu(I) to Cu(II) oxidation while the second is the oxidation of the ferrocene to ferrocenium.
The CV experiment was then carried out in the presence of two equivalents of 2,2 0 :6 0 ,2 00 -terpyridine (5).The anodic potential sweep showed the two peaks corresponding to the Cu II/I and ferrocenyl oxidation processes.On the reverse scan, only the reduction of the ferrocenium back to ferrocene is observed.No peak corresponding to the reduction of the [Cu 2 (1a)(3) 2 ] 4+ complex was observed.Instead proceeding in the cathodic direction, a peak corresponding to the reduction of the pentacoordinated [Cu(5)(3)](PF 6 ) 2 complex was observed at À0.10 V (Fig. 4 black trace).This electrochemical behaviour is very similar to that observed by Sauvage et al. 21in their MIAs systems.
Herein we have shown that readily synthesised 2,2 0 -bipyridineappended ferrocene rotor molecules can be switched from a folded/stacked (syn) to an extended/unstacked (anti) conformation upon the addition of [Cu(CH 3 CN) 4 ](PF 6 ) and 6,6 0 -dimesityl-2,2 0 -bipyridine to the rotor units.This switching is driven by a combination of electrostatic and steric repulsion which destabilizes the syn conformation upon copper(I) complexation and generates the anti rotamer.This extension and contraction process was completely reversible and could be triggered either chemically or electrochemically.The chemically driven process can be repeated multiple times but a large amount of Cu(II)-cyclam waste slowly builds up in solution.The switching of the rotor can also be triggered electrochemically.Oxidative bulk electrolysis of the [Cu 2 (1a)(3) 2 ](PF 6 ) 2 rotor complex in the presence of two equivalents of 2,2 0 :6 0 ,2 00 -terpyridine (5) was accompanied by a colour change from deep red/brown to yellow indicative of the translocation of the [Cu(3)] 2+ fragment to the 2,2 0 :6 0 ,2 00 -terpyridine (5) ligands and the formation of syn-1a.Reduction of the mixture restored the original red/brown colour suggesting the [Cu(3)] + fragment has moved from the 2,2 0 :6 0 ,2 00 -terpyridine (5) back to the rotor 1a regenerating the extended anti-[Cu 2 (1a)(3) 2 ](PF 6 ) 2 rotor complex.This electrochemical process provides a clean reversible method for the extension and contraction of the ferrocene rotors.
The findings presented here could enable the generation of new, readily accessible, non-interlocked, electrochemically switchable bistable molecular machines or catalysts. 25Inspired by the molecular ''folding ruler'' of Takeuchi and co-workers 26 we are currently attempting to generate longer ferrocene oligomers that will undergo a larger molecular extension and contraction and exploit these molecules as (nano)molecular actuators.

Fig. 1
Fig. 1 Generic stimuli responsive switching with ferrocene (Fc) rotor molecules and the compounds 1a-b and 2a-b studied herein.

Fig. 3
Fig. 3 ORTEP 18 diagrams showing the solid state structures of the rotor 1a (a), and the Cu(I) complex [Cu 2 (1a)(3) 2 ](PF 6 ) 2 (b).The ellipsoids are shown at the 50% probability level.Counterions and hydrogen atoms of the complex are omitted for clarity.Additional crystallographic details can be found in the ESI.†
SØS and ABSE thank the University of Otago for a PhD scholarship.CJM thanks the NZ Ministry of Business, Innovation and Employment Science Investment Fund (Grant no.UOOX1206) for financial support.AN thanks Scientific Research at King Saud University for funding (RGP-VPP-236).JDC thanks the Marsden Fund (Grant no.UOO1124) and the University of Otago (Division of Sciences Research Grant) for supporting this work.