A ferrocene based switchable molecular folding ruler

Article abstract of DOI:10.1039/c7cc03358c

A 2,2′-bipyridine-appended bis(ferrocene) three tiered molecular folding ruler, can be switched from a folded conformation to an extended conformation by the addition of [Cu(CH₃CN)₄](PF₆) and 6,6′-dimesityl-2,2′-bipyridine

Full text of DOI:10.1039/c7cc03358c

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A bis(ferrocene) three tiered molecular folding ruler can be induced to undergo a large scale extension and
contraction process either chemical or better electrochemical methods.

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A Ferrocene Based Switchable Molecular Folding
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A
2,2O-bipyridine-appended bis(ferrocene) three tiered bis(porphyrinate) polymer adopts an extended conformation, and the
addition of 1,4-diazabicyclo[2.2.2]octane (DABCO) was supposed to
molecular folding ruler, can be switched from a folded
conformation to an extended conformation by the addition of
Cu(CH CN) ](PF ) and 6,6'-dimesityl-2,2'-bipyridine. This
3 4 6
extension and contraction process could be triggered either
chemically or electrochemically and was reversible.
induce the folding of the —ruler“ into a contracted stacked form.
Unfortunately, while UV-vis spectroscopy confirmed the binding
DABCO to the zinc porphyrin sites, dynamic light scattering
experiments indicated that there was little contraction observed, with
the average polymer size remaining approximately 12 nm in diameter.
The re-—extension“ of the polymer was achieved via protonation of
the DABCO guest with trifluoroacetic acid.
[
Nanoscale molecular machines are critical for the functioning of
16e
1
Herein, inspired by the design of the polymeric folding ruler,
biological systems. Inspired by these systems chemists have begun
we build on our previous work with ferrocene rotors1
4e, 14f
and report
2
to create synthetic analogues. Molecular machines based on both
a triple tiered diferrocene molecular folding ruler (Figure 1, difcbipy).
Additionally, we examine the switching from a stacked (syn, syn) to
an extended (anti, anti) conformation, driven by the complexation and
decomplexation of a copper 6,6'-dimesityl-2,2'-bipyridine fragment to
the 2,2O-bipyridyl (bipy) binding sites of difcbipy. The system was
studied using a combination of nuclear magnetic resonance (NMR),
and UV-vis spectroscopies, cyclic voltammetry (CV) and density
functional theory (DFT). Similar to our previously reported
monoferrocene rotary switch, a combination of electrostatic and
steric repulsion destabilises the syn, syn conformation of difcbipy
upon complexation generating the anti, anti rotamer. This extension
and contraction process was completely reversible and could be
triggered either chemically or electrochemically.
mechanically interlocked architectures (MIAs) and non-interlocked
systems have been developed and the pioneering work in this area led
to the award of the 2016 Nobel prize in Chemistry to Sauvage,
3
Stoddart and Feringa. While MIAs have been exploited to generate
some impressive synthetic machines, including synthetic rotary
4
5
motors, molecular muscles, and
a sequence-specific peptide
synthesiser6 there have been increasing efforts to generate non-
interlocked systems, as they are potentially synthetically more
accessible. In particular, systems incorporating metal ions are
14f
becoming more and more common due to the useful kinetic and
thermodynamic properties of the coordination bonds.7 Non-
interlocked synthetic machines have been exploited to develop rotary
motors,8 molecular walkers
7c,
9
10
and robots. More recently, non-
interlocked rotary motors have also been used to —drive“ nanocars
The triple tiered diferrocene folding ruler (difcbipy) was
across surfaces,11 and contract and expand gels.
12
13
synthesised
from
1-(5-yl-ethynyl-2,2•-bipyridine)-1•-iodo-
ferrocene1 and 1,4-diethynyl-2,5-bis(hexyloxy)benzene in modest
4f
(fc),14
Sandwich complexes such as ferrocene
1
5
16
yield (12%) synthesised using modified literature procedures
metallocarboranes and lanthanide bis(porphyrinate)s have been
used as rotary components in a range of non-interlocked molecular
(
ESI†).19 1H NMR spectroscopy, density functional theory (DFT)
1
7
18
calculations and X-ray crystallography were used to determine the
preferred conformation of difcbipy (ESI†). The proton signals
associated with the internal pyridyl (Hd-f, û/ = 0.1-0.12 ppm) and
phenyl rings (û/ = 0.18 ppm) of difcbipy were shifted upfield relative
to those of the singly armed model compounds (1-(5-yl-ethynyl-2,2‘-
bipyridine)ferrocene and 1,4-diethynyl-2,5-bis(hexyloxy)benzene
machines due to their molecular ball-bearing properties. However,
the vast majority of these sandwich systems only contain one
molecular ball-bearing unit and switch from a stacked (syn) to
unstacked (anti) state (Figure 1a). The exception is the elegant
polymeric molecular folding ruler of Takeuchi and co-workers.16e The
authors developed a polymer containing zinc porphyrin units as a
recognition site for a ditopic guest molecule, and cerium(IV)
bis(porphyrinate) rotatable joints. The system was designed to extend
and contract upon treatment with a chemical stimulus in a similar
fashion to muscles. In its natural state the cerium(IV)
(
S4), suggesting that difcbipy adopted a stacked (syn, syn)
conformation in solution (ESI†). This was further supported by
computational modelling of the system. DFT calculations (CAM-
B3LYP/6-31G(d), DMF solvent field) were used to determine the
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DOI: 10.1039/C7CC03358C
relative energies of a series of different rotational conformers of from X-ray crystallography (Fig. 2b and ESI†). The molecular
difcbipy (the angle (.) between the bipy and phenyl —arms“ of structure of difcbipy clearly showed that the bipyridyl and phenyl
difcbipy was varied from -180˘ to 180˘ and the compound energy substituents are stacked (centroid-centroid distance 3.737 Å) and the
minimised). These calculations showed, as expected, that the stacked, ferrocene cyclopentadiene (Cp) rings are eclipsed, with a dihedral
1
fully eclipsed (syn, syn) rotamer (initial angle . = 0˘) was the lowest angle of 3.98˘ between the substituents, consistent with the H NMR
-
1
energy conformation for difcbipy by 10 kJ mol (Fig. 2a, ESI†). spectral and computational data (Fig. 2b and ESI†).
Additional evidence for stacked (syn, syn) conformation was obtained
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•
î
ꢀ~tC
ò
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î
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ï ï ð ò
ꢀ
ð
We chose to use the copper(I) 6,6'-dimesityl-2,2'-bipyridine20 observed for the model compound S4 indicating that the phenyl arm
+
complex [Cu(dimesbipy)(CH
3
CN)
2
] developed by Schmittel and co- of the dicopper(I) complex is no longer Œ-stacked (ESI†).
workers to unfold the difcbipy (Fig. 1b). The complex is well known
to cleanly form heteroleptic complexes such
[
DFT calculations (CAM-B3LYP/6-31G(d), DMF solvent field)
as were used to examine the relative energies of different rotational
Cu(bipy)(dimesbipy)] and we and others21 have previously used conformers
+
14f
of
the
dicopper(I)
complex
the motif to generate switchable systems. Reaction of difcbipy in [Cu
2
(difcbipy)(dimesbipy)
2
](PF ) (ESI†). The resulting energy plot
6 2
acetone solution with [Cu(CH CN) ](PF ) and the sterically bulky revealed that the anti, anti rotamer (. = 180˘) was the lowest energy
3
4
6
2
0
bipyridine ligand 6,6'-dimesityl-2,2'-bipyridine (dimesbipy) was conformation (ESI†). Futhermore, as the angle (.) between the central
accompanied by a colour change from orange to red indicative of phenyl and terminal bipy —arms“ of the folding ruler complex was
1
4f, 22
1
13
complexation (Fig. 1b and ESI†).
H NMR, C NMR, and IR reduced the energy increased. The energies of conformations with . ≤
spectroscopy, combined with HRESI-MS and elemental analysis, 100˘ were so high that these rotamers would not be readily accessible,
confirmed the formation of the expected dicopper(I) complex presumably due to the electrostatic and steric repulsion (ESI†). Thus
[
2 2 6 2
Cu (difcbipy)(dimesbipy) ](PF ) . The mass spectrum of the it can be surmised that the bipy arms of the rotor must have extended
complex displayed two peaks at m/z
=
981.3261 to at least 100˘ away from the central phenyl arm in solution,
2107.6128 generating an anti, anti rotamer. Disappointingly, we were unable to
[
[
Cu
Cu
2
(difcbipy)(dimesbipy)
(difcbipy)(dimesbipy)
2
]2+
and
m/z
=
)]+ consistent with the clean generate
X-ray
(difcbipy)(dimesbipy)
Having confirmed that difcbipy (syn, syn) and the corresponding
(anti, anti) would adopt two different
would switch conformation from the syn, syn to the anti, anti conformations we next examined if we could cleanly and reversibly
quality
single
crystals
of
2
2
(PF
6
formation of the dicopper(I) complex (ESI†).
[Cu
2
2
](PF
6 2
) despite extensive efforts.
Due to the combination of electrostatic and steric repulsion
generated upon complex formation it was expected that the difcbipy dicopper(I) complex
rotamer.1 An examination of the H NMR spectra of the complex switch between the folded and extended conformations. Addition of
Cu (difcbipy)(dimesbipy) ](PF , the diferrocene folding ruler two equivalents of 1,4,8,11-tetraazacyclotetradecane (cyclam) to an
difcbipy and the corresponding model compound S4 provided chloroform (CDCl ) solution of [Cu (difcbipy)(dimesbipy) ](PF
4f
1
[
2
2
6 2
)
3
2
2
6 2
)
experimental support for this conformation switch on complexation. caused an immediate colour change from red to orange (Fig. 1b (ii),
As discussed above, the proton resonance of the central phenyl unit of ESI†) and the precipitation of a white solid, indicating that the Cu(I)
difcbipy was shifted upfield relative to those of the model compound ions had been removed from the ferrocene complex.2
1a, 21b 1
H NMR
S4, indicating that the triple tiered diferrocene folding ruler phenyl and UV-vis spectroscopy and ESMS (ESI†) confirmed that only the
rings adopts a stacked (syn, syn) conformation in solution (ESI†). free difcbipy and dimesbipy were present in solution.
+
Upon complexation to the [Cu(dimesbipy)] the proton signals of the
3 4 6
Addition of two equivalents of [Cu(CH CN) ](PF ) to the solution
phenyl unit shift downfield to a value that was almost identical to that regenerated the characteristic red colour of the complex
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DOI: 10.1039/C7CC03358C
2 2 6 2
(difcbipy)(dimesbipy) ](PF )
, and the 1H NMR spectrum by us with our monoferrocene rotor system14f and Sauvage et al.24a, 24b
[
(
Cu
ESI†) indicated that the complex had quantitatively re-formed. This in their molecular machines.
Bulk electrolysis in the presence of two equivalents of terpy was
chemically driven syn, syn-to-anti, anti switching process could be
repeated a number of times (>5) indicating that switching was accompanied by a colour change from deep red/brown to orange, the
completely reversible (ESI†). However, the method led to the build- same as was observed for the chemical switching process with cyclam
up of waste by-products (Cu(II)-cyclam) which could potentially lead (ESI†). Spectroelectrochemistry (acetone, 0.1 M Bu
4
NPF
6
, OTTLE
6 2
) complex
2
3
to the loss of function. Therefore we also examined whether the cell, ESI†) of the [Cu
2
(difcbipy)(dimesbipy)
2
](PF
electrochemical Cu(I)/Cu(II) switching process developed by confirmed that during the bulk oxidation the MLCT band (ꢀmax = 460-
Sauvage et al.24 could be exploited to extend and contract difcbipy.
470 nm) decreases until a spectrum consistent with —free“ difcbipy is
generated. ESI-MS analysis of the terpy and
Cu (difcbipy)(dimesbipy) ](PF mixture before bulk electrolysis
displayed peaks due to the [Cu
[
2
2
6 2
)
+
2
(difcbipy)(dimesbipy)
2
(PF
and [terpy+H] ions. After oxidation
the mass spectrum only contain peaks consistent with
6
)] ,
2+
+
[
2 2
Cu (difcbipy)(dimesbipy) ]
2+
+
+
[
Cu(terpy)(dimesbipy)] , [difcbipy+H] and [difcbipy+Na] ions.
Combined these results suggest that, upon oxidation or reduction
n+
of the copper ions, the [Cu(dimesbipy)] fragments are set in motion,
14f
as was observed in the monoferrocene rotor system. Upon oxidation
2+
4+
of [Cu
2
(difcbipy)(dimesbipy)
2
]
2 2
to [Cu (difcbipy)(dimesbipy) ] ,
the resulting tetrahedrally coordinated Cu(II) ions are unstable and
this leads to the migration of the [Cu(dimesbipy)]2 fragment to the
terpy ligands generating the more stable five-coordinate
+
Cu(terpy)(dimesbipy)]2 complex.
+
14f
Similarly, upon reduction of
[
+
the Cu(II) back to Cu(I) the [Cu(terpy)(dimesbipy)] complex is
unstable as the copper(I) ions are in the pentacoordinate trigonal
+
bipyramidal environment. As such the [Cu(dimesbipy)] fragments
migrate back to the difcbipy regenerating the complex
2+
[
2 2
[Cu (difcbipy)(dimesbipy) ] , and returning the copper(I) ions to
the preferred tetrahedral coordination environment. This results in an
electrochemically induced extension and contraction of the
diferrocene molecular folding ruler difcbipy.
Herein we have shown that a 2,2O-bipyridine-appended triple
tiered diferrocene molecular folding ruler difcbipy can be switched
from a folded/stacked (syn, syn) to an extended/unstacked (anti, anti)
3 4 6
conformation upon the addition of [Cu(CH CN) ](PF ) and
dimesbipy to the rotor units. This switching is driven by a
combination of electrostatic and steric repulsion which destabilizes
the folded conformation upon copper(I) complexation and generates
the extended rotamer. The 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 electrochemical switching, in the presence
of terpy, provides a clean reversible method for the extension and
contraction of the diferrocene molecular folding ruler. Furthermore,
the extension and contraction of the diferrocene molecular folding
ruler is of a larger magnitude than that observed for previously
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1
CV (100 mV s , acetone, 0.1 M NBu
that the [Cu (difcbipy)(dimesbipy) ](PF
chemically reversible processes at 0.66 and 0.80 V respectively (Fig.
red/brown trace) while the difcbipy displays a single ferrocenyl
based chemically reversible processes at 0.73 V. By comparison with
4
PF
6
) experiments showed
2
2
6 2
) complex displays two
1
8
reported ferrocene switches.
3
2
2, 25
the related previously prepared model systems
process of [Cu (difcbipy)(dimesbipy) ](PF
Cu couple while the second oxidation process is ferrocenyl based.
The CV experiment with [Cu (difcbipy)(dimesbipy) ](PF was
the first oxidation
2
2
6 2
) was assigned to the
II/I
2
2
6 2
)
then carried out in the presence of two equivalents of 2,2•:6•,2••-
terpyridine (terpy). The anodic potential sweep showed the two peaks
II/I
corresponding to the Cu 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
4
+
[
Cu
proceeding in the cathodic direction, a peak corresponding to the C]PµŒꢁ ïX /ë• ~íìì uë • U ꢀꢂꢁš}vꢁU ìXí a .µ
reduction of the pentacoordinated [Cu(dimesbipy)(terpy)](PF
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rí
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1
6e
design of Takeuchi and co-workers could potentially be exploited
to develop stimuli responsive (nano)molecular actuators. We are
currently trying to improve the synthesis of the ferrocene based
folding ruler unit and develop methods to incorporate them into
condensed phases in order to transfer the molecular level motion into
useful work.
SØS and JEB thank the University of Otago for PhD scholarships.
CJM thanks the NZ Ministry of Business, Innovation and
Employment Science Investment Fund (Grant No. UOOX1206) for
financial support. JDC thanks the Marsden Fund (Grant no.
UOO1124) and the University of Otago (Division of Sciences
Research Grant) for supporting this work. The New Zealand eScience
Infrastructure (NeSI) is thanked for computational time.
1
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1
Notes and references
a
Department of Chemistry, University of Otago, PO Box 56, Dunedin, New
Zealand. Fax: +64
3 479 7906; Tel: +64 3 479 7731; E-mail:
jcrowley@chemistry.otago.ac.nz
b
MacDiarmid Institute for Advanced Materials and Nanotechnology, New
Zealand
†
Electronic Supplementary Information (ESI) available: the
1
supplementary information contains the experimental procedures, H and 15. Metallacarboranes have also been used to develop similar molecular
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