A Six-Crossing Doubly Interlocked [2]Catenane with Twisted Rings, and a Molecular Granny Knot

Article abstract of DOI:10.1002/anie.201807135

A molecular 6₃² link (a six crossing, doubly interlocked, [2]catenane with twisted rings) and a 3₁#3₁ granny knot (a composite knot made up of two trefoil tangles of the same handedness) were constructed by ring-closing olefin metathesis of an iron(II)-coordinated 2×2 interwoven grid. The connections were directed by pendant phenyl groups to be between proximal ligand ends on the same faces of the grid. The 6₃² link was separated from the topoisomeric granny knot by recycling size-exclusion chromatography. The identity of each topoisomer was determined by tandem mass spectrometry and the structure of the 6₃² link confirmed by X-ray crystallography, which revealed two 82-membered macrocycles, each in figure-of-eight conformations, linked through both pairs of loops.

Full text of DOI:10.1002/anie.201807135

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Accepted Article
Title: A Six-Crossing [2]Catenane with Twisted Rings (a Molecular
6-3-2 Link), and a Molecular Granny Knot
Authors: David Leigh, Jonathan Danon, Simone Pisano, Alberto
Valero, and Iñigo J. Vitorica-Yrezabal
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201807135
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A Six-Crossing [2]Catenane with Twisted Rings (a Molecular ꢀꢀꢀ
Link), and a Molecular Granny Knot
Jonathan J. Danon, David A. Leigh,* Simone Pisano, Alberto Valero and Iñigo J. Vitorica-Yrezabal
Abstract: A molecular 6!! link (a six crossing, doubly interlocked,
[2]catenane with twisted rings) and a 31#31 granny knot (a composite
knot made up of two trefoil tangles of the same handedness) were
constructed by ring-closing olefin metathesis of an iron(II)-
coordinated 2×2 interwoven grid. The connections were directed by
pendant phenyl groups to be between proximal ligand ends on the
same faces of the grid. The 6!! link was separated from the
of David catenane (6!! link) but with a different crossing pattern.
The topology can be depicted as two figure-of-eight shapes
linked through both pairs of loops (Figure 1). The crossing
pattern for this link can, in principle, be achieved from a 2×2
interwoven grid by connecting the ends of ligand strands on the
same face of the grid that are coordinated to adjacent metal ions.
However, if the grid is a near-perfect square then each strand
end is equidistant to the two closest ligand-ends and a mixture
of two topoisomers will be obtained. The relative orientation of
the connections on the two faces of the grid plane dictates the
topology formed: a 6!! link (the joined ends on one face are
roughly perpendicular to the joined ends on the other face) or a
granny knot (the connected groups on each face are close to
parallel with those on the other) (Figure 1).
topoisomeric
granny
knot
by
recycling
size-exclusion
chromatography. The identity of each topoisomer was determined by
tandem mass spectrometry and the structure of the 6!! link confirmed
by X-ray crystallography, which revealed two 82-membered
macrocycles, each in figure-of-eight conformations, linked through
both pairs of loops.
Links (generally termed ‘catenanes’ when referring to
molecular systems) are mechanically connected closed loops
(macrocycles).[1] Complex links are found in some proteins[2] and
DNA[3]. Topologies with two or three small-molecule components
that have succumbed to synthesis include Hopf links[4] (the
simplest [2]catenane topology; a 2!! link in Alexander-Briggs
notation[5]), Solomon links (4!!),[6] a Star of David catenane (6!!),[7]
[9]
[10]
Borromean rings ( 6!! ),[8] and 6!!
and 9!!
links.[11] The
topologies with two components and fewer than seven crossings
that have yet to be prepared in small-molecule form are the
Whitehead link ( 5!! ),[12] and 6!! and 6!! links (types of two-
component links with twists in the rings).[13] Most of the complex
molecular links synthesized to date are derived from metal
coordination complexes that assemble ligand strands in spatial
arrangements that direct the ring closing reactions to generate
the required topology.[14] Types of motifs employed include
orthogonally positioned ligand complexes,[15] linear[16] and
circular[17] metal helicates, and interwoven molecular grids[6m]. A
2×2 interwoven grid was previously used to synthesize a
Solomon link by connecting the ends of parallel ligand
strands.[6m] However, the possibility of accessing different
topologies from alternative ligand connectivities using similar
types of scaffold remained unexplored. Here we report on the
designed synthesis of one of the topologies missing from the
molecular lexicon, a 6!! link, via a 2×2 interwoven grid. Also
formed is a 31#31[18] (granny) composite[10,19] knot.
Figure 1. The topologies obtained by connecting the closest ligand ends on
the same face of a 2×2 interwoven square grid. The knot and link topologies
are both chiral,[1,14,18] formed as a racemic mixture from a racemic grid.
The 6!! link[13] consists of two constitutionally identical
macrocycles involving six crossings, the same number as a Star
To achieve the required connectivity of the ligand strands,
[*]
Dr. J. J. Danon, Prof. D. A. Leigh, S. Pisano, A. Valero, Dr. I. J.
Vitorica-Yrezabal
School of Chemistry, University of Manchester
Oxford Road, Manchester M13 9PL (UK)
E-mail: david.leigh@manchester.ac.uk
Homepage: http://www.catenane.net
we designed bis-tridentate ligand
1
based on the
scaffold
bis(pyridylbenzimidazolyl)thiazolo[5,4-d]thiazole
previously used to assemble a Solomon link (Figure 2).[6m] The
meta-substituted phenyl groups (attached as part of a biphenyl
unit) direct the pendant alkene chains above or below the grid
plane, the biphenyl linkage significantly restricting the space
Supporting information for this article is given via a link at the end of
the document.
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accessible by the chain ends. The chain length was chosen to
avoid over-reaching, permitting only the required strand
connections. Ethyl groups were used to increase the solubility of
the benzimidazole units. These are smaller than the isopentyl
groups used in the Solomon link synthesis[6m] to avoid sterically
hindering the conformations of the alkene chains that lead to the
required connections.
Figure 2. Metal-directed synthesis of 6!! link 2 and granny knot 3 from ligand 1.
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To confirm that the modifications of the ligand would not
hamper grid formation, we prepared a model ligand with methyl
groups in place of the alkene chains (Supporting Information,
section 2.5). Complexation of the ligand with an equimolar
amount of iron(II) or zinc(II) tetrafluoroborate proceeded
smoothly in a mixture of dichloromethane and acetonitrile at
room temperature to quantitatively afford the corresponding 2×2
interwoven grids, which were characterised by NMR
spectroscopy and X-ray crystallography (Supporting Information,
sections 2.5 and 3). The alkene-bearing 2×2 interwoven grid
[Fe414](BF4)8 was synthesized in analogous fashion, using ligand
1 and a small excess of iron(II) tetrafluoroborate (Supporting
Information, section 2.2). The 1H NMR spectrum of [Fe414](BF4)8
showed significant downfield shifts of the pyridine protons
compared to the parent ligand (1), and a significant upfield shift
of the benzimidazole proton Hs (Supporting Information, p. 9 and
Figure S55), which should face the pyridine ring of the
perpendicular ligand strand in [Fe414](BF4)8 and therefore is
indicative of the anticipated grid structure.
Grubbs second generation catalyst[20] under microwave heating
in a sealed vial (Figure 2). After 3 hours H NMR indicated
1
complete consumption of the starting material, and electrospray
ionization mass spectrometry (ESI-MS) showed multiply charged
peaks corresponding to the successive loss of tetrafluoroborate
counter anions from the fully-closed grid (Supporting Information,
Figure S3). The crude product was demetallated (Figure 2) by
treatment with a saturated aqueous solution of tetrasodium
ethylenediaminetetraacetate (Na4EDTA) to afford a mixture of
the metal-free knot and link and other reaction byproducts,
which were separated by preparative recycling size-exclusion
chromatography (GPC) (Supporting Information, sections 2.2
and 2.3).
The non-interlocked macrocycle and other, non-ring-closed,
compounds obtained from the RCM reaction were removed from
the knot and link in the first GPC cycles to give pristine 2/3 in
18
% overall yield. However, the separation of the two
topoisomers, which appeared as a single peak during the first
few GPC cycles, proved to be far more challenging. After
exhaustive recycling (15-17 cycles), two distinct peaks were
obtained, the identities of which were determined by tandem
ESI-MS. The faster eluting compound gave a series of tandem
MS peaks (Figure 3b) that correspond to linear fragments that
can only arise from the composite knot (fragmentation of the
same bonds in a link would lead to dethreading and lower
masses corresponding to the intact charged component
macrocycle[21]). Under the same conditions, the equivalent ion of
the slower eluting fraction fragmented to singly- and doubly-
charged macrocycle without any higher mass fragments,
behaviour only consistent with a link topology (Figure 3a). Both
topoisomers were also characterised by 1H and 13C NMR, giving
distinct but complex spectra (Supporting Information, section
1
2.3). The broad H NMR spectra are typical of slow reptation of
interwoven strands in large and complex molecular knots and
links (Supporting Information, Figure S10).[7,22,23]
The individual topoisomers were each remetallated by
treatment with excess iron(II) tetrafluoroborate in
a
chloroform/acetonitrile mixture under heating (Figure 2 and
Supporting Information, section 2.4). In each case this
proceeded with full conversion to the corresponding metallated
1
complex, [Fe42](BF4)8 or [Fe43](BF4)8 respectively. The H NMR
spectrum of each complex was significantly sharper than that of
the mixture originally obtained from the RCM reaction
(Supporting Information, Figures S2 and S40).
We attempted to grow crystals from
a
sample of
[Fe42/3](BF4)8 obtained by remetallation of a mixture of the
organic topoisomers that had been separated from other species
by GPC (Supporting Information, section 2.2). However, slow
diffusion of toluene into a solution of [Fe42/3](BF4)8 in acetone
yielded single crystals of, to our surprise, the demetallated 6!!
link. The coordination of thiazolo[5,4-d]thiazole ligands to Fe(II)
is rather weak[6m] and it appears that the poor solubility of metal-
free 2 causes it to crystallize from solution as it reversibly forms
at very low concentration. The X-ray crystal structure of 2
(Figure 4 and Supporting Information, section 3) shows two 82-
membered macrocycles, each with one twist, mechanically
joined through both pairs of loops to form the six-crossing 6!! link
Figure 3. ESI-MS/MS tandem mass spectra of the [M+3H]3+ ion for a) 6!! link 2
and b) granny knot 3.
The 2×2 interwoven grid [Fe414](BF4)8 was subjected to ring
closing olefin metathesis (RCM) as a dilute solution (1 mM) in
dichloromethane/nitromethane (3:1), employing the Hoveyda-
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topology (Figure 4). A ~3:1 E/Z mixture of alkenes is present in
the alkyl chains. The aromatic regions of the ligand strands
stack at the center of the structure, with an average distance of
~3.5 Å between strands. From inspection of models it is clear
that the 6!! link is topologically chiral;[1,18] both enantiomers are
present in the unit cell.
Keywords: molecular grids • catenanes • coordination chemistry
• chemical topology • supramolecular chemistry
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The designed assembly of different molecular topologies is
one of the last forms of isomerism still to be properly mastered
through synthetic chemistry. Like natural product synthesis,[24]
the way the field is advanced is to demonstrate the synthesis of
previously difficult to make or inaccessible structures. In doing
so
a toolbox of strategies, tactics and methodologies is
developed that can be used to tackle ever more complex targets.
The current work demonstrates that carefully designed ligand
extensions can be used to direct the end-groups of interwoven
strands on a 2×2 grid for connections that generate both a six-
crossing two-component prime link and a granny composite knot.
Acknowledgements
We thank the Engineering and Physical Sciences Research
Council (EPSRC; EP/P027067/1) for funding, and the Diamond
Light Source (UK) for synchrotron beamtime on I19. DAL is a
Royal Society Research Professor.
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Entry for the Table of Contents
COMMUNICATION
Jonathan J. Danon, David A. Leigh,*
Simone Pisano, Alberto Valero, and
Iñigo J. Vitorica-Yrezabal
Page No. – Page No.
A Six-Crossing [2]Catenane with
Twisted Rings (a Molecular ꢀꢀꢀ Link),
and a Molecular Granny Knot
Linked in. The synthesis of a molecular 6!! link and a 31#31 granny knot from a 2x2
interwoven grid is described. The two topoisomers were separated by exhaustive
recycling size-exclusion chromatography and their identities determined by tandem
mass spectrometry. The X-ray crystal structure of the metal-free 6!! link was
obtained.
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