Neutral interlocked assemblies from anionic pseudorotaxanes coordinated to Sn(IV) and Cu(I) metallic centers

Article abstract of DOI:10.1016/j.ica.2016.10.006

Anionic [2]pseudorotaxanes have been prepared by threading an axle consisting of two dithiophosphonate donor groups attached to a 1,4,5,8-naphthalenediimide (NDI) core through the cavity of the bis-1,5-(dinaphtho)-38-crown-10 ether (15DN38C10). The subseq

Full text of DOI:10.1016/j.ica.2016.10.006

Accepted Manuscript
Research paper
Neutral interlocked assemblies from anionic pseudorotaxanes coordinated to
Sn(IV) and Cu(I) metallic centers
Robert R. Baum, Jedidiah J. Veach, Radu F. Semeniuc, Kraig A. Wheeler, Perry
J. Pellechia
PII:
S0020-1693(16)30648-X
http://dx.doi.org/10.1016/j.ica.2016.10.006
ICA 17299
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
17 August 2016
2 October 2016
5 October 2016
Please cite this article as: R.R. Baum, J.J. Veach, R.F. Semeniuc, K.A. Wheeler, P.J. Pellechia, Neutral interlocked
assemblies from anionic pseudorotaxanes coordinated to Sn(IV) and Cu(I) metallic centers, Inorganica Chimica
Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.10.006
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Neutral interlocked assemblies from anionic pseudorotaxanes coordinated to
Sn(IV) and Cu(I) metallic centers
Robert R. Baum,a Jedidiah J. Veach,a Radu F. Semeniuc,*a Kraig A. Wheeler,a and Perry J.
Pellechiab
a Department of Chemistry, Eastern Illinois University, Charleston, IL 61920, USA.
b Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA
Corresponding author:
Radu F. Semeniuc
Department of Chemistry
Eastern Illinois University
Charleston, IL 61920, USA.
Tel: +1 217 581 5422; E-mail: rsemeniuc@eiu.edu
Abstract
Anionic [2]pseudorotaxanes have been prepared by threading an axle consisting of two dithiophosphonate
donor groups attached to a 1,4,5,8-naphthalenediimide (NDI) core through the cavity of the bis-1,5-
(dinaphtho)-38-crown-10 ether (15DN38C10). The subsequent coordination of these interlocked,
negatively charged species to R3SnCl (R = Me, Ph) and (Ph3P)2CuNO3 yields neutral (pseudo)rotaxanes
of the general formula NDI[(CH2CH2O)n(An)PS2“M“]2 ⊂ 15DN38C10 (n = 1, 2; An = anisole; “M“ =
R3Sn or (Ph3P)2Cu). Solution and solid state studies of the rotaxanation process show that there are two
factors influencing the interaction between the axles and the wheel: the size of the stopper and the length
of the side-arms of the axles. When the axle termini are bulky enough to prevent threading, the length of
the side-arms has little influence over the self-assembly process. In contrast, when the size of the stopper
does not preclude the threading of the axle through the cavity of the crown, the length of the side-arms is
the determining factor in the rotaxanation process. These results presented here provide an alternative
method for the synthesis of counter-ion free interlocked species, based on anionic axles with donor
capabilities that would produce a neutral interlocked architecture upon threading and coordination to
cationic metal-containing centers.
Keywords:
Pseudorotaxanes; Supramolecular chemistry; X-ray crystal structures; Phosphor-1,1-dithiolates; Metal
complexes.
Highlights:
•
•
•
•
Anionic pseudorotaxanes have been prepared.
Their propensity to coordinate to Sn(IV) and Cu(I) centers has been explored.
Neutral bimetallic interlocked architectures have been obtained.
Solution and solid state (X-Ray diffraction) studies were performed.

1. Introduction
The last decades witnessed an increased interest in the synthesis and characterization of molecular
machines and devices based on mechanically interlocked molecules [1]. These species are composed of
two or more distinct components connected in such a way that their dissociation is not possible without
breaking one (or more) of their covalent bonds. Examples of such architectures include rotaxanes,
catenanes, knots, and suitanes [1, 2]. Among these, rotaxanes are heavily studied, since the position of
one component relative to the other can be controlled via various external inputs (photons, electrons,
protons or chemicals) [3]. Pseudorotaxanes and rotaxanes consist of one or more macrocycles threaded
into one or more axles (linear, dumbbell shaped molecules). Pseudorotaxanes are compounds that exist in
equilibrium with their unthreaded counterparts (the macrocyclic component is free to slip off of the axle);
in contrast, rotaxanes are interlocked compounds in which the crown is permanently trapped onto the axle
by bulky stoppers at the axle termini [4-9].
The formation of (pseudo)rotaxanes depends on the presence of complementary recognition sites on
both the axle and wheel. As extensively demonstrated by Stoddart [3a, 4] and others [5], a secondary
dialkyl-ammonium center (Scheme 1a) can be used as a recognition site because it threads into crown
ethers like dibenzo-24-crown-8 (DB24C8) and bis-p-phenylene-34-crown-10 (BPP34C10) with the
formation of (pseudo)rotaxanes. Another set is based on the bis(pyridinium)ethane dication (Scheme 1b)
and DB24C8, template that was used by Loeb to prepare a large variety of pseudo(rotaxanes) [6]. The
threading of viologen species (Scheme 1c) through the cavities of crown ethers such BPP34C10 and bis-
1,5-(dinaphtho)-38-crown-10 (15DN38C10) to form [(viologen)⊂(crown ether)]2+ [2]pseudorotaxanes
has been studied by Stoddart [7] and others [8] and lies at the foundation of a wide range of mechanically
interlocked molecules in general, and pseudorotaxane and rotaxane systems, in particular. Rotaxanes and
pseudorotaxanes can also form from the naphthalenediimide π-deficient group (Scheme 1d) and the π-rich
15DN38C10 crown ether, as demonstrated by Sanders [9].
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
DB24C8
R
BPP34C10
15DN38C10
O
O
N
R N
N R
O
R
N
H2
R
R N
N R
R
N
O
a
c
b
d
Scheme 1. Crown ethers and axle components used in the preparation of pseudorotaxanes and
rotaxanes.
An important research direction in this field is the use of pseudorotaxanes as ligands. It is believed that
the incorporation of metals within interlocked architectures would lead to a synergistic effect, thus
enhancing the properties of the resulting species. As such, several metal-containing interlocked
architectures have been prepared and characterized [6a,b, 8d-f, 9a, 10]. Usually, their design is based on
2

an axle having a recognition site for pseudorotaxane formation and equipped with donor groups for
subsequent coordination to metallic centers. The resulting species could provide a blueprint for the future
organization of more complex species into higher-order arrays, thus generating crystalline materials
comprised of interlocked molecular components that could be independently manipulated [6a, b].
Despite the advances in this field, the rational synthesis of metal-based interlocked architectures still
remains a challenging issue in synthetic chemistry. This situation stems from the fact that, while several
complementary recognition sites could be used in the template-based synthesis of a “rota-ligand”, not all
of them are robust enough to withstand the ensuing coordination process, which often requires either
strongly donating polar solvents that disrupt the non-covalent interactions holding the pseudorotaxane
together, or elevated temperatures. In addition, both pseudorotaxanes and metallic centers are frequently
accompanied by counter-ions that often interfere with either the self-assembly process, or with the desired
properties of these species.
An elegant strategy toward counter-ion free metal-based interlocked architectures was developed by
Loeb [11], employing zwitterionic pseudorotaxanes as ligands and connecting them to neutral metallic
centers. Mezei showed that neutral bimetallic pseudorotaxanes can also be obtained from the interaction
between positively charged pseudorotaxanes (2+) and two anionic (1-) metallic nodes [8d,e].
We propose here an alternative method for the synthesis of these counter-ion free species, based on
anionic axles with donor capabilities that would produce a neutral species upon threading and
coordination to cationic metal-containing centers.
Recently, we have described the synthesis and characterization of two new ditopic ligands having
dithiophosphonate donor groups linked to a 1,4,5,8-naphthalenediimide (NDI) core through ether spacers
of different length (1-[NH4]2, Scheme 2) [12]. The reaction of these ligands with Ph3SnCl and
(PPh3)2CuNO3 produced bimetallic complexes, of the general formula 2-[M]2, aslo depicted in Scheme 2.
Structural investigations of these compounds revealed that the different length of the ligand’s side-arms
influence the relative position of the -(An)PS2[M] groups (An = anisole) with respect to the central NDI
moiety: a short side-arm (compounds of type 2a in Scheme 2) places the -(An)PS2[M] moiety in close
proximity of the NDI group, while a longer side-arm (compounds of type 2b in Scheme 2) directs the
metallic centers away from the NDI moiety, thus influencing their molecular and supramolecular
structures.
S
[M]
S
Me
-
+
S
NH4
S
Me
P
n
O
P
n
O
O
N
O
O
N
O
O
N
O
O
N
O
O
O
O
S
O
S
n
n
O
P
O
P
Me
[M]
-
+
S
Me
NH4
S
n = 1: 2a-[SnPh3]2; 2a-[Cu(PPh3)2]2
n = 2: 2b-[SnPh3]2; 2b-[Cu(PPh3)2]2
n = 1: 1a-[NH4]2
n = 2: 1b-[NH4]2
Scheme 2. Structural description of ditopic dithiophosphonate ligands and their metal complexes.
It is in this context that we decided to investigate the formation of anionic pseudorotaxanes using our
NDI-based dithiophosphonate ligands 1-[NH4]2 in combination with 15DN38C10, and to explore their
propensity to form neutral interlocked species upon coordination to metallic centers. We have also looked
3

at the interaction of the neutral 2-[M]2 complexes with the same crown ether, to ascertain which metallic
group would act as a stopper for the crown and which would allow the crown to slip off of the axle
component (rotaxane formation vs. pseudorotaxane formation). The results of our investigations are
reported in this contribution.
2. Results and discussion
2.1. Strategy and syntheses
As a general strategy, to build anionic pseudorotaxanes that could also act as ligands, one would need
a recognition motif that would not be affected by the presence of donor groups within the body of the
rota-ligands. Since an anionic axle is soluble only in polar solvents, the use of recognition motifs based on
hydrogen bonds would have limited success, as polar solvents interfere with the hydrogen bonds between
the axle and the wheel. With these restrictions in mind, we decided to explore a π-π stacking based
template, since these interactions are not affected by the presence of donor Lewis bases and are known to
form even in polar solvents. As such, we have chosen 15DN38C10 ether (3) as wheel, because of its well-
known π-electron donating capabilities toward various π-electron deficient substrates [9].
On another hand, dithiophosphonate-based ligands [(RO)R’PS2]¯ are known for their rich coordination
chemistry: the presence of soft Lewis acidic centers on these ligands, i.e. sulfur, opens the possibility of
supramolecular association through secondary bonds [13]. As such, combining this donor set with the π-
deficient NDI moiety should produce an axle with a strong affinity for the 15DN38C10 wheel, with the
added benefit of the versatility of the dithiophosphonate ligand.
The ditopic dithiophosphonate axles were synthesized as reported earlier [12], by the reaction of
1,4,5,8-naphthalene-tetracarboxylic dianhydride with two equivalents of either 2-aminoethanol or 2-(2-
aminoethoxy)ethanol in DMF, followed by the reaction of the resulting diols with Lawesson’s Reagent in
benzene. The ammonium salts NDI[(CH2CH2O)n(An)PS2NH4]2 (n = 1: 1a-[NH4]2; n = 2: 1b-[NH4]2) were
precipitated by passing dry gaseous NH3 through the cold benzene solution of the dithiophosphonic acids.
The axles coordinate to R3SnCl (R = Me, Ph) or (Ph3P)2CuNO3 (Scheme 2), yielding the bimetallic
compounds 2a-[SnMe3]2; 2a-[SnPh3]2; 2a-[Cu(PPh3)2]2 (for n = 1) and 2b-[SnMe3]2; 2b-[SnPh3]2; 2b-
[Cu(PPh3)2]2 (for n = 2). The molecular and supramolecular structures of 2a-[SnPh3]2, 2a-[Cu(PPh3)2]2
and 2b-[SnPh3]2 were described elsewhere [12]; their general structural characteristics feature the -PS2
group coordinated in an either mono- (2a-[SnPh3]2 and 2b-[SnPh3]2) or bidentate fashion (2a-
[Cu(PPh3)2]2). The most important structural differences, arising from the different length of the ligand’s
side arms, is the position of the (An)PS2[M] groups with respect to the central NDI moiety: in 2a-
[SnPh3]2 and 2a-[Cu(PPh3)2]2 they virtually surround the NDI group, while in 2b-[SnPh3]2 they are
positioned away from the NDI moiety.
To gain insight into the rotaxanation process involving these species, two routes toward neutral
bimetallic interlocked architectures were investigated, Scheme 3. Path I involves anionic pseudorotaxane
ligand formation as a first step, followed by coordination of the preformed “rota-ligand” to metallic
centers. This investigation would afford information on the stability of the anionic pseudorotaxanes, as
well as on their coordination behavior towards metallic centers. Path II is based on the slippage of the
crown onto the preformed neutral metallic axles. Employing metallic centers with various additional
ligands of different sizes, it is possible to establish which metallic group would allow the crown to be
threaded by the bimetallic axle, and which would block the rotaxanation process. Applying this
4

information to route I, one could obtain (depending on the size of the axle termini) either rotaxanes or
pseudorotaxanes.
O
O
c
b
S
S
a
P
n
P
O
N
O
N
O
O
N
O
N
O
[M]+
SNH4
S [M]
O
O
n
H4NS
S
[M] S
S
O
O
DCM / H2O
n
O
n
O
P
P
O
O
n = 1: 2a-[SnMe3]2; 2a-[SnPh3]2; 2a-[Cu(PPh3)2]2
n = 2: 2b-[SnMe3]2; 2b-[SnPh3]2; 2b-[Cu(PPh3)2]2
n = 1: 1a-[NH4]2
n = 2: 1b-[NH4]2
O
O
O
Path I
Path II
O
O
f
e
Acetone/Water
0oC
CHCl3
3
3
d
O
O
O
O
O
O
O
S
S
O
O
O
O
P
n
P
O
O
O
O
O
O
N
[M]+
SNH4
S [M]
O
O
N
O
O
O
N
O
O
O
O
O
N
n
O
O
[M] S
S
O
H4NS
S
O
O
O
Acetone/Water
0oC
n
n
P
P
O
O
O
O
O
O
O
O
n = 1: 2a-[SnMe3]2 3; 2a-[SnPh3]2 . 3; 2a-[Cu(PPh3)2]2
.
n = 2: 2b-[SnMe3]2 3; 2b-[SnPh3]2 3; 2b-[Cu(PPh3)2]2
3
.
.
.
n = 1: 1a-[NH4]2
n = 2: 1b-[NH4]2
3
3
.
.
.
3
Scheme 3. Two synthetic pathways toward neutral bimetallic pseudorotaxanes starting from
anionic axles; also given is the proton labelling mode used in NMR studies.
2.2. Solution Studies
Upon mixing equimolar solutions of axles 1a-[NH4]2 or 1b-[NH4]2 and 15DN38C10 (3) in acetone-d6
: D2O (5 : 0.6, v/v; the water is needed to help dissolving the anionic axle), the slow rate of association
and dissociation of the components on the NMR timescale at room temperature permits the observation of
three different sets of peaks in their NMR spectra (Fig. 1), corresponding to the free axles and crown
ether, and the [2]pseudorotaxane species. The pseudorotaxane formation is indicated by the upfield shift
of the NDI (Ha) and the crown aromatic peaks (Hd-f) due to aromatic shielding effects resulting from the
NDI-crown interaction, while the anisole peaks (Hb-c) shifted downfield, indicative of an “outer sphere”
interaction of this group with the crown 3 (for proton labeling mode see also Scheme 3).
5

1
Fig. 1. Partial H-NMR spectra of a) crown 3, b) axle 1a-[NH4]2, c) pseudorotaxane 1a-[NH4]2 ⊂ 3, d)
+
pseudorotaxane 1a-[NH4]2 ⊂ 3 / excess NH4 , e) axle 1b-[NH4]2, f) pseudorotaxane 1b-[NH4]2 ⊂ 3 and g)
+
pseudorotaxane 1b-[NH4]2 ⊂ 3 / excess NH4 ; H and H’ represent protons bound to uncomplexed and
complexed species, respectively (for proton labeling mode see Scheme 3); the peak at 7.33 ppm (marked
with *) is C6H6 carried on from the synthesis of the axles.
The observation of signals for the resonances of threaded and unthreaded species in the NMR spectra
of 1a-[NH4]2 or 1b-[NH4]2 and 3 allows the measurement of their association constants (Ka) and free
energies of complexation (∆G°) (Table 1) using the convenient single-point method [4a]. It should be
noted that, since the NH4⁺ ion forms inclusion compounds with a large variety of crown ethers, the
apparent Ka values reported here for pseudorotaxanes 1a/b-[NH4]2 ⊂ 3 were calculated considering a 1 : 1
reaction of the anionic axles with the [3·NH4]⁺ inclusion complex. To avoid the formation of this
undesired inclusion complex, we attempted to neutralize the dithiophosphonic acids using Me4NOH and
n-Bu4NOH as bases, instead of gaseous ammonia. Unfortunately, we observed decomposition of the axles
in all cases. Anion exchange using R4NCl (R = Me, n-Pr and n-Bu) also failed. To the best of our
+
knowledge, there is no information on the interaction between the NH4 cation and 15DN38C10 and their
+
complexation ratio. Comparing the available information on the association of NH4 cation with crown
ethers of similar size [14], we expected a 1 : 1 [NH4·3]+ compound, although a 2 : 1 ratio cannot be ruled
+
out a priori. To study the NH4 : 3 complexation ratio, we performed an ESI+/MS experiment onto a 5 : 1
mixture of NH4PF6 and 15DN38C10. Analysing the spectrum, we have observed the base peak at 654 m/z
corresponding to the 1 : 1 [NH4·3]+ inclusion complex. There are three more peaks present, corresponding
to the 1 : 1 [H·3]+ (637 m/z, 50 %), 1 : 1 [Na·3]+ (659 m/z, 10 %), and 1 : 1 [K·3]+ (675 m/z, 4 %)
inclusion complexes, respectively. Importantly, there is no peak at 336 m/z that would correspond to a 2 :
+
1 [(NH4)2·3]2+ inclusion complex. Since the dissolution of the axle produces two equivalents of NH4
6

ions, it is reasonable to assume that in our experiments the formation of the [NH4·3]+ complex is
essentially complete.
It is also conceivable that both NH4⁺ cations and NDI-based axles compete for the formation of the
[NH4·3]+ inclusion complex and 1a/b-[NH4]2 ⊂ 3 pseudorotaxanes, respectively. As such, when the NMR
spectra of the axles 1a/b-[NH4] and 15DN38C10 were recorded in the presence of excess NH4PF6 (Fig. 1
d and g), a small decrease of the Ka values was observed (Table 1). VT-NMR studies also showed that the
overall concentration of the interlocked species increases with decreasing the temperature, as shown in
Fig. 2 for 1b-[NH4]2 ⊂ 3, either in the absence or presence of excess NH4⁺ .
Fig. 2. 1H-VT-NMR (acetone-d6/D2O, 5:0.6 v/v) of 1b-[NH4]2 ⊂ 3, (top), and 1b-[NH4]2 ⊂ 3 with excess
NH4PF6 (bottom).
Addition of two equivalents of R3SnCl (R = Me, Ph) or (Ph3P)2CuNO3 to pseudorotaxanes 1a/b-
[NH4]2 ⊂ 3 (path I) resulted in the formation of mixtures of unthreaded bimetallic axles and bimetallic
pseudorotaxanes. Unfortunately, the only species soluble in this solvent system (acetone/H2O, 5:0.6 v/v)
are the 2b-[SnMe3]2 ⊂ 3 pseudorotaxane and its corresponding unthreaded 2b-[SnMe3]2 complex; all
other compounds precipitated out of this solution. Solubilization of the precipitates can be achieved by the
addition of more acetone to the solvent system. This solubility mismatch precludes a thorough
comparative study of the bimetallic interlocked species with their parent 1a/b-[NH4]2 ⊂ 3
pseudorotaxanes (an acetone – water ratio higher than 5: 0.6 led to the precipitation of the anionic axles);
as such, only the 2b-[SnMe3]2 ⊂ 3 pseudorotaxane is included in Table 1.
The 1H-NMR spectra recorded after the addition of the metallic salts to the “rota-ligands” are similar
with the spectra of the 1a/b-[NH4]2 ⊂ 3 pseudorotaxanes described above: three different sets of peaks
can be observed, corresponding to the free 2a/b-[M]2 complexes, the unthreaded crown ether, and the
bimetallic 2a/b-[M]2 ⊂ 3 interlocked species. While the signals associated with the organic groups
attached to the metallic center (Me3Sn, Ph3Sn, (PPh3)2Cu) are observed, it is not possible to differentiate
between the groups belonging to the unthreaded bimetallic complexes and those appended to the
bimetallic interlocked species, presumably because they are too far from the NDI ⊂ 15DN38C10 region.
7

These results show that the anionic 1a/b-[NH4]2 ⊂ 3 pseudorotaxanes are sufficiently robust to withstand
the ensuing coordination process to metallic centers, with the formation of neutral bimetallic
(pseudo)rotaxanes.
Table 1. Association constants (Ka)a and free energies of complexation (∆G°)b for pseudorotaxanes 1-
[NH4]2 ⊂ 3 and 2-[M]2 ⊂ 3 at 25°C.
Solvent
Pseudorotaxane
1a-[NH4]2 ⊂ 3
1a-[NH4]2 ⊂ 3c
Ka (M-1)
896.5
758.7
1259.1
1131.5
562.7
88.3
∆G° (kJmol-1)
– 16.9
– 16.4
– 17.7
– 17.4
– 15.7
– 11.1
– 16.5
– 10.4
– 11.3
-
Acetone-d6 : D2O
5.0/0.6 (v/v)
1b-[NH4]2 ⊂ 3
1b-[NH4]2 ⊂ 3c
2b-[SnMe3]2 ⊂ 3
2a-[SnMe3]2 ⊂ 3
2b-[SnMe3]2 ⊂ 3
2a-[SnPh3]2 ⊂ 3
2b-[SnPh3]2 ⊂ 3
2a-[Cu(PPh3)2]2 ⊂ 3d
2b-[Cu(PPh3)2]2 ⊂ 3d
CDCl3
787.8
66.1
95.9
-
-
-
a Determined by the single-point method using initial concentrations of axle and wheel of 1.66·10-3 M
and the integral values of the Ha resonances for threaded and unthreaded axles; errors are estimated to be
b
c
10% or less; calculated using the formula ∆G° = - RTlnKa; measured in the presence of excess of
NH4PF6; d no pseudorotaxane peaks were observed.
Pseudorotaxane formation starting from the neutral, bimetallic axles (path II) was studied in CDCl3.
Upon mixing equimolar amounts of axles and wheel and recording their NMR spectra, pseudorotaxane
formation was observed in only a few cases, Fig. 3. As a general trend, the coordination of the -PS2 donor
set to the metallic centers leads to very low Ka values or no pseudorotaxane formation at all (Fig. 3, Table
1), 2b-[SnMe3]2 ⊂ 3 being the only exception to the rule.
The results from investigating paths I and II toward pseudorotaxane formation, combined with the
structural characteristics of the unthreaded bimetallic compounds [12], show that there are two factors
influencing the interaction between the axles and the wheel: the size of the stopper and the length of the
side-arms of the axles. While for both anionic axles 1a-[NH4]2 and 1b-[NH4]2 the size of the (An)PS2
group alone is not bulky enough to prevent threading, in the case of 1a-[NH4]2 the shorter side-arms of
the axle place the dithiophosphonate groups in close proximity to the NDI core, thus protecting it from
the crown ether 3, which has as a consequence a lower Ka value for 1a-[NH4]2 ⊂ 3 compared to that for
1b-[NH4]2 ⊂ 3 (896 M-1 vs. 1259 M-1).
It is clear that in the case of 2a/b-[Cu(PPh3)2]2 axles the stoppers are bulky enough to prevent the
slippage of crown 3 regardless of the length of the side-arms. This implies that using (PPh3)2CuNO3 as the
metal source in path I, the formation of a rotaxane can be expected. In the case of 2a/b-[SnPh3]2 axles,
the size of the (An)PS2SnPh3 groups permits to a very small extent the threading process. However, the
length of their side-arms has little influence over the Ka values (66 M-1 vs. 95 M-1), suggesting that in this
8

case the size of the stopper is the limiting factor in pseudorotaxane formation. In the case of 2a/b-
[SnMe3]2 axles, the smaller size of the (An)PS2SnMe3 stopper makes threading possible, as indicated by
the Ka value of 787 M-1 for the 2b-[SnMe3]2 ⊂ 3 pseudorotaxane. However, 2a-[SnMe3]2 ⊂ 3 shows a
very small Ka value (88 M-1), explained by the shorter side-arms of the axle which would place the
(An)PS2SnMe3 stoppers in the vicinity of the NDI core, thus hampering the rotaxanation process.
Comparing the association constants for the 1b-[NH4]2 ⊂ 3 / 2b-[SnMe3]2 ⊂ 3 pair in the acetone-water
solvent system (2b-[SnMe3]2 is the only compound soluble in this solvent system), it can be observed that
the coordination of the -PS2 group to the Me3Sn(IV) center (and consequently the increase in the
bulkiness of the axle termini) significantly lowers the Ka value (1259 M-1 vs. 562 M-1, see Table 1).
Fig. 3. 1H-NMR spectra of equimolar mixtures of bimetallic axles and crown 3 in CDCl3.
2.3. Solid state studies
As mentioned above, addition of the metallic salts to the acetone-water solutions of the anionic
pseudorotaxanes resulted in the formation of mixtures of unthreaded bimetallic axles and bimetallic
pseudorotaxanes. In the case of 2b-[SnPh3]2 ⊂ 3, vapor diffusion of Et2O into the aqueous acetone
solution of the components produced deep red single crystals, which were used in X-ray diffraction
experiments. There are two independent interlocked molecules in the unit cell, both having the -PS2
groups coordinated in a monodentate fashion to the tin atom. A notable difference between the two
independent molecules is that in one the Sn center is tetra-coordinate (Fig. 4a), while in the other, a water
molecule expands the tin coordination number to five (Fig. 4b). There are two angular parameters (τ4 and
τ5) used to describe the geometry around four and five-coordinated metallic centers [15]. The formula for
calculating the τ4 parameter is based on the sum of the two largest angles subtracted from 360, then all
divided by 141 [15a]. The obtained values would range from 1.00 for a perfect tetrahedral geometry to
0.00 for a perfect square planar geometry. The τ5 parameter is calculated by dividing the difference
between the two largest angles by 60; a trigonal bipyramidal structure would produce a value of 1.00,
while a square pyramidal structure would produce a value of 0.00 [15b]. Applying these formulae to our
9

case, the tin atoms are very close to a perfect tetrahedral (τ4 = 0.95) and trigonal bipyramidal (τ5 = 0.91)
geometries, respectively.
Comparing the coordination of the PS2 donor group to the Sn(IV) centers, only small differences
between their bond lengths and angles can be observed. The tetrahedral tin shows a shorter Sn – S
covalent bond at Sn1B – S1B = 2.443 Å, while the corresponding value in the case of the penta-
coordinated tin is Sn1A – S1A = 2.563 Å. The second sulfur atoms of the PS2 group are involved in
secondary bonds [16]: the tin – sulfur distances (Sn1B – S2B = 3.560 Å and Sn1A – S2A = 3.745 Å) are
longer than the sum of their covalent radii (2.48 Å), but shorter than the sum of their van der Waals radii
(3.97 Å), suggesting weak interactions between the sulphur and tin atoms.
The common structural feature of the independent molecules of the pseudorotaxane 2b-[SnPh3]2 ⊂ 3
(Fig. 4) consists of a 15DN38C10 wheel threaded by the 2b-[SnPh3]2 axle, with the π-rich aromatic
regions of the wheel stacked around the π-deficient NDI core. In the case of the tetra-coordinate tin
pseudorotaxane, the perpendicular distance between the NDI and 15DN38C10 rings is 3.50 Å, while for
the penta-coordinate tin pseudorotaxane the corresponding value is 3.48 Å.
Examining the crystal packing of 2b-[SnPh3]2 ⊂ 3, no supramolecular interactions can be identified
for the pseudorotaxane containing the tetra-coordinated tin atom. In contrast, looking at the penta-
coordinated tin center, it can be observed that the hydrogen atoms from the water molecule bonded to the
tin atom are involved in two hydrogen bonds with two oxygen atoms within a crown ether wheel from a
neighboring pseudorotaxane molecule, see Fig. 5. The O···H distances are 1.91 Å and 1.73 Å, with the O
– H – O angles of 173.8° and 170.1° respectively. This interaction contributes to different orientation of
the side-arms of the two independent pseudorotaxanes. In the case of the tetra-coordinated tin atom, the
side-arms are oriented toward the NDI-wheel recognition motif, while in the other case, the SnPh3 and
anisole moieties are oriented away from 15DN38C10 (see Fig. 4 for a space filling representation of the
two molecules, emphasizing the two different orientations).
Another difference between the two molecules (presumably another consequence of the hydrogen
bonding interaction described above) is the orientation of the central NDI core relative to the aromatic
region of the crown ether, Fig. 6. This orientation can be measured by the torsion angle defined by the
NNDI – CtNDI – Ct15DN38C10 – C15DN38C10 angle.6a In the case of the tetracoordinate tin pseudorotaxane, the
torsion angle is 86.8° and for the pentacoordinate tin pseudorotaxane is 78°.
10

Fig. 4. Molecular structures of the two independent molecules of 2b-[SnPh3]2 ⊂ 3: a) the four-
coordinate and b) the five-coordinate tin atom; selected bond lengths (Å) and angles (°): Sn1A – S1A =
2.5626(14); Sn1A – O11A = 2.457(4); Sn1A – C19A = 2.132(5); Sn1A – C25A = 2.140(5); Sn1A –
C31A = 2.136(5); S1A – P1A = 2.0462(19); S2A – P1A = 1.934(2); Sn1B – C31B = 2.128(5); Sn1B –
S1B = 2.4427(14); Sn1B – C19B = 2.135(6); Sn1B – C25B = 2.116(5); S1B – P1B = 2.0758(19); S2B –
P1B = 1.9387(19); S1A – Sn1A – C19A = 101.35(14); S1A – Sn1A – C25A = 90.58(13); S1A – Sn1A –
C31A = 99.05(15); S1A – Sn1A – O11A = 175.54(9); S1A – P1A – S2A = 117.18(10); S1B – Sn1B –
C31B = 111.35(14); S1B – Sn1B – C19B = 100.82(15); S1B – Sn1B – C25B = 109.68(15); S1B – P1B –
S2B = 115.00(8); c) and d) space filling representation of the two independent molecules; color code: C –
green/yellow, H – black/grey, O – red, N – blue, S – yellow, P – orange, Sn – purple.
Fig. 5. Hydrogen bonding involving the coordinated water molecule and the crown ether: a) two
pseudorotaxanes held together by hydrogen bonding; b) detail of the hydrogen bonds; c) space filling of
the atoms involved in hydrogen bonding.
11

Fig. 6. The different relative orientation between the NDI core relative and the aromatic region of the
crown ether: a) the tetra-coordinate tin pseudorotaxane; b) the penta-coordinate tin pseudorotaxane.
3. Conclusions
We reported here the synthesis and characterization of two negatively charged axles with a NDI core,
capable of interacting with the 15DN38C10 crown ether to form anionic pseudorotaxanes. These
interlocked species interact with Sn(IV) and Cu(I) metallic centers, with the formation of neutral
(counter-ion free) bimetallic (pseudo)rotaxanes. To the best of our knowledge, these are the first examples
of anionic pseudorotaxanes with donor properties that generate, upon coordination to cationic metal
centers, such interlocked neutral species.
The successful synthesis of these species shows the potential of this method for the future preparation
of more complex metal-based interlocked architectures. For example, the use of metallic systems with an
overall charge of 2+ should yield at least mono-dimensional rotaxane-based coordination polymers
through the coordination of two -PS2 groups from two different pseudorotaxanes to the same metal (the
side-arms of the axles are not long enough to allow the formation of the corresponding monometallic
catenanes).
Importantly, the NDI group can be easily functionalized with other negatively charged side-arms,
containing the carboxylate and/or dithiocarbamate donor groups, thus opening the door for the synthesis
of a large variety of neutral, interlocked arrays, either discrete species self-assembled via secondary bonds
or “classic,” coordinatively bonded coordination polymers.
4. Experimental
4.1. General considerations
Unless otherwise noted, all operations were carried out in open air. Operations requiring inert
(nitrogen) atmosphere were carried out using standard Schlenk techniques and an MBraun LabStar dry-
box. When needed, solvents were dried by conventional methods and distilled under a dry N2 atmosphere
12

immediately prior to use. Elemental analyses were performed by Robertson Microlit Laboratories, NJ.
NMR spectra were recorded by using a 400 MHz Bruker Avance FT-NMR Spectrometer and a Varian
Mercury/VX 400 Spectrometer. Electrospray ionization mass spectrometry data were obtained on a
MicroMass QTOF spectrometer. Clusters assigned to specific ions show appropriate isotopic patterns as
calculated for the atoms present. The axles 1a-[NH4]2, 1b-[NH4]2, 2a-[SnPh3]2, 2a-[Cu(PPh3)2]2, 2b-
[SnPh3]2, and 2b-[Cu(PPh3)2]2 [12], and the 15DN38C10 crown ether [17] were prepared according to
published procedures. All other reagents are commercially available (Sigma-Aldrich) and were used
without further purification.
4.2. Synthesis of NDI-[CH2CH2O(An)PS2(SnMe3)2]2 (2a-[SnMe3]2)
A 250 mL flask was charged with a stirring bar and a solution of Me3SnCl (0.0498 g, 0.25 mmol) in
DCM (75 mL). To this colorless solution was added, under vigorous stirring, an orange aqueous solution
of 1a (0.0991 g, 0.125 mmol) in 75 mL water, and the mixture was stirred for an additional 12 hours. The
completion of the reaction was indicated by the loss of the color of the aqueous phase and the appearance
of an orange color in the organic phase. The organic phase was separated, and the aqueous phase
extracted with DCM (3 x 10 mL). The combined organic phases were dried over anhydrous Na2SO4 and
removed under reduced pressure, to afford the desired compound as an orange solid (0.0722 g, 53.3 %).
1H-NMR (400 MHz, CDCl3): δ = 8.73 (s, 4H, NDI), 7.80 (m, 4H, MeO-C6H4-P), 6.82 (m, 4H, MeO-
C6H4-P), 4.61 (m, 4H, -CH2-), 4.46 (m, 4H, -CH2-), 3.79 (s, 6H, -O-CH3), 0.53 (s, JH-Sn = 27.8 Hz, 18H,
Sn(CH3)3). Anal. Calcd. for C38H44N2O8P2S4Sn2: C = 42.09, H = 4.09, N = 2.58; Found: C = 42.21, H =
3.89, N = 2.77.
4.3. Synthesis of NDI-[CH2CH2OCH2CH2O(An)PS2(SnMe3)2]2 (2b-[SnMe3]2)
A 250 mL flask was charged with a stirring bar and a solution of Me3SnCl (0.0498 g, 0.25 mmol) in
DCM (75 mL). To this colorless solution was added, under vigorous stirring, an orange aqueous solution
of 1b (0.110 g, 0.125 mmol) in 75 mL water, and the mixture was stirred for an additional 12 hours. The
completion of the reaction was indicated by the loss of the color of the aqueous phase and the appearance
of an orange color in the organic phase. The organic phase was separated, and the aqueous phase
extracted with DCM (3 x 10 mL). The combined organic phases were dried over anhydrous Na2SO4 and
removed under reduced pressure, to afford the desired compound as an orange solid (0.0952 g, 64.9 %).
1H-NMR (400 MHz, CDCl3): δ = 8.69 (s, 4H, NDI), 7.81 (m, 4H, MeO-C6H4-P), 6.84 (m, 4H, MeO-
C6H4-P), 4.47 (m, 4H, -CH2-), 4.14 (m, 4H, -CH2-), 3.88 – 3.83 (m, 10H, 4H -CH2- overlap with 6H -O-
CH3), 3.77 (m, 4H, -CH2-), 0.59 (s, JH-Sn = 28.2 Hz, 18H, Sn(CH3)3). Anal. Calcd. for
C42H52N2O10P2S4Sn2: C = 43.02, H = 4.47, N = 2.39; Found: C = 43.39, H = 4.38, N = 2.62.
4.4. General procedure for pseudorotaxane synthesis and crystal growing procedure
As indicated by the VT-NMR experiments, the association constant of the pseudorotaxanes is
significantly increased at 0° C. Therefore, acetone-water solutions of the axle and crown (0.1 mmol each)
were stirred for 2 hours in an ice-water bath. To the red solutions, solid R3SnCl (R = Me, Ph) or
(Ph3P)2CuNO3 was added, and the stirring was continued overnight. To the resulting suspensions enough
acetone was added dropwise (ca. 1 mL) to dissolve the precipitates. The resulting red solutions were each
13

divided in about 0.5 mL portions and transferred in vials which were subsequentely placed in larger vials
already filled with about 15 mL of diethyl ether. The large vials were tightly closed and kept at 0o C for
several days, during which time, in the case of 2b-[SnPh3]2 ⊂ 3, red crystals formed and used for single
crystal X-ray diffraction analysis.
4.5. Crystallographic studies
Crystallographic details for compounds 2b-[SnPh3]2 ⊂ 3 are summarized in Table 2. The X-ray data
were collected at 100 K on a Bruker SMART APEXII CCD diffractomer equipped with Cu Kα radiation.
Intensities were collected using phi and omega scans and were corrected for Lorentz polarizatrion and
absorption effects. The X-SEED software platform [18], equipped with SHELXS and SHELXL modules on
a PC computer [19], was used for all structure solution and refinement calculations and molecular
graphics. The structure was solved by direct methods, and refined by anisotropic full-matrix least-squares
for all non-hydrogen atoms. All hydrogen atoms were placed in calculated positions and refined using a
riding model with fixed thermal parameters.
Table 2. Crystal data and details for 2b-[SnPh3]2 ⊂ 3.
2b-[SnPh3]2 ⊂ 3
Empirical Formula
FW, g · mol-1
Crystal System
Space Group
T (K)
C216H220N4O42P4S8Sn4
4399.24
Triclinic
P1
100(2)
a, (Å)
b, (Å)
11.6038(4)
16.3167(6)
27.3486(9)
88.799(2)
78.172(2)
83.306(2)
5033.6(3)
1
c, (Å)
α, (o)
β, (o)
γ, (o)
V, (Å3)
Z, Z’
Dx (mg·m-3)
1.451
F(000)
R/Rw (obs data)
2268
0.054/0.138
1.05
2
S
no. of reflns / params
∆ρmax/min (e·Å-3)
17677/1330
+ 0.98/ – 0.61
14

Acknowledgments
The authors thank Eastern Illinois University for financial support through a PFRCA grant; the Single
Crystal Diffractometer at EIU was purchased using funds provided by the NSF (CHE-0722547).
Appendix A. Supplementary Material
CCDC 773640 contains the supplementary crystallographic data for the compound described in this
paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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17

Graphical Abstract
Neutral (pseudo)rotaxanes have been prepared from anionic axles threaded through the cavity of the bis-
1,5-(dinaphtho)-38-crown-10 ether and subsequent coordination to cationic Sn(IV) and Cu(I) metallic
centers.
18

Highlights:
•
•
•
•
Anionic pseudorotaxanes have been prepared.
Their propensity to coordinate to Sn(IV) and Cu(I) centers has been explored.
Neutral bimetallic interlocked architectures have been obtained.
Solution and solid state (X-Ray diffraction) studies were performed.
19