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

Article abstract of DOI:10.1039/c1dt10569h

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) no

Full text of DOI:10.1039/c1dt10569h

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COMMUNICATION
Complexes of a [2]rotaxane ligand with terminal terpyridine groups†
Darren J. Mercer and Stephen J. Loeb*
Received 3rd April 2011, Accepted 9th May 2011
DOI: 10.1039/c1dt10569h
Permanently interlocked [2]rotaxane ligands can be created
by capping a pyridine terminated [2]pseudorotaxane with ter-
pyridine containing stoppers. The robust nature of the re-
sulting [2]rotaxane ligand allows coordination to inert metals
such as Ru(II) not possible under standard self-assembly
conditions.
A practical method of incorporating inert metal ions directly
into an interlocked species would be to construct a ligand that was
9
itself a permanently interlocked rotaxane. 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
Combining the physical properties of transition metals (electronic,
magnetic and catalytic) with the dynamic properties of mechan-
ically interlocked molecules (MIMs) has the potential to create
chemical systems with a variety of unique applications, the scope of
10
2+
ligand by stoppering the [2]pseudorotaxane [1ÃDB24C8] with
¢-(4-tolyl)-2,2¢,6¢,2¢¢-terpyridine groups to produce the ro-
4
4+
taxane ligand [2ÃDB24C8] as outlined in Scheme 1. 4¢-
11
(
4-Bromobenzyl)-2,2¢,6¢,2¢¢-terpyridine was prepared in 35%
1
which, are just beginning to be explored. For example, a number
11
yield by bromination of 4¢-(4-tolyl)-2,2¢,6¢,2¢¢-terpyridine using
of MIM systems have been reported in which a transition metal
2
acts as 1) a templating ion to facilitate interpenetration, 2) a re-
3
porter group to sense binding of a guest, 3) an additive that elicits
4
molecular motion or 4) a building block to create metal organic
5
frameworks (coordination polymers). 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
6
coordination to create an interlocked [2]rotaxane. We originally
2
+
stoppered the [2]pseudorotaxane [1ÃDB24C8] with the Pd(II)
+
pincer complex [Pd(C
6
H
3
(CH
2
SPh)
2
)] and later reported this
could easily be accomplished using simple anionic fragments such
-
-
7
[
MnBr
3
] and [CoBr ] yielding neutral, zwitterionic complexes.
3
Since the preparation of these metal stoppered rotaxanes re-
quired 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
8
complexes with labile transition metal ions 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.
Department of Chemistry and Biochemistry, University of Windsor, Windsor,
Ontario, Canada, N9B 3P4. E-mail: loeb@uwindsor.ca; Fax: 1 519 973
7
098; Tel: 1 519 253 3000
Electronic supplementary information (ESI) available: Details of synthe-
Scheme
1
Preparation of the [2]rotaxane ligand: i)
3 equiv. of
†
4
¢-(4-bromobenzyl)-2,2¢,6¢,2¢¢-terpyridine, MeNO
2
, room temperature,
ses, 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
7 days followed by anion exchange by stirring the MeNO
NaOTf(aq) for 10 h; isolated yield 31%.
2
layered with
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N-bromosuccinamde in CCl
resulting benzylbromide derivative used to alkylate the terminal
pyridine groups of the [2]pseudorotaxane [1ÃDB24C8] produc-
ing [2]rotaxane [2ÃDB24C8] in 31% yield.
4
solution and two equivalents of the
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
2
+
4
+
10
In order to explore the coordination of both labile and inert
most commonly observed for this templating motif. The complex
4
+
˚
metal ions to [2ÃDB24C8] , complexes of Zn(II) and Ru(II)
were prepared. For Zn(II), [2ÃDB24C8][OTf] was stirred with
Zn(H O) ][OTf] in MeCN solution for 12 h at room temperature
yielding an orange solution. The H NMR spectrum of the result-
is over 4 nm in length with a Zn(II) ◊ ◊ ◊ Zn(II) distance of 37.1 A.
4
For
Ru(II),
[2ÃDB24C8][OTf]
4
was refluxed with
12
[
2
6
2
[RuCl
3
(terpy)] in a 1 : 1 EtOH/H O solution for 24 h yielding
2
1
a deep red solution. These are the standard conditions for
the preparation of heteroleptic terpyridine complexes such as
4
+
ing complex showed only broadened resonances for [2ÃDB24C8]
due to rapid metal ligand exchange.
2
+
12
1
[Ru(terpy)(terpy¢)] . The H NMR spectrum of the resulting
Diffusion of iso-propyl ether into a MeNO
2
solution of the
complex (Fig. 2) clearly shows formation of the binuclear product
8
+
complex produced essentially quantitative yield of an orange crys-
talline 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
[(Ru(terpy)) (2ÃDB24C8)] ; NMR assignments were confirmed
2+
2
by 2D COSY and NOESY spectra. The axle 2 exhibits
significant chemical shift changes due to both interpenetration
through the DB24C8 wheel as well as coordination to Ru(II).
The interpenetration effects methylene protons a and aromatic
protons b which are involved in hydrogen bonding to oxygen
atoms of the crown ether resulting in Dd of 0.33 and 0.30 ppm
8
+
of one of the [(Zn(H
2
O) ) (2ÃDB24C8)] cations. The dumbbell
3
2
10
respectively while p-stacking between the electron rich aromatic
rings of DB24C8 and the electron poor pyridinium groups on the
axle results in significant shielding and Dd of -0.21 and -0.18 ppm
10
for protons c and d respectively. Rotation of the pyridine groups
2
+
of (2ÃDB24C8) into favourable conformations for coordination
and bonding to the Ru(II) centre results in coordination shifts for
i–m ranging from a Dd of -1.31 ppm for m to +0.26 ppm for i.
1
Fig. 2 The H NMR spectrum of the binuclear [2]rotaxane complex
[
(Ru(terpy))
2
8 3
(2ÃDB24C8)][OTf] in MeCN-d solution at 298 K. The
labelling scheme is shown in Scheme 1.
The UV-visible absorption spectrum of [(Ru(terpy)) -
2
8
+
(
2ÃDB24C8)] in MeCN solution showed a single MLCT absor-
ption peak at lmax = 485 nm. This is comparable to lmax = 483 nm
2
+ 13
observed for [Ru(terpy)(4¢-(4-tolyl)terpy)] . The red shifts from
2
+
l
max = 474 nm observed for [Ru(terpy)
presence of the electron donating tolyl group.
Diffusion of iso-propyl ether into a MeNO
(Ru(terpy)) (2ÃDB24C8)][OTf] yielded X-ray quality crystals.
Fig. 3 shows a ball-and-stick-representation of the complex cation,
2
] can be attributed to the
14
2
solution of
‡
[
2
8
Fig. 1 A ball-and-stick representation of the cationic portion of the X-ray
8+
2 3 2
crystal structure of [(Zn(H O) ) (2ÃDB24C8)] . The complex occupies a
8
+
[
(Ru(terpy)) (2ÃDB24C8)] . As was observed for the Zn(II)
2
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).
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
6
386 | Dalton Trans., 2011, 40, 6385–6387
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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
1
5
‡
(
Crystal
data
for
[(Zn(H
2
O)
3
)
2
(2ÃDB24C8)][(Zn(H
, M = 5971.88,
/c, a = 24.665(14), b =
2 4 2
O)(BF ))
2ÃDB24C8)][OTf]
7
2 3 210 2 50 26 82 4
(MeNO ) : C200H B F N O S14Zn
T = 173(2)K, monoclinic, space group P2
1
6.270(15), c = 21.104(12) A˚ , b = 108.612(7), V = 12959(13) A˚ ,
3
2
-
3
-
1
r
1
0
c
= 1.530 g cm , m = 0.603 mm , Z = 2, reflections collected =
= 0.1241, wR
= 0.4726, GoF = 0.934
21682 (Rint = 0.7726), final R indices [I > 2sI]: R
.2584, R indices (all data): R = 0.4350, wR
1
2
=
1
2
with data/variables/restraints = 22775/1531/476. Crystal data for
(Ru(terpy)) (2ÃDB24C8)][OTf] [Cl] .(MeNO
Ru , M = 3311.84, T = 173(2)K, triclinic, space group P1, a = 14.056(3),
[
2
6
2
2
)
4
:
130 2 18 20 34
C H118Cl F N O
¯
2
S
6
b = 14.073(3), c = 22.616(5) A˚ , a = 78.454(4), b = 83.369(4), g = 62.894(3),
V = 3900.2(16) A˚ , r
3
= 1.401 g cm , m = 0.406 mm , Z = 1, reflections
16
-3
-1
c
collected = 37004 (Rint = 0.1452). Before SQUEEZE: R indices [I >
2
0
sI]: R
.4588, GoF = 1.297 with data/variables/restraints = 11259/928/355.
= 0.1212, wR = 0.2830,
= 0.3350, GoF = 0.890 with
data/variables/restraints = 13674/928/502.
1
= 0.1627, wR
2
= 0.3859, R indices (all data): R
1
= 0.3037. wR
2
=
After SQUEEZE: R indices [I > 2sI]: R
R indices (all data): R = 0.2567. wR
1
2
1
2
1
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7
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7
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8
G. J. E. Davidson and S. J. Loeb, Dalton Trans., 2003,
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4319.
8+
9 G. J. E. Davidson, S. J. Loeb, P. Passaniti, S. Silvi and A. Credi, Chem.-
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2
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).
1
0 S. J. Loeb, J. Tiburcio, S. J. Vella and J. A. Wisner, Org. Biomol. Chem.,
2
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1
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to Ru(II) in a perpendicular fashion with bond distances and
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12 B. P. Sullivan, J. M. Calvert and T. J. Meyer, Inorg. Chem., 1980, 19,
1
404.
1
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˚
distance of 36.5 A.
1
1
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3
.2, CRYSTAL IMPACT, (2011) Postfach 1251, D-53002, Bonn,
4
+
Preparation of a binuclear Zn(II) complex of [2ÃDB24C8] with
labile co-ligands (H O) shows that this ligand has the potential
Germany.
16 P. v. d. Sluis and A. L. Spek, Acta Cryst., 1990, A46, 194.
2
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Dalton Trans., 2011, 40, 6385–6387 | 6387