Close Encounters of the Weak Kind: Investigations of Electron-Electron Interactions between Dissimilar Spins in Hybrid Rotaxanes

Article abstract of DOI:10.1021/jacs.9b05590

We report a family of hybrid [2]rotaxanes based on inorganic [Cr₇NiF₈(O₂C^tBu)₁₆]^- ("{Cr₇Ni}") rings templated about organic threads that are terminated at one end with pyridyl gr

Full text of DOI:10.1021/jacs.9b05590

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Article
Close encounters of the weak kind: Investigations of electron-
electron interactions between dissimilar spins in hybrid rotaxanes
Selena J Lockyer, Alistair J. Fielding, George F. S. Whitehead,
Grigore A. Timco, Richard E. P. Winpenny, and Eric J.L. McInnes
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05590 • Publication Date (Web): 14 Aug 2019
Downloaded from pubs.acs.org on August 14, 2019
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Close encounters of the weak kind: Investigations of electron-
electron interactions between dissimilar spins in hybrid rotax-
anes
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Selena J. Lockyer,a Alistair J. Fielding,b George F. S. Whitehead,a Grigore A. Timco,a Richard E. P.
Winpennya and Eric J. L. McInnes*a
aSchool of Chemistry and Photon Science Institute, The University of Manchester, Oxford Road, Manchester M13
9PL, U.K.
b School of Pharmacy and Biomolecular sciences, Liverpool John Moores University, Liverpool L3 5UX
KEYWORDS: Supramolecular chemistry • rotaxanes • exchange coupling • EPR
ABSTRACT: We report a family of hybrid [2]rotaxanes based on inorganic [Cr7NiF8(O2CtBu)16]- (“{Cr7Ni}”) rings templat-
ed about organic threads that are terminated at one end with pyridyl groups. These rotaxanes can be coordinated to
[Cu(hfac)2] (where hfac = 1,1,1,5,5,5-hexafluoroacetylacetone), to give 1:1 or 1:2 Cu:{Cr7Ni} adducts: {[Cu(hfac)2](py-
CH2NH2CH2CH2Ph)[Cr7NiF8(O2CtBu)16]},
{[Cu(hfac)2][py-CH2NH2CH2CH3][Cr7NiF8(O2CtBu)16]},
{[Cu(hfac)2]([py-
CH2CH2NH2CH2C6H4SCH3][Cr7NiF8(O2CtBu)16])2}, {[Cu(hfac)2]([py-C6H4-CH2NH2(CH2)4Ph][Cr7NiF8(O2CtBu)16])2}, and
{[Cu(hfac)2]([3-py-CH2CH2NH2(CH2)3SCH3][Cr7NiF8(O2CtBu)16])2}, the structures of which have been determined by X-ray
diffraction. The {Cr7Ni} rings and CuII ions both have electronic spin S = ½, but with very different g-values. Continuous-
wave EPR spectroscopy reveals the exchange interactions between these dissimilar spins, and hence the communication
between the different molecular components that comprise these supramolecular systems. The interactions are weak
such that we observe AX or AX2 type spectra. The connectivity between the {Cr7Ni} ring and thread terminus is varied
such that the magnitude of the exchange interaction J can be tuned. The coupling is shown to be dominated by through-
bond rather than through-space mechanisms.
approach can be built into coordination cages exploiting su-
pramolecular chemistry.15 This is based on systems of the gen-
eral formula (cation)[Cr7NiF8(O2CR’)16],16,17 which consist of a
ring of seven CrIII and one NiII ion, with each edge bridged by
one fluoride and two carboxylates; the cation is typically a pro-
tonated secondary amine. {Cr7Ni} rings have a well-isolated S =
½ ground state, arising from strong antiferromagnetic nearest
neighbour exchange,18 with coherence times at low tempera-
tures that can be optimised by choice of carboxylate (R’) and
cation.19,20 The chemical robustness resulting from the CrIII (d3)
oxidation state enables a remarkable range of chemistry to be
performed on these rings,17 for example, site-specific ligand
substitutions, introducing functional groups that allow building
into more complex structures. The largest we have character-
ised to date contains twenty-four {Cr7Ni} rings centred on a
Pd12 cage.21
INTRODUCTION
There is a growing literature on the design, synthesis and
study of molecular systems as potential qubits for Quantum
Information Science (QIS) using electron spins. The first such
proposal was for a Mn12 cluster,1 a single-molecule magnet, but
most subsequent studies have focussed on S = ½ systems that
provide a simple two-level quantum system. Such molecules
require sufficiently long phase memory (or decoherence) times
(Tm) to allow spin manipulation without loss of quantum phase
information to the environment, and many studies have fo-
cussed on maximising Tm. Systems studied include nitroxyls,2
fullerenes,3 lanthanide ions,4,5 VIV and CuII complexes.6-8 Such
work has recently been extended to examine a molecular Qudit
which allows the implementation of the Grover algorithm in a
TbIII sandwich complex via manipulation of the metal hyperfine
states.9
In the (cation)[Cr7NiF8(O2CR’)16] structures, the cation sits in
the centre of the {Cr7Ni} ring. If suitable extended and sterically
demanding R groups are chosen then [2]rotaxane structures
are formed. If instead an extended diammonium thread is used,
for example [RNH2(CH2)nNH2R]2+, then a {Cr7Ni} ring can form
on each ammonium group to form [3]rotaxanes which are
{Cr7Ni}2 homodimers.22 We have previously shown that we can
detect the very weak {Cr7Ni}…{Cr7Ni} interactions in a range of
these [3]rotaxanes by pulsed EPR methods.23,24 More recently,
we used a related approach to couple {Cr7Ni} to a hetero-spin
centre.15 This involved introducing a pyridyl (py) head group to
an extended amine py-CH2NHCH2CH2Ph (thread A, Table 1)
which can then form the [2]rotaxane (HA)[Cr7NiF8(O2CtBu)16]
(1) where the head of the thread is now functionalised (Figure
The next challenge for molecular systems is to move beyond
single qubit gates, requiring the introduction of controllable
coupling between two or more qubits. In principle, such cou-
pling between molecular spin qubits can be controlled very
precisely via the chemistry. Attempts have been made towards
this in systems including organic radicals,2,10 lanthanide di-
mers,11 vanadyl dimers12 and fullerene dimers.13 For some QIS
proposals it is a requirement to be able to address the individu-
al spin qubits selectively.2,10 One approach is to couple two
different spins such that the interaction (J) is much weaker than
the difference in Zeeman energies (|J| << ΔgµBB), defining a
weakly coupled ‘AX’ spin system. Takui has referred to this as
“g-engineering”.10,14 We recently demonstrated that such an
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S1a). This can be reacted with Lewis acid metal species, for
The average Cu…{Cr7Ni} distance (average over individual
example
[Cu(hfac)2]
(Hhfac
=
1,1,1,5,5,5-
form
metal ions) is 8.46(3) Å with an average Nam-Cu-M angle (θ) of
1
hexafluoroacetylacetone),
to
31.38(15)°.
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{[Cu(hfac)2](HA)[Cr7NiF8(O2CtBu)16]} (2). We showed by contin-
uous wave EPR that the S = ½ CuII ion and the S = ½ {Cr7Ni}
ring (which have g-values of ca. 2.1 and 1.8, respectively), are
weakly coupled (at Q-band magnetic fields) giving distinct res-
onances for the two components, each split by a J-coupling in a
manner more commonly associated with NMR spectroscopy.
Hence, we demonstrated that we had an AX electron spin sys-
tem based on supramolecular chemistry principles. However,
the mechanism for this interaction was unclear, being too
strong and isotropic for through-space dipolar interactions, yet
with no obvious exchange pathway other than through H-
bonding between the ring F-atoms and the ammonium groups.
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Table 1. List of Threads: A - E
Thread Structure
A
Figure 1. Crystal structure of 2, the red arrows show the
unique z axis for the CuII and {Cr7Ni}. For 2 they are parallel
with one another. Atom colours: blue (N), red (O), grey (C),
green (Cr), lilac (Ni), navy (Cu), yellow (F). Bu(pivalate)
groups and hydrogens omitted for clarity
t
B
In order to vary the Cu…{Cr7Ni} separations, four more
[2]rotaxanes were synthesised with varying threads (Figure
S1b-e). Similar chemistry with thread B (Table 1) gives
{[HB][Cr7NiF8(O2CtBu)16]}
3
(Figure
S1b)
then
{[Cu(hfac)2][HB][Cr7NiF8(O2CtBu)16] 4 (Figure 2). Threads A and
B only differ by the end group at the far end of the chain from
the pyridyl, with A terminated with a phenyl group, providing
significant steric bulk to prevent slippage of the {Cr7Ni} ring off
the thread (as shown by EPR, see later). In HB the terminus is
a methyl group: despite the smaller size presenting no obvious
steric barrier to dissociation (hence making 3 and 4 “pseudo-
rotaxanes”), again solution EPR proves the integrity of 4 in
solution (see later). Presumably this is a result of the H-bonding
between the fluorides of the {Cr7Ni} ring and the ammonium
protons. 4 crystallises in the tetragonal P4/ncc space group,
resulting in disorder of the pyridyl…[Cu(hfac)2] over four
equivalent sites.
C
D
E
In this work we systematically vary the thread that links the
{Cr7Ni} ring and CuII site in order to probe the mechanism(s) of
the spin-spin interaction. This introduces a number of changes
including the through-space and through-bond distances, and
the types of bonds (saturated or unsaturated) between the S =
½ centres.
SYNTHESIS
AND
STRUCTURAL
CHARACTERISATION
Figure 2. Crystal structure of 4, the red arrows show the
unique z axis for the CuII and {Cr7Ni}. For 4 they are perpen-
dicular to one another. Colors as for Figure 1.
Four of the five threads used were prepared by Schiff-base
condensations followed by reduction, adapting literature meth-
ods.25 The fifth thread (B) was commercially available. Thread
A (Table 1) forms the [2]rotaxane {[HA][Cr7NiF8(O2CtBu)16]} 1
X-ray crystallography shows that, in contrast to 2, the thread
pyridyl group does not bind at the apical site of the square py-
ramidal geometry at Cu. Instead, it binds in an equatorial site,
with one of the Cu…O bonds now defining the apical site.
Hence, the Cu coordination geometry is rotated by 90o with
respect to the {Cr7Ni} ring cf. 2 (at least in the solid state). Giv-
en the otherwise similar linkages there are minimal structural
differences to 2, with through-bond and through-space Nam…Cu
distances of 9.09(2) and 7.14(3) Å, respectively. However, the
(Figure
S1a)
and
subsequently
its
adduct
{[Cu(hfac)2][HA][Cr7NiF8(O2CtBu)16]} 2 (Figure 1) via a 1:1 reac-
tion with [Cu(hfac)2]. Crystals of 2 can be grown from warm
acetone, and crystallise in the orthorhombic Pnna space group.
The pyridyl group of the thread binds to [Cu(hfac)2] at the apical
site of a square-pyramidal geometry at the CuII ion. The ammo-
nium of HA lies at the {Cr7Ni} centroid with a Nam…Cu through-
space distance of 7.16(3) Å [8.99(2) Å through-bond distance].
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average Cu…{Cr7Ni} distance is 8.31(3) Å [average θ of
32.06(18)°], slightly shorter than in 2.
space Nam…Cu distances are 14.76(4) and 11.44(10) Å, re-
spectively, with an average Cu…{Cr7Ni} distance of 12.30(4) Å
(average θ of 20.95(15)°).
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2
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5
Finally, thread E was synthesised (Table 1), which has the
same amine…py linkage as in C except the pyridyl group is
now substituted in the 3-position. Thread
E produces
{[HE][Cr7NiF8(O2CtBu)16]} 9 (Figure S1e), which then reacts
6
7
8
9
with
[Cu(hfac)2]
to
produce
the
2:1
adduct
{[Cu(hfac)2]([HE][Cr7NiF8(O2CtBu)16])2} 10 (Figure 5). 10 crystal-
lises in the monoclinic space group C2/c. Again, in contrast to
6, the two molecules of 9 bind cis to one another at the six-
coordinate CuII ion. This is accommodated by buckling of the
two threads. The through-bond and though-space Nam…{Cr7Ni}
distances are 9.47(4) and 7.74(4) Å, respectively, with an aver-
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age Cu…{Cr7Ni} distance of 9.49(4)
27.64(14)°).
Å (average θ =
Figure 3. Crystal structure of 6. Colors as for Figure 1, with
dull yellow (S).
To extend the Cu…{Cr7Ni} distance an extra CH2 group was
introduced between the pyridyl and amine groups in C (Table
1), which forms the [2]rotaxane {[HC][Cr7NiF8(O2CtBu)16]} 5
(Figure S1c). In contrast to the systems based on A and B,
rotaxane 5 reacts with [Cu(hfac)2] to produce the 2:1 complex
{[Cu(hfac)2]([HC][Cr7NiF8(O2CtBu)16])2} 6 (Figure 3). 6 crystallis-
es in the monoclinic I2/a space group. The increased
py…amine separation allows a second molecule of 5 to bind at
the CuII site, giving trans-{(py)2(hfac)2} octahedral coordination
at the CuII ion. The through-bond and through-space Nam…Cu
distances are 10.57(8) Å and 8.59(8) Å respectively, with an
average Cu…{Cr7Ni} distance of 9.69(3) Å (average θ of
27.35(14)°).
Figure 5. Crystal structure of 10. Colors as for Figure 3.
The synthetic work produces five new heterospin systems,
which vary in the coordination number and geometry at the
copper site, and in the relative orientation of the z-axis at the
Cu site, and the unique axis of the ring, which is the direction
normal to the plane defined by the eight metal sites. In 2 the
CuII ion has a 5-coordinate square-based pyramidal environ-
ment with the local z axis pointing to the apex where the
[2]rotaxane pyridine is bound. Hence the unique axes of the
ring and Cu are almost co-linear. In 4, the [2]rotaxane is bound
in the equatorial plane of the Cu square-based pyramid; hence
the unique axes of the Cu and {Cr7Ni} are almost perpendicu-
lar. In 6, which has a trans-{(py)2(hfac)2} six coordination at Cu,
there is a marked elongation along one of the O-Cu-O axes
(Cu-O 2.20 Å), which defines the Cu z-axis, and this is orthog-
onal to the Cu-N direction and to the unique axes of the {Cr7Ni}
rings. In both 8 and 10, which have cis-{(py)2(hfac)2} arrange-
ments, there is a marked elongation along the sole trans O-Cu-
O direction (2.215-2.280 Å; other Cu-O and Cu-N distances
1.980-2.069 Å); therefore here the Cu z-axis is again orthogo-
nal to the unique {Cr7Ni} axes.
Figure 4. Crystal structure of 8. Colors as for Figure 1.
To further increase the Cu…{Cr7Ni} distance, thread D was
prepared, incorporating an p-C6H4 arene spacer between the
pyridyl and amine (Table 1). Thread D gives the [2]rotaxane
{[HD][Cr7NiF8(O2CtBu)16]} 7 (Figure S1d). Reaction of 7 with
[Cu(hfac)2] gives {[Cu(hfac)2]([HD][Cr7NiF8(O2CtBu)16])2} 8 (Fig-
ure 4). 8 crystallises in the orthorhombic P21/c space group. As
with 6, the greater separation of the [Cu(hfac)2] and {Cr7Ni}
fragments enables formation of the 2:1 product. However, in
contrast to 6, in 8 the two molecules of 7 bind in a cis fashion at
the six-coordinate CuII ion. The through-bond and through-
EPR SPECTROSCOPY
Table 2. Structural Data for compounds 2, 4, 6, 8 and 10
2
4
6
8
10
Cu-Nam dist. via space (Å)
Cu-Nam dist. via bonds (Å)
Ave. Cu-M dist. via space (Å)
Ave. θ Nam-Cu-M (deg)
7.16(3)
8.99(2)
8.46(3)
31.38(15)
7.14(3)
9.09(2)
8.31(3)
32.06(18)
8.59(8)
10.60(8)
9.69(3)
27.35(14)
11.44(10)
14.76(4)
12.30(4)
20.95(15)
7.79(4)
9.21(4)
9.49(4)
27.64(14)
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Continuous wave (CW) Q-Band EPR (ca. 34 GHz) spectros-
Because gz(Cu) > gx,y(Cu) and gz(Cr7Ni) < gx,y(Cr7Ni) this is
copy measurements were performed on the supramolecular
complexes 2, 4, 6, 8 and 10 as powders and as 3 mM solutions
in dry (1:1) CH2Cl2:toluene at 5 K. The solution spectra have
significantly narrower linewidths than those from powders (Fig-
ure S5). At 5 K the EPR spectra of the isolated (cation){Cr7Ni}
species are dominated by the S = ½ ground state of the {Cr7Ni}
ring.18
consistent with the crystal structure model.
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However, there is no guarantee that this structure is retained
in solution and EPR spectra of 2 and 4 in frozen solution are
very similar (Figures 7 and 8, respectively). Each component of
both g(Cu) and g(Cr7Ni) is now further split into spectroscopic
doublets. This arises from the J-coupling between the dissimilar
spins (Figure S6). The J-coupling, being well resolved, does not
serve to broaden the linewidths (which, therefore, are very simi-
lar to those of 8) and the Cu hyperfine interaction on gz(Cu) is
still well resolved. Analysis of the multi-line pattern shows that it
now consists of two overlapping hyperfine quartets; hence the
J-coupling is of similar magnitude to this component of the Cu
hyperfine interaction. The spectra can be simulated using a
simple spin-Hamiltonian incorporating only the individual g-
matrices, the Cu hyperfine interaction, and an isotropic ex-
change interaction:
We start by describing the simplest spectrum, which is ob-
served for 8 (Figure 6). Complex 8 has the longest separation
of the Cu and {Cr7Ni} components, and the CW EPR spectrum
appears as a simple superposition of the independent spectra
of the components. Simulation26 (with a 1:2 weighting of the
Cu:{Cr7Ni} components) gives g-values that are typical for the-
se species: gx,y,z(Cu) = 2.041, 2.041, 2.287, and gx,y,z(Cr7Ni) =
1.778, 1.755, 1.714, where x,y,z refer to the local g-frames of
the two components. The g(Cu) values are consistent with te-
tragonal coordination with gz corresponding to the elongated O-
Cu-O direction (see above). For isolated {Cr7Ni} rings, single
crystal studies show that gz (the unique axis, approximating to
axial symmetry) corresponds to the normal to the {Cr7Ni}
plane.18 The spectra are very sharp, and there is good resolu-
tion of the 63,65Cu hyperfine (I = 3/2) interaction on the gz(Cu)
component, giving a well-defined quartet with Az = 450 MHz.
Since there is no effect of any Cu…{Cr7Ni} coupling in these
CW spectra, simulations are independent of the relative orien-
tation of g(Cu) and g(Cr7Ni).
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Ĥ = µBŜCu·gCu·B + µBŜCr7Ni·gCr7Ni·B + ŜCu·ACu·ÎCu - 2JŜCu·ŜCr7Ni
All g-values (and hyperfines) were initially fixed using values
from equivalent isolated Cu and {Cr7Ni} species, with J adjust-
ed as the only free variable; all parameters were then refined
within narrow limits to produce the best agreement. For 2, the
apical Cu…py direction is parallel to the {Cr7Ni} normal, hence
we have set the gz components of the two spins to be co-
parallel. However, we find that the calculated spectra are in-
sensitive to this relative orientation for the small (with respect to
the difference in Zeeman energy) and isotropic |J| values that
we observe here. Introducing a slight g-rhombicity gives a bet-
ter fit to the relative intensities. The final parameters are:
gx,y,z(Cr7Ni) = 1.782, 1.767, 1.712; gx,y,z(Cu) = 2.065, 2.045,
2.327 and Az(Cu) = 450 MHz. Excellent agreements with the
experimental spectra of 2 and 4 are found with isotropic ex-
change interactions of J = -0.015 and -0.013 cm-1 (450 and 390
MHz), respectively (Table 3). The calculated relative intensities
within the exchange split doublets are sensitive to the sign of J,
and we find better agreement with an anti-ferromagnetic inter-
action.
Figure 6. CW Q-Band (ca. 34 GHz) EPR spectrum of 8 in
solution at 5 K (black) and simulation (blue).
Complexes 2 and 4, based on a very similar thread, have the
shortest Cu…{Cr7Ni} separations. The crystal structure of 4
gives the apical position of the Cu coordination sphere as per-
pendicular to the Cu…py direction and hence, surprisingly,
different to 2. However, this is complicated by disorder of the
hfac groups in the structure. We have performed single crystal
EPR measurements on 4 to test the orientation of gz(Cu) with
respect to the gz(Cr7Ni). An indexed crystal was aligned such
that a plane of data could be measured where the applied
magnetic field (B) could be rotated from perpendicular to the
{Cr7Ni} ring to being in the {Cr7Ni} plane. The resulting angular
variation (Figure S12) shows that the maxima in g(Cu) (i.e.
lowest resonance fields) correspond the maxima in g(Cr7Ni).
Figure 7. CW Q-Band (ca. 34 GHz) EPR spectra of 2 in solu-
tion at 5 K (black) and simulation (blue).
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The observation of an exchange-split spectrum for the pseu-
a 1:2 Cu:{Cr7Ni} species). Given the latter (the strongest and
weakest coupled systems studied here) it is safe to assume
that the broadening is a manifestation of the intramolecular
Cu…{Cr7Ni} interactions (all species are studied at the same
concentrations). The broadening arises because |J| is of a simi-
lar magnitude to the intrinsic linewidths. [For example, the Cu
hyperfine interaction is more poorly resolved for 6 in solution
than in, for example, 2 and 8 in the solid state; Figure S5.]
Hence, we have estimated |J| for 6 by simulations, with fixed
linewidths, where |J| is increased systematically from zero. The
Hamiltonian is now:
do-rotaxane 4 is definitive proof that the thread and ring do not
dissociate in solution (in this solvent system), despite the lack
of a bulky stopper on the thread. This is an indication of the
importance of the H-bonding interactions between the second-
ary ammonium protons of the thread with the bridging fluorides
of the {Cr7Ni} ring. The very similar J-coupling for 2 and 4 also
demonstrates that the relationship between the Cu and {Cr7Ni}
are near identical in the two complexes. This either implies that
they relax to the same geometry at Cu in solution, or that the
exchange coupling is not very sensitive to the position of the
pyridyl group binding site at Cu. The latter seems unlikely, giv-
en that the Cu magnetic orbital (dx2−y2 in the local axis system)
would either be orthogonal to or in the plane of the Cu…{Cr7Ni}
interaction. Hence, the former explanation seems more likely,
we can further speculate that the very slightly smaller |J| in 4
than in 2 might reflect a minimal partial slippage, or greater
flexibility, in 4 due to the lack of a bulky stopper. However, even
if true, this is a very minor effect.
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Ĥ = µBŜCu·gCu·B + Σi=1,2µBŜCr7Ni,i·gCr7Ni,i·B + ŜCu·ACu·ÎCu
2JΣi=1,2ŜCu·ŜCr7Ni,i
−
Figure 9. CW Q-Band (ca. 34 GHz) EPR spectrum of 6 in
solution at 5 K (black) and simulation (blue).
The gz(Cu) and gz(Cr7Ni) orientations are assumed to be
perpendicular, given that the elongated O-Cu-O axis is perpen-
dicular to the unique {Cr7Ni} axis. However, as above, the cal-
culated spectra are only sensitive to this for much larger |J|
(>0.05 cm-1, or 1.5 GHz, with our linewidths). We find reasona-
ble agreement with |J| = 0.005 cm-1 (150 MHz), although this is
obviously less well defined than in 2 and 4 (and is insensitive to
the sign of J, Figure S7). Note that 6 is behaving as an AX2
spin system and, in principle the Cu spectrum is now being split
into 1:2:1 triplets by the weak J-coupling with the two equivalent
{Cr7Ni} rings. We have previously reported spectra of
{[Cu(NO3)2(Me2CO)]([pyCH2NH2CH2CH2Et][Cr7NiF8(O2CtBu)16])
2},15 where |J| is bigger at -0.010 cm-1 (300 MHz; note the short-
er link), and the triplet structure is resolved more clearly.
Figure 8. CW Q-Band (ca. 34 GHz) EPR spectrum of 4 in
solution at 5 K (black) and simulation (blue).
The simple form of the spectra for 2 and 4 arises because
the difference in Zeeman energies of the two spins (ΔgµBB) is
much greater than the exchange interaction between them, and
the latter acts as a perturbation on the former. Approximating to
g = 2.1 and 1.8 for Cu and {Cr7Ni}, respectively, for an applied
field of B = 1.2 T (roughly g = 2.0 for 34 GHz) ΔgµBB = 0.17 cm-
1 (or 5 GHz), which is five to six times larger than |2J|. Hence, 2
and 4 are well described as AX spin systems under these con-
ditions.
Spectra of 6 (Figure 9) do not show the clear resolution of J
that is evident in 2 and 4. Hence the Cu…{Cr7Ni} interactions
are significantly weaker than in 2 and 4, due to the additional
CH2 group between the ammonium and pyridyl groups in
thread C. However, the spectrum is noticeably broadened
compared to that of 2 and 4, and also to that of 8 (which is also
Finally, spectra of 10 (Figure 10), where the position of sub-
stitution at the pyridyl group is changed, have resolution that
lies between those observed for 6 and 8: again, this is seen
most clearly in the Cu hyperfine structure. Simulations, follow-
ing the same model and method as above, give |J| = 0.003 cm-1
Table 3. EPR CW Q-Band spectroscopy data; 3 mM solutions in CH2Cl2:toluene at 5 K for compounds 2, 4, 6, 8 and 10
2
4
6
8
10
gx,y,z{Cr7Ni}
gx,y,z(Cu)
1.780, 1.765, 1.710
2.065, 2.045, 2.325
1.782, 1.767, 1.712
2.065, 2.045, 2.327
1.772, 1.750, 1.717
1.778, 1.755, 1.714
1.772, 1.750, 1.698
2.041, 2.030, 2.278
2.041, 2.041, 2.287
2.041, 2.030, 2.278
ACuz (MHz)
J (MHz)
450
-450
450
-390
450
±150
450
<±30
450
±90
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Page 6 of 10
(90 MHz). The results and modelling show that complexes 6
ture, showing that Tm is still limited by intermolecular effects at
and 10 are close to the detection limit of |J| by CW EPR, with
our experimental linewidths of ca. 12 mT. This intrinsic lin-
ewidth can then be used to define an upper limit for |J| in 8, and
calculated spectra show that |J| must be < ≈0.001 cm-1 (30
MHz) for this complex.
the higher concentration. On cooling the 0.2 mM solution to 3
K, Tm increases to 912 and 469 ns for Cu and {Cr7Ni}, respec-
tively (Figures S9,S10). Spin-lattice relaxation (T1) times were
determined by inversion recovery measurements [π-T-π/2-τ-π-
τ-echo] under the same conditions (Table 4, Figure S11), giving
T1 = 281 and 126 µs, for the Cu and {Cr7Ni} components, re-
spectively.
1
2
3
4
5
6
The {Cr7Ni} relaxation time constants (T1 and Tm) are similar
to those for isolated (cation)[Cr7NiF8(O2CtBu)16] rings (Tm 400-
700 ns, depending on the cation;20 T1 of ca. 100 µs for
(Me2NH2)[Cr7NiF8(O2CtBu)16]).19 Data for isolated square-
pyramidal [Cu(hfac)2(py)] or trans-[Cu(hfac)2(py)2] complexes,
equivalent to our Cu moieties in 2 and 8, have not been report-
7
8
9
10
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ed but
a
related monomeric six-coordinate complex
[Cu(hfac)2(4,4’-Me2-2,2’-bipy)] has Tm ≈ 3 µs and T1 ≈ 1000
µs.27 These are substantially longer than {Cr7Ni}, as expected
given than the latter are strongly exchange coupled clusters of
high-spin ions (CrIII and NiII). They are also longer than the Cu
ions in 2 and 8, although we cannot compare directly given that
structural factors can have significant effects on relaxation in
CuII monomers.28 (We are unable to measure the Cu spin over
a wide enough temperature range in order to fit T1 to a mecha-
nistic model; see below.) The important observation is that the
slower relaxation of the monometallic Cu than the {Cr7Ni} ring is
preserved in the supramolecular adducts.
The effect of a faster relaxing spin on a slower relaxing spin
depends critically on the relative magnitude of the coupling and
the difference in resonance frequency of the two spins.27 If the
coupling is substantial in this regard then the 1/T1 relaxation
rate of the slow relaxing system will be enhanced to match that
of the fast relaxing spin. We do not observe this here, hence
the relaxation behavior of these supramolecular adducts is also
consistent with description as weakly coupled AX(2) spin sys-
tems. However, even when the interaction is weak it can still
significantly enhance the relaxation of the slow spin (see the
equations given in reference 27) and it is possible that this is
the reason that the Cu components in 2 and 8 relax faster than
in monomeric [Cu(hfac)2(4,4’-Me2-2,2’-bipy)].
Figure 10. CW Q-Band (ca. 34 GHz) EPR spectrum of 10 in
solution at 5 K (black) and simulation (blue).
RELAXATION MEASUREMENTS BY PULSED
EPR
We have previously reported electron spin relaxation studies
on compound 2, which has the shortest linker between the Cu
and {Cr7Ni} groups and the largest |J| of the compounds stud-
ied here. In order to test if there is significant variation in relaxa-
tion behaviour with |J| and/or linker we now report such meas-
urements on compound 8 which has the longest linker and the
weakest |J|.
Given this, we performed complementary X-band relaxation
measurements on 8 (Figures S13,S14): the lower resonance
fields give a smaller difference in resonance frequency which
should enhance the effect of the interaction on the slow relax-
ing Cu spin. At 3 K, we find T1 for the Cu component (meas-
ured at the equivalent B0) does indeed decrease, from 281 to
175 µs (Q- and X-band, respectively). However, surprisingly,
we find that T1 for the {Cr7Ni} component also decreases: there
is clearly more than one contributing factor to the relaxation
behaviour and this will be studied in depth in a future paper.
The phase memory time (Tm) was determined by a standard
Hahn echo decay sequence [π/2-τ-π-τ-echo] at Q-band for 8 in
dry toluene:DCM (1:1) at 5 K for 3 mM solutions, and at 3 and 5
K for 0.2 mM solutions (Table 4). Measurements were taken at
different static magnetic fields (B0; Table 4). Here, we discuss
those taken at the maxima in the echo detected field swept
(EDFS) spectra (Figure S8) for both the Cu (B0 = 1162 mT) and
{Cr7Ni} components (B0 = 1353 mT), corresponding to orienta-
The transverse relaxation times of compound 8 are compa-
rable to those of 2 which has Tm ca. 1 µs and 600 ns for Cu and
{Cr7Ni}, respectively, under similar conditions.15 (Note care
needs to be taken in comparing the 1:1 adduct 2 with the 2:1
adduct 8 as this changes the local concentrations of spins
which can effect relaxation.29) This implies that both com-
pounds are in the “slow exchange” regime30 where the 1/T1
relaxation rate of the fast relaxing spins is slow compared to the
interaction frequency. This is consistent with 1/T1 {Cr7Ni} ≈ 0.01
MHz (T1 ≈ 100 µs at base temperature) and interactions in the
MHz regime. In this case Tm of the slow relaxing spin is deter-
mined by other factors, as for the isolated spin. On increasing
the temperature, 1/T1 for {Cr7Ni} increases rapidly, enhancing
1/Tm of the Cu. Hence, we can only measure the Cu spin over a
limited temperature range.
tions in the local gx,y plane for either component. We find Tm
=
250 and 254 ns for the Cu and {Cr7Ni} components, respective-
ly, for the 3 mM solution at 5 K: these extend to 514 and 327
ns, respectively, for the 0.2 mM solution at the same tempera-
Table 4. Relaxation times for 8 from Q-Band pulsed EPR.
Features
gz(Cu)
Mid gz(Cu) /
x,y(Cu)
gx,y(Cu)
gx,y{Cr7Ni}
g
Field
Position (mT)
1067
1115
1162
1353
Concentration,
temperature
3 mM, 5K
0.2 mM, 5K
0.2 mM, 3K
Tm (ns)
T1 (μs)
-
273(15)
503(5)
822(1)
254(7)
514(3)
912(1)
254(4)
327(1)
469(0.4)
493(5)
739(1)
DISCUSSION
A series of new hybrid [2]rotaxanes (1, 3, 5, 7 and 9) has
been made, each comprising a {Cr7Ni} ring about an asymmet-
0.2 mM, 3K
345(7)
400(8)
281(4)
126(1)
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Journal of the American Chemical Society
ric, long-chain secondary ammonium thread functionalised with
boxylate on the ring,34 providing a through-bond pathway en-
tirely via covalent (or coordination) bonds. In that case, we ob-
served strong coupling in the EPR spectra (i.e. |2J| >> ΔgµBB;
with |2J| = -0.44 cm-1 or 13 GHz).
a pyridyl group at one end. Each of these [2]rotaxanes binds to
[Cu(hfac)2] to form the extended [2]rotaxane adducts 2, 4, 6, 8
and 10, respectively. In these structures, the distance between
the CuII ion and {Cr7Ni} ring(s) is dictated by the spacing be-
tween the secondary ammonium site and the pyridyl group in
the thread (Table 1). These separations also control the stoi-
chiometry of the adducts. In 2 and 4, the spacer is a single CH2
group, and the resulting proximity of the bulky {Cr7Ni} pivalates
and the hfac-groups only allows a single [2]rotaxane to coordi-
nate at Cu, forming 1:1 adducts. The resulting square pyrami-
dal geometry at Cu has the hfac groups folded away from the
{Cr7Ni}, preventing coordination of a further [2]rotaxane at Cu.
[For 2 and 4, the square pyramidal geometries at Cu have dif-
ferent orientations (and apical ligands) in the solid state, but
may relax to the same structure in solution.] In contrast, in 6, 8
and 10 the spacers in the threads are longer [(CH2CH2) or
(C6H4CH2)]. This relieves the strain between {Cr7Ni} and Cu.
such that a second [2]rotaxane can coordinate at Cu to form
2:1 adducts, with pseudo-octahedral coordination at Cu. There
is no obvious reason why, of the 2:1 adducts, 6 should favour
trans and 8 and 10 should favour cis geometries at Cu.
1
2
3
4
5
6
7
8
The anomalous coupling in compound 10 is consistent with a
through-bond mechanism. These two complexes have similar
py-CH2CH2NH2R connectivity between Cu and {Cr7Ni} other
than the position of substitution of the pyridyl: this is para for 6
and meta for 10. It is well understood from studies of electronic
coupling via conjugated pathways that stronger coupling is
observed for para than meta linked centres.35,36 We have noted
related effects in a family of covalently bound {Cr7Ni} dimers
linked via di-immines coordinated directly to the Ni ions.37 For
8, which has the longest linker, we observe no evidence of
coupling in the CW EPR spectra and this defines an upper limit
to any possible interaction (|2J| <0.002 cm-1 or 60 MHz). The
largest magnitude component of the calculated dipolar matrix
for 8 is -0.0012 cm-1 (40 MHz; Table S2), hence for this length
of linker the dipolar interactions are likely to be dominant or at
least significant with respect to J.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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60
Very weak interactions, exchange and/or dipolar, are poten-
tially measurable by pulsed dipolar EPR spectroscopy. We
have previously measured the interactions between two {Cr7Ni}
rings in different dimeric structures using double electron-
electron resonance (DEER) spectroscopy,23 with interaction
frequencies in the range ca. 6 - 1 MHz ({Cr7Ni}…{Cr7Ni} sepa-
rations 16-31 Å). In the materials studied here, the spectral
separation of the two components is too large for DEER meth-
ods. An alternative technique is relaxation induced dipolar
modulation (RIDME) which measures the oscillations in the
echo decay of a slow relaxing spin due to T1 flipping of a faster
relaxing spin. We have previously used RIDME to measure the
interaction in a [2]rotaxane containing a {Cr7Ni} ring about a
CW EPR gives the magnitude, or upper limit, of the ex-
change coupling between the hetero-spin (S = ½) CuII ion and
{Cr7Ni} ring(s). These order as 2 ≈ 4 > 6 > 10 > 8, with the
range spanning |2J| = 0.030 to <0.002 cm-1 (900 to <60 MHz).
Given that these interactions are much weaker than the differ-
ence in Zeeman energy (ca. 5 GHz at Q-band magnetic fields),
these are well described as AX or AX2 spin systems. Resolu-
tion of such spectra are actually rather rare in CW EPR be-
cause, in general, when these conditions are met the J-splitting
tends to lie within the experimental linewidth.31 In the materials
studied here the resolution is aided by the very different intrin-
sic g-values of the {Cr7Ni} ring and CuII ion. The weak coupling
in all these systems is consistent with the electron spin relaxa-
tion behaviour, being very similar between the strongest (2) and
weakest (8) coupled materials, and the retention of the slower
relaxation times of Cu in the presence of the faster relaxing
{Cr7Ni} rings.
tempo-terminated
thread
([Ph(CH2)2NH2CH2(C6H4)2-
tempo][Cr7NiF8(O2CtBu)16]).38 This gave a dipole-dominated
frequency of 9 MHz (0.0003 cm-1; tempo…{Cr7Ni} distance of
16.8 Å) with a vanishingly small exchange interaction.
We attempted Q-band RIDME measurements on 8, observ-
ing on the slower relaxing Cu spectrum (five-pulse π/2-τ1-π-τ1-t-
π/2-T-π/2-τ2-t-π-τ2-refocued echo sequence; 0.2 mM in 1:1
CH2Cl2:toluene at 3 K). However, we did not observe any oscil-
lations. The strength of interaction that can be detected by such
methods is limited by the excitation bandwidth of the pulses,39
but we observed no oscillations for pulses as short as 12 ns.
Hence, we believe the Cu…{Cr7Ni} interaction for 8 lies in a
“blind-spot” between being detectable by CW EPR line broad-
ening and RIDME experiments with conventional microwave
pulses. This blind-spot is more significant for these sorts of
materials than in organic diradicals40 because of the smaller
intrinsic linewidths of the latter. It may be possible to overcome
this by exploiting broad-band excitation methods enabled by
pulse shaping technologies.41 We are also currently designing
Cu…{Cr7Ni} rotaxanes with longer and more rigid linkers that
should put the interaction energy in the range appropriate for
RIDME experiments, and diamagnetic analogues42 that will
allow such experiments on aligned single crystals.43
The magnitude of the Cu…{Cr7Ni} interaction |J| varies in-
versely with the distance between the two components (Table
3). The exception to this trend is compound 10 where |J| is
smaller than for 6 despite the shorter Cu…{Cr7Ni} distances
(see later). This trend would be expected of both through-space
(dipolar) or through-bond (exchange) interactions. We have
calculated the dipolar interactions in 2, 4, 6, 8 and 10 based on
the crystal structures, and using projection factors for the S = ½
ground state onto the individual metal ions of the {Cr7Ni} ring
calculated32 by ITO (Irreducible Tensor Operator) techniques,
with spin Hamiltonian parameters previously defined for the
parent (Me2NH2)[Cr7NiF8(O2CtBu)16] ring,18 and that match ex-
perimentally determined values from 53Cr NMR.33
Even for 4, with the shortest Cu…{Cr7Ni} distances, the
largest magnitude component of the dipolar matrix is -0.0034
cm-1 (100 MHz) (Table S2), almost an order of magnitude
smaller than the experimentally observed |2J|. Calculated CW
EPR spectra including only the dipolar interaction matrix are
indistinguishable from the sum of the isolated spins for all com-
pounds studied here (using a fixed linewidth of 12 mT). Hence,
dipolar coupling is not dominating in these compounds. Moreo-
ver, for 2 and 4, where there is clear resolution of the ex-
change-splitting in both gz and gx,y components of the spectra,
we can experimentally observe the isotropic nature of J: an
isotropic interaction is not consistent with dipolar coupling. The
separation of Cu and {Cr7Ni} is too far for direct orbital overlap,
hence the exchange must be through-bond. These pathways
involve hydrogen bonds (of two fluorides of the ring to the two
ammonium protons on the thread), which must limit the magni-
tude of the exchange. In contrast, we have previously reported
{Cr7Ni} rings bound to CuII via a functionalised −O2C-py car-
CONCLUSION
In conclusion, we have reported a family of hybrid inorganic-
organic rotaxanes based on paramagnetic inorganic rings tem-
plated about organic threads. The rotaxanes can be can be
coordinated to CuII complexes, via a pyridyl terminal group on
the threads, to provide dissimilar electron spin S = ½ species
on the ring and the thread components. CW EPR studies show
these electron spins – localised on the different molecular com-
ponents of the supramolecular structures – to behave as weak-
ly coupled AX(2) systems, at the magnetic fields associated with
Q-band EPR spectroscopy. The interaction is shown to be
through-bond and this must mean it is mediated by the H-
bonding between the secondary ammonium site of the thread
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Page 8 of 10
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1
2
3
4
5
6
7
8
9
We have shown that by design of the thread we can control
the magnitude of this supramolecular interaction. In future we
will extend the series into the regime of weaker couplings that
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ASSOCIATED CONTENT
Supporting Information. The supporting information is
available free of charge on the ACS publications website at
DOI:… Experimental details, X-ray crystallography, further
structural figues, powder cw EPR, EPR simulations, dipolar
calculations, relaxation data, single crystal EPR spectra.
AUTHOR INFORMATION
Corresponding Author
* Prof Eric J. L. McInnes, School of Chemistry and Photon
Science Institute, The University of Manchester. Email: er-
ic.mcinnes@manchester.ac.uk
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
Funding Sources
This work was supported by the EPSRC through the award of
a doctoral training grant to SJL and funding of the EPSRC
National EPR Facility at Manchester (NS/A000055/1). This
work was supported by the EPSRC(UK) (grant number
EP/L018470/1) and
a
Established Career Fellowship
(EP/R011079/1) to REPW, We also thank EPSRC (UK) for
funding an X-ray diffractometer (EP/K039547/1). We also
thank the European Research Council for an Advanced Grant
(ERC-2017-ADG-786734). We thank Diamond Light Source
for access to synchrotron X-ray facilities.
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