Cobalt complexes with click-derived functional tripodal ligands: Spin crossover and coordination ambivalence

Article abstract of DOI:10.1021/ic200246v

We demonstrate the use of a Cu(I) catalyzed "Click" reaction in the synthesis of novel ligands for spin crossover complexes. The reaction between azides and alkynes was used to synthesize the reported tripodal ligand tris[(1-benzyl-1H-1,2,3-triazol-4-yl)m

Full text of DOI:10.1021/ic200246v

ARTICLE
pubs.acs.org/IC
Cobalt Complexes with “Click”-Derived Functional Tripodal Ligands:
Spin Crossover and Coordination Ambivalence
David Schweinfurth,† Fritz Weisser,† Denis Bubrin,† Lapo Bogani,*,‡ and Biprajit Sarkar*,†
†Institut f€ur Anorganische Chemie, Universit€at Stuttgart, Pfaffenwaldring 55, D-70550, Stuttgart, Germany
‡Physikalisches Institut, Universit€at Stuttgart, Pfaffenwaldring 57, D-70550, Stuttgart, Germany
S Supporting Information
b
ABSTRACT: We demonstrate the use of a Cu(I) catalyzed
“Click” reaction in the synthesis of novel ligands for spin
crossover complexes. The reaction between azides and alkynes
was used to synthesize the reported tripodal ligand tris[(1-benzyl-
1H-1,2,3-triazol-4-yl)methyl]amine, TBTA, and the new ligands
tris[(1-cyclohexyl-1H-1,2,3-triazol-4-yl)methyl]amine, TCTA,
and tris[(1-n-butyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBuTA.
Reactions of TBTA with Co(ClO4)2 lead to complexes of the form [Co(TBTA)(CH3CN)3](ClO4)2, 1, and [Co(TBTA)2](ClO4)2,
2, where complex formation can be controlled by the metal/ligand ratio and the complexes 1 and 2 can be chemically and reversibly
switched from one form to another in solution resulting in coordination ambivalence. The benzyl substituents of TBTA in 2 show
intramolecular CꢀH—π T-stacking that generates a chemical pressure to stabilize the low spin (LS) state at lower temperatures. The
structural parameters of 2 are consistent with a JahnꢀTeller active LS Co(II) (elongation) ion showing four short and two long bonds. 2
shows spin-crossover (SCO) behavior in the solid state and in solution with a high T0 close to room temperature which is driven by the
T-stacking. 1remains high spin (HS) between 2 and 400 K. Reversible chemical switching is observed between 1and 2at room temperature,
with an accompanying change in the spin state from HS to LS. The importance of the intramolecular T-stacking in driving the SCO behavior
is proven by comparison with two analogous compounds that lack an aromatic substituent and remain HS down to very low temperatures.
’ INTRODUCTION
(SCO) compounds. Metal centers with an electronic configuration of
4ꢀ7 in an octahedral ligand field can exist either in the high spin (HS)
d
The 1,3-dipolar cycloaddition reaction between alkynes and
azides was reported by Huisgen et al. decades ago.1 The Cu-
catalyzed version of this reaction, recently reported by two
independent groups, and now known as the “Click” reaction,
has many advantages compared to the noncatalyzed form.2 The
catalyzed version leads selectively to only one regioisomer of
the triazole, works under mild conditions in environmentally
benign solvents, and the work up steps are simpler. The above-
mentioned points, together with the excellent functional group
tolerance of this reaction, have recently made it one of the most
important tools in synthetic strategies. Since the initial discovery
of the “Click reaction”, this method has found use in areas of
chemistry as diverse as dendrimers and polymers,3 drug
discovery,4 material science,5 bioconjugation,6 and metal respon-
sive flourophores,7 just to name a few. Anyway, its use and
advantages in the creation of functional ligands for magnetically
bistable metal complexes is still largely unexplored.8 This is
surprising because of the advantages that this approach has to
offer: “click reactions” now allow using tripodal ligands with
1,2,3-triazole rings with a large choice of functional groups
perfectly arranged in the desired geometry. The freedom of
choice of the appendages also allows studying in detail the effects
of weak interactions like π or T-stacking (Scheme 1), both
intramolecular and intermolecular.
or low spin (LS) forms, depending on the ligand field stabilization
and spin pairing energies. SCOs are characterized by the fact that the
spin ground state changes with temperature T,fromHStoLS,leading
to a characteristic variation of the magnetic behavior around a
transition temperature T0.9 Light is also a frequently used as external
stimulus to induce such transitions.10 Reversible photoinduced spin
state switching has recently been observed for d8 metal centers as
well11 and iron complexes are well-known for showing SCO.12
Certain Co(II) complexes also undergo SCO, but the energy range
in which SCO is observed is much narrower than their iron counter-
parts, thus requiring fine-tuning of the electronic states.13a,b The
majority of octahedral Co(II) SCO complexes are based on the
terpyridine ligands which impose a tetragonal compression in the
JahnꢀTeller active d7, LS Co(II) center. Examples of SCO in Co(II)
complexes where a tetragonal elongation is observed in the LS form
are also known. In most of these cases it is the ligand rigidity that
provides the high enthalpic barrier required for SCO.13,14 Ligands
produced with the click chemistry approach might provide possibi-
lities for weak interactions that could then drive the SCO in relevant
systems.
In the following we present the first use of Click-derived 1,2,3-
triazole tripods for SCOs and the creation of four new Co(II)
Such characteristics are particularly useful for metal complexes that
can undergo transitions from one state to another, like spin-crossover
Received: February 4, 2011
Published: June 09, 2011
r
2011 American Chemical Society
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Scheme 1. Structures of the 1,2,3-Triazoles Ligands Used and of the Allowed Conformational Freedoma
a (a) Scheme of the reversible chemical transformation between Co(II) complexes 1 and 2. (b) TBTA ligand; (c) TCTA ligand; (d) TBuTA ligand.
complexes. The effect of soft-donor click ligands on the magnetic
states of Co(II) systems is investigated, showing the presence of
SCO behavior at high temperatures. The enhanced structural
and chemical flexibility is then exploited to investigate the role of
the functional groups and different coordination modes. The use
of click reaction allows substituting the functionalities with
different ones, clearly showing the effect of intra- and intermo-
lecular stacking interactions. Eventually the enhanced properties
of the ligands are highlighted by the possibility of having, at the
same time, coordination ambivalence, high SCO temperatures,
and sufficient flexibility for a reversible ligand exchange, rever-
sibly switching the system from the HS to the LS state in solution.
and most of the systems considered use strong donors based on P,
diimine, or rigid terpyridine ligands.13
Here we pursue a different strategy, made possible by the use of
click-derived ligands, using 1,2,3-triazole tripods. Such ligands have
more binding modes and a larger structural flexibility than the
widely investigated terpyridine ligands, thus allowing for easier
ligand exchange. The N2 and N3 atoms of triazole ligands
(Scheme 1) can bind to metal centers, and their differing basicities
can be used for selective metal coordination. The relatively high
acidity of the ring CꢀH proton of the 1,2,3-triazoles makes this an
ideal candidate to generate hydrogen bonding networks, and this
strategy has been extensively used for anion binding with 1,2,3-
triazoles as anion receptors.16 The use of 1,4-substituted 1,2,3-
triazoles as ligands in coordination chemistry has seen a recent
surge,17 and for SCOs this can provide additional opportunities for
metal coordination, hydrogen bond formation, and stacking inter-
actions. N2 and N3 can also be used for generating weak interac-
tions. In this work we show the importance of such functional sites
in the creation of SCO compounds using these ligands. The
enhanced flexibility of these ligands allows coordinating the metal
ion in different ways, contrarily to previously used systems. This
should lead to the possibility of reversibly changing the coordina-
tion environment of the metal center (Scheme 1a).
’ RESULTS AND DISCUSSION
Ligand Design and Synthesis. Targeted ligand design is at
the heart of generating SCO metal complexes. Tuning of the
steric and electronic properties (including weak interactions) of
ligands is fundamental for the creation of SCO compounds with
high transition temperatures and thermal hysteresis.10d,15 Intra-
molecular interactions that produce a more rigid ligand shell
normally induce an increase in the transition enthalpy, thus raising
T0. Intermolecular interactions, which give rise to cooperative
effects, can produce thermal hysteresis in the SCO behavior.
Up to now the strategy to obtain high transition temperatures
has relied on the use of very rigid cages and ligands that strongly
bind onto the metal center. Fe-based SCO compounds are
mostly based on rather rigid ligands with N-containing coordinat-
ing groups. Co-basedSCOmaterials are relativelyless investigated,
The ligand we chose is the tripodal ligand tris[(1-benzyl-1H-1,2,3-
triazol-4-yl)methyl]amine, TBTA (literature known, Scheme 1b),
which provides additional coordination possibilities through the
central amine nitrogen and multiple sites for generating weak
interactions. The synthetic path used (Experimental Section) is
described in Scheme 2.
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Scheme 2. Synthesis of the Tripodal Click Ligands
Table 1
compound
1
2
3
chemical formula
Mr
C
36H39N13Co C2H3N.2(ClO4)
C60H60N20Co.2(ClO4)
C54H84N20Co.2(ClO4)
1271.4
3
952.69
1319.11
crystal system, space group
temperature (K)
a, b, c (Å)
triclinic, P1
triclinic, P1
orthorhombic, Pbca
100
173
173
12.405(2), 12.482(2), 17.162(3)
10.182(2), 11.401(2), 13.982(2)
17.990(1), 18.387(2), 18.396(1)
R,β, γ (deg)
V (Å3)
79.928(14), 78.168(11), 60.612(13)
94.385(11), 104.667(9), 93.441(12)
90, 90, 90
6085.30(16)
4
2257.5(7)
2
1560.4(4)
1
Z
density (g/cm3)
1.402
1.404
1.388
F(000)
986
685
2692
radiation type
Mo KR
Mo KR
Mo KR
μ (mmꢀ1
)
0.7107
0.7107
0.7107
crystal size (mm)
0.5ꢁ 0.45 ꢁ 0.2
9047, 8625, 6495
0.0357
0.55ꢁ 0.3 ꢁ 0.2
7538, 7137, 5042
0.0398
0.1ꢁ 0.09ꢁ 0.09
14056, 7367, 5684
0.0276
meas., indep. and obsvd [I > 2σ(I)] refl.
Rint
R[F2 > 2σ(F2)], wR(F2), S
0.051, 0.1398, 0.963
0.771, ꢀ0.604
0.0453, 0.1243, 0.917
0.633, ꢀ0.438
0.0503, 0.1153, 1.079
0.696, ꢀ0.619
ΔFmax, ΔFmin (e Åꢀ3
)
The ligands TCTA and TBuTA (Schemes 1c and 1d) were
also synthesized (Experimental Section) by the copper catalyzed
Click reaction between tris-propargyl amine and cyclohexyl azide
or n-butyl azide respectively. These ligands possess no aromatic
substituents, so that π- and T-stacking interactions should be
suppressed, both within the molecule and among molecules. This
allows investigating the importance of the interactions in 1,2,3-
triazole-based SCO compounds.
Complexes 1ꢀ3 could be obtained as single crystals suited for
X-ray inspection. Compounds 1 and 2 were crystallized by slow
diffusion of diethylether into acetonitrile solutions, while 3
crystallized upon slow cooling of the methanol solution. Measure-
ments at 173 K showed that both 1 and 2 crystallize in the triclinic
P1 space group, whereas 3 crystallizes in the orthorhombic Pbca
space group. Crystallographic details are given in Table 1.
The cobalt center in compound 1 (Figure 1) is coordinated by
the nitrogen atoms of the three triazole rings of TBTA with
average CoꢀN distances of 215 ( 1 pm. Three other positions
are occupied by acetonitrile molecules positioned at 222 (
1, 224 ( 2, and 250 ( 1 pm from the Co center. There is a
seventh coordination to the central amine nitrogen of TBTA
which is at a distance of 265 ( 1 pm from the Co center. The
capping bond is longer than the others, but consistent with other
heptacoordinate complexes and also significantly shorter than
what is expected for noncoordinating nitrogens (about 280 pm
for Co(II)-N(amine).18 To our knowledge this is the first
example of such coordination geometry with Co(II) complexes
using tetradentate tripodal ligands. This shall be ascribed to the
coordination flexibility that these ligands bring with them. The
resulting capped octahedron shows that the CH3CN molecules
are placed in an almost perfect triangular structure (Supporting
Information, Figure S1), as well as the three triazoles. The
coordination around the cobalt center in 1 is thus that of a rare
cappedoctahedron with CoꢀN distances typical of a HS Co(II)
Synthesis and Structure of the Complexes. Reactions of
Co(ClO4)2 6H2O with equimolar amounts of TBTA at room
3
temperature resulted in the formation of [Co(TBTA)(CH3CN)3]
(ClO4)2, 1. An analogous procedure afforded [Co(TCTA)2]
(ClO4)2, 3, or [Co(TBuTA)2](ClO4)2, 4, by reacting Co-
(ClO4)2 6H2O with 2 equiv of TCTA or TBuTA, respectively.
3
Compound [Co(TBTA)2](ClO4)2, 2, could be prepared by two
different procedures. Reaction of 1 equiv of Co(ClO4)2 6H2Owith
3
2 equiv of TBTA led to the formation of 2. Alternatively, the
addition of 1 equiv of TBTA to 1 leads to the formation of 2 through
the substitution of the three labile acetonitrile ligands. One of
the TBTA ligands of 2 can be substituted back by acetonitrile
molecules by the addition of Co(ClO4)2 6H2O which gives back 1
3
(Scheme 1a). It is to be noticed that similar procedures are not
possible for most Fe complexes that have high T0 temperatures.
Here, on the contrary, the different coordination modes allowed by
the enhanced flexibility of the ligand can be switched back and forth,
demonstrating the flexibility and advantages of this approach.
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Figure 1. Structures of the compounds, with hydrogens and counterions omitted for clarity. Coordination bonds and stacking interactions are
represented as fragmented. (a) Structure of 1, showing the heptacoordinate Co(II) center. (b) Structure of 2, showing the disposition of the two binding
arms of TBTA and the nonbinding one. (c) View of the intramolecular and intermolecular stacking interactions. Only two molecules of 2 and the benzyl
rings of adjacent molecules are represented. The involved benzyl rings and centroids are yellow, the triazoles purple, and the relevant H atoms white.
center.13a There are no significant intermolecular interactions in
case of 1, the only significant weak interaction being that between
the CꢀH of the triazole rings and the perchlorate ions. Such
CꢀH anion interactions have been made use of as using triazole
3 3 3
containing molecules as anion receptors.16
In contrast, the cobalt center in 2 is coordinated by two TBTA
ligands resulting in hexacoordination with complete absence of
acetonitrile molecules (Figure 1). Each TBTA binds to the cobalt
center through two of the triazole nitrogen atoms at relatively
short CoꢀN distances of 193 ( 1 pm. These four triazole
nitrogen thus occupy the equatorial positions of the octahedron.
The axial positions are occupied by the amine nitrogen atoms of
each TBTA, and the CoꢀN distance here is 236 ( 1 pm. The
resulting geometry is an axially elongated octahedron, as ex-
pected for a LS JahnꢀTeller distorted Co(II) center, in which
one triazole arm of TBTA is left noncoordinating.13a
Figure 2. View of the molecular structure for 3. Average CoꢀN-
This remaining arm allows the formation of a rather complex
(triazole) distances = 210 ( 1 pm and average CoꢀN(amine) distance =
pattern of stacking interactions, both intramolecular and inter-
molecular (Figure 1). The benzyl ring of one of the coordinating
arms forms two intermolecular interactions with two adjacent
molecules. One is a T-stacking between one of the benzyl
hydrogens and the 1,2,3-triazole ring of the uncoordinating arm
of an adjacent molecule (H-ring center distance of 280 ( 1 pm).
The other interaction is a weak π-stacking (planes distance of
380 ( 1 pm at an angle of 11ꢀ) with the benzyl appendage of
another adjacent molecule. This latter benzyl ring is further
T-stacked in an intramolecular fashion with the hydrogen atom
(H-ring center distance of 269 ( 4 pm) of a coordinating arm of
the other TBTA ligand. The resulting pattern helps stabilize the
225 ( 1 pm.
complex and make it more rigid but, at the same time, it does not
hinder the internal flexibility. The ligand allows different coordina-
tion geometries and coordination ambivalence, as shown by 1 and 2.
The arrangements of molecules are also completely different, with 1
showing no significant intra- or intermolecular weak interactions
whereas 2 engage in such interactions as discussed above.
The cobalt center in 3 is coordinated by two TCTA molecules, in
a manner similar to 2 (Figure 2). However, the average CoꢀN-
(triazole) distances of 210 ( 1 pm are longer than the corresponding
distances in 2, and the average CoꢀN(amine) distances of 225 ( 1
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On the contrary, 2 shows a very sharp decrease of the χT value
from 2.0 emu K/mol at 400 K to 1.3 emu K/mol at 300 K to reach
a constant value of 0.47 emu K/mol below 150 K, which agrees
well with a LS Co(II) center.13 (Figure 3a) The observed
behavior is quite typical of SCO compounds, and indicates a
high T0, comparable to those obtained with the very rigid
terpyridine ligands. The regular solution model describes well
the data by
χT ¼ χLST þ ðχHST ꢀ χLSTÞηHS
ð1Þ
where χLS = 0.5 emu K/mol and χHS = 2.5 emu K/mol
correspond to the LS and HS susceptibilities, respectively, and
ηHS is the HS molar fraction at a given T. ηHS is related to the
thermodynamic quantities governing the transition by ln(1 ꢀ
ηHS/ηHS) = [ΔH ꢀ TΔS ꢀ Γ(1ꢀ2ηHS)]/RT, where ΔH and
ΔS are the enthalpy and entropy variations, R is the universal gas
constant, and Γ accounts for cooperative effects. Solution of the
transcendental equation with graphical methods (Supporting
Information, Figure S3 and S4) and least-squares fitting
(agreement R2 = 0.9995) leads to ΔH = 13.9 ( 0.2 kJ/mol,
ΔS = 43 ( 1 J Kꢀ1 molꢀ1, Γ = 0.4 kJ/mol. This corresponds to
ηHS = 0.4 at 300 K. ΔH and ΔS values are appreciably higher to
those found for more rigid ligands in Co-based SCOs.13 In
particular the ΔS value is larger than expected for the spin change
of a Co(II) ion, 5.8 J Kꢀ1 molꢀ1. The remainder entropy should
be assigned to vibrational and conformational changes in the
molecule, made easier by the TBTA ligand. The very high ΔH
value that drives T0 toward room temperature cannot be justified
by a structural rigidity of the ligand, as in studies of terpyridine
based complexes, and shall rather be justified by the presence of
the intramolecular T-stacking interaction, whose energy scale is
of the order of tens of kJ/mol.19 The Γ factor is much smaller
than 2RT0, and thus no thermal hysteresis is observed, consistent
with the lack of hydrogen bonds and the presence of weaker
stacking interactions.
Figure 3. Magnetic properties of the compounds. (a) Temperature
dependence of the χT value for 1 (red triangles), 2 (black circles), 3
(blue squares), and 4 (green pentagons) in the 2ꢀ400K range. Lines are
fits to the data (see text). The chemical switching equilibrium is
highlighted. (b) X-band EPR spectra of 1, f = 9.6727, and (c) of 2, f =
9.6726, as polycrystalline powder acquired at 110 K, together with the
corresponding simulations (see text).
pm are shorter than the corresponding distances in 2. There are also
no significant noncovalent interactions observed for 3 as is the case for
2. The CoꢀN distances in 3 are compatible with those of a HS
Co(II) center. Like 1, 3 also do not show any significant inter-
ꢀ
molecular interactions other than the CꢀH (triazole) ClO4
3 3 3
interactions as in the case of 1.
The uncoordinated triazole arms of the TBTA ligands in 2
participate in intra- and intermolecular noncovalent interactions. In
particular, the intramolecular T-stacking between the benzyl sub-
stituents of different triazole rings creates a chemical pressure within
the molecule which results in a LS Co(II) state for 2 at lower
temperature as seen from the CoꢀN bond lengths as well as
SQUID and electron paramagnetic resonance (EPR) spectroscopic
measurements (see discussion below). Such interactions are missing
in 1 and hence the Co(II) center in 1 is in the HS state even at low
temperatures. That such interactions are important for generating a
chemical pressure and hence altering the spin state of the Co(II)
center is proven by the properties of complex 3. This compound is
analogous to 2 but lacks the possibilities of a T-stacking since the
cyclohexyl rings are not aromatic. Thus, in spite of having the same
first coordination sphere as 2, 3 shows bond lengths and magnetic
properties that are typical of a HS Co(II) center.
Magnetic Behavior. The magnetic properties of the samples
of 1, 2, 3, and 4 were investigated with a SQUID magnetometer
in the 2ꢀ400 K temperature (T) range. The T dependence of the
χT product, where χ is the molar static magnetic susceptibility,
defined as the ratio between the molar magnetization and the
applied magnetic field, is reported in Figure 3a for all compounds.
1 shows a room temperature χT value of about 2.6 emu K/mol, in
good agreement with the reported values for an isolated, HS
Co(II) center,13 with spin state S = 3/2 (ground state 4T1g). χT
remains constant down to 23 K, where the effect of depopulation
of the higher Kramers levels becomes appreciable and χT then
decreases to 2.0 emu K/mol at 2 K. The behavior could be fitted
by considering a zero-field splitting of the spin ground state S =
3/2 with the Hamiltonian H = D[S2z ꢀ S(S þ 1)/3] þ HgS/kBT,
where Sz is the z-component of the spin and kB is the Boltzmann
constant, affording |D| = 8.2 cmꢀ1. Addition of intermolecular
interactions did not improve the fit.
The importance of the stacking interactions in driving the
transition is clearly shown by 3 and 4 whose room temperature
χT value (2.9 emu K/mol) remains constant down to 115 K,
when χT starts dropping, reaching a value of 1.8 emu K/mol at
2 K (Figure 3a). The behaviors for both compounds are remark-
ably similar, with the curves almost superimposing one over
another. Appreciable effects of the depopulation of Co(II) levels
are normally observed below 60 K, and this behavior could be
due to zero field splitting effects. Should it be assigned to a low-
temperature SCO effect, fitting of the curve for 3 with eq 1 (R2 =
0.9995), yields ΔH = 1.0 ( 0.5 kJ/mol, ΔS = 15 ( 3 J Kꢀ1 molꢀ1
,
Γ = 0.5 kJ/mol. Whatever the case the sharp drop of T0 clearly
reveals the importance of the intramolecular interactions in
driving the SCO transition. The two ligands have very different
steric encumbrances and an expansion of the ligand sphere
because of unfavorable intramolecular steric pressure cannot
account for the high-spin nature. On the contrary, both com-
pounds cannot lead to stacking interactions, as they lack aromatic
substituents.
EPR Spectroscopy. The X-band EPR spectra of 1, 2, 3, and 4
recorded at 110 K as a polycrystalline powder (Figure 3 and
Supporting Information, Figure S5) show the presence of a HS
state for 1, 3, and 4 and LS state for 2, consistently with the
magnetization data. The spectra could be simulated with g-values
of 5.35, 3.65, and 1.87 for 1 and 5.60, 3.70, and 2.10 for 3. This
leads to a gav value of the high spin system of 3.62 and 3.80
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which is driven by the T-stacking. 1 remains HS between 2 and
400 K. The chemical switching between 1 and 2 in solution is
accompanied by a complete switching of the spin state of Co(II)
(Figure 4). This result accompanied by the fact that 2 shows SCO
behavior in solution suggests the importance of weak intramolecular
interactions in driving such processes. The switching between 1 and
2 is also accompanied by a color change as seen from differences in
their absorption spectra (Supporting Information, Figure S6).
’ CONCLUSIONS
Click-chemistry allows for an enormous enrichment of the
variety of triazole ligands, and an unprecedented rational tuning
of the groups, and opens up new possibilities for generating
ligands for bistable complexes as mentioned above. 2 shows SCO
behavior with a high T0. Reversible chemical switching is observed
between 1 and 2 at room temperature with an accompanying
change in the spin state from HS to LS. The importance of the
intramolecular T-stacking in driving the SCO behavior is proven
by the magnetic properties of [Co(TCTA)2](ClO4)2, 3, and
[Co(TBuTA)2](ClO4)2, 4, which are analogous to 2 but lack an
aromatic substituent and remain HS until low temperatures. On
the other hand, the SCO behavior of 2 is maintained in the solid
state as well as in solution. We have created a new Co(II) complex
in which the structural flexibility of the ligand is assisted by internal
T-stacking interactions in creating a room-temperature SCO
transition. Unlike the more rigid terpyridine ligands, the magnetic
properties of these complexes can be chemically switched in a fully
reversible way. The structural flexibility of the ligands allows for a
change in their coordination mode hence making them coordina-
tion ambivalent. These results afford insights into the role of
different interactions in SCOs and highlight the unexplored poten-
tial of increasingly popular “Click-based” ligands in the chemistry
of Co(II) complexes and for magnetic bistability.
Figure 4. Magnetic characterization of 2 in solid state and in solution.
Temperature dependence of the χT value for microcrystals of 2 (black
squares) and for a solution of 2 in CH3CN. The diamagnetic contribu-
tion from the solution and sample holder was subtracted.
respectively. The extremely broad signal for 4 precluded a reason-
able simulation and g value determination for this compound. The
LS complex 2 could be simulated with g^ = 2.221 (A^ = 30G) and
g = 2.020 (A = 80G). Compounds 1, 3, and 4 which are in the HS
state show broad EPR lines because of fast relaxation processes
owing to high spinꢀorbit coupling associated with HS Co(II)
centers.19e,20 4 (Supporting Information, Figure S5) shows much
broader lines compared to 1 and 3, and this probably indicates the
influence of the n-butyl substituents in 4 in enhancing the relaxa-
tion phenomenon.
These EPR data corroborate the susceptibility measurements.
1, 3, and 4 show EPR spectra at 110 K that are typical of a HS
Co(II) center. The transition observed in this case arises from the
low-lying S = 1/2 manifold. Transitions related to the S = 3/2
state are usually not observed in the X-band because of the large
zero field splitting associated with a HS Co(II) centers. The
broad lines and large g-anisotropy are an indication of fast
relaxation processes as is brought about by the large spinꢀorbit
coupling of the HS Co(II) centers. 2 shows a much narrower
EPR signal than 1 and 3 because of the slower relaxation times of
the LS Co(II) center of 2. The narrower lines also result in the
resolution of the hyperfine coupling to the 59Co nucleus, which
has a nuclear spin, I = 7/2.
Reversible Switching Behavior. A reversible chemical switch-
ing is possible between 1 and 2 in solution, as shown in Scheme 1.
The complex formation can be controlled by metal/ligand ratio
and the complexes 1 and 2 can be reversibly switched from one
form to another in solution, resulting in coordination ambivalence.
Structural characterization at 173 K shows CoꢀN distances for 1
that are typicalfor a high spinCo(II) complex. Incontrast, 2 shows
CoꢀN distances typical for a JahnꢀTeller distorted low spin
Co(II) center with TBTA coordinating through two of the triazole
nitrogens and central amine nitrogen atom with one triazole arm
being free. The benzyl substituents of the free triazole arms of 2
participate in intramolecular CꢀH---π T-stacking that generates a
chemical pressure to stabilize the LS state at lower temperatures. 2
shows SCO behavior in the solid state and in solution, as probed by
SQUID susceptometry, with a high T0 close to room temperature
’ EXPERIMENTAL SECTION
General Procedures and Materials. All manipulations were
carried out under air. Trispropargylamine and Co(ClO4)2 6H2O were
3
purchased from Sigma Aldrich and used as received. TBTA and
cyclohexylazide were prepared according to the literature procedures.21
Caution! Perchlorate salts of metal complexes with organic ligands are
potentially explosive. Heating of dried samples must be avoided; handling of
small amounts has to proceed with great caution using protection.
Synthesis of Tris[(1-cyclohexyl-1H-1,2,3-triazole-4-yl)me-
thyl]amine (TCTA). Cyclohexylazide, 2.86 g (22.9 mmol), and
tripropargylamine, 1.00 g (7.62 mmol), were dissolved in a mixture of
CH2Cl2/H2O/tert-butanol (25 mL/25 mL/50 mL). CuSO4 5H2O,
3
0.286 g (1.14 mmol), sodium ascorbate, 0.905 g (4.57 mmol), and
TBTA, 0.121 g (0.23 mmol), were added, and the solution was stirred for
5 days at 50 ꢀC. The reaction mixture was poured into water (100 mL)
and was extracted with CH2Cl2 (3 50 mL). The combined organic
layers were washed several times with water (3 50 mL), dried over
3
3
Na2SO4, and the solvent was evaporated. The product was recrystallized
from methanol to yield white crystals (2.51 g, 65%). Anal. Calcd for
C27H42N10: C, 64.00; H, 8.35; N, 27.64; Found C, 63.88; H, 8.40; 27.62.
HRMS (ESI) Calcd for C27H42N10Na ([M þ Na]þ): m/z 529.3486;
1
found 529.3495. H NMR (250 MHz, CDCl3, δppm): 1.30 (m, 3H,
cyclohexyl); 1.39ꢀ1.52 (m, 6H, cyclohexyl); 1.71ꢀ1.83 (m, 9H,
cyclohexyl); 1.92 (d, 3JHꢀH = 13.6 Hz, 6H, cyclohexyl); 2.21 (d, 3JHꢀH
=
13.3 Hz, 6H, cyclohexyl); 3.77 (s, 6H, NCH2); 4.43 (t, 3JHꢀH = 11.8 Hz,
3H, cyclohexyl CH); 7.79 (s, 3H, 5-triazole-H). 13C NMR (100 MHz,
CDCl3, δppm): 25.3; 25.3; 33.7; 47.3; 60.2; 121.9; 128.1. The protocols
for the click reaction were not optimized, but the formation of three
6119
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Inorganic Chemistry
ARTICLE
triazole cycles in one molecule consumes more time than just a regular
click reaction.
Tris[(1-n-butyl-1H-1,2,3-triazole-4-yl)methyl]amine (TBuTA).
Butyl iodide (4.51 g, 24.5 mmol) and sodium azide (1.75 g, 30.0
mmol) were dissolved in H2O/tert-butanol (25 mL/50 mL). The
solution was refluxed overnight. Then the solution was allowed to cool
22.04. Found: C, 50.98; H, 6.67; N, 22.10. UVꢀvis (CH3CN): λmax
(ε [Mꢀ1 cmꢀ1] 235 nm (5160), 493 nm (81), 515 nm (84), 610 nm
(33) (4T1 f 4T2, higher energy bands are MLCT and IL in origin).
[Co(TBuTA)2](ClO4)2 (4). Co(ClO4)2 6H2O (42.7 mg, 0.12
3
mmol) was dissolved in CH2Cl2 (5 mL). TBuTA (100 mg, 0.23 mmol)
was added, and the solution was stirred at room temperature for 2 h. The
solvent was evaporated, and the bright pink solid was dried at high
vacuum to yield the desired product (120 mg, 90%). Anal. Calcd. for
down, and tripropargylamine (500 mg, 3.82 mmol), CuSO4 5H2O
3
(143 mg, 0.57 mmol), sodium ascorbate (454 mg, 2.29 mmol), and
TBTA (30.4 mg, 0.06 mmol) were added. Afterward the mixture was
stirred for additional 5 days at 50 ꢀC. The reaction mixture was poured
C42H72Cl2CoN20O8 CH2Cl2: C, 43.04; H, 6.22; N, 23.35; Found C,
3
43.15; H, 6.27; 23.10. HRMS (ESI) Calcd for C42H72CoN20 ([M ꢀ 2
(ClO4ꢀ)]2þ): m/z 457.7785; found 457.7791. UVꢀvis (CH3CN):λmax
(ε [Mꢀ1 cmꢀ1] 230 nm (5050), 480 nm (90), 520 nm (75), 615 nm
(35) (4T1 f 4T2, higher energy bands are MLCT and IL in origin).
Crystallography. Single crystals were grown as mentioned above. A
suitable crystal was selected under a layer of viscous hydrocarbon oil,
attached to a glass fiber, and instantly placed in a low-temperature N2-
stream. The X-ray intensity data were collected at 173 K using a Siemens P4
diffractometer. Calculations were performed with the SHELXTL PC 5.03
and SHELXL-97 program.22 The structures were solved by direct methods
into water (100 mL) and was extracted with CH2Cl2 (3 50 mL). Then
3
the combined organic layers were washed several times with water
(3 50 mL), dried over Na2SO4, and the solvent was evaporated.
3
Finally, the compound was cleaned by flash chromatography on silica
gel (DCM/MeOH). The product was obtained as a white solid
(950 mg) in 58% yields. Anal. Calcd. for C21H36N10: C, 58.85; H,
8.47; N, 32.68; Found C, 58.60; H, 8.23; 32.79. HRMS (ESI) Calcd for
1
C21H37N10 ([M þ H]þ): m/z 429.3197; found 429.3191. H NMR
(250 MHz, CDCl3): δ 0.95 (t, 3JHꢀH = 7.3 Hz, 9H, CH3); 1.36 (sextet,
3JHꢀH = 7.6 Hz, 6H, CH2); 1.89 (quintet, 3JHꢀH = 7.3 Hz, 6H, CH2);
3.74 (s, 6H, NCH2); 4.35 (t, 3JHꢀH = 7.3 Hz, 6H, CH2); 7.75 (s, 3H,
5-triazole-H). 13C NMR (62.5 MHz, CDCl3): δ 13.5; 19.6; 32.4; 47.1;
50.1; 123.8; 143.9.
2
and refined on Fo by full-matrix least-squares refinement. Anisotropic
thermal parameters were included for all non-hydrogen atoms. CCDC
778709ꢀ778711 and the Supporting Information contain the cif files of the
structures. All these data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_requests/cif.
Preparation of [Co(TBTA)(CH3CN)3](ClO4)2 (1). Co(ClO4)2
3
1
General Instrumentation. H NMR spectra were recorded at
6H2O, 0.366 g (1.00 mmol) was dissolved in 20 mL of CH3CN. TBTA,
0.531 g (1.00 mmol), was added, and the solution was stirred at room
temperature for 2 h. Ten milliliters of diethylether was added, and the product
was precipitated by cooling the solution down to ꢀ20 ꢀC overnight. The
bright pinksolid was filtered and washed with diethylether to yield the desired
product (0.866 g, 95%). X-ray quality crystals were grown by slow diffusion of
ether into an acetonitrile solution. Anal. Calcd for C36H39Cl2CoN13O8: C,
47.43; H, 4.31; N, 19.97. Found: C, 47.35; H, 4.51; N, 20.04. MS (ESI) Calcd
for C30H30ClCoN10O4 ([M ꢀ (ClO4ꢀ) ꢀ 3(CH3CN)]þ): m/z 688.1;
Found 688.1. UVꢀvis (CH3CN): λmax (ε [Mꢀ1 cmꢀ1]) 269 nm (17800),
497nm(87), 536nm(87), 698nm(6) (4T1 f4T2, higherenergybands are
metal-to-ligand charge-transfer (MLCT) and IL in origin).
250.13 MHz on a Bruker AC250 instrument. X-band EPR spectra were
recorded with an EMX Bruker System connected with an ER 4131 VT
variable temperature accessory, and simulations were done using the
Bruker Simfonia software. Elemental Analyses was performed on a
Perkin-Elmer Analyzer 240. Mass spectrometry was carried out on a
BRUKER Daltronics Microtof Q mass spectrometer.
Magnetic Characterization. The magnetic susceptibility mea-
surements were performed on solid polycrystalline samples with a
Quantum Design MPMS superconducting Quantum Interference De-
vice (SQUID) magnetometer and were all corrected for the diamagnetic
contribution as calculated with Pascal’s constants. Data were corrected
for the magnetism of the sample holder, which was independently
determined at the same temperature and fields.
Preparation of [Co(TBTA)2](ClO4)2 (2). The compound could
be prepared by two alternative procedures and the identity of the
product was proven by X-ray crystallography. a) Co(ClO4)2 6H2O
3
0.73.0 g (0.20 mmol) was dissolved in 10 mL of CH3CN. TBTA, 0.212 g
(0.4 mmol) was added and the solution was stirred at room temperature
for 2 h. Ten mL of diethylether was added and the product was
precipitated by cooling the solution down to ꢀ20 ꢀC overnight. The
bright pink solid was filtered and washed with diethylether to yield the
desired product (0.171 g, 65%). X-ray quality crystals were grown by
slow diffusion of diethylether into an acetonitrile solution. b) [Co-
(TBTA)(CH3CN)3](ClO4)2 0.130 g (0.14 mmol) and TBTA, 0.074 g
(0.14 mmol) were dissolved in 15 mL of CH3CN. The resulting pink
solution was stirred for 2 h at room temperature. Ten mL of diethylether was
added and the product was precipitated by cooling the solution down
to ꢀ20 ꢀC overnight. The light pink solid was filtered and washed with
diethylether to yield the desired product (0.148 g, 80%). X-ray quality
crystals were grown by slow diffusion of ether into an acetonitrile solution.
Anal. Calcd for C60H60Cl2CoN20O8: C, 54.63; H, 4.58; N, 21.24. Found: C,
54.41; H, 4.62; N, 21.23. MS (ESI) Calcd for C60H60CoN20 ([M ꢀ 2
(ClO4ꢀ)]2þ): m/z 559.7; Found 559.7. UVꢀvis (CH3CN): λmax
(ε [Mꢀ1 cmꢀ1] 265 nm (10800), 481 nm (59) sh, 515 nm (66), 615 nm
(48) (2E f 2T2, 2T1, higher energy bands are MLCT and IL in origin).
’ ASSOCIATED CONTENT
S
Supporting Information. Further details are given in
b
Figure S1ꢀS6. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: sarkar@iac.uni-stuttgart.de.
’ ACKNOWLEDGMENT
We thank Corinna Schelzel for experimental help and Dr.
F. Lissner for crystal measurements. This work was financed by
FCI (Chemiefondsstipendium for DS), DFG, SFB-TRR21, the
Baden-W€urttemberg Stiftung, and the Humboldt Stiftung (Sofja
Kovalevskaja award of the Bundesministerium f€ur Bildung und
Forschung).
Preparation of [Co(TCTA)2](ClO4)2 (3). Co(ClO4)2 6H2O,
3
0.050 g (0.14 mmol), and TCTA, 0.142 g (0.28 mmol), were dissolved
in 25 mL of methanol, and the solution was refluxed for 1 h. The solution
was allowed to cool down overnight, and the product was collected by
filtration (0.133 g, 75%) as light pink crystals that allowed for X-ray
diffraction. Anal. Calcd for C54H84Cl2CoN20O8: C, 51.02; H, 6.66; N,
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