A two-in-one pincer ligand and its diiron(II) complex showing spin state switching in solution through reversible ligand exchange

Article abstract of DOI:10.1002/anie.201408966

A novel pyrazolate-bridged ligand providing two {PNN} pincer-type compartments has been synthesized. Its diiron(II) complex LFe₂(OTf)₃(CH₃CN) (1; Tf=triflate) features, in solid state, two bridging triflate ligands, with a terminal triflate and a MeCN ligand completing the octahedral coordination spheres of the two high-spin metalions. In MeCN solution, 1 is shown to undergo a sequential, reversible, and complete spin transition to the low-spin state upon cooling. Detailed UV/Vis and ¹⁹F NMR spectroscopic studies as well as magnetic measurements have unraveled that spin state switching correlates with a rapid multistep triflate/MeCN ligand exchange equilibrium. The spin transition temperature can be continuously tuned by varying the triflate concentration in solution.

Full text of DOI:10.1002/anie.201408966

Angewandte
Chemie
DOI: 10.1002/anie.201408966
Spin Switching
A Two-in-one Pincer Ligand and its Diiron(II) Complex Showing Spin
State Switching in Solution through Reversible Ligand Exchange**
Subhas Samanta, Serhiy Demesko, Sebastian Dechert, and Franc Meyer*
Abstract: A novel pyrazolate-bridged ligand providing two
{PNN} pincer-type compartments has been synthesized. Its
diiron(II) complex LFe2(OTf)3(CH3CN) (1; Tf = triflate)
features, in solid state, two bridging triflate ligands, with
a terminal triflate and a MeCN ligand completing the
octahedral coordination spheres of the two high-spin metal
ions. In MeCN solution, 1 is shown to undergo a sequential,
reversible, and complete spin transition to the low-spin state
upon cooling. Detailed UV/Vis and 19F NMR spectroscopic
studies as well as magnetic measurements have unraveled that
spin state switching correlates with a rapid multistep triflate/
MeCN ligand exchange equilibrium. The spin transition
temperature can be continuously tuned by varying the triflate
concentration in solution.
ing or with reversible substitution of solvent ligands by weakly
coordinating anions.[9,10] Spin state switching in solution has
thus been discussed for solution-based chemosensing or MRI
contrast applications.[7] As a prominent example, Herges and
Tuczek et al. recently developed a porphyrin-based nickel(II)
system that shows magnetic bistability in solution, induced by
the light-driven reversible coordination of a tethered ligand to
the nickel ion.[11]
Here we report a diiron(II) complex that undergoes
a reversible and essentially complete high-spin/low-spin
transition in solution through a multistep temperature-
dependent ligand exchange reaction. The diiron(II) complex
is based on a novel type of binucleating ligand scaffold that is
composed of two {PNN} pincer-type subunits, and that can be
viewed as a dinucleating version of the bipyridine-based
{PNN} pincer ligand I (Scheme 1).[12,13] Complexes with
T
he spin crossover (SCO) phenomenon has received wide-
spread attention in transition metal chemistry.[1] It is most
frequently observed for iron(II) complexes, and its relevance
ranges from the role of metal ions in biology[2] to magnetic
device applications.[3] Spin crossover is mostly observed for
bulk materials in the solid state, in which intermolecular
cooperative effects play important roles for achieving com-
plete, often abrupt, and in some cases even hysteretic high-
spin/low-spin transitions in response to external perturba-
tions, such as irradiation with light or changes in temperature
or pressure.[4,5] In contrast, spin transitions of molecules in
solution are relatively rare, and they are usually characterized
by gradual SCO following a Boltzmann distribution and
resulting in a spin state equilibrium because of the lack of any
cooperativity.[6–8] As a consequence, in many cases spin
transitions in solution are not complete within the phase
limitations of the solvent. In addition to genuine SCO, which
does not involve any first sphere ligation changes, spin state
sensitivity in solution may arise from changes in the metal
ionꢀs chemical environment, for example from ligand disso-
ciation or substitution reactions, or from triggering events in
the second coordination sphere.[6,7] In few cases, spin state
variations have been associated with reversible solvent bind-
Scheme 1. Complexes I with {PNN} pincer ligand and pyrazolate-
bridged binuclear systems II with two pincer-type {PNN} subunits.
tridentate pincer ligands are currently receiving much atten-
tion,[14] and the new ligand was designed to allow for
cooperativity effects in chemical transformations mediated
by two adjacent metal ions that are hosted in pincer-type
binding pockets. To this end the pincer motif was merged with
a pyrazolate bridging unit (II); compartmental pyrazolate-
based ligands with chelating side arms in the 3- and 5-
positions of the heterocycle have indeed been used benefi-
cially for achieving bimetallic reactivity,[15] and also for
inducing cooperativity and multistability in oligonuclear
SCO complexes.[16,17]
The preparation of the new proligand HL was achieved
[*] Dr. S. Samanta, Dr. S. Demesko, Dr. S. Dechert, Prof. Dr. F. Meyer
Institut fꢀr Anorganische Chemie
through
a
two-step phosphorylation of the previously
3,5-bis(6-methylpyridyl-2-yl)pyrazole (A,
reported[18]
Georg-August-Universitꢁt Gçttingen
Scheme 2); details of the synthetic procedure are provided
in the Supporting Information (SI). In the first step, depro-
tonation of A by nBuLi (2.5 equiv) followed by the addition
of di(tert-butyl)chlorophosphine (1 equiv) resulted in the
formation of B in 90% yield. Repeating the sequence with
4.5 equivalents of nBuLi for deprotonation in the second step
followed by the addition of another equivalent of di(tert-
butyl)chlorophosphine gave the desired proligand HL in 42%
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: franc.meyer@chemie.uni-goettingen.de
Homepage: http://www.meyer.chemie.uni-goettingen.de
[**] S.S. is grateful to the Alexander von Humboldt foundation for
postdoctoral fellowship support. This work has been carried out in
the framework of the European COSTaction CM 1305 (ECOSTBIO).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408966.
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In the solid state, 1 features a distorted octahedral geometry
for both ferrous ions. Each of the pincer cavities binds an iron
atom in a meridional fashion along with two triflate counter-
ions that serve as bridges within the bimetallic pocket. The
coordination environment is completed by a terminal triflate
at Fe1 and an acetonitrile molecule at Fe2. Overall, this
represents an unsymmetrical binuclear complex with two
pincer-type subunits, and with weak and potentially labile
ligands filling all remaining coordination sites.
A Mçßbauer spectrum of solid 1 at 80 K (Figure 2, top)
shows two closely spaced doublets with isomer shifts of 1.15
and 1.18 mmsꢀ1 and large quadruple splittings DEQ of 3.20
and 3.53 mmsꢀ1, respectively, in accordance with two ferrous
Scheme 2. Preparation of proligand HL from A and reaction of HL with
Fe(OTf)2(CH3CN)2 to form complex 1.
yield. A two-fold phosphorylation of A in a single-step
procedure proved unsuccessful.
The constitution of HL was confirmed by single-crystal X-
ray diffraction (see SI). The 1H NMR spectrum of HL in
[D6]acetone solution shows a characteristic sharp singlet at
7.4 ppm for the pyrazole-H4, but otherwise broadened
resonances that sharpen upon lowering the temperature,
likely because of tautomerism of the pyrazole-NH. In line
with this result, the 31P NMR spectrum at 253 K shows two
signals at 35.6 and 34.8 ppm that coalesce at around 323 K
(see Figures S1–S4).
Reaction of HL with two equivalents of Fe(OTf)2-
(CH3CN)2 (OTf = triflate) in acetonitrile and in the presence
of 10 equivalents of NEt3 (for capturing the pyrazole-N-
bound proton) gave a reddish yellow solution from which
light yellow crystalline LFe2(OTf)3(CH3CN) (1) could be
isolated in 60% yield. 1 is an air sensitive complex that is
soluble in common organic solvents. The electrospray ioniza-
tion mass spectrum (ESI-MS) of an acetonitrile solution of
1 shows a dominant peak at m/z 948 amu corresponding to the
ion [MꢀOTfꢀꢀCH3CN]+, suggesting that some triflate ions
remain associated with the [LFe2]3+ scaffold in solution, but
that both triflate and MeCN ligands are labile.
Figure 2. Mçßbauer spectra of 1 in the solid state (top) and in frozen
MeCN solution (bottom) at 80 K.
ions in slightly different environment,[19] as was found
crystallographically. Variable temperature magnetic suscept-
ibility data confirm the high-spin FeII configuration (Figure 3,
left). The decrease of cM T at very low temperatures likely
reflects weak intramolecular antiferromagnetic exchange and
zero-field splitting (a simulation gave g = 2.16, J = ꢀ0.2 cmꢀ1,
j D j= 5.7 cmꢀ1; see SI for details).
Compound 1 was characterized by X-ray diffraction
analysis. The molecular structure is shown in Figure 1, and
structural parameters are provided in the SI (Tables S1–S3).
1
Resonances of 1 in the H NMR spectrum in CD3CN at
room temperature are broadened and paramagnetically
shifted (Figure S5). Though it was not possible to assign all
Figure 3. Variable temperature magnetic susceptibility of 1 determined
by SQUID magnetometry. Left: in solid state; the black line represent
the simulation (see SI for details); Right: in MeCN solution; the gray
line represents the simulation in the range 350–263 K; the black line
represents the simulation from 263–212 K (see text for details).
Figure 1. Molecular structure of 1 at 133 K determined by X-ray
diffraction (30% probability thermal ellipsoids). Hydrogen atoms are
omitted for clarity.
2
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protons, the broad singlet signal for a large number of protons
at 8.1 ppm can be unambiguously attributed to the tert-butyl
groups of the ligand scaffold in the complex. Surprisingly,
upon cooling the paramagnetically shifted resonances grad-
ually disappear and the broad signal for the tert-butyl groups
undergoes a shift to higher field, with a final chemical shift of
1.8 ppm at 238 K; the latter chemical shift is indicative of
a diamagnetic compound at that temperature. This observa-
tion prompted to collect Mçßbauer data of 1 in frozen MeCN
solution at 80 K (Figure 2, bottom). Under those conditions
the spectrum shows a single narrow quadrupole doublet with
an isomer shift of 0.46 mmsꢀ1 and DEQ = 0.59 mmsꢀ1 charac-
teristic for a low-spin iron(II) complex.[19] Both 1H NMR and
Mçßbauer data thus suggest that an almost complete temper-
ature-induced spin state change of the complex occurs in
MeCN solution, and that both ferrous ions have identical
coordination environments at low temperatures.
Figure 5. Variable-temperature UV/Vis spectral changes at 529 nm and
308 nm.
At room temperature in MeCN solution, compound
We surmised that ligand exchange, namely the substitu-
1
shows intense UV/Vis absorptions at 261 nm (e =
tion of triflates by MeCN, takes place in solution upon
lowering the temperature. MeCN exhibits a stronger ligand
field and induces the switch to the low-spin state. To
corroborate this assumption, variable temperature 19F NMR
spectra in [D3]MeCN solution were recorded. At 323 K
a single relatively broad 19F NMR signal is observed at
ꢀ22.8 ppm, indicating a rapid exchange of the bridging and
terminal triflates on the NMR time scale. Upon cooling, the
signal broadens further with a concomitant shift to higher
fields (see the upper ten spectra from 323–238 K in Figure 6).
21700mꢀ1 cmꢀ1) and 308 nm (e = 20800mꢀ1 cmꢀ1; Figure 4).
Figure 4. Variable temperature UV/Vis spectral changes upon cooling
from 323 K to 233 K in acetonitrile solution (conc. 4.5ꢂ10ꢀ4 m).
These UV/Vis bands are tentatively assigned to metal-to-
ligand charge transfer (MLCT) and intraligand transitions.
Upon cooling to 233 K the solution color changes from pale
yellow to intense red, and the UV/Vis spectrum shows drastic
changes. Most prominently, absorptivity in the high-energy
region increases with the generation of two intense bands at
263 nm (e = 35300mꢀ1 cmꢀ1) and 241 nm (e = 38900mꢀ1 cmꢀ1).
Notably, low-spin iron(II) with its filled t2g set of orbitals and
singlet electronic state is expected to feature more intense
MLCT transitions. In addition a shoulder at 409 nm and
a weak band at 529 nm (e = 374mꢀ1 cmꢀ1) arises upon cooling,
likely attributable to d–d transitions.[10,20] More detailed
spectral monitoring of the solution, in particular in the 285–
350 nm region, suggests a reversible multistep transformation
(at least two steps) upon varying the temperature, with an
intermediate predominating at around 270 K (Figures 5 and
S7c). It should be noted that solutions of 1 in other solvents
such as acetone or THF do not show any such temperature-
dependent changes.
Figure 6. Variable-temperature 19F NMR spectra of the complex 1 in
acetonitrile. Spectra from the top are on cooling from 323 to 238 K
and then on subsequent warming from 238 to 323 K.
These changes suggest rapid exchange equilibria of triflates
and MeCN ligands. Below 253 K the 19F NMR signal becomes
sharp again, and it is finally found at ꢀ79.2 ppm at 238 K.
Notably this d(19F) chemical shift corresponds to uncoordi-
nated triflate, because NaOTf in MeCN solution shows an
essentially identical 19F NMR resonance (ꢀ79.4 ppm). These
changes are fully reversible upon warming (see the lower nine
spectra from 243–323 K in Figure 6). Careful evaluation of the
temperature dependence of the 19F NMR chemical shift
suggests a three-step process, with the slope of the d(19F)
versus T curve changing at around 283 K and 263 K (Fig-
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ure S9). This likely reflects the sequential dissociation of the
three triflates and is fully in accordance with the UV/Vis
results.
Variable temperature magnetic susceptibility measure-
ments were performed by both Evans method[21] as well as
SQUID magnetometry (SQUID = superconducting quantum
interference device) for complex 1 in MeCN solution, which
allows following the changes of its magnetic moment. Results
obtained using the two methods are consistent (Figure S9);
SQUID data are depicted in Figure 3 (right side) and reveal
almost complete spin state switching and transformation to
the low-spin iron(II) complex at 212 K. Magnetic data are
fully congruent when measured in cooling and heating modes,
indicating full reversibility of the spin transition process.
Comparison of the temperature dependence of the 19F NMR
data, which reflect the ligand exchange processes, and the
changes of the magnetic moment are consistent, thereby
indicating that the spin transition is indeed correlated with the
ligand exchange reactions. Thermodynamic parameters of the
SCO transition were calculated according to[22,23]
Figure 7. Temperature dependence of cM T for 1 in MeCN solution with
different equivalents of NaOTf added.
to avoid freezing of the solution. Plots of the variable
temperature cT data for different concentrations of NaOTf
are shown in Figure 7.
It is evident that the spin state transition shifts to lower
temperatures with increasing NaOTf concentration, which is
in line with the above discussion: the ligand exchange
equilibrium is shifted toward the high-spin species with
bound triflate in the presence of a high triflate loading.
Furthermore the magnetic moment at 320 K slightly increases
with increasing triflate concentration, indicating that neat
1 (without added NaOTf), because of the equilibrium in
solution, is only around 96% in the high-spin state at that
temperature. Plots of the percentage of molecules in their
high-spin state at different temperatures and different triflate
concentrations are shown in Figure S9. The results confirm
that the triflate/MeCN ligand exchange is directly associated
with the spin state switching process (Scheme 3).
1
g ¼
ð1Þ
1 þ expðDH=RT ꢀ DS=RÞ
in which DH and DS are the enthalpy and entropy parameters,
and R is the universal gas constant. It turned out that
simulation of the complete temperature range with only one
set of parameters does not give a good overall fit. However,
good agreement with experimental data was found for the
temperature range 350 to 263 K with parameters DH =
52.6 kJmolꢀ1 and DS = 197 Jmolꢀ1 Kꢀ1 (gray line in Figure 3,
right side), and for the temperature range 263 to 212 K with
parameters DH = 32.4 kJmolꢀ1 and DS = 121 Jmolꢀ1 Kꢀ1
(black line in Figure 3, right side). Therefore, simulation of
the magnetic data suggests a two-step spin switching phe-
nomenon, in which the first step is completed at around
263 K. Notably the experimental value of cM T at 263 K
matches with the one expected for a situation with one low-
spin iron(II) and one high-spin(II) ion in the complex (LS–
HS, 3.15 cm3 molꢀ1 K). The large experimental values for DH
and DS, in particular for the step at higher temperatures, are
beyond the typical range for genuine SCO without any
dissociation or exchange of ligands (usually DH < 30 kJmolꢀ1
and DS < 130 Jmolꢀ1 Kꢀ1).[6,7] Combining the results from
variable temperature UV/Vis, 19F NMR spectroscopy, and
magnetic susceptibility measurements thus suggests that spin
state switching of one ferrous ion occurs above 263 K and is
associated with the substitution of two triflate ligands by
MeCN, whereas substitution of the third triflate proceeds
below 263 K and causes spin state switching of the second
ferrous ion to give the LS–LS species.
Scheme 3. Triflate/MeCN ligand exchange and spin state switching.
In conclusion, a novel pyrazolate-bridged ligand with two
tridentate pincer-type {PNN} compartments is presented. Its
diiron(II) complex LFe2(OTf)3(CH3CN) (1), which in the
solid state is found in the all-high-spin configuration (HS–
HS), is shown to undergo reversible and complete spin state
switching in MeCN solution, which has been rationalized on
the basis of a temperature-dependent triflate/MeCN ligand
exchange equilibrium. Thorough analysis of variable temper-
ature UV/Vis spectroscopy, 19F NMR spectroscopy, and
magnetic susceptibility data have elucidated a sequential
two-step spin transition process that correlates with the
To further substantiate the presence of a ligand exchange
equilibrium inducing the spin transition, variable temperature
magnetic susceptibility data were measured for different
triflate ion concentrations in solution after adding 6, 12, 18,
24, and 30 equiv of NaOTf. Due to the fact that data obtained
by SQUID measurements and Evans method gave essentially
identical results in the above studies, the latter was used for
these experiments. Data have been collected down to 238 K
4
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Chemie
[11] a) S. Venkataramani, U. Jana, M. Dommaschk, F. D. Sçnnichsen,
substitution, by MeCN ligands, of two triflates (in the first
step) and of the third triflate (in the second step) upon
lowering the temperature, generating a mixed-spin (LS–HS)
species [LFe2(OTf)(CH3CN)x]2+ around 263 K and a LS–LS
species [LFe2(CH3CN)y]3+ at even lower temperatures.
Although the spin crossover of iron(II) compounds in the
solid state, induced by light or by changes in temperature or
pressure, is very well established and heavily studied, systems
that undergo complete spin switching in solution are rela-
tively scarce, and spin transitions in solution have rarely been
traced back to specific molecular events. One may speculate
about potential sensor applications of such bi- and multistable
systems, but we find the prospect of tuning chemical reactivity
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Received: September 10, 2014
Published online: && &&, &&&&
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Keywords: diiron complexes · ligand exchange ·
magnetic properties · pincer ligands · spin crossover
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[25] See for example: a) P. Bhattacharya, H. Guan, Comments Inorg.
Chem. 2011, 32, 88 – 112; b) K. L. Fillman, E. A. Bielinski, T. J.
Schmeier, J. C. Nesvet, T. M. Woodruff, C. J. Pan, M. K. Takase,
N. Hazari, M. L. Neidig, Inorg. Chem. 2014, 53, 6066 – 6072; c) P.
Ghosh, S. Samanta, S. K. Roy, S. Demeshko, F. Meyer, S.
Goswami, Inorg. Chem. 2014, 53, 4678 – 4686; d) I. Koehne, T. J.
Schmeier, E. A. Bielinski, C. J. Pan, P. O. Lagaditis, W. H.
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Inorg. Chem. 2014, 53, 2133 – 2143.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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.
Angewandte
Communications
Communications
Spin Switching
S. Samanta, S. Demesko, S. Dechert,
F. Meyer*
&&&& — &&&&
A Two-in-one Pincer Ligand and its
Diiron(II) Complex Showing Spin State
Switching in Solution through Reversible
Ligand Exchange
A pyrazolate-bridged ligand with two
{PNN} pincer-type compartments has
been developed. Its diiron(II) complex
LFe2(OTf)3(NCMe) shows a sequential,
reversible, and complete spin state
switching between the all-high-spin and
all-low-spin states in solution, which
correlates with a rapid multistep triflate/
MeCN ligand exchange equilibrium. The
spin transition temperature can be con-
tinuously tuned by varying the triflate
concentration.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!