Bispidine platform grants full control over magnetic state of ferrous chelates in water

Article abstract of DOI:10.1002/chem.201300604

A bicyclic ligand platform for iron(II), which allows total control over the complex's magnetic properties in aqueous solution simply by varying one of the six coordination sites of the bispidine ligand, is reported. To achieve this, an efficient synthetic route to an N5 bispidine framework (ligand L4) that features an unsubstituted N-7 site is established. Then, by choosing appropriate N-7-coordinating substituents, the spin state of choice can be imposed on the corresponding ferrous complexes under environmentally relevant conditions in water and near-room temperature. Importantly, the first low-spin and diamagnetic iron(II) chelates in the bispidine series, both in the solid state and in aqueous solution, are reported. The eradication of head-on steric clashes between pendent coordinating arms is at the origin of this success. A new pair of constitutionally similar ferrous coordination compounds of a multidentate ligand system is obtained, which exhibits a distinctly binary (off-on) magnetic relationship. The new synthetic intermediate L4 may be substituted in just one step by any desired pendent arm, thus allowing access to complexes with finely tuned magnetic properties. All under control: Structural variation of bispidines allows for the preparation of ferrous complexes that adopt magnetic properties of choice under environmentally benign conditions in water at room temperature. Pairs of constitutionally similar, robust chelates with a binary off-on magnetic relationship are obtained (see figure). Copyright

Full text of DOI:10.1002/chem.201300604

FULL PAPER
DOI: 10.1002/chem.201300604
Bispidine Platform Grants Full Control over Magnetic State of Ferrous
Chelates in Water
Jacek Lukasz Kolanowski,[a] Erwann Jeanneau,[b] Robert Steinhoff,[a] and
Jens Hasserodt*[a]
Abstract: A bicyclic ligand platform
for iron(II), which allows total control
over the complexꢀs magnetic properties
in aqueous solution simply by varying
one of the six coordination sites of the
bispidine ligand, is reported. To ach-
ieve this, an efficient synthetic route to
an N5 bispidine framework (ligand L4)
that features an unsubstituted N-7 site
is established. Then, by choosing ap-
propriate N-7-coordinating substitu-
ents, the spin state of choice can be im-
posed on the corresponding ferrous
complexes under environmentally rele-
vant conditions in water and near-room
temperature. Importantly, the first low-
spin and diamagnetic iron(II) chelates
in the bispidine series, both in the solid
state and in aqueous solution, are re-
ported. The eradication of head-on
steric clashes between pendent coordi-
nating arms is at the origin of this suc-
cess. A new pair of constitutionally
similar ferrous coordination com-
pounds of a multidentate ligand system
is obtained, which exhibits a distinctly
binary (off–on) magnetic relationship.
The new synthetic intermediate L4
may be substituted in just one step by
any desired pendent arm, thus allowing
access to complexes with finely tuned
magnetic properties.
Keywords: bispidines · chelates ·
iron
·
magnetic
properties
· N ligands
Introduction
studies due to their elevated stability.[8–10] Binary complexes
require access to hexadentate ligands, but their syntheses
are often far from straightforward and do not allow simple
variation of the periphery for tuning of the magnetic proper-
ties of the resulting complexes. In view of the difficulties in
devising adequate multidentate chelating systems,[10] only a
handful of solution-phase studies of ferrous complexes with
tetradentate or higher ligands at different magnetic states
have yet been reported.[8,10–12]
Water is arguably the most intriguing solvent given its
omnipresence in biological and environmentally friendly
processes. Near-room temperatures are particularly interest-
ing because of their biological relevance and because of the
associated simplicity in designing devices that do not require
extensive cooling or heating. We have been especially inter-
ested[3,13,14] in obtaining a binary system of constitutionally
related ferrous complexes that withstand the hydrolytic/pro-
tolytic nature of aqueous media at room temperature, while
adopting either a decidedly low-spin[15] or high-spin state by
only slight constitutional variation. Polyaza macrocycles
would appear to be the ligands of choice because they not
only confer very high stability onto their corresponding fer-
rous complexes,[16] but are also known to exercise higher
ligand fields than their open-chain counterparts.[17] Similar
benefits may be obtained from the few ligands that are
based on a more rigid, multicyclic framework displaying sev-
eral nitrogen-donor sites.
Mononuclear coordination compounds of iron(II) most com-
monly adopt one of two distinct spin states: low spin (S=0;
diamagnetic) and high spin (S=2; paramagnetic), although
a borderline case is possible: spin crossover (SCO).[1] This
set of three spin states gives access to a wide range of prop-
erties and makes ferrous complexes highly attractive for ap-
plications as switches and sensors,[2–4] as nonheme enzyme
models for biomimetic studies,[5] and as magnetic resonance
imaging contrast agents.[6] There are numerous examples
showing different spin states in the solid phase, including
both distinct molecules and coordination polymers.[2,7]
However, solution-phase studies of iron(II) complexes for
which the magnetic properties can be finely tuned are
scarce, even though they would greatly facilitate the devel-
opment of the above-mentioned applications that often re-
quire compounds to operate in solution. Complexes with
multidentate ligands (ꢀ4 coordinating atoms), and especial-
ly binary complexes, are of prime interest for solution-phase
[a] J. L. Kolanowski, R. Steinhoff,+ Prof. Dr. J. Hasserodt
Laboratoire de Chimie, ꢁcole Normale Supꢂrieure de Lyon
46 allꢂe dꢀItalie, 69364 Lyon (France)
Fax : (+33)472 72 88 60
E-mail: jens.hasserodt@ens-lyon.fr
[b] Dr. E. Jeanneau
Centre de Diffractomꢂtrie Henri Longchambon, Universitꢂ Lyon 1
We became interested in the class of bispidines
(Scheme 1) for which hexadentate ligands have been report-
ed.[18,19] This bicyclic system (which is part of the more gen-
eral class of diazabicyclononanes/sparteines) is of elevated
5 rue de la Doua, 69100 Villeurbanne (France)
[+] M.Sc. intern on leave from the Technical University of Munich.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300604.
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Results and Discussion
Bispidines were first reported by Mannich et al. in 1930 who
also coined their name.[21] Bispidinones (Scheme 1) can be
readily constructed by two one-pot Mannich condensation
reactions from a suitably substituted cyclohexanone. Only
3 years later, the underlying bicyclic structure (3,7-diazabicy-
clononane) was discovered in the alkaloid sparteine.[22]
Haller et al. reported on the structural diversification of bis-
pidinones in a series of papers from 1965 to 1989;[23,24] they
were also the first to explore their metal coordination chem-
istry.[23,25] Further such reports appeared sporadically in the
literature,[26] but it was the group of Comba et al. who put
bispidine coordination chemistry on a solid foundation in
the past 15 years.[18,27] Bispidinone CuII complexes were con-
sidered for positron emission tomography imaging because
of their elevated stability and the opportunity for facile bio-
conjugation.[28] Bispidinone metal chelates were also ex-
plored as catalysts for aziridination,[29] olefin oxidation,[30]
CH group hydroxylation,[31] halogenation,[32] and sulfoxida-
tion,[33] as well as in nonheme enzymatic models.[34]
Iron(II) complexes of bispidine-like ligands have attracted
particular attention, giving rise to 70 publications in total
and 65 in the past 10 years (including 36 patents). Among
them, bispidinone chelates are the most important with
47 references. None of these, however, include an example
of a truly low-spin complex, either in solution or in the solid
state. Only two binary, hexacoordinate bispidine–iron(II)
complexes have been reported, but their magnetic proper-
ties were not investigated.[19] However, their color (yellow
Scheme 1. The examined ligands and underlying bispidine backbone.
pdz=pyridazine, oxdz=oxadiazole.
rigidity with respect to at least four of its N-donor sites. It
has been declared particularly suitable for the formation of
highly stable complexes and it exercises a high ligand
field.[20] It also offers the unique opportunity to fine-tune
the corresponding complexes in regard to total charge, hy-
drosolubility, and potential affinity to external binding part-
ners by taking advantage only of the periphery of the mole-
cule (the polar “underbelly” consisting of three oxygen-
bearing functional groups). Such an opportunity is largely
absent for established macrocyclic ligands. Despite the
recent progress in the characterization and practical use of
bispidine-based ferrous chelates, only their high-spin
iron(II) complexes have so far been described.
We now report that bispidines are a suitable platform for
creating mononuclear ferrous complexes that adopt the spin
state of oneꢀs choice in aqueous solution and at ambient
temperature. We elucidate the structural dependence of the
spin state, which will enable future workers to fine-tune the
magnetic properties of comparable bispidine chelates. Al-
though high-spin bispidine ferrous complexes have been
known since 2002, we have now obtained the first ever low-
spin bispidine–iron(II) complexes by introducing two rarely
used heteroaromatic coordinating moieties (pyridazine or
oxadiazole) into the pentadentate bispidine backbone (L4,
Scheme 1). The key to the success of our present approach
resides in the efficient synthetic access from a single
common precursor of bispidines, varying by just one pen-
ꢁ
or light brown) and bond lengths in the solid state (Fe N
bonds of ca. 2.2 ꢄ) suggest a high-spin state. The earliest
hexadentate ligand reported by Comba et al. (L1,
Scheme 1)[35] gave a ferrous complex with a solid-state struc-
ture revealing its preference for coordinating its counterion
(sulfate) rather than the “dangling” pyridylmethyl pendent
arm (py4, Scheme 1), and this in spite of the highly favor-
AHCTUNGTREGaNNNU ble five-membered chelate ring that would thus have been
formed. The authors claimed, however, that some coordina-
tion of this pendent arm can be observed spectroscopically
in solution (in all likelihood acetonitrile or methanol).
Based on modeling experiments with a semiempirical da-
taset covering iron (AM1), we concluded that a severe steric
clash between the two coordinating arms facing one another
is at the origin of this destabilization of the hexacoordinate
state and the resulting preference for a ternary complex. We
thus hypothesized that replacement of one of the two pyri-
dylmethyl pendent arms with a spatially less demanding one
would eradicate this clash and allow the hexadentate ligand
to exercise its full ligand field so as to render the iron center
low-spin. We identified the unusual pyridazinylmethyl (L2)
and oxadiazolylmethyl (L3) moieties as the candidates most
likely to fulfill this requirement because they do not feature
any substituent alpha to the coordinating nitrogen atom (in
contrast to the pyridylmethyl groups on L1, see Scheme 1).
ACHTUNGTRENNUNGdent arm, and in the resulting subtle tuning of the ligand
field. Thorough analyses in the solid state (five X-ray che-
late structures plus two ligand structures, and superconduct-
ing quantum interference device (SQUID) measurements)
and in solution (magnetic moments, redox potentials, and
temperature-dependent proton NMR spectroscopy) form a
reliable basis for our conclusions.
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Magnetic State of Ferrous Chelates
FULL PAPER
Solid-state structural analysis: Ferrous complexes of all four
ligands presented in Scheme 1 were prepared. All were
found to exist as geminal diols (hydrated ketone at position
C-9) in the solid state, a phenomenon already observed by
Comba et al. The deliberate use of a noncoordinating coun-
ease of approach of the monodentate ligand acetonitrile, as
can be concluded for the oxadiazole moiety in [FeL3]. Ac-
cordingly, all three structures also show bond lengths fully
compliant with a low-spin state. Thus, the first four low-spin
and diamagnetic crystals of bispidine ferrous complexes are
reported.
ꢁ
terion (BF4 ) allowed us to obtain monocrystals of
[FeL1]·2BF4. Its structural analysis revealed a fully low-spin
complex (iron–nitrogen bond lengths of ca. 2.0 ꢄ), albeit se-
verely distorted (Figure 1, see the deflection angle of the pi-
It is instructive to compare these structures with a high-
spin bispidine ferrous complex to appreciate the radial ex-
pansion that the iron center experiences when switching
from low to high spin (Figure 2A). Ternary complex [FeL4-
Figure 1. X-ray structural analysis. Illustration by CPK formulae of de-
creasing steric strain in complexes [FeL1] (diamagnetic in the solid state/
“intermediate spin” in aqueous solution), [FeL2] (diamagnetic/diamag-
netic), [FeL3] (diamagnetic/diamagnetic), and [FeL4ACTHNUTRGNE(UNG CH3CN)] (diamag-
netic/intermediate spin); torsional angles are given for the two approach-
ing heterocyclic planes grafted onto N-3 and N-7.
colyl planes of py3 and py4), rather than a high-spin com-
plex as previously observed for [FeL1ACTHNUTRGNEUNG
(SO4)].[35] Inspection
of the space-filling model of the diffraction data in Figure 1
immediately confirms our prior assumption that steric clash
of the opposing coordinating arms on nitrogen atoms N-3
and N-7 is at the origin of the destabilization of the hexa-
coordinate isomer in solution, and in the solid state as re-
ported.[35] When monocrystals of [FeL2] and [FeL3] were
subjected to X-ray diffraction analysis, the resulting struc-
tures revealed a much reduced steric clash for binary com-
plex [FeL2] and none for [FeL3] (Figure 1, compare deflec-
tion angles between the planes of py3 and pdz and oxdz, re-
spectively). This tendency towards a strainless complex
when going from [FeL2] to [FeL3] can be explained with
the change from a six-membered heterocycle in [FeL2] (py-
Figure 2. A) Ferrous-center radial expansion in
a high-spin complex
([FeL4(SO4)]; right) compared with a low-spin complex ([FeL3]; left);
AHCTUNGTRENNUNG
hydrogen atoms omitted for clarity. B) X-ray structures of uncomplexed
ligands L1 (left) and L3 (right; all hydrogen atoms omitted for clarity)
illustrate extensive preorganization for metal chelation.
AHCTUNTGREGN(NUN SO4)] exhibits an N5O1 coordination motif that exercises a
ligand field sufficiently reduced to turn the iron center fully
high spin. Capped-stick presentation of the structures of
complexes [FeL3] and [FeL4ACHTNUTRGNEUN(G SO4)] illustrate the departure
from largely collinear coordination bonds for the low-spin
state (Npy2-Fe-Npy3 and N7-Fe-Npy1) in favor of a signifi-
cantly bent orientation in the high-spin state. This is due to
all coordination bonds now adopting a length of approxi-
mately 2.2 ꢄ.
Finally, examination of the solid-state structures of the un-
complexed ligands L1 and L3 (Figure 2B) reveals the highly
preorganized nature of the hexadentate bispidine system.
The lone pairs of nitrogen atoms N-3 and N-7 are already
locked in the orientation found in their respective metal
ACHTUNGTRENNUNGridACHTUNGTRENNUNGazine) to a five-membered one in [FeL3] (oxadiazole),
which has an even more favorable approach angle. We were
delighted to observe that our measure of moving to an azole
coordinating arm did not hinder total access to the low-spin
state in spite of the lower basicity of azoles. Although 3-
methyl-5-phenyloxadiazole sports a pKa value of 2.6, those
of pyridazine and pyridine are found at 2.3 and 5.2, respec-
tively.[36] For comparison, ternary complex [FeL4
also prepared in the context of this work, shows the same
ACHTUNGTERN(NUNG CH3CN)],
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J. Hasserodt et al.
chelates. The lone pairs of nitrogen atoms Npy2 and Npy3
from cis-pyridyl moieties require only rotation of one single
bond to adopt the orientation in the chelate. In the nonche-
lated form they are found pointing towards the oxygenated
underbelly of the ligand, probably for reasons of dipole
compensation in the crystal lattice or electrostatic repulsions
between them, which disappear once coordinated to the
metal ion. This aspect of high preorganization, paired with a
hole size more adapted to the oxidation state 2 than 3 of
iron,[37] bodes well for studies on stability in various aqueous
media.
Magnetic moments in aqueous solution: All five complexes
were studied in aqueous solution by dissolving crystalline
samples for maximal measurement accuracy. Their cationic,
saline nature made it possible to establish suitable concen-
trations that facilitated NMR experiments (final concentra-
tions varied slightly around 5 mm). The magnetic moments
were determined by the method of Evans[3,38] using a super-
conducting NMR spectrometer (500 MHz/11.7 T).[39] Solvent
compositions of either H2O/D2O (85:15) or pure CD3CN
and containing an additional 2% tBuOH as reference sub-
stance were used (Figure 3, see Experimental Section and
Supporting Information). Values of magnetic moments in
Bohr magnetons (BM) were determined from shifts in the
tBuOH signal and were corrected for temperature-depend-
ent density changes of the solvent (see also the Experimen-
tal Section and Supporting Information).[40] For the high-
spin ternary complex [FeL4ACHTNUGRTENUNG(SO4)], a value of 4.96 BM is
found at room temperature, which does not change upon
heating (Figure 3); this value is not affected over a period of
12 h under exclusion of air, but exposure to air clearly
causes the sample to slowly degrade. On the other hand,
binary complexes [FeL2] and [FeL3] give distinctly diamag-
netic aqueous samples and show a small diamagnetic (up-
field) shift with respect to the reference tBuOH signal. The
corresponding magnetic moments hardly change with tem-
perature: 0.59–0.69 BM/25–808C for [FeL2] and 0.54–
0.71 BM/25–608C for [FeL3] in D2O/[D4]MeOH, 2:1 (the re-
duced aqueous solubility of [FeL3] required the move to a
solvent mixture including methanol; see the Experimental
Section). These values correspond well with those for other
low-spin ferrous complexes[13] and do not change over a
week of exposure to air.
Figure 3. A) Chemical shifts of tBuOH and corresponding magnetic mo-
ments found for aqueous solutions (ca. 5 mm) with respect to an external
reference at two different temperatures (Evansꢀ method); the less intense
signal corresponds to tBuOH contained in the reference solution. B) Sol-
vent dependence. C) Concentration dependence for [FeL4ACHTUNGTRENNUNG(CH3CN)].
Stars refer to deviations from indicated conditions (see the text and Ex-
perimental Section for details).
ure 3B); at higher temperature: signal coalescence). For the
[FeL1] complex, the influence of rising temperature is much
more pronounced: 1.31–2.96 BM at 20–808C (Figure 3A,
Figure 4A, and the Supporting Information). In other
words, dissolution of [FeL1] in water at room temperature
already leads to slight paramagnetism. This proves the exis-
tence of some type of equilibrium that moves towards para-
magnetic species upon heating, without, however, reaching,
even at 808C, a value that would correspond to the entire
population of iron centers being high spin. In contrast to
In contrast to the clear-cut behavior described above, the
magnetic moment of an aqueous sample of ternary complex
[FeL4
ACHTUNGTRENNUNG
4.16–5.12 BM/25–808C.
A
brings about a significant change in magnetic moment
(4.66 BM at 0.5 mm to 2.74 BM at 15 mm, see Figure 3C).
These two phenomena indicate the exchange of monoden-
tate acetonitrile with bulk water as seen for other nonbinary
iron(II) complexes,[41,42] which results in a switch from an N6
to the usual N5O1 coordination motif as found also for
[FeL4ACTHNUTRGNEUN(G CH3CN)], the magnetic moment of an [FeL1] sample
in CD3CN does change upon changes in temperature: 0.80–
2.82 BM/ꢁ20–608C (Figure 3A).
Magnetic moments in the solid state: Magnetometric mea-
[FeL4
G
G
ACHUTGTNRENsNUG ureHCATUNGTRENNmUGN ents on crystalline, finely grained samples of the low-
tonitrile solution remain diamagnetic, even upon heating
(Figure 3B: 0.65–0.56 BM/25–508C (look for ** in Fig-
spin, high-spin, and SCO complexes were carried out with a
SQUID instrument. As expected, values for [FeL2] and
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Magnetic State of Ferrous Chelates
FULL PAPER
tion and Supporting Information). A D2O sample of [FeL1]
gives a spectrum that covers a 1–22 ppm interval at room
temperature, and a 1–63 ppm interval at 808C (Figure 5 and
Figure 5. Temperature-dependent evolution of the proton NMR spectrum
of [FeL1] in D2O.
the Supporting Information), with only one set of signals ob-
served at both temperatures. This tendency is maintained in
CD3CN (from 1–10 ppm at ꢁ408C to 1–60 ppm at 608C)
and in [D6]acetone (1–9.5 and 1–50 ppm at ꢁ50 and +508C,
respectively). The spectral profile in both solvents is almost
identical despite significant differences in the coordination
properties and ligand-field strengths exerted by these sol-
vents. The presence of only a single set of signals in all spec-
tra suggests that the equilibrium present in solution is fast
compared to the NMR timescale, even at low temperatures.
No signals stemming from a species with a decoordinated
pendent arm were observed; in conjunction with the solvent
insensitivity of the [FeL1] spectrum, this strongly suggests
the presence of a SCO phenomenon. In contrast to this be-
Figure 4. A) Plot of SQUID-derived magnetic moment versus tempera-
ture for [FeL1] in the two solid-state forms (the initial one turns into the
second one upon heating over 365 K, and refers to the loss of a solvent
molecule). B) Plot of NMR-derived magnetic moment versus tempera-
ture for [FeL1] in solution; solid lines represent the best theoretical fit of
the curves.
[FeL3] were very close to zero and the samples remained di-
amagnetic within a temperature range between 300 and
100 K, in agreement with the solution data. The high-spin
[FeL4
G
havior of [FeL1], a solution of [FeL4ACHTGNUTERNU(NG CH3CN)] in CD3CN
consistent with the presence of a high-spin FeII center and a
spin of 2. Upon cooling to 100 K, its magnetic moment de-
creases subtly, which can be caused by zero-field splitting
(ZFS; see also Figure S23 in the Supporting Information).
In congruence with the solution behavior, a microcrystalline
sample of [FeL1] exhibits a spin transition when cycling be-
tween 350 and 100 K (black triangles in Figure 4A). Regard-
less of the heating/cooling direction, the transition graphs
are superimposable, that is, no hysteresis is present. Howev-
er, when heating above 365 K, the graph corresponding to
subsequent cooling adopts an altered profile (white triangles
in Figure 4A). This may be explained by the loss of a water
molecule from the crystal lattice. Our conclusion that
[FeL1] shows true SCO in solution is thus confirmed in the
solid state.
gives a spectrum that occupies the diamagnetic interval of
1–10 ppm, whether at room temperature or at 608C because
[FeL4ACHTNUGTRENNGU(CH3CN)] and [FeL4HCATUNGTNER(NUGN CD3CN)] are the only species
possible. In [D6]acetone, the coordination of acetonitrile at
0–108C is proved by a proton NMR spectrum covering the
diamagnetic window of 1–10 ppm. Upon heating, however,
severe broadening of the signals is observed (already at
258C). This indicates the acceleration of the exchange proc-
ess of CH3CN. Subsequently, paramagnetic signals appear
and gradually increase at higher temperatures (2–200 ppm
at 508C), thereby indicating the replacement of the coordi-
nating acetonitrile molecule.
Redox potentials: Cyclic voltammetry (CV) was performed
on samples in acetonitrile prepared from crystalline material
and containing 0.1m nBu4NClO4. Neither samples in water
nor in water/acetonitrile solutions (50:50) could be prepared
due to insufficient solubility, probably caused by the high
but required salinity of the solution. CV waves of all studied
complexes were reversible (see also Figure S28 in the Sup-
porting Information) at all five investigated scan rates (50,
100, 150, 200, 250 mVsꢁ1), thus suggesting no significant var-
iation of the coordination geometry. This can be explained
by both the well-described rigidity of the bispidine platform
1
Analysis of H NMR spectra: The diverging magnetic sus-
ceptibilities are also manifest in the corresponding NMR
spectra. Although proton spectra in D2O for [FeL2] and
[FeL3] at room temperature fit into the habitual chemical-
shift window for diamagnetic samples (1.0–10.5 ppm for
[FeL2] and 1–10 ppm for [FeL3]), that of the fully high-spin
ternary complex [FeL4ACHTUNGTRENN(UGN SO4)] occupies the expected para-
magnetic window of 3–200 ppm (see the Experimental Sec-
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J. Hasserodt et al.
2
and its significant tolerance for the size of the metal
center.[43] Interestingly, the binary complexes [FeL1] (SCO)
and [FeL2] (low-spin) and the ternary complex [FeL4-
1
meff ¼ fmLS ½expðꢁDHꢂ=RTÞ expðDSꢂ=RÞ þ 1ꢃꢁ1
ð1Þ
1
2
=
þmHS ½expðDHꢂ=RTÞ ꢄ expðꢁDSꢂ=RÞ þ 1ꢃꢁ1
g
2
ACHTUNGTRENNUNG(CH3CN)] (low-spin) all showed comparable E = redox po-
2
tentials of approximately 650 mV (Table 1), similar to the
In an ideal situation the magnetic moments of the low-
spin (mLS) and/or high-spin (mHS) form are known or can be
obtained by cooling or heating the solution until only one
spin state is present. This would allow for a two (or three)-
parameter fit of meff(T) from which the enthalpy and entropy
of the spin transition process could be extracted directly.[45]
However, in practice a unique spin state (and the associated
magnetic moment) can often not be reached. This then re-
quires a four-parameter fit,[44] which is more prone to gener-
ate significant errors in the estimated values.[46] This situa-
tion is present for [FeL1] in water and in acetonitrile, for
which it was not possible to push the spin equilibrium to
either extremity of the process. Thus, the four-parameter fit
(Figure 4B) to the experimental data (293–353 and 253–
333 K in water and acetonitrile, respectively) led to the fol-
Table 1. Redox potentials of the low-spin complexes (cyclic voltamme-
try).
1
Complex
[FeL1]·2BF4
[FeL2]·2BF4
[FeL3]·2ClO4
[FeL4(CH3CN)]·2BF4
Epa [V]
Epc [V]
DEp [mV]
E = [a] [V]
2
N
0.682
0.677
0.800
0.689
0.619
0.619
0.737
0.626
63
58
63
63
0.651
0.648
0.769
0.658
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
[a] 1 mm in acetonitrile at RT with 0.1m nBu4NClO4 as electrolyte (scan
rate: 100 mVsꢁ1). For more details see the Experimental Section and
Supporting Information.
previously reported 661 mV for [FeL1]·2ClO4.[35] This value
for [FeL1] (SCO) could explain the same prolonged stability
towards air oxidation as we observed for fully low-spin com-
plex [FeL2]. By contrast, the higher redox potential of
[FeL3] (769 mV) indicates the greater inertness conferred
by L3 on the iron(II) center relative to the other ligands
studied in this work, probably stemming from its significant-
ly softer nature, which may fit the iron(II) state more favor-
ably.
lowing approximate values: 1) for water: DH8=26.8–
ꢁ1
29.5 kJmolꢁ1, DS8: 68.1–80.6 Jmolꢁ1 K , and T = =378 K;
1
2
and 2) for acetonitrile: DH8: 23.9–29.7 kJmolꢁ1, DS8: 58.3–
85.8 Jmolꢁ1 K , and T = =371 K (Table 2 and Figure S14 in
ꢁ1
1
2
the Supporting Information). In acetonitrile, the values of
mLS ((0.41ꢅ0.13) BM) and mHS ((5.40ꢅ1.22) BM) suggest a
significant contribution of the orbital factor (ZFS) but are
still within a typical range for iron(II) complexes, albeit with
Thermodynamic characterization of [FeL1] spin variation in
solution: Unlike the solvent-dependent high-spin/low-spin
character of [FeL4] and that of previously reported bispi-
dine–iron(II) complexes, the intermediate magnetic mo-
ments of [FeL1] and their relative insensitivity to the nature
of the solvent may be explained by the presence of a SCO
phenomenon, as already suggested by Comba et al.[35] How-
ever, a partial contribution by a coordination/decoordination
equilibrium cannot be ruled out (Scheme 2).
To gain further insight into the low-spin–high-spin equili-
brium in the solution state of [FeL1], we estimated DH8 and
DS8 of the thermal low-spin–high-spin transition by per-
forming a fit of experimentally determined temperature-de-
pendent solution magnetic moments to the Equation (1).[44]
Table 2. Estimated values of spin equilibrium parameters for [FeL1] in
solution.[a]
[b]
1
Solvent
mLS
mHS
DH8
[kJmolꢁ1
DS8
T =
2
[BM]
[BM]
G
]
[Jmolꢁ1 Kꢁ1
]
[K]
CD3CN
Water
Water
Water
CD3CN
CD3CN
0.41ꢅ0.13
0.00ꢅ0.05
0.06ꢅ0.04
0.00ꢅ0.05
0.46ꢅ0.05
0.41ꢅ0.05
5.40ꢅ1.22
5.02ꢅ0.51
4.90[b]
26.79ꢅ2.93
28.11ꢅ1.34
28.47ꢅ0.42
27.23ꢅ0.22
28.18ꢅ0.55
26.80ꢅ0.53
72.16ꢅ13.60
74.35ꢅ6.28
75.98ꢅ1.28
70.13ꢅ0.68
75.54ꢅ1.67
72.18ꢅ1.59
371
378
375
388
359
371
5.40[b]
4.90[b]
5.40[b]
[a] Fit of experimentally determined magnetic moments at different tem-
peratures to the exponential Equation (1); see the main text and Sup-
porting Information. [b] mHS fixed at the limiting values for magnetic mo-
ments of the high-spin iron(II) ion with S=2 (for more details see the
main text and Supporting Information).[12]
a
relatively high error. In
water, these parameters are lo-
cated close to the spin-only
values (0 and 4.9 BM) with a
lower error than in acetonitrile
(mLS: (0.00ꢅ0.05) BM and mHS
:
(5.02ꢅ0.51) BM). To verify
the reliability of these results
and to estimate the values with
higher precision, we examined
the three-parameter best fits as
previously proposed by Martin
et al.[12] These are obtained by
Scheme 2. Possible spin equilibria for [FeL1] in solution. LS=low spin, HS=high spin.
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Magnetic State of Ferrous Chelates
FULL PAPER
fixing mHS at 4.90 BM (the lower limit of the spin-only value
of high-spin iron(II), estimated on the basis of the spin-only
formula m=
reach a 20% yield upon continued practice by the experi-
menter. This modest value can be explained in part by the
formation of the kinetically favored configurational isomer
(2’-trans-dipyridyl; see the Supporting Information), which
has already been observed for bispidinones.[48] On prolonged
heating, however, the proportion of the desired cis isomer
can be maximized as reported for analogous compounds,[49]
albeit to the detriment of yield. In this context it should be
stressed that great amounts (40 g and more) of the starting
material 1 can be safely introduced into this bicycle-forming
step because of the ease of preparation of 1. The respective
multicomponent reaction to form intermediate 1 starts from
simple precursors (2-pyridylcarbaldehyde, 2-picolylamine,
dimethyl acetone-1,3-dicarboxylate) and is high yielding (ꢀ
80%). We were thus able to prepare batches of more than
10 g of 2. Straightforward deprotection of bispidinone 2 by
1
2
2[S
A
expected for the 5T2 state). The values of thermodynamic
parameters obtained from this treatment (DH8: 27.0–28.9
and 26.3–28.7 kJmolꢁ1
;
DS8: 69.5–77.3 and 70.6–
77.2 Jmolꢁ1 Kꢁ1 for water and acetonitrile, respectively) are
consistent with the results of the four-parameter fit but sig-
nificantly more precise, thereby increasing its reliability. Our
rough estimation of enthalpy and entropy of the spin-transi-
tion process in solution places the complex [FeL1] within
the typical range cited for iron(II) chelates exhibiting an in-
termediate magnetic moment in solution (DH=(30ꢅ
15) kJmolꢁ1, DS=(70ꢅ22) Jmolꢁ1 Kꢁ1).[9] For a truly “un-
1
5
contaminated” A1g$ T2g SCO in octahedral ferrous com-
plexes, the enthalpy should usually be found between 15 and
25 kJmolꢁ1, a value that increases when another phenomen-
on such as coordinative isomerism is involved.[47] In the case
of [FeL1], the observed values slightly exceed the upper
limit, which suggests that a small contribution from a coordi-
nation–decoordination equilibrium is involved.
treatment with trifluoroacetic acid (TFA) in dichloro
ane then leads to intermediate L4 with a 56% yield of the
isolated product.
ACHTUNGTRENNUNGmeth-
AHCTUNGTRENNUNG
Finally, introduction of the pyridazinylmethyl and oxadia-
zolmethyl groups requires access to the respective chlorides.
Although the latter chloride is commercially available, the
former can be prepared readily by chlorination of commer-
cially available methylpyridazine with trichloroisocyanuric
acid.[50] Alkylation of ligand L4 in position N-7 by these
chlorides over 24–72 h furnishes L2 and L3, which can be
purified by flash column chro-
Ligand and chelate synthesis: Hexadentate bispidine ligands
L2 and L3 were prepared via a common intermediate, pen-
tadentate L4 (Scheme 3). A protecting-group strategy, rarely
matography on neutral alumi-
na. Such an alkylation is also
an alternative route to L1 by
using picolyl chloride. The cor-
responding chelates were pre-
pared at room temperature in
dry, degassed solvents (aceto-
nitrile for [FeL1], [FeL2],
[FeL3], and [FeL4
and methanol for [FeL4
over 1–12 h by using the fol-
lowing salts: Fe(BF4)2·6H2O
for [FeL1], [FeL2], and [FeL4-
(CH3CN)]; Fe(ClO4)2·xH2O
for [FeL3]; and FeSO4·7H2O
for [FeL4(SO4)]. Importantly,
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
completion of the reaction was
monitored by mass spectrome-
Scheme 3. Preparation of the ligands L1–L4. DMB=dimethoxybenzyl, TFA=trifluoroacetic acid, DIEA=di-
try. The fairly pure precipitates
ACHTUNGTRENNUNGisopropylethylamine.
thus obtained were then re-
crystallized by liquid/liquid dif-
seen in multidentate ligand synthesis, allowed us to obtain
the three ligand variations in just one simple alkylation step
so as to carry through as much of the precious precursor
(L4) as possible. However, this measure forced us to careful-
ly adapt the synthetic protocol reported for L1[35] to permit
the introduction of a dimethoxybenzyl (DMB) protecting
group instead of the picolyl group. The double Mannich re-
action from piperidinone 1 to protected bicyclic intermedi-
ate 2 is associated with low to modest yields, but can reliably
fusion of diethyl ether into solutions in acetonitrile/metha-
nol to obtain samples suitable for X-ray analysis and for all
other characterization.
Conclusion
We have prepared the first known low-spin, diamagnetic,
ferrous bispidine complexes, both in solution and in the
Chem. Eur. J. 2013, 00, 0 – 0
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Hasserodt et al.
“Laboratoire des Multimatꢂriaux et Interfaces (UMR 5615)” at Univer-
sitꢂ Claude Bernard Lyon 1 (France) by using a SQUID magnetometer
(Quantum Design model MPMS-XL) in an applied magnetic field of
0.1 T. Compounds [FeL2], [FeL3], and [FeL4] were measured by using a
capsule in the temperature range 100–300 K. Compound [FeL1] was
measured in an aluminum sample holder to allow heating at 400 K. All
data were corrected for the contribution of the sample holder and dia-
magnetism of the samples estimated from Pascalꢀs constants.[5,52]
solid phase at room temperature. We initially synthesized
1) a binary compound showing evidence of strain induced in
parts of the ligand ([FeL1]·2BF4), and 2) a ternary one that
comprises an extra monodentate ligand ([FeL4-
ACHTUNGTRENNUNG(CH3CN)]·2BF4). However, in water these complexes adopt
a spin equilibrium with partial population of the high-spin
version; unsurprisingly, this tendency becomes more pro-
nounced with increasing temperature. For the ternary com-
plex, simple ligand exchange is responsible for this effect,
whereas the intermediate spin of binary [FeL1] most likely
results from true thermal SCO with potentially some residu-
al contribution from ligand exchange. We then prepared
hexadentate bispidine ligands (L2 and L3) that do not suffer
induction of strain upon complexation with iron(II), and
which therefore give perfectly low-spin iron(II) complexes
even in water. We thus show that the electronic spin of
iron(II) can be fully or partially muted in solution, as well
as in the solid state, by the judicious choice of an appropri-
ate bispidine derivative. The high stability of this suite of
constitutionally related chelates bodes well for their future
use in the design of responsive off-on probes; only a handful
of systems can claim a similar performance. Our bispidine
platform should thus find a wide range of solution-phase ap-
plications in the biomedical and technological arena.
Crystallographic data: CCDC-902550 (L1), CCDC-902551 (L3), CCDC-
902554 ([FeL1]), CCDC-902552 ([FeL2]), CCDC-904020 ([FeL3]),
CCDC-902553 ([FeL4ACTHNUTRGENNUG(CH3CN)]), CCDC-902296 ([FeL4AHCUTGNTREN(NNGU SO4)]) contain
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Compound [FeL1]·2BF4: Chemical formula: C33H38B2F8FeN6O8; Mr =
876.16 gmolꢁ1; crystal dimensions 0.105ꢅ0.179ꢅ0.473 mm; monoclinic;
space group P21/c; unit cell dimensions a=10.3903(6), b=18.170(1), c=
19.079(1) ꢄ; a=90, b=101.85(1), g=908; V=3525.2(3) ꢄ3; Z=4; 1calcd
=
1.651 mgmꢁ3
; ; MoKa radiation, l=0.7107 ꢄ; T=100 K;
m=0.53 mmꢁ1
qmax =29.58, qmin =3.48; no. of measured reflections: 74517; no. of inde-
pendent reflections: 9196; Rint =0.055; RACTHNUTRGNEU[GN F2>2s(F2)]=0.039; wR(F2)=
0.090; D1max =0.58 eꢄꢁ3; D1min =ꢁ0.60 eꢄꢁ3
.
Compound [FeL2]·2BF4: Chemical formula: C34H38B2F8FeN8O7; Mr =
900.18 gmolꢁ1
; crystal dimensions 0.179ꢅ0.248ꢅ0.349 mm; triclinic;
¯
space group P1; unit cell dimensions a=10.6244(6), b=12.3593(7), c=
15.2033(9) ꢄ; a=103.178(5), b=103.007(5), g=98.123(5)8; V=
1854.5(2) ꢄ3; Z=2, 1calcd =1.612 mgmꢁ3; m=0.51 mmꢁ1; MoKa radiation,
l=0.7107 ꢄ; T=110 K; qmax =29.58, qmin =3.58; no. of measured reflec-
tions: 40829; no. of independent reflections: 9362;
R
;
int =0.042;
D1min
R
A
=
ꢁ0.76 eꢄꢁ3
.
Experimental Section
Compound [FeL3]·2ClO4: Chemical formula: C36H35Cl2FeN7O15; Mr =
932.45 gmolꢁ1
; crystal dimensions 0.096ꢅ0.195ꢅ0.289 mm; trigonal;
¯
space group R3, unit cell dimensions a=23.982(2), b=23.982(2), c=
39.047(3) ꢄ; a=90, b=90, g=1208; V=19449(3) ꢄ3; Z=18; 1calcd
1.433 mgmꢁ3 m=0.54 mmꢁ1
MoKa radiation, l=0.7107 ꢄ; T=100 K;
qmax =29.48, qmin =3.68; no. of measured reflections: 54907; no. of inde-
General materials and procedures: Reagents and solvents were pur-
chased from Aldrich, Acros, and Alfa Aesar and used without further pu-
rification. Solvents were dried by standing the commercial, dry HPLC-
grade solvents for 24 h on thermally activated molecular sieves (3 ꢄ for
MeOH and EtOH, 4 ꢄ for CH3CN and Et2O, sieve activation by 24 h of
heating at 3158C) and degassed by a freeze–pump–thaw method (se-
quence repeated 3–5 times) for complexation reactions. All NMR spectra
=
;
;
pendent reflections: 11013; Rint =0.099, RACTHNUTRGNE[NUG F2>2s(F2)]=0.120; wR(F2)=
0.225; D1max =2.33 eꢄꢁ3; D1min =ꢁ1.87 eꢄꢁ3
.
Compound
[FeL4
(CH3CN)]·2BF4:
Chemical
formula:
C34H38B2F8FeN8O6; Mr =872.17 gmolꢁ1; crystal dimensions 0.123ꢅ0.287ꢅ
were acquired on
a Bruker AVANCE 500 instrument (500.10 and
125.76 MHz for 1H and 13C, respectively) at 298 K (unless otherwise
stated). Chemical shifts (d) are reported in ppm (s=singlet, d=doublet,
t=triplet, m=multiplet, br=broad, eq.=equatorial, ax.=axial) and ref-
erenced to the residual solvent peak. NMR coupling constants (J) are re-
ported in hertz (Hz). UV/Vis spectra were recorded on a V-670 Jasco
spectrophotometer. High-resolution mass spectrometry measurements
and elemental analyses were performed at the “Centre Commun de
Spectrometrie de Masse” of the University Claude Bernard in Lyon
(France) and the Service Central dꢀAnalyse of the CNRS in Solaize
(France). Cyclic voltammetry (CV) experiments were conducted in a
standard one-compartment, three-electrode electrochemical cell with a
biologic ESP300 potentiostat. Tetra-n-butylammonium perchlorate
(TBAP) was used as supporting electrolyte (0.1m) in acetonitrile. A vitre-
ous carbon working electrode (Ø=3 mm, ALS Co.) was polished with
1 mm diamond paste before each recording. The counter electrode was a
Pt wire. Electrode potentials are referred to a Ag/AgNO3 reference elec-
trode (ALS Co., 0.01m AgNO3, 0.1m TBAP in acetonitrile). The concen-
trations of the complexes were established around 1 mm and the meas-
urements were performed at five different scan rates (50, 100, 150, 200,
250 mVsꢁ1), all giving similar reversible cyclovoltammograms. Only crys-
talline batches of bispidine ferrous complexes were used.
¯
0.362 mm; triclinic; space group P1; unit cell dimensions a=11.0566(8),
b=13.7715(9), c=14.718(1) ꢄ; a=66.375(6), b=69.925(6), g=
71.325(6)8; V=1884.5(2) ꢄ3; Z=2, 1calcd =1.537 mgmꢁ3; m=0.50 mmꢁ1
;
MoKa radiation, l=0.7107 ꢄ; T=100 K; qmax =29.58, qmin =3.58; no. of
measured reflections: 38176; no. of independent reflections: 9388; Rint
=
0.050, RACHTUNGTRNEUNG
[F2>2s(F2)]=0.060; wR(F2)=0.111; D1max =1.79 eꢄꢁ3; D1min
=
ꢁ0.85 eꢄꢁ3
.
Compound [FeL4ACHTNUTGRNEUN(G SO4)]: Chemical formula: C57H70Fe2N10O23S2; Mr =
1439.07 gmolꢁ1; crystal dimensions 0.030ꢅ0.242ꢅ0.293 mm; orthorhom-
bic; space group Pbca; unit cell dimensions a=16.741(3), b=17.538(2),
c=20.854(2) ꢄ; a=90, b=90, g=908; V=6123.0(1) ꢄ3; Z=4, 1calcd
=
1.561 mgmꢁ3
; ; CuKa radiation, l=1.5418 ꢄ; T=150 K;
m=5.22 mmꢁ1
qmax =66.78, qmin =3.48; no. of measured reflections: 24753; no. of inde-
pendent reflections: 5387; Rint =0.062; RACTHNUTRGNEU[GN F2>2s(F2)]=0.065; wR(F2)=
0.138; D1max =1.90 eꢄꢁ3; D1min =ꢁ0.69 eꢄꢁ3
.
Synthesis: Compound 1 was synthesized according to known proce-
dures.[53] Preparation of new bispidinone 2 and already reported ligand
L1[35] was performed by modifying the reported methodology. Ligands L2
and L3 were synthesized by alkylation of compound L4 by standard pro-
cedures. All complexations were performed under inert argon atmos-
pheres and with dry and degassed HPLC-grade solvents. For more details
please see the Supporting Information.
Magnetic susceptibilities in solution were determined by the Evans
method[38, 51] by using a coaxial NMR tube and tBuOH (2%) as refer-
ence.[3,44] Varied-temperature magnetic susceptibilities were corrected
with respect to the effect of solvent volume expansion/contraction upon
heating/cooling;[40,41] for more details see the text and Supporting Infor-
mation. Solid-state magnetic susceptibility data were collected in the
Intermediate 2: Formaldehyde (37% in H2O; 17.47 g; 215.3 mmol;
2.3 equiv) and dimethoxybenzylamine (18.00 g; 107.6 mmol; 1.15 equiv)
were added to a solution of piperidinone 1 (43.10 g; 93.6 mmol) in etha-
nol (600 mL) at 608C. The reaction mixture was stirred under reflux for
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Magnetic State of Ferrous Chelates
FULL PAPER
8 h and all volatiles were removed under reduced pressure. The residue
was dissolved in a minimal quantity of methanol (50 mL) with the help
of heating and was then treated with diethyl ether (150 mL). A white
crystalline solid appeared after several hours at room temperature, was
isolated by filtration, and then washed three times with diethyl ether. A
repeat of the procedure on the resulting filtrate yielded additional mate-
rial, which was combined with the first crop to give a total of 12.89 g
(21%) of pure product. M.p. 187–1908C; 1H NMR (500 MHz, CDCl3):
the desired complex. 1H NMR (500 MHz, D2O): d=3.58 (6H, CO2CH3),
5.65 (2H, s; CH2-6/8 eq.), 6.65 (1H, s; py1-H5), 7.85 (2H, s; py2/3-H5),
8.09 (1H, s), 9.5 (1H, s), 9.78 (2H, s), 10.73 (1H, s), 11.10 (4H, s), 11.61–
11.73 (3H, m), 13.20 (1H, s), 14.98 (1H, s), 18.54 (3H, brs; py1/2/3-H3),
20.57–21.28 ppm (4H, m; CH-2/4, CH2-6/8 eq.); UV/Vis (0.1 mm sol in
water): lmax (e)=554 (334), 458 (5560), 403 (4680), 380 (4390), 253 nm
(13296mꢁ1 cmꢁ1); SQUID suggests SCO, moderately paramagnetic; solu-
tion-state meff (H2O, 5 mm, 298 K): 1.47 BM; solution-state meff (CH3CN,
d=2.49 (2H, d, 2J(H,H)=11.6 Hz; CH2-6/8 eq.), 3.07 (2H, d, 2J
ACHTUNGTRENNUNG AHCTUNGTRNEN(UGN H,H)=
5 mm, 298 K): 1.77 BM; CV (E = , CH3CN, 100 mVsꢁ1): 651 mV; HRMS
1
2
(ESI): m/z calcd for C33H34FeN6O6: 333.0939 [M2+]; found: 333.0934; ele-
mental analysis calcd (%) for C33H34B2F8FeN6O6·2H2O: C 45.24, H 4.37,
11.9 Hz; CH2-6/8 ax.), 3.37 (2H, s; N7-CH2Ar), 3.66 (2H, s; N3-CH2py),
3.78 (6H, s; CO2CH3), 3.86 (3H, s; Ar4-OCH3), 3.98 (3H, s; Ar2-OCH3),
5.35 (2H, s; CH-2/4), 6.43 (1H, dd, 3J(H,H)=8.12, 4J
ACTHNUGTRENNUGN ACHTUNGTRENNNUG
N 9.59; found: C 45.53, H 4.24, N 9.66.
Ar-H5), 6.59 (1H, d, 4J
N
ACHTUNGTRENNUNG
AHCTUNGTRE[GNUNN FeL2]·2BF4: The ligand (100 mg; 0.168 mmol) was dissolved in de-
A
ACHTUNGTRENNUNG
gassed, anhydrous ethanol (10 mL) and a solution of iron(II) tetrafluoro-
borate hexahydrate (60 mg; 0.140 mmol) in anhydrous, degassed ethanol
(2 mL) was added dropwise with stirring under argon. The solution im-
mediately turned dark brown and a precipitate appeared at the end of
the addition. The reaction was continued for 4 h until the total consump-
tion of the ligand was observed (by MS analysis), and the precipitate was
isolated by filtration under argon, and washed several times with ethanol
and diethyl ether to yield pure product (102 mg) in the form of a dark
brown powder. It was then recrystallized by a liquid/liquid diffusion of di-
ethyl ether into a wet (5% v/v of H2O) acetonitrile solution of the prod-
uct. Dark brown crystals suitable for X-ray analysis were isolated from
the mother liquor, washed with ethanol, and dried to yield 52 mg (36%)
of a crystalline product used for further analyses. Treating the remaining
AHCTUNGTRENNUNG
G
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
8.51 ppm (3H, m; py2/3-H3, py1-H3); jmod 3C NMR (126 MHz, CDCl3):
d=52.5 (CO2CH3), 55.5 (Ar3-OCH3), 55.5 (Ar5-OCH3), 55.8 (N3-
CH2py1), 58.0 (N7-CH2Ar), 59.4 (C-6/8), 62.2 (C-2/4), 70.6 (C-1/5), 98.7
(Ar-C3), 103.8 (Ar-C5), 117.6 (Ar-C1), 121.6 (py1-C4), 122.6 (py2/3-C4),
124.2 (py1-C6), 124.7 (py2/3-C6), 133.1 (Ar-C6), 135.5 (py1-C5), 135.6
(py2/3-C5), 148.8 (py2/3-C3), 149.3 (py1-C3), 156.4 (py1-C1), 158.5 (Ar-
C2), 159.2 (Ar-C4), 160.7 (py2/3-C1), 168.8 (CO2CH3), 204.1 ppm (C-9);
ꢁ
IR (solution in dichloromethane) nmax =3051–2835 (C H), 1738 (C=O),
1612 (Ar), 1589 (Ar), 1510 cmꢁ1 (Ar); HRMS (ESI): m/z calcd for
C36H38N5O7: 652.2766 [M+H+]; found: 652.2768; elemental analysis
calcd (%) for C36H38N5O7: C 66.35, H 5.72, N 10.75; found: C 65.72, H
6.01, N 10.38.
1
mother liquor may lead to an additional crop. H NMR (500 MHz, D2O):
d=2.74 (2H, d, J=13.29 Hz; CH2-6/8 eq.), 3.22 (2H, d, J=13 Hz; CH2-6/
8 ax.), 3.78 (6H, s; -CO2CH3), 4.43 (2H, s; N-CH2), 4.97 (2H, s; N-CH2),
5.78 (2H, s; CH-2/4), 6.84 (1H, d, J=7.79 Hz; CHarom), 7.12–7.15 (3H,
m; CHarom), 7.36–7.43 (3H, m; CHarom), 7.67–7.72 (3H, m; CHarom), 7.86
(1H, dd, J=8.25, 5.04 Hz; CHarom), 8.28 (2H, d, J=5.50 Hz; py2/3-H3),
9.58 (1H, d, J=3.67 Hz; py1-H4), 10.18 ppm (1H, d, J=5.96 Hz; pdz-
H4); UV/Vis (0.1 mm in water): lmax (e)=455 (10870), 366 (3479),
253 nm (13770mꢁ1 cmꢁ1); SQUID indicates low-spin, diamagnetic (gꢆ0);
Ligand L4: Trifluoroacetic acid (20 mL) was added to a solution of 2
(4.03 g; 6.18 mmol) in dry dichloromethane (100 mL). The reaction vessel
was sealed and the mixture was stirred for 12 h at 35–408C. Then the re-
action was basified on cooling with 2m sodium hydroxide solution and
the organic phase was separated. The water phase was extracted with di-
chloromethane (2ꢅ150 mL) and the organic fractions were combined.
After evaporation of the solvent under reduced pressure, the residue was
dissolved in a minimal quantity of hot acetonitrile and the solution was
filtered before precipitating the desired product. After 1 h at room tem-
perature, the resulting white solid was isolated by filtration and washed
with isopropanol and diethyl ether. Working up the filtrate led to addi-
tional material, which was combined with the first crop to yield 1.75 g
(56%) of pure product. M.p. 191–1928C; 1H NMR (500 MHz, CDCl3):
d=3.28 (2H, t, J=13.68 Hz; CH2-6/8 eq.), 3.50 (2H, s; N3-CH2py1), 3.64
(6H, s; -CO2CH3), 4.19 (2H, d, J=12.82 Hz; CH2-6/8 ax.), 4.59 (1H, brs,
1H; N7-H), 5.28 (s, 2H; CH-2/4), 6.93 (d, J=7.69 Hz, 1H; py1-H6), 6.99
(dd, J=6.63, 4.92 Hz, 1H; py1-H4), 7.14–7.16 (m, 2H; py2/3-H4), 7.33 (d,
J=6.84 Hz, 2H; py2/3-H6), 7.42 (td, J=7.69, 1.71 Hz, 1H; py1-H5), 7.60
(td, J=7.69, 1.71 Hz, 2H; py2/3-H5), 8.31 (d, J=4.27 Hz, 1H; py1-H3),
8.60 ppm (d, J=4.70 Hz, 2H; py2/3-H3); 13C NMR (126 MHz, CDCl3):
d=52.1 (-CO2CH3), 55.4 (C-6/8), 56.8 (N3-CH2py1), 64.6 (C-2/4), 70.6
(C-1/5), 121.4 (py1-C4), 123.1 (py2-C4, py3-C4), 124.0 (py1-C6), 125.8
(py2-C6, py3-C6), 135.4 (py1-C5), 136.1 (py2-C5, py3-C5), 148.4 (py1-
C3), 149.7 (py2-C3, py3-C3), 156.8 (py2-C1, py3-C1), 157.8 (py1-C1),
168.9 (-CO2CH3), 203.2 ppm (C-9); IR (solution in dichloromethane):
1
solution-state meff (H2O, 5 mm, 298 K): 0.59 BM; CV (E = , CH3CN,
2
100 mVsꢁ1): 648 mV; HRMS (ESI) calcd for C32H33FeN7O5: 333.5916
[M2+]; found: 333.5910; elemental analysis calcd (%) for
C32H33B2F8FeN7O6·H2O: C 44.74, H 4.11, N 11.44; found: C 44.31, H
4.16, N 11.83.
AHCTUNGTRE[GNNNU FeL3]·2ClO4: The ligand L3 (128 mg; 0.193 mmol) was dissolved in de-
gassed, anhydrous acetonitrile (20 mL) and a solution of iron(II) perchlo-
rate hydrate (70 mg; 0.256 mmol) in anhydrous, degassed acetonitrile
(2 mL) was added dropwise with stirring under argon. The solution im-
mediately turned dark brown and a precipitate appeared after 1 h. The
reaction was continued with heating to approximately 508C overnight to
ensure the full consumption of the ligand (by direct injection MS analy-
sis). The resulting precipitate was washed with diethyl ether, redissolved
in wet acetonitrile (5% v/v of H2O), and set up for vapor diffusion of di-
ethyl ether. Volatiles were removed from the filtrate and the residue was
set up for crystallization as above. Both batches together yielded 45 mg
(25%) of a crystalline (dark brown) material suitable for X-ray analysis,
which was used for further characterizations. 1H NMR (500 MHz, D2O):
d=2.90 (2H, d, J=13.38 Hz; CH2-6/8 eq.), 3.61 (2H, q, J=7.14 Hz; CH2-
6/8 ax.), 3.78 (6H, s; -CO2CH3), 4.56 (2H, s; N-CH2), 5.09 (2H, s; N-
CH2), 5.89 (2H, brs; CH-2/4), 6.99 (1H, d, J=8.03 Hz; CHarom), 7.22
(2H, t, J=6.69 Hz; CHarom), 7.36 (1H, brs; CHarom), 7.44 (2H, d, J=
7.36 Hz; CHarom), 7.56 (1H, t, J=7.70 Hz; CHarom), 7.75 (4H, t, J=
7.70 Hz; CHarom), 7.84 (2H, t, J=7.36 Hz; CHarom), 8.35–8.42 (2H, m;
CHarom), 8.52 (2H, brs; py2/3-H3), 9.47 ppm (1H, brs; py1-H3); UV/Vis
(0.1 mm in water): lmax (e)=532 (774), 443 (12570), 254 nm
(28704mꢁ1 cmꢁ1); SQUID indicated low-spin, diamagnetic (gꢆ0); solu-
ꢁ
nmax =3311 (NH), 3053–2852 (C H), 1732 (C=O ester), 1710 (C=O
ketone), 1589 (Ar), 1570 cmꢁ1 (Ar); HRMS (ESI): m/z calcd for
C27H28N5O5: 502.2085 [M+H+]; found: 502.2069; elemental analysis
calcd (%) for C36H38N5O7: C 64.66, H 5.43, N 13.96; found: C 64.68, H
5.43, N 13.72.
ACHTUNGTRENNUNG[FeL1]·2BF4: The ligand (253 mg, 0.427 mmol) was dissolved in degassed,
anhydrous acetonitrile (10 mL) and a solution of iron(II) tetrafluorobo-
rate hexahydrate (151 mg; 0.448 mmol) in anhydrous, degassed acetoni-
trile (5 mL) was added dropwise with stirring under argon. Upon addi-
tion, the solution immediately turned dark brown. The reaction was con-
tinued for 4 h until no more ligand signal was observed by direct injec-
tion mass spectrometry. Then, approximately 5% v/v of degassed water
was added and the solution was set up for slow gas diffusion of diethyl
ether. The resulting dark red (almost black) crystalline material ready for
X-ray analysis was collected, washed, and dried to yield 278 mg (71%) of
1
tion-state meff (H2O, 5 mm, 298 K): 0.54 BM; CV (E = , CH3CN,
2
100 mVsꢁ1): 769 mV; HRMS (ESI): m/z calcd for C36H35FeN7O7:
366.5968 [M2+]; found: 366.5976; elemental analysis calcd (%) for
C36H35B2F8FeN7O7·2H2O: C 45.84, H 4.17, N 10.40; found: C 45.71, H
3.96, N 10.51.
Chem. Eur. J. 2013, 00, 0 – 0
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E
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0.217 mmol) in methanol (4 mL) was added dropwise to a heated (608C)
solution of the ligand (109 mg; 0.217 mmol) in dry, degassed methanol
(8 mL). The solution turned yellow and after 30 min at 608C, no more
ligand was observed by mass spectrometry, thus indicating that the reac-
tion had finished. Hence, it was stopped and approximately 5% v/v of de-
gassed water was added, followed by the addition of diethyl ether. After
several days pale-yellow square-like cubic crystals suitable for X-ray
analysis were formed and collected to yield 65 mg (39%) of the pure
product. Rf =0.37 (Al neutral, preconditioned in MeOH/CH3COOꢁNH4
(0.1m in water) 3:1; run in CHCl3/MeOH 12.5:1; pure by UV/Vis light
and after SCNꢁ staining); 1H NMR (500 MHz, D2O): d=3.97 (s), 22.96
(s), 23.64 (s), 30.65 (v br), 42.14 (s), 44.12 (s), 50.12 (s), 152.62 (v br),
191.16 (v br), 196.29 (v br); UV/Vis (0.25 mm): lmax (e)=414 (1188),
255 nm (10120mꢁ1 cmꢁ1); SQUID indicated high-spin, paramagnetic (g=
2.18); solution-state meff (H2O, 5 mm, 298 K): 4.96 BM; HRMS (ESI): m/z
calcd for C27H29FeN5O6: 287.5728 [M2+]; found: 287.5725; elemental
analysis calcd (%) for C27H29FeN5O10S·2H2O: C 45.84, H 4.70, N 9.90;
found: C 45.93, H 4.63, N 9.58.
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ACHTUNGTRENNUNG[FeL4ACHTUNGTRENNUNG(CH3CN)]·2BF4: A solution of iron(II) tetrafluoroborate hexahy-
drate (105 mg; 0.312 mmol) in dry degassed acetonitrile (1 mL) was
added dropwise under argon to a suspension of the ligand (150 mg;
0.300 mmol) in dry, degassed acetonitrile (3.5 mL) at room temperature.
Upon addition the color of the solution turned deep brown and all solids
were dissolved. After 1.5 h at room temperature, no more ligand was ob-
served on the mass spectrum, so the solution was directly set up for
vapor diffusion of diethyl ether. After a week, crystals suitable for X-ray
analysis were isolated by filtration and washed with isopropanol and di-
ethyl ether to yield 39 mg (17%) of the pure product, which was used for
further characterizations. 1H NMR (500 MHz, [D3]acetonitrile): d=2.36
(2H, d, J=13.05 Hz; CH2-6/8 eq.), 3.13 (2H, d, J=13.72 Hz; CH2-6/8
ax.), 3.74 (6H, s; CO2CH3), 4.67 (1H, brs; NH), 4.76 (2H, s; N3-
CH2py1), 5.02 (1H, s; C9-OH), 5.54 (3H, s; CH-2/4, C9-OH), 6.88 (1H,
d, J=8.03 Hz; py1-H6), 7.17 (1H, t, J=6.53 Hz; py1-H4), 7.24–7.40 (4H,
m; py2/3-H4/6), 7.49 (1H, t, J=7.70 Hz; py1-H5), 7.76 (2H, t, J=
7.53 Hz; py2/3-H5), 8.70 (1H, d, J=5.35 Hz; py1-H3), 8.86 ppm (2H, d,
J=5.02 Hz; py2/3-H3); UV/Vis (0.25 mm in acetonitrile): lmax (e)=521
(772), 444 (7144), 380 nm (3828mꢁ1 cmꢁ1); solution-state meff (H2O, 5 mm,
298 K): 4.16 BM; solution-state meff (CH3CN, 5 mm, 298 K): 0.65 BM; CV
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Acknowledgements
A PhD fellowship (2010–2012) for J.L.K. from La Ligue Contre Le
Cancer is gratefully acknowledged. We thank FayÅal Touti for technical
advice, Oliver Thorn-Seshold for manuscript revision, Sandrine Denis-
Quanquin for assistance with NMR experiments, Philippe Maurin for val-
uable discussions, Christophe Bucher for expert assistance in the cyclic
voltammetry experiments, and Dominique Luneau and Ruben Checa for
SQUID measurements.
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Received: February 16, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
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J. Hasserodt et al.
All under control: Structural variation
of bispidines allows for the preparation
of ferrous complexes that adopt mag-
netic properties of choice under envi-
ronmentally benign conditions in water
at room temperature. Pairs of constitu-
tionally similar, robust chelates with a
binary off–on magnetic relationship
are obtained (see figure).
Chelates
J. L. Kolanowski, E. Jeanneau,
R. Steinhoff, J. Hasserodt* .. &&&& — &&&&
Bispidine Platform Grants Full
Control over Magnetic State of
Ferrous Chelates in Water
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These are not the final page numbers!