Spin Switching with Triazolate-Strapped Ferrous Porphyrins

Article abstract of DOI:10.1021/acs.inorgchem.9b00349

Fe(III) porphyrins bridged with 1,2,3-triazole ligands were synthesized. Upon deprotonation, the triazolate ion coordinates to the Fe(III) ion, forming an overall neutral high-spin Fe(III) porphyrin in which the triazolate serves both as an axial ligand a

Full text of DOI:10.1021/acs.inorgchem.9b00349

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Spin Switching with Triazolate-Strapped Ferrous Porphyrins
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Morten K. Peters, Sebastian Hamer, Torben Jakel, Fynn Rohricht, Frank D. Sonnichsen,
Carolina von Essen,§ Manu Lahtinen,§ Christian Naether,‡ Kari Rissanen,*,§ and Rainer Herges*,†
†Otto-Diels-Institut fu
̈
r Organische Chemie, Christian Albrechts-Universitat, Otto-Hahn-Platz 4, 24098 Kiel, Germany
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‡
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Anorganische Chemie, Christian-Albrechts-Universitat, Max-Eyth-Strasse 2, 24118 Kiel, Germany
§
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University of Jyvaskyla, Department of Chemistry, P.O. Box 35, 40014 Jyvaskyla, Finland
S
* Supporting Information
ABSTRACT: Fe(III) porphyrins bridged with 1,2,3-triazole ligands were synthesized. Upon deprotonation, the triazolate ion
coordinates to the Fe(III) ion, forming an overall neutral high-spin Fe(III) porphyrin in which the triazolate serves both as an
axial ligand and as the counterion. The second axial coordination site is activated for coordination and binds p-methoxypyridine,
forming a six-coordinate low-spin complex. Upon addition of a phenylazopyridine as a photodissociable ligand, the spin state of
the complex can be reversibly switched with ultraviolet and visible light. The system provides the basis for the development of
switchable catalase- and peroxidase-type catalysts and molecular spin switches.
[Fe(III)porphyrin, the counterion, two oxygen ligands, and two
nitrogen ligands] that are in equilibrium in solution.
Notwithstanding the high switching efficiency (76%), caused
by the multicomponent character, the system is quite sensitive to
changes in composition and environment. Here we report a
novel two-component system, which includes a covalent
triazole-porphyrin unit and only one separate ligand.
Fe(III)tetraphenylporphyrin chloride (FeTPPCl) is commer-
cially available and in principle meets the structural require-
ments for spin switching. The FeTPPCl is five-coordinate and a
high-spin complex with a chloride ion serving as the counterion
and axial ligand (Figure 2).19 However, the chloride ligand is
strongly bound, and the second axial coordination site binds
only very strong nitrogen ligands, such as 1-methylimidazole
(Figure 2a). Moreover, the mixed chloride/imidazole complex is
still a high-spin complex.20,21 Upon replacement of the chloride
ligand in FeTPPCl with azide (FeTPPN3), the iron(III) ion is
closer to its spin crossover point. The azide complex FeTPPN3 is
also five-coordinate and high-spin, but in contrast to FeTPPCl, it
is considerably more apt to bind a second axial ligand, leading to
a low-spin complex (Figure 2b).9,22,23
INTRODUCTION
■
Ferrous porphyrins with anionic ligands serving both as an axial
ligand and as counterions are abundant in nature. They act as
catalysts in selective CH activation (cytochrome P450),1−4
hydrogen peroxide decomposition (catalases),5 oxidation of a
variety of substrates with hydrogen peroxide (peroxidases),6 and
a number of further biologically important processes (Figure
1).7−9
Controlled spin switching is essential for the function of these
enzymes.10 During its catalytic cycle, cytochrome P450 switches
between the six-coordinate low-spin resting state and the five-
coordinate high-spin state.11 Spin state control, particularly of
iron complexes, is also a prerequisite in a number of further
biocatalytic processes such as C−H activation by methane
monooxygenases.12 The design of artificial systems whose spin
states are responsive to external stimuli is the first step toward
molecular machine-type catalysts13 that perform particularly
difficult processes such as conversion of hydrocarbons to
alcohols under ambient conditions.14 Single-molecule spin state
switching is also the prerequisite to molecular spintronics,15
responsive contrast agents,16,17 and a number of further
applications.18 Recently, we presented the first artificial ferrous
porphyrin system that changes its spin state between high-spin
5
1
(S = /2) and low-spin (S = /2) states upon irradiation with
Received: February 7, 2019
light.19 However, the complex consists of six components
© XXXX American Chemical Society
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Figure 1. Heme units of ferrous porphyrin-containing enzymes.
Figure 2. Spin states of Fe(III)porphyrins with one anionic axial ligand, with and without 1-methylimidazole as the second axial ligand: (a) with a
chloride ligand (1), (b) with azide (2), and (c) schematic representation of a ferrous porphyrin strapped with an anionic ligand (target structure for
spin switching).
Figure 3. Coordination of Fe(III)porphyrin chloride with triazole (DMTDH) and triazolate (DMDT−). (a) FeTPPCl 1 reacts with triazolate but not
with triazole to form a 2:1 complex. (b) Strapped porphyrin 3 does not react with the triazolate anion.
However, FeTPPN3 2 has several disadvantages. The azide
ligand is easily replaced with Cl− forming FeTPPCl 1 and with
OH− leading to the μ-oxo compound (FeTPP)2O.24 To avoid
these problems, we designed a strapped porphyrin including an
anionic ligand L− in the bridge (Figure 2c), complying with the
following preconditions. (1) L− should be strongly binding. (2)
B
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The five-coordinate complex should be high-spin. (3) The free
axial coordination site should bind ligands with a low or medium
ligand field strength. (4) Binding of the second axial ligand
should induce a spin switch to the low-spin state.
RESULTS AND DISCUSSION
■
Several Fe(III)porphyrins with covalently linked anionic ligands
are known.25−29 However, strapped anionic nitrogen ligands
have not been realized so far. Among the noncovalently
coordinated nitrogen ligands, imidazolate, isocyanate,30 and
azide22 are the most common.31 Unfortunately, azide and
isocyanate cannot be tethered to the porphyrin because there is
no free valence available to form a covalent bond. Imidazolate
anions suffer from the fact that only one position is available to
attach a tether (the C atom distant from the coordinating N
atom) and imidazolates are easily protonated (pKa = 14.3);32
hence, the corresponding complexes would not be water stable.
We therefore considered triazoles as anionic ligands for
strapping from positions 4 and 5. The parent triazole has a
pKa of 8.8.33 The 4,5-diester substituted triazole unit should be
considerably more acidic (pKa ∼ 5) and when coordinated to
Fe(III) should be inert to protonation (water stable) at neutral
pH.
Figure 4. PBE/def2svp-optimized structures of triazolate-strapped
target compounds (b) 7 and (c) 8 and (a) a structural reference with a
corresponding non-covalently attached ligand. Hydrogen atoms and
the meso 2,5-dichlorophenyl substituents have been omitted for the sake
of clarity. The bond lengths of the Fe−N coordination bonds indicate
the accuracy of the fit of the tether lengths. The reference compound
exhibits the optimum Fe−N distance because the porphyrin and the
ligand were optimized without further geometry constraints. The
reference compound (a) was constructed by cutting the structure in
panel b at the wavy lines and complementing the free valences by
methyl groups.
To test whether triazolates are suitable anionic ligands for an
integration into the bridge of a strapped porphyrin, we
performed preliminary experiments with 4,5-dimethyl-1,2,3-
triazole-4,5-dicarboxylate (DMTDH) (Figure 3). In a prelimi-
nary experiment, a solution of iron(III) tetraphenylporphyrin
chloride (FeTPPCl 1, 100 μM) in acetonitrile was treated with
an 1000-fold excess of the 4,5-dimethyl-1,2,3-triazole-4,5-
dicarboxylate (DMTDH). Changes in nuclear magnetic
resonance (NMR) or ultraviolet (UV) spectra were not
observed, indicating that there is no coordination of the free
ligand DMTDH to the Fe(III) ions. Obviously, the coordination
power of neutral DMTDH is insufficient for coordination.
However, upon deprotonation of the triazole (DMTDH) with
solid K2CO3, the color of the FeTPPCl/DMTDH solution
changes from brown to yellow and the UV−visible (UV−vis)
spectrum exhibits characteristic absorption bands of low-spin
FeTPPL2 (550 nm, 575 nm shoulder, 640 nm) (Figure 3; see
Figure S2).34 Resonances in the NMR spectrum at −14 ppm
further confirm the [FeTPP(DMTD)2]− low-spin state. Density
functional theory (DFT) calculations verify that the triazolate
anion coordinates from its nitrogen at position 2 to the Fe(III)
ion forming a symmetrical complex (Figure S1). An FeTPP-
(DMTD) (1,1 complex) is very unlikely, because it is known
that FeTPP+ cations with one anionic nitrogen ligand are high-
spin (see Figure 3a).10,11,35−37 The triazolate (DMTD−),
however, does not coordinate to a strapped, Fe(III)porphyrin
6 whose second axial coordination side is sterically shielded
(Figure 3b).8 This lack of reactivity proves that the triazolate has
to attack from the opposite axial side to replace the chloride
ligand. We conclude that a strapped triazolate should thus bind
to the Fe(III) ion.
coordination bond, and compound 8 with the longer tether has a
longer Fe−N distance. Obviously, the tether in structure 7 is
somewhat too small, pulling the triazolate ligand toward the
Fe(III) ion, while the tether in 8 is slightly too long. A
conformational analysis of 7 revealed that the tethers are still
flexible. The O−CH2−CH2−O units adopt gauche and anti
conformations, which are similar in energy (Figure S1).
Nevertheless, the geometry fit is quite good, and 7 and 8 were
selected as target structures for synthesis.
Conceptually, three strategies for synthesizing strapped
porphyrins (capped,38 hindered,1 or bridged porphyrins39) are
possible: (a) bridging a prefabricated porphyrin, e.g., reaction of
a 5,15-bis(2-hydoxyphenyl) porphyrin with a chainlike building
block with a leaving group at both ends, and (b and c) using
building blocks that already include the bridge to prepare the
porphyrin (mixed aldehyde synthesis). There are two options
for the latter strategy:1,40 (b) using a bridged bis-aldehyde and 2
equiv of dipyrromethane1,40−42 or (c) using a bridged
bisdipyrromethane and 2 equiv of an aldehyde (see Figure 5).43
Synthetic pathway a was not successful. Therefore, we
pursued strategy b. Salicylaldehyde (9) and chloroethanol
(10a) were reacted to give 2-(2-hydroxyethoxy)benzaldehyde
(11a).44 Acetylene dicarboxylic acid (12) with 11a yields
dialdehyde 13a (85%). Dialdehyde 13a was used in Lindsay-
type cyclization reactions with different 5-substituted dipyrro-
methanes45 to afford acetylene-bridged strapped porphyrins
(see Figure 6, bottom right) with yields of ≤35%.
The yield of 15b is low (1%) because of severe scrambling.46
The main product is tetraphenylporphyrin (TPP). 15c is formed
in a 15% yield. A crystal structure analysis reveals a strained
structure with an approximate C2 symmetry. The acetylene unit
is not exactly linear, and the porphyrin ring is distorted toward a
ruffled-type geometry (see Figure 5 and Figure S7).
We also prepared an analogously strapped porphyrin 15e with
longer −O−CH2−CH2−CH2− bridges (extended by an
additional CH2 group) in a 35% yield, which is more than
double compared to that of 15c, obviously because the longer
bridge imposes less strain. A similar trend albeit with lower yields
was observed with the bis(pentafluoro)-substituted strapped
The DFT model reactions predict that an −O−CH2−CH2−
or −O−CH2−CH2−CH2− chain connecting the ester function
of the DMTDH with the ortho position of the meso phenyl rings
should provide a suitable preorientation of the central nitrogen
atom of DMTD− for coordination with Fe(III) [7 and 8 (panels
b and c of Figure 4, respectively)]. As compared to a structurally
similar reference compound (Figure 4a) with an unconstrained
triazolate ligand and optimal coordination geometry, structure 7
with the short tether exhibits a somewhat shorter Fe−N
C
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Figure 5. Three strategies for synthesizing strapped porphyrins.
Figure 6. Stepwise synthesis of the strapped porphyrins (Figure 5, pathway b).
porphyrins 15d (3%) and 15f (5%). Scrambling was not
detected in any strapped porphyrin except 15b.
Electron deficient acetylenes are known to undergo 1,3-
dipolar cycloaddition with azide.47 Reaction of the strapped
porphyrins 15a−f with trimethylsilyl azide afforded the
corresponding triazoles 16a−f, respectively, in moderate to
excellent yields (34−96%). A crystal structure analysis of 16e
confirms the structural assignment. The porphyrin ring is planar,
which is in agreement with our assumption that the bridge does
not impose strain (see Figure 8).
The X-ray structure of 16e shows an orientation of the “strap”
markedly different from that in 15c (Figure 7). Due to the bulky
Figure 7. Crystal structure of 15c, with thermal ellipsoids shown at the
50% probability level: (A) side view and (B) top view (for a van der
Waals plot, see Figure S7).
triazole unit, the orientation of the ester groups is inverted with
respect of the porphyrin plane, and the carbonyl oxygens in 16e
point toward the porphyrin plane. This esters-in conformation
creates a larger steric demand (see Figure S8 for van der Waals
plots) of the strap toward the porphyrin plane and is
compensated by tilting of the strap toward one of the
dichlorobenzene substituents of the porphyrin skeleton (Figure
D
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8B) by 26° from orthogonality. The triazole hydrogen is located
at position N2 and not at N3 as this is usually the case. This
trifluoroacetic acid was added. If coordinated, the triazole should
decoordinate upon protonation. Changes in the UV−vis
spectrum were not observed.
Because the free triazole ligand DMTDH coordinates to
FeTPPCl after deprotonation (Figure 3), a similar intra-
molecular coordination was expected for the deprotonated,
strapped DMTDH ligand in 17c. Upon addition of K2CO3 to
chloride complex 17c, the four high-spin signals of the pyrrole
protons at 85.7, 83.0, 80.2, and 78.4 ppm change to form two
broad signals at 78.9 and 84.9 ppm (Figure S3). No signals are
visible in the low-spin range, and the magnetic moment was
determined to be 5.62 μB (see the Supporting Information, page
S27), confirming the high-spin S = 5/2 state for a solution of 0.77
mM 17c with K2CO3 in acetone-d6. Characteristic changes are
also observed in the UV−vis spectrum. A strong Q-band at 576
nm appears, which is typical for five-coordinate nitrogen
Fe(III)TPP complexes (see Figure 9).51 Protonation of five-
coordinate strapped triazolate complex 18 with HCl leads back
to chloride complex 17c (Figure 9).
The p-methoxypyridine acts both as a base and as a ligand
(Figure 9b). Upon addition of 100 equiv of p-methoxypyridine,
triazole is deprotonated and replaces the chloride. Starting from
both 17c and 18, the same six-coordinate complex 20 is formed.
The pyrrole proton chemical shifts (−0.61 and −26.08 ppm)
clearly indicate that complex 20 is low-spin (S = 1/2) (see Figure
9 and Figure S5), being in agreement with the magnetic moment
of 2.56 μB (see Supporting Information, page S27). Hence, the
overall neutral five-coordinate, strapped triazolate complex 18
performs spin switching upon coordination to an additional axial
anion (Figure 9a) as well as to a neutral nitrogen ligand (Figure
9b).
Figure 8. Crystal structure of 16e with thermal ellipsoids shown at the
50% probability level. (A) Front view and (B) side view showing the
tilting of the strap.
might be due to the strong hydrogen bond (N−H···O, 1.76 Å;
N···O, 2.75 Å; N−H···O angle, 175°) to the solvent diisopropyl
ether.
Upon a solution of the free base porphyrins with FeCl2 in
acetonitrile being heated to 80 °C under nitrogen and
subsequent oxidation upon exposure to air, the Fe(III)-
porphyrins were obtained in 12−69% yields.48 NMR analysis
reveals that all iron(III) porphyrins are high-spin (S = 5/2). The
pyrrole protons resonate at 70−80 ppm, which is characteristic
for high-spin Fe(III)porphyrins.49 No signals were observed at 0
to −30 ppm, which would indicate the formation of low-spin (S
= 1/2) complexes.50 Obviously, the strapped, protonated triazole
units do not coordinate to the Fe3+ ion, in agreement with the
corresponding free protonated triazoles [DMTDH (Figure 3)].
To confirm the noncoordinating behavior of the triazole unit,
Figure 9. Switching the spin state of triazole-strapped porphyrins. Deprotonation of the triazole leads to coordination of the triazolate, the spin state
remaining high-spin (S = 5/2). Additional coordination of an external ligand, (a) triazolate (DMTD−) or (b) p-methoxypyridine, switches the spin state
to low-spin (S = 1/2). The change in coordination number, charge, and spin state leads to a change in the UV spectrum of 16 that makes it similar to that
of 5 (see Figure 3 and Figures S2 and S4).
E
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As with the addition of methoxypyridine to 17c (Figure 9),
the addition of the photodissociable ligand (PDL)52−54 trans-4-
methoxy-3-(3,5-di-tert-butylphenyl)azopyridine (50 equiv)
leads to the formation of a low-spin complex. Upon irradiation
with 365 nm, the ligand isomerizes from the trans to the cis
configuration. However, the ligand remains coordinated.
Obviously, the steric hindrance in the cis geometry is not
sufficient to overcome the electronically strong binding power.
We therefore applied trans-4-chloro-3-(3,5-di-tert-butylphenyl)-
azopyridine 22 (50 equiv) as a weaker photodissociable ligand.
22 leads to the formation of a low-spin complex 21 with the
typical 1H NMR signals at −26.6 ppm and a magnetic moment
of 3.44 μB (Figure 10). The magnetic moment is higher than
switch to the low-spin state. Upon addition of a photoswitchable
ligand, the spin state of the complex can be reversibly switched
with light. Our five-coordinate high-spin complexes 18 might
serve as model compounds for a number of natural iron(III)-
porphyrins, such as cytochrome P450, and as potential
photoswitchable catalases. Further potential applications
include responsive contrast agents16,58 and spintronics.15
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00349.
All one-dimensional NMR spectra, UV−vis spectra, ESI-
MS, MALDI-MS, Evans measurements, experimental
data of the spin switch, data of the crystal structure, and
DFT calculation (PDF)
Accession Codes
CCDC 1884544 and 1885475 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 10. Assumed mechanism of spin state switching of strapped
ferrous porphyrin 21 in the presence of photodissociable ligand 22.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: rherges@oc.uni-kiel.de.
*E-mail: kari.t.rissanen@jyu.fi.
ORCID
■
expected for a pure low-spin complex, because coordination is
incomplete and 22 is a weaker ligand than p-methoxypyridine
(Figure 9, complex 20). Irradiation for 15 min (20.0 mW/cm2)
with UV light (365 nm) switches the trans-azopyridine to the cis
configuration (PSS = 72% cis, 28% trans), which in turn gives rise
to a decrease in the intensity of the low-spin signals and an
increase in the magnetic moment to 4.3 μB (see Supporting
Information, page S27). Irradiation for 15 min (20.0 mW/cm2)
with 430 nm light restores the low-spin signals in 1H NMR (PSS
= 86% trans, 14% cis) (see Figure S6). We attribute the change in
magnetic moment to the dissociation of the ligand in the cis
configuration and recoordination of the trans isomer. The
moderate spin state switching efficiency (∼25%) is mainly due
to the low photochemical trans−cis conversion of ligand 22.
Further improvement of the switching efficiency must involve
a covalently attached, photoswitchable ligand according to the
“record player” design.55−57
̈
Frank D. Sonnichsen: 0000-0002-4539-3755
Manu Lahtinen: 0000-0001-5561-3259
Christian Naether: 0000-0001-8741-6508
Kari Rissanen: 0000-0002-7282-8419
Rainer Herges: 0000-0002-6396-6991
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the collaborative research
center SFB 677 (Function by Switching) and the University of
■
̈
̈
Jyvaskyla for financial support. The authors thank Dr. Jan
Krahmer for Evans measurements.
REFERENCES
■
CONCLUSION
In summary, we report on the design, synthesis, and
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