Structural Differences and Redox Properties of Unsymmetric Diiron PDIxCy Complexes

Article abstract of DOI:10.1002/ejic.201901173

We present two bimetallic iron complexes, [Fe₂(PDIeCy)(OTf)₄] (1) and [Fe₂(PDIpCy)(THF)(OTf)₄] (2) coordinated by an unsymmetric ligand. The new ligand, PDIeCy (PDI = pyridyldiimine; e = ethyl; Cy = cyclam), is

Full text of DOI:10.1002/ejic.201901173

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DOI: 10.1002/ejic.201901173
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Unsymmetric Diiron Complexes
Structural Differences and Redox Properties of Unsymmetric
Diiron PDIxCy Complexes
Andreas J. Hofmann,[a] Christian Jandl,[a] and Corinna R. Hess*[a]
Abstract: We present two bimetallic iron complexes, two iron centers of the PDIpCy-based 2 remain unconnected,
[Fe2(PDIeCy)(OTf)4] (1) and [Fe2(PDIpCy)(THF)(OTf)4] (2) coordi- directly, under all conditions examined. Both compounds con-
nated by an unsymmetric ligand. The new ligand, PDIeCy (PDI = tain electronically non-coupled high-spin (S = 2) ferrous centers,
pyridyldiimine; e = ethyl; Cy = cyclam), is a variant of the previ- as established by Mössbauer spectroscopy and magnetic sus-
ously reported PDIpCy (p = propyl) ligand, featuring a shorter ceptibility studies. Cyclic voltammetry demonstrates the rich
linker between the two metal coordination sites. The structural redox chemistry of the compounds, involving both ligand and
and electronic properties of 1 and 2, both in the solid and metal-centered redox processes. Moreover, we synthesized the
solution state, were analyzed by means of X-ray crystallography, two-electron reduced [Fe2(PDIeCy)]2+ form of 1, which contains
and spectroscopic methods, including 19F-NMR. The two ligand the dianionic PDI2– ligand, and represents a two-electron
platforms yield markedly different diiron structures: the PDIeCy charge localized complex.
ligand permits formation of a bridged, μ-OTf complex, while the
Introduction
group, to support dinuclear complexes thereof (Scheme 1).[29]
We reported the synthesis of non-coupled homo- and heterobi-
metallic Ni- and Zn-containing PDIpCy complexes. We now
have focused our attention on diiron compounds. Iron offers
Binuclear iron sites are employed by enzymes for varied biologi-
cal processes that entail dioxygen activation (hemerythrin,
MMO, RNRs), NO reduction (FNORs), and H2 chemistry, and fea-
ture in the active sites of hydrolases (PAPs, GliJ).[1–10] The combi-
nation of two redox-active iron centers in close proximity con-
fers advantages for catalysis, including cooperative binding and
activation of substrates, and the use of both metal ions for
multi-electron processes. Many biomimetic diiron complexes
have been generated as spectroscopic and mechanistic probes
for the enzymes, and as synthetic catalysts for related chemis-
try.[10–19] A notable feature among several of the aforemen-
tioned enzyme active sites is the asymmetric coordination envi-
ronment. Differing ligand environments, correlated with distinct
electronic properties, allow each metal center to adopt a
unique role in the overall reaction.[19,20] Accordingly, a number
of bio-inspired, unsymmetric complexes — not only of iron, but
with other metals — have been reported in recent years, which
merge unique coordination environments.[21–28]
We recently developed the unsymmetric PDIpCy ligand,
which combines a pyridine diimine (PDI) and a cyclam (Cy)
[a] Department of Chemistry and Catalysis Research Center, Technische
Universität München,
Lichtenbergstraße 4, Garching 85748, Germany
E-mail: corinna.hess@ch.tum.de
https://www.department.ch.tum.de/bioinorganic/home/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201901173.
© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
Scheme 1. Synthesis of 1 and 2.
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access to a wider range of oxidation states, and Fe-PDI and Fe2···Oavg = 2.31 Å). This trend was observed for PDIpCy com-
-cyclam complexes are uniquely suited for a variety of redox plexes containing other metals and for 2.[29] The bridging tri-
and organometallic reactions, including several of the afore- flate thus binds in an asymmetric fashion in 1, closer to the PDI
mentioned biologically relevant reactions.[30–46] We envisioned site.
that the combined scope of reactions facilitated by the two
types of Fe compounds might give rise to unique activities.
Furthermore, we have generated a variant of the previously
reported PDIpCy ligand, PDIeCy, in which the propyl-linker of
the molecule is replaced by a shorter ethyl group. We expected
that the PDIeCy might allow a closer approach of the two metal
centers. In this work, we describe our initial studies to character-
ize the two diiron complexes, using X-ray crystallography, spec-
troscopic methods and cyclic voltammetry. The structural prop-
erties and metal···metal interactions of the Fe2-PDIpCy and
-PDIeCy were compared, as these features are relevant to poten-
tial cooperative reactivity within these systems. Examination of
the redox properties provides insight into the electron donor
and acceptor capacity of the combined PDI/Cy system.
Figure 1. Molecular structure of 1 (left) and 2 (right) in the solid state (50 %
probability ellipsoids). Hydrogen atoms, solvent molecules and partial disor-
der are omitted for clarity.
Results and Discussion
The structural differences between 1 and 2 influence the
packing features in the solid state. The N–H groups of the cyc-
lam moiety in 1 only form intramolecular hydrogen bonds with
coordinated triflate anions on both Fe atoms (Figure S32). In
contrast, the crystal structure of 2 shows intermolecular
hydrogen bonds between the cyclam N-Hs and triflates of adja-
cent complexes, or co-crystallized THF, leading to chain forma-
tion (Figure S33).
Synthesis and Solid State Characterization
PDIeCy was generated according to a similar procedure as re-
ported for the synthesis of PDIpCy, involving the reaction of
2,6-diacetylpyridine with 2,6-diisopropylaniline and 1-(2-amino-
ethyl)-1,4,8,11-tetraazacyclotetradecane, respectively (Scheme
S1). The diiron complexes of the two ligand scaffolds,
[Fe2(PDIeCy)(OTf)4] (1; OTf = trifluoromethansulfonate) and
[Fe2(PDIpCy)(THF)(OTf)4] (2), were synthesized upon addition of
two equiv. of Fe(OTf)2 to the respective ligand in MeCN at room
temperature (Scheme 1). The composition of the bimetallic
complexes was verified by mass spectrometry and elemental
analysis.
Single crystals of both 1 and 2 were successfully obtained
from a tetrahydrofuran (THF)/pentane solution (CCDC 1912170,
CCDC 1912171). Although the crystalline sample of 2 was signif-
icantly twinned, we were able to solve the crystal structure and
to model the (disordered) molecular structure of the dinuclear
complex. We therefore refrain from a detailed discussion of geo-
metrical parameters and use this data only as structural proof.
The structure of 2 (Figure 1) resembles the previously reported
complexes [Zn2(PDIpCy)(THF)(OTf)4] and [NiZn(PDIpCy)(THF)-
(OTf)4]. The metal centers of 2 are separated by roughly 8 Å,
and each iron adopts a pseudo-octahedral geometry. The Fe
residing in the PDI unit is additionally ligated by two κO-triflate
ligands (FePDI–Oavg = 2.14 Å) and a THF molecule. The FeCy coor-
dination sphere also is completed by two κO-triflate ligands
(FeCy–Oavg = 2.26 Å) arranged trans to each other. The shorter
linker of the PDIeCy results in significant changes to the binu-
clear structure. The Fe centers of 1 are bridged by a μ-triflate
molecule that coordinates via one O-atom to each metal ion.
Consequently, the Fe···Fe distance in 1 is reduced to 5.6 Å (Fig-
ure 1). Each iron center of 1 again adopts a six-coordinate ge-
ometry, with additional terminal triflate ligands in the remain-
ing coordination positions. Overall, the Fe1–O bond lengths are
Electronic Structure
Mössbauer spectroscopy and magnetic susceptibility studies
were carried out on solid samples of 1 and 2 to examine the
electronic structures of the diiron complexes, and to determine
whether differences in the structure influence the M···M interac-
tions. Figure 2 shows the Mössbauer data of 1; a two-
Figure 2. Mössbauer spectrum (80 K) of 1. Experimental data shown as black
circles. The red line represents the combined fit of the individual fits for each
Fe center; green line affords δ(|ΔEQ|): 1.19 (1.80) mm s–1, blue line affords
shorter than the Fe2–O distances (Fe1···Oavg
=
2.11 Å;
δ(|ΔEQ|): 1.14 (2.41) mm s–1
.
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component fit reveals comparable parameters for the two iron and otherwise, only MeCN molecules that can complete the
sites, with δ(|ΔEQ|): 1.19 (1.80) mm s–1 and δ(|ΔEQ|): 1.14 (2.41) iron coordination sphere but are unlikely to bridge the two iron
mm s–1. Similar values were obtained for 2 (δ(|ΔEQ|): 1.21 (2.48) centers. The complex thus acts as a benchmark for evaluation
mm s–1 and δ(|ΔEQ|): 1.06 (2.55) mm s–1; Figure S6), demonstrat- of the spectroscopic and redox properties of the triflate-based
ing that the triflate bridge does not significantly affect the indi- analogues.
vidual FeII centers. The Mössbauer data clearly denote the pres-
The absorption spectra of 1 and 2 in the non-coordinating
ence of two high-spin, S = 2, ferrous ions in each complex.
Magnetic susceptibility measurements verified the spin state
of the complexes (Figure 3 and Figure S7). The effective mag-
netic moment of 1 remains constant at 7.3 μB between 300 K
and 70 K and decreases to a final value of 4.1 μB at 2 K. The
room temperature magnetic moment is close to the spin-only
value for two non-coupled high-spin iron(II) ions (6.9 μB), and
well below that for a strongly coupled S = 4 system (8.9 μB).
The data were fit according to the spin Hamiltonian, including
exchange coupling and zero-field splitting terms (see SI for de-
tails, and Figure S8),with two S = 2 centers. The best fit was
obtained with g = 2.1, J = 0.01 cm–1, |D| = 17.5 cm–1, and E/D =
0.32. The negligible exchange coupling constant demonstrates
that the two iron centers in 1 are non-coupled, despite the
bridging triflate. Similar J-values were reported for other μ-tri-
flate diiron complexes.[47–49] The magnetic susceptibility data
for 2 gave similar values, where the best fit was obtained with
g = 2.17, J = 0.07 cm–1, |D| = 17.7 cm–1, and E/D = 0.33 (Figures
S7 and S9).
solvent DCM (Figure S10) are dominated by transitions of the
PDI site. The absorptions in the UV region (λmax = 300 nm) are
due to π–π* transitions, while the broad band in the visible
region (400–700 nm) is assigned to PDI-based MLCT transi-
tions.[50,51] The solid state spectrum of 1 (Figure S11) closely
resembles the solution state spectrum. However, the spectrum
of 3 in DCM is noticeably different, lacking features from 550–
700 nm (Figure S12).
To further probe the coordination chemistry of the triflate
ligands, 19F-NMR were obtained for 1 and 2. The 19F-NMR spec-
trum of 1 in DCM (Figure S13) at 20 °C exhibits seven signals
that appear between 75 and –57 ppm. These signals divide
upon cooling of the sample, which may be due to greater asym-
metry or slower ligand exchange at low temperature. As the
resonances of bridging triflates are shifted downfield with re-
spect to those of terminally bonded triflates,[52,53] the signal at
75 ppm at 20 °C can be assigned to the former.
The spectra of 2 also exhibit multiple signals (Figure S14),
however the resonance at 75 ppm is not observed, indicating
that a triflate bridged conformation is absent for the PDIpCy
complex. The broad signals at –57 ppm (20 °C) in the spectra
of both complexes can be assigned to non-coordinated triflate
counterions.[52,54] This suggests multiple conformations are
available in which one or both sites may be less than six-coordi-
nate in solution. The various resonances in the NMR spectra
could not be precisely assigned. However, the data clearly show
differences between the spectra of 1 and 2, and indicate that
the bridged conformation of 1 persists in DCM.
The electronic spectra of all three diiron complexes are virtu-
ally identical in MeCN (Figure 4). The bands in the visible region
are red-shifted (λ = 410–570 nm) with respect to their position
in the DCM spectra of 1 and 2. The transitions in the spectrum
of 3 have a slightly lower intensity than those of 1 and 2 —
which may reflect an additional effect due to the counter-
anion — however, the overall shape and band positions are the
same.
Figure 3. Magnetic susceptibility data for 1 (2–300 K). Circles represent the
experimental data; red line represents the simulation, which affords: S1 = S2 =
2, g1 = g2 = 2.10, J = 0.01 cm–1, |D1| = |D2| = 17.5 cm–1, E/D1 = E/D2 = 0.26,
TIP = 2.39 × 10–3 emu.
Interestingly, upon cooling, a marked change in the absorp-
tion bands of all three diiron complexes is observed (Figure 4,
Figure S15 and Figure S16); the transitions in the visible region
increase in intensity and become more distinct. The changes in
the absorption spectra coincide with an isosbestic point at
371 nm (Figure 4 bottom, inset), indicating clean conversion
Solution State Characterization
While the minor modification to the ligand backbone leads to between two forms of the molecule. The original spectrum also
differences in the molecular structures of 1 and 2, we ques- is fully restored upon return of the sample to room tempera-
tioned whether the coordination of the triflate molecules per- ture, following the variable temperature measurements. We at-
sists in solution. The solution state behavior of the bimetallic tribute the behavior to changes occurring at the Fe-PDI site,
complexes is relevant to their potential use as homogeneous since the spectrum of the monometallic [(iPrPDI)Fe(OTf)2][55]
catalysts. To support the solution state characterization of 1 also increases in intensity upon cooling (Figure S17). However,
and 2, a related complex, [Fe2(PDIeCy)(MeCN)3](PF6)4 (3), was the changes in the absorption features of the mononuclear
synthesized upon reaction of PDIeCy with two equivalents of complex are less significant than those of the bimetallic com-
[Fe(MeCN)6](PF6)2. Compound 3 includes the PDIeCy backbone, pounds, suggesting that the second metal site may play a role
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valence tautomerism between the Fe center and the redox-ac-
tive PDI) offer possible explanations.
Redox Properties
Cyclic voltammetry was carried out to examine the redox prop-
erties of the bimetallic complexes. The CV of 1 shows five redox
couples in the range of –1.7 to 1.2 V vs. Fc+/0 (Figure 5; Table 1).
Two reductions occur at –1.2 and –1.6 V. The potentials are
similar to the values observed in the CV of
[Zn2(PDIpCy)(OTf)4],[29] and are likewise assigned to the PDI0/−
and PDI–/2– couples of the diiron complex. The assignment is in
accord with the redox behavior established for other Fe-PDI
complexes, in which ligand-centered reduction occurs prior to
metal-centered reduction.[57,58] The CV of 1 additionally exhibits
a series of seemingly reversible oxidative couples that were re-
solved using differential pulse voltammetry (DPV; Figure 5). The
oxidative processes consist of two closely spaced events at 0.4
and 0.6 V, followed by a third oxidation at 1.0 V. The first two
couples are tentatively assigned to the oxidation of the two
divalent Fe centers to their ferric forms. The final oxidative event
would then correspond to formation of an FeIV species. FeIV-
cyclam species are well-established,[59] and the FeIV/III couple of
Figure 4. Top: electronic spectra of 1 (black), 2 (red) and 3 (blue) in MeCN at
room temperature. Bottom: temperature dependent absorption spectra of 1.
in this phenomenon. The 19F-NMR spectra for 1 and 2 show
only one resonance in the temperature range of –40 °C to
+80 °C (Figure S18 and Figure S19). This signal, due to the unco-
ordinated triflates, shifts moderately from –78.8 ppm to
–63.5 ppm with increasing temperature. Thus, the absorption
spectra and 19F-NMR data indicate that the triflate ligands of 1
and 2 are readily displaced by acetonitrile, and coordination
equilibria involving the triflate ions cannot account for the tem-
perature dependent changes in the absorption spectra. More-
over, the same temperature dependent changes in the absorp-
tion spectrum also are observed for 3 (Figure S16), which lacks
any triflate ions.
The solution state magnetic susceptibilities of 1 and 2
(MeCN-d3 or propionitrile; see SI for details) were determined
by Evan's method, to probe whether changes in the spin state
of the bimetallic complex might occur. The room temperature
effective magnetic moments of 7.0 μB (1) and 7.1 μB (2) deter-
mined by NMR are comparable to the values obtained from the
solid-state SQUID data. The data confirm that both iron centers
remain high-spin in solution, even upon coordination of aceto-
nitrile. The magnetic moments remain fairly constant from 90
to –20 °C, but decrease at lower temperatures yielding final
values of 5.8 μB (1) and 5.7 μB (2) at –90 °C (Table S1; Figures
S20 – S22). Since the absorption bands of 1 and 2 continue to
decrease in intensity above –20 °C, spin state changes do not
appear to fully account for the absorption spectral changes.
Although we have not identified the origin of the spectral
changes, differences in the coordination number (and thus ge-
ometry) at the Fe-PDI site,[56] or in the electronic structure (e.g.
Figure 5. Top: CV of 1 (MeCN, 0.1
region. Bottom: differential pulse voltammetry (DPV) and cyclic voltammetry
(CV) of 1 (MeCN, 0.1
[N(nBu)4]PF6, 20 mV s–1) showing the oxidative events.
M
[N(nBu)4]PF6, 0.1 V s–1) in the reductive
M
Table 1. Electrochemical data for 1–3 (MeCN, 0.1
M
[N(nBu)4]PF6, 0.1 V s–1).
Redox process
E1/2, V vs. Fc+/0
1
2
3
PDI0/PDI–
PDI–/PDI2–
–1.5
–1.2
0.4, 0.6
–1.6
–1.2
0.4, 0.5
–1.5
–1.2
0.5
III/II
III/II
Cy
Fe & Fe
PDI
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