Direct NO Reduction by a Biomimetic Iron(II) Pyrazolate MOF

Article abstract of DOI:10.1002/anie.202108095

A novel metal-organic framework (MOF) containing one-dimensional, Fe²⁺ chains bridged by dipyrazolate linkers and N,N-dimethylformamide (DMF) ligands has been synthesized. The unusual chain-type metal nodes feature accessible coordination sites on adjacent metal centers, resulting in motifs that are reminiscent of the active sites in non-heme diiron enzymes. The MOF facilitates direct reduction of nitric oxide (NO), producing nearly quantitative yields of nitrous oxide (N₂O) and emulating the reactivity of flavodiiron nitric oxide reductases (FNORs). The ferrous form of the MOF can be regenerated via a synthetic cycle involving reduction with cobaltocene (CoCp₂) followed by reaction with trimethylsilyl triflate (TMSOTf).

Full text of DOI:10.1002/anie.202108095

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Accepted Article
Title: Direct NO Reduction by a Biomimetic Iron(II) Pyrazolate MOF
Authors: Zhongzheng Cai, Wenjie Tao, Curtis E. Moore, Shiyu Zhang,
and Casey R. Wade
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Direct NO Reduction by a Biomimetic Iron(II) Pyrazolate MOF
Zhongzheng Cai, Wenjie Tao, Curtis E. Moore, Shiyu Zhang, and Casey R. Wade*
[*]
Dr. Z. Cai, Dr. W. Tao, Dr. C. E. Moore, Prof. Dr. S. Zhang, Prof. Dr. C. R. Wade
Department of Chemistry and Biochemistry
The Ohio State University
100 West 18th Ave, Columbus, OH 43210
E-mail: wade.521@osu.edu
Supporting information for this article is given via a link at the end of the document.
Abstract: A novel metal-organic framework (MOF) containing one-
dimensional, Fe2+ chains bridged by dipyrazolate linkers and N,N-
dimethylformamide (DMF) ligands has been synthesized. The
unusual chain-type metal nodes feature accessible coordination sites
on adjacent metal centers, resulting in motifs that are reminiscent of
the active sites in non-heme diiron enzymes. The MOF facilitates
direct reduction of nitric oxide (NO), producing nearly quantitative
yields of nitrous oxide (N2O) and emulating the reactivity of flavodiiron
nitric oxide reductases (FNORs). The ferrous form of the MOF can be
regenerated via a synthetic cycle involving reduction with cobaltocene
(CoCp2) followed by reaction with trimethylsilyl triflate (TMSOTf).
generally considered too stable to promote direct NO
reduction, although a ferrous NO reduction pathway has
been demonstrated for flavodiiron proteins.[16–18]
In contrast to small molecule complexes, the biomimetic
reactivity of iron-based MOFs toward NO has not been
thoroughly explored.[2,4] Coordinatively unsaturated iron
sites in MIL-88(Fe), MIL-101(Fe), and Fe2(dobdc) have
been shown to exhibit strong and reversible interactions
with NO (Figure 1a).[19–21] In addition, Fe2+-exchanged
MOF-5 and CuI-MFU-4l have been found to mediate NO
disproportionation to generate M–NO2 species and N2O
(Figure 1b).[22–24] Unlike FNORs, in which the diiron active
site is poised to facilitate N−N bond coupling, the isolated
Fe2+ and Cu+ sites at the SBUs of the these MOFs are
incapable of direct, two-electron reduction of NO.
Here, we report the synthesis and NO reduction
reactivity of a Fe2+-pyrazolate MOF (1) featuring chain-type
SBUs with accessible coordination sites on adjacent Fe2+
centers. These motifs resemble the active sites of non-
heme diiron enzymes, such as FNORs, that use
cooperative substrate activation to carry out multielectron
redox processes. 1 generates N2O in nearly quantitative
yield upon exposure to stoichiometric amounts of NO,
emulating the reactivity of FNORs and representing a rare
example of direct NO reduction from ferrous sites (Figure
1c). Moreover, sequential reaction of the oxidized MOF
product with a reductant and Lewis acid regenerates the
Fe2+ form of the MOF, which maintains NO reduction
activity.
Metal-organic frameworks (MOFs) are promising
platforms for the design of biomimetic materials.[1–4] They
are often assembled using linkers with weak-field donor
groups such as carboxylates or azolates that are similar to
those found in the active sites of metalloenzymes. MOF-
supported reactive sites also benefit from site-isolation
while the porous framework provides substrate access and
a
tunable microenvironment. However, metal-metal
cooperativity, a key feature of multimetallic enzyme active
sites, can be difficult to emulate in highly porous MOFs
owing to the large spatial separation between reactive
structural components. In this regard, MOFs assembled
from secondary building units (SBUs) comprised of one-
dimensional metal chains are potentially well-equipped to
facilitate cooperative substrate activation at adjacent metal
sites and enable multi-electron processes by distributing
the metal redox burden. However, reported examples of
cooperative and multi-electron reactivity in MOFs with
chain-type SBUs remain rare, in part due to their tendency
to contain coordinatively saturated metal sites.[5,6]
Nitric oxide (NO) acts a critical signaling molecule in
biological systems, but becomes toxic at elevated
concentrations.[7,8] Flavodiiron nitric oxide reductases
(FNORs) are a family of enzymes found in pathogenic
bacteria that reduce NO to nontoxic N2O as a protective
mechanism.[9,10] The FNOR active site consists of a non-
heme diiron cluster (Fe–Fe distance of 3.2-3.6 Å)
coordinated by histidine and carboxylate ligands, and
several model complexes have been designed to
investigate its NO reduction mechanism.[11–13] Recently,
Lehnert and coworkers reported the only example of a
diferrous, non-heme model complex capable of reducing
NO to N2O via N–N bond coupling between {FeNO}7
groups.[14,15] Prior to this report, {FeNO}7 species were
Figure 1. Reactivity of iron-based MOFs with NO.
Solvothermal reaction of Fe(OTf)2(THF)2 (OTf = triflate,
THF
=
tetrahydrofuran) and H2bppdi (2,6-bis(1H-
pyrazolyl)pyromellitic diimide) in the presence of
triethylamine generates 1 as an orange, microcrystalline
powder after washing with DMF and THF (Figure 2a). Solid
1 rapidly oxidizes upon exposure to air, as indicated by a
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color change from orange to bright red, and gradually
decomposes under ambient conditions. 1H NMR spectra of
μ-OH ligands. Notably, the Fe···Fe distance between
adjacent metal sites (3.28 Å) is similar to that found in the
acid-digested samples of 1 show a H2bppdi:DMF:THF ratio diiron active site of FNORs (3.2-3.6 Å).[28] The framework
of 1:0.5:3, which supports an empirical formula of
structure of 2-OH is reminiscent of Fe2(bdp)3 (bdp2– = 1,4-
Fe(bppdi)(DMF)0.5(THF)3 (Figure S2). Zero-field, 57Fe benzenedipyrazolate).[29,30] However, while Fe2(bdp)3
Mꢀssbauer spectroscopy was used to investigate the
metal oxidation state in 1 (Figure 4a, see Supporting
contains octahedral Fe3+ centers coordinated by six
pyrazolate linkers, each Fe3+ site in 2-OH is only 4-
Information for additional details). The spectrum measured connected with μ-hydroxide ligands replacing an absent
at 4 K displays a broad quadrupole doublet with an isomer
shift (δ) of 1.09 mm/s and quadrupole splitting parameter
(ΔEQ) of 3.01 mm/s that corresponds to the presence of
high-spin iron(II) species. The broad absorption signal
suggests some variability in the metal coordination
environment, which has been observed for other iron-
based MOFs and proposed to arise from framework
distortions or differences in solvent coordination.[25,26]
dipyrazolate linker. Moreover, 2-OH is quite unique in that
the μ-hydroxide groups bridging adjacent pairs of atoms
are mutually cis, resulting in a disphenoidal arrangement
of pyrazolate N donors and the formation of helical
chains.[31]
Figure 3. PXRD patterns of 1 measured under air-free conditions in a sealed
capillary and upon exposure to air along with the pattern of 1-O and that
simulated from the single crystal structure of 2-OH.
The powder X-ray diffraction (PXRD) pattern of 1
matches that simulated from the single-crystal structure of
2-OH, confirming that the MOFs adopt similar framework
structures (Figure 3). However, the presence of Fe2+
centers in 1 requires coordination of neutral bridging
ligands at the chain-type SBUs rather than the μ-hydroxide
1
groups found in 2-OH. H NMR and TGA analysis of 1
show a 2:1 H2bppdi:DMF ratio (Figures S3 and S6), and
the redshifted DMF v(CO) band observed in the IR
spectrum signals its coordination to a metal (Figure S4).[32]
Together these data indicate that DMF solvent molecules
act as bridging ligands in the chain-type SBUs. The 2:1
linker:DMF ratio also suggests that the DMF ligands only
occupy half of the bridging coordination sites.
Consequently, the coordination sites between alternating
pairs of Fe2+ sites along the chain-type SBUs should
remain open or occupied by weakly coordinating THF
molecules. 1 was gently activated by heating at 60 °C
under vacuum (10-4 bar), and a subsequent N2 adsorption
isotherm (77 K) gave a calculated Brunauer-Emmett-Teller
(BET) surface area of 1330 m2 g-1 (Figure S5). The acid-
digested 1H NMR spectrum of the activated sample shows
that the H2bppdi:DMF ratio is largely unchanged, further
supporting the role of DMF as a bridging ligand (Figure S6).
Subsequent attempts to activate 1 at higher temperatures
resulted in volatilization of the bridging DMF molecules and
framework collapse (Figure S7 and S8). This behavior
Figure 2. a) Synthesis of 1 and 2-OH. b) Single crystal structure of 2-OH.
Although we have been unable to obtain a single crystal
of 1 suitable for X-ray diffraction, the framework structure
has been established from single crystals of an oxidized
Fe3+ analogue, 2-OH (Figure 2b).[27] A few small red
crystals of the latter were obtained during reaction
optimization, with oxidation attributed to the presence of
adventitious O2/H2O. 2-OH crystallizes in the P61 space
group with helical, chain-type SBUs linked by the linear
dipyrazolates to form triangular channels ~10 Å in diameter.
Each Fe3+ center adopts a pseudo-octahedral geometry
via coordination of four different pyrazolate units and two
2
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clearly indicates that the DMF molecules provide comparable to 1, but similar features are not observed in
necessary structural support.
the 1600-1700 cm-1 region of its DRIFT spectra upon
exposure to NO, confirming their assignment as DMF v(CO)
bands (Figures S12 and S33).
The N2O yield drops significantly when 1 is treated with
excess NO. Reaction with 2 equiv. of NO per Fe2+ afforded
only 67 % N2O yield after heating at 60 °C for 2 h. GC
monitoring of the reaction of 1 with 3 equiv. of NO revealed
even greater inhibition, with less than 10 % N2O yield
observed over the course of 1 h at room temperature. The
IR spectrum of 1 upon exposure to 5 equiv. of NO (1-exNO)
shows the appearance of broad v(NO) bands around 1851
and 1832 cm-1 as well as a new feature at 1573 cm-1
(Figure S13). These new signals redshift by 25-50 cm-1
when 15NO is used, confirming their association with NO-
derived species (Figure S14). The frequency of the v(NO)
bands is unusually high for {FeNO}7 species and indicates
the presence of weakly activated NO ligands.[11,33,34] On the
other hand, the band at 1573 cm-1 appears below the
typical N–O stretching region for mono-nitrosyl complexes.
It is reminiscient of v(NO) bands observed for diiron -
nitrosyl species, but such an assignment is tenuous given
that the Fe···Fe distances in 2-OH (3.28 Å) are significantly
longer than those found in structurally characterized -
nitrosyl complexes (2.4-2.6 Å).[35,36] Moreover, the band is
inconsistent with hyponitrite (–O–N=N–O–) or nitro/nitrito
Figure 4. a) 57Fe Mꢀssbauer spectra (4 K) of 1 and 1-O. b) Room temperature
DRIFTS spectra measured for 1 upon exposure to dilute 14NO (~25% vol).
–
(NO2 ) groups which typically exhibit N–O and N=N
stretching bands at lower frequencies.[37,38] Although the
identity of the species formed upon reaction of 1 with
excess NO remains ambiguous, the dramatic decrease in
Scheme 1. Synthetic cycle for reaction of 1 with NO and regeneration of 1’.
reactivity supports
a
direct reduction rather than
disproportionation mechanism since the latter should be
promoted by increasing NO concentration.[22,23] This
behavior also suggests that key N–N coupling or N2O
release steps may be inhibited by binding of excess NO at
the Fe2+ sites. Several plausible mechanisms have been
proposed for N–N coupling in diiron proteins and model
complexes (Figure S15).[9,18] Among these, there is strong
evidence that supports a pathway involving diferrous
dinitrosyl intermediates.[17,39] In the case of 1, a similar
diferrous dinitrosyl pathway seems feasible given the short
the Fe···Fe distances the accessibility of coplanar
coordination sites. However, the present data preclude
distinction among the mechanistic possibilities, and more
detailed kinetic and computational studies will be needed
to interrogate the mechanism operative for 1.
The oxidized MOF product, 1-O, maintains crystallinity
and exhibits a BET surface area of 1110 m2g–1, only slightly
lower than that of 1 (Figure S5). The IR spectrum shows
no indication of residual Fe–NO species or Fe–NO2 groups
that would result from NO disproportionation (Figure S18).
The 57Fe Mꢀssbauer spectrum exhibits a broad signal that
could be fitted as two overlapping doublets (79 %, δ1 = 0.36
mm/s, ΔEQ1 = 1.78 mm/s and 21 %, δ2 = 0.23 mm/s, ΔEQ2
= 0.40 mm/s, Figure 4a). Again, the broad absorption
profile observed for this material suggests a distribution of
slightly different local coordination environments.[25,26] The
isomer shifts of both components are characteristic of Fe3+
and consistent with data reported for oxo-bridged
complexes.[40] The appearance of two distinct Fe3+ sites
may be related to the partial release of coordinated DMF
solvent molecules upon reaction with NO. Nevertheless,
these data support direct reduction of NO to N2O with
Motivated by the resemblance of the putative ferrous
diiron motifs in 1 to the FNOR active site, we set out to
investigate its reactivity toward NO (Scheme 1). Solid 1
undergoes an immediate color change from orange to
black upon exposure to 1.0 equiv. of NO gas per Fe2+ in a
sealed vial at room temperature. The black solid gradually
turns to red when left at ambient temperature, but the color
change could be accelerated with mild heating. GC
analysis of the headspace 1 min after the addition of NO
revealed formation of N2O in 27 % yield (Figure S42). N2O
production gradually increased over time, reaching 60 %
yield in the headspace after 1h. Nearly quantitative N2O
yields (91-99%, Table S2) were obtained for multiple
batches of 1 when the reaction was heated at 60 °C for 1
h and N2O adsorption by the MOF was taken into account
(see Supporting Information for details). In situ diffuse
reflectance infrared Fourier transform spectroscopy
(DRIFTS) experiments were carried out by dosing 1 with
dilute NO or isotopically labeled 15NO (Figure 3b and S9).
Some minor intensity changes are observed in the in the
1700-1750 cm-1 region where the NO stretch of {Fe(NO)}7
species might be expected to appear, but comparison of
the difference spectra do not clearly reveal the buildup of
any intermediates (Figure S10). The DRIFT spectra do
show a shift in intensity of the bridging DMF v(CO) band
from 1652 cm-1 to 1675 cm-1. This shift is consistent with
solvent decoordination, insinuating reaction of NO at the
Fe2+ sites.[32] Notably,
analogue (1-DMSO) shows NO reduction activity
a DMSO solvent-exchanged
3
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concomitant formation of Fe3+–O–Fe3+ species at the
chain-type SBUs of 1-O.
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solid that closely resembles 1. PXRD and N2 adsorption
measurements show that the MOF maintains crystallinity
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confirms the complete departure of [CoCp2]+ from the
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spectrum of 1’ exhibits a quadrupole doublet (δ = 1.04 mm
s-1 and ΔEQ = 3.07 mm/s) that matches the signal observed
for 1 (Figure S31). Gratifyingly, reaction of 1’ with 1 equiv.
NO under the same conditions employed for 1 produced
N2O in 73% yield (Figure S37), confirming successful
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Acknowledgements
This work was supported by an Early Career Faculty
grant (80NSSC18K1504) from the NASA Space
Technology Research Grants Program. The authors
acknowledge the Surface Analysis Laboratory (NSF DMR-
0114098) of the Ohio State University Department of
Chemistry and Biochemistry. We also thank Tom Rayder
and Jordon Hilliard for assistance with gas adsorption
measurements.
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Keywords: Bioinspired materials • Metal-organic frameworks •
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4
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Entry for the Table of Contents
A novel iron(II) MOF containing accessible coordination sites on adjacent metal centers facilitates the direct reduction of NO to N2O.
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5