Gradual Release of Strongly Bound Nitric Oxide from Fe2(NO)2(dobdc)

Article abstract of DOI:10.1021/ja5132243

An iron(II)-based metal-organic framework featuring coordinatively unsaturated redox-active metal cation sites, Fe₂(dobdc) (dobdc^4- = 2,5-dioxido-1,4-benzenedicarboxylate), is shown to strongly bind nitric oxide at 298 K. Adsorption isotherms indicate an adsorption capacity greater than 16 wt %, corresponding to the adsorption of one NO molecule per iron center. Infrared, UV-vis, and M?ssbauer spectroscopies, together with magnetic susceptibility data, confirm the strong binding is a result of electron transfer from the Fe^II sites to form Fe^III-NO^- adducts. Consistent with these results, powder neutron diffraction experiments indicate that NO is bound to the iron centers of the framework with an Fe-NO separation of 1.77(1) ? and an Fe-N-O angle of 150.9(5)°. The nitric oxide-containing material, Fe₂(NO)₂(dobdc), steadily releases bound NO under humid conditions over the course of more than 10 days, suggesting it, and potential future iron(II)-based metal-organic frameworks, are good candidates for certain biomedical applications.

Full text of DOI:10.1021/ja5132243

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Gradual Release of Strongly-Bound Nitric Oxide from Fe(NO)(dobdc)
Eric D. Bloch, Wendy L. Queen, Sachin Chavan, Paul S. Wheatley, Joseph M. Zadrozny,
Russell Morris, Craig M. Brown, Carlo Lamberti, Silvia Bordiga, and Jeffrey R. Long
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5132243 • Publication Date (Web): 24 Feb 2015
Downloaded from http://pubs.acs.org on March 2, 2015
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Gradual Release of Strongly-Bound Nitric Oxide from
Fe2(NO)2(dobdc)
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Eric D. Bloch,† Wendy L. Queen,‡,¶ Sachin Chavan,§ Paul S. Wheatley,# Joseph M. Zadrozny,† Russell
Morris,# Craig M. Brown,¶,∆,$ Carlo Lamberti,§,☐Silvia Bordiga,§ Jeffrey R. Long*†
†Department of Chemistry, University of California, Berkeley, CA 94720, USA.
¶Center of Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
‡The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
§Department of Chemistry, NIS, CrisDi, and INSTM Centre of Reference, University of Turin, Via Quarello 15, Iꢀ10135 Toꢀ
rino, Italy
#EaStChem School of Chemistry, University of St Andrews, Purdie Building, St Andrews KY16 9ST, U.K.
∆Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA.
$The Bragg Institute, Australian Nuclear Science and Technology Organization, PMB1, Menai, NSW, Austrualia.
☐Southern Federal University, Zorge Street 5 344090 RostovꢀonꢀDon, Russia.
Supporting Information Placeholder
adsorption and release of nitric oxide by frameworks
featuring coordinativelyꢀunsaturated metal centers. The
first investigation involved the widely studied metalꢀ
ABSTRACT: An iron(II)ꢀbased metalꢀorganic frameꢀ
work featuring coordinativelyꢀunsaturated redoxꢀactive
metal cation sites, Fe2(dobdc) (dobdc4– = 1,4ꢀdioxidoꢀ
2,5ꢀbenzenedicarboxylate), is shown to strongly bind
nitric oxide at 298 K. Adsorption isotherms indicate an
adsorption capacity greater than 16 wt %, corresponding
to the adsorption of one NO molecule per iron center.
Infrared, UVꢀvis, and Mössbauer spectroscopies, togethꢀ
er with magnetic susceptibility data, confirm the strong
binding is a result of electron transfer from the FeII sites
to form FeIIIꢀNO– adducts. Consistent with these results,
powder neutron diffraction experiments indicate that NO
is bound to the iron centers of the framework with an
Fe–NO separation of 1.77(1) Å and an Fe–N–O angle of
150.9(5)°. The nitric oxideꢀcontaining material,
Fe2(NO)2(dobdc), steadily releases bound NO under
humid conditions over the course of more than 10 days,
suggesting it, and potential future iron(II) based metalꢀ
organic frameworks, are good candidates for certain biꢀ
omedical applications.
organic framework Cu3(btc)2 (btc3–
=
1,3,5ꢀ
benzenetricarboxylate, HKUSTꢀ1).6,7 This compound
adsorbs nearly 4.0 mmol/g of NO, a significant imꢀ
provement over zeolites, which can adsorb NO with
maximum capacities of 1 mmol/g under similar condiꢀ
tions.8 Upon exposing HKUSTꢀ1 to humid air, a slow
release of a fraction of the coordinated NO occurs over
the course of an hour. Although the amount released is
limited to just ~2 ꢁmol/g, it proved sufficient to inhibit
platelet aggregation in biological experiments, which is
necessary to prevent blood clotting. In addition to anꢀ
tithrombotic applications, porous materials that can store
and deliver the critical biological signaling molecule NO
may be useful for antibacterial and wound healing appliꢀ
cations.9
In order to improve upon the NO release properties of
HKUSTꢀ1, storage and release by Ni2(dobdc) and
Co2(dobdc) were subsequently studied.10,11 These mateꢀ
rials feature exceptionally high densities of coordinativeꢀ
lyꢀunsaturated metal cations and are unable to form the
MI–NO+ adducts likely responsible for the poor NO reꢀ
lease displayed by HKUSTꢀ1. Indeed, the frameworks
adsorbed close to 7 mmol/g NO at room temperature and
released the entire quantity within 15 h of exposure to
humid air. These frameworks, however, suffer from bioꢀ
compatibility issues, as they are based on cobalt or nickꢀ
el. The NO storage and release properties of a family of
biocompatible MILꢀ88(Fe) based metalꢀorganic frameꢀ
works were recently reported.12 However, these materiꢀ
Metalꢀorganic frameworks, which have received a
great deal of attention for gas storage and molecular
separations,1 have also recently shown promise for apꢀ
plications in the biomedicine, typically for drug storage
and delivery.2 Although a number of structures have
been synthesized from biologically active ligands,3 bioꢀ
active molecules can also be incorporated into a metalꢀ
organic framework postꢀsynthetically4 or produced via
the catalytic decomposition of precursor molecules such
as Sꢀnitrosothiols.5 An important example of this is the
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Figure 2. Infrared spectra of Fe2(dobdc) in the presence of inꢀ
Figure 1. Adsorption of NO in Fe2(dobdc) at 298 K; closed and
open symbols represent adsorption and desorption, respectively.
Saturation is nearly achieved by 7 mbar (inset).
creasing NO pressure at room temperature (0ꢀ0.04 mbar interval),
the IR spectrum of activated Fe2(dobdc) has been subtracted. Inꢀ
set: Diffuse reflectance UV–Vis–NIR spectra of Fe2(dobdc) (red)
and Fe2(NO)2(dobdc) (black).
als adsorb and release < 0.35 mmol of NO per gram,
significantly less than the M2(dobdc) frameworks.
ing to a significantly bent M–N–O angle. In the case of
{FeꢀNO}7, these values typically fall in the range of
1700ꢀ1860 cm–1.14b,d Accordingly, Fe2(NO)2(dobdc) disꢀ
plays a single peak at low NO coverage of 1782 cm–1,
the small peak around 1810 cm–1 is an effect of backꢀ
ground subtraction (see Figure 2). The observed red shift
is significantly larger than that measured on the isostrucꢀ
tural Ni2(dobdc) (1845 cm–1) system,11 and is even larger
than that observed upon dosing NO on very active surꢀ
face sites such as CrII (1810 cm–1)15a or FeII (1845ꢀ1860
cm–1)14a on silica or CuI (1790ꢀ1810 cm–1)15bꢀe in zeoꢀ
lites. This indicates a substantial charge transfer from the
FeII centers in Fe2(dobdc) to NO.
One of the more recently discovered members of the
M2(dobdc) series, Fe2(dobdc),13 combines the adꢀ
vantages of both systems, exhibiting the high density of
accessible sites found in all M2(dobdc) compounds and
the biocompatibility of iron. Moreover, it has been
demonstrated that, unlike other members of the
M2(dobdc) family, Fe2(dobdc) displays strong, selective
interactions with O2 based on the accessible redox nature
of the FeII centers in the material. Given these promising
attributes, we sought to investigate NO storage and reꢀ
lease in Fe2(dobdc).
The NO adsorption isotherm at 298 K, shown in Figꢀ
ure 1, offers an initial indication that Fe2(dobdc) may be
of utility for NO storage and release. Under these condiꢀ
tions the isotherm is extraordinarily steep, approaching
saturation near 0.007 bar. At this very low pressure, the
adsorption corresponds to 0.95 NO molecules per iron
site and reaches 1.00 at 1.2 bar. In contrast, the adsorpꢀ
tion of NO in Co2(dobdc) and Ni2(dobdc) reaches one
molecule per accessible metal site at pressures greater
than 0.2 bar.10 All three frameworks show significant
hysteresis in their adsorption isotherms. While the cobalt
and nickel analogues desorb approximately 1ꢀ2 mmol/g
at low pressure, Fe2(dobdc) retains 0.95 NO/Fe (6.21
mmol/g) upon desorption to 0.007 mbar. Further experꢀ
iments, in which the sample is placed under dynamic
vacuum or flowing N2, indicate NO remains bound until
at least 353 K. Similar to the adsorption of O2 in this
framework, the bright green sample turns dark brown
upon adsorption of NO, indicating oxidation of the FeII
sites.13
The highest coverage spectrum displayed in Figure 2
corresponds to an NO equilibrium pressure (PNO) of 0.04
mbar. Further increases in PNO reveal that the band at
1782 cm–1 is already saturated, as it does not move in
frequency (figure S1), reflecting the isolated nature of
the FeIII–NO– oscillator inside the framework.14d The
steep increase of the FeIII–NO– band in the low PNO reꢀ
gion, fully supports the adsorption data reported in Figꢀ
ure 1. No additional bands ascribable to nitrosylic speꢀ
cies were observed at any pressure, indicating that the
monoꢀnitrosyl complex does not evolve into iron polyꢀ
nitrosyls, as observed in zeolites.14a This confirms the
1:1 stoichiometry measured in the volumetric adsorption
experiment. After NO adsorption, O2 was introduced to
the framework to probe whether any accessible iron(II)
sites remain. As shown in Figure S1 of the Supporting
Information, the spectrum is nearly unchanged with a
minimal amount of hydroxyl formation observed. This
additional experiment demonstrates that the NO coordiꢀ
nated to Fe prevents oxidation by O2 and the decomposiꢀ
tion otherwise observed upon exposing Fe2(dobdc) to
air.13 Evacuation of Fe2(NO)2(dobdc) at 353 K does not
result in any appreciable NO desorption, while the NꢀO
band disappears almost completely upon evacuation at
433 K (orange and red spectra in Figure S1).
Infrared spectroscopy offers a convenient means of
investigating the nature of NO binding and has been
widely employed to probe the interaction of NO with
iron(II) centers.14 The N–O stretching frequency of free
nitric oxide occurs at 1880 cm–1, and can range from
1500 to 1900 cm–1 upon coordination to a transition
metal. The specific energy depends on the mode of coꢀ
ordination, with lower frequencies usually correspondꢀ
The formation of a stable FeIII–NO– complex with a
bent Fe–N–O geometry is also suggested by the UVꢀvis
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absorption spectra, as shown in the inset of Figure 2.
These spectra reveal the disappearance of the dꢀd bands
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around 5,000 and 12,000 cm–1, which is accompanied by
the appearance of three new electronic transitions at
14,000, 17,000, and 20,000 cm–1 that are likely related to
dꢀd transitions with mixed p character. Extensive mixing
of the metal and ligand wave functions in the molecular
orbitals of the complex does not allow a distinction beꢀ
tween ligand field and charge transfer for these transiꢀ
tions.
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Both Mössbauer spectroscopy and dc magnetic susꢀ
ceptibility measurements confirm the charge transfer
from iron(II) to NO. At room temperature the Mössbauer
spectrum of Fe2(dobdc) features a simple doublet. This
doublet exhibits an isomer shift and quadrupole splitting
of 1.094(3) and 2.02(1) mm/s, respectively, both conꢀ
sistent with highꢀspin iron(II).13 The Mössbauer spectra
of Fe2(NO)2(dobdc) indicates oxidized iron cations with
substantially reduced isomer shift and quadrupole splitꢀ
ting (see Figure S3). At 298 K these values, 0.563(4) and
1.057(7) mm/s, respectively, are consistent with highꢀ
spin iron(III) in an octahedral coordination environment.
Although Mössbauer studies on octahedral {FeꢀNO}7
systems are relatively rare,16 the parameters for
Fe2(NO)2(dobdc) are in good agreement, for example,
with the reported values of δ = 0.68 mm/s and ꢂE = 1.03
mm/s for deoxy hemerythrin.17 The room temperature
Figure 3. Structures of Fe2(dobdc) and Fe2(NO)2(dobdc) as deꢀ
termined from Rietveld analysis of neutron powder diffraction
data. Orange, blue, red, gray, and white spheres represent Fe, N,
O, C, and H atoms, respectively. Values in parentheses give the
estimated standard deviation in the final digit of the number.
Figure 4. Release of NO by Fe2(NO)2(dobdc) upon contact with
11 % relative humidity N2 at 310 K. The red curve quantifies the
total NO released in mmol/g, while the black data indicates that
NO is still evolving after 10 days.
χ
MT value of 3.40 cm3K/mol indicates an S = 3/2 sysꢀ
tem, consistent with an S = 5/2 FeIII center antiferroꢀ
magnetically coupled to an S = 1 NO– moiety (Figure
S4).
leased approximately twoꢀthirds of the strongly bound
NO. A reproducible increase in NO concentration is seen
after approximately 3 days, which may correspond to
some structural rearrangement or partial collapse as NO
is released. Although the total quantity of 4.0 mmol/g
released was less than released by both Ni2(dobdc) and
Co2(dobdc) (7.0 and 6.7 mmol/g, respectively) tested
under the same conditions,10 it was delivered much more
slowly, still releasing PPB concentrations after 10 days.
As there is approximately 2.0ꢀ2.5 mmol/g of NO reꢀ
maining, this material has the possibility of releasing
NO for an even longer period of time. A similar release
behavior was observed at 298 K (see Figure S5).
We turned to powder neutron diffraction to further inꢀ
vestigate the binding of NO within Fe2(dobdc). A
Rietveld refinement was performed against data collectꢀ
ed for a sample of Fe2(NO)2(dobdc) at 4 K. From this
model a single adsorption site is apparent with coordiꢀ
nated nitric oxide exhibiting a refined occupancy of
0.970(5) NO molecules per Fe site. As expected, NO is
Nꢀbound with a bent geometry, featuring an Fe–N disꢀ
tance of 1.785(7) Å and an Fe–N–O angle of 151.4(6)°
(see Figure 3). Both of these values are consistent with
previously reported FeIII–NO– species.18 As observed
previously in Fe2(OH)2(dobdc),19 the Fe–O bond trans to
the open coordination site elongates substantially upon
NO coordination, shifting from 2.13(2) to 2.31(1) Å.
The N–O distance of 1.17(1) Å is slightly elongated
from that of free NO (1.154),20 consistent with the forꢀ
mation of NO–.
The foregoing results demonstrate that Fe2(dobdc) can
serve as a potentially biocompatible platform for the
slow, moistureꢀtriggered release of NO. The strong bindꢀ
ing of nitric oxide to the coordinativelyꢀunsaturated, reꢀ
doxꢀactive FeII sites within Fe2(dobdc) is attributed to
the formation of FeIII–NO– adducts, as characterized
spectroscopically by various methods and structurally by
powder neutron diffraction. Most significantly, as a reꢀ
sult of the strong binding, Fe2(NO)2(dobdc) continues to
steadily release NO over the course of at least 10 days.
These results demonstrate the potential utility of iron(II)
based metalꢀorganic frameworks for the strong binding
and slow release of nitric oxide
To monitor NO release, a sample of Fe2(NO)2(dobdc)
(see the Supporting Information for full experimental
procedures) was exposed to N2 at 11% relative humidity
at 310 K, and the amount of NO released was quantified
by chemiluminescence. As shown in Figure 4, the mateꢀ
rial initially releases a short burst of NO, which is likely
attributable to a small amount of physisorbed gas. Over
the duration of the experiment, the compound slowly reꢀ
ASSOCIATED CONTENT
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Supporting Information. Experimental details, additional
Page 4 of 5
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IR and Mössbauer spectra, magnetic susceptibility, powder
diffraction pattern, and room temperature triggered release.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
jrlong@berkeley.edu
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This research was supported through the Center for Gas
Separations Relevant to Clean Energy Technologies, an
Energy Frontier Research Center funded by the U.S. Deꢀ
partment of Energy, Office of Science, Office of Basic Enꢀ
ergy Sciences under award DEꢀSC0001015. We thank
Arkema and Gerald K. Branch for fellowship support of
E.D.B. and Ateneo Project 2011 ORTO11RRT5 for finanꢀ
cial support of S.B., C.L., and S.C.
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