Pushing the photodelivery of nitric oxide to the visible: Are {FeNO}7 complexes good candidates?

Article abstract of DOI:10.1039/c7dt03142d

Photodelivery of NO requires stable compounds which can be made reactive by irradiation with (visible) light. Traditional {MNO}⁶ complexes require a substantial ligand design to shift their absorption spectra to the appropriate region of the electromagnetic spectrum. [Fe((CH₂Py₂)₂Me[9]aneN₃)(NO)](BF₄)₂ is a new {FeNO}⁷ octahedral coordination compound, which is thermally and air-stable in solution. Illumination with a 450 nm light source induces significant photodetachment of the coordinated NO (_NO = 0.52 mol einstein^-1), suggesting that {FeNO}⁷ compounds can be in fact suitable compounds for therapeutic NO-photorelease.

Full text of DOI:10.1039/c7dt03142d

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Pushing the photodelivery of nitric oxide to the
visible: are {FeNO}7 complexes good candidates?†
Cite this: DOI: 10.1039/c7dt03142d
Natalia Levin, a Julián Perdoménico, a Eckhard Bill,b Thomas Weyhermüllerb
a
and Leonardo D. Slep
*
Photodelivery of NO requires stable compounds which can be made reactive by irradiation with (visible)
light. Traditional {MNO}6 complexes require a substantial ligand design to shift their absorption spectra to
the appropriate region of the electromagnetic spectrum. [Fe((CH2Py2)2Me[9]aneN3)(NO)](BF4)2 is a new
{FeNO}7 octahedral coordination compound, which is thermally and air-stable in solution. Illumination
with a 450 nm light source induces significant photodetachment of the coordinated NO (ϕNO = 0.52
mol einstein−1), suggesting that {FeNO}7 compounds can be in fact suitable compounds for therapeutic
NO-photorelease.
Received 24th August 2017,
Accepted 26th October 2017
DOI: 10.1039/c7dt03142d
rsc.li/dalton
lation of a 3MLCT state that leads to the generation of an
unstable intermediate {RuIII-NO•} and finally of the photopro-
Introduction
NO releasing compounds were already used therapeutically for duct [RuIII-solv]n+ (eqn (1)).
the treatment of illnesses1 long before the discovery of the
hν;solv
nþ
½MNOꢀ ꢁꢁꢁꢁꢁꢁ! ½MðsolvÞꢀnþ þ NO
ð1Þ
multiple physiological activities that nitric oxide (NO) presents
in mammals,2 and before the proposal of an endogenous syn-
thetic path for it.3 The findings of the role of NO in cellular
apoptosis in the early 1990s promoted the search of appropri-
ate NO-releasing molecules for use in antitumor therapies.
One of the strategies developed was focused on designing NO-
photoreleasing species to be employed in photodynamic
therapy (PDT).4 The quest for robust nitrosyl metallic com-
plexes thermally stable under physiological conditions and
sensitive only to a specific incident irradiation source (if poss-
ible in the visible range of the spectrum) while having at the
same time high quantum yields of NO release (ϕNO) fuelled
the field of photochemistry of inorganic nitrosyl complexes.5
In this regard, ruthenium nitrosyls emerged as good candi-
dates and have been the main focus of this kind of investi-
gation, particularly constrained to {RuNO}6 species, due to
their stability towards O2 and ease of manipulation.5,6 The
abundant literature on the subject offers a general consensus
through which the release of a NO molecule involves the popu-
The strong π-acceptor nature of the nitrosyl moiety in
{MNO}6 species is responsible for the main complication in
this kind of substance. The covalent interaction with the t2g
set of metal orbitals shifts the whole absorption spectrum to
the UV region making most of the known substances unsuita-
ble for any practical application. Some of the most interesting
reports on the field explore in fact creative synthetic variations
intended to red-shift the absorption profile.7
The exploration of {MNO}n NO-releasing species with
n greater than 6 has only scarcely appeared more recently in
the literature. This is surprising, given the fact that these
reduced NO holding species are expected to display lower
energy electronic transitions without the need for extensive co-
ligand design due to the lower covalency of the M–NO bond.
In 2005, Sauaia et al. reported the photochemistry of a
{RuNO}7 species (cis-[Ru(bpy)2(4-pic)(NO)]2+, 4-pic = 4-picoline)
generated in situ by electrochemical reduction in aqueous
medium.8 We reported later the first XRD characterization of a
{RuNO}7 complex ([Ru(Me3[9]aneN3)(bpy)(NO)]2+) whose solu-
tions proved sensitive to a 365 nm irradiation source leading
to NO release.9 In both cases, the processes were interpreted
according to eqn (2), where the oxidation state of the metal
center remains unchanged upon NO release.
aDepartamento de Química Inorgánica, Analítica y Química Física,
Facultad de Ciencias Exactas y Naturales, and INQUIMAE, Universidad de Buenos
Aires – CONICET, Pabellón 2, 3er piso, Ciudad Universitaria, C1428EHA,
Ciudad Autónoma de Buenos Aires, Argentina. E-mail: slep@qi.fcen.uba.ar
bMax-Planck-Institut für Chemische Energiekonversion, Stiftstraße 34-36,
D-45470 Mülheim an der Ruhr, Germany
hν;solv
nþ
½MIINOꢀ ꢁꢁꢁꢁꢁꢁ! ½MIIðsolvÞꢀnþ þ NO
ð2Þ
†Electronic supplementary information (ESI) available. CCDC 1565106. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c7dt03142d
To our knowledge there has been only one investigation of
another {FeNO}7 species, [Fe(N3PyS)(NO)]2+ (N3PyS = N,N-bis(2-
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pyridylmethyl)-(N-(bis-2-pyridylmethyl)amine)),
that
also 13C-DEPT-NMR (Fig. S2†) (CDCl3, 500 MHz, 298 K): δ (ppm)
shows the release of NO in acetonitrile when interacting with 45.11 (CH3), 54.42 (CH2), 46.35, 45.99 (CH2).
an irradiation source, following the same reaction stoichio-
metry depicted in eqn (2).10 Noticeably, the photodetachment
Synthesis of (CH2Py2)2Me[9]aneN3
of NO from the latter seems to be induced under low-intensity
The preparation of the ligand starting from Me[9]aneN3 has
been previously described.14 0.526
of Me[9]aneN3
visible light, though no overall quantum yield has been
reported. The lack of additional reports on {FeNO}7 photo-
active species, combined with the fact that the compound
[Fe(N4Py)(NO)]2+ (N4py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)
methylamine) also explored in the same report behaves as a
non-photoactive species, might be wrongly considered as
enough evidence to claim that [Fe(N3PyS)(NO)]2+ shows an
exceptional behavior, when in fact this subject has not been
sufficiently explored. We report herein the synthesis and com-
plete characterization of a new {FeNO}7 species holding a
coordination sphere inspired from our previous report of a
{RuNO}7 complex. The new compound, as well as its
Ru analog, shows NO release photoreactivity when exposed to
a 365 nm light source but also efficient photorelease of NO
upon irradiation in the visible range.
g
(3.67 mmol) were dissolved in 30 mL of water together with
1.22 g of picolyl chloride (7.44 mmol). The pH of the solution
was brought to ca. 11 by the addition of aliquots of a 2 M solu-
tion of NaOH. The solution was stirred for three days during
which the pH was kept at the same value by further additions
of the basic solution. After this period, NaOH (2 M) solution
was added to the mixture to achieve pH 13. Upon this, the
reaction medium was extracted with CHCl3 (6 × 30 mL). The
combined organic layers were dried over anhydrous Na2SO4,
filtered and evaporated in a rotavap to afford 0.957 g of an
orange oil (2.94 mmol, 80%), which contained 90.2% of the
desired compound according to a quantitative GC determi-
nation employing a RTx-5 Amine S-77 column, length 15 m
and diameter 0.25 mm, along with a FID detector (Fig. S3†).
The product eluted with a retention time of 23.07 min using
H2 as the gas carrier (0.42 bar, 1.7 mL min−1) and a tempera-
ture ramp of 8 °C per minute starting at 70 °C until 300 °C.
13C-DEPT-NMR (Fig. S3†) (CD3CN, 500 MHz, 298 K): δ =
47.13, 56.91, 58.33, 59.18, 65.78, 123.17, 124.49, 137.57,
150.02 ppm, in accordance with the 13C-NMR spectrum col-
lected previously.14
Materials and experimental procedures
Materials and reagents. The reagents employed in the syn-
thetic procedures were purchased from Sigma-Aldrich, and
were used without further purification. All organic solvents
employed in synthetic procedures or physical determinations
were dried and freshly distilled before use following the stan-
dard procedures.11 A vacuum line and Schlenk glassware
(or alternatively a glovebox) were employed when the manipu-
lation required the exclusion of air. The precursor 1,4,7-triaza-
tricyclo[5.2.1.04.10]decane (the orthoamide of 1,4,7-triazacyclo-
Synthesis of [Fe((CH2Py2)2Me[9]aneN3)(NO)](BF4)2
([1-NO](BF4)2)
nonane) of the pentadentate ligand employed in the synthesis 0.99 g of Fe(BF4)2·5H2O (3.10 mmol) were dissolved in a
of the complex was prepared according to an already published mixture of dry and degassed solvents (6 mL of methanol
procedure.12
(MeOH) and 2 mL of acetonitrile (CH3CN)). The initially
almost colorless solution turned dark brown upon the
Synthesis of Me[9]aneN3
addition of 1.12 g of (CH2Py2)2Me[9]aneN3 (3.44 mmol) pre-
The procedure was inspired by
a
previous report.13 dissolved in 2 mL of MeOH and 1 mL of CH3CN. After two
Modifications described here simplify the synthesis without hours of stirring, the solution was cooled down with an ice
the loss of yield: 0.720 g of 1,4,7-triazatricyclo[5.2.1.04.10
bath. NO(g) (purchased from Air Liquide, 99.5% pure, passed
decane (5.18 mmol) were dissolved in 10 mL of THF to afford through a KOH(s) trap before use) was bubbled into the vigor-
pale yellow solution. Subsequently, 1.30 of CH3I ously stirred solution, which turned immediately into a darker
]
a
g
(9.16 mmol) were added dropwise to the mixture, upon which shade, almost black. The NO bubbling was maintained for five
a white precipitate was formed that turned pale yellow over minutes after which Ar was circulated through the system for
time. The reaction medium was stirred for 4 hours and the pre- another ten minutes, to eliminate the excess of reagent. The
cipitate was then separated by filtration and suspended in ice bath was removed and the reaction medium was concen-
10 mL of water. The mixture was brought to reflux inducing trated overnight by means of a subtle Ar flow. After this period
the full dissolution of the solid. Once at room temperature, red crystals shaped like prisms were found in the round flask
the yellow solution was kept at 4 °C overnight. The following surrounded by a red oil and recovered under an Ar atmosphere
day, 1.47 g of NaOH was added under stirring to the cold solu- for XRD investigation. The oil was treated in the glove box with
tion, after which the reaction mixture was refluxed for three cold degassed solvents (3 mL MeOH and 1 mL CH3CN) and
hours. After reaching room temperature, the solution was left overnight at −30 °C under N2 upon which a red precipitate
extracted with CHCl3 (6 × 30 mL). The combined organic layers was found in the reaction medium yielding 72.2 mg of a red
were dried over anhydrous Na2SO4, filtered and evaporated in solid (0.124 mmol, 39.7%). Anal. calcd for [1-NO](BF4)2,
a rotavap to yield 0.526 g of a pale yellow oil (3.67 mmol, C19H27N6O1B2F8Fe (Mr = 584.92 g mol−1): C 38.9, H 5.0, N 14.3.
1
70.9%). H-NMR (Fig. S1†) (CDCl3, 500 MHz, 298 K): δ (ppm) Found: C 37.8, H 4.9, N 13.7. MS (ESI+) (Fig. S4†) m/z: 205 =
2.35 (s, 3H, CH3), 2.48 (m, 4H, CH2), 2.68 (m, 8H, CH2). [M − 2BF4−]2+; 190.6 = [M − NO − 2BF4−]2+.
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Physical determinations. Microanalytical data for C, H and colored species were obtained by means of a chemometric
N were obtained with a Carlo Erba EA 1108 analyzer. UV-vis procedure recently described elsewhere.17
spectra were recorded with either an HP8453 or an HP8452A
The crystal structure of compound [1-NO](BF4)2 was deter-
diode array spectrophotometer. IR spectral measurements mined using a Bruker-Nonius Kappa Mach3/APEX II diffract-
(KBr pellets) were carried out using a Nicolet iS10. The 1H and ometer equipped with a Mo IμS anode and INCOATEC Helios
13C NMR spectra were recorded with a 500 MHz Bruker mirror optics (λ = 0.71073 Å). Diffraction data were collected at
AVANCE III spectrometer. ESI spectra were obtained with a 100 K in a nitrogen cryo-stream. Final cell constants were
standard ESI source and Q Exactive Plus from ThermoFisher obtained from least squares fits of several thousand strong
Scientific equipment. X-band EPR spectra were recorded at reflections. Intensities of redundant reflections were used to
85 K with a Bruker ESP 300 spectrometer equipped with an correct for absorption using the SADABS program.18 The struc-
Oxford ESR 910 liquid-helium cryostat and an Oxford temp- ture was readily solved by Patterson methods and subsequent
erature controller. X-band EPR spectra were simulated with difference Fourier techniques. The Siemens ShelXTL software
EasySpin 5.0.9.18 Cyclic voltammograms were recorded in package19 was used for solution and artwork of the structures,
dried and degassed CH3CN with 0.2 M of Bu4NPF6 as a and ShelXL-201320 was used for structure refinement. All non-
support electrolyte in a three-electrode cell. The working elec- hydrogen atoms were anisotropically refined and hydrogen
trode was a 3 mm diameter vitreous carbon disc, the counter atoms bound to carbon were placed at calculated positions
electrode was a Pt wire and the reference electrode an Ag wire. and refined as riding atoms with isotropic displacement para-
−
The measured potentials were corrected by the use of an meters. The BF4 anions were found to be disordered on two
internal standard (ferrocene or decamethylferrocene).15 The positions. A split atom model with restrained atomic displace-
potential of the working electrode was controlled with a ment parameters and bond lengths was refined using the
BioLogic SP300 potentiostat. Combined controlled potential ShelXL EADP, SADI and ISOR instructions. The occupation
coulometry and UV-Vis spectroelectrochemistry experiments factor ratios refined to values of about 0.6 : 0.4 and 0.67 : 0.33,
were performed in dried and degassed CH3CN (0.1 M respectively. CCDC 1565106† contains the supplementary crys-
Bu4NPF6 as a support electrolyte) in a three-electrode home- tallographic data for this paper.
made cell coupled to a quartz cuvette (1 cm path). The
Theoretical calculations. All calculations reported in this
working electrode was a 1 cm2 Pt mesh, the counter electrode paper were performed with the program package Gaussian
was a Pt wire immersed in CH3CN/0.1 M Bu4NPF6 and the 09.21 The geometry optimizations were carried out at the BP86
reference electrode a Ag wire immersed in CH3CN/0.1 M level22 of DFT. This functional has repeatedly proved to be
Bu4NPF6/0.01 M AgNO3. An internal ferrocene (Fc) standard reliable in predicting accurate structures of transition metal
was employed as a reference15 at the end of the experiments complexes. We employed the all-electron Gaussian basis sets
(using a 2 mm diameter vitreous carbon disc as the working reported by the Ahlrichs group. For all the atoms accurate
electrode). The system was maintained at −20
1 °C by triple-ζ valence basis sets with one set of polarization (TZV(P))
means of an RC6 LAUDA thermostat. The potential of the functions were used.23 We employed tight SCF convergence cri-
working electrode was controlled with a TEQ-03 potentiostat. teria and default settings in the geometry optimizations. The
Photochemical experiments were performed with either nature of the resulting stationary points was in all cases tested
3.0 mL or 3.3 mL of solutions contained in a 1.00 cm path by computing the vibrational spectra.
length fluorescence cuvette employing respectively a 365 nm
or a 450 nm light emitting diode. The intensity of the 450 nm
LED (2.70 × 10−6 einstein s−1 dm−3) was determined by refer-
ence to a 365 nm LED source employing a FieldMaster-
Coherent power meter with a LM-2UV photodiode as a light
sensor. The 365 nm LED was calibrated by actinometry
Results and discussion
Synthesis and characterization of [Fe((CH2Py2)2Me[9]aneN3)
(NO)](BF4)2 ([1-NO](BF4)2)
employing a standard solution of K3[Fe(C2O4)3] by means of The pentadentate ligand (CH2Py2)2Me[9]aneN3 chosen for the
an already described procedure,16 yielding an intensity of preparation of the iron nitrosyl complex has already been
1.59 × 10−6 einstein s−1 dm−3. The spectral evolution was employed in the preparation of acetonitrile complexes of Ru
monitored spectrophotometrically along the course of the and Fe in the context of water oxidation catalyst investi-
reaction in a 90° configuration. The evolution of NO was gation.24 The strategy involved here in its preparation is
qualitatively revealed by its reaction with reduced myoglobin different from the reported one, making its synthesis easier.
(Mb).7g,h An argon stream was passed through the irradiated Addition of the free ligand to a solution of Fe(BF4)2·5H2O in
solution and collected afterwards in a ca. 1 × 10−6 M solution CH3CN/MeOH under anaerobic conditions led to an immedi-
of Mb at pH = 7.2 (30 mM phosphate buffer), which was pre- ate color change of the solution suggesting that the coordi-
viously prepared by the reaction of metMb with sodium nation had taken place at room temperature. Bubbling of
dithionite under argon. The coordination of NO to the Fe(II) NO(g) into the ice cold mixture drastically turned the solution
center of Mb was monitored spectrophotometrically into almost black. Slow evaporation with an Ar stream yielded
(Fig. S9†).7g,h Numerical values for the quantum yields for the red prisms suitable for XRD investigation and allowed the iso-
photoreactions (ϕNO) and deconvoluted spectral profiles of the lation of the bulk product by treatment of the rest of the batch.
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to 2.096(3) Å in [1-NO]2+, which is caused by the NO-induced σ-
trans effect. This behavior has been observed in other {MNO}7
systems and is relevant in the field of bioinorganic chem-
istry.1,26 The {FeNO} moiety is noticeably bent, with an Fe–
N(1)–O(1) angle of 148.3(4)°, which is within the range charac-
teristic of {MNO}7 species (137°–155°).27,28 Particularly, bond
lengths and angles agree well with those observed in
[Fe(N3PyS)(NO)](BF4) and [Fe(N4Py)(NO)](BF4)2.10 The {MNO}
moiety also resembles the one in the only report of an hexa-
coordinated {RuNO}7 species, [Ru(Me3[9]aneN3)(bpy)(NO)]
(BF4)2 ([2-NO](BF4)2).9 Table 1 also collects structural para-
meters for the DFT optimized structures of [1-NO]2+ and
[1-CH3CN]2+. Since [Fe(N3PyS)(NO)]+ revealed temperature-
dependent high-spin (S = 3/2) and low-spin (S = 1/2) equili-
brium,29 the computations of [1-NO]2+ have been performed
considering both possibilities. The computed S = 3/2 spin state
Fig. 1 Structure of the cation (30% probability level thermal ellipsoids)
in the crystal of [1-NO](BF4)2.
The XRD investigation of the single crystals showed that the results 55 kJ mol−1 higher in energy than the S = 1/2 state. This
complex crystallizes in the chiral space group P212121 with a fact combined with the comparison between the experimental
unit cell comprised of four [1-NO]2+ cations and eight BF4
and DFT optimized parameters tends to discard the high
−
anions (Fig. S5 and Table S1†). The counterions are partially spin situation, which displays much longer Fe–N bond lengths
disordered on two positions. The Fe2+ atom lies at the center than the experimental ones due to the presence of a HS Fe(II)
of a distorted octahedron; the connectivity pattern and spatial center. Above all, DFT provides a deeper understanding of the
distribution of the atoms in the first coordination sphere electronic structure of the compound. The σ-trans effect arises
confirm the presence of the asymmetric geometric from the partial delocalization of electronic density from the
isomer (asym isomer) (Fig. 1), as was the case for other com- NO-centered π* orbital into the metal centered dz2, and to a
plexes reported with the same pentadentate ligand.24 The lower degree the dxz orbital, all of which contribute to the
refined Flack-parameter (0.017(5)) strongly supports that SOMO of the molecule (Fig. 2, inset). The antibonding charac-
the correct absolute configurations have been determined and teristic of this orbital with respect to the trans bond results in
that the measured crystal was composed of just one the elongation of the Fe–N(2) bond. In addition to these fea-
enantiomer.25
tures, the IR spectrum of the species, collected by an ATR
Table 1 reports the most significant structural parameters, measurement shows an intense peak at 1660 cm−1 (Fig. S6†),
together with those of the previously characterized [1-CH3CN] computed by DFT at 1692 cm−1, characteristic of the NO
(OTf)2 species.24b The substitution of CH3CN by NO has strong stretching vibration in {MNO}7 octahedral species,27,28 and is
implications. Whereas acetonitrile acts mainly as a σ-donor in agreement with the significant π*NO nature of the SOMO,
ligand, nitric oxide possesses the double effect of being both a for it is a strong indicator of the presence of the unpaired elec-
σ-donor and π-donor, therefore strengthening the Fe–N(1) tron in a mainly NO-centered orbital. This description is also
bond, which shortens from 1.929(2) Å in [1-CH3CN]2+ to compatible with the X-band EPR spectrum of [1-NO]2+
1.731(3) Å in [1-NO]2+, while the trans-standing Fe–N(2) bond recorded in a CH3CN frozen glass at 30 K (Fig. 2), which con-
shows pronounced elongation from 1.992(2) Å in [1-CH3CN]2+ firms the spin state assignment. The spectrum shows the
Table 1 Selected experimental and calculated bond lengths, angles and NO stretching frequenciesa
b
d
[1-NO](BF4)2
[1-NO]2+ S = 1/2c
[1-NO]2+ S = 3/2 c
[1-CH3CN](OTf)2
[1-CH3CN]2+ S = 0c
[1-CH3CN]2+ S = 2c
Distances/Å
Fe–N(1)
Fe–N(2)
Fe–N(3)
Fe–N(4)
Fe–N(5)
Fe–N(6)
N(1)–O(1)
1.731(3)
2.096(3)
1.987(3)
2.031(4)
1.992(3)
1.991(4)
1.143(6)
1.736
2.145
2.004
2.108
2.004
2.027
1.183
1.741
2.226
2.141
2.265
2.163
2.246
1.170
1.929(2)
1.992(2)
1.988(2)
1.994(2)
1.970(2)
2.047(2)
1.891
2.028
1.972
2.084
1.994
2.024
2.117
2.285
2.153
2.278
2.183
2.263
Angle/°
Fe–N(1)–O(1)
148.3(4)
1660
145.5
1692
151.8
1765
—
νNO/cm−1
a See Fig. 1 for atom labeling. b This work. c Computed employing DFT at the BP86/TZV(P) level of theory. d Ref. 24b.
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AgCl/NaCl) (Fig. 3). Controlled potential coulometry and UV-
vis spectroelectrochemistry confirm the one-electron nature of
both processes. The spectral changes associated with the one-
electron oxidation and reduction of {FeNO}7 (Fig. 4 and
Table S3†) are consistent with the previous reports of conver-
sion into {FeNO}6 and {FeNO}8, respectively in related
species.30,32
Photochemical reactivity
When irradiated with a 450 nm light source, solutions of
[1-NO]2+ in acetonitrile undergo significant spectral changes,
marked by the appearance of a new band in the visible range
(400 nm) (Fig. 5) with the concomitant production of NO,
which was detected qualitatively by its reaction with reduced
myoglobin (Mb).7g,h This pronounced variation of the absorp-
tion profile occurs with a well-defined isosbestic point,
suggesting that the process can be interpreted as a clean con-
version of the reagent into a single colored photoproduct.
A
multiwavelength chemometric analysis of the spectral
changes17 in the region between 300 and 700 nm allowed us to
calculate the concentration profile of the reaction and to
deconvolute the spectrum of the product, which matches per-
fectly the one of [1-CH3CN]2+.24b This suggests that [1-NO]2+
Fig. 2 X-band EPR spectrum of [1-NO]2+ in 0.2 M CH3CN/[Bu4N]PF6 at
30 K. Experimental conditions: Microwave frequency 9.6349 GHz,
modulation of amplitude 1 G. Bottom: Computer-simulated spectrum
(details in the text). Inset: Calculated SOMO of the species (DFT).
typical S = 1/2 signature of the {MNO}7 fragment9,10,30,31 with
rhombic g-values and hyperfine interaction with a single 14N
nucleus (I = 1). The simulation and fitting of the spectrum
affords the spin-Hamiltonian g-values: g = [2.0383
2.0137 0.0001, 1.9605 0.0001], and the diagonal values of the
A-tensor: A = [0.77 0.04, 23.89 0.06, 8.5 0.5] × 10−4 cm−1
0.0003,
.
The electrochemical exploration by cyclic voltammetry in
CH3CN reveals two waves at 0.964 V and −0.327 V (vs. 3 M Ag/
Fig. 4 UV-Vis spectra obtained by spectroelectrochemistry of [1-NO]
(BF4)2 in CH3CN/0.1 M [Bu4N]PF6 at 253 K under anaerobic conditions
(Ar). Top: One-electron oxidation at 1.2 V vs. 3 M Ag/AgCl/NaCl (inset:
zoom in). Bottom: One-electron reduction at −0.6 V vs. 3 M Ag/AgCl/
NaCl.
Fig. 3 Cyclic voltammogram of [1-NO](BF4)2 in CH3CN/0.2 M [Bu4N]
PF6 at 298 K. Scan rate 100 mV s−1. E1/2 detected at 0.964 V and −0.327
V (vs. 3 M Ag/AgCl/NaCl).
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this compound can be seen as a valuable example of a new
kind of NO-photoreleasing system that deserves exploration,
not only in connection to its bioinorganic relevancy, but also
to provide insight into fundamental aspects of inorganic
photochemistry. We seek to close this gap and a systematic
study of related compounds with changes in the coordination
sphere is underway to assess the effect these changes may have
on the NO-photorelease quantum yield.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Fig. 5 Spectral changes in the UV-vis spectra of an acetonitrile solution
of [1-NO](BF4)2 interacting with a light source (λirr = 450 nm) under
anaerobic conditions (Ar). Inset: Calculated concentration profiles for
[1-NO]2+ (black) and [1-CH3CN]2+ (red).
This work has been supported by grants from CONICET,
ANPCyT and the University of Buenos Aires (UBA). LDS is a
member of the scientific staff of CONICET. NL and JP are doc-
toral fellows of the same institution. We wish to thank
Dr A. Roitberg for providing access through collaboration to
the computational resources of the National Center for
Supercomputing Applications.
undergoes a rupture of the Fe–NO bond, photoreleasing a
nitric oxide molecule, with the incorporation of a solvent mole-
cule into the coordination sphere, as described in eqn (3):
hν; CH3CN
2þ
½1 ꢁ NOꢀ ꢁꢁꢁꢁꢁꢁ! ½1 ꢁ CH3CNꢀ2þ þ NO:
ð3Þ
References
This behavior parallels the one observed in the only photo-
active {FeNO}7 species reported in the literature.10 We don’t
detect any evidence of recombination of the photoreleased NO
and estimate ϕNO as 0.52 mol einstein−1, affording therefore a
remarkably higher value than the values reported in the two
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the estimated ϕNO becomes only slightly smaller (0.40
mol einstein−1, Fig. S7†).
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Whereas it is well established that {MNO}6 species can easily
photorelease NO by interaction with light of an appropriate
wavelength, the photochemistry of {MNO}n (n ≠ 6) species has
so far been barely explored in the literature. This might lead to
the wrong impression that these species are not accessible to
the photorelease of NO. This above discussed new {FeNO}7
species shows that the photodetachment of a nitric oxide
molecule from an octahedral {FeNO}7 compound is possible
and most probably a normal process as for {MNO}6 complexes.
At this point, however, we cannot find a rational explanation to
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may have a significant effect on the value of ϕNO. In any case,
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