Bioinspired oxidation of oximes to nitric oxide with dioxygen by a nonheme iron(II) complex

Article abstract of DOI:10.1007/s00775-019-01726-6

The ability of two iron(II) complexes, [(Tp^Ph2)Fe^II(benzilate)] (1) and [(Tp^Ph2)(Fe^II)₂(NPP)₃] (2) (Tp^Ph2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate, NPP-H = α-isonitrosopropiophenone), of a monoanionic facial N3 ligand in the O₂-dependent oxidation of oximes is reported. The mononuclear complex 1 reacts with dioxygen to decarboxylate the iron-coordinated benzilate. The oximate-bridged dinuclear complex (2), which contains a high-spin (Tp^Ph2)Fe^II unit and a low-spin iron(II)–oximate unit, activates dioxygen at the high-spin iron(II) center. Both the complexes exhibit the oxidative transformation of oximes to the corresponding carbonyl compounds with the incorporation of one oxygen atom from dioxygen. In the oxidation process, the oxime units are converted to nitric oxide (NO) or nitroxyl (HNO). The iron(II)–benzilate complex (1) reacts with oximes to afford HNO, whereas the iron(II)–oximate complex (2) generates NO. The results described here suggest that the oxidative transformation of oximes to NO/HNO follows different pathways depending upon the nature of co-ligand/reductant.

Full text of DOI:10.1007/s00775-019-01726-6

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https://doi.org/10.1007/s00775-019-01726-6
ORIGINAL PAPER
Bioinspired oxidation of oximes to nitric oxide with dioxygen
by a nonheme iron(II) complex
Shrabanti Bhattacharya1 · Triloke Ranjan Lakshman1 · Subhankar Sutradhar1 · Chandan Kumar Tiwari1 ·
Tapan Kanti Paine1
Received: 18 April 2019 / Accepted: 24 September 2019
© Society for Biological Inorganic Chemistry (SBIC) 2019
Abstract
The ability of two iron(II) complexes, [(TpPh2)FeII(benzilate)] (1) and [(TpPh2)(FeII)2(NPP)3] (2) (TpPh2 = hydrotris(3,5-
diphenylpyrazol-1-yl)borate, NPP-H=α-isonitrosopropiophenone), of a monoanionic facial N3 ligand in the O2-dependent
oxidation of oximes is reported. The mononuclear complex 1 reacts with dioxygen to decarboxylate the iron-coordinated
benzilate. The oximate-bridged dinuclear complex (2), which contains a high-spin (TpPh2)FeII unit and a low-spin iron(II)–
oximate unit, activates dioxygen at the high-spin iron(II) center. Both the complexes exhibit the oxidative transformation of
oximes to the corresponding carbonyl compounds with the incorporation of one oxygen atom from dioxygen. In the oxidation
process, the oxime units are converted to nitric oxide (NO) or nitroxyl (HNO). The iron(II)–benzilate complex (1) reacts with
oximes to aford HNO, whereas the iron(II)–oximate complex (2) generates NO. The results described here suggest that the
oxidative transformation of oximes to NO/HNO follows diferent pathways depending upon the nature of co-ligand/reductant.
Graphic abstract
Keywords Oxidation · Oximes · Nitric oxide · Iron · Nonheme
Electronic supplementary material The online version of this
Introduction
article (https://doi.org/10.1007/s00775-019-01726-6) contains
supplementary material, which is available to authorized users.
Nitric oxide (NO) acts as a bio-messenger [1] and governs
many intracellular phenomena such as blood vessel dilation,
neuronal signal transmission, cytotoxicity against pathogens
and tumors, cellular respiration activity, etc. [2–8]. Investi-
gations suggest that excess or lack of nitric oxide production
* Tapan Kanti Paine
ictkp@iacs.res.in
1
School of Chemical Sciences, Indian Association
for the Cultivation of Science, 2A & 2B Raja S. C. Mullick
Road, Jadavpur, Kolkata 700032, India
Vol.:(0123456789)
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may cause diabetes, infammation, myocardial ischemia
and physiological imbalances [9–13]. Considering the bio-
logical and therapeutic importance of this small molecule,
understanding of the biosynthesis route of nitric oxide has
been the central focus of many investigations. Furthermore,
nitroxyl (HNO), the one-electron reduced and protonated
form of NO, has received attention in pharmacology and
biology for its therapeutic potential [14–16].
33]. Based on model studies with iron(II)–porphyrin com-
plexes, a radical-type autoxidation process involving oxi-
dation of oximes to an iminoxyl radical intermediate has
been proposed for NOS-catalyzed oxidation of NHA with O2
[29]. An iron(III)–α-nitrosoalkylperoxide species, proposed
as a key intermediate, eventually generated an iron(IV)–oxo
complex, NO and l-citrulline. A (salen)CoII complex has
been reported to oxidize acetophenone oxime with tert-
butylhydroperoxide in the presence of dioxygen. The oxi-
dation reaction involves initial oxidation of the oxime to
the corresponding iminoxy radical followed by the reaction
with O2 generating NO via a putative α-nitrosoalkylperoxo
intermediate [34]. Therefore, similar to nitrites/nitrates and
nitrosothiols, oximes can be used as precursors for NO and
HNO generation [16, 35].
Nitric oxide is generated from l-arginine in the reaction
catalyzed by the heme monooxygenase, nitric oxide synthase
(NOS) [17–20]. The structure of NOS reveals a redox active
heme site similar to cytochrome P450 with the bound sub-
strate and cofactor tetrahydrobiopterin (H4B) [21, 22]. In the
frst step of the reaction of NOS, l-arginine is converted to
N-hydroxyarginine (NHA) by molecular oxygen [23–25]. In
the subsequent step, 3-electron oxidation of NHA generates
NO radical and l-citrulline (Scheme 1). In the reaction, one
oxygen atom from molecular oxygen is incorporated into the
hydroxylamine group of NHA, from which NO is formed
[26, 27]. The frst hydroxylation step is well-established for
heme oxygenases, where an iron(IV)–oxo-porphyrin radi-
cal cation is involved as the active oxidant. However, the
mechanistic details of the second step remain a matter of
debate [28–30]. A nucleophilic iron(III)–hydroperoxide
intermediate has been implicated to initiate the reaction
via nucleophilic attack at the oxime carbon of NHA [31].
Inspired by the enzymatic reactions, simple substrates such
as aldoximes/ketoximes have been used to generate NO in
the reaction of iron–porphyrin complex with dioxygen [32,
The enzyme, NOS not only utilizes tetrahydrobiopterin
cofactor, but also receives electrons for the reductive activa-
tion of dioxygen and subsequent oxidation of NHA. With an
objective to develop an understanding of the mechanism of
the oxidation of oximes with dioxygen, we have explored
the reactivity of a nonheme iron complex with metal-bound
2-electron reductant toward a series of oximes and of an
iron(II)–oximate complex. We reported an iron(II)–benzilate
complex, [(TpPh2)FeII(benzilate)] (1), of a monoanionic
facial N3 ligand (TpPh2 = hydrotris(3,5-diphenylpyrazol-
1-yl)borate) that reacted with dioxygen to display oxida-
tive decarboxylation reactions. A nucleophilic iron–oxygen
species was intercepted in the decarboxylation pathway. On
the basis of mechanistic studies of the oxidation of diferent
substrates by the iron(II)–benzilate complex and dioxygen,
a nucleophilic iron(II)–hydroperoxide intermediate was
proposed as the active oxidant [36–39]. Herein, we report
the reactivity of the iron(II)–benzilate complex (1) toward
diferent oximes in the presence of dioxygen (Chart 1). An
iron(II)–oximate complex, [(TpPh2)(FeII)2(NPP)3] (2) (NPP
=α-isonitrosopropiophenone), was isolated and structurally
characterized to compare its reactivity with 1. The dioxygen
reactivity of 2 along with the mechanistic studies on the
oxidation of oximes by the iron complexes are presented in
this work.
Scheme 1 Reactions catalyzed by nitric oxide synthase (NOS)
Chart 1 a Iron(II) complexes
and b oximes used in this study
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Na[FeII(NPP)3] (3)
Materials and methods
Ammonium iron(II) sulfate hexahydrate (0.130 g,
0.3 mmol) was dissolved in 10 mL degassed water. To
the solution, sodium salt of α-isonitrosopropiophenone
(0.185 gm, 1 mmol) was added and stirred for 1 h. A deep
blue solid precipitated, which was isolated by fltration and
dried under vacuum. Yield: 0.19 g (67%). Anal. calcd for
C27H24FeN3NaO6 (565.09 g/mol): C, 57.36; H, 4.28; N,
7.43. Found C, 57.02; H, 3.94; N, 7.39. IR (KBr, cm−1):
3250 (br), 2924 (w), 1661 (vs), 1597 (m), 1452 (s), 1323
(s), 1184 (s), 1001 (vs), 897 (m), 712 (s), 665 (m). UV–Vis
in dichloromethane λ, nm (ε, M−1 cm−1): 336 (2480), 625
(1120).
Commercially available chemicals were used without
further purification unless otherwise stated. Solvents
were distilled and dried before use. Preparation and han-
dling of air-sensitive compounds were carried out in an
inert atmosphere glove box. Fourier transform infrared
spectra were performed on a Shimadzu FT-IR 8400S
instrument using KBr pellets. Elemental analyses were
carried out on a Perkin Elmer 2400 series II CHN ana-
lyzer. UV–Vis spectra (single and time-dependent) were
recorded on an Agilent 8453 diode array spectrophotom-
eter. All room temperature NMR spectra were recorded
on a Bruker Avance 500/300 MHz spectrometer and were
referenced to residual deuterated solvents. X-band EPR
spectra were collected on a JEOL JES-FA 200 instru-
ment. GC–MS measurements were performed with a
Perkin Elmer Clarus 600 instrument using an Elite 5 MS
(30 m × 0.25 mm × 0.25 μm) column with a maximum
temperature of 300 °C. Labeling experiments were car-
ried out with the 18O2 gas (99 atom%) and H218O from
Icon Services Inc.
Dioxygen reactivity
The iron(II) complex (0.02 mmol) was dissolved in dry
benzene (10 mL). Bubbling O2 gas through the solution
of 1 and 2 resulted in a color change from blue to red
for 2 and from colorless to green for 1. The solution was
allowed to stir at room temperature (15 min for 1 and 8 h
for 2). After the reaction, the solvent was evaporated to
dryness and the residue was treated with 3 M HCl solu-
tion (10 mL). The organic products were extracted with
diethyl ether (3 × 15 mL) and the organic layer was dried
over anhydrous sodium sulfate. After removal of the sol-
The KTpPh2 ligand [40] and the iron complex 1 [36]
were synthesized following the reported procedures.
Although no problem was encountered during the syn-
thesis of the complexes, perchlorate salts are potentially
explosive and should be handled with care!
1
vent, the residue was analyzed by GC–MS and H NMR
[(TpPh2)FeII(NPP)3] (2)
spectroscopy. The reactions were performed in triplicate.
2
In a 50-mL round-bottomed flask, iron perchlorate
hexahydrate (0.36 g, 1 mmol) was dissolved in 10 mL
methanol. The ligand KTpPh2 (0.35 g, 0.5 mmol) was
added to the reaction mixture and was allowed to stir
for 2–3 min. To the suspension was added a mixture of
α-isonitrosopropiophenone (0.25 g, 1.5 mmol) and trieth-
ylamine (209 μL, 1.5 mmol in 1 mL of methanol) with
constant stirring. The solution immediately turned blue,
which was stirred for overnight under nitrogen environ-
ment. The reaction mixture was concentrated to dryness,
the residue was dissolved in dichloromethane, and fil-
tered. Evaporation of filtrate afforded blue microcrystal-
line solid. Crystals suitable for elemental analysis and
X-ray diffraction were grown from a solvent mixture of
dichloromethane and methanol. Yield: 0.485 g (76%).
Anal. calcd for C72H58BFe2N9O6 (1267.79 g/mol): C,
68.21; H, 4.61; N, 9.94. Found C, 68.44; H, 4.42; N,
10.42. IR (KBr, cm−1): 3448 (br), 3059 (w), 2924 (w),
2619 (w), 1547 (w), 1479 (m), 1460 (m), 1331 (vs), 1285
(vs), 1175 (m), 1003 (m), 762 (m), 698 (vs). UV–Vis in
benzene λ, nm (ε, M−1 cm−1): 350 (3580), 680 (2000).
Reaction of complex 1 with oximes
Complex 1 (0.02 mmol) was dissolved in dry benzene
(1 mL) under nitrogen atmosphere. To the solution was
added the oxime (5 equiv) and pure oxygen gas was bub-
bled through the solution for 2 min. The reaction solu-
tion was allowed to stir for 20 min. After the reaction,
the resulting solution was passed through a 15-cm silica
(60–120 mesh size) column and washed with diethyl ether
(10 mL). The organic phase was then analyzed by GC–MS
and 1H NMR spectroscopy. For GC analyses, naphthalene
was used as an internal standard and the products were
identifed by comparison of their GC retention times and
GC–MS with those of authentic compounds.
Control experiment
A control experiment was performed with iron(II) per-
chlorate hexahydrate (0.02 mmol) and oxime (1 mmol) in
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dioxygen saturated benzene. No oxidized product of oxime
was detected in the reaction solution.
1H NMR (500 MHz, CDCl3, 295 K) of organic
products
Benzophenone: δ 7.80 (d, 4H), 7.59 (t, 2H), 7.50 (t, 4H);
benzoic acid: δ 8.13 (d, 2H), 7.64 (t, 1H), 7.42 (t, 2H); ben-
zil: δ 7.99 (d, 4H), 7.69 (t, 2H), 7.49 (t, 4H); 1-phenylpro-
pane-1,2-dione: δ 7.89 (d, 2H), 7.73 (t, 1H), 7.64 (t, 2H),
2.17 (s, 3H); acetophenone: δ 7.94 (d, 2H), 7.64 (t, 1H), 7.56
(t, 2H), 2.5 (s, 3H).
Detection of nitric oxide
Fig. 1 ORTEP plot of complex 2 with 40% thermal ellipsoid param-
eters. All hydrogen atoms except that on B have been omitted for
clarity. Average distances: Fe1–N=2.209 Å, Fe2–N=1.89 Å,
Fe1–O=2.095 Å and Fe2–O=1.953 Å
Nitric oxide (or HNO), formed in the reaction with iron(II)
complexes, reacts with oxygen and water to form nitrous
acid which is quantitatively converted to a diazonium salt
by reaction with sulfanilic acid (Griess B) in acid solution.
The diazonium salt is then coupled to N-(1-naphthyl)eth-
ylenediamine (Griess A), forming an azo dye that can be
spectrophotometrically quantifed based on its absorbance
at 548 nm [41, 42].
equipped with charge-coupled device (CCD) area detector.
Data collection, data reduction and structure solution/refne-
ment were performed using the APEX II software package
[44]. The structure of the complex was solved by intrinsic
method and refned by the full-matrix least-squares method
based on F2 with all observed refections. All the non-hydro-
gen atoms were treated anisotropically. At the end of the
refnement cycles, some disordered electron densities were
located in the asymmetric unit. SQUEEZE calculations indi-
cated the presence of 71 electrons per unit cell [45]. Crystal-
lographic data for the complex are summarized in Table S1.
Reaction of the complexes (1 and 2)
with triphenylphosphine
A mixture of the iron(II) complex 1 (0.04 mmol) and oxime
(0.2 mmol) (or iron(II) complex only for 2) was taken in a
Schlenk tube. To the mixture was added oxygen saturated
benzene (1 mL) by a syringe and the reaction fask was
closed fully to avoid loss of HNO/NO from the solution.
After 20 min of stirring, triphenylphosphine (0.08 mmol)
dissolved in benzene (500 μL) was added by a syringe. The
reaction solution was allowed to stir for 24 h and passed
through a 15-cm silica (60–120 mesh) column and washed
with diethyl ether (10 mL). The dark-green solution was
dried under vacuum and was analyzed by 31P NMR spec-
troscopy using H3PO4 as a standard. For the detection of NO
and HNO, the method reported by King and co-workers were
followed [43]. In the reaction with complex 1 and oxime,
the resonances for aza-ylide and triphenylphosphine oxide
appear at 18.1 ppm and 28.6 ppm, respectively. For com-
plex 2, the resonance for triphenylphosphine oxide appears
at 27.0 ppm. Resonance signal for triphenylphosphine from
the reaction with 1 and oxime appears at −4.2 ppm, whereas
it appears at −5.7 ppm for complex 2.
Results and discussion
Synthesis and characterization
Complex 1 was prepared following the protocol reported
earlier by our group, [36] while the dimeric complex 2 was
isolated from the reaction of KTpPh2, iron(II) perchlorate
hydrate and a basic solution of α-isonitrosopropiophenone
(NPP-H) in methanol under nitrogen atmosphere (see
1
Experimental section). The H NMR spectrum of 2 in
CDCl3 shows paramagnetically shifted resonances indicat-
ing the presence of high-spin iron(II) center in the complex
(Fig. S1). The blue solution of complex 2 exhibits intense
and broad bands at 494 nm, 590 nm and 680 nm, attrib-
utable to Fe(II)-to-nitroso charge-transfer (CT) transitions
(Fig. S3) [46]. Similar CT bands are been observed in the
tris(oximato)iron(II) complex Na[Fe(NPP)3] (3) (Experi-
mental and Fig. S3). It is important to mention here that
complex 3 displays proton resonances in the region between
0 ppm and 8 ppm typical of low-spin iron(II) complex (Fig.
S2).
X‑ray crystallographic data collection
and refnement and solution of the structure
X-ray single crystal data of 2 was collected using Mo Kα
(λ=0.7107 Å) radiation on a SMART APEX difractometer
1 3