Highly selective binding of nitric oxide by CoIII and Fe III complexes

Article abstract of DOI:10.1039/c2dt32708b

In order to construct compounds with highly selective binding activity for NO, two Co^III and two Fe^III complexes with square-planar N₂O₂-type donor sets, N-[2-(2-hydroxybenzylamino)ethyl]-2- hydroxybenzamide (H₃L1) and 1,2-bis(2-hydroxybenzoylamino)ethane (H₄L2), [Co^III(L1)] (1), Na[Co^III(L2)] (2), [Fe^III(L1)] (3), and (PPh₄)[Fe^III(L2)] (4), were designed and synthesized. These compounds were characterized by electronic absorption, FT-IR, ¹H-NMR spectroscopies, ESI-mass spectrometry, and elemental analyses. The redox potentials of the Co^III and Fe ^III complexes with L1, 1 and 3, have quasi-reversible waves at -0.51 and -0.49 V, respectively, and those with L2, 2 and 4, afforded reversible and irreversible waves at -0.96 and -1.04 V, respectively. Interestingly, all complexes quickly react with NO under an Ar atmosphere to form nitrosyl complexes, as monitored by UV-vis spectroscopy. The formation of nitrosyl complexes was confirmed by the appearance of the N-O stretching vibration at about 1650 cm^-1; 1649 for 1, 1651 for 2, 1648 for 3, and 1650 cm ^-1 for 4. The reactivity of each of these complexes with other small molecules such as NO₂^-, NO₃^-, CO, and O₂ was also studied. None of the complexes react with CO and O₂. Co^III complexes 1 and 2 react with NO₂ ^-, while Fe^III complexes, 3 and 4, do not react with small amounts of NO₂^-. Complex 3 reacts with NO ₂^- at concentrations above 100 equiv. of NO ₂^-. We succeeded in preparing complexes with highly selective reactivity for NO. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt32708b

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Highly selective binding of nitric oxide by CoIII and FeIII
complexes†
Cite this: Dalton Trans., 2013, 42, 4470
Zizheng Zhang, Tatsuya Suwabe, Mai Ishikawa, Yasuhiro Funahashi,
Tomohiko Inomata, Tomohiro Ozawa and Hideki Masuda*
In order to construct compounds with highly selective binding activity for NO, two CoIII and two FeIII com-
plexes with square-planar N2O2-type donor sets, N-[2-(2-hydroxybenzylamino)ethyl]-2-hydroxybenz-
amide (H3L1) and 1,2-bis(2-hydroxybenzoylamino)ethane (H4L2), [CoIII(L1)] (1), Na[CoIII(L2)] (2), [FeIII(L1)]
(3), and (PPh4)[FeIII(L2)] (4), were designed and synthesized. These compounds were characterized by
electronic absorption, FT-IR, 1H-NMR spectroscopies, ESI-mass spectrometry, and elemental analyses. The
redox potentials of the CoIII and FeIII complexes with L1, 1 and 3, have quasi-reversible waves at −0.51
and −0.49 V, respectively, and those with L2, 2 and 4, afforded reversible and irreversible waves at −0.96
and −1.04 V, respectively. Interestingly, all complexes quickly react with NO under an Ar atmosphere to
form nitrosyl complexes, as monitored by UV-vis spectroscopy. The formation of nitrosyl complexes was
confirmed by the appearance of the N–O stretching vibration at about 1650 cm−1; 1649 for 1, 1651 for
2, 1648 for 3, and 1650 cm−1 for 4. The reactivity of each of these complexes with other small molecules
such as NO2−, NO3−, CO, and O2 was also studied. None of the complexes react with CO and O2. CoIII
complexes 1 and 2 react with NO2−, while FeIII complexes, 3 and 4, do not react with small amounts
Received 13th November 2012,
Accepted 4th January 2013
DOI: 10.1039/c2dt32708b
−
of NO2−. Complex 3 reacts with NO2 at concentrations above 100 equiv. of NO2−. We succeeded in
www.rsc.org/dalton
preparing complexes with highly selective reactivity for NO.
lead to the onset of vascular diseases such as arteriosclerosis.
It is also suspected that the reduction of the activities of eNOS
1. Introduction
Nitric oxide is a mono-radical molecule, generally known as a and nNOS is indirectly related to the incidence of Alzheimer’s
highly toxic molecule, which recently has been recognized as disease and Parkinson’s disease. The third NOS, iNOS, is
an important small molecule in biological systems.1 The NO induced by exogenous agents and is involved in an immuno-
molecule is produced in vivo from members of the nitric oxide logical effect which leads to production of higher concen-
synthase (NOS) family of enzymes.2,3 The members of the NOS trations of NO than eNOS and nNOS. When bacteria invade
family are classified into three groups: neuronal NOS (nNOS), the body, a high concentration of nitric oxide will be generated
endothelial NOS (eNOS), and inducible NOS (iNOS).4,5 nNOS is in the whole body. This causes a state of septic shock with a
expressed in synapses in brain tissue and plays an important severe drop in blood pressure, which can lead to death. There-
role as a neurotransmitter.6 eNOS is expressed in endothelial fore, methods for obtaining quantitative measurements of
cells and plays a role in the vascular relaxation effect.7 The nitric oxide are needed for preventive medicine and for treat-
concentration of nitric oxide in biological systems is regulated ment of diseases associated with changes in concentration
by eNOS and nNOS. The incidence of disorders related to levels of nitric oxide in vivo.
metabolic syndrome has been increasing in recent years.
Several methods for measuring NO concentrations have
These disorders are characterized by an increase in the con- been developed.8–12 Fluorescence spectroscopic and electro-
centrations of active oxygen species in vivo and an inhibition chemical methods have gained recent attention. Fluorescence
of the activities of eNOS and nNOS, resulting in an imbalance methods have higher sensitivity and high selectivity for NO
of NO concentration.1 Abnormal concentrations of nitric oxide detection, but are not reusable or adaptable. Electrochemical
methods have also been investigated and found to have
high sensitivity, reusability, and good adaptability.13 Various
Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute
devices, such as metal electrodes and semiconductor electro-
of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan.
des, have been investigated.13,14 However, the electrochemical
E-mail: masuda.hideki@nitech.ac.jp
methods are subject to detection inhibition due to the pres-
ence of the nitrite ion, an oxidized derivative of the NO
†Electronic supplementary information (ESI) available: IR spectra, cyclic voltam-
mograms, and UV-vis spectra found in the text. See DOI: 10.1039/c2dt32708b
4470 | Dalton Trans., 2013, 42, 4470–4478
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
molecule. In order to improve the selectivity for NO, ion-
exchange membranes such as Nafion® have been coated on
electrodes to prevent interference by the nitrite ion.13,15
However, it is difficult to construct a homogeneous ion-
exchange membrane and this causes a decrease in NO detec-
tion sensitivity. Catalysts such as metal porphyrin derivatives
have also been studied,16 and it has been found that these cat-
alysts cannot overcome the shortcomings which are similar to
those of the electrochemical methods.
Metal complexes (including the porphyrin complexes
described above) often interact with NO to form nitrosyl
species. Nitrosyl complexes with amidate nitrogen, [Fe(PaPy3)-
(NO)](ClO4)2,17 [Ru(PaPy3)(NO)](BF4)2,18 (PaPy3H = N,N′-bis(2-
pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide)
and
[Mn(PaPy3)(NO)]ClO4,19 have been recently prepared by
Mascharak et al. who investigated the photo-activated release
of the coordinated nitrosyl molecule as NO using visible or
near UV light. These nitrosyl complexes were synthesized from
octahedral starting compounds. This suggests that substi-
tution by NO is required. This is unfavorable for highly sensi-
tive NO detection. We previously prepared square planar20 and
trigonal bipyramidal21 Co(III) complexes coordinated with two
amidate nitrogens and two or three thiolate sulfur atoms,
respectively. These structures with vacant sites are advan-
tageous for NO detection because no substitution reactions are
required. There are some reports on the development of NO
sensors using metal complexes. In 2005, Narayanaswamy et al.
reported that bis[(2,9-dimethyl-1,10-phenanthroline)]copper(II)
complex, Cu(dmp)22+, has high selectivity for NO over a
number of potential interfering compounds such as nitrite,
H2O2, peroxynitrite anion and superoxide anion. It was
Scheme 1 N2O2-type ligands developed as NO sensors.
interacts with the metal centre. Unfortunately, the metal
complex was found to be unstable in water.
In this study, in order to overcome the instability, phenol
ligands with basicity similar to that of thiols have been intro-
duced as alternatives to thiol donors to increase the stability of
the Fe(III) complex by taking hard and soft acid base (HSAB)
rules into account. We employ two ligands, N-[2-(2-hydroxy-
benzylamino)ethyl]-2-hydroxybenzamide (H3L1) and 1,2-bis(2-
hydroxybenzoylamino)ethane (H4L2), as shown in Scheme 1.
These ligands are expected to serve as the N2O2-type donating
ligands with 3 and 4 negative charges (3- and 4-donors),
respectively, and to form a stable square-planar complex. By
supplying such a strongly negative charge to the metal cations,
the Lewis acidity of the metal ion will be lowered and weaken
the reactivity with ions and molecules found in the environ-
demonstrated that nitrite and peroxynitrite do not interfere in
2+ 22
the reaction of NO with Cu(dmp)2
.
In 2011, Lippard et al.
reported that the copper complex of seminaphthofluorescein-
based fluorescent probes has highly selective reactivity with
NO or NO donors. Treatment with reactive oxygen and nitro-
gen species such as NO3−, NO2−, ONOO−, HNO, H2O2, or OCl−
does not produce fluorescence enhancement, although nitro-
gen dioxide (NO2) affords minimal fluorescence.23 In 2012,
Olson et al. reported that five-coordinate Fe(II) heme proteins
ligated with a neutral proximal histidine ligand have strong
affinity for gaseous ligands, such as NO, CO, and O2, in the
order NO ≫ CO ≫ O2 due to a combination of back bond-
ing and spin coupling effects. All heme proteins with a
neutral proximal histidine and an apolar distal pocket have
been shown to have much higher affinities for NO than
for CO and O2.24 However, heme model compounds do not
have selective reactivity for NO. We therefore recognized a
need to develop a new sensor with highly selective reactivity
for NO.
ment, such as water, Cl−, NO3 and NO2−. These complexes
−
are thus expected to react only with highly reactive monoradi-
cal molecules such as NO.
2. Experimental section
2.1 Reagents
The reagents used for preparation of ligands and metal com-
plexes were purchased from Wako Chemical Co. and Tokyo
Kasei Industry and used without further purification. Solvents
were obtained from Wako Chemical Co. in reagent grade and
used after distillation.
We also prepared an Fe(III) complex with an N2S2-type
ligand and investigated its reactivity with NO.25 The Fe(III)
complex has a distorted square pyramidal structure with one
chloride anion at the apical position. With introduction of
2.2 Measurements
NO gas into a CH3CN solution of the complex, its colour dras- Elemental analysis was performed with a Perkin-Elmer 2400II
tically changes from orange to pale green, suggesting that NO CHNS/O full-automatic analyzer.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 4470–4478 | 4471

View Article Online
Paper
Dalton Transactions
ESI-mass spectra were acquired with an LCT mass triethylamine (3.31 g, 0.033 mol), followed by stirring for
spectrometer equipped with an ionspray interface (Micromass 1 hour. Triethylamine (3.31 g, 0.033 mol) was added to
Limited, Manchester, UK). Instrument settings, data acqui- another acetonitrile solution (70 ml) of AEMP (5.44 g,
sition, and data processing were controlled by a computer with 0.033 mol). These two acetonitrile solutions were mixed
the Windows NT operating system. Samples were introduced together and stirred overnight. After removal of acetonitrile by
with a single syringe pump (KD scientific Inc., USA) fitted with evaporation, a small amount of CHCl3 was added to the
1
Hamilton syringes (Hamilton Co, Reno, NE). H-NMR spectra residual solid. The solution was loaded onto a silica gel
of the complexes in CDCl3 or DMSO-d6 were recorded with a column (5 cm × 40 cm) using a 50 : 1 mixture of CHCl3 and
Varian Gemini-300 FT-NMR instrument and
a BRUKER MeOH. The eluents were characterized by thin-layer chromato-
AVANCE 600 FT-NMR instrument. The chemical shifts graphy. The solution was evaporated to dryness to give HBEB
were estimated from TMS as an internal standard reference. as a yellow paste.
Electronic absorption and FT-IR spectral measurements were
D. Synthesis of ligand N-[2-(2-hydroxybenzylamino)ethyl]-2-
performed using a Jasco V-530 instrument and FT/IR-4200 or hydroxybenzamide (H3L1). Pd–C was added twice by a micro-
FT/IR-410 instruments, respectively. Cyclic voltammetry spatula into a methanol solution containing N-[2-(2-hydroxy-
measurements were carried out using an ALS/CH Instruments benzylamino)ethyl]-2-benzyloxybenzamide (HBEB) (1.59 g,
Electrochemical Analyzer Model 600A. A glassy-carbon elec- 4.2 mmol). The benzyl group of HBEB was removed by hydro-
trode, an Ag/Ag+ electrode, and a Pt-wire electrode were used genation at 3 atm. After the reaction, the solution was filtered.
as working, reference, and counter electrodes, respectively. The remaining solid was sufficiently washed with methanol.
A 1 mM sample was prepared by dissolving appropriate The solution was dried after filtration with Celite. Preparation
1
amounts of a complex in CH3CN with (n-Bu)4NClO4 (TBAP) of H3L1 was confirmed by FT-IR, ESI-MS and H-NMR spectral
(0.1 M) as an electrolyte.
measurements.
1H-NMR (300 MHz; δ ppm from TMS in DMSO-d6): 2.72 (t,
2H, –CH2–CH2–), 3.44 (q, 2H, –CH2–CH2–), 3.82 (s, 2H, CH2),
6.6–7.9 (m, 8H, aryl), 8.85 (s, 2H, NH). ESI-TOF mass: m/z
2.3 Synthesis of N-[2-(2-hydroxybenzylamino)ethyl]-2-
hydroxybenzamide (H3L1)
A. Synthesis of 2-[(2-aminoethylamino)methyl]phenol 287.2 [H3L1 + H+]+. FT-IR (KBr, cm−1): ν 3154 (N–H amide),
(AEMP). AEMP was synthesized according to procedures out- 1624 (CvO), 1545 (N–H amide, amine), 1460 (CvC), 1411
lined in a previous report26 as follows; salicylaldehyde (24.4 g, (O–H), 1201 (C–O), 751 (C–H).
0.20 mmol) was added dropwise to a methanol solution con-
2.4 Synthesis of 1,2-bis(2-hydroxybenzoylamino)ethane
(H4L2)
taining ethylenediamine (60.0 g, 1.0 mmol) under an N2
atmosphere in an ice bath, and stirred for 30 minutes. Sodium
borohydride was then slowly added to the solution followed by A mixture of ethyl salicylate (3.04 g, 20 mmol) and ethylenedi-
stirring for 30 minutes at room temperature. After the reaction, amine (0.6 g, 10 mmol) was refluxed in an oil bath for 1 hour
methanol and excess ethylenediamine were removed under and then cooled to room temperature. A white oily substance
reduced pressure. The resulting oily matter was extracted with was obtained, which was dissolved in methanol at 70 °C. Dis-
ethyl acetate (50 ml) at pH 9. The ethyl acetate layer was dried tilled water heated to about 70 °C was added to the solution
with anhydrous sodium sulfate (Na2SO4) overnight. After gradually. In addition, a few drops of ethanol were added to
removal of ethyl acetate by evaporation, AEMP was obtained as the solution, which was then left standing at room temp-
a brown oil.
erature. After filtration, white needle-shaped crystals of H4L2
B. Synthesis of 2-benzyloxybenzoic acid. To a sodium ethox- were obtained. Preparation of H4L2 was confirmed by
ide solution, obtained by reaction of Na in ethanol, was added elemental analysis and FT-IR, ESI-MS, and 1H-NMR spectral
methyl salicylate (13.68 g, 0.09 mol). After 10 minutes, benzyl measurements.
chloride (11.4 g, 0.09 mol) was also added and the solution
Anal. Calcd for C16H16N2O4: C, 63.99; H, 5.37; N, 9.33%.
1
was refluxed overnight. The solution was then filtered to Found: C, 64.06; H, 5.31; N, 9.32%. H-NMR (600 MHz; δ ppm
remove the white precipitate of NaCl. 1 N NaOH aqueous solu- from TMS in DMSO-d6): 3.50 (s, 4H, CH2) 6.89–7.83 (m, 8H,
tion (90 ml) was gradually added to the filtrate. The ethanol aryl), 8.98 (s, 4H, NH, OH). ESI-Mass: m/z 323.3 [H4L2 + Na+]+.
was removed under vacuum. The residual aqueous solution FT-IR (KBr, cm−1): ν 3406 (N–H amide), 1643 (CvO), 1551
was washed with diethyl ether. Concentrated HCl solution was (N–H amide), 1502 (CvC), 1371 (O–H), 1253 (C–O), 745 (C–H).
added dropwise into the aqueous solution under cooling in an
2.5 Preparation of CoIII and FeIII complexes
ice bath. A white precipitate of 2-benzyloxybenzoic acid was
then obtained by suction filtration.
1H-NMR (600 MHz, δ ppm from TMS in DMSO-d6) δ = 5.20 amino)ethyl]-2-hydroxybenzamide
(s, 2H, CH2) 7.00–7.79 (m, 9H, aryl). 0.18 mmol) was dissolved in 30 ml dry DMF under Ar gas.
2.5.1 Preparation of [CoIII(L1)]. N-[2-(2-Hydroxybenzyl-
(H3L1) (52.46 mg,
C. Synthesis of N-[2-(2-hydroxybenzylamino)ethyl]-2-benzyl- Sodium hydride (13.2 mg, 0.55 mmol) was added to the solu-
oxybenzamide (HBEB). To a solution of acetonitrile (120 ml) tion as a deprotonation agent and the solution was stirred for
was added 2-benzyloxybenzoic acid (7.47 g, 0.033 mol), 30 min. Then CoCl2 (23.8 mg, 0.18 mmol) was added to the
2-chloro-N-methylpyridinium iodide (8.36 g, 0.033 mol) and solution and the solution was stirred for 5 hours at room
4472 | Dalton Trans., 2013, 42, 4470–4478
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
temperature. The air was passed through the resultant solu-
2.5.4 Preparation of (PPh4)[FeIII(L2)]. 1,2-Bis(2-hydroxy-
tion, and this caused the colour change from blue to brownish benzoylamino)ethane (H4L2) (70.1 mg, 0.23 mmol) was dis-
green. The solution was evaporated under vacuum and then a solved in 30 ml dry DMF under Ar gas. Sodium hydride
brownish green solid was obtained, which was dissolved in dry (22.4 mg, 0.93 mmol) was added and the solution was stirred
ethanol. This solution was filtered with Celite to remove NaCl. for 30 min. Then FeCl3 (11.4 mg, 0.07 mmol) was added gradu-
The brownish green powder was obtained through recrystalli- ally and the solution was stirred for 5 hours in an ice bath.
zation from ethanol/dichloromethane. Preparation of After addition of PPh4Cl to the solution, the solution was evap-
[CoIII(L1)] was confirmed by FT-IR and ESI-Mass spectral orated to dryness. The residues were dissolved in CH2Cl2,
measurements and elemental analysis.
which was filtered with Celite to remove NaCl. The residues
Anal. Calcd for [CoIII(L1)]·H2O·0.5DMF: C, 52.97; H, 5.21; N, were dissolved in ether and then a red powder was obtained.
8.82%. Found: C, 53.13; H, 5.17; N, 8.71%. ESI-MS: m/z 340.8 Preparation of (PPh4)[FeIII(L2)] was confirmed by FT-IR,
[CoIII(L1)]-H+. FT-IR (KBr, cm−1): ν 1598 (CvO). Electronic ESI-MS measurements and elemental analysis. The sample in
absorption spectrum in ethanol (λmax, nm (ε[M−1 cm−1])): 387 the IR spectrum has been analyzed using the sodium salt in
(sh), 532 (540), 578 (400).
spite of (PPh4)[FeIII(L2)].
2.5.2 Preparation of Na[CoIII(L2)]. 1,2-Bis(2-hydroxybenzoyl-
Anal. Calcd for (PPh4)[FeIII(L2)]·0.5DMF: C, 68.46; H, 4.91;
amino)ethane (H4L2) (54.5 mg, 0.18 mmol) was dissolved in N, 4.81%. Found: C, 68.35; H, 5.20; N, 4.47%. ESI-MS: m/z
10 ml dry DMF under Ar gas. Sodium hydride (17.4 mg, 352.3 [FeIII(L2)]−. FT-IR (KBr, cm−1): ν 1616 (CvO). Electronic
0.72 mmol) was added as a deprotonation agent to the solu- absorption spectrum in ethanol (λmax, nm (ε[M−1 cm−1])): 307
tion and then the solution was stirred for 30 minutes. CoCl2 (8200), 434(2500).
(23.6 mg, 0.18 mmol) was added gradually to the solution, and
2.6 Reactions with small molecules and ions
immediately the colour of the solution changed from clear to
dark blue. The solution was then continuously stirred for
5 hours. The air was passed through the resultant solution
and the solution colour changed from dark blue to purple.
This solution was evaporated under vacuum, and then a
purple solid was obtained. The solid was dissolved in MeCN,
and filtered with Celite to remove NaCl. The purple powder
was obtained by recrystallization from acetonitrile/dichloro-
methane. Preparation of Na[CoIII(L2)] was confirmed by FT-IR
and ESI-MS measurements and elemental analysis.
2.6.1 Reaction with NO. When nitric oxide gas was
bubbled into a DMF solution of CoIII or FeIII complex, the solu-
tion colour changed immediately. The coordination of NO to
the metal was determined by electronic absorption and FT-IR
spectra.
2.6.2 Reactions with NO2−, NO3−, O2, and CO. The reac-
tions with NO2−, NO3−, O2, and CO were also monitored in
DMF solution by UV-vis spectroscopy, in which they are added
as NaNO2, NaNO3, air and CO gas.
Anal. Calcd for Na[CoIII(L2)]·3H2O: C, 44.46; H, 4.20;
N, 6.48%. Found: C, 44.37; H, 4.03; N, 6.27%. 1H-NMR
(600 MHz; δ ppm from TMS in DMSO-d6): 16.1 (s, aryl), 28.0
(s, aryl). ESI-Mass: m/z 355.2 [CoIII(L2)]−. FT-IR (KBr, cm−1):
ν 1597 (CvO). Electronic absorption spectrum in ethanol
(λmax, nm (ε[M−1 cm−1])): 320 (8900), 558 (1150).
3. Results and discussion
3.1 Preparation and characterization of CoIII and FeIII
complexes with L1 or L2
2.5.3 Preparation of [FeIII(L1)]. N-[2-(2-Hydroxybenzyl- The N2O2-type ligands, H3L1 and H4L2, were synthesized to
amino)ethyl]-2-hydroxybenzamide (H3L1) (20.1
mg, construct CoIII and FeIII complexes that selectively bind NO.
0.07 mmol) was dissolved in 30 ml dry DMF under Ar gas. These ligands are expected to form a four- or five-coordinate
Sodium hydride (5.04 mg, 0.21 mmol) was added and the solu- metal complex, namely a metal complex with a vacant site, as
tion was stirred for 30 min. FeCl3 (11.4 mg, 0.07 mmol) was shown in metal complexes with structurally similar ligand,
added and the solution was stirred for a few hours. A red 1,2-bis(2-mercapto-2-methylpropioylamino)ethane (L), (PPh4)-
powder was obtained through ether diffusion. The filtered red [CoIII(L)]20 and [FeLCl].25 So the site can be used as a coordi-
powder was dissolved in dehydrated ethanol, and then the nation space of the NO molecule. The successful syntheses of
ethanol was evaporated under vacuum. The resultant solid was these ligands were confirmed by elemental analyses as well as
dissolved in DMF. After desalination using the gel filtration IR and 1H-NMR spectral analyses.
chromatography (G-10), the resultant solution was evaporated
The CoIII complexes, [CoIII(L1)] (1) and Na[CoIII(L2)] (2),
under vacuum and the resultant solid was dissolved in were prepared by air oxidation of the corresponding CoII
ethanol. A red powder was obtained through ether diffusion. complex derived from the reaction of CoCl2 and the ligands,
Preparation of [FeIII(L1)] was confirmed by FT-IR and ESI-TOF H3L1 and H4L2, respectively, in DMF. The complex 2 showed
MS measurements.
absorption bands in the near ultraviolet region and at about
Anal. Calcd for [FeIII(L1)]·7NaCl·C2H5OH: C, 27.22; H, 2.66; 550 nm, which are tentatively assignable to a π–π* transition
N, 3.53%. Found: C, 27.26; H, 2.48; N, 3.80%. ESI-MS: m/z band of the ligand and a ligand-to-metal charge transfer
338.2 [FeIII(L1)]-H+. FT-IR (KBr, cm−1):
ν
1608 (CvO). (LMCT) band from phenol to CoIII centre, respectively, as
Electronic absorption spectrum in ethanol (λmax, nm (ε[M−1 shown in Fig. 1b and listed in Table 1. Appearance of the latter
cm−1])): 427(2200).
band suggests to form a square planar CoIII complex.20,27 In IR
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 4470–4478 | 4473

View Article Online
Paper
Dalton Transactions
Fig. 1 (a) Electronic absorption spectral change of [CoIII(L1)] (1) (solid line) with addition of NO (dashed line) in DMF (sample concentration: 1 × 10−4 M). (b) Elec-
tronic absorption spectral change of Na[CoIII(L2)] (2) (solid line) with addition of NO (dashed line) in DMF (sample concentration: 1 × 10−4 M). (c) Electronic absorp-
tion spectral change of [FeIII(L1)] (3) (solid line) with addition of NO (dashed line) in DMF (sample concentration: 1 × 10−4 M). (d) Electronic absorption spectral
change of (PPh4)[FeIII(L2)] (4) with addition of NO (dashed line) in DMF (sample concentration: 1 × 10−4 M).
in 1 (Fig. 1a), suggesting that it is also a square planar
Table 1 Electronic absorption spectral data of 1, 2, 3 and 4
λmax/nm (ε mol−1
complex. To confirm the estimated structures, elemental ana-
1
lyses, H-NMR and ESI-MS were measured. Elemental analysis
Compound
dm3 cm−1
)
Tentative assignment
of the complex 1 revealed that [CoIII(L1)] was prepared as
[CoIII(L1)]·H2O·0.5DMF. The ESI-MS spectrum also exhibits a
parent peak assignable to [[CoIII(L1)]-H+]− in the negative
mode without any other strong peaks throughout the measure-
ment range (m/z: 100–1500). Unfortunately, an NMR spectrum
of this complex could not be measured because of the low
solubility of this complex in all solvents tested. This indicates
the formation of [CoIII(L1)].
[CoIII(L1)] (1)
Na[CoIII(L2)] (2)
[FeIII(L1)] (3)
387 (1750)
π–π* transition
532 (540) 578 (ε 400) LMCT (phenolato O to CoIII
)
)
320 (8900)
558 (1150)
427 (2200)
π–π* transition
LMCT (phenolato O to CoIII
LMCT (phenolato O to FeIII
π–π* transition
)
(PPh4)[FeIII(L2)] (4) 307 (8200)
434 (2500)
LMCT (phenolato O to FeIII
)
1
The H-NMR spectrum of the complex 2 gave two peaks at
δ = 16.1 (s, aryl) and 28.0 (s, aryl) in the paramagnetic region,
indicating formation of a square-planar CoIII complex with an
intermediate spin state as reported previously.28 Elemental
analysis showed that Na[CoIII(L2)] was prepared as Na-
[CoIII(L2)]·3H2O, and the ESI-MS spectrum also gave a parent
peak at m/z: 355.2 which can be assigned as [CoIII(L2)]−
without any other peaks in negative mode in the region
measured. These results indicate the formation of a square
planar complex, Na[CoIII(L2)].
Table 2 FT-IR spectral data of H3L1, H2L2, 1, 2, 3 and 4
Compound
νCvO/cm−1
δOH/cm−1
H3L1
H4L2
1624
1643
1598
1597
1608
1616
1411
1371
No signal
No signal
No signal
No signal
[CoIII(L1)] (1)
Na[CoIII(L2)] (2)
[FeIII(L1)] (3)
(PPh4)[FeIII(L2)] (4)
Electronic absorption spectra of the FeIII complexes,
[FeIII(L1)] (3) and (PPh4)[FeIII(L2)] (4), are similar to each other.
Two clear absorption bands were obtained for 4: the band in
spectra of H4L2 and 2, the shift of the CvO stretching band to shorter wavelength can be assigned as a π–π* transition band29
smaller frequency regions was observed as listed in Table 2. and another one is an LMCT band from phenol oxygen to FeIII
These spectral results suggest that the CoIII centre has a centre, which are depicted in Fig. 1c and 1d and listed in
square planar geometry. A similar behavior was also observed Table 1. The absorption spectral feature is similar to that of
4474 | Dalton Trans., 2013, 42, 4470–4478
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
(PPh4)2[FeLCl].25 This result suggests that complex 4 has a those of the complexes with ligand L1. This is quite reasonable
square pyramidal structure although the peak maximum of in light of the difference in the electron donating charac-
the LMCT band has largely shifted to a shorter wavelength teristics of the two ligands, because L2 has two amidato N
region because of the different donating atoms (O or S). On atoms and two phenolato O atoms and L1 has one amidato N
the other hand, the elemental analysis demonstrates that and two phenolato O atoms. The d orbitals of CoIII and FeIII
(PPh4)[FeIII(L2)] was prepared as (PPh4)[FeIII(L2)]·0.5DMF, complexes with L2 which have four negatively-charged donat-
which suggests the formation of a square planar structure. ing atoms are destabilized compared to those of L1 which have
Additionally, the ESI-MS spectrum has a parent peak at 355.2 three negatively-charged donating atoms. Considering that
in the negative mode without any other strong peaks. This these metal complexes have square-planar geometry, the com-
355.2 peak is assigned to [FeIII(L2)]−. Therefore, we estimated plexes have structural and electrochemical conditions suitable
the structure of the complex 4 is a square planar. As for the for binding of NO.
complex 3, although the elemental analysis of the precipitate
The redox potential of the N2S2-type CoIII complex with two
was inconclusive due to unsuccessful desalination (see the amidato N and two thiolato S atoms, (PPh4)[CoIII(L)] (L = 1,2-
Experimental section), the ESI-MS spectrum has a parent peak bis(2-mercapto-2-methylpropioylamino)ethane, which was
at 338.2 in the negative mode, which is assignable to reported previously by our group, is −1.62 V vs. NHE.20 This
[[FeIII(L1)]-H+]− without any other strong peaks. Furthermore, value is not suitable for applications in measurements of NO
the electronic absorption spectral feature was similar to the in the human body because reduction of water must precede
one of 4 as well as the IR spectral behaviour. These results the detection of NO. It is 0.66 V higher in the negative region
indicate the formation of [FeIII(L1)] with a square planar than that of Na[CoIII(L2)] (2), even though both complexes
structure.
have two amidato N and two thiolato S or phenolato O atoms.
This effect may be caused by the different sizes of the chelate
rings; [CoIII(L1)] has 6,5,6-membered chelate rings and (PPh4)-
[CoIII(L)] has 5,6,5-membered chelate rings. Therefore, we
have succeeded in preparing stable metal complexes with
more positive redox potentials than the (PPh4)[CoIII(L)]
complex which was reported previously, using the ligands L1
and L2.
3.2 Electrochemical properties
It is very important to construct a metal complex with low
Lewis acidity in considering the selective and specific reaction
with NO. It is particularly important for the complex to adopt
square-planar or square-pyramidal geometry, because the axial
sites must be vacant for binding of NO. This designed complex
is expected to adopt square-planar geometry with electron-
donating ligands such as H3L1 and H4L2. In order to confirm
utility of this strategy, the electrochemical properties of
[CoIII(L1)] (1), Na[CoIII(L2)] (2), [FeIII(L1)] (3), and (PPh4)-
[FeIII(L2)] (4) were investigated using cyclic voltammetry. The
cyclic voltammograms of 1 and 3 in DMF have a quasi-
reversible wave assignable to CoIII/II and FeIII/II couples,
respectively, as shown in Fig. S1 and S2,† and those of 2 and 4
in DMF showed reversible and irreversible waves assignable to
CoIII/II and FeIII/II couples, respectively, as shown in Fig. S1 and
S2.† The electrochemical data of 1, 2, 3, and 4 are listed in
Table 3. Interestingly, the E1/2 values of the CoIII and FeIII com-
plexes, which were mainly affected by the ligands, have similar
values according to the type of ligand; those of complexes 1
and 3 with ligand L1 are −0.51 and −0.49 V, respectively, and
those of complexes 2 and 4 with ligand L2 are −0.96 and
−1.04 V, respectively. The redox potentials of the complexes
with ligand L2 were detected in the more negative region than
3.3 Reactivity of 1, 2, 3, and 4 with NO as measured by UV-vis
and IR spectra
The reactivities of these complexes with NO were investigated
by UV-vis and IR spectral methods. Nitric oxide gas was
bubbled into DMF solutions of the metal complexes under Ar.
A colour change was observed before and after addition of NO.
The colour of the solutions of complexes 1 and 2 changes from
green to pale yellow upon reaction with NO, and those of com-
plexes 3 and 4 both change from reddish orange to yellow.
Introduction of NO gas into solutions of the metal complexes
leads to a change in the electronic absorption spectra as
shown in Fig. 1a to 1d for 1, 2, 3, and 4, respectively. The
absorption bands at 532 and 578 nm for 1, 558 nm for 2,
427 nm for 3, and 434 nm for 4 all decrease during the reac-
tion with NO. The UV-vis spectral changes of 1 and 2 are very
similar to that of the four coordinate square-planar N2S2-type
CoIII complex, (PPh4)[CoIII(L)].20
IR spectral measurement was performed to estimate the
binding character of NO to the CoIII complexes, 1 and 2, and
FeIII ones, 3 and 4. The spectra are shown in Fig. S3a to S3d†
and the NO stretching vibration frequencies are listed in
Table 3 and 4. Characteristic νNO bands assignable to the N–O
stretching vibration were detected at 1651 cm−1 for 1,
1649 cm−1 for 2, 1648 cm−1 for 3, and 1650 cm−1 for 4. These
frequencies are all lower than that of the free NO molecule
(1840 cm−1),30 suggesting that all of the complexes have bound
NO. The N–O stretching vibration values of 1 (1649 cm−1) and
2 (1651 cm−1) are listed in Table 4 together with those of
Table 3 Redox potentials of CoIII and FeIII complexes, 1, 2, 3 and 4, in DMFa
Compound
Epc/V
Epa/V
ΔE/V
E1/2/V
[CoIII(L1)] (1)
−0.54
−0.99
−0.54
−1.20
−0.47
−0.93
−0.43
−0.87
0.07
0.06
0.33
0.33
−0.51
−0.96
−0.49
−1.04
Na[CoIII(L2)] (2)
[FeIII(L1)] (3)
(PPh4)[FeIII(L2)] (4)
a The potentials were standardized using Fc/Fc+.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 4470–4478 | 4475

View Article Online
Paper
Dalton Transactions
3.4 Electrochemical behaviour in reactions of 1, 2, 3, and 4
with NO
Table 4 Comparison of νNO values for Co(NO) complexes
Compound
n : {Co(NO)}n
νNO/cm−1
Ref.
Cyclic voltammograms for complexes 1, 2, 3, and 4 were
measured in DMF before and after the reaction with NO
(Fig. S4a to S4d†). For 1, the oxidation and reduction waves
observed at −0.47 and −0.54 V, respectively, disappear and a
new reduction wave appears at −1.09 V upon addition of NO.
The reduction wave of complex 1 was shifted by 550 mV to the
negative region upon reaction with NO. Complexes 2 and 4
show the same changes as complex 1 as follows; for 2, the oxi-
dation and reduction waves at −0.93 and −0.99 V, respectively,
disappear and a new reduction wave appears at −1.19 V. This
new reduction wave is accompanied by a negative shift of
about 200 mV. For 4, oxidation and reduction waves at −0.87
and −1.20 V, respectively, disappear and a new reduction wave
appears at −1.48 V accompanied by a negative shift of about
280 mV. Complexes 1, 2 and 4 undergo the same change as
that of the previously reported N2S2-type CoIII complex.20
Although complex 3 also exhibits the same behavior, the oxi-
dation and reduction waves at −0.43 and −0.54 V, respectively,
do not disappear and are slightly weakened. This is
accompanied by the appearance of a new reduction wave at
Co(salen)(NO)
Co(salphen)(NO)
Co(TPP)(NO)
N2S2-type Co(NO)
[CoIII(L1)] (1) + NO
Na[CoIII(L2)] (2) + NO
8
8
8
7
1640
1660
1689
1650
1649
1651
31
32
33
39
This work
This work
TPP: meso-tetraphenylporphyrinato.
Table 5 Comparison of νNO values for Fe(NO) complexes
Compound
n : {Fe(NO)}n
νNO/cm−1
Ref.
34
Fe(salen)(NO)
Fe(salphen)(NO)
7
7
S = 3/2
S = 1/2
S = 3/2
S = 1/2
1710
1630
1724
1643
1670
1766
1767
1822
1809
1648
1650
34
Fe(TPP)(NO)
7
6
6
6
35
36
37
38
Fe(OEP)(NO)(CH3)
Fe(OEC)(NO)
[FeS2Me2N3(Pr,Pr)(NO)]+
[FeIII(mmppa)Cl]2− + NO
[FeIII(L1)] + NO
25
This work
(PPh4)[FeIII(L2)] + NO
This work −1.39 V. This is explained as follows: a fraction of the total Fe
complex is coordinated by a residual chloride ion in the solu-
TPP: meso-tetraphenylporphyrinato; OEP: octaethylporphyrin; OEC:
2,3,7,8,12,13,17,18-octaethylcorrlo.
tion, which is inert with respect to substitution of Cl− for NO.
3.5 Reactions with NO2−, NO3−, O2 and CO
previously reported CoII and CoIII–NO complexes. These
stretching vibrations are very similar to those of CoII(salen)-
(NO) (n = 8), CoII(salphen)(NO) (n = 8), CoII(tpp)(NO) (n = 8),
and N2S2-typeCoIII(NO) (n = 7), regardless of their electron
numbers. Here, the number n is defined as the total number
of d-electrons of the metal ion and one electron in the radical
molecule. This result makes us difficult to discuss the elec-
tronic states of coordinated nitric oxide and the metal ion. The
Nitrite, carbon monoxide, dioxygen, and other small mole-
cules are present in the environment. These molecules may
interfere with detection of NO. The binding activity of these
compounds should be examined. In this work, we studied the
−
binding behavior of CO, O2, NO2−, and NO3 for complexes
1, 2, 3, and 4 by UV-vis spectroscopy.
First, the reactivity of complexes 1, 2, 3 and 4 for CO was
studied. When CO gas was flowed into DMF solutions of com-
plexes 1, 2, 3 and 4, their UV-vis spectra did not show any
changes in the 400–700 nm region (Fig. S5a to S5d†), indicat-
ing that these complexes do not react with CO.
Second, the reactivity of complexes 1, 2, 3, and 4 with O2
was investigated. The exposure of DMF solutions of complexes
to air did not produce any UV-vis spectral changes for 2, 3 and
4, while the absorption bands for 1 at 532 nm and 578 nm
were reduced in intensity (Fig. S6a to S6d†). This is considered
to be due to the coordination of H2O in air to the Co centre, as
reported previously.40
N–O stretching vibration values of complexes 3 (1648 cm−1
)
and 4 (1650 cm−1) are listed in Table 5 together with those of
the previously reported FeII and FeIII–NO complexes. Interest-
ingly, the previous data indicate that the N–O stretching
vibration values of the NO molecule coordinated to iron ions
depend on the oxidation and spin states of iron ions; low-spin
FeII(salen)(NO) (electron number, n = 7), low-spin FeII(salphen)-
(NO) (n = 7), and FeII(tpp)(NO) (n = 7) have values around
1650 cm−1, whereas high-spin FeII(salen)(NO) (n = 7), high-
spin FeII(salphen)(NO) (n = 7), FeIII(oep)(NO)(CH3) (n = 6), and
FeIII(oec)(NO) (n = 6) have values at approximately 1750 cm−1
.
Third, for the reaction with NaNO2, CoIII and FeIII com-
plexes were found to have different reactivities from each other
when 10 equiv., 100 equiv. and excess amounts of NaNO2 were
added (Fig. S7a to S7d†). The UV-vis spectrum of 1 undergoes
a decrease in the absorption bands at 532 nm and 578 nm
with an isosbestic point at 463 nm. The same behavior was
observed in the UV-vis spectrum of 2; the absorption bands at
558 nm decrease with an isosbestic point at 412 nm. These
In our case, the wavenumbers of NO stretching vibrations are
near the n = 7 state, which indicates the formation of an FeII-
nitrosyl complex reduced by the NO molecule or that of the
two-NO-coordinated FeIII complex. The former is not a reason-
able assignment because the reduction potential is too nega-
tive to lead to reduction of the metal centre (vide infra). We
therefore conclude that two molecules may be coordinated to
the FeIII centre to form octahedral complexes, [FeIIIL1(NO)2]−
and [FeIIIL2(NO)2]−.
findings indicate that NO2 coordinates to the CoIII centre. On
−
the other hand, the spectrum of 3 exhibits a slight decrease in
4476 | Dalton Trans., 2013, 42, 4470–4478
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
the absorption bands at 427 nm when 100 equiv. or more of selective reactivity. This complex 4 is expected also to have not
NaNO2 is added, although it does not undergo a significant only selectivity but also sensitivity for NO. We are therefore
spectral change when 10 equiv. of NaNO2 is added. This indi- developing it as a NO sensor device.
−
cates that NO2 partly coordinates to the FeIII centre of 3 but
the interaction is weak. Interestingly, the spectrum of 4 does
not undergo any changes even if large amounts of NaNO2 are
added. Among complexes 1, 2, 3, and 4 with NaNO2, only 4
Acknowledgements
was found to be unreactive with NaNO2.
We gratefully acknowledge support of this work by a Grant-in-
−
Fourth, the reactivity with NO3 was investigated by Aid for Scientific Research from the Ministry of Education,
addition of 10 equiv., 100 equiv. and an excess amount of Culture, Sports, Science, and the Knowledge Cluster Project.
NaNO3 to the DMF solutions of complexes 3 and 4 (Fig. S8a
and S8b†). However, their UV-vis spectra did not show any
spectral changes in the 400–700 nm region, indicating that
Notes and references
−
these complexes do not interact with NO3
.
Each of the complexes studied in this work is capable of
binding NO. However, CoIII complexes 1 and 2 react also with
NO2−, although they do not react with CO and O2. 1 also reacts
with water molecules in air. Previously, other Co model com-
plexes, such as [CoII(salen)], have been reported to react with
NO41 as well as other small molecules, such as H2O, O2 and
1 P. L. Feldman, O. W. Griffith and D. J. Stuehr, Chem. Eng.
News, 1993, 20, 26–38.
2 R. M. J. Palmer, D. D. Rees, D. S. Ashton and S. Moncado,
Biochem. Biophys. Res. Commun., 1988, 153(3), 1251–1256.
3 A. M. Leone, R. M. J. Palmer, R. G. Knowles, P. L. Francis,
D. S. Ashton and S. Moncado, J. Biol. Chem., 1991, 266(35),
23790–23795.
− 42–44
−
NO2
.
Complexes 1 and 2 react with NO2 and the N2S2-
type CoIII complex (PPh4)[CoIII(L)] does not react with NO2
.
4 C. Nathan, FASEB J., 1992, 6, 3051–3064.
− 34
These different behaviours can be explained by the differences
in their redox potentials. The CoII/CoIII redox potentials of 1
and 2 have lower values of −0.51 and −0.96 V, respectively.
However, the corresponding redox potential of (PPh4)[CoIII(L)]
is −1.62 V. The redox potential depends on the electron
donating character of the ligand, consequently, the Lewis
acidity of the metal centre should be closely related with the
redox potential.20 Namely, the redox potential of the metal
centre shifts to a lower potential region when a strong donat-
5 H. M. Abu-Soud, K. Ichimori, A. Presta and D. J. Stuehr,
J. Biol. Chem., 2000, 275(23), 17349–17357.
6 A. Ohta, H. Takagi, T. Matsui, J. Hamai, S. Iida and
H. Esumi, Neurosci. Lett., 1993, 158, 33–35.
7 L. J. Ignarro, G. M. Buga, K. S. Wood, R. E. Byrns and
G. Chaudhuri, Proc. Natl. Acad. Sci. U. S. A., 1987, 84, 9265–
9269.
8 K. Shibuki, Neurosci. Res., 1990, 9(1), 69–76.
9 T. Malinski and Z. Taha, Nature, 1992, 358, 676–678.
ing ligand, which reduces the Lewis acidity of the metal ion, is 10 K. Funazo, M. Tanaka and T. Shono, Anal. Chim. Acta,
introduced because it causes the increase in the energy level of 1980, 119, 291–297.
d-orbitals. So it can be clearly understood that the Lewis 11 J. J. B. Kalyanaraman and J. S. Hyde, Biochem. Biophys. Res.
acidities of 1 and 2 are smaller than that of (PPh4)[CoIII(L)].
Consequently, the former complexes interacted with the NO2
Commun., 1993, 192(2), 926–934.
12 M. Sato, T. Nakajima, M. Goto and Y. Umezawa, Anal.
Chem., 2006, 78, 8175–8182.
−
ion.
On the other hand, the FeIII complexes, 3 and 4, have 13 H. Dacres and R. Narayanaswamy, Sens. Actuators, B, 2005,
−
specific reactivity for NO. They do not react with CO and NO3
,
107, 14–23.
but they have different reactivities with NO2−. Complex 3 reacts 14 M. R. Vilar, J. El-Beghdadi, F. Debontridder, R. Naaman,
−
with NO2 at a concentration greater than 100 equiv. Complex
A. Arbel, A. M. Ferraria and A. M. Botelho Do Rego, Mater.
Sci. Eng., C, 2006, 26, 253–259.
4 does not react with NO2−. These findings show that complex
4 has much greater specific selectivity for NO than the other 15 S. Mochizuki, Seitaikougaku, 2005, 43(1), 32–35.
complexes investigated in this work.
16 V. Ryabova, A. Schulte, T. Erichsen and W. Schuhmann,
Analyst, 2005, 130, 1245–1252.
17 A. K. Patra, R. K. Afshar, M. M. Olmstead and
P. K. Mascharak, Angew. Chem., Int. Ed., 2002, 41, 2512–
2515.
Conclusion
In order to develop metal complexes which selectively bind 18 A. K. Patra and P. K. Mascharak, Inorg. Chem., 2003, 42,
NO, two CoIII and two FeIII complexes with square-planar
7363–7365.
N2O2-donor sets, N-[2-(2-hydroxybenzylamino)ethyl]-2-hydroxy- 19 K. Ghosh, A. A. Eroy-Reveles, B. Avila, T. R. Holman,
benzamide (H3L1) and 1,2-bis(2-hydroxybenzoylamino)ethane
(H4L2), were designed as metal complexes with NO molecule-
M. M. Olmstead and P. K. Mascharak, Inorg. Chem., 2004,
43, 2988–2997.
accessible vacant sites on the apical sides of the complex. 20 T. Yano, H. Arii, S. Yamaguchi, Y. Funahashi, K. Jitsukawa,
Although all complexes have selective reactivity for NO, the
T. Ozawa and H. Masuda, Eur. J. Inorg. Chem., 2006, 3753–
3761.
FeIII complex 4 with the L2 ligand exhibits a particularly high
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 4470–4478 | 4477

View Article Online
Paper
Dalton Transactions
21 T. Yano, T. Ozawa and H. Masuda, Chem. Lett., 2008, 37(7), 32 B. S. Tovrog, S. E. Diamond and F. Mares, J. Am. Chem.
672–677. Soc., 1981, 103, 3522–3526.
22 H. Dacres and R. Narayanaswamy, Sens. Actuators, B, 2005, 33 W. R. Scheidt and J. R. Hoard, J. Am. Chem. Soc., 1973, 95,
107, 14–23. 8281–8288.
23 M. D. Pluth, M. R. Chan, L. E. McQuade and S. J. Lippard, 34 J. A. McCleverty, Chem. Rev., 2004, 104, 403–418.
Inorg. Chem., 2011, 50, 9385–9392.
24 A.-L. Tsai, V. Berka, E. Martin and J. S. Olson, Biochemistry,
2012, 51, 172–186.
35 W. R. Scheidt and M. E. Frisse, J. Am. Chem. Soc., 1975, 97,
17–21.
36 R. Guilard, G. Lagrange, A. Tabard, D. Lancon and
K. M. Kadish, Inorg. Chem., 1985, 24, 3649–3656.
25 Z. Zhang, T. Yano, T. Shibayama, T. Suwabe, T. Inomata,
Y. Funahashi, T. Ozawa and H. Masuda, Trans. Mater. Res. 37 M. Autret, S. Will, E. V. Caemelbecke, J. Lex,
Soc. Jpn., 2009, 34, 529–532.
26 W. Adam and C. R. Saha-Moeller, Indian J. Chem., 2004,
46A, 56–62.
27 D. Schnieders, A. Hammerschmidt, M. Merkel,
F. Schweppe and B. Krebs, Z. Anorg. Allg. Chem., 2008, 634,
2933–2939.
J. P. Gisselbrecht, M. Gross, E. Vogel and K. M. Kadish,
J. Am. Chem. Soc., 1994, 116, 9141–9149.
38 D. Schweitzer, J. Ellison, S. C. Shoner, S. Lovell and
J. A. Kovacs, J. Am. Chem. Soc., 1998, 120, 10996–
10997.
39 T. Ozawa, T. Yano, Z. Zizheng, T. Inomata, Y. Funahashi
and H. Masuda, ECS Trans., 2008, 16(11), 545–549.
28 C. A. Jimenez, J. B. Belmar, F. S. Delgado, M. Lopez-Rodri-
guez, O. Pena, M. Julve and C. Ruiz-Perez, Dalton Trans., 40 A. Debuigne, Y. Champouret, R. Jerome, R. Poli and
2007, 2135–2144. C. Detrembleur, Chem.–Eur. J., 2008, 14, 4046–4059.
29 M. Izakovic and J. Sima, Acta Chim. Slov., 2004, 51, 427– 41 T. Shioya and T. M. Swager, Chem. Commun., 2002, 1364–
436. 1365.
30 Y. Mitsuura and T. Ozawa, Kikan Kagaku Sosetsu 30: NO— 42 J. J. Chapman and M. E. Welker, Organometallics, 1997, 16,
Kagaku to Seibutsu, ed. T. Nagano, I. Ohkura, S. Ohta, 747–755.
T. Ozawa and T. Yoshimura, Japan Scientific Press, 1996, 43 K. Ishii and K. Aoki, Anal. Chem., 1983, 55, 604–
ch. 1., pp. 3–7. 608.
31 J. G. Nguyen, C. A. Johnson, B. Subramaniam and 44 M. R. Ganjali, M. Rezapour, M. R. Pourjavid and
A. S. Borovik, Chem. Mater., 2008, 20, 5939–5941.
M. Salavati-Niasari, Anal. Sci., 2003, 19, 1127–1131.
4478 | Dalton Trans., 2013, 42, 4470–4478
This journal is © The Royal Society of Chemistry 2013