Six-coordinate ferric porphyrins containing bidentate N-t-butyl-N-nitrosohydroxylaminato ligands: Structure, magnetism, IR spectroelectrochemisty, and reactivity

Article abstract of DOI:10.1039/c5dt03074a

NONOates (diazeniumdiolates) containing the [X{N₂O₂}]^- functional group are frequently employed as nitric oxide (NO) donors in biology, and some NONOates have been shown to bind to metalloenzymes. We report the preparation, crystal structures, detailed magnetic behavior, redox properties, and reactivities of the first isolable alkyl C-NONOate complexes of heme models, namely (OEP)Fe(η²-ON(t-Bu)NO) (1) and (TPP)Fe(η²-ON(t-Bu)NO) (2) (OEP = octaethylporphyrinato dianion, TPP = tetraphenylporphyrinato dianion). The compounds display the unusual NONOate O,O-bidentate binding mode for porphyrins, resulting in significant apical Fe displacements (+0.60 ? for 1, and +0.69 ? for 2) towards the axial ligands. Magnetic susceptibility and magnetization measurements made from 1.8-300 K at magnetic fields from 0.02 to 5 T, yielded magnetic moments of 5.976 and 5.974 Bohr magnetons for 1 and 2, respectively, clearly identifying them as high-spin (S = 5/2) ferric compounds. Variable-frequency (9.4 GHz and 34.5 GHz) EPR measurements, coupled with computer simulations, confirmed the magnetization results and yielded more precise values for the spin Hamiltonian parameters: g_avg = 2.00 ± 0.03, D = 3.89 ± 0.09 cm^-1, and E/D = 0.07 ± 0.01 for both compounds, where D and E are the axial and rhombic zero-field splittings. IR spectroelectrochemistry studies reveal that the first oxidations of these compounds occur at the porphyrin macrocycles and not at the Fe-NONOate moieties. Reactions of 1 and 2 with a histidine mimic (1-methylimidazole) generate RNO and NO, both of which may bind to the metal center if sterics allow, as shown by a comparative study with the Cupferron complex (T(p-OMe)PP)Fe(η²-ON(Ph)NO). Protonation of 1 and 2 yields N₂O as a gaseous product, presumably from the initial generation of HNO that dimerizes to the observed N₂O product.

Full text of DOI:10.1039/c5dt03074a

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ARTICLE
Six-coordinate Ferric Porphyrins Containing Bidentate N-t-Butyl-
N-nitrosohydroxylaminato Ligands: Structure, Magnetism, IR
spectroelectrochemisty, and Reactivity
Nan Xu,* a,b Jonathan H. Christian,c Naresh S. Dalal,*c Erwin G. Abucayon,a Colin Lingafelt,b Douglas
R. Powell,a and George B. Richter-Addo*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
–
www.rsc.org/
Abstract: NONOates (diazeniumdiolates) containing the [X{N2O2}] functional group are frequently employed as nitric
oxide (NO) donors in biology, and some NONOates have been shown to bind to metalloenzymes. We report the
preparation, crystal structures, detailed magnetic behavior, redox properties, and reactivities of the first isolable alkyl C-
NONOate complexes of heme models, namely (OEP)Fe(η2-ON(t-Bu)NO) (1) and (TPP)Fe(η2-ON(t-Bu)NO) (2) (OEP =
octaethylporphyrinato dianion, TPP = tetraphenylporphyrinato dianion). The compounds display the unusual NONOate
O,O-bidentate binding mode for porphyrins, resulting in significant apical Fe displacements (+0.60 Å for 1, and +0.69 Å for
2) towards the axial ligands. Magnetic susceptibility and magnetization measurements made from 1.8–300 K at magnetic
fields from 0.02 to 5T, yielded magnetic moments of 5. 976 and 5.974 Bohr magnetons for 1 and 2, respectively, clearly
identifying them as high-spin (S = 5/2) ferric compounds. Variable-frequency (9.4 GHz and 34.5 GHz) EPR measurements,
coupled with computer simulations, confirmed the magnetization results and yielded more precise values for the spin
Hamiltonian parameters : gavg = 2.00 ± 0.03, |D| = 3.89 ± 0.09 cm-1, and E/D = 0.07 ± 0.01 for both compounds, where D
and E are the axial and rhombic zero-field splittings. IR spectroelectrochemistry studies reveal that the first oxidations of
these compounds occur at the porphyrin macrocycle and not at the Fe-NONOate moieties. Reactions of 1 and 2 with a
histidine mimic (1-methylimidazole) generate RNO and NO, both of which may bind to the metal center if sterics allow, as
shown by a comparative study with the Cupferron complex (T(p-OMe)PP)Fe(η2-ON(Ph)NO). Protonation of 1 and 2 yields
N2O as a gaseous product, presumably from the initial generation of HNO that dimerizes to the observed N2O product.
gfgf
1 Introduction
The NONOate (diazeniumdiolate) functional group formally
contains two NO moieties linked to an organic fragment X as
sketched in Figure 1. The most widely-known NONOates
contain carbon-bound (C-NONOates) and nitrogen-bound (N-
NONOates) organic fragments, although several NONOates
containing S-bound and O-bound fragments are also known.1
C-NONOates have attracted a lot of attention from the
viewpoint of their coordination chemistry. For example, the
–
Cupferron anion (i.e., [Ph{N2O2}] ) binds to several metal ions,
and has been widely used historically as an analytical reagent
for the colorimetric detection of Fe and Cu.2 However, there is
also biological interest in the chemistry of C-NONOate
Fig. 1 Top: Sketch of the NONOate functional group
showing the common resonance forms. Bottom: Chemical
structures of the NONOate-containing natural products
alanosine and poecillanosine.
compounds due to the fact that the C-NONOate functional group is
present in several natural products including the antitumor
antibiotic alanosine isolated from Streptomyces alanosinicus3, 4 and
the free radical scavenger poecillanosine isolated from the marine
sponge Poecillastra spec. aff. tenuilaminaris (Figure 1),5 and several
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others such as dopastin and fragin.1 C-NONOates such as Cupferron analysis (as judged by Cl analysis), but it was not detectable by X-ray
and the related N-NONOates are frequently employed as NO diffraction.
donors in physiological media.6
Preparation of (TPP)Fe(η2-ON(t-Bu)NO) (2). The tetraphenyl-
NONOates bind to metals in coordination compounds almost porphyrin derivative of 1 above, namely (TPP)Fe(η2-ON(t-Bu)NO)
exclusively via the bidentate O,O-binding mode.2 The only (2), was obtained similarly in 68% isolated yield. Anal. Calcd for
structurally characterized exception of an observed monodentate C48H37N6O2Fe.0.74CH2Cl2 (MW 933.44): C, 68.99; H, 4.57; N, 9.90;
O-binding mode is that of
a
Cu complex containing the Cl, 6.18. Found: C, 69.23; H, 4.86; N, 9.61; Cl, 6.25. IR (KBr, cm-1):
1163 (sh) and 961 (m) cm-1 assigned to the ONNO moiety. X-ray
7
–
([Et2N{N2O2}] ) ligand.
Given the biological importance of C-NONOates, we were quality crystals were obtained by
a slow evaporation of a
interested in determining what kind of interactions would be CH2Cl2/cyclohexane (2:1) solution of the complex under nitrogen.
present if C-NONOates encounter heme. We note that Cupferron is Fractional CH2Cl2 was present in the sample sent for elemental
a good substrate for horseradish peroxidase (releasing PhNO and analysis (as judged by Cl analysis), but it was present in full
NO),8 and that a direct binding of an N-NONOate to the metal occupancy in the structure obtained by X-ray diffraction.
center of cobalamin has been shown to precede the release of NO.9
Preparation of (OEP)Fe(η2-ON(Ph)NO) (3). The synthesis was
Interestingly, a related direct binding of the natural product performed under reduced laboratory lighting to minimize the light-
Dopastin via its NONOate functional group to the di-Cu active site of induced decomposition of the product. To a Schlenk tube charged
mushroom tyrosinase has been proposed to precede the observed with (OEP)FeCl (23.1 mg, 0.04 mmol) in CH2Cl2 (10 mL) was added
inhibition of this enzyme.10
an excess of freshly prepared Ag[ON(Ph)NO] (22.3 mg, 0.09 mmol).
We now report the preparation, crystal structures, detailed The mixture was stirred for 15 min during which time the color
magnetic behavior, redox properties, and reactivities of two alkyl C- changed from brown-red to bright red-orange. The solution was
NONOate complexes of heme models. Combined with our initial then filtered via cannula and the solvent removed in vacuo. The
report on a Cupferron complex,11 these remain the only isolable dark residue was washed with hexane and dried in vacuo to give
NONOate complexes of heme models reported to date.
(OEP)Fe(η2-ON(Ph)NO) (3) (15.7 mg, 59% yield). IR (KBr, cm-1): 1340
(m), 1283 (s), 940 (m) cm-1, assigned to υN=N, υNO and δONNO
,
respectively.
Preparation of (T(p-OMe)PP)Fe(η2-ON(Ph)NO) (4).11 The (T(p-
OMe)PP) derivative of 3 above was obtained similarly in 63% yield
(46.0 mg) from the reaction of (T(p-OMe)PP)FeCl (65.0 mg, 0.08
mmol) in CH2Cl2 (10 mL) and excess of freshly prepared
Ag[ON(Ph)NO)] (64.3 mg, 0.26 mmol) under reduced laboratory
lighting. IR (KBr, cm-1): δONNO = 937 cm-1. The IR frequencies due to
N=N and N=O vibrations were not detected due to the intense
porphyrin ring signals in the region where those peaks are normally
expected.
2 Experimental
2.1 Synthesis
The reactions were performed anaerobically using nitrogen as
the inert gas. Standard Schlenk glassware and an inert atmosphere
glove box were utilized for the syntheses. Solvents were distilled
under nitrogen from appropriate drying agents or collected under
nitrogen from a Pure Solv 400-5-MD Solvent Purification System
(Innovative Technology). N-t-butyl-N-nitrosohydroxylamine (HON(t-
Bu)NO) was prepared by literature methods.12 The Cupferron salt
Ag[ON(Ph)NO] was prepared as described previously.13 FT-IR
spectra for compound characterization were recorded on a Bio-Rad
FTS 155 spectrometer. Elemental analyses were performed by
Atlantic Microlab, Inc., Norcross, GA.
Reactions of the (por)Fe(η2-ON(R)NO) Complexes (por = OEP,
T(p-OMe)PP; R = t-Bu, Ph) with 1-MeIm. The following reactions
performed under reduced laboratory lighting are representative.
(i) To a CDCl3 (3 mL) solution of compound 1 (16 mg, 0.013
mmol) was added excess 1-MeIm (28.2 mg, 0.34 mmol). The
reaction mixture was stirred at room temperature for 1 hr during
which time the color gradually changed from purple to red-purple.
The solvent and volatiles were transferred by vacuum to a separate
flask. The remaining solid in the reaction flask was washed with
hexane and dried in vacuo to give a mixture of the known
(OEP)Fe(NO) (υNO 1672 cm-1) compound14 and unreacted 1 as
Preparation of (OEP)Fe(η2-ON(t-Bu)NO) (1). To a CH2Cl2 (10
mL) solution of [(OEP)Fe]2(ꢀ-O) (25 mg, 0.021 mmol) under an
atmosphere of nitrogen was added a CH2Cl2 solution (2.5 mL) of
excess N-t-butyl-N-nitrosohydroxylamine (68 mg, 0.58 mmol, ~28x
excess). The solution was stirred for 1 h during which time the
color changed gradually from brown to purple-red. The solvent was
removed under vacuum. Hexane (~5 mL) was added to the residue,
and the mixture stirred for ~ 1 min and left to stand for 5-10 min.
The light red supernatant was discarded and the remaining dark
purple solid dried in vacuo overnight to give (OEP)Fe(η2-ON(t-
Bu)NO) (1) (16 mg, 0.022 mmol, 55% isolated yield). Anal. Calcd for
C40H53N6O2Fe.0.055CH2Cl2 (MW 710.40): C: 67.72; H, 7.53; N,
11.83; Cl, 0.55. Found: C, 67.73; H, 7.58; N, 11.73; Cl, 0.53. IR (KBr)
spectroscopy revealed a new peak at 1166 cm-1 assigned to the
ligand. X-ray quality crystals were obtained by a slow evaporation
of a CH2Cl2/hexane (2:1) solution of the complex under nitrogen.
Residual CH2Cl2 was present in the sample sent for elemental
1
judged by IR spectroscopy. A H NMR spectroscopic analysis of the
trapped volatiles revealed the formation of t-BuNO [δ ppm: 1.6 (s,
(t-BuNO)2 dimer) and 1.2 (s, t-BuNO monomer)]15 as the primary
byproduct of the reaction in ~50% unoptimized yield based on 1.
(ii) To a CH2Cl2 (5 mL) solution of (T(p-OMe)PP)Fe(η2-ON(Ph)NO)
(4; 20.0 mg, 0.02 mmol) was added excess 1-MeIm, and the
reaction mixture stirred for ~1 h at –45 °C during which time the
color changed from bright red-orange to red-purple. The solvent
was removed in vacuo and the residue was washed with hexane
and the product dried in vacuo overnight. Crystallization of the
residue from the slow evaporation of its CH2Cl2/hexane (1:1)
solution gave a mixture (~9.0 mg) of the crystalline products (T(p-
2 | J. Name., 2012, 00, 1-3
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OMe)PP)Fe(NO) (23% yield; υNO 1670 cm-1)14 and (T(p- IR spectrum of the solid product showed a peak at 1347 (sh) cm-1
OMe)PP)Fe(PhNO)(1-MeIm) (23% yield; υMeIm 1539 (m), and υNO assigned to an Fe-bound PhNO ligand.
1346 cm-1) in a 1:1 ratio.
Chemical oxidation of (TPP)Fe(η2-ON(t-Bu)NO). To a CDCl3 (2.5
The corresponding reaction of (OEP)Fe(η2-ON(Ph)NO) with mL) solution of (TPP)Fe(η2-ON(t-Bu)NO) (5 mg, 0.01 mmol) under
excess 1-MeIm similarly gave a 1:1 mixture of (OEP)Fe(NO) (υNO reduced laboratory lighting was added AgBF4 (1.5 mg, 0.01 mmol).
1672 cm-1) and (OEP)Fe(PhNO)(1-MeIm) (υMeIm 1523 (m), and υNO The reaction mixture was stirred for 15 min during which time the
1337 cm-1)16 as products in ~21% yield for each product.
color of the solution changed from purple-red to a bright red. IR
Reactions of the (por)Fe(η2-ON(t-Bu)NO) Complexes (por = spectral analysis of the product solution under N2 showed the
OEP, TPP) with nitric oxide. Solutions of the (por)Fe(η2-ON(t- formation of a new band at 1293 cm-1 indicative of a TPP-type π-
Bu)NO) (por: OEP, 1; TPP, 2) complexes in CH2Cl2 (3 mL) were radical cation product.20
exposed to NO gas (via bubbling of the gas through the solutions)
2.2 Magnetic and EPR Measurements
for 5 min during which time the color of the solutions changed from
purple to bright red-purple. The unreacted NO gas in the headspace
was then replaced by sparging with N2 gas for ~5 min. The IR
spectrum of the product solution from the reaction of 1 with NO in
CH2Cl2 displayed a new strong υNO band at 1883 cm-1 (υ NO =1848
cm-1) assigned to the six-coordinate (OEP)Fe(NO)(η1-ON(t-Bu)NO).
The IR spectrum of the product of the reaction of 2 with NO yielded
a strong υNO band at 1888 cm-1 (υ NO =1851 cm-1) assigned to the
Temperature-dependent magnetic susceptibility measurements
on polycrystalline samples of 1 and 2 were made using a Quantum
Design MPMS-XL-5 SQUID magnetometer over a temperature range
of 1.8 to 300 K at a measuring field of 0.02 T. Magnetization
measurements at variable temperature and variable field (VTVH)
were performed at 1, 3, and 5 T over the temperature range of 1.8
to 300 K. A diamagnetic correction of 4.585 x 10-4 emu/mol for 1
and 5.139 x 10-4 emu/mol for 2 was calculated using Pascal
constants21 and was applied to the experimental data along with a
contribution from the gelatin sample holder and straw. The
six-coordinate
(TPP)Fe(NO)(η1-ON(t-Bu)NO).
Attempted
crystallization of the OEP adduct led to the formation of the known
five coordinate (OEP)Fe(NO) compound identified by IR
spectroscopy and by X-ray crystallography.
Protonation of the (por)Fe(η2-ON(R)NO) Complexes. The
following reactions performed under reduced laboratory lighting
are representative.
magnetic properties were evaluated using the following standard
5
spin Hamiltonian of a system with S =
/
and zero-field splitting
2
(zfs) terms, D and E.
(i) (TPP)Fe(η2-ON(t-Bu)NO) (2): Triflic acid (20 ꢀL, 0.2 mmol)
was added dropwise to a CDCl3 (3.0 mL) solution of 2 (13.1 mg, 0.02
mmol) at room temperature. The bright red-purple solution
immediately changed to an orange-black color. The reaction
mixture was stirred for 1 h in a sealed Schlenk tube. The headspace
gases were then vacuum transferred into a gas IR cell. The gas
phase IR spectrum revealed bands at 2237/2212 and 1276/1261
cm-1 assigned to ν(antisym) and ν(sym) of N2O, respectively.17
ꢌ
ꢌ
ꢌ
ꢀ = ꢂꢀ ∙ ꢅꢆ ∙ ꢇ + ꢉ ꢊꢇꢋ − ꢍ /3ꢎ + ꢏ ꢊꢇꢐ − ꢇꢑꢌꢎ
(1)
ꢁ
ꢃꢄ
ꢈ
ꢈ
ꢁ
ꢈ
ꢈ
ꢃꢄ
In this Hamiltonian, β is the Bohr-magneton, ꢀ is the magnetic field
vector, ꢅꢆ is the Zeeman tensor, D and E are the axial and rhombic
zfs parameters, respectively, and the ꢇ terms are spin operators.
Since magnetic susceptibility measurements are typically insensitive
towards the small energy of E, evaluation of both compounds
began with the assumption of axial symmetry, thus E was assumed
to be zero. Simulation of the experimental magnetic data was
performed with the julX program.23 It was necessary to include a
small temperature-independent paramagnetism (TIP) term in the
simulations according to χcalc = χ + TIP.
22
ꢈ
The remaining reaction solution was vacuum transferred to
another flask for 1H NMR spectroscopic analysis. 1H NMR (CDCl3,
253 K, 400 MHz): δ 7.27 (s, CDCl3), 5.32 (s, CH2Cl2), 2.21 (s, acetone),
1.75 (s, H2O), 1.59 (s, (t-BuNO)2 dimer) and 1.27 (s, t-BuNO
monomer),15 1.22 (s, unassigned) and 0.06 (s, silicone grease
impurity).18 The solid residue was dried in vacuo and crystallized
from the slow evaporation of a CH2Cl2-hexane solution of this
residue. The resulting crystals were identified from an X-ray
structural analysis as the known five coordinate complex
(TPP)Fe(OSO2CF3).19
(ii) (T(p-OMe)PP)Fe(η2-ON(Ph)NO) (4): Triflic acid (20 ꢀL, 0.2
mmol) was added dropwise to a CH2Cl2 (10.0 mL) solution of 4 (18.2
mg, 0.02 mmol) at room temperature. The bright red-purple
solution immediately changed to a dark orange color. The reaction
mixture was stirred for 12 h in a sealed Schenk tube, and the
headspace gases were then vacuum transferred into a gas IR cell.
The gas phase IR spectrum revealed the formation of N2O.17 The
solvent was then removed from the residual mixture in the Schlenk
tube in vacuo. Diethyl ether was added to the oily residue, and the
mixture was stirred overnight and then left to stand for several
minutes to allow the resulting solid particles to separate. The
solvent was discarded and the solid product was dried in vacuo. The
Room temperature electron paramagnetic resonance (EPR)
spectra were recorded on a Bruker E500 spectrometer equipped
with
X and Q-band microwave sources (9.4 and 34.5 GHz,
respectively). The frequency was recorded with a built-in digital
frequency counter and the magnetic field was calibrated using a
2,2-diphenyl-1-picrylhydrazyl standard (DPPH, g = 2.0036).23 All
samples were measured in quartz tubes that were sealed with
approximately 1 inch of N-grease and lids. Signals from the
instrument cavity and quartz tubes (Supporting Information Figure
S1) were measured separately and were subtracted from the
spectra of our samples. The spectra were analyzed by visual
comparison with a locally developed computer simulation program,
as described elsewhere.24, 25
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2.3 Electrochemistry and Spectroelectrochemistry
Cyclic voltammograms were recorded using a BAS CV-50W
Voltammetric Analyzer equipped with a three-electrode cell (3 mm
Pt disk working electrode, Pt wire auxiliary electrode and a Ag/AgCl
or Ag wire quasi-reference electrode) as described previously.26
Solutions were 1 mM in analyte and 0.1 M in [NBu4]PF6 in CH2Cl2.
Ferrocene (Fc) was used as an internal reference standard, with
potentials (V) reported relative to the Fc/Fc+ couple.27
(2)
moiety.32, 33 For compound
2
, in addition to the new related band
at 1163 cm-1, a new medium intensity band at 961 cm-1 is observed
IR spectroelectrochemical measurements were recorded using a
Bruker Vector 22 FT-IR spectrometer equipped with a Remspec mid-
IR fiber-optic dip probe and a liquid nitrogen cooled MCT detector.
In our adaptation of the probe for these measurements,28 the
stainless steel mirror on the liquid transmission head of the fiber-
optic dip probe was replaced with a 3 mm Pt disk working electrode
and equipped with a custom-made electrochemical cell including a
Pt wire auxiliary electrode and a Ag/AgCl or Ag wire quasi-reference
electrode as described previously.26, 29
that is in a region associated with δ
(ONNO).32, 34 Other IR bands of
the bidentate ligand are not readily discernable, presumably due to
overlap with the porphyrin bands.
3.2 Molecular Structures
The crystal structures of
1 and 2, the latter crystallizing as a
CH2Cl2 solvate, are shown in Figure 2, with
(a)
2.4 X-ray Crystallography
Intensity data for the crystals at 100(2) K were collected using a
31
diffractometer with a Bruker APEX CCD area detector30,
and
graphite-monochromated Mo K
α radiation (λ = 0.71073 Å). A
summary of the crystal and refinement data is shown in Table S1.
The X-ray crystal structural data have been deposited with the
Cambridge Crystallographic Data Center, with reference codes
CCDC 1040731 (compound
CCDC 1040734 (compound
1
), CCDC 1040732 (compound
2), and
6). These data can be obtained free of
charge via www.ccdc.cam.ac.uk/ data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ,
UK; Fax: +44 1223 336033.
(b)
3 Results and Discussion
Synthesis
We are interested in the reactions of NO donors such as the
general class of NONOate compounds with iron porphyrins as
possible models for the interactions of these NONOates with
various heme active sites. We previously reported the preparation
of the aryl NONOate Cupferron iron porphyrin complexes
(por)Fe(η2-ON(Ph)NO) (por = TPP, T(p-OMe)PP) from the reactions
of the (por)FeCl precursors with Ag[ON(Ph)NO].11 We have now
Fig. 2 Molecular structure of (a) (OEP)Fe(η2-ON(t-Bu)NO) (1) (only the major
disordered ligand (67% occupancy) orientation for 1 is shown), and (b)
(TPP)Fe(η2-ON(t-Bu)NO) (2). One of the porphyrin phenyl rings (with the
C41 atom) is disordered; only the major component (85% occupancy) is
employed
a more convenient route to prepare the hitherto
unknown alkyl NONOate complexes (por)Fe(η2-ON(t-Bu)NO) (por =
OEP, TPP) using the readily obtainable porphyrin oxo-dimer
precusors and N-t-butyl-N-nitrosohydroxylamine as shown in eq. 2.
The analytically pure products were obtained in reasonable yields. shown. H atoms have been omitted for clarity. Thermal ellipsoids are
drawn at 35%.
The solids appear to be stable in air for short periods, but their
solutions are sensitive to air as judged by IR spectroscopy. The IR
selected bond lengths and angles collected in Table 1. The most
striking features of the structures are the bidendate O,O-binding
modes of the ligand to the porphyrin Fe centers resulting in
spectrum of
1 as a KBr pellet reveals, in addition to the porphyrin
bands, a new strong band at 1166 cm-1 associated with the N2O2
substantial apical Fe displacements (ꢁFe) of +0.60 Å (for 1) and
4 | J. Name., 2012, 00, 1-3
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Table 1. Selected bond lengths (Å), bond angles (°) for
and •(CH2Cl2).
1
Table 2. Structural data for η2-O,O liganded iron(III) porphyrins.
2
ꢁ
Fe (Å)b
0.60
1
2•(CH2Cl2)
Fe-O (Å)
2.113(6)
2.118(5)
2.051(2)
2.050(2)
2.044(3)
2.091(2)
2.019(4)
2.323(8)
2.123(3)
2.226(3)
2.064(6)
2.067(6)
O-Fe-O (°)
68.0(3)
1a
Fe−O1
Fe−O2
O1−N5
O2−N6
N5−N6
2.118(5)
[2.116(5)]a
2.113(6)
[2.114(6)]a
1.231(7)
[1.232(8)]a
1.340(9)
[1.342(9)]a
1.250(8)
[1.252(9)]a
1.529(9)
2.051(2)
2.050(2)
1.306(3)
1.313(3)
1.261(4)
1.495(4)
2
72.31(9)
71.52(9)
51.6(2)
0.69
0.69
0.60
0.61
0.80
4
(TPP)Fe(NO3)
(TpivPP)Fe(NO3)
(OEP)Fe(O2C7H5)
57.75(10
N6−C(CMe3)
Fe−N(por)
73.1(2)
2.073(4)
–
2.085(3)
2.144(3)
72.31(9)
–
a
b
For the major disordered component. apical displacement from
2.104(4)
68.0(3)
the 24-atom porphyrin planes.
∠
∠
O2−Fe−O1
N5−O1−Fe
[69.9(6)]a
122.3(8)
[120.3(14)]
120.0(2)
The X-ray crystal structural data for
1 and 2 are indicative of
a
these high-spin ferric complexes, using the stereochemical/spin-
state relationships described by Scheidt and Reed.41 To probe
possible changes in spin-states and associated spin-crossover42,
113.6(5)
[111.2(8)]a
112.3(10)
115.0(2)
111.8(3)
∠
∠
N6−O2−Fe
N6−N5−O1
43
temperatures of complexes
1 and 2, we embarked on detailed
[113.6(19)]
variable-temperature magnetic studies of these complexes.
a
118.6(8)
[118.8(13)]
120.8(3)
121.4(3)
117.7(3)
∠
∠
∠
N5−N6−O2
3.3 Magnetic Behavior
a
Magnetic susceptibility (χ) measurements were undertaken to
125.3(7)
[128.8(12)]
N5−N6−C(CMe3)
O2−N6−C(CMe3)
establish the electronic spins of
magnetic moments of and were found to be 5.976 µB and 5.974
µB, respectively at room temperature, that is, close to the spin-only
1 (por = OEP) and 2 (por = TPP). The
1
2
a
114.6(6)
[110.5(8)]a
5
value for an S =
/
system (5.916 µB for g = 2). The magnetic
2
moments of both compounds remain nearly constant down to 20 K,
but decrease sharply below 20 K, as shown in Figure 4. The sharp
decrease in µeff suggested that both compounds have significant
a The data in brackets are for the disordered (second) component.
zero-field splitting (zfs) resulting from spin-spin and spin-orbit
5
coupling, as expected for an
environment.44
S
=
/
2
state in noncubic
+0.69 Å (for
2) from the 24-atom porphyrin planes towards the
ligands. The Fe−O bond lengths are near equivalent at ~2.11 Å for
1
The data in Figure 3 were well simulated using the parameters
S = 5/2, D = 4.3 ± 0.2 cm-1, and gavg = 2.02 ± 0.01. Further
confirmation of these parameters was achieved by variable
temperature/variable field (VTVH) magnetization measurements
(1.8–300 K). The VTVH data for both compounds are shown in the
and ~2.05 Å for
(with 69.9(6)
72.3(1)
derivative
2
, with respective O−Fe−O bite angles of 68.0(3)
°
°
for the minor component) for compound
1
and
°
for compound
2. It is interesting to note that for the OEP
1
, the terminal N5−O1 bond length of 1.231(7) Å
(1.232(8) Å for minor component) is significantly shorter than the
N6-O2 bond length (1.340(9) Å), whereas the related N−O bond
lengths in the TPP derivative are closer to each other (1.306(3) and
inset of Figure 3. These data were well simulated using S = 5/2, gavg
=
2.02 ± 0.01 and D = 3.6 ± 0.2 cm-1, which is in good agreement with
the susceptibility results. In both compounds the magnetic data are
1.313(3) Å, respectively). The ONNO moiety in
angle of 0.6(4)°) is nearly planar, compared with the wider torsion
angle of 9.2(15) (–19(3)° for the minor component) of compound
, although both compounds possess N6 atoms with essentially
planar geometries. The N−N bond length of is 1.250(8) Å, and is
2 (ONNO torsion
5
consistent with the assignment of an S =
/ high-spin (HS) ferric
2
(Fe3+) ion. Since magnetization and magnetic susceptibility are not
the most accurate methods for determining D or g, a detailed EPR
study was undertaken.
°
1
1
Figure 4 shows the room temperature, X-band (9.4 GHz)
powder EPR spectra of both compounds (middle segment, blue
1.261(4) Å in 2.
The η2-O,O binding mode of the axial ligands in
1, 2, and 4 is
uncommon;35,
there are only three other iron porphyrin
36
(compound
1) and black (compound 2)). Also shown are the
complexes that display this η2-O,O binding mode, namely the high-
spin ferric nitrato complexes (TPP)Fe(NO3)37 and (TpivPP)Fe(NO3),38,
39 and the ferric tropolonate complex (OEP)Fe(O2C7H5)40 (Table 2).
simulated energy level diagrams with the magnetic field orientation
parallel to the principal symmetry axis of the molecule (H || z) and
parallel to the x,
y directions (top and bottom segments
respectively). The simulated energy level diagrams and EPR spectra
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Fig.
3
Temperature dependence of µeff (main figure) and VTVH
and labeled (a) and (b)
measurements (inset) for compounds
1
2
respectively. The solid lines represent the best fit simulations to the
5
experimental data using S = /2, gavg = 2.02 ± 0.01, and D = 4.3 ± 0.2 cm-1for
µeff , and S = 5/2, gavg = 2.02 ± 0.01 and D = 3.6 ± 0.2 cm-1 for VTVH
measurements.
were calculated using a locally developed computer program,45, 46
which diagonalizes the Hamiltonian matrix of equation 1
.
At X-band (9.4 GHz), two distinct features were seen, at g’ =
7.60 and g’ = 4.15. Computer simulations of the X-band spectra
(middle segment, red trace), using the parameters: S = 5/2, gz =2.03
± 0.03 , gx,y =1.97 ± 0.03, |D| = 3.89 cm-1 ± 0.09, and E/D = 0.07 ±
0.01, are in good agreement with the experimental spectra with
minor artifacts due to an imperfectly random distribution of
crystallites in the used powder sample, not an uncommon feature
in powder EPR studies. The EPR peaks are rather broad, thus
suggesting g-anisotropy due to the electrostatic field from
neighboring porphyrin molecules. Similarly broadened spectra have
been previously reported for ferric nitrato porphyrinates.39 A small,
sharp peak was observed at g = 2.02, which is tentatively ascribed
to a minor S = ½ impurity or decomposed product (see Supporting
Information).
Fig. 4 Top: X-band (9.4 GHz) EPR experimental and simulation spectra of 1
(por = OEP) and 2 (por = TPP) at room temperature. Botom Q-band (34.5
GHz) EPR experimental and simulation spectra of 1 and 2. The top and
bottom portions in each figure show the energy level diagrams, for the H ||
z and H || x, y directions respectively. The red numbers in the top figure
represent the Ms quantum numbers in the high-field limit. In the bottom
figure; however, they are just labels for energy levels since the field strength
was not enough to be in the ‘ high-field’ limit. Red arrows mark the EPR
transition assignment.
X-band analysis is confirmed by additional measurements at Q-
band (34.5 GHz), a four-fold higher microwave frequency. The
improved resolution provided by Q-band (34.5 GHz), splits the
spectra into three peaks at g’ = 7.59, 5.58, and 4.15 as shown in
Figure 4 (middle segment). The Q-band spectra were well fit using
the same parameters as the X-band data. At both frequencies we
label the g values with prime symbols indicating that they are only
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effective values, which are skewed from the usual high-spin Fe3+ cyclic voltammetry experiments, with the applied potential held
47-49
values (g ~2.002–2.009),22,
due to large zfs, a result that has slightly above the Epa for the first oxidation. The resulting
been seen in many high-spin Fe3+ systems.22, 47-49 For example, the difference IR spectrum is shown in Figure 6. Importantly, an intense
report of large g-values has been seen in biological systems with new band at 1541 cm-1 grew in after the first oxidation; this band is
high spin ferric ions like the heme proteins47-51 and in other Fe3+ in the region associated with the characteristic bands for OEP
porphyrinates where the effective g values have been shown to containing π-radical monocations.20 No new band was detected in
range from ~7.7 to 1.8.(ref 40).
the range typical for ferric-NO moieties.14
The EPR data, combined with magnetic susceptibility,
unambiguously show that the iron center in these compounds
exists as a high-spin ferric ion, with a large axial zfs and non
vanishing rhombic zfs.
3.4 Redox Behavior and IR Spectroelectrochemistry
The redox behavior of compounds
investigated by cyclic voltammetry
spectroelectrochemistry. The cyclic voltammagrams are shown in
1
and
2
in CH2Cl2 was
and infrared
Fig. 6 Difference FTIR (spectrum) showing formation of the porphyrin
radical product during the first oxidation of (OEP)Fe(η2-ON(t-Bu)NO).
Figure 5. Both compounds
1
and
2
show well-defined reverisble
is oxidized at an Eo' potential
first oxidations. The OEP derivative
1
of +0.27 V versus the Fc/Fc+ couple, lower than that of the TPP
analog that displays its redox couple at +0.44 V. The difference in
the magnitude of redox couples is reflective of the influence of the
electron-donating capacity of the macrocycles (OEP > TPP),52
suggestive of electrooxidations at a site in close proximity to or on
the porphyrin macrocycles (vide infra).
Based on both the cyclic voltammetry and spectroelectrochemistry
results, we conclude that the bound diazeniumdiolate ligand is
retained upon oxidation, and that the oxidation occurs on the
macrocycle and not on the axial metal-ligand fragment (eq. 3).
_
(por)Fe(η2-ON(t-Bu)NO)
→
(por )Fe(η2-ON(t-Bu)NO) + e (3)
•+
The OEP derivative
at −1.60 V and an associated small return peak at −0.99 V. In
contrast, the TPP analogue exhibits a reversible reduction couple
1 displays an irreversible reduction with Epc
The
difference
IR
spectrum
obtained
during
2
spectroelectrochemical oxidation of
2
does not show any new band
with E°' at −1.26 V. The peak separation of this reduction couple
formed in the 2000−1500 cm-1 region. We note that the
characteristic bands of tetraarylporphyrin radicals at 1295−1270
cm-1 in the IR spectra20 are outside of the accessible IR spectral
window of our instrumentation. However, chemical oxidation of
(TPP)Fe(η2-ON(t-Bu)NO) using AgBF4 (see Experimental Section)
results in a product with a new band at 1293 cm-1, consistent with
(0.33 V) is larger than that of the Fc/Fc+ couple (0.14 V) under
identical conditions, indicative of
a quasi-reversible reduction
and/or slow electron transfer during the reduction process.
the generation of the (TPP )Fe(η2-ON(t-Bu)NO) radical cation
•+
product as described in eq. 6.
3.5 Reactivity
The final products obtained when (OEP)Fe(η2-ON(t-Bu)NO) was
reacted with 1-MeIm were not the expected six-coordinate
compounds shown in eq. 4, but rather the known nitrosyl
compound (OEP)Fe(NO) (υNO = 1665 cm-1) and the organic t-BuNO
product, the latter characterized by 1H NMR spectroscopy. No
stable six-coordinate product was obtained in this case.
Fig. 5 Cyclic voltammograms of the (por)Fe(η2-ON(t-Bu)NO) (por = OEP,
TPP) compounds in CH2Cl2 containing 0.1 M NBu4PF6 and at a scan rate of 0.2
V/s at room temperature.
(4)
IR spectroelectrochemistry of compound
1 upon oxidation was
investigated under the same experimental conditions used for the
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We propose that the reaction of eq. 4 results in the elimination
Journal Name
of both NO and t-BuNO, with NO being the only reagent capable of
re-binding to Fe, as the t-BuNO ligand is likely too bulky to re-bind
effectively with the Fe porphyrin.
Indeed, to examine this proposed reaction pathway further, we
prepared the related Cupferron analog (OEP)Fe(η2-ON(Ph)NO) (
3)
and performed a similar reaction with 1-MeIm. In this case, we
were successful at isolating both the known five-coordinate
(OEP)Fe(NO) compound and the six-coordinate derivative
(OEP)Fe(PhNO)(1-MeIm) that we identified by IR spectroscopy and
X-ray crystallography.16
OMe)PP)Fe(η2-ON(Ph)NO) (
The analogous reaction of (T(p-
4
) with 1-MeIm generated the nitrosyl
Scheme 1. Proposed reaction pathways of (por)Fe(η2-ON(R)NO). (A)
Reaction with 1-MeIm, (B) Reaction with NO, (C) Protonation
reaction with triflic acid (HOTf).
(or 15NO) to
cm-1
1 in CH2Cl2 yielded a product with a υNO band at 1883
(
υ15NO 1848 cm-1) assigned to the desired nitrosyl
(OEP)Fe(NO)(η1-ON(t-Bu)NO) product (Scheme 1; path B). The
related nitrosyl (TPP)Fe(NO)(η1-ON(t-Bu)NO) product displayed its
υNO at 1888 cm-1 υ15NO 1851 cm-1). Although there are no isolable
(
Fig. 7. Molecular structure of (T(p-OMe)PP)Fe(PhNO)(1-MeIm) (6). H atoms (por)Fe(NO)(alkoxide) compounds reported in the literature, the
υNO of (TPP)Fe(NO)(η1-ON(t-Bu)NO) at 1888 cm-1 is lower than the
υNO of the neutral nitrosyl trifluroacetate compound
have been omitted for clarity. Thermal ellipsoids are drawn at 50%. Selected
bond lengths and angles: Fe1–N5 = 1.795(2) Å, Fe1–N6 = 2.044(2) Å, Fe1–
(TPP)Fe(NO)(OC(=O)CF3) at 1907 cm-1.55 Unfortunately, all attempts
N5–O5 = 123.4(2)°, N5–Fe1–N6 = 178.8(1)°, N4–Fe1–N5–O5 = 38.1(2)°.
to isolate the (por)Fe(NO)(η1-ON(t-Bu)NO) products from solution
compound (T(p-OMe)PP)Fe(NO) and the new six-coordinate (T(p-
resulted in their decomposition to the known five-coordinate
OMe)PP)Fe(PhNO)(1-MeIm) (6) derivative that were characterized
(por)Fe(NO) derivatives.
by IR spectroscopy and X-ray crystallography (Figure 7).
Another important biologically-relevant reaction that we
examined was the protonation of the (por)Fe(η2-ON(R)NO)
complexes. Similar to the reactions with 1-MeIm described above,
these protonation reactions were performed under reduced
laboratory lighting to minimize the light-induced decomposition of
The C-N-O plane of the PhNO ligand bisects adjacent porphyrin
N
atoms, and the axial ligands are essentially mutually
perpendicular. Similar axial ligand orientations have been observed
previously for (OEP)Fe(i-PrNO)(py),53 (por)Fe(i-PrNO)(1-MeIm) (por
= TPP, TTP),53 and (TPP)Fe(PhNO)2,54 although parallel (or near-
parallel) orientations are present in the crystal structures of
(OEP)Fe(i-PrNO)(1-MeIm),53 and related complexes.16
the starting (por)Fe(η2-ON(R)NO) compounds.
Reaction of
(OEP)Fe(η2-ON(t-Bu)NO) (
1
) with anhydrous triflic acid generated
the known (OEP)Fe(OTf) that was characterized by X-ray
Based on the products formed from the reactions of the
NONOate compounds with 1-MeIm (top of Scheme 1; path A), we
propose the initial binding of 1-MeIm trans to the NONOate group
crystallography, with t-BuNO as the organic product that was
identified by 1H NMR spectroscopy (path
C in Scheme 1).
Interestingly, gas-phase IR spectroscopic analysis of the collected
headspace gases revealed the formation of N2O. The analogous
protonation reaction with the Cupferron complex (T(p-
OMe)PP)Fe(η2-ON(Ph)NO) (
4) likewise yielded N2O gas as a product.
Based on these results, we propose that protonation likely
occurs at the "exposed" N5 atoms, as both O-atoms of the
NONOate group are involved in binding to the ferric center in the
(por)Fe(η2-ON(R)NO) complexes (i.e., pathway C in Scheme 1).
Similar N-protonations have been considered in the pH-
dependent decompositions of NONOates to help explain the
would likely need to be coincident with
a bidentate-to-
monodentate binding shift of the NONOate group, analogous to the
products of eq. 4. Surprisingly, there is only a single structural
precedent for such a monodentate NONOate binding (in a Cu
complex).7
Decomposition of this (1-MeIm)Fe(η1-ON(R)NO)
intermediate then results in the release of RNO and Fe-bound NO.
We note that we obtained the (por)Fe(RNO)(1-MeIm) and
(por)Fe(NO) products in relatively low yields (~50% total isolated
yield based on Fe) and were not successful in identifying the other
products of the reaction, hence this proposed path for the reaction
with 1-MeIm is likely not the complete picture for this reaction
pathway.
release of HNO in these systems.56, 57 This protonated [(por)Fe(η2
-
ON(R)N(H)O)]+ intermediate would then release both RNO and
HNO, the latter decomposing according to its known dimerization58,
59 to the observed N2O gas.
We also explored a possible adduct formation between the
(por)Fe(η2-ON(t-Bu)NO) compounds and NO to generate the six-
coordinate (por)Fe(NO)(η1-ON(t-Bu)NO) products. Addition of NO
8 | J. Name., 2012, 00, 1-3
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14. L. Cheng and G. B. Richter-Addo in The Porphyrin Handbook,
Vol. 4; R. Guilard, K. Smith and K. M. Kadish, Eds.; Academic
Press, New York, 2000, pp 219-291.
4 Conclusions
This report shows that the alkyl NONOates
1 and 2 are isolable
and characterizable by X-ray structural, spectroelectro-
15. J. P. Freeman, J. Org. Chem., 1963, 28, 2508-2511.
chemical and magnetic measurements. These species may 16. N. Godbout, L. K. Sanders, R. Salzmann, R. H. Havlin, M.
Wojdelski and E. Oldfield, J. Am. Chem. Soc., 1999, 121, 3829-
3844.
17. A. Lapinski, J. Spanget-Larsen, J. Waluk and J. G. Radziszewski,
J. Chem. Phys., 2001, 115, 1757-1764.
18. H. E. Gottlieb, V. Kotlyar and A. Nudelman, Journal of Organic
Chemistry, 1997, 62, 7512-7515.
19. J. A. Gonzalez and L. J. Wilson, Inorg. Chem., 1994, 33, 1543-
1553.
20. E. T. Shimomura, M. A. Phillippi, H. M. Goff, W. F. Scholz and
C. A. Reed, J. Am. Chem. Soc., 1981, 103, 6778-6780.
21. G. A. Bain and J. F. Berry, J. Chem. Educ., 2008, 85, 532-536.
22. R. Boča, Coord. Chem. Rev., 2004, 248, 757-815.
23. J. Krzystek, A. Sienkiewicz, L. Pardi and L. C. Brunel, J. Magn.
Reson., 1997, 125, 207-211.
24. P. P. Samuel, K. C. Mondal, N. A. Sk, H. W. Roesky, E. Carl, R.
Neufeld, D. Stalke, S. Demeshko, F. Meyer, L. Ungur, L. F.
Chibotaru, J. Christian, V. Ramachandran, J. van Tol and N. S.
Dalal, J. Am. Chem. Soc., 2014, 136, 11964-11971.
25. W. J. Liu, J. H. Christian, R. Al-Oweini, B. S. Bassil, J. van Tol, M.
Atanasov, F. Neese, N. S. Dalal and U. Kortz, Inorg. Chem.,
2014, 53, 9274-9283.
thus be more stable than previously thought. The detailed
magnetic studies showing high-spin (S = 5/2) formulations for
these ferric compounds are consistent with the significant
apical displacements of the Fe centers towards the axial O,O-
bidentate ligands. Although
1 and 2 are thermodynamically
stable, they are reactive towards a histidine mimic (1-MeIm),
NO, and protons, shedding additional insight into C-NONOate
decomposition pathways in the presence of metals.
Acknowledgements
We are grateful to the National Science Foundation (CHE-
1213674 to GBR-A) for funding for this work. J.H.C. and N.S.D.
wish to thank Catherine Kent and Tiffany Brooks for valuable
assistance with the magnetic measurements.
Supporting Information
Tables of X-ray crystallography data and figures for the crystal
structures of
1, 2, and 6. Figures for the X-band and Q-band empty
26. Z. N. Zahran, M. J. Shaw, M. A. Khan and G. B. Richter-Addo,
Inorg. Chem., 2006, 45, 2661-2668.
cavity and quartz tube profiles, Boltzmann populations and full Q
and X-band EPR spectra for 1 and 2.
27. N. G. Connelly and W. E. Geiger, Chem. Rev., 1996, 96, 877-
910.
28. M. J. Shaw, R. L. Henson, S. E. Houk, J. W. Westhoff, M. W.
Jones and G. B. Richter-Addo, J. Electroanal. Chem., 2002, 534
47-53.
29. S. M. Carter, J. Lee, C. A. Hixson, D. R. Powell, R. A. Wheeler,
M. J. Shaw and G. B. Richter-Addo, Dalton Trans., 2006, 1338-
1346.
,
References
1. J. A. Hrabie and L. K. Keefer, Chem. Rev., 2002, 102, 1135-
1154.
2. R. C. Mehrotra in Hydroxamates, Cupferron and Related
Ligands, Vol. 2 (Ed. G. Wilkinson), Pergamon Press, U. K., 1987
pp. 505-514 (Chapter 15.9).
,
30. APEX2 Software Reference Manual, Bruker-AXS, Madison, WI,
2007
.
3. H. N. Jayaram, A. K. Tyagi, S. Anandaraj, J. A. Montgomery, J.
A. Kelley, J. Kelley, R. H. Adamson and D. A. Cooney, Biochem.
Pharmacol., 1979, 28, 3551-3566.
4. Y. K. S. Murthy, J. E. Thiemann, Coronell.C and P. Sensi,
Nature, 1966, 211, 1198-1199.
5. T. Natori, Y. Kataoka, S. Kato, H. Kawai and N. Fusetani,
Tetrahedron Letters, 1997, 38, 8349-8350.
6. P. G. Wang, M. Xian, X. Tang, X. Wu, Z. Wen, T. Cai and A. J.
Janczuk, Chem. Rev., 2002, 102, 1091-1134.
7. J. L. Schneider, J. A. Halfen, V. G. Young and W. B. Tolman,
New J. Chem., 1998, 22, 459-466.
8. T. A. Alston, D. J. T. Porter and H. J. Bright, J. Biol. Chem., 1985,
260, 4069-4074.
9. H. A. Hassanin, L. Hannibal, D. W. Jacobsen, M. F. El-Shahat,
M. S. A. Hamza and N. E. Brasch, Angew. Chem. Int. Ed., 2009,
48, 8909-8913.
31. Data Reduction: SAINT Software Refence Manual. Bruker-AXS,
Madison, WI. , 2007.
32. I. J. Casely, Y. Suh, J. W. Ziller and W. J. Evans,
Organometallics, 2010, 29, 5209-5214.
33. L. K. Keefer, J. L. Flippen-Anderson, C. George, A. P. Shanklin,
T. A. Dunams, D. Christodoulou, J. E. Saavedra, E. S. Sagan and
D. S. Bohle, Nitric Oxide: Biol. Chem., 2001, 5, 377-394.
34. A. Deak, I. Haiduc, L. Parkanyi, M. Venter and A. Kalman, Eur.
J. Inorg. Chem., 1999, 1593-1596.
35. W. R. Scheidt, J. Porph. Phthalocyanines, 2008, 12, 979-992.
36. W. R. Scheidt in The Porphyrin Handbook, Vol. 3; K. M. Kadish,
K. M. Smith and R. Guilard, Eds.: Academic Press, New York,
2000, Chapter 16, pp 49-112.
37. M. A. Phillippi, N. Baenziger and H. M. Goff, Inorg. Chem.,
1981, 20, 3904-3911.
38. O. Q. Munro and W. R. Scheidt, Inorg. Chem., 1998, 37, 2308-
2316.
39. G. R. A. Wyllie, O. Q. Munro, C. E. Schulz and W. R. Scheidt,
Polyhedron, 2007, 26, 4664-4672.
40. L. Cheng, M. A. Khan, D. R. Powell, R. W. Taylor and G. B.
Richter-Addo, Chem. Commun., 1999, 1941-1942.
41. W. R. Scheidt and C. A. Reed, Chem. Rev., 1981, 81, 543-555.
42. T. Ikeue, Y. Ohgo, O. Ongayi, M. G. H. Vicente and M.
Nakamura, Inorg. Chem., 2003, 42, 5560-5571.
10. M. Shiino, Y. Watanabe and K. Umezawa, Bioorg. Med. Chem.,
2001, 9, 1233-1240.
11. G.-B. Yi, M. A. Khan and G. B. Richter-Addo, Inorg. Chem.,
1995, 34, 5703-5704.
12. N. Arulsamy, D. S. Bohle, J. A. Imonigie and E. S. Sagan, J. Am.
Chem. Soc., 2000, 122, 5539-5549.
13. M. Ahmed, A. J. Edwards, C. J. Jones, J. A. McCleverty, A. S.
Rothin and J. P. Tate, J. Chem. Soc., Dalton Trans., 1988, 257-
263.
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43. T. Ikeue, Y. Ohgo, T. Yamaguchi, M. Takahashi, M. Takeda and 52. K. M. Kadish, E. Van Caemelbecke and G. Royal in The
M. Nakamura, Angew. Chem. Int. Ed., 2001, 40, 2617-2620.
44. J. A. Weil and J. R. Bolton, Electron Paramagnetic Resonance
Elementary Theory and Practical Applications, 2nd Ed.: Wiley-
Porphyrin Handbook, Vol. 8; K. M. Kadish, K. M. Smith and R.
Guilard, Eds.: Academic Press, San Diego, CA, 2000, Chapter
55.
Interscience, Hoboken, NJ, 2007
45. J. Krzystek, S. A. Zvyagin, A. Ozarowski, S. Trofimenko and J.
Telser, J. Magn. Reson., 2006, 178, 174-183.
46. J. Krzystek, A. Ozarowski and J. Telser, Coord. Chem. Rev.,
2006, 250, 2308-2324.
47. V. V. Laguta, M. D. Glinchuk, I. P. Bykov, Y. L. Maksimenko, J.
Rosa and L. Jastrabik, Phys. Rev. B, 1996, 54, 12353-12360.
.
53. C. D. Sohl, J. Lee, S. S. Alguindigue, M. A. Khan and G. B.
Richter-Addo, J. Inorg. Biochem., 2004, 98, 1238-1246.
54. L.-S. Wang, L. Chen, M. A. Khan and G. B. Richter-Addo, Chem.
Commun., 1996, 323-324.
55. N. Xu, L. E. Goodrich, N. Lehnert, D. R. Powell and G. B.
Richter-Addo, Angew. Chem. Int. Ed. Engl., 2013, 52, 3896-
3900.
48. R. G. Pontin, E. F. Slade and D. J. E. Ingram, J. Phys. C Solid 56. D. J. Salmon, C. L. T. de Holding, L. Thomas, K. V. Peterson, G.
State Phys., 1969,
49. A. Priem, P. J. M. van Bentum, W. R. Hagen and E. J. Reijerse,
Appl. Magn. Reson., 2001, 21, 535-548.
50. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance
of Transition Ions, Oxford Press, Oxford, 1970, pp 430-433.
2
, 1146-1150.
P. Goodman, J. E. Saayedra, A. Srinivasan, K. M. Davies, L. K.
Keefer and K. M. Miranda, Inorg. Chem., 2011, 50, 3262-3270.
57. A. S. Dutton, C. P. Suhrada, K. M. Miranda, D. A. Wink, J. M.
Fukuto and K. N. Houk, Inorg. Chem., 2006, 45, 2448-2456.
58. K. M. Miranda, Coord. Chem. Rev., 2005, 249, 433-455.
51. J. R. Pilbrow, Transition Ion Electron Paramagnetic Resonance, 59. V. Shafirovich and S. V. Lymar, J. Am. Chem. Soc., 2003, 125
,
Clarendon Press, Oxford, 1990 6547-6552.
.
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