High-pressure infrared spectroscopic study of the nitric oxide complex of iron(II)-meso-tetraphenyl porphyrinate

Article abstract of DOI:10.1016/j.ica.2005.09.055

The infrared spectrum of iron(II)-meso-tetraphenylporphyrinate (FeTPP(NO)) has been measured as a function of pressure up to 3.1 GPa. The N-O stretching frequency decreases with increasing pressure, as expected for the bonding model for nitric oxide bound to iron in porphyrin complexes. Other peaks in the spectrum show positive pressure dependence.

Full text of DOI:10.1016/j.ica.2005.09.055

Inorganica Chimica Acta 359 (2006) 3089–3091
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Note
High-pressure infrared spectroscopic study of the nitric oxide
complex of iron(II)-meso-tetraphenyl porphyrinate
*
*
´
Mirabelle Premont-Schwarz, D. Scott Bohle , Denis F.R. Gilson
Department of Chemistry, McGill University, 810 Sherbrooke Street, W. Montreal, QC, Canada H3A 2K6
Received 16 August 2005; accepted 24 September 2005
Available online 8 November 2005
Dedicated to Brian James on the occasion of his 70th birthday.
Abstract
The infrared spectrum of iron(II)-meso-tetraphenylporphyrinate (FeTPP(NO)) has been measured as a function of pressure up to
3.1 GPa. The N–O stretching frequency decreases with increasing pressure, as expected for the bonding model for nitric oxide bound
to iron in porphyrin complexes. Other peaks in the spectrum show positive pressure dependence.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Iron porphyrinate; High-pressure; Infrared; N–O stretch
1. Introduction
the recent structure of the heme binding domain of sGC
contains a highly distorted heme [8], and this has attendant
affects on NO selectivity and affinity. In prior model system
studies, we found, NO substitution kinetics can vary over
six orders of magnitude with porphyrin ring distortion
being an important factor leading to NO lability and facile
substitution kinetics [9]. Porphyrin ring distortion has both
steric and electronic consequences and these remain the key
aspects of our research program.
Bonding models proposed to explain nitrosyl bending
and tilting [10,11] involve a rotation of the iron dz2 orbital
with respect to the porphyrin plane in order to increase the
overlap between this orbital and the NO p* orbital. The N–
O stretching frequency has a well-defined infrared absorp-
tion that is sensitive to the local environment and the
stretching frequency decreases as the Fe–N–O angle
increases [12,13]. This is consistent with a weakening of
the N–O bond with the donation of electron density into
the anti-bonding orbital. The application of high external
pressure can induce changes in bond lengths and angles
and it is anticipated that an increase in applied pressure will
lead to a shortening of the Fe–N(O) bond and/or changes
in the tilt angle and Fe–N–O bond angle, leading to
increased donation into the NO antibonding orbital thus
There are many surprising relationships between iron
proteins and nitric oxide. For example, heme proteins are
involved in the biosynthesis (the NOS isoforms) and the
biosensing (soluble Guanylate Cyclase) of physiological
levels of nitric oxide. Although many metalloprophyrins
bind NO, few have either the affinity or redox ambiguity
that the iron porphyrins have. In the highest affinity redox
combination, often described as a ferrous form [1],
{FeNO}7 [2], the NO binds in a bent fashion with the
NO adopting an unusual tilted off axis geometry [3,4]. In
the one electron oxidized ensemble, a ferric or {FeNO}6
form, the NO is linear and much more labile [5]. Although
many model systems have been structurally [6] and spectro-
scopically characterized [7], the fundamental problem of
how sGC is able to reversibly bind NO remains unan-
swered. How is sGC able to distinguish between NO and
O2? How does it lose NO to regenerate the NO responsive
enzyme? With regard to these problems, it is notable that
*
Corresponding authors.
E-mail addresses: scott.bohle@mcgill.ca (D.S. Bohle), denis.gilson@
mcgill.ca (D.F.R. Gilson).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.09.055

3090
M. Pre´mont-Schwarz et al. / Inorganica Chimica Acta 359 (2006) 3089–3091
weakening the N–O bond and leading to a negative depen-
dence of the vibrational frequency with pressure. Nor-
mally, vibrational frequencies increase with an increase in
applied pressure but in some unusual bonding situations,
such as the dp–pp backbonding in ZeiseÕs salt [14] and
the ‘‘dihydrogen bond’’ in the ammonia–borane complex
[15], the frequencies decrease.
We have examined the effect of high applied pressure on
the infrared spectrum of the nitrosyl complex of iron(II)-
meso-tetraphenylporphyrinate (FeTPP(NO)), which has
been a well-studied model compound, to provide spectro-
scopic evidence of the bonding model to complement the
structural arguments. The crystal structure of FeTPP(NO)
consists of statistically disordered chains of molecules
arranged along a crystallographic fourfold axis, with the
NO groups disordered over eight positions [16]. Vibrations
below 600 cmÀ1, i.e., those involving motions of the iron
atom, have been reported by Sage and co-workers [17]
and several modes at 56 cmÀ1 and below have been
assigned to torsional motions involving the nitrosyl group.
Thus the barrier to reorientation is low and the disorder
may be dynamic, as is the case for the cobalt complex [18].
1700
1600
1500
1400
wavenumbers, cm-1
Fig. 1. FT-IR spectra at different pressures. Upper 0.33 GPa, middle
1.53 Gpa, lower 2.96 GPa.
increased with increasing pressure, Fig. 2. The pressure
dependence for both peaks is negative, in agreement with
the proposed the bonding model. The low barrier to tor-
sional motion of the nitroxyl group should be increased
as the interatomic distances are decreased with increasing
pressure, which could explain the appearance of a second
N–O stretching vibration if a different local environment
occurs. The appearance of a second N–O stretching vibra-
tion is also consistent with the results of a Mo¨ssbauer study
[22] of Fe(TPP)NO, which showed a quadrupole doublet
with significant broadening of the lower-velocity line. This
was attributed to the disorder.
Fig. 2 and Table 1 show the pressure dependences of
other strong peaks in the spectrum. Certain peaks in the
vibrational spectra of metal–porphyrin compounds have
been identified as indicators of spin state and oxidation
state [23,24]. These structure-sensitive peaks include the
2. Experimental
The meso-porphyrin was prepared according to the
method developed by Adler [19] and used without further
purification in the metallation reaction [20]. The nitrosyl
iron porphyrin was synthesized by literature methods [9],
which briefly involves dissolving 0.2 g of the Fe(TPP)Cl
compound in 30 mL of dichloromethane in a Fisher-Porter
bottle with 10 drops each of 2,6-lutidine and methanol to
catalyze the oxidation and the reaction vessel was pressur-
ized with 30 psi of nitric oxide gas overnight. After purging
the reaction vessel with nitrogen, the product was precipi-
tated from the solution by the addition of methanol and
dissolved in chloroform and recrystallized from hexanes.
The purity of the product was verified by TLC, IR, and
UV–Vis spectroscopy.
High-pressure studies were performed with the use of a
diamond anvil cell purchased from High Pressure
Diamond Optics Inc., Tucson, Arizona, with type IIA
diamonds. Samples were contained in 200 lm thick
stainless-steel gaskets. IR spectra were recorded on a
Bruker IFS48 FT spectrometer equipped with a Bruker
A-590 infrared microscope (objective 15·) and a liquid
nitrogen cooled MCT detector. The spectra were collected
at 4 cmÀ1 resolution and 100 scans. The pressures were
measured using the shift of the anti-symmetric N–O
stretching mode of sodium nitrite in a sodium bromide
matrix [21] included with the sample.
1800
1600
1400
1200
1000
800
3. Results and discussion
0
1
2
3
The pressure dependence of the N–O stretching fre-
quency is shown in Fig. 1. At about 1.5 GPa, a shoulder
appeared on the low wavenumber side and the separation
Pressure, GPa
Fig. 2. Pressure dependence of vibrations of FeTPP(NO).

M. Pre´mont-Schwarz et al. / Inorganica Chimica Acta 359 (2006) 3089–3091
3091
Table 1
Pressure dependence of vibrational frequencies
References
Frequency (cmÀ1
)
dm/dP (cmÀ1 GPaÀ1
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porphyrin core mode at 1351 cmÀ1, a ring deformation
mode mixed with Cb-H in-plane bending at 800 cmÀ1 and
a pyrrole ring breathing mode at 1004 cmÀ1. These peaks
showed no breaks or changes in slope over the same pres-
sure range and, therefore, the splitting of the NO stretching
vibration is not the result of any change in structure or a
solid state phase transition. The peak at 1447 cmÀ1
,
assigned as the Ca–Cm stretching vibration, has a much
lower pressure dependence than the other vibrations. A
high pressure X-ray crystallographic study of cobalt(II)tet-
raphenylporphyrinate revealed a phase transition from a
tetragonal to triclinic structure at 0.49 GPa [25]. This study
also included infrared spectroscopic results, which showed
pressure dependences similar to those reported here.
Acknowledgment
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(1987) 5042.
This work was supported by grants from the Natural
Sciences and Engineering Research Council (Canada) in
the form of a Discovery grant to DSB and a Summer
Award to MPS.