Nitric oxide addition to the ferrous nitrosyl porphyrins Fe(P)(NO) gives trans-Fe(P)(NO)2 in low-temperature solutions [15]

Full text of DOI:10.1021/ja000308q

6516
J. Am. Chem. Soc. 2000, 122, 6516-6517
Nitric Oxide Addition to the Ferrous Nitrosyl
Porphyrins Fe(P)(NO) Gives trans-Fe(P)(NO)2 in
Low-Temperature Solutions
Ivan Lorkovic´ and Peter C. Ford*
Department of Chemistry and Biochemistry
UniVersity of California, Santa Barbara
Santa Barbara, California 93106
ReceiVed January 28, 2000
ReVised Manuscript ReceiVed April 24, 2000
Ferrous porphyrins of the type FeII(TPP) (TPP ) meso-
tetraphenylporphinato dianion) undergo NO addition to give the
paramagnetic mononitrosyl complex Fe(TPP)(NO) (1).1,2 This has
analogy in the reactions of NO with various ferrous heme proteins3
including soluble guanylyl cyclase, NO’s target in its role as a
natural bioregulator in the cardiovascular system.4 There are,
however, significant ambiguities regarding the subsequent reac-
tions of Fe(TPP)(NO) with excess NO. For example, this complex
was reported to promote NO disproportionation5 to give N2O and
the nitrosyl nitro complex Fe(TPP)(NO)(NO2) in a manner similar
to ruthenium porphyrins that were mechanistically probed in this
laboratory.6 Recently, however, we have demonstrated that
ambient temperature solutions of Fe(TPP)(NO) display no changes
in IR or optical spectra when treated with NO carefully cleaned
of higher NOx impurities.7 The latter observation is consistent
with an earlier report by Wayland and Olson,8 who, nonetheless,
did conclude that Fe(TPP)(NO) reacts with NO reversibly to give
the dinitrosyl Fe(TPP)(NO)2 (2) in low-temperature solution, the
supporting evidence being reversible disappearance of the ESR
signal for Fe(TPP)(NO) under 400 Torr NO as T was lowered.
Another early report9 suggests 2 to be formed by reaction of NO
with 1 in ambient temperature toluene, although the optical
spectral data appear consistent with formation of Fe(TPP)(NO)-
(NO2). To address these inconsistencies, we report here optical,
infrared and NMR spectral data for NO reactions with Fe(TPP)-
(NO) and with Fe(TmTP)(NO), a more soluble analogue (TmTP
) meso-tetra-m-tolyl-porphinato dianion). Spectra acquired in
toluene-d8, chloroform, and methylcyclohexane confirm the
formation of a dinitrosyl complex Fe(P)(NO)2 (P ) TPP, P )
TmTP) in low-temperature solutions. Furthermore, these data
argue for characterization of Fe(P)(NO)2 as a centrosymmetric
complex with equivalent trans nitrosyls (eq 1).
Figure 1. Bottom: 1H NMR spectrum of Fe(TmTP)(NO) (∼10 mM) in
toluene-d8 at 179 K. Top: 1H NMR spectrum of this same solution with
added NO (10 mM) at 179 K. The peak at 5.32 ppm represents a small
amount of CH2Cl2 impurity. Both spectra represent the same number of
scans obtained with identical parameters and are presented with the same
vertical scale. The toluene-d7 and CH2Cl2 resonances are slightly
broadened in the presence of Fe(TmTP)(NO) alone.
impurity proton peaks appear at 7.15, 7.07, 6.98 (internal
reference), and 2.07 ppm. In the absence of NO, the NMR
spectrum shows considerable paramagnetic broadening, as ex-
pected for a {FeNO}7 species. The only distinguishable peaks
are broadened methyl (2.1 ppm), meta (8.2 ppm) and para (6.7
ppm) protons of the tolyl moiety; the ortho tolyl protons and
â-pyrrole protons are too broad to locate. In the presence of NO
(10 mM), however, all proton resonances sharpen and shift to
nearly diamagnetic values, the â-pyrrole, and the ortho, meta,
para, and methyl tolyl protons being observed at 8.94 (8H), 7.94
(8H), 7.39 (4H), 7.19 (4H), and 2.25 ppm (12H), respectively.
This observation is indicative of the formation of a diamagnetic
species we characterize as Fe(TmTP)(NO)2. In contrast, no
differences were observed between the 295 K spectra of Fe-
(TmTP)(NO) without and with added NO (10 mM); thus, there
was no appreciable formation of a dinitrosyl complex at that
temperature.
For spectra recorded between 294 and 179 K, exchange
between Fe(TmTP)(NO) (3) and Fe(TmTP)(NO)2 (4) appears fast
on the NMR time scale, since a single set of porphyrin protons
were observed in the presence of NO at each T. These spectra
also contain quantitative information regarding the equilibrium
constant K1. At a given T, the degree of peak sharpening arising
from the presence of NO can be used to deduce K1 using the
expression: K1 ) [4] [3]-1[NO]-1 ) {(w - wD)/(wNO - wD) -
1}[NO]-1, where w ) the peak width (at half-height) of
Fe(TmTP)(NO) in the absence of NO, wNO is the width in the
1
Figure 1 shows H NMR spectra of low-temperature (179 K)
solutions prepared from Fe(TmTP)(NO) (∼10 mM) in toluene-
d8 alone and in the presence of NO (10 mM). The toluene-d7
* Corresponding author. E-mail: ford@chem.ucsb.edu.
(1) (a) Wayland, B. B.; Olson, L. W. J. Chem. Soc., Chem. Commun. 1973,
897-898. (b) Scheidt, W. R.; Frisse, M. E. J. Am. Chem. Soc. 1975, 97, 17-
21.
(2) {MNO}7 in the Enemark-Feltham formalism. Enemark, J. H.; Feltham,
R. D. J. Am. Chem. Soc. 1974, 96, 5002-5004.
(3) (a) Traylor, T. G.; Sharma, V. S. Biochemistry 1992, 31, 2847-2849.
(b) Radi, R. Chem. Res. Toxicol. 1996, 5, 828-835. (c) Feelisch, M.; Stamler,
J. S. (Eds.) Methods in Nitric Oxide Research; Wiley: Chichester, UK, 1996,
and refs therein.
(6) (a) Lorkovic´, I. M.; Ford, P. C. Inorg. Chem. 1999, 38, 1467-1473.
(b) Miranda, K. M.; Bu, X.; Lorkovic´, I. M.; Ford, P. C. Inorg. Chem. 1997,
36, 4838-4848. (c) Lorkovic´, I. M.; Ford, P. C. J. Chem. Soc., Chem.
Commun. 1999, 1225-1226.
(4) (a) Kim, S.; Deinum, G.; Gardner, M. T.; Marletta, M. A.; Babcock,
G. T. J. Am. Chem. Soc. 1996, 118, 8769-8770. (b) Burstyn, J. N.; Yu, A.
E.; Dierks, E. A.; Hawkins, B. K.; Dawson, J. H. Biochemistry 1995, 34,
5896-5903.
(5) (a) Yoshimura, T. Inorg. Chim. Acta 1984, 83, 17-21. (b) Settin, M.
F.; Fanning, J. C. Inorg. Chem. 1988, 27, 1431-1435. (c) Ellison, M. K.;
Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 1999, 38, 100-108.
(7) Lorkovic´, I. M.; Ford, P. C. Inorg. Chem. 2000, 39, 632-633.
(8) Wayland, B. B.; Olson, L. W. J. Am. Chem. Soc. 1974, 96, 6037-
6041.
(9) Lanc¸on, D.; Kadish, K. M. J. Am. Chem. Soc. 1983, 105, 5610-5617.
10.1021/ja000308q CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/23/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 27, 2000 6517
1695 cm-1 with twice the intensity (∼1600 M-1 cm-1), and
another much weaker band at 1777 cm-1. The positions and
relative intensities of these are consistent with a trans arrangement
of equivalent nitrosyl ligands, that is, Fe(TPP)(NO)2.
An analogous experiment was carried out with the more soluble
Fe(TmTP)(NO) and a 50:50 mixture of 14NO/15NO (8 mM total
NO) in methylcyclohexane (Figure 2). The room-temperature
spectrum shows two νNO bands at 1683 and 1654 cm-1 consistent
with a ∼50/50 mixture of Fe(TmTP)(14NO) and Fe(TmTP)(15NO),
respectively. Cooling to ∼173 K gave an IR spectrum with
overlapping bands at 1694, 1676, and 1665 cm-1 in an intensity
ratio (∼ 1:2:1) consistent with formation of unlabeled, mixed
labeled, and fully labeled Fe(TmTP)(NO)2 in a statistical distribu-
tion. Low-temperature IR spectra of Fe(TmTP)(NO)2 prepared
with pure 14NO and with pure 15NO are also shown.11
Supplemental Figure S-1 shows UV-vis spectra obtained from
cooled (∼173 K) methylcyclohexane solutions of Fe(TmTP)(NO)
(∼200 µM) in the absence of NO and of Fe(TmTP)(NO)2 in the
presence of NO (8 mM). The Q and Soret bands for Fe(TmTP)-
(NO)2 occur at 540 nm (1.5 × 104 M-1 cm-1) and 416 nm (1.4
× 105 M-1 cm-1), respectively, compared to 536 nm (9 × 103
M-1 cm-1) and 404 nm (1.0 × 105 M-1 cm-1) for Fe(TmTP)-
(NO). The Soret band for Fe(TmTP)(NO)2 is clearly distinguish-
able from that for Fe(TmTP)(NO)(NO2) (432 nm).7 Analogous
experiments using Fe(TPP)(NO) in CHCl3 give analogous results
(supplemental Figure S-2). It is important to note that the NMR,
IR, and UV-vis spectral features of Fe(TPP)(NO)2 and Fe-
(TmTP)(NO)2 revert to those of Fe(TPP)(NO) and Fe(TmTP)-
(NO) upon warming to room temperature.
Figure 2. Top: Infrared spectra of solution prepared from Fe(TPP)-
(NO) (2.3 mM) and NO (8 mM) in CHCl3 at 295 K (dashed line) and at
213 K (solid line). Bottom: FTIR spectra of a methylcyclohexane solution
of Fe(TmTP)(NO) (1 mM) in the presence of 1:1 14NO:15NO (8 mM
total) at 295 K (dashed line) and at ∼179 K (solid line), showing the
transformation of isotopically mixed Fe(TmTP)(NO) (3) to isotopically
mixed Fe(TmTP)(NO)2 (4). The ∼179 K spectra of 4 prepared using
pure 15NO and pure 14NO (thin dashed lines) at 1696 and 1665 cm-1
,
respectively, are also shown. Bands in bottom spectra are sharper owing
to the hydrocarbon solvent.
To summarize, we have observed reversible formation of Fe-
(P)(NO)2 from NO plus Fe(P)(NO) in low-temperature solutions
by NMR, IR, and UV-vis spectroscopy. The NMR data indicate
this species to be diamagnetic, while the IR data point to a
centrosymmetric trans-dinitrosyl configuration. However such
solutions do not undergo further reaction under these conditions.
Although it is not clear why the behavior of the Fe(P)(NO) system
is different from that of the analogous ruthenium porphyrins6 and
that recently reported for the non-heme iron complex,12 the present
data show that the ferro-heme center does not support NO
disproportionation to N2O plus Fe(P)(NO)(NO2).
presence of NO, and wD is the width of a diamagnetic species
such as the monoprotio solvent impurity. From the half-height
widths of the tolyl methyl proton resonance for a 1 mM solution
of Fe(TmTP)(NO) ([NO] ) 10 mM), values of K1 were estimated
at T between 253 K (23 M-1) and 179 K (3100 M-1) and the
thermodynamic parameters for eq 1 found to be ∆H° ) -6.7 (
0.8 kcal/mol and ∆S° ) -21 ( 4 cal (K/mole)-1.10 Similar results
were seen upon cooling a CDCl3 solution of Fe(TPP)(NO) to 210
K in the presence and absence of NO.
Figure 2 shows FTIR spectra recorded for solutions of Fe-
(TPP)(NO) (2.3 mM) in CHCl3 at room temperature and at 213
K. In the absence of added NO, cooling this solution had no effect
on the nitrosyl stretching band (νNO ) 1681 cm-1, ꢀ ) 800 M-1
cm-1), except for a slight band intensification due to thermal
solvent contraction. Furthermore, addition of NO (8 mM) to a
room-temperature solution of Fe(TPP)(NO) had no effect on the
shape or intensity of this band.7 However, cooling the latter
solution to 213 K resulted in the appearance of a new band at
Acknowledgment. This work was supported by the National Science
Foundation (CHE 9726889). We thank Dr. Steve Massick and Jon
Marhenke for expertise in assembly of the low temperature IR/UV-vis
cell.
Supporting Information Available: Included is Figure S-1 displaying
the optical spectra of Fe(TmTP)(NO) and Fe(TmTP)(NO)2 in 179 K
methylcyclohexane, Figure S-2 (in black and white and in color)
displaying the reversibility of optical spectral changes as a CHCl3 solution
of Fe(TPP)(NO) and NO (8 mM) is cooled then rewarmed (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
(10) (a) The [NO] for the equilibrated toluene solution at 295 K was
calculated to be 10 mM from published solubility data,10b and [NO] was
assumed to be the same value at the lower T as well. Assumption of minimal
net transport between headspace and solution can be rationalized on two bases.
First, the gas/liquid interface is small relative to the liquid volume in the closed
NMR tube, and thus transport will be slow. Furthermore, although NO
solubility in toluene (units: mol L-1 atm-1) increases with decreasing T, PNO
correspondingly drops so that increases in [NO]equilibrium are minor (<10% from
295 to 213 K). Since no hysteresis was observed in peak width measurements
as T was decreased and increased, we conclude that net transport leading to
changes in [NO] is minimal under these conditions. (b) Shaw, A. W.; Vosper,
A. J. J. Chem. Soc. Faraday Trans. 1977, 73, 1239-1244. These authors
measured NO solubility in toluene for 22 temperatures between 213 and 289
K. Values outside that temperature range were estimated by extrapolation.
JA000308Q
(11) (a) Peak separations corresponding to different degrees of 14NO/15NO
labeling is greater between unlabeled and singly labeled (18 cm-1) than
between singly and fully labeled (11 cm-1). An analogous pattern was observed
for the correspondingly labeled Ru(TmTP)(NO)2 which displayed bands at
1641, 1624, and 1613 cm-1, respectively.10b (b) Lorkovic´, I. M.; Ford, P. C.
manuscript in preparation.
(12) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 10504-
10512.