Reactivity of the iron porphyrin Fe(TPP)(NO) with excess no. Formation of Fe(TPP)(NO (NO2) occurs via reaction with trace NO2

Article abstract of DOI:10.1021/ic9910413

Fe(TPP)(NO) (TPP = tetraphenylporphyrinato dianion) does not react observably with excess NO in organic solvents at ambient pressure and temperature; however, adventitious NO₂ rapidly leads to the nitrosyl nitro product Fe(TPP)(NO)(NO₂

Full text of DOI:10.1021/ic9910413

632
Inorg. Chem. 2000, 39, 632-633
Reactivity of the Iron Porphyrin Fe(TPP)(NO) with Excess NO. Formation of Fe(TPP)(NO)(NO2) Occurs
via Reaction with Trace NO2
Ivan M. Lorkovic´ and Peter C. Ford*,†
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106
ReceiVed August 27, 1999
Nitrosyl complexes of iron(II) and iron(III) porphyrins play
key roles in biological chemistry ranging from the activation of
s-guanylyl cyclase by nitric oxide to the preservation of meat by
sodium nitrite.1 Thus, it is important to understand the chemical
properties of such complexes, including further reactions with
NO and other nitrogen oxides. The original goal of this study
was to elucidate the dynamics of the reported2-4 reaction of the
nitrosyl complex Fe(TPP)(NO) (1) (TPPH2 ) meso-tetraphen-
ylporphine) with NO to give the nitrosyl nitro species Fe(TPP)-
(NO)(NO2) (2) for comparison to the stopped flow kinetics
investigation of analogous ruthenium complexes.5 However, this
work has uncovered surprisingly different reactivity patterns for
Fe(II) compared to Ru(II); 1 apparently does not react with NO
to give 2 in room temperature toluene or chloroform over the
course of hours. Instead, NO2, a common impurity in NO sources,
is shown to be the likely reactant responsible for formation of 2
from 1 under most conditions.
Figure 1 shows the UV-vis and IR spectra of a chloroform
solution of 1 (∼1.7 mM, 0.5 mL) in a 1.0 mm CaF2 IR cell. The
electronic spectrum displays characteristic porphyrin Q-band peaks
at 538 nm (ꢀ ) 8700 M-1 cm-1) and 606 nm (2800),6 and the IR
spectrum shows the nitrosyl stretch νNO at 1682 cm-1 (800).
Exposure of this solution to excess NO (7 mM) scrupulously
scrubbed of other nitrogen oxides7 led to no change in the
electronic or IR spectra over several hours. Thus, there appears
to haVe been no reaction of 1 with NO under these conditions
(eq 1).8 Furthermore, the IR spectrum demonstrated that N2O
(2221 cm-1, ꢀ ) 910 M-1 cm-1 in CHCl3) was not formed in
detectable concentrations (Figure 1b, inset). After 3 days, the IR
spectrum revealed some N2O (∼6 µM), but the yield was <1%
Figure 1. (a) UV-vis and (b) IR spectra of Fe(TPP)(NO) in CHCl3 (1
assuming the stoichiometry 1 + 3NO f 2 + N2O.
mm CaF2): under Ar (solid line); after 3 h in the presence of 7 mM NO,
before (gray dotted) and after (dashed) addition of 150 µL of air; same
sample after dilution in CHCl3, degassing, and reconcentration (gray dot-
dash). Inset shows absence of N2O after 3 h exposure of 1 to NO (7
mM). The broad absorption at 1876 cm-1, obscured by imperfect solvent
subtraction, is due to solution NO (∼20 M-1 cm-1).
This (lack of) reactivity contrasts sharply with the behavior of
similar Ru(II) complexes Ru(P)(CO) (PH2 ) various porphyrins),
† E-mail: ford@chem.ucsb.edu.
(1) (a) Legzdins, P. A.; Richter-Addo, G. B. Metal Nitrosyls; Oxford
University Press: New York, 1992. (b) Cheng, L.; Richter-Addo, G. B.
Chapter 33. In The Prophyrin Handbook; Academic Press: Kadish, K.
M., Smith, K. M., Guilard, R., Eds.; New York, in press. Personal
communication from G. B. Richter-Addo.
(2) Yoshimura, T. Inorg. Chim. Acta 1984, 83, 17-21.
(3) Settin, M. F.; Fanning, J. C. Inorg. Chem. 1988, 27, 1431-1435.
(4) Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 1999, 38,
100-108.
(5) (a) Lorkovic, I. M.; Ford, P. C. Inorg. Chem. 1999, 38, 1467-1473. (b)
Lorkovic, I. M.; Ford, P. C. Chem. Commun. (Camridge) 1999, 1225-
1226. (c) Lorkovic, I. M.; Miranda, K. M.; Lee, B. L.; Bernhard, S.;
Schoonover, J. M.; Ford, P. C. J. Am. Chem. Soc. 1998, 120, 11674-
11683.
Fe(TPP)(NO) + NO f
no observable reaction at room temperature (1)
which react readily with NO to give the nitrosyl nitrito species
Ru(P)(NO)(ONO).9 Stopped-flow kinetics studies indicate the
dinitrosyl Ru(P)(NO)2 to be an intermediate, which reacts with
an additional 2 equiv of NO via third-order kinetics to give Ru-
(P)(NO)(ONO) plus N2O.5
(6) Scheidt; W. R.; Frisse, M. E. J. Am. Chem. Soc. 1975, 97, 17-21.
(7) NO was purified by being passed through an Ascarite scrubber and then
(8) (a) There have been reports that 1 reacts with excess NO to give Fe-
(TPP)(NO)2. This was first described by Wayland and Olson,8b who
presented evidence that low-T toluene solutions of 1 under NO
demonstrate reversible changes in the EPR and optical spectra consistent
with the formation of the dinitrosyl. NMR experiments in this laboratory
are in agreement with the observation that such a species may be formed
at low temperature but not at ambient temperature. It is also notable
that there were no indications in Wayland’s spectra that 1 reacts further
with NO to give 2 at room or lower T. (b) Wayland, B. B.; Olson, L. W.
J. Am. Chem. Soc. 1974, 96, 6037-6041.
3
through an activated silica (80-100 mesh) packed column (5 ft, /8 in
stainless steel tubing coil) at -78 °C. The latter procedure removed N2O
and any residual NO2 and N2O3. Warning: at -78 °C, considerable NO
absorbs in the column and precaution should be taken for the controlled
NO release when the system warms to room temperature. Solutions were
prepared and transferred to cells with gastight syringes inside an inert
atmosphere “drybox”. Materials in contact with NO solutions (septa,
stopcocks, and plungers) were deaerated by storage in the drybox for at
least 18 h prior to solution exposure.
10.1021/ic9910413 CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/27/2000

Communications
Inorganic Chemistry, Vol. 39, No. 4, 2000 633
Scheme 1
solution of 1 ([Fe] ) 8 µM) in toluene ([NO] ) 8 mM) (Figure
1). Stepwise addition of O2 led to decreased absorbance in the
Soret band region at 406 nm due to 1 and increases at 432 nm
due to 2 as well as increased absorbance at 306 nm indicating
the buildup of N2O3 (ꢀ ) 2 × 103 M-1 cm-1).11 The spectral
changes at 432 nm and calculated [N2O3] values were used to
determine K3 ) 160 ( 20 according to Hoshino’s method12
(Supporting Information, Figure S-1). One can also estimate the
K2 for the reaction of NO2 with 1 from the relationship K2 )
K1K3. If one assumes K1 in toluene to be similar to that reported
in acetonitrile (6.7 × 103 M-1),13 then K2 is ∼1.1 × 106 M-1 in
toluene.
Further manipulations of 2 demonstrated that this easily
undergoes dissociation of NO. For example, the solution of 2 for
which the optical and IR spectra displayed in Figure 1 were
recorded was removed from the IR cell, diluted under Ar with
CHCl3, and then evaporated in vacuo back to the original volume.
The IR spectrum of the resulting solution (Figure 1b) shows no
Figure 2. Formation of 2 from 1 in toluene at 22 ( 1 °C, obtained by
addition of air (5, 10, 15, 20, 30, 40, 50, 100 µL) to a 117 mL cell
containing 1 (8 µM) in toluene (25 mL), with NO (500 Torr, 8 mM).
The initial spectrum of 1 in the absence of NO is shown as a dotted line.
The spectrum of 1 in the presence of 8 mM NO, the first solid line, is
nearly the same, with subsequent spectra representing addition of air.
It is difficult to reconcile the present results with reports that
1 reacts under these conditions to give 2, unless the previous
observations were due to trace higher nitrogen oxides in the NO
gas streams of (Scheme 1). This hypothesis was tested by adding
air (∼150 µL, 2.6 µmol of O2) directly to the NO/Fe(TPP)(NO)
solution described above. The resulting UV-visible and IR
spectra are identical to those reported for 2 (Figure 2).2,4 NO
autoxidation in aprotic solvents gives NO2,10 and this reacts with
excess NO to give N2O3, so it is the equilibrium mixture of these
species (K1 ) [N2O3]/[NO][NO2]) to which the oxidation of 1 is
attributed. Preliminary stopped-flow kinetics studies show the
reaction of 1 with N2O3 to have a second-order rate constant >106
M-1 s-1 (Scheme 1).
ν
NO band, and the Q-band spectrum (Figure 1a) displays features
(510 nm (ꢀ ∼ 104 M-1 cm-1), 576 (3.5 × 103, sh), 650 (1.7 ×
103), and 688 (2 × 103)) characteristic of an FeIII(TPP)(X) type
complex.14 Since 2 is re-formed promptly upon reintroduction of
NO, we assign this species as Fe(TPP)(NO2) (3), formed by NO
dissociation (eq 2).
Fe(TPP)(NO )
Fe(TPP)(NO)(NO2) h
+ NO
(2)
2
3
Analogous spectral changes were observed upon controlled
removal of NO from the headspace of a toluene solution of 2,
NO, and a known amount of NO2, followed by equilibration. This
procedure allowed estimation of K4 at (3 ( 2) × 10-2 M-1 very
similar to the value previously reported for dissociation of NO
from Fe(TpivPP)(NO2) in CH3CN.14a
The equilibrium constant K3 was evaluated from the spectral
changes as microliter aliquots of air were introduced into a dilute
(9) (a) Kadish, K. M.; Adamian, V. A.; Van Caemelbecke, E.; Tan, Z.;
Tagliatesta, P.; Bianco, P.; Boschi, T.; Yi, G.-B.; Khan, M. A.; Richter-
Addo, G. B. Inorg. Chem. 1996, 35, 1343-1348. (b) Miranda, K. M.;
Bu, X.; Lorkovic, I. M.; Ford, P. C. Inorg. Chem. 1997, 36, 4838-
4848.
In summary, Fe(TPP)(NO), unlike Ru(P),5,9 does not ap-
preciably bind a second NO in room temperature solutions nor
does it promote NO disproportionation to give 2.15 The latter is
formed readily from Fe(TPP)(NO) by reaction with (the much
more soluble) NO2 (N2O3) present in many NO gas streams or
formed by NO reaction with trace O2. Thus, unless care is taken,
NOx impurities are likely to play a major role in the observed
chemistry of transition metal complexes whenever NO is used in
large excess. Qualitatively, NO is more readily dissociated from
2 than is NO2.
(10) (a) Nottingham, W. C.; Sutter, J. R. Int. J. Chem. Kinet. 1993, 25, 375-
381. (b) Ford, P. C.; Wink, D. A.; Stanbury, D. M. FEBS Lett. 1993,
326, 1-3.
(11) Lorkovic, I. M.; Ford, P. C. Unpublished results.
(12) (a) Hoshino, M.; Ozawa, K.; Seki, H.; Ford, P. C. J. Am. Chem. Soc.
1993, 115, 9568-9575. (b) A lower [N2O3] is required to effect the
conversion of 1 to 2 at the same [NO] in CHCl3 than in toluene, so K3
must have a larger value in the former.
(13) (a) Shaw, A. W.; Vosper, A. J. J. Chem. Soc. A 1971, 1592-1595. (b)
Redmond, T. F.; Wayland, B. B. J. Phys. Chem. 1968, 72, 1626-1629.
(14) (a) Frangione, M.; Port, J.; Baldiwala, M.; Judd, A.; Galley, J.; DeVega,
M.; Linna, K.; Caron, L.; Anderson, E.; Goodwin, J. A. Inorg. Chem.
1997, 36, 1904-1911. (b) Munro, O. Q.; Scheidt, W. R. Inorg. Chem.
1998, 37, 2308-2316. (c) Phillippi, M. A.; Baenziger, N.; Goff, H. M.
Inorg. Chem. 1981, 20, 3904-3911.
Acknowledgment. This work was supported by the National
Science Foundation (CHE 9726889) and by a CULAR grant from
Los Alamos National Laboratory.
(15) (a) It has been suggested15b that trace H2O may interact with 1 to inhibit
its reactivity with NO. However, solutions of 1 plus NO prepared in an
oven-dried silanized UV/vis cell by vacuum line transfer of toluene from
highly activated 3 Å molecular sieves (to minimize such traces)
demonstrated no differences in reactivity. (b) Farmer, P. J. Personal
communication.
Supporting Information Available: Supplemental Figure S-1 that
demonstrates the determination of K3. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC9910413