Six-coordinate nitro complexes of iron(III) porphyrins with trans S-donor ligands. Oxo-transfer reactivity in the solid state

Article abstract of DOI:10.1021/ic901722g

Spectroscopic studies demonstrate that the 5-coordinate O-nitrito complexes Fe(Por)(η¹-ONO) (Por - mesotetraphenyl- or meso-tetra-p-tolyl- porphyrinato dianions) react with the thioethers (R₂S) dimethylsulfide and tetrahydrothiophene

Full text of DOI:10.1021/ic901722g

11236 Inorg. Chem. 2009, 48, 11236–11241
DOI: 10.1021/ic901722g
Six-Coordinate Nitro Complexes of Iron(III) Porphyrins with trans S-Donor Ligands.
Oxo-Transfer Reactivity in the Solid State
Tigran S. Kurtikyan,*,† Astghik A. Hovhannisyan,† Alexei V. Iretskii,‡ and Peter C. Ford*,§
†Molecule Structure Research Centre (MSRC) NAS, 0014, Yerevan, Armenia, ‡Department of Chemistry,
Environmental Sciences, Geology and Physics, Lake Superior State University, Sault Ste Marie, Michigan 49783,
and §Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara,
California 93106-9510
Received August 31, 2009
Spectroscopic studies demonstrate that the 5-coordinate O-nitrito complexes Fe(Por)(η1-ONO) (Por - meso-
tetraphenyl- or meso-tetra-p-tolyl-porphyrinato dianions) react with the thioethers (R2S) dimethylsulfide and
tetrahydrothiophene to give the 6-coordinate N-nitrito complexes Fe(Por)(R2S)(NO2). These reactions were
conducted in low-temperature porous layered solids formed in a cryostat; however, with excess R2S in the
atmosphere, the same species are moderately stable at room temperature. Six-coordinate O-nitrito isomers were
not observed with the R2S proximal ligands, even though DFT calculations for the Fe(P)(DMS)(η1-ONO) and
Fe(P)(DMS)(NO2) models (P=porphinato dianion, DMS=dimethyl sulfide) show the latter to be only modestly lower
energy (∼8 kJ/mol) than the former. Leaving this system at room temperature in the presence of excess R2S leads
eventually to the appearance in the FTIR spectra of the ν(NO) band characteristic of the ferrous nitrosyl Fe(Por)(NO).
Concomitantly, the mass spectrum of the gas phase demonstrated the molecular peaks of the sulfoxides R2SO,
indicating oxygen atom transfer reactivity for the ferric porphryinato complexes of nitrite.
Introduction
nitro-isomers may be sufficiently small that hydrogen-bond-
ing interactions may change the mode of coordination.6 In
this context, fundamental questions remain regarding how
systemic parameters, such as the nature of the proximal
ligand L, influence the coordination of the nitrite as well as
its reactivity.
-
The nitrite ion NO2 is present throughout the mamma-
lian organism1 and has been reported to help regulate
vasodilation during hypoxic events and modulate ischemia-
reperfusion tissue injury.2 Much of the growing interest in the
physiological chemistry of nitrite has focused on its interac-
tions and reactions with ferro- and ferri-heme proteins.3
Nitrite is ambidentate and can bind to the metal through
an oxygen as a “nitrito” ligand or through the nitrogen as a
“nitro” ligand, and both forms have been observed not only
for iron-porphyrin models4 but also for heme proteins.5
Furthermore, the difference in binding energy for nitrito- and
*To whom correspondence should be addressed. E-mail: kurto@
netsys.am. Phone: 37410 287423 (T.S.K.). E-mail: ford@chem.ucsb.edu.
Phone: 805 893 2443 (P.C.F.).
(1) Feelisch, M.; Rassaf, T.; Mnaimneh, S.; Singh, N.; Bryan, N. S.;
Jourd’Heuil, D.; Kelm, M. FASEB J. 2002, 16, 1775–85.
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Discov. 2008, 7, 156. (b) Feelisch, M.; Fernandez, B. O.; Bryan, N. S.; Garcia-
Saura, M. F.; Bauer, S.; Whitlock, D. R.; Ford, P. C.; Janero, D. R.; Rodriguez, J.;
Ashrafian, H. J. Biol. Chem. 2008, 283, 33927–33934.
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42, 157–167. (b) Heinecke, J.; Ford, P. C. Coord. Chem. Rev. in press.
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Scheidt, W. R. Inorg. Chem. 1992, 31, 4459–4467.
In this context, we have investigated the coordination of
nitrite (and other NOx species) with the iron centers of heme
model compounds by preparing microporous layers of
Fe(Por) (Por=meso-tetraphenyl- or meso-tetra-p-tolyl-por-
phyrinato dianions) on solid substrates in a cryostat.7 Vola-
tile reactants are added in specific sequences, and the reaction
chemistry of these systems is probed spectroscopically at
tunable and carefully controlled temperatures in a solvent-
free environment. These studies have shown that nitrite
linkage isomerism is very sensitive to the nature of L.
(5) Copeland, D. M.; Soares, A. S.; West, A. H.; Richter-Addo, G. B.
J. Inorg. Biochem. 2006, 100, 1413–1425.
(6) Silaghi-Dumitrescu, R. Inorg. Chem. 2004, 43, 3715–3718.
r
pubs.acs.org/IC
Published on Web 11/03/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11237
The reaction of NO2 with Fe(Por) gives the 5-coordinate
nitrito complexes Fe(Por)(η1-ONO) (1),7a which react further
with volatile Lewis bases to form the 6-coordinate species for
which the N-bound nitro form Fe(Por)(L)(η1-NO2) is gen-
erally favored.7b However, when the reaction of 1 with L
was carried out at very low temperatures (<140 K), it was
possible to spectrally characterize the metastable nitrito
isomers Fe(Por)(L)(η1-ONO) where L=NO,7a,c NH3,7b or
tetrahydrofuran.7d With the first two, warming led to the
nitrito f nitro isomerization, whereas with THF, both
isomers were present over a narrow low temperature interval,
but THF dissociated upon further warming to restore the
nitrito complex 1.7d
Numerous heme proteins have proximal S-donor ligands,
either a cysteine thiolate or a methionine residue.8 In the
present study, the effect of the proximal thioethers dimethyl-
sulfide (DMS) and tetrahydrothiophene (THT) on heme
nitrite coordination and reactivity is addressed. In the course
of these studies, we also observed apparent oxygen atom transfer
(OAT) at ambient temperature from the nitrite complexes of
the ferric porphyrins to the R2S to give the corresponding
sulfoxides R2SO.
Fe(Por)(ONO) and this was confirmed by FTIR measure-
ments.7a The layered film of Fe(Por)(ONO) was cooled by
liquid nitrogen and small portions of DMS or THT were
introduced into the cryostat. The film was slowly warmed and
IR or UV-visible spectra (using CaF2 windows) were mea-
sured at different substrate temperatures determined by a
thermocouple. After the reaction was completed the cryostat
was shortly pumped out to remove the excess of supplied
ligands for obtaining FTIR spectra relatively free from the
bands of adsorbed S-donors.
In the experiments pursuing the study of oxo-transfer
reactivity of I measured quantities of the sulfur bases were
introduced into cryostat at room temperature and FTIR
spectra of the layers were measured over the course of time.
The layer was maintained under S-donors vapors overnight
after which the gaseous content of the cryostat was analyzed
by FTIR spectroscopy and mass spectrometry. For the first
purpose the gaseous content of the cryostat was deposited
through the injector to the cold (77 K) KBr substrate of
anothercryostat. The depositedlayer thusformedwas heated
in course of pumping until the temperatures at which the
excess R2S was completely eliminated from the deposit. The
FTIR spectra of remaining products were then measured.
The mass spectrometric measurements were performed using
a residual gas analyzer. At this point also the main part of the
excess R2S was pumped out from the cryostat at lowered
temperatures then the remaining mixture was introducedinto
the chamber of the gas analyzer through the variable leak
valve.
Heme nitrite complexes with proximal S-donor ligands
have been characterized only for sterically protected
porphyrins.4 An example is the ferrous “picket fence” por-
phyrinato complex [FeII(TpivPP)(PMS)(NO2)]- (TpivPP=
meso-tetrakis(o-pivalamido-phenyl)porphyrinato dianion,
PMS is the thioether pentamethylene sulphide.4a In this case,
The NO2 (15NO2) was obtained by oxidizing with excess
dioxygen NO (15NO) that was purified according to proce-
dure described elsewhere.7g This was then purified by frac-
tional distillation until a pure white solid was obtained. 15NO
with 98.5% enrichment was purchased from the Institute of
Isotopes, Republic of Georgia. DMS (Aldrich, 99%) and
THT(99þ%)werestoredovernightoversodiuminagastight
flask to remove trace quantities of thiols and water and were
vacuum distilled before using.
-
the NO2 is in the pocket formed by the four pivalamide
groups and is N-coordinated (nitro). Notably, the O-atoms
of the nitrite ion ligand are positioned at distances from NH-
groups of the pivalamide chains appropriate for H-bonding
interactions that might stabilize one isomer or another. In
this context, it was of interest to evaluate what structures
would be realized without such a stabilizing factor.
The FTIR spectra and UV-visible spectra were respec-
tively recorded using “Nexus” and “Helios γ” spectrophot-
ometersoftheThermoNicolet Corporation andmassspectra
were recorded by residual gas analyzer “RGA-200” of Stan-
ford Research Systems. The spectral slit width during FTIR
Experimental Section
Low temperature sublimates of the ferrous porphyrinates
Fe(Por) were prepared as described previously7a by heating
the hexacoordinateFe(Por)(B)2 complexes (B is pyridine (Py)
or piperidine) in a Knudsen cell at ∼470 K under high
vacuum (P = 3 ꢀ 10-5 Torr) to eliminate the labile axial
ligands. The Knudsen cell was then heated to 520 K, where-
upon Fe(Por) sublimed onto the 77 K surface of the KBr or
CaF2 substrate cooled by liquid nitrogen to give sponge-like
metallo-arylporphyrinato layers with high microporosity.7e
Once Fe(Por) layers of thickness sufficient for UV-visible
and IR spectral studies were formed (0.3-2.0 h), they were
heated to room temperature under dynamic vacuum. Small
increments of NO2 (15NO2) gas were then introduced for ∼30s
after which the apparatus was evacuated. During this procedure
the redFe(Por) film turnedbrown indicating the formation of
measurements was 2 cm-1
.
Unrestricted density functional theory (DFT) optimiza-
tion calculations were performed with the Spartan’04 (Wave
function, Inc.) orJaguar6.0(Schrodinger, Inc.)attheB3LYP
level using the LACVP* basis set without symmetry con-
straints. The optimized structures represent the equilibrium
geometries of the molecules in gas phase. The calculations
were performed on a Dell Precision 490 dual core Xeon 3.0 GHz
CPU workstation or a custom built core 2 duo 3.0 Ghz
Pentium CPU workstation.
Results and Discussion
To study the reactions with the thioethers, the 5-coordinate
nitrito complexes Fe(Por)(η1-ONO) (1) were prepared on the
KBr or CaF2 substrates in the vacuum cryostat as described
above and the temperature was lowered to <140 K. Small
amounts of a thioether, either dimethyl sulfide or tetrahy-
drothiophen, were then introduced into the cryostat as gas
whereupon they condensed onto the substrate covered with
the microporous layers of 1. For temperatures <140 K for
DMS and <160 K for THT there were no changes in the
intensities of FTIR bands characteristic of the coordinated
nitrito moiety of 1, and the only new spectral features were
those attributed to the S-donors adsorbed on the layers.
(7) (a) Kurtikyan, T. S.; Ford, P. C. Angew. Chem., Int. Ed. 2006, 45, 492–
496. (b) Kurtikyan, T. S.; Hovhannisyan, A. A.; Gulyan, G. M.; Ford, P. C. Inorg.
Chem. 2007, 46, 7024–7031. (c) Kurtikyan, T. S.; Hovhannisyan, A. A.;
Hakobyan, M. E.; Patterson, J. C.; Iretskii, A.; Ford, P. C. J. Am. Chem. Soc.
2007, 129, 3576–3585. (d) Kurtikyan, T. S.; Hovhannisyan, A. A.; Iretskii, A. V.;
Ford, P. C. Aust. J. Chem. 2009, 62, 1226–1230. (e) Kurtikyan, T. S.; Ford, P. C.
Coord. Chem. Rev. 2008, 252, 1486–1496. (f) Kurtikyan, T. S.; Gasparyan, A. V.;
Martirosyan, G. G.; Zhamkochyan, G. A. J. Appl. Spectrosc. 1995, 65, 62–66.
(g) Martirosyan, G. G.; Azizyan, A. S.; Kurtikyan, T. S.; Ford, P. C. Inorg. Chem.
2006, 45, 4079–4087. (h) Kurtikyan, T. S.; Martirosyan, G. G.; Lorkovic', I. M.;
Ford, P. C. J. Am. Chem. Soc. 2002, 124, 10124–10129.
(8) Turano, P.; Lu, Y. In Handbook on Metalloproteins Bertini, I., Sigel, A.,
Sigel, H., Eds.; Marcel Dekker: New York, 2001; Chapter 9, pp 269-356.

11238 Inorganic Chemistry, Vol. 48, No. 23, 2009
Kurtikyan et al.
Figure 1. FTIR spectral changes observed in the layered Fe(TPP)(η1-ONO) (solid line) after supplying into cryostat 0.4 Torr equivalent DMS and
warming from 140 to 170 K (dashed and dotted lines). The bands of excess DMS are denoted by asterisks.
Table 1. Infrared Spectral Data (in cm-1) for Coordinated Nitrite and for Spin Sensitive Bands of the Fe(Por)(R2S)(NO2) Adducts and the Starting Pentacoordinate Nitrito
Complexes (Data Listed in Parentheses Represent the Bands Observed for the Reaction Products of Fe(Por)(η1-O15NO) plus R2S)
compounds
νa(NO2) or ν(NdO)
νs(NO2) or ν(N-O)
δ(ONO) or δ(NO2)
spin-sensitive bands
Fe(TTP)(ONO)a
1528 (1496);
1527 (1495);
1401 (1374);
1399 (1373);
1405 (1376);
1407 (1377);
901 (878);
900 (877);
1300 (1281);
1302 (1283);
1302 (1283);
1303 (1284);
751 (747);
∼750 (∼746)*
809 (∼803);
810 (∼803);
808 (∼802);
807 (∼802);
1340, 428
1340, 436
1348, 456
1347, 465
1349, 457
1347, 465
Fe(TPP)(ONO)a
Fe(TTP)(DMS)(NO2)
Fe(TPP)(DMS)(NO2)
Fe(TTP)(THT)(NO2)
Fe(TPP)(THT)(NO2)
a Reported previously in ref 7a b Overlapped with the intense porphyrin band and seen in difference spectra.
.
However, when T was further raised from 140 to 170 K for
DMS and 160 to 190 K for THT, several key spectral changes
were evident. Specifically, the ν(NdO) and ν(N-O) stretch-
ing bands for the nitrito moiety at 1525 cm-1 and 906 cm-1
disappeared, while new bands at 1399, 1302, and 810 cm-1
grew in intensity (Figure 1, Table 1). All these bands have
lower frequency isotopic counterparts that were apparent
when DMS or THT was allowed to react with layers of the
isotopically labeled Fe(Por)(η1-O15NO) (Figure 2, Table 1).
The electronic absorption spectra also underwent notice-
able changes upon interaction of Fe(Por)(η1-ONO) with R2S
(Figure 3), indicating the formation of new compounds.
The infrared spectral changes upon reaction of R2S with 1
are analogous to those seen when 6-coordinate nitro com-
plexes are formed upon interaction of the ligands NH3,
pyridine, 1-methylimidazole (1-MeIm), and THF with 1.
Nitro and nitrito isomers can be differentiated from the
high-frequency stretching modes that are much closer to each
other for the former than for nitrito species.9 The asymmetric
and symmetric stretching modes νa(NO2) and νs(NO2) for the
hexacoordinate nitro-complexes are located in the vicinity of
1400 and 1300 cm-1, while δ(NO2) is at ∼810 cm-1. In this
context, the bands at 1399, 1302, and 810 cm-1 seen for
the DMS adduct of Fe(TPP)(ONO) and at 1373, 1283, and
∼803 cm-1 for that of Fe(TPP)(O15NO) can be attributed to
formation of the nitro isomers as indicated in Scheme 1.
The spectral changes observed upon low-temperature
interaction of R2S with the Fe(Por)(η1-ONO) differ signifi-
cantly from those obtained when NO,7a NH37b, or THF7d
were used as the ligands. For the first two ligands, disap-
pearance of the nitrito bands at low temperature was accom-
panied by the growth of the bands slightly shifted from the
ν(NdO) and ν(N-O) bands of 1, the former being shifted to
lower frequency, the latter shifted to higher frequency. These
data were reasonably interpreted as resulting from the
low-temperature formation of the metastable 6-coordinate
nitrito-complexes, namely, Fe(Por)(NO)(η1-ONO) and Fe-
(Por)(NH3)(η1-ONO). Upon warming these each isomerized
to the nitro isomers Fe(Por)(NO)(NO2) and Fe(Por)(NH3)-
(NO2) that were stable evenat room temperature when excess
NO(g) and NH3(g) were present, respectively. However, in
the case of the S-donor ligands, it was not possible to detect
the 6-coordinate nitrito species that would be the anticipated
intermediate upon reaction of R2S with 1. Instead, only
6-coordinate nitro complexes were observed, even at the
lowest temperatures where a reaction of 1 with R2S was
apparent. Hence, the barrier for nitrito to nitro linkage
(9) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination
Compounds, 3rd ed.; Wiley & Sons: New York, 1978; pp 244-247.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11239
Figure 2. FTIR spectra of samples obtained after low-temperature DMS interaction with sublimed layers containing Fe(TPP)(η1-ONO) (solid line) and
Fe(TPP)(η1-O15NO) (dashed line).
Figure 4. Structure of computed (B3LYP/LACVP*) porphinato com-
plex Fe(P)(DMS)(NO2).
conserve the 6-coordinate Fe(Por)(THT)(NO2) complexes.
In the presence of 12 Torr THT at ambient conditions,
they decompose within a few hours. However, the reaction
Figure 3. UV-visible spectra of Fe(TPP) (solid line), Fe(TPP)(η1-ONO)
between 1 and R2S is also fully reversible since pumping at
(dashed line), and after interaction with DMS (dotted line). The spectra
elevated temperatures led to complete restoration of the
original spectral properties of 1.
were taken after FTIR measurements of the same samples confirming
the indentity of each species.
Certain ring vibration modes of Fe(TPP) complexes, for
example, bands at ∼1350 cm-1 (ν(Ca-Cm) mixed with
ν(Cm-phenyl)) and at ∼450 cm-1 (δ(Pyr. rotation)) have
been shown to be sensitive indicators of the metal center spin
state.10 For the intermediate spin state Fe(TPP),11 these
bands lie at 1346 and 464 cm-1. Upon reaction with NO2
to give Fe(TPP)(η1-ONO), they shift to 1341 and 434 cm-1
consistent with a high-spin state. (For Fe(TTP), the analo-
gous bands appear at 1346 and 455 cm-1 and correspond-
ingly shift upon reaction with NO2 to 1340 and 428 cm-1). In
previous studies, it was shown that the 6-coordinate nitro
complexes Fe(TPP)(L)(NO2) formed by the porous layer
reactions of Fe(TPP)(η1-ONO) with NO or a nitrogen base
such as NH3 or Py consistently display these porphyrin bands
Scheme 1
isomerization of the nitrite ligand must be sufficiently low
that this pathway is facile at the lowest temperatures where
R2S is sufficiently mobile to penetrate into the porous layers
of 1 and form the 6-coordinate complexes.
(10) (a) Oshio, H.; Ama, T.; Watanabe, T.; Kincaid, J.; Nakamoto, K.
Spectrochim. Acta 1984, 40A, 863–870. (b) Paulat, F.; Praneeth, V. K. K.;
Nather, C.; Lehnert, N. Inorg. Chem. 2006, 45, 2835–2856.
(11) Collman, J. P.; Hoard, J. L.; Kim, N.; Lang, G.; Reed, C. A. J. Am.
Chem. Soc. 1975, 97, 2676–2681.
In the presence of excess DMS (P > 40 Torr), the Fe-
(Por)(DMS)(NO2) nitro complexes formed in this manner
are stable at room temperature. For the less volatile THT
ligand it was impossible to have enough THT pressure to

11240 Inorganic Chemistry, Vol. 48, No. 23, 2009
Kurtikyan et al.
Scheme 2. Possible pathways for oxygen atom transfer from nitrite to R2S mediated by FeIII(Por)
at ∼1350 and 460 cm-1, indicating a Fe(III) center in the low-
spin electronic state.7a-c The IR spectra of the Fe(Por)(R2S)-
(NO2) products formed from the reaction of Fe(Por)(η1-
ONO) with DMS and THT show new bands in the vicinity of
1350 and 460 cm-1 (see Table 1) indicating a low spin state
for these Fe(III) complexes.
DFT calculations for the porphinato analogs are consis-
tent with these experimental observations. The (gas phase)
structures of isomeric FeIII(P)(DMS)(NO2) and FeIII(P)-
(DMS)(ONO) (P=porphinato dianion) complexes at differ-
ent spin states were computationally optimized at B3LYP
level using the LACVP* basis set without symmetry con-
straints. The lowest energy species was FeIII(P)(DMS)(NO2)
in the doublet spin state (Figure 4), and this was about 8 kJ/
mol lower in energy than themost stable nitrito isomer, which
was also in the doublet spin state. The results of these
computations are given in the Supporting Information
(Table S1).
Notably, when the cryostat containing excess DMS in the
gas phase (5 Torr) and sublimed layers of Fe(TPP)(η1-ONO)
and the R2S adduct was maintained at room temperature, a
new band at 1676 cm-1 appeared in the IR spectrum of the
iron porphyrin layer over a period of several hours. (See
Supporting Information Figure S-1). This band was attrib-
uted to the ν(NO) frequency of the 5-coordinate ferrous
nitrosyl complexes Fe(TPP)(NO) which has been shown in
earlier experiments to appear at 1677 cm-1 for this complex as
a result of NO interaction with sublimed layer of Fe(TPP).7h
In addition, bands attributed to the coordinated nitrite ion
correspondingly decreased in intensity (Supporting Informa-
tion Figure S1). To determine other possible products, the
gaseous content of the cryostat after reacting overnight was
transferred through the injector into another cryostat and
deposited onto a KBr substrate cooled by liquid nitrogen. The
FTIR spectra were recorded during a warming/pumping
procedure. According to these measurements pumping at
180 K completely removed the excess DMS from the sub-
strate, and the IR spectrum of remaining compound was
identical to that of solid DMSO layer obtained by depositing
directly on the substrate (Supporting Information Figure S2).
The same results were obtained when the layered Fe(TTP)
was used as an iron-porphyrin.
of the isotopically labeled DMS18O was registered. When
THT was used instead of DMS, the behavior was fully
analogous and the m/z molecular peaks of THT-oxide at
104 and of THT18O at 106 were respectively observed for the
reactions of THT with 1 and with 18O labeled 1 (see Supporting
Information Figure S3). These observations provide clear
evidence that the iron porphyrins are somehow facilitating
oxygen atom transfer (OAT) reactions from coordinated nitrite
to the substrates DMS and THT.
In previous studies, it has been shown that solutions of
FeIII(Por) complexes with added nitrite salts mediate oxygen
atom transfers to various substrates with concomitant for-
mation of ferrous nitrosyls FeII(Por)(NO). This reaction was
observed both in organic solvents12 and in aqueous media at
physiologically relevant pH values.13 In the latter case, the
use of DMS as substrate, the water-soluble ferric porphyin
FeIII(TPPS)(H2O)2, and sodium nitrite gave the correspond-
ing products DMSO and FeII(TPPS)(NO).13 The observa-
tion of this reaction in the much simpler system described
here, containing only the layered iron-porphyrin nitrite
complexes and gas phase substrate suggests the generality
of such oxygen transfers for these heme model systems when
nitrite is present. Similar oxygen atom transfer activity of thin
layers was previously observed in our laboratory for the Co
porphyrin nitro complex Co(TPP)(NO2).14
However, at this stage it is not possible to identify con-
clusively which ferric porphyrin species might be responsible
for the promotion of R2S oxidation. FTIR spectral analysis
of the layers containing iron-porphyrin nitrite complexes at
room temperature in the presence of excess R2S indicate
that both 5-coordinate Fe(Por)(η1-ONO) and 6-coordinate
Fe(Por)(R2S)(NO2) are present. Furthermore, the computa-
tional studies have found only modest differences in energy
between Fe(P)(η1-ONO) and its linkage isomer Fe(P)(NO2)7c
(1 kJ, the high spin state of the former being insignificantly
more stable according to the calculation) and between
Fe(P)(DMS)(NO2) and Fe(P)(DMS)(η1-ONO) (8 kJ, the
doublet state of the former being the more stable as described
above).7c While the calculations are for the gas phase
porphinato complexes and therefore have to be con-
sidered qualitative, they clearly suggest that all four of these
Mass spectral analysis of the gaseous content of this
cryostat by a residual gas analyzer shows the m/z molecular
peaks of DMSO at 78. When the analogous experiments were
conducted using the isotopically substituted Fe(Por)-
(η1-18ON18O) as the parent nitrito species, the mass spectrum
(12) O’Shea, S. K.; Wang, W.; Wade, R. S.; Castro, C. E. J. Org. Chem.
1996, 61, 6388–6395.
(13) Khin, C.; Heinecke, J.; Ford, P. C. J. Am. Chem .Soc. 2008, 130,
13830–13831.
(14) Kurtikyan, T. S.; Mardyukov, A .N.; Goodwin, J. A. Inorg. Chem.
2003, 42, 8489–8593.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11241
species need to be taken into account when consider-
ing possible mechanisms for the oxygen atom transfer as
illustrated in Scheme 2. As we have seen, the nitrite ion
linkage isomerization pathways seem to be reasonably facile,
so we would not expect the potential transformations be-
tween these four species to be rate limiting in the slow OAT
reactions.
Summary
This study shows that low-temperature interaction of
S-donor thioethers DMS and THT with 5-coordinate iron-
porphyrin O-nitrito complexes Fe(Por)(η1-ONO) leads to
formation of 6-coordinate N-nitrito species Fe(Por)(R2S)-
(NO2). Hence, coordination of the proximal S-donor ligand
leads to O-nitrito f N-nitrito isomerization of the nitrite
ligand. In the presence of gas phase R2S donors these six-
coordinate complexes can be observed even at room tempera-
ture. In contrast to high spin 5-coordinate O-nitrito complexes,
the 6-coordinate species are in the low spin state. DFT
calculations for these 6-coordinate species give minimal
energies for the low-spin doublet states for the N-nitrito and
O-nitrito isomers with the N-nitrito isomer only 8 kJ/mol more
stable than O-nitrito one. There are several indications of oxo-
transfer reactivity of solid-state iron-porphyrin nitrite system in
the presence of gas phase R2S: appearance of the iron-nitrosyl
band in the layered solid and formation of corresponding
sulfoxides demonstrated by the FTIR and mass spectrometric
measurements together with 18O isotope labeling experiments.
Since the predominant species present in the ambient
temperature solids with excess R2S present are the penta-
coordinate nitrito complex 1 and the hexacoordinate nitro
complex Fe(Por)(R2S)(NO2), OAT pathways 1 and 3 might
seem to be the most reasonable. However, a problem with
pathway 3 is that if the oxygen atom transferred in a
concerted pathway was the uncoordinated O in the γ posi-
tion, the isonitrosyl species FeII(Por)(ON) would be an
intermediate along the reaction coordinate. However, our
DFT computations for the porphine analogs show the
optimized structure of the isonitrosyl isomer to be 67 kJ
higher in energy than the product nitrosyl species. Thus it
seems more likely that 1 would isomerize to the energetically
accessible 5-coordinate nitro isomer, followed by OAT from
this species to the substrate, that is, pathway 2. A similar
argument can be used to discount the importance of pathway
4 to the oxygen atom transfer mechanism. Lastly, with regard
to possible OAT from the 6-coordinate nitro complex,
concerted OAT and R2S dissociation from the proximal
position would be one possible pathway for this to occur
(step 1), but given the relative slowness of these OAT
reactions, the stepwise dissociation of R2S to give Fe(Por)-
(NO2) followed by step 2 seems equally likely. Indeed, an
earlier computational study by Conradie and Ghosh15 has
suggested that oxygen atom transfer to DMS via pathway 2
would give a lower energy activation barrier than the con-
certed reaction indicated by pathway 1. Better identifying
which of these species and pathways are responsible for the
oxygen atom transfer reactivity is the subject of ongoing
investigation.
Acknowledgment. P.C.F. acknowledges support from the
U.S. National Science Foundation (grant CHE-0749524).
Supporting Information Available: Figure S1 demonstrating
formation of nitrosyl iron-porphyrin upon standing O-nitrito
complex Fe(TPP)(η1-ONO) in the atmosphere of DMS; Figure
S2 representing both the FTIR spectra of the gaseous content of
cryostat where the layer of Fe(TPP)(η1-ONO) has been under
DMS vapors overnight and solid DMSO; Figure S3 represent-
ing mass spectra of THT, and gaseous content of cryostat where
the layer of Fe(TPP)(η1-ONO) has been under vapors of THT
overnight; DFT calculations including total energies for all spin
states of the 6-coordinate ferric porphinato complexes Fe(P)-
(DMS)(η1-ONO) and Fe(P)(DMS)(NO2) (Table S1); selected
bond distances computed for the doublet states of the 6-coordi-
nate ferric porphinato complexes Fe(P)(DMS)(η1-ONO) and
Fe(P)(DMS)(NO2) (Table S2); and final geometries for Fe(P)-
(DMS)(η1-ONO) and Fe(P)(DMS)(NO2). This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(15) Conradie, J.; Ghosh, A. Inorg. Chem. 2006, 45, 4902–4909.