In situ FT-IR and UV-vis spectroscopy of the low-temperature NO disproportionation mediated by solid state manganese(II) porphyrinates

Article abstract of DOI:10.1021/ic051824q

The heterogeneous reaction between NO gas and sublimed layers of manganese(II) porphyrinato complexes Mn(Por) (Por = TPP (tetraphenylporphyrinato dianion), TMP (tetramesitylporphyrinato dianion), or TPP_d20 (perdeuterated tetraphenylporphyrinato

Full text of DOI:10.1021/ic051824q

Inorg. Chem. 2006, 45, 4079−4087
In Situ FT-IR and UV−vis Spectroscopy of the Low-Temperature NO
Disproportionation Mediated by Solid State Manganese(II) Porphyrinates
Garik G. Martirosyan,† Arsen S. Azizyan,† Tigran S. Kurtikyan,*,†,‡ and Peter C. Ford*,§
Armenian Research Institute of Applied Chemistry (ARIAC), 375005, YereVan, Armenia, Molecule
Structure Research Centre NAS, 375014, YereVan, Armenia, and Department of Chemistry and
Biochemistry, UniVersity of California, Santa Barbara, California 93106
Received October 21, 2005
The heterogeneous reaction between NO gas and sublimed layers of manganese(II) porphyrinato complexes
Mn(Por) (Por TPP (tetraphenylporphyrinato dianion), TMP (tetramesitylporphyrinato dianion), or TPPd20
)
(perdeuterated tetraphenylporphyrinato dianion)) has been monitored by IR and optical spectroscopy over the
temperature range of 77 K to room temperature. These manganese porphyrins promote NO disproportionation to
NO2 species and N2O, and the reaction proceeds via several distinct stages. At 90 K, the principal species observed
spectrally are the nitric oxide dimer, cis-ONNO, two manganese nitrosyls, the simple NO adduct Mn(Por)(NO), and
another intermediate (1) that is apparently critical to the disproportionation mechanism. This key intermediate is
formed prior to N2O evolution, and proposals regarding its likely structure are offered. When the system is warmed
to 130 K, the disproportionation products, N2O and the O-coordinated nitrito complex Mn(Por)(NO)(ONO) (2), are
formed. IR spectral changes show that, upon further warming to 200 K, 2 isomerizes into the N-bonded nitro
linkage isomer Mn(Por)(NO)(NO2) (3). After it is warmed to room temperature, the latter species loses NO and
converts to the known 5-coordinate nitrito complex Mn(Por)(ONO) (4).
Introduction
to influence catalytic cycles dependent on the transformations
among various NOx.4
The reactions of metal complexes with nitric oxide
(nitrogen monoxide) and the metal-mediated transformations
of NO to other NOx species have long been of interest. While
these reactions have demonstrated importance in biological
media,1 they have also drawn considerable attention in
catalytic chemistry where oxygen atom transfers from metal-
coordinated NOx may have utility in the selective oxidations
of organic substrates2 and in environmental chemistry where
of NOx removal from effluent gas streams has importance.3
One such reaction is the disproportionation of NO to nitrous
oxide and nitrogen dioxide (eq 1). Disproportionation, the
source of major impurities in gaseous NO commercially
supplied in high-pressure steel tanks, is relevant to mecha-
nisms for the enzymatic reduction of NO to N2O and is likely
3NO f N2O + NO2
(1)
There are numerous literature reports describing activation
of NO disproportionation by metal complexes to give N2O
and metal nitrite complexes.5 For example, Tolman and co-
workers have described the reaction of NO with copper(I)
tris(pyrazolyl)borate to give N2O and a Cu(II) nitrito
species.5f Similar reactivity has been described by Franz and
Lippard for manganese and iron tropocoronand complexes.6
(2) Goodwin, J.; Bailey, R.; Pennington, W.; Rasberry, R.; Green, T.;
Shasho, S.; Yongsavanh, M.; Echevarrria, V.; Tiedeken, J.; Brown,
C.; Fromm, G.; Lyerly, S.; Watson, N.; Long, A.; De Nitto, N. Inorg.
Chem. 2001, 40, 4217-4225. (b) Remias, J. E.; Sen, A. J. Mol. Catal.
A 2003, 201, 63-70. (c) Tovrog, B. S.; Mares, F. M.; Diamond, S. E.
J. Am. Chem. Soc. 1980, 102, 6616-6618. (d) Diamond, S. E.; Mares,
F. M.; Szalkiewicz, A.; Muccigrosso, D. A.; Solar, J. P. J. Am. Chem.
Soc. 1982, 104, 4266-4268. (e) Castro, C. E.; O’Shea, S. K. J. Org.
Chem. 1995, 60, 1922-1923. (f) Castro, C. E. J. Org. Chem. 1996,
61, 6388-6395.
(3) Trogler, W. C. J. Chem. Educ. 1995, 72, 973-976. (b) Iwamoto, M.;
Yahiro, H. Catal. Today. 1994, 22, 5-18.
(4) Wasser, I. M.; de Vries, S.; Moenne-Loccoz, P.; Schroder, I.; Karlin,
K D. Chem. ReV. 2002, 102, 1201-1234.
* To whom correspondence should be addressed. E-mail: kurto@netsys.am
(T.S.K.); ford@chem.ucsb.edu (P.C.F.).
† Armenian Research Institute of Applied Chemistry.
‡ Molecule Structure Research Centre NAS.
§ University of California.
(1) Delwich, C. C. Denitrification Nitrification and Atmospheric Nitrous
Oxide; Wiley: New York, 1981. (b) Averill, B. A. Chem. ReV. 1996,
96, 2951-2964.
10.1021/ic051824q CCC: $33.50
Published on Web 04/19/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 10, 2006 4079

Martirosyan et al.
for spectroscopic studies because of the absence of solvent
interference. After sample preparation, a known quantity of NO
measured by a mercury manometer was slowly deposited onto the
77 K Mn(Por) layers. IR or UV-vis spectra were measured for
these mixtures at different substrate temperatures controlled by a
thermocouple.
The critical step of NO disproportionation reactions appears
to be the N-N coupling required to form a precursor of N2O.
It has been proposed in those studies that metal nitrosyls
reacted with NO to form a cis-dinitrosyl intermediate, which
then converts into a hyponitrito M(N2O2)2- complex, fol-
lowed by O-atom abstraction by a third NO. Among metal
porphyrins NO disproportionation has been reported for
ruthenium and osmium complexes, M(Por)(CO), which react
with NO to give N2O and the nitrosyl nitrito complexes
M(Por)(NO)(ONO) (M ) Ru, Os; Por ) TPP, OEP).7
Although these systems were the subjects of kinetics studies,
the detailed mechanisms of the N-N bond formation leading
to N2O have not been fully characterized.
This paper reports the IR and UV-vis spectroscopic
studies monitored over the temperature range from 77 K to
room temperature (RT) for the reaction of NO with films of
the manganese(II) porphyrinato complexes Mn(Por) (Por )
TPP (tetraphenylporphyrinato dianion), TMP (tetramesi-
tylporphyrinato dianion), or TPPd20 (perdeuterated tetraphe-
nylporphyrinato dianion)) that promote NO disproportion-
ation. In this work, we report vibrational and optical spectra
of an intermediate that is apparently the direct precursor of
the N-N bond formation, and we also report the IR spectra
of several other previously unknown Mn-NOx complexes.
The nitric oxide and 15NO (Institute of Isotopes, Republic of
Georgia, with isotopic enrichment 98.5%) were purified as follows.
First, NO was passed multiple times through a column containing
KOH pellets and through butylbromide/liquid N2 (-119 °C) cooled
traps to remove N2O and NO2 impurities. Although the N2O
contamination after this procedure was estimated to be less than
0.2%, this level of purity was not satisfactory for the low-
temperature FTIR experiments. Weak bands at 2240 and 1290 cm-1
attributed to N2O were still seen after NO deposition on the KBr
substrate at 7 K, so additional purification was necessary to
eliminate interference from N2O impurities. In the second step, the
glassy bulb containing prepurified NO was submerged into a dewar
flask filled with liquid N2 and connected with cryostat cooled by a
helium closed-cycle refrigeration system (ARS DE202). NO
evaporating from the 77 K bulb was deposited onto the 7 K substrate
of the cryostat (at 77 K, the NO vapor pressure is about 10-1 Torr
while that for N2O is 10-6 Torr),11 and the purity of the condensate
was checked by IR spectroscopy. Then the cryostat was allowed
to warm to 110 K, and the NO was condensed into another glass
bulb submerged in liquid N2. Considerable precautions were taken
to prevent inadvertent air contamination during the gas transfers
and purification steps for each experiment described here.
Gas analyses were performed by gas chromatography on a GCHF
18.3 instrument equipped with a thermal conductivity detector and
a 300 cm column packed with Porapac Q (80-100 mesh). The
retention times (min) for various gases were NO (1.11), NO2 (3.6),
and N2O (4.6-4.8), using H2 as a carrier gas with flow rate 40
mL/min at 40 °C. In a typical experiment, the cryostat with prepared
Mn(TPP) sample was attached to a high-vacuum line; a known
quantity of NO measured with a mercury manometer was added,
and the unit was sealed with a vacuum valve. After the reaction,
the headspace gas of the cryostat was carefully transferred under
vacuum into a flask fitted with liquid nitrogen finger, manometer,
and adapter, which allowed for syringe access and protected the
sample from air with a rubber septum. The pressure was equilibrated
to atmospheric pressure with H2, and the gas mixture was sampled
with a Hamilton gastight syringe for GC injections.
Experimental Section
Complexes Mn(TPP)(Pip) and Mn(TMP)(Pip) (Pip ) piperidine),
synthesized according to published methods,8 were the precursors
of the manganese(II) porphyrinato complexes Mn(Por) used to
prepare the sublimed layers. Mn(TPPd20)(Pip) was synthesized
following the procedure reported in ref 9. The Mn(Por) sublimates
on the KBr or CaF2 substrates of the optical cryostats were prepared
under continuous vacuum conditions, according to a procedure
described elsewhere.10 Such thin layers of metallo-tetraarylporphy-
rins sublimed onto a low-temperature (77 K) surface are spongelike
and have high microporosity that allows potential ligands to diffuse
easily across the bulk.10b,c The species thus formed are convenient
(5) Gans, P. J. Chem. Soc. A 1967, 943-946. (b) Gwost, D.; Caulton, K.
Inorg. Chem. 1974, 13, 414-417. (c) Settin, M. F., Fanning, J. C.
Inorg. Chem. 1988, 27, 1431-1435. (d) Paul P. P.; Karlin K. D. J.
Am. Chem. Soc. 1991, 113, 6331-6332. (e) Ruggiero, C. E.; Carrier,
S. M.; Tolman, W. B. Angew. Chem., Int. Ed. 1994, 33, 895-897. (f)
Schneider, J. L.; Carrier, S. M.; Ruggiero, C. E.; Young V. G.; Tolman,
W. B. J. Am. Chem. Soc. 1998, 120, 11408-11418.
(6) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc. 1998, 120, 9034-9040.
(b) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 10504-
10511.
(7) Miranda, K.; Bu, X.; Lorkovic, I.; Ford, P. C. Inorg. Chem. 1997, 36,
4838-4848. (b) Kadish, K. M.; Adamian, V. A.; Caemelbecke, E.
V.; Tan, Z.; Tagliatesta, P.; Blanco, P.; Boschi, T.; Yi, G. B.; Khan,
M. A.; Richter-Addo, G. B. Inorg. Chem. 1996, 35, 1343-1348. (c)
Bohle, D. S.; Goodson, P. A.; Smit, B. D. Polyhedron 1996, 15, 3147-
3150. (d) Yi, G. B.; Khan, M. A.; Richter-Addo, G. B. Inorg. Chem.
1996, 35, 3453-3454.
(8) Kobayashi, H.; Yanagawa, Y. Bull. Chem. Soc. Jpn. 1972, 45, 450-
454.
(9) Lindsey, J. S.; Hsu, H. C.; Schreiman, I. C. Tetrahedron Lett. 1986,
27, 4969-4970. (b) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.;
Kearney, P. C.; Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827-
836.
(10) Martirosyan, G. G.; Azizyan, A. S.; Kurtikyan, T. S.; Ford, P. C. J.
Chem. Soc., Chem. Commun. 2004, 13, 1488-1489. (b) Byrn, M. P.;
Curtis, C. J.; Hsiou, Y.; Khan, S. I.; Sawin, P. A.; Tendick, S. K.;
Terzis, A.; Strouse, C. E. J. Am. Chem. Soc. 1993, 115, 9480. (c)
Kurtikyan, T. S.; Gasparyan, A. V.; Martirosyan, G. G.; Zhamkochyan,
G. H. J. App. Spectrosc. 1995, 62, 62 (Russian).
Quantitative analysis of the N2O content in the headspace was
performed by comparison of the average peak areas of three
injections with previously prepared standard curves with CO2 used
as an internal standard. The standards were analyzed in the same
way as the reaction samples. Three injections were made for each
standard to obtain GC calibration curves for NO or N2O. Infrared
spectra were measured on Specord M-80 and Nicolet “Nexus” FTIR
spectrometers. The UV-vis spectra were measured on a Specord
M-40 spectrophotometer.
Results and Discussion
Low-Temperature Reaction of Mn(TPP) with NO.
When sublimed layers of Mn(TPP) at 77 K were exposed to
excess NO that had been carefully purified in the manner
described above, new absorption bands were observed at
1853 s and 1752 vs. We have assigned these bands to the
(11) Knunjants, Ed., Handbook of Chemistry; Soviet Encyclopedia: Mos-
cow, 1988.
4080 Inorganic Chemistry, Vol. 45, No. 10, 2006

Low-Temperature NO Disproportionation
symmetric and asymmetric NO stretching modes of the cis-
ONNO dimer on the basis of the following reasoning. The
NO dimers have been extensively studied in gas and
condensed phases, as well as in different low-temperature
matrixes.12 In the pure solid at 12 K, the ν(NO)sym and
ν(NO)asym bands of the more stable cis-ONNO isomer were
identified as appearing at 1865 and 1760 cm-1, respectively.
In contrast, the trans dimer, having the center of symmetry,
exhibits only a single infrared active ν(NO) band, which has
been observed at 1760 cm-1 in a dinitrogen matrix13 and at
1740 cm-1 in a carbon dioxide matrix;14 however, the
sensitivity of these bands to the environment is well-known.15
It is significant that the positions of the two bands seen upon
deposition of NO on 77 K Mn(TPP) layers are quite similar
to those seen in our experiments with metallo porphyrinato
layers of other metals (Co-, Fe-, Cu-, Ni-, and Zn(TPP))
under analogous conditions. This indicates that the bands
seen in the Mn(TPP) layers are independent of the metal
center and, hence, are characteristic of cis-ONNO itself in
these media and not of coordinated (NO)2 dimers.
Subsequent warming of the samples to ∼90 K resulted in
the formation of a new band at 1712 cm-1 and a very weak
one near 1820 cm-1, in addition to the bands at 1853 s and
1752 vs attributed to cis-ONNO (Figure 1a). It should be
noted that the band at 1820 cm-1 overlaps with another small
absorbance at 1812 cm-1, which belongs to a porphyrin
moiety overtone and occurs in the IR spectra of all tetra-
aryl-substituted MPs. When this experiment was carried out
with 15NO, the same pattern of NO-dependent bands was
seen but with the frequencies shifted to 1820 s, 1716 vs,
1784 vw, and 1680 m cm-1, respectively. The intensity ratio
of the asymmetric/symmetric bands (integrated absorbances)
seen in Figure 1a is ∼4, which is somewhat higher than the
reported intensity ratio (∼2.8) of these IR bands for cis-
ONNO;15 however, the higher relative intensity of the 1752
cm-1 band in Figure 1a may be caused by an overlap with
the ν(NO) band 1740 cm-1 (1705 cm-1 for 15NO) of Mn-
(TPP)(NO),16 or another 6-coordinate species containing
coordinated NO (see below), which is also formed under
these conditions. In our control experiments with M(TPP)
(M ) Zn, Ni) that do not form nitrosyls or for those where
the ν(NO) of M(TPP)(NO) (Co and Fe) do not overlap the
ν(NO)asym band of cis-ONNO, this ratio is closer to the
reported value.
Figure 1. IR and UV-vis spectra of the same sample of Mn(TPP) before
and after treatment with NO. In the top panel, the dashed line represents
Mn(TPP) at 77 K, the thin line indicates the spectrum after the sample was
exposed to NO and warmed to 90 K, and the thick line shows the same for
15NO. In the bottom panel, the dashed line shows Mn(TPP) and the solid
line shows it after it was exposed to NO and warmed to 90 K.
The bands at 1820 and 1712 cm-1 (1784 and 1680 for
15NO) are attributed to the formation of a new intermediate
1. The difference between spectra recorded before and after
the addition of approximately 1 equiv of NO to the Mn(TPP)
layers at ∼90 K shows initial formation of a nitrosyl band
at 1740 cm-1, assigned to the mononitrosyl complex
Mn(TPP)(NO). As more NO equivalents were added, there
was concomitant formation of bands at 1820 and 1712 cm-1
(1) and at 1853 and 1752 cm-1 (cis-ONNO). The difference
between the spectra recorded before and after this step also
showed the decrease in the 1740 cm-1 band of the mono-
nitrosyl complex. This indicates that the latter underwent
conversion to 1 when the system was exposed to the
additional NO under these conditions. Cooling of this
sample to 77 K, followed by a prolonged evacuation,
ultimately results in decreased intensities of the bands
attributed to the dimer and to 1. Attempts to com-
pletely remove the cis-ONNO dimer by evacuating the
system, without disruption of intermediate 1 were unsuc-
cessful.
(12) Kukolich, S. G. J. Am. Chem. Soc. 1982, 104, 4715-4716. (b) East,
A. L. L.; McKellar, A. R. W.; Watson, J. K. G. J. Chem. Phys. 1998,
109, 4378-4383. (c) Lipscomb, W. N.; Wang, F. E.; May, W. R.;
Lippert, E. L. Acta Crystallogr. 1961, 14, 1100-1101. (d) Kometer,
R.; Legay, F.; Legay-Sommire, N.; Schwentner, N. J. Chem. Phys.
1994, 100, 8737-8745. (e) Nour, E. M.; Chen, L. H.; Strube, M. M.;
Laane, J. J. Phys. Chem. 1984, 88, 756-763.
(13) Legay, F.; Legay-Sommire, N. Chem. Phys. Lett. 1993, 211, 516-
521
(14) Fateley, W. G.; Bent, H. A.; Crawford, B. J. Chem. Phys. 1959, 31,
204-217.
(15) Krim, L.; Lacome, N. J. Phys. Chem. A 1998, 102, 2289-2296.
(16) Wayland, B. B.; Olson, L. W.; Siddiqui, Z. U. J. Am. Chem. Soc.
1976, 98, 94-98. (b) Hoshino, M.; Konishi, S. Chem. Phys. Lett. 1985,
115, 511-516. (c) Scheidt, W. R.; Hatano, K.; Rupprecht, G. A.;
Piciulo, P. L. Inorg. Chem. 1979, 18, 292-299, (d) Zahran, Z. N.;
Jonghyuk, L.; Alguindigue, S. S.; Khan, M. A.; Richter-Addo, G. B.
J. Chem. Soc., Dalton Trans. 2004, 189-196.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4081

Martirosyan et al.
Table 1. IR Frequencies (cm-1) of Mn(Por)(NO)(ONO) Complexesa
compound
ν(NO)
ν(NdO)
ν(N-O)
δ(ONO)
Mn(TPP)(NO)(ONO)
1812 (1778) 1480 (1454) 971 (952) 822 (817)
Mn(TPPd20)(NO)(ONO) 1811 (1778) 1481 (1453) 976 (957)
Mn(TMP)(NO)(ONO) 1809 (1777) 1484 (1455) 968 (948) 822 (818)
a Values in parentheses represent the frequencies observed after the
reaction with 15NO.
Figure 2. FT-IR monitoring of N2O and Mn(TPP)(NO)(ONO) formation
upon warming of the Mn(TPP) sample from 100 to 130 K under excess
NO (ca. P ) 15 Torr).
When a sample prepared by stepwise addition of NO to
Mn(TPP) layers at ∼90 K instead was warmed to T > 100
K, the bands attributed to the NO dimer and that at 1712
cm-1 underwent a gradual disappearance. At the same time,
formation of N2O was detected by the appearance of the
characteristic IR band at 2221 cm-1. Notably, under analo-
gous experimental conditions, the other metalloporphyrins
listed above do not display an IR band similar to that of 1
nor the formation of N2O. This circumstantial evidence
suggests that chemical involvement of manganese center is
necessary requirement for formation of 1, which plays a role
in the production of N2O.
Optical spectroscopy is a sensitive probe of the metal
oxidation state in manganese porphyrin complexes because
of the extensive mixing of the metal eg and porphyrin ring
eg(π) orbitals.17a-c Figure 1b shows the optical spectra of
the material obtained by exposure of Mn(TPP) to excess NO
at 77 K and slowly warmed to 90 K. The IR spectrum of
this same material is dominated by the band at 1820 and
1712 cm-1; in other words, it is intermediate 1. Notably,
the optical spectrum is close to that reported for the
mononitrosyl complex Mn(TPP)(NO) in frozen solutions;16a
thus it appears that the oxidation state in 1 is the same as
that in Mn(TPP)(NO), since oxidation to Mn(III) should
show an absorption at λmax ) 480 nm.
Figure 3. In situ UV-vis reaction spectra of Mn(TPP) under excess NO
when the sample was warmed from ∼90 to ∼130 K. The appearance of
the absorbance band at 480 nm upon formation of 2 indicates oxidation of
the metal center to Mn(III) state.
changes in the region characteristic to the ONO- vibrations
are shown in Supporting Information Figure S1. Three new
bands appear at 1488, 970, and 821 cm-1, and they are
consistent with the ν(NdO), ν(N-O), and δ(ONO) modes
of an oxygen-coordinated ONO moiety. When the experi-
ment was carried out with 15NO, the nitrosyl and nitrito
ligand bands were located at 1778, 1450, 950, and 818 cm-1,
respectively.
Analogous infrared spectral changes were also observed
when the same reactions with NO were carried out with
sublimed layers of Mn(TPPd20) and Mn(TMP). Frequencies
observed for the nitrosyl and nitrito groups in the various
Mn(Por)(NO)(ONO) products are listed in Table 1.
Irreversible conversion of intermediate 1 to Mn(Por)(NO)-
(ONO) was also indicated by the changes in the visible
spectra (Figure 3). When the sample was warmed to 130 K,
the absorption band of 1 at 539 nm shifted to 546 nm,
together with formation of a broad shoulder at 560 nm and
a new band at 478 nm. The latter suggests oxidation of the
metal center upon formation of 2, since this absorption is
characteristic of other Mn(III) porphyrin nitrosyl com-
plexes.16a
Figure 2 shows the IR spectral changes upon warming a
Mn(TPP) sample under excess NO from 100 to 130 K, where
it is seen that decreases in the bands attributed to cis-ONNO
and 1 are accompanied by the emergence of new bands
centered at 2221 and 1812 cm-1. The former band undoubt-
edly represents the formation of N2O, while the latter band
was assigned to the ν(NO) of the linear nitrosyl in the
nitrosyl-nitrito species Mn(TPP)(NO)(ONO) (2), which was
characterized in an earlier communication.10a IR spectral
Figure 4 shows the temporal progression of the integrated
IR absorbance peak of N2O produced by a sample maintained
at 130 K. The highest yield of nitrous oxide was found by
keeping the sample at 130 K from 2 to 3 h. Spontaneous
warming of the sample from 80 K to room temperature gives
decreased yields of N2O. The formation of N2O was
confirmed by gas chromatography. Quantitative analysis of
the reaction gas revealed formation of 0.4-0.7 equiv of N2O
(17) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138-163. (b) Gouterman,
M. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978;
1-166. (c) Boucher, L. J. Coord. Chem. ReV. 1972, 7, 289-311. (d)
Williamson, M. M.; Hill, C. L. Inorg. Chem. 1986, 25, 4668-4671.
(e) Arasasingham, R. D.; Bruice, R. D. Inorg. Chem. 1990, 29, 1422-
1426.
4082 Inorganic Chemistry, Vol. 45, No. 10, 2006

Low-Temperature NO Disproportionation
Figure 4. Production of N2O (integrated IR absorbances of the band at
2221 cm-1) depending on the time that the sample was maintained at 130
K. The N2O produced when the cryostat spontaneously warmed to RT is
denoted by triangle.
Figure 5. Changes in the IR spectrum of 2 upon the increase of T from
130 K to RT. (PNO ) 5 Torr).
per manganese, depending on the reaction conditions.
However, such analysis revealed negligible N2O formation
when the same system was studied at room temperature.
Isotope Exchange Experiments. We also studied the
reaction of Mn(15NO)(TPP) with an excess of 14NO at 77-
130 K. Upon exposure to excess 14NO, the ν(NO) band of
Mn(15NO)(TPP) at 1707 cm-1 decreased rapidly and, finally,
almost completely disappeared, and according to the IR
spectra, the major products were Mn(TPP)(14NO)(O14NO)
and 14N2O. Thus, the 15NO/14NO exchange on the metal
center must be much faster than the subsequent dispropor-
tionation. Furthermore, although the nitrosyl complex Mn(T-
PP)(NO) is labile in room-temperature solutions,18 in sub-
limed layers, it does not lose NO under long-term continuous
evacuation, even when the sample was heated to ∼350 K.
In this context, an NO exchange mechanism dependent upon
the presence of excess NO must be in effect under these
low T conditions.
Such a mechanism might be functioning via the formation
of a dinitrosyl intermediate Mn(Por)(NO)2 analogous to those
reported for Ru- and Fe-porphyrins.19a,b To test this
hypothesis, the reaction of Mn(TPP) with a 1:1 mixture of
15NO and 14NO was examined at low-temperature (77-100
K). It was anticipated that under these conditions a dinitrosyl
complex Mn(TPP)(NO)2 would demonstrate ν(NO) bands20
because of the formation of Mn(TPP)(14NO)2, Mn(TPP)-
(15NO)2, and Mn(TPP)(14NO)(15NO). Unfortunately, interfer-
ence from the broad bands of the mixed (NO)2 dimers
prevented this assessment. On the other hand, the IR
spectrum from this experiment displayed four bands of equal
intensity in the nitrous oxide region at 2221 (14N2O), 2199
(14N15NO), 2175 (15N14NO), and 2152 (15N2O) cm-1,21
consistent with complete scrambling of the nitrogen labels
in this disproportionation product (Figure S2).
Figure 6. Changes in the UV-vis spectra of the NO/Mn(TPP) system as
the temperature was increased from 200 K to RT.
5. All bands attributed to Mn(TPP)(NO)(ONO) diminished
in intensity and finally disappeared, while new bands
developed at 1448 cm-1 (1425 cm-1 for the reaction with
15NO) and ∼1000 cm -1. This spectral behavior is consistent
with the loss of NO from 2 upon warming of the sample to
give the previously characterized22 nitrito complex Mn(TPP)-
(ONO) (4), which displays IR bands at 1444 and 1029 cm-1
assigned to the ν(NdO) and ν(N-O) of the coordinated
nitrito ligand. However, the latter band is not well resolved
in the spectrum of our amorphous sample. The assignment
of the final product as Mn(TPP)(ONO) was strengthened by
reacting the Mn(TPP) layers with very low concentrations
of NO2 at RT. The result was an FT-IR spectrum identical
to that obtained after the products of the low-temperature
reaction of Mn(TPP) with excess NO were warmed to room
temperature.
The changes in the optical spectrum are in agreement with
this conclusion. The UV-vis spectrum displayed, upon
warming of 2 to room temperature, new bands at ∼385, 476,
583, and 620 nm (Figure 6), consistent with those of
Mn(TPP)(ONO).22 The remaining absorbance at 540 nm is
the result of the incomplete conversion of Mn(TPP)(NO) into
2 during the low T reaction; this feature is also shown by
the small 1740 cm-1 band seen in the IR spectra after the
sample reached RT.
Further Reactivity. The FTIR spectral changes during
further warming of 2 from 130 K to RT are shown in Figure
(18) Zavarine, I. S.; Kini, A. D.; Morimoto, B. H.; Kubiak, C. P. J. Phys.
Chem. B 1998, 102, 7287-7292.
(19) Lorkovic, I.; Ford, P. C. Inorg. Chem. 1999, 38, 1467-1473. (b)
Lorkovic, I.; Ford, P. C. J. Am. Chem. Soc. 2000, 122, 6516-6517.
(c) Kurtikyan, T. S.; Gulyan, G. M. Ford, P. C. Book of Abstracts;
XVI All-Russian Symposium on Modern Chemical Physics, Tuapse,
Russia; Institute of Chemical Physics: Moscow, 2004; p 179.
(20) Andrews, L.; Citra, A. Chem. ReV. 2002, 102, 885-911.
(21) Lapinski, A.; Spanget-Larsen, J.; Waluk, J.; Radziszewski, J. J. Chem.
Phys. 2001, 115, 1757-1764.
The conversion of 2 to 4 described above might be
accomplished by the simple dissociation of NO from
(22) Suslick, K. S.; Watson, R. A. Inorg. Chem. 1991, 30, 912-919.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4083

Martirosyan et al.
Figure 7. FT-IR spectra when 2 was warmed from 130 to 200 K: (top)
Mn(TPPd20)(14NO)(O14NO) and (bottom) same process for Mn(TPPd20)-
(15NO)(O15NO) (ν1 vibration of N2O is denoted by *).
Figure 8. FT-IR spectral changes observed when 2 was warmed from
200 K to RT: (top) Mn(TPPd20)(14NO)(O14NO) and (bottom) Mn(TPPd20)-
(15NO)(O15NO).
Mn(TPP)(NO)(ONO) (eq 2). However, careful inspection of
the temporal IR spectra upon warming of 2 revealed the
formation of another transient species.
O-coordinated nitrito ligand of 2 to give an N-bonded nitro
complex Mn(TPP)(NO)(NO2) (3) (eq 3). Similar nitrito-
nitro isomerizations are known for pentaammine and ethyl-
enediamine cobalt complexes in the solid state24 and proposed
in the recombination processes after photodissociation of NO2
from Co(TPP)(NO2) in solution.25 An analogous isomeriza-
tion has been recently documented for Fe (Por)(NO)(ONO)
complexes.26 A nitrosyl nitro complex analogous to 3 is the
iron(III) species Fe(TPP)(NO)(NO2), which displays IR
bands for the asymmetric and symmetric modes of coordi-
nated NO2 at 1450 and 1300 cm-1,27a close to the positions
of the bands attributed to 3. Other six-coordinate porphyrin
nitro complexes show similar IR bands. For example, in
sublimed films, those for Co(TPP)(NO2) are located at 1468
and 1280 cm-1, while the addition of piperidine or pyridine
(to give six-coordinate complexes) shifts these bands to 1437
and 1308 cm-1 or 1439 and 1310 cm-1, respectively.28
Mn(Por)(NO)(ONO) Mn(Por)(ONO) + NO
f
(2)
(2)
(4)
Figures 7 and 8 show the temporal IR spectral changes in
the region corresponding to ONO- vibrations, upon warming
of the sample from 130 K to RT. As can be seen from Figure
7, warming of 2 from 130 to 200 K led to a gradual decrease
in the intensity of the bands at 1481 and 970 cm-1 (1454
and 950 cm-1 for 15NO) (the 970 band is not shown).
Concomitantly, two new bands, correlated in intensity,
appeared at 1421 and 1304 cm-1 (1392 and 1286 cm-1 for
15NO), reaching their maximal intensities at 200 K. This
conversion was irreversible; cooling of the sample did not
lead to an increase of the intensities of the bands of 2.
Spectral changes observed upon further warming of the
sample from 200 K to RT are shown in Figure 8. The new
bands at 1421 and 1304 cm-1,23 together with bands assigned
to the nitrito vibrations of 2, disappeared, while two other
new bands appeared at 1448 cm-1 (1425 cm-1 for 15NO)
and in vicinity of 1000 cm-1. These bands and a small one
at 1740 cm1 indicate that some Mn(TPP)(NO) persisted after
the sample reached RT.
Mn(Por)(NO)(NO )
Mn(Por)(NO)(ONO)
f
(3)
2
(2)
(3)
The apparent isomerization of 2 to 3 is reflected in a shift
of ν(NO) to ∼1805 cm-1 (1770 cm-1for 15NO), as the bands
(24) Grente, I.; Nordin, E. Inorg. Chem. 1979, 18, 1109-1116. (b) Grente,
I.; Nordin, E. Inorg. Chem. 1979, 18, 1869-1874.
(25) Seki, H.; Okada, K.; Iimura, Y.; Hoshino, M. J. Phys. Chem. A 1997,
101, 8174-8178.
The appearance of the transient IR bands at 1421 and 1304
cm-1 is consistent with the linkage isomerization of the
(26) Kurtikyan, T. S.; Ford, P. C. Angew. Chem., Int. Ed. 2006, 45, 92-
96.
(23) The band at 1304 cm-1 appears as a shoulder of porphyrin band at
1300 cm-1, the intensity of which (as well as the intensities of few
other porphyrin bands 1600, 1233, 1204 cm-1) shows temperature
dependence. The same behavior of the 1300 cm-1 band (more intensive
at lower temperatures) for 15NO can be seen in Figures 7 and 8.
(27) Yoshimura, T. Inorg. Chem. Acta 1984, 83, 17-21. (b) Kurtikyan, T.
S.; Martirosyan, G. G.; Lorkovic, I.; Ford, P. C. J. Am. Chem. Soc.
2002, 124, 1012-1017.
(28) Stepanyan, T. H.; Akopyan, M. E.; Kurtikyan, T. S. Russ. J. Coord.
Chem. 2000, 26, 425-428.
4084 Inorganic Chemistry, Vol. 45, No. 10, 2006

Low-Temperature NO Disproportionation
at 1421 and 1304 cm-1 (1392 and 1286 cm-1 for 15NO) reach
their maxima. Although the exact ν(NO) position might be
affected by overlap with the ν(NO) band of 2 still present
in sample, the shift to lower frequency is in agreement with
the trend reported for Fe(TPP) complexes. The Fe(TPP)-
(NO)(ONO) generated by UV photolysis at low temperature
displayed a higher frequency ν(NO) band than did the Fe-
(TPP)(NO)(NO2) precursor.29
Assignment of 3 as Mn(TPP)(NO)(NO2) is further sup-
ported by experiments where increments of O2 were delib-
erately added into a cryostat containing Mn(TPP)(NO)
prepared under excess NO at 200 K (Figure S3).30 The
incremental O2 resulted in decreased intensity of the nitrosyl
band and the appearance of small absorbances at 1860, 1590,
and 1295 cm-1, characteristic of N2O3.31 The sample was
kept at 200 K for a several hours which led to complete
consumption of N2O3 and the appearance of three new bands
at 1806, 1422, and 1304 cm-1 (1771, 1392 and 1286 cm-1
for 15NO), indicating the formation of 3. Warming of the
sample to room temperature shows conversion of 3 into
Mn(TPP)(ONO). However, we were unable to obtain a pure
material because of the incomplete conversion of Mn(TPP)-
(NO) into Mn(TPP)(NO)(NO2). The addition of more O2
ultimately results in the complete disappearance of the
nitrosyl band and formation of a nitrate complex with IR
bands at 1470 and 1284 cm-1.22
warmed from 130 to 200 K during continuous high-vacuum
evacuation and the IR spectrum monitored. Spontaneous NO2
dissociation from 2 should result in the appearance of
Mn(TPP)(NO), but this was not seen. Instead, the IR spectral
changes were consistent with the intramolecular transforma-
tion of 2 to 3. The stable nitrito product Mn(TPP)(ONO)
(4) is formed upon further warming to RT. If an Mn(III)
nitro species were the result of NO dissociation from 3, this
would initially give the pentacoordinate nitro intermediate
Mn(TPP)(NO2) which would undergo linkage isomerization
to the more stable nitrito form. The same intermediate has
been invoked in solution-phase flash photolysis of Mn(TPP)-
(ONO) under excess NO where time-resolved UV-vis spec-
troscopy detected a species thought to be Mn(TPP)(NO2),
which underwent unimolecular isomerization to 4 (kisom
)
14 s-1 in 298 K toluene).32 Notably, the loss of NO from 2
(or 3) appears to be irreversible, since recooling of the Mn-
(TPP)(ONO) sample under excess NO does not reform 2.
2 f Mn(Por)(NO) + NO2 (N2O3 under excess NO) f 3 (5)
Some additional information can be also drawn from
Figures 7 and 8. Earlier studies established that several IR
and Raman bands for Fe(TPP) axial complexes are sensitive
to the spin and oxidation states of iron center.33 The bands
in the ranges of 1350-1330 cm-1 (ν(Ca-Cm) mixed with
ν(Cm-phenyl)) and 469-432 cm-1 (porphyrin core defor-
mation mode) lie at higher frequencies in low-spin com-
plexes. Similar behavior of these bands was noted in studies
of dioxygen and nitrate complexes of Mn(TPP).34 For MnII-
(TPPd20), these bands lie at 1326 and 422 cm-1. Formation
of the nitrosyl complex Mn(TPPd20)(NO) is accompanied by
transition from the high- to low-spin state,16a and this is
accompanied by shifts of these bands to 1336 and 450 cm-1,
respectively. No further shifts of these bands were observed
upon generation of 2, indicating that the Mn center remains
in the low-spin state, in agreement with other 6-coordinate
manganese nitrosyl porphyrinato complexes.16a These bands
do not shift further when the samples are warmed from 130
to 200 K, thus 3 is also likely to be in a low-spin state.
However, upon formation of the nitrito complex 4, the band
at 1336 cm-1 shifts back to 1326 cm-1 (Figure 8), suggesting
that this species is high spin as has been reported for the
nitrato analogue Mn(TPP)(ONO2).22
The above results clearly suggest that the transformation
of 2 to 4 upon warming of samples of 2 from 130 K to room
temperature may be more complicated than the simple NO
dissociation depicted in eq 2. The nitrosyl nitro species 3 is
formed as a transient species, and it is possible that at least
part of the 2 to 4 transformation occurs via the sequence 2
f 3 f 4 (eq 4). Alternatively, the formation of 3 may be a
“dead-end” equilibrium.
The formation of 3 from 2 might have several routes, one
being an intramolecular nitrito- to nitro- linkage isomeriza-
tion, the other being NO2 dissociation from 2 to give
Mn(Por)(NO) followed by reaction of the latter with NO2
or with N2O3 under excess NO (the typical experimental
condition) to give 3. The following experiments were
initiated to test the likely mechanism. A sample of Mn(TPP)-
(NO)(ONO) (2) was evacuated at 130 K; then 15NO was
introduced into the cryostat, and that sample was allowed to
warm to RT. If eq 5 were operational, Mn(TPP)(O15NO)
should be formed as one of the final products; however, none
was detected. In a separate experiment, a sample of 2 was
Possible Disproportionation Mechanisms. There are
several key observations that need to be addressed when
speculating about prospective mechanisms for NO dispro-
portionation.
(a) At low T, intermediate 1 is formed upon exposure of
Mn(Por) to NO prior to formation of N2O, and it displays
(32) Hoshino, M.; Nagashima, Y.; Seki, H.; DeLeo, M.; Ford, P. C. Inorg.
Chem. 1998, 37, 2464-2469.
(33) Oshio, H.; Ama, T.; Watanabe, T.; Kincaid, J.; Nakamoto, K.
Spectrochim. Acta A 1984, 40, 863-870. (b) Chottard G.; Battioni,
P.; Battioni, J- P.; Lange, M.; Mansuy, D. Inorg. Chem. 1982, 20,
1718-1722. (c) Stong, J. D.; Spiro, T. G.; Kubaska, R. J.; Shupak, S.
I. J. Raman Spectrosc. 1980, 9, 312-318.
(34) Kurtikyan, T. S.; Stepanyan, T. H.; Martirosyan, G. G.; Kazaryan, R.
K.; Madakyan, V. N. Russ. J. Coord. Chem. 2000, 26, 345-348. (b)
Kurtikyan, T. S.; Stepanyan, T. G.; Martirosyan, G. G.; Kazaryan, R.
K.; Madakyan, V. N. Russ. Chem. Bull. 2000, 49, 1540-1543.
(29) Lee, J.; Kovalevsky, A. Y.; Novozhilova, I.; Bagley, K.; Coppens,
P.; Richter-Addo, G. B. J. Am. Chem. Soc. 2004, 126-127.
(30) Exposure of the Mn(TPP) to excess NO at room temperature followed
by slow cooling to 200 K gave an IR spectrum (ν(NO) ∼1740 cm-1
)
showing only the formation of Mn(TPP)(NO).
(31) Laane, J.; Ohlsen, J. R. Prog. Inorg. Chem. 1986, 28, 465-469.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4085

Martirosyan et al.
Scheme 1. Hypothetical O-Atom Transfer from Dinitrogen Dioxide
to Coordinated NO
Scheme 2. Possible Geometries of O-Coordinated Dinitrogen Dioxide
isotope sensitive IR bands at 1820 and 1712 cm-1. The
formation of 1 is somewhat reversible, since evacuating the
system not only removes the NO dimers present at these
temperatures but also depletes 1.
1020 cm-1 (IR).39 In the present study, we used different
porphyrins to free the spectral ranges where hyponitrito bands
might be expected but did not detect isotope sensitive bands
in these regions. As discussed below, a complex of dinitrogen
dioxide is a likely candidate for 1, but the IR spectra suggest
the extent of charge transfer from the metal is too small to
consider the resulting ligand to be a hyponitrite anion.
Intermediate 1 appears prior to N2O formation and shows
isotope-sensitive IR bands at 1820 and 1712 cm-1 in the
region associated with terminal nitrosyls.40 For a dinitrosyl
complex, the competition between the two π-acceptor ligands
would shift the ν(NO) frequencies to higher values relative
to the mononitrosyl analogue Mn(TPP)(NO) (1740 cm-1).
For example, Fe(TPP)(NO) shows ν(NO) at 1681 cm-1 in
CHCl3, while cooling to 213 K resulted in the appearance
of a new band at 1695 cm-1 with twice the intensity and
another much weaker band at 1776 cm-1 attributed to
Fe(TPP)(NO)2.19b A similar spectrum (1692 s and 1772 w
cm-1) was obtained in solid-state conditions at low T.19c The
pattern for 1 is different, so it seems unlikely that this species
is a dinitrosyl complex.
A plausible alternative for 1 would be the Mn(II) complex
of a nearly neutral dinitrogen dioxide ligand, since 1 is
generally observed under conditions where the nitric oxide
dimers are also seen. Modest charge transfer from the
electron-rich metal center to the π* orbitals of ONNO should
result in lowered frequencies for the NO stretches of that
ligand.41 However, since the electronic spectrum of this
complex appears to be that of a Mn(II) porphyrin complex,
we would argue that such charge transfer is much less than
implied by designating this ligand as hyponitrite. Further-
more, 1 might be a six-coordinate nitrosyl adduct of the type
trans-Mn(Por)(NO)(ONNO). In analogy, an EPR study of
the reaction of NO with Co(TPP)(NO) in toluene solution
has been interpreted in terms of the initially formed Co(TPP)-
(NO)2 converting to Co(TPP)(NO)(N2O2) upon an increase
in NO pressure.42 As noted in point c, the product formed
initially is the O-bonded nitrito complex Mn(Por)(NO)-
(ONO), and since nitro to nitrito isomerization does not
appear to be facile under these conditions, it seems likely
that the precursor to 2 is also an O-coordinated adduct.
Scheme 2 displays some possible structures of O-coordinated
dinitrogen dioxide complexes.
(b) At room temperature, sublimed layers of Mn(Por) form
Mn(Por)(NO) (ν(NO) ) 1740 cm-1) when exposed to NO
but do not promote NO disproportionation. No evidence of
the nitrito complex was observed in the FT-IR or UV-vis
spectra of Mn(Por) exposed to excess NO at RT.
(c) The nitrito nitrosyl complex Mn(Por)(NO)(ONO) (2)
undergoes linkage isomerization to give the nitro complex
Mn(Por)(NO)(NO2) (3) as T is increased, but the reverse was
not seen. This suggests that the disproportionation mechanism
must lead directly to the O-bound nitrito complex, since 2
is formed concurrently with the appearance of N2O.
The key step for NO disproportionation is the N-N
coupling to form N2O. Various scenarios can be envisioned.
One might involve oxygen atom transfer from the dinitrogen
dioxide dimer to a coordinated NO as illustrated in Scheme
1 and as suggested in solution studies for the reaction of
NO with Ru(Por)((CO).35 However, such a pathway would
immediately generate an N-coordinated nitro complex. If
this were 5-coordinate, isomerization to the pentaco-
ordinate nitrito complex 4 would be favorable, but the rate
should be slow at low T, and a nitro intermediate would be
observed. Furthermore, point c discounts the possibility of
the direct formation of 3 at low T. Nonetheless, the unusual
feature of the disproportionation being observable at low T
but not at RT argues for a key species having a negative
enthalpy but a very positive entropy of formation. This would
be a likely characteristic of intermediates involving two or
more moles of NO gas, such as the NO dimer or a complex
thereof.
In this context, the metal center is a possible template for
the formation of a hyponitrite complex, M(N(O)NO). An
intermediate might be the dinitrosyl species cis-Mn(Por)-
(NO)2, although the hyponitrito ligand could also be formed
by direct attack of free NO on a coordinated NO. A
hyponitrito complex that has been rigorously established is
4+ 36
[Co(NH3)5(N2O2)]2
, and this displays IR bands at 1136,
1046, and 932 cm-1.37 Another, Pt(N2O2)(PPh3)2, is reported
to show strong bands at 1285 and 1240 cm-1.38 In sodium
hyponitrite, where the N2O22- anion has a trans configuration,
the corresponding bands occur at 1383, 1115 (Raman), and
(35) Ford, P. C.; Lorkovic, I. Chem. ReV. 2002, 102, 993-1017.
(36) Hoskins, B. F.; Whillams, F. D.; Dale, D. H.; Hodkin, D. C. Chem.
Commun. 1969, 69-70.
(39) Millen, D. J; Polydoropoulos, C. N.; Watson, D. J. Chem. Soc. 1960,
687-692.
(40) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 3rd ed.; Willey: New York, 1978.
(41) Snis, A.; Panas, I. Chem. Phys. 1997, 221, 1-10.
(42) Wayland, B. B.; Minkiewicz, J. V. J. Chem. Soc., Chem. Commun.
1976, 1015-1016.
(37) Mercer, E. E.; McAllister, W. A.; Durig, J. R. Inorg. Chem. 1967, 6,
1816-1822.
(38) Bhaduri, S.; Johnson, B. F.; Pickard, A.; Raithby, P. R.; Sheldrick,
G. M.; Zuccaro, C. I. J. Chem. Soc., Chem. Commun. 1977, 354-
355.
4086 Inorganic Chemistry, Vol. 45, No. 10, 2006

Low-Temperature NO Disproportionation
Scheme 3. Prospective Sequence of Reactions in the Course of the Disproportionation of NO by Mn(Por) to Give N2O and Mn(Por)(ONO) in Low-
Temperature Sublimed Layers
Six-coordinate complexes, as shown in Scheme 2, would
be expected to display an ν(NO) band for the coordinated
nitrosyl. As noted above, the overlap of other absorbances
with this (NO)2 band is a likely reason for the unusually
high-intensity ratio of asymmetric/symmetric bands for the
NO dimers in the present experiments. Among these, III
would appear to be the more consistent with the IR bands
attributed to 1 given that peak separation and relative
intensities are similar to those of the cis-ONNO dimer. Such
a coordination mode, however, might be expected to pull
the Mn away from the mean porphyrin plane toward the
(NO)2 ligand, weakening the Mn-NO bond and thereby
changing the ν(NO) frequency. However, in contrast to the
other metal porphyrin systems, the addition of trans ligand
to the Mn(II) porphyrin nitrosyls does not significantly
changes the ν(NO) frequency (1).16c,d
Scheme 3 is a proposed mechanism for the transformations
taking place in the course of the interaction of excess NO
with sublimed layers of Mn(Por) at low T. Intermediate 1 is
believed to be responsible for the assembly of 3 equiv of
NO: two as the ONNO ligand and one as a nitrosyl ligand.
The formation of the nitrito nitrosyl complex 2 is thought to
be the result of the attack of free NO on the O atom of the
coordinated dimer with concomitant formation of N2O. This
would satisfy the apparent requirement that a metastable
O-coordinated nitrito product, 2, is initially formed in the
disproportionation sequence, but this is quite speculative. The
data do clearly show that as the temperatures of solids
containing 2 are slowly increased, linkage isomerization
occurs to give the nitro isomer 3. Further temperature
increases lead to NO loss, giving the pentacoordinate nitrito
complex Mn(TPP)(ONO) (4); however, it is not clear
whether this occurs directly from 2 or via the intermediacy
of 3.
Summary
The sublimed layers of manganese(II) porphyrins display
interesting reactivity with NO. The room temperature reaction
of Mn(Por) with excess NO leads only to the known nitrosyl
complex Mn(Por)NO; however, when the same reaction is
executed at low temperature, NO disproportionation occurs.
We have used in situ FTIR and optical spectroscopy to
monitor these transformations from 77 K to RT. A reaction
intermediate spectrally characterized at 90 K is thought to
be an O-coordinated dinitrogen dioxide complex Mn(Por)-
(NO)(ONNO) (1). Further reaction with NO shows evolution
of N2O and formation of the nitrosyl nitrito complex
Mn(Por)(NO)(ONO) (2), which is stable at 130 K. The latter
undergoes linkage isomerization into the N-bounded nitrosyl
nitro form Mn(TPP)(NO)(NO2) when the sample warms from
130 to 200 K, but the reverse process was not seen. When
the system was warmed further (to room temperature), the
known nitrito complex Mn(TPP)(ONO) is the final manga-
nese product.
Acknowledgment. Financial support from CRDF (Grant
AC2-2520-TB-03) is gratefully acknowledged. P.C.F. also
acknowledges a grant (DE-FG02-04ER15506) from the
Chemical Sciences Division, Office of Basic Energy Sci-
ences, U.S. Department of Energy.
Supporting Information Available: Figures showing the IR
spectra of the formation of 2, the IR spectra of a sample with an
excess of a 1:1 mixture of 15NO/14NO, and the FT-IR spectra of
Mn(TPP)(NO)(NO2) derivatives in the sublimed layers. This
material is available free of charge via the Internet at
http://pubs.acs.org.
IC051824Q
Inorganic Chemistry, Vol. 45, No. 10, 2006 4087