Tracking reactive intermediates by FTIR monitoring of reactions in low-temperature sublimed solids: Nitric oxide disproportionation mediated by ruthenium(II) carbonyl porphyrin Ru(TPP)(CO)

Article abstract of DOI:10.1021/ic400102q

Interaction of NO (¹⁵NO) with amorphous layers of Ru(II) carbonyl porphyrin (Ru(TPP)(CO), TPP^2- = meso-tetraphenylporphyrinato dianion) was monitored by FTIR spectroscopy from 80 K to room temperature. An intermediate spectrally char

Full text of DOI:10.1021/ic400102q

Article
pubs.acs.org/IC
Tracking Reactive Intermediates by FTIR Monitoring of Reactions in
Low-Temperature Sublimed Solids: Nitric Oxide Disproportionation
Mediated by Ruthenium(II) Carbonyl Porphyrin Ru(TPP)(CO)
†
,†
†
,‡
Arsen S. Azizyan, Tigran S. Kurtikyan,* Garik G. Martirosyan, and Peter C. Ford*
†Molecule Structure Research Center (MSRC) of the Scientific and Technological Centre of Organic and Pharmaceutical Chemistry,
National Academy of Sciences, 0014,Yerevan, Armenia
‡
Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106-9510, United
States
*
S Supporting Information
1
5
ABSTRACT: Interaction of NO ( NO) with amorphous layers of Ru(II) carbonyl
2
‑
porphyrin (Ru(TPP)(CO), TPP = meso-tetraphenylporphyrinato dianion) was
monitored by FTIR spectroscopy from 80 K to room temperature. An intermediate
−
1
spectrally characterized at very low temperatures (110 K) with ν(CO) at 2001 cm
−1
−1
15
and ν(NO) at 1810 cm (1777 cm for NO isotopomer) was readily assigned to
the mixed carbonyl−nitrosyl complex Ru(TPP)(CO)(NO), which is the logical
precursor to CO labilization. Remarkably, Ru(TPP)-mediated disproportionation of
NO is seen even at 110 K, an indication of how facile this reaction is. By varying the
quantity of supplied NO, it was also demonstrated that the key intermediate
responsible for NO disproportionation is the dinitrosyl complex Ru(TPP)(NO)2,
supporting the conclusion previously made from solution experiments.
the second stage was very slow owing to the [NO]2
1
. INTRODUCTION
dependence) and with modest concentrations of added CO
(0.8−2.5 mM), it was possible to show that the first stage leads
to reversible formation of a dinitrosyl intermediate (C, eq 2)
Ruthenium(II) carbonyl porphyrin complexes Ru(Por)(CO)
1, Por2− = meso-tetraphenyl (TPP)-, meso-tetra(4-tolyl)
(
(
TTP)-, or octaethyl (OEP)- porphyrinato dianions) have
5
4
with equilibrium constants (K ) of 2.0 × 10 and 6.3 × 10 for
2
been shown to react with NO to give the corresponding O-
nitrito nitrosyl complexes Ru(Por)(NO)(ONO) (2) both in
2
Por = TTP and OEP, respectively. The dinitrosyl complex C
1
,2
3
was subsequently characterized by infrared spectroscopy and
solution and in thin porous layers of solid Ru(Por)(CO).
6,7
computation.
Since stoichiometric quantities of nitrous oxide are also formed,
this reaction is the result of NO disproportionation mediated
by the ruthenium carbonyl porphyrin (e.g., eq 1). Similar
Ru(Por)(CO) + 2NO ⇌ Ru(Por)(NO) + CO
2
(C)
(2)
4
reactivity has been seen for the osmium analog and for other
5
metal centers.
The proposed pathway to formation of 2 from 1 is illustrated
in Scheme 1. The first step would be reversible addition of one
NO to give the carbonyl nitrosyl intermediate A. Carbon
monoxide dissociation from A (the steady state concentration
of which is first order in [NO] at low [NO]) to give the
mononitrosyl species Ru(Por)(NO) (B) would be rate limiting
2
under these conditions. Flash photolysis studies have shown
8
−1 −1
8
that B is rapidly trapped by NO to form C (k ≈ 10 M s ).
C
As a result, in the absence of added CO, reaction of 1 with
excess NO to give C is first order in [NO].
II
The kinetics of the Ru (Por)-mediated NO disproportiona-
However, although logic clearly suggests the intermediacy of
A, this nitrosyl carbonyl complex has not previously been
directly observed. Here we use the sublimed layers method-
tion were studied in dry, deaerated toluene solution at 298 K
using a stopped-flow spectrophotometer. With excess NO two
II
stages were observed, each being first order in [Ru (Por)]
9
,10
Por2 = OEP or TTP ): an initial rapid reaction to give an
−
2−
2−
2
ology
to identify and characterize this species using FTIR
(
observable intermediate with a rate that was first order in [NO]
and a slower transformation to 2 that was second order in
Received: January 15, 2013
[
NO]. By probing the reaction at relatively low [NO] (where
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic400102q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Proposed Mechanism for Reaction of Excess NO
with Ru(Por)(CO) in 298 K Toluene Solution (ref 2)
2
−
Figure 1. FTIR spectral changes accompanying reaction of NO (4
Torr equivalent) with a sublimed layer of Ru(TPP)(CO) upon
warming from 80 to 110 K.
spectroscopy of low-temperature (80−110 K) solids for Por
2
−
=
TPP . We were also able to demonstrate formation of
dinitrosyl complex C along the reaction coordinate leading to 2.
1
1
temperature. These bands slowly decreased in intensity as the
system was warmed. Simultaneously, the intensity of the
ν(CO) band at 1944 cm− also decreased, and two new bands
EXPERIMENTAL SECTION
■
Ru(TPP)(CO)·EtOH was purchased from Midcentury Chemicals Inc.
1
(
Posen, IL). It was placed in a Knudsen cell and heated to about 470 K
−1
−5
of about equal intensity grew in at 2001 and 1810 cm (Figure
1). When NO was used, the latter band shifted to 1777 cm
under high vacuum (P = 10 Torr) for about 1 h to eliminate solvated
EtOH. Then liquid nitrogen was poured into the cryostat, and
sublimation of 1 with an appropriate rate required heating to about
15
−1
while the position of former slightly shifts to lower frequency to
1999 cm− (Figure 2), as would be anticipated due to a
1
5
50 K. For obtaining layers with thicknesses convenient for IR spectral
studies, sublimation was carried out for about 2 h. For reactions with
nitric oxide, NO was fed to the cryostat from a vessel provided with a
mercury manometer to measure the quantity of this gas. NO and
1
5
NO (with isotopic enrichment 98.5% purchased from Institute of
Isotopes, Republic of Georgia) were purified by passing through a trap
containing KOH pellets immersed in an acetone/dry ice slurry to
remove higher nitrogen oxides and trace quantities of water. Purity was
checked by IR measurements of the layer obtained by slow deposition
of NO on the cooled substrate of the optical cryostat (80 K), which
did not show characteristic IR bands of N O, N O , or H O, thus
2
2
3
2
indicating the absence of these species. (The absence of N O also
2
3
indicates that no NO impurities were present in the NO introduced
2
to the cryostat.) Infrared spectra were recorded with a Nexus (Thermo
−1
Nicolet Corp.) FTIR spectrometer in the range 400−4000 cm with a
−1
spectral resolution of 2 cm .
RESULTS AND DISCUSSION
Thin layers of metal tetra-arylporphyrinates obtained by
sublimation onto a low-temperature (77 K) surface are
■
9
,10
amorphous and have high microporosity.
Thus, the
metalloporphyrins in these layers can readily interact with
volatile reactants, and adducts thus formed can be studied by IR
spectroscopy over a wide temperature range without the
interference of solvent absorptions. In this context, it was
Figure 2. (Solid line) FTIR spectrum at 110 K of a sublimed layer of
Ru(TPP)(CO). (Dashed line) Spectrum after introduction of NO (4
Torr equivalent) into a cryostat at 80 K and warming to 110K.
(Dotted line) Spectrum resulting from the same procedure performed
with 15NO.
5
shown earlier that reaction of excess NO with sublimed layers
of Ru(TPP)(CO) (1) leads to formation of O-nitrito−nitrosyl
complex Ru(TPP)(NO)(ONO) (2) in analogy to solution
experiments.
coupling of the CO and NO stretching vibrations. Analogous
1
5
The IR spectrum of 1 in sublimed layers prepared as
described at 80 K displays an intense band at 1944 cm− that
can be attributed to the carbonyl stretch ν(CO) (Figure 1,
initial spectrum). With the cryostat at 80 K, a small amount of
NO was introduced; the system was slowly warmed, leading to
the spectral changes shown in Figure 1. Introduction of NO led
experiments using a 1/1 mixture of NO and NO are
1
described in the Supporting Information.
These spectral data clearly indicate that the low-temperature
reaction of 1 with NO leads to formation of the carbonyl−
nitrosyl complex Ru(TPP)(CO)(NO) (A, Scheme 1) with the
high-frequency band near 2000 cm− assigned to the carbonyl
1
−
1
−1
to the appearance of IR bands at ∼1760 and 1860 cm
stretch ν(CO) and the lower frequency band near 1800 cm
15
representing the asymmetric and symmetric ν(NO) bands of
cis-ONNO, which forms spontaneously from NO at low
assigned to the nitrosyl stretch ν(NO) {ν( NO)}. As noted in
the Introduction, the existence of such a species at the initial
B
dx.doi.org/10.1021/ic400102q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
stage of reaction was suggested previously but never
confirmed.
Spectra shown in Figures 1 and 2 also display weak bands of
−
1
growing intensities at 1636 and 1845 cm that appear at lower
−
1
15
frequency (∼1600 and 1810 cm ) in experiments with NO
isotopomer. The lower frequency band can be assigned as
ν(NO) of the dinitrosyl complex Ru(TPP)(NO) (C) and the
6
2
other to ν(NO) of Ru(TPP)(NO)(ONO) (2), the product of
eq 1. Thus, it is notable that, even at 110 K, disproportionation
apparently is occurring.
The carbonyl−nitrosyl intermediate A is thermally unstable
and begins to decompose at temperatures above 110 K.
Simultaneously, when excess NO is present, the rate of
disproportionation is apparently markedly increased, as
manifested by the growth of bands at 1845(1810) {ν(NO)},
−
1
1
(
516(1489) {ν(-NO)}, and 928(908) cm {ν(O−N)}
1
5
numbers in parentheses represent data for N isotopomer).
Figure 4. Formation of differently labeled nitrito−nitrosyl complexes
These new bands indicate formation of the nitrito−nitrosyl
15
Ru(TPP)(NO)(ONO) upon interaction of an equimolar NO/ NO
5
−1
complex 2 (Figure 3). Notably, the absorbance at 2220 cm
mixture with Ru(TPP)(CO).
Spectra of the various N and 15N species described in this
article are summarized in Table 1. Notably, the ν(NO)
Table 1. Axial Ligands’ Vibrational Frequencies in
Corresponding Ru(TPP)(L)(L′) Complexes
ν(O
complexes
ν(CO) ν(NO) ν(NO)
N)
T, K
Ru(TPP)(CO)
Ru(TPP)(CO)2
1944
2009
1636
1607
1616
293
200
293
293
293
Ru(TPP)(NO)
2
15
Ru(TPP)( NO)
Ru(TPP)(NO)
2
1
5
(
NO)
Ru(TPP)(CO)
NO)
Ru(TPP)(CO)
2001
1999
1810
1777
1845
1810
110
110
293
293
(
15
(
NO)
Ru(TPP)(NO)
ONO)
1516
1489
929
908
(
Figure 3. FTIR spectral changes demonstrating decomposition of
15
Ru(TPP)( NO)
15
15
15
15
Ru(TPP)(CO)( NO) and formation of Ru(TPP)( NO)(O NO)
(O NO)
upon warming of the layer from 110 to 180 K.
frequency at 1810 cm− for the nitrosyl−carbonyl intermediate
A is in a range that is often seen for linearly coordinated
nitrosyl complexes, which would be certainly unexpected for a
1
2151 cm− for reaction with 15NO) also increases. This is
1
(
II
evidence for formation of N O that is adsorbed in the layer. As
compound with formulation Ru (TPP)(NO)(CO). Linear NO
2
the layers are warmed, this absorbance is maintained until ∼150
coordination is typical of Ru(III) and Fe(III) complexes where
NO coordination leads to charge transfer to the metal center to
K, but on further warming N O passes into the gas phase. This
2
II
+
observation unambiguously confirms that the transformations
of the ruthenium complexes are due to nitric oxide
disproportionation and are not the result of accidental O2
contamination in the course of experiment. We recognize that
give (formally) diamagnetic M (NO ) species. Such a
resonance form would not be expected for A, since charge
transfer to the Ru(II) center would be much less favorable. For
comparison, the ν(NO) frequencies of 6- and 5-coordinate
II
such contamination could generate NO , and reaction of that
nitrosyl complexes of ferrous porphyrin complexes Fe (Por)-
2
II
species with Ru(TPP)(NO) would also form Ru(TPP)(NO)-
(L)(NO) and Fe (Por)(NO) fall in the range 1625−1690
1
2
−1,
(
ONO), as observed for analogous Fe−porphyrins. In
cm and these have Fe−N−O bond angles in the range 137−
14
experiments with excess nitric oxide in the form of a 1:1
148°. The IR spectrum of the analogous dinitrosyl species
15
II
−1
NO/ NO mixture the O-nitrito−nitrosyl complexes with
differently labeled nitrogen in nitrito and nitrosyl moieties are
formed (Figure 4). In this case, also four bands of equal
intensity (not shown) of the differently labeled nitrous oxide
grow in spectra at 2220, 2197, 2174, and 2151 cm that belong
to asymmetrical stretching of N O, NNO, N NO, and
Ru (TPP)(NO) displays only a single ν(NO) at 1642 cm in
2
solution, and computational studies suggest that both Ru−N−
7
O angles are in the range of 130°. Given the relatively weak
interaction between 1 and NO, there may be some analogy
between A and the copper nitrosyl complex [Cu-
(CH NO ) (NO)][PF ] reported by Hayton and co-work-
−1
1
4
15
15
2
3
2 5
6 2
1
5
13
13
−1
N O correspondingly.
2
ers, where ν(NO) was found to be 1933 cm , even higher
C
dx.doi.org/10.1021/ic400102q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
−
1
than that of free NO (1875 cm ), but that the Cu−N−O bond
angle was quite acute (121.0°). Given the very weak
coordination of the NO in that complex, these workers
concluded that the resonance form making the greatest
pathway would appear to be a reasonable mechanism. Unseen
in the earlier solution-phase studies, however, was the mixed
carbonyl−nitrosyl complex Ru(TPP)(CO)(NO) (A), the
logical precursor to CO labilization. Described in the present
study is the use of FTIR spectroscopy to probe the
transformations following reaction of small quantities of NO
with low-temperature sublimed layers of 1 where this unstable
intermediate was readily identified and characterized.
II
contribution had formulation Cu (NO).
For A, the ν(CO) band appears at higher frequency than that
of 1 (Table 1). This suggests that there is some charge transfer
from the metal center to the NO, since this would decrease the
backbonding from metal d orbitals to the π* orbitals of CO.
π
The effect on ν(CO) is less for A than for the dicarbonyl
ASSOCIATED CONTENT
Supporting Information
FTIR spectral changes demonstrating low-temperature (80−
10K) interaction of equimolar NO/ NO mixture with
■
complex Ru(TPP)(CO) , however (Table 1).
2
*
S
For reaction of NO with Ru(Por)(CO) in hydrocarbon
solutions, infrared and optical spectroscopic evidence obtained
by stopped-flow techniques demonstrated ambient temperature
formation of a reactive intermediate concluded to be the
15
1
sublimed layers of Ru(TPP)(CO); comments regarding the
15
equimolar NO/ NO mixture interaction with layered Ru-
2
dinitrosyl complex Ru(Por)(NO)2. This conclusion is
(TPP)(CO). This material is available free of charge via the
Internet at http://pubs.acs.org.
completely supported by the studies reported here for reaction
in the layered solid. At low NO pressure when the quantity of
NO has not been enough to effectively promote NO
disproportionation, the dashed FTIR spectra demonstrated in
Figure 5 was obtained. It contains an intense band at 1636
AUTHOR INFORMATION
Corresponding Author
E-mail: kurto@netsys.am (T.S.K.); ford@chem.ucsb.edu
P.C.F.).
■
*
(
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the SCS RA (Project # 11-1d052)
and by the Chemistry Division of the U.S. National Science
Foundation (Grant # CHE-1058794).
REFERENCES
■
(
1) (a) Kadish, K. M.; Adamian, V. A.; Caemelbecke, E. V.; Tan, Zh.;
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.
́
(
5
c) Bohle, D. S.; Hung, Ch.-H.; Smith, B. D. Inorg. Chem. 1998, 37,
798−5806.
2) Lorkovic,
3) Kurtikyan, T. S.; Martirosyan, G. G.; Lorkovic,
J. Am. Chem. Soc. 2002, 124, 10124−10129.
4) Leal, F. A.; Lorkovic, I. M.; Ford, P. C.; Lee, J.; Chen, L.; Torres,
Figure 5. (Solid line) FTIR spectra of layered RuTPP(CO). (Dashed
line) After storage under NO (0.7 Torr) overnight. (Dotted line) After
additional overnight storage under NO at 4 Torr of NO.
(
(
́
I. M.; Ford, P. C. Inorg. Chem. 1999, 38, 1467−1473.
I. M.; Ford, P. C.
́
(
cm− that has its analogue at 1642 cm−1 in solution and should
1
L.; Khan, M. A.; Richter-Addo, G. B. Can. J. Chem. 2003, 81, 872−888.
(5) For examples, see: (a) Gargano, M.; Giannoccaro, P.; Rossi, M.;
Sacco, A.; Vasapollo, O. G. Gazz. Chim. Ital. 1975, 105, 1279−1290.
be unequivocally assigned to dinitrosyl complex Ru(TPP)-
NO) (C). In the solid state, this species is fairly stable at
room temperature (RT), in contrast to Fe(TPP)(NO) , which
loses its sixth ligand upon warming to RT both in solution
and in amorphous solid. Addition of the new NO portion into
the cryostat at room temperature slowly converts dinitrosyl
complex into the O-nitrito−nitrosyl species 2 (Figure 5, dotted
line). This observation is consistent with the conclusion of a
kinetic study demonstrating that the dinitrosyl complex
Ru(TPP)(NO) is the key intermediate responsible for NO
disproportionation.
In summary, NO reaction with amorphous layers of
Ru(TPP)(CO) (1) follows the pattern seen in solution. The
dinitrosyl intermediate C is formed, and this undergoes further
reaction with excess NO to give the nitrosyl nitrito complex 2.
Remarkably, Ru(TPP)-mediated disproportionation of NO is
seen even at 110 K, an indication of how facile this reaction is.
In the solution-phase study, it was suggested that oxidation of
C to 2 may occur by O-atom transfer from N O formed by
NO dimerization. Since N O formation is favored by low
(
2
(b) Ruggiero, C. E.; Carrier, S. M.; Tolman, W. B. Angew. Chem., Int.
2
14
Ed. 1994, 33, 895−897. (c) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc.
15
1998, 120, 9034−9040.
(
(
6) Lorkovic’, I. M.; Ford, P. C. Chem. Commun. 1999, 1225−1226.
7) Patterson, J. C.; Lorkovic, I. M.; Ford, P. C. Inorg. Chem. 2003,
4
(
2, 4902−4908.
8) Lorkovic, I. M.; Miranda, K. M.; Lee, B.; Bernhard, S.;
Schoonover, J.; Ford, P. C. J. Am. Chem. Soc. 1998, 120, 11674−11683.
(9) Kurtikyan, T. S.; Gasparyan, A. V.; Martirosyan, G. G.;
Zhamkochyan, G. A. J. Appl. Spectrosc. 1995, 62, 62−66 (Russ).
(10) Kurtikyan, T. S.; Ford, P. C. Coord. Chem. Rev. 2008, 252,
2
2
1
(
(
486−1496.
11) (a) 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.
1
998, 109, 4378−4383. (c) Kometer, R.; Legay, F.; Legay-Sommire,
N.; Schwentner, N. J. Chem. Phys. 1994, 100, 8737−4587. (d) Nour, E.
M.; Chen, L. H.; Strube, M. M.; Laane, J. J. Phys. Chem. 1984, 88,
2
7
56−763.
2
2
(12) Lorkovic’, I. M.; Ford, P. C. Inorg. Chem. 2000, 39, 632−633.
2
2
(13) Lapinsky, A.; Spanget-Larsen, J.; Waluk, J.; Radziszewski, J. J.
temperature and is seen in the present studies (see above), this
Chem. Phys. 2001, 115, 1757−1764.
D
dx.doi.org/10.1021/ic400102q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
1
(
1
(
6
(
14) Wyllie, G. R. A.; Scheidt, W. R. Chem. Rev. 2002, 102, 1067−
089.
15) Wright, A. M; Wu, G.; Hayton, T. W. J. Am. Chem. Soc. 2010,
32, 14336−14337.
16) Lorkovic’, I. M.; Ford, P. C. J. Am. Chem. Soc. 2000, 122, 6516−
517.
17) Azizyan, A. S. Chem. J. Arm. 2009, 62, 11−17 (Russ).
E
dx.doi.org/10.1021/ic400102q | Inorg. Chem. XXXX, XXX, XXX−XXX