Redox properties of ruthenium nitrosyl porphyrin complexes with different axial ligation: Structural, spectroelectrochemical (IR, UV-Visible, and EPR), and theoretical studies

Article abstract of DOI:10.1021/ic702371t

Experimental and computational results for different ruthenium nitrosyl porphyrin complexes [(Por)Ru(NO)(X)]ⁿ⁺ (where Por^2- = tetraphenylporphyrin dianion (TPP^2-) or octaethylporphyrin dianion (OEP^2-) and X = H₂O (n = 1,2,3) or pyridine, 4-cyanopyridine, or 4-N,N-dimethylaminopyridine (n = 1,0)) are reported with respect to their electron-transfer behavior. The structure of [(TPP)Ru(NO)(H₂O)]BF₄ is established as an {MNO} ⁶ species with an almost-linear RuNO arrangement at 178.1(3)°. The compound [(Por)Ru(NO)(H₂O)]BF₄ undergoes two reversible one-electron oxidation processes. Spectroelectrochemical measurements (IR, UV-vis-NIR, and EPR) indicate that the first oxidation occurs on the porphyrin ring, as evident from the appearance of diagnostic porphyrin radical-anion vibrational bands (1530 cm^-1 for OEP^?- and 1290 cm^-1 for TPP^?-), from the small shift of ～20 cm^-1 for ν_NO and from the EPR signal at g _iso ≈ 2.00. The second oxidation, which was found to be electrochemically reversible for the OEP compound, shows a 55 cm^-1 shift in ν_NO, suggesting a partially metal-centered process. The compounds [(Por)Ru(NO)(X)]BF₄, where X = pyridines, undergo a reversible one-electron reduction. The site of the reduction was determined by spectroelectrochemical studies to be NO-centered with a ca. -300 cm^-1 shift in ν_NO. The EPR response of the NO^? complexes was essentially unaffected by the variation in the substituted pyridines X. DFT calculations support the interpretation of the experimental results because the HOMO of [(TPP)Ru(NO)(X)]⁺, where X = H ₂O or pyridines, was calculated to be centered at the porphyrin π system, whereas the LUMO of [(TPP)Ru(NO)(X)]⁺ has about 50% π*(NO) character. This confirms that the (first) oxidation of [(Por)Ru(NO)(H₂O)]⁺ occurs on the porphyrin ring wheras the reduction of [(Por)Ru(NO)(X)]⁺ is largely NO-centered with the metal remaining in the low-spin ruthenium(II) state throughout. The 4% pyridine contribution to the LUMO of [(TPP)Ru(NO)(py)]⁺ is correlated with the stability of the reduced form as opposed to that of the aqua complex.

Full text of DOI:10.1021/ic702371t

Inorg. Chem. 2008, 47, 7106-7113
Redox Properties of Ruthenium Nitrosyl Porphyrin Complexes with
Different Axial Ligation: Structural, Spectroelectrochemical (IR,
UV-Visible, and EPR), and Theoretical Studies
Priti Singh,† Atanu Kumar Das,† Biprajit Sarkar,† Mark Niemeyer,† Federico Roncaroli,‡
Jose´ A. Olabe,‡ Jan Fiedler,§ Stanislav Za´lisˇ,§ and Wolfgang Kaim*,†
Institut fu¨r Anorganische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart,
Germany, Departamento de Qu´ımica Inorga´nica, Anal´ıtica y Qu´ımica F´ısica and INQUIMAE,
Facultad de Ciencias Exactas y Naturales, UBA, Pabello´n 2, Ciudad UniVersitaria, C1428EHA Buenos
Aires, Republic of Argentina, and J. HeyroVsky´ Institute of Physical Chemistry, V.V.i., Academy of
Sciences of the Czech Republic, DolejsˇkoVa 3, CZ-18223 Prague, Czech Republic
Received December 5, 2007
Experimental and computational results for different ruthenium nitrosyl porphyrin complexes [(Por)Ru(NO)(X)]n+
-
(where Por2- ) tetraphenylporphyrin dianion (TPP2 ) or octaethylporphyrin dianion (OEP2-) and X ) H2O (n )
1, 2, 3) or pyridine, 4-cyanopyridine, or 4-N,N-dimethylaminopyridine (n ) 1, 0)) are reported with respect to their
electron-transfer behavior. The structure of [(TPP)Ru(NO)(H2O)]BF4 is established as an {MNO}6 species with an
almost-linear RuNO arrangement at 178.1(3)°. The compound [(Por)Ru(NO)(H2O)]BF4 undergoes two reversible
one-electron oxidation processes. Spectroelectrochemical measurements (IR, UV-vis-NIR, and EPR) indicate
that the first oxidation occurs on the porphyrin ring, as evident from the appearance of diagnostic porphyrin radical-
anion vibrational bands (1530 cm-1 for OEP•- and 1290 cm-1 for TPP•-), from the small shift of ∼20 cm-1 for νNO
and from the EPR signal at giso ≈ 2.00. The second oxidation, which was found to be electrochemically reversible
for the OEP compound, shows a 55 cm-1 shift in νNO, suggesting a partially metal-centered process. The
compounds [(Por)Ru(NO)(X)]BF4, where X ) pyridines, undergo a reversible one-electron reduction. The site
of the reduction was determined by spectroelectrochemical studies to be NO-centered with a ca. -300 cm-1
shift in νNO. The EPR response of the NO• complexes was essentially unaffected by the variation in the
substituted pyridines X. DFT calculations support the interpretation of the experimental results because the
HOMO of [(TPP)Ru(NO)(X)]+, where X ) H2O or pyridines, was calculated to be centered at the porphyrin
π system, whereas the LUMO of [(TPP)Ru(NO)(X)]+ has about 50% π*(NO) character. This confirms that the
(first) oxidation of [(Por)Ru(NO)(H2O)]+ occurs on the porphyrin ring wheras the reduction of [(Por)Ru(NO)(X)]+
is largely NO-centered with the metal remaining in the low-spin ruthenium(II) state throughout. The 4% pyridine
contribution to the LUMO of [(TPP)Ru(NO)(py)]+ is correlated with the stability of the reduced form as opposed
to that of the aqua complex.
Introduction
group. Soluble guanylate cyclase (sGC) is one of the other
heme-containing enzymes that act as biological receptors for
NO.2b In addition to its significance in physiology, the
interaction of NO with heme is also important in the nitrogen
cycle.2c Because of the general importance of the heme-NO
interaction, a large amount of research has been carried out
toward the synthesis of corresponding model systems. These
investigations employ synthetic porphyrins such as tetraphe-
nylporphyrin (TPP) or the more electron-rich octaethylpor-
The interaction of nitric oxide with heme proteins plays a
very important role in many physiological processes.1,2 Nitric
oxide is biosynthesized by a class of enzymes called nitric
oxide synthases (NOSs),2a which contain heme as a prosthetic
* Corresponding author: E-mail. kaim@iac.uni-stuttgart.de.
†
Universita¨t Stuttgart.
Universidad de Buenos Aires.
Academy of Sciences of the Czech Republic.
‡
§
7106 Inorganic Chemistry, Vol. 47, No. 16, 2008
10.1021/ic702371t CCC: $40.75  2008 American Chemical Society
Published on Web 07/23/2008

Redox Properties of Ruthenium Nitrosyl Porphyrin Complexes
phyrin (OEP), and iron nitrosyl complexes of these synthetic
porphyrins have been extensively studied.3 Many investiga-
tions on iron nitrosyl porphyrins have included variations
of the axial ligand in the trans position to NO, ranging from
N donors,3c,d such as pyridine, imidazole, or piperidine, to
S donors, such as thiolates,3e,h in order to understand the
role of axial ligands in the properties of coordinated NO.
The heavier analogues of iron, ruthenium nitrosyl por-
phyrins, have also been anticipated to be promising models
in the study of the interactions of NO with heme because of
their enhanced stability relative to iron nitrosyl complexes.4
However, in contrast to the several reports on the
syntheses4c,d,5 and structural studies6 of ruthenium nitrosyl
porphyrins, there have been far fewer investigations on the
electrochemistry7 and spectroelectrochemistry8,9 of ruthenium
nitrosyl porphyrins despite the fact that these would be
essential in understanding electron-transfer processes. In
addition, the unambiguous assignment of the NO oxidation
state10 for NO-coordinated ruthenium porphyrin complexes
[(Por)Ru(NO)(X)] with different porphyrins (Por) and various
axial ligands (X) can be useful in understanding the electron-
transfer processes. All three components, the porphinato
ligands (Por-/2-/3-), the metal (Ru2+/3+), and the NO system
(NO+/0/-) are redox-active in the central redox potential
region so that the determination of individual oxidation state
combinations is not trivial.
In this Article, we report experimental and theoretical
studies of two different ruthenium nitrosyl porphyrin com-
plexes in which we vary the axial ligand from an aqua ligand
to acceptor- or donor-substituted pyridines. The effect of axial
ligands on the redox properties of ruthenium nitrosyl
porphyrin complexes has been investigated by means of
electrochemical and various spectroelectrochemical methods,
including EPR. In a recent report, the small variance in EPR
parameters for very different compounds containing the
{RuNO}7 configuration was noted.11
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Experimental Section
Instrumentation. EPR spectra in the X band were recorded with
a Bruker System EMX. IR spectra were obtained using a Nicolet
6700 FT-IR instrument; solid-state IR measurements were per-
formed with an ATR unit (smart orbit with diamond crystal).
UV-vis-NIR absorption spectra were recorded on J&M TIDAS
and Shimadzu UV 3101 PC spectrophotometers. Cyclic voltam-
metry was carried out in 0.1 M Bu4NClO4 solutions using a three-
electrode configuration (glassy carbon working electrode, Pt counter
electrode, and Ag wire as a pseudoreference) and a PAR 273
potentiostat and function generator. The ferrocene/ferrocenium (Fc/
Fc+) couple served as an internal reference. Spectroelectrochemistry
was performed by our use of an optically transparent low-
temperature cell.12a A two-electrode capillary served to generate
intermediates for X-band EPR studies.12b
Synthesis. The complex [(TPP)Ru(NO)(H2O)]BF4 was prepared
according to the literature7a and was additionally characterized by
single-crystal X-ray crystallography. The complex [(OEP)Ru(NO)-
(H2O)]BF4 was prepared according to the literature.13 The reduction
studies for [(TPP)Ru(NO)(X)]BF4 and [(OEP)Ru(NO)(X)]BF4,
where X ) pyridine, 4-cyanopyridine, or 4-N,N-dimethylaminopy-
ridine, were performed after an electrocatalyzed exchange in a
solution of the aqua complex in an excess (ca. 10-fold) of the
respective pyridine.7
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X-ray Structure Determination of [(TPP)Ru(NO)(H2O)]BF4 ·
2H2O. Dark-red crystals were grown by the slow diffusion of
hexane into a dichloromethane solution at 269 K. A suitable crystal
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Inorganic Chemistry, Vol. 47, No. 16, 2008 7107

Singh et al.
Table 1. Crystallographic Data for [(TPP)Ru(NO)(H2O)]BF4 ·2H2O
refine freely, whereas hydrogen atoms on the two cocrystallized
water molecules could not be located. Final R values are listed in
Table 1. Important bond parameters are given in Table S1.
DFT Calculations. The electronic structures of all complexes
were calculated by density functional theory (DFT) methods using
the Gaussian 0315a and ADF2006.0115b,c program packages. To
simplify some of the calculations, the ethyl substituent was replaced
by methyl in [(OEP)Ru(NO)(H2O)]n, leading to [(OMP)Ru-
(NO)(H2O)]n. The calculations of the vibrational frequencies were
performed at optimized geometries. Low-lying excited states were
calculated by the time-dependent DFT (TD-DFT) method.
The hybrid functional of Perdew, Burke, and Ernzerhof16a
(PBE0) was used within Gaussian (G03/PBE0) with 6-31G*
polarized double-ꢀ basis sets16b for C, N, H, and O atoms and
effective core pseudopotentials and with corresponding optimized
sets of basis functions for Ru atoms.16c During the geometry
optimization of the complex [(TPP)Ru(NO)(H2O)]+ the triple-ꢀ
basis augmented by diffuse functions (aug-cc-pvtz)16d was used
for O within the H2O ligand. The vibrational analysis was done
with the “pure” density functional BPW91.16e,f The polarizable
conductor calculation model (CPCM) was used for the modeling
of the solvent influence in TD-DFT calculations.
Slater type orbital (STO) basis sets of triple-ꢀ quality with two
polarization functions for the Ru atom and of triple-ꢀ quality with
one polarization function for the remaining atoms were employed
in ADF2006.01. The inner shells were represented by the frozen
core approximation (1s for C, N, and O and 1s-3d for Ru were
kept frozen). The calculations were performed with the functional
including Becke’s gradient correction16e to the local exchange
expression in conjunction with Perdew’s gradient correction16g to
the local correlation (ADF/BP). The scalar relativistic (SR) zeroth-
order regular approximation (ZORA) was used within ADF
calculations. The g tensor was obtained from a spin-nonpolarized
wave function after incorporating the spin-orbit (SO) coupling. A
and g tensors were obtained by first-order perturbation theory from
a ZORA Hamiltonian in the presence of a time-independent
magnetic field.17a,b Core electrons were included in calculations
of A tensors.
empirical formula
fw
C44H30BF4N5O4Ru
880.61
T (K)
173(2)
λ (Å)
0.71073
monoclinic
P21/n
cryst syst
space group
Z
4
a (Å)
b (Å)
c (Å)
15.558(3)
15.255(3)
17.567(3)
93.355(14)
4162.3(12)
1.405
0.443
3-53
8952
ꢁ (deg)
V (Å3)
Fcalcd (g·cm-3
)
µ (mm-1
)
2θ range (deg)
collected data
unique data/Rint
no. of data >2σ
no. of parameters
GOFa
8632/0.029
6155
569
0.975
R1, wR2 (I > 2σ)b
R1, wR2 (all data)b
resd dens (e/Å3)
0.0437, 0.1151
0.0645, 0.1205
0.999/-0.902
a GOF ) {∑[w(Fo2 - Fc2)2]/(n - p)}1/2, where n and p denote the number
of data and parameters. b R1 ) ∑(|Fo| - |Fc|)/∑|Fo| and wR2 ) {∑[w(Fo
2
2 2
2
- Fc2)2]/∑[w(Fo ) ]}1/2 where w ) 1/[σ2(Fo ) + (aP)2 + bP] and P ) [(max;
2
2
0, Fo ) + 2Fc ]/3.
was selected under a layer of viscous hydrocarbon oil, attached to
a glass fiber, and instantly placed in a low-temperature N2 stream.14a
The X-ray intensity data were collected at 173 K using a Siemens
P4 diffractometer. Crystal data are given in Table 1. Calculations
were performed with the SHELXTL PC 5.0314b and SHELXL-
9714c program systems installed on a local PC. The structures were
2
solved by direct methods and refined on Fo by full-matrix least-
squares refinement. An absorption correction was applied by using
semiempirical ψ scans. Anisotropic thermal parameters were
included for all nonhydrogen atoms. The disordered BF4 anion was
refined with split positions and a side occupation factor of 0.50.
The B-F distances were restrained with SADI commands. Hy-
drogen atoms on the coordinated water molecule were allowed to
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Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
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O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross,
J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann,
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J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
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Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
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Results and Discussion
Crystal Structure. The complex [(TPP)Ru(NO)(H2O)]BF4
was synthesized by a reported method7a and characterized
additionally by X-ray crystallography (Table 1, Figure 1).
The molecular structure in the crystal confirms the {RuNO}6
configuration10b as [(TPP2-)RuII(NO+)(H2O)]BF4 via the
nearly linear (178.1(3)°) RuNO arrangement (Figure 1, Table
S1) and the typical6,10,18 Ru-N (1.726(3) Å) and N-O
(1.143(4) Å) bond lengths. The experimental geometry is
reasonably described by DFT calculations, as shown in Table
S1. Both G03/PBE0 and ADF/BP methods yield an almost
linear Ru-N-O bond. Wheras the calculations overestimate
the Ru-H2O distance, the Ru-N bond lengths are repro-
duced within 0.02 Å. The calculations with the 6-31G*
double-ꢀ basis resulted in Ru-O bonds that were too long,
but after adding diffuse functions and enlarging the basis
set, the calculated Ru-O bond length converged to the
experimental value.
Cyclic Voltammetry. Cyclic voltammetry has been em-
ployed in the study of the precursor compounds [(TPP)-
(18) Scheidt, W. R.; Ellison, M. K. Acc. Chem. Res. 1999, 32, 350.
7108 Inorganic Chemistry, Vol. 47, No. 16, 2008

Redox Properties of Ruthenium Nitrosyl Porphyrin Complexes
behavior. In contrast to the oxidations, even the first reduction
of [(Por)Ru(NO)(H2O)]+ complexes was irreversible7 and
thus not investigated further. However, as reported earlier,7
the addition of pyridine to solutions of [(TPP)Ru-
(NO)(H2O)]+ forms [(TPP)Ru(NO)(py)]+ (Figure S1), which
can be reversibly reduced at E1/2(red1) ) -0.79 V versus
Fc+/0. By varying the electronic effect of the pyridine by
means of a substitution in the para position, we observed
corresponding shifts of the potential for reduction (Table 2).
A better electron acceptor than pyridine, 4-cyanopyridine,
shifts the potential for the first reduction of [(TPP)-
Ru(NO)(X)]+ to a less-negative value (-0.68 V versus
Fc+/0), whereas 4-N,N-dimethylaminopyridine, a better elec-
tron donor but poorer π-electron acceptor than unsubstituted
pyridine, makes the complex more difficult to reduce by
shifting the potential to a more negative value, -0.90 V
versus Fc+/0. A similar trend was observed for the reduction
of [(OEP)Ru(NO)(X)]+ with different pyridines (Table 2).
The sites of the reversible electron-transfer processes, the
two oxidations of [(Por)Ru(NO)(H2O)]+ and the reduction
of [(Por)Ru(NO)(X)]+, were determined by spectroelectro-
chemical measurements.
Figure 1. Molecular structure of the cation in the crystal of
[(TPP)Ru(NO)(H2O)]BF4 ·2H2O at 173 K.
IR Spectroelectrochemistry. The accessibility of oxidized
forms of [(Por)Ru(NO)(H2O)]+ and of the reduced forms of
[(Por)Ru(NO)(X)]+ allowed us to investigate the location of
the redox processes by IR spectroelectrochemistry. Table 3
summarizes the data on the vibrational frequency of NO in
different states together with DFT-calculated results. The
oxidation of the aqua complexes causes comparatively small
positive shifts of ∆ν(NO) ) 27 and 18 cm-1 for [(TPP)Ru-
(NO)(H2O)]+/2+ and [(OEP)Ru(NO)(H2O)]+/2+, respectively,
suggesting that the oxidation occurs on neither NO nor Ru
but on the porphyrin ring (Figure 3, top). Similar shifts were
noted for CO analogues.9a DFT calculations give a ca. 20
cm-1 positive shift of the NO stretching frequencies for both
TPP and OEP complexes, which is in agreement with the
experiments. Moreover, the appearance of porphyrin radical
anion (“radical cation”)19 diagnostic ring vibrational bands20
at 1531 cm-1 for OEP•- and at 1290 cm-1 for TPP•- after
the spectroelectrochemical oxidation of the aqua complexes
also confirms that the porphyrin ring is the target of the
electron-transfer process of oxidation.9,20 DFT calculations
found intense, almost-degenerate pairs of ring vibration bands
at 1548 and 1549 cm-1 or 1295 and 1298 cm-1 for the
oxidized OMP or TPP complex, respectively.
Figure 2. Cyclic voltammogram of [(OEP)Ru(NO)(H2O)]BF4 in CH2Cl2/
0.1 M n-Bu4NClO4 at 25 °C (oxidation); scan rate ) 200 mV/s.
Table 2. Redox Potentials of Complexesa
complex
E1/2(ox1) E1/2(ox2) E1/2(red1)
[(TPP)Ru(NO)(H2O)]BF4
[(TPP)Ru(NO)(py)]BF4
[(TPP)Ru(NO)(4-CN-py)]BF4
[(TPP)Ru(NO)(4-N,N-Me2N-py)]BF4
[(OEP)Ru(NO)(H2O)]BF4
[(OEP)Ru(NO)(py)]BF4
[(OEP)Ru(NO)(4-CN-py)]BF4
[(OEP)Ru(NO)(4-N,N-Me2N-py)]BF4
a Potentials in volts versus [Fe(C5H5)2]+/0 from cyclic voltammetry at
100 mV/s in CH2Cl2/0.1 M Bu4NClO4 solutions. b Irreversible process.
c Only partially reversible.
0.78
b
b
b
0.71
b
b
1.14c
n.o.
n.o.
n.o.
1.13
n.o.
n.o.
n.o.
b
-0.79
-0.68
-0.90
b
-0.95
-0.67
-1.00
b
Ru(NO)(H2O)]+ and [(OEP)Ru(NO)(H2O)]BF4 and the ana-
logues containing pyridines instead of H2O in the trans
position to NO. The exchange could be affected electrocata-
lytically by setting the working electrode potential to ca. 200
mV before the cathodic peak maximum (Figure S1). A cyclic
voltammogram for the oxidation of [(OEP)Ru(NO)(H2O)]+
in CH2Cl2/0.1 M n-Bu4NClO4 is shown in Figure 2. The
compound undergoes two one-electron reversible oxidation
processes at E1/2(ox1) ) 0.71 V and E1/2(ox2) ) 1.13 V
versus Fc+/0. The redox potentials shift to slightly higher
values for [(TPP)Ru(NO)(H2O)]+ (Figure S2, Table 2), as
expected from the exchange of ethyl donors by electron-
withdrawing phenyl groups in TPP. Accordingly, the second
oxidation is no longer fully reversible for the TPP compound,
which is also recognized from the spectroelectrochemical
The second oxidation causes a positive shift of ∆ν(NO) )
55 cm-1 for [(OEP)Ru(NO)(H2O)]+ in CH2Cl2/0.1 M
n-Bu4NClO4 (Figure 3, middle). A high-energy shift of 64 cm-1
is also observed for the 1531 cm-1 ring vibrational band.
∆ν(NO) ) 55 cm-1 may be compared to ∆ν(NO) ) 79 cm-1
observed for the RuII
f
RuIII transition in [Ru-
(NO)Cl5](2-)f(-) 21e suggesting the relevance of a presumably
,
spin-paired [(OEP•-)RuIII(NO+)(H2O)]3+ configuration as an
(19) One-electron oxidized porphyrins are often referred to as “radical
cations” because they often contain coordinated metal dications.
However, the ligands themselves are then oxidized from their normal
dinegative forms, Por2-, to radical anions, Por•-.
(20) Shimomura, E. T.; Phillippi, M. A.; Goff, H. M.; Scholz, W. F.; Reed,
C. A. J. Am. Chem. Soc. 1981, 103, 6778.
Inorganic Chemistry, Vol. 47, No. 16, 2008 7109

Singh et al.
Table 3. Experimental and G03/BPW91-Calculated NO Stretching Frequencies for [(Por)Ru(NO)(X)]n+ Complexes
n ) 2 n ) 1
n ) 0
calcd.
exptl.
calcd.
exptl.
calcd.
exptl.
ν/cm-1
ν/cm-1
ν/cm-1
ν/cm-1
ν/cm-1
ν/cm-1
[(TPP)Ru(NO)(H2O)]n+
[(TPP)Ru(NO)(Py)]n+
[(OEP)Ru(NO)(H2O)]n+
[(OEP)Ru(NO)(Py)]n+
1922
1902
n.o.
1904
1903
1901
1901
1875
1885
1877
1876
n.o.
1584
n.o.
1665
1664
1921a
1895b
n.o.
1568
a 1941 cm-1 calculated for n ) 3. 1950 cm-1 observed for n ) 3.
b
Figure 4. EPR spectrum of oxidized [(OEP)Ru(NO)(H2O)]BF4 (top, 298
K) and reduced [(TPP)Ru(NO)(py)]BF4 (bottom, 110 K) in CH2Cl2/0.1 M
n-Bu4NClO4.
counterions can change the relative energies of these close-
lying states, possibly reversing their order.
The vibrational stretching band of NO shifts to lower
values by a much larger amount (ca. -300 cm-1 for
[(Por)Ru(NO)(X)]+ during the reduction of the pyridine (X)
complexes. Such large negative shifts in the vibrational
frequency of NO are typical21 for the reduction involving
mainly electron uptake by nitrosyl-based orbitals (Figure 3,
bottom). For both complexes, the DFT calculations indicate
the formation of the typical bent Ru-N-O structure
(Ru-N-O angle of 140.5° for both TPP and OMP com-
plexes) during the reduction accompanied by negative shifts
of the calculated NO-stretching frequencies by about 240
cm-1.
Figure 3. IR spectroelectrochemical response for the first oxidation (top)
and second oxidation (middle) of [(OEP)Ru(NO)(H2O)]BF4 and for the
reduction of [(TPP)Ru(NO)(py)]BF4 (bottom) in CH2Cl2/0.1 M n-Bu4NClO4
at 298 K.
alternative to [(OEPo)RuII(NO+)(H2O)]3+. A corresponding
claim was earlier made for systems [(TPP)Ru(CO)(X)]2+.9a The
combination of mono-oxidized porphyrin ligands with oxidized
metal centers is of greater general interest with respect to active
intermediates of heme enzymes (e.g., peroxidases);22 S ) 0 or
1 ground states can be considered but are difficult to distinguish,
for instance, by EPR, when there is a lack of isolated material.
Metal/porphyrin π overlap and thus partial covalency would
result in shifts of both ν(NO) and of porphyrin ring vibrational
(21) (a) Wanner, M.; Scheiring, T.; Kaim, W.; Slep, L. D.; Baraldo, L. M.;
Olabe, J. A.; Za´lisˇ, S.; Baerends, E. J. Inorg. Chem. 2001, 40, 5704.
(b) Sarkar, S.; Sarkar, B.; Chanda, N.; Kar, S.; Mobin, S. M.; Fiedler,
J.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2005, 44, 6092. (c) Singh,
P.; Sarkar, B.; Sieger, M.; Niemeyer, M.; Fiedler, J.; Za´lisˇ, S.; Kaim,
W. Inorg. Chem. 2006, 45, 4602. (d) Videla, M.; Jacinto, J. S.; Baggio,
R.; Garland, M. T.; Singh, P.; Kaim, W.; Slep, L. D.; Olabe, J. A.
Inorg. Chem. 2006, 45, 8608. (e) Sieger, M.; Sarkar, B.; Za´lisˇ, S.;
Fiedler, J.; Escola, N.; Doctorovich, F.; Olabe, J. A.; Kaim, W. Dalton
Trans. 2004, 1797.
3
bands. The DFT-calculated frontier orbitals for the lowest A
state of the model [(OMP)Ru(NO)(H2O)]3+ are shown in Figure
S6. Although the singly occupied orbitals have OMP character,
the energy difference between the SOMO and the almost-
degenerate HOMOs is only 0.13 eV. The next triplet state,
where electron density is partially delocalized over Ru and NO,
is energetically close. Solvent effects and the influence of
(22) Kaim, W.; Schwederski, B. Bioinorganic Chemistry: Inorganic
Elements in the Chemistry of Life: An Introduction and Guide; Wiley:
Chichester, U.K., 1994.
7110 Inorganic Chemistry, Vol. 47, No. 16, 2008

Redox Properties of Ruthenium Nitrosyl Porphyrin Complexes
Table 4. Experimental and Calculated EPR Parameters for [(Por)Ru(NO)(X)]0
axial ligand (X)
gxx
gyy
gzz
Ayy(14N)/G
4-cyanopyridine
2.036 (TPP)
2.009 (OEP)
2.036 (TPP)
2.027 (TPP)a
2.002 (OEP)
2.028 (OMP)a
2.036 (TPP)
2.017 (OEP)
1.985 (TPP)
1.986 (OEP)
1.985 (TPP)
1.983 (TPP)a
1.984 (OEP)
1.984 (OMP)a
1.985 (TPP)
1.985 (OEP)
1.886 (TPP)
1.881(OEP)
1.880 (TPP)
1.923 (TPP)a
1.877 (OEP)
1.916 (OMP)a
1.878 (TPP)
1.872 (OEP)
33 (TPP)
32 (OEP)
33 (TPP)
pyridine
31.8 (TPP)a
32 (OEP)
31.8 (OMP)a
33 (TPP)
4-N,N-dimethylaminopyridine
32 (OEP)
a Calculated values.
EPR Spectroelectrochemistry. EPR spectroscopy sup-
ports the above interpretations. The reversibly-obtained
oxidized forms [(Por)Ru(NO)(H2O)]2+ show signals at giso
) 2.0002 (TPP complex) and 2.0017 (OEP complex) and
line widths of about 25 G (Figure 4, top; Figure S3). Such
EPR signals with giso ≈ 2.00 and without noticeable g
anisotropy in the frozen state at X-band frequency (9.5 GHz)
are typical for organic radicals and here for paramagnetic
species containing the spin almost exclusively in the
conjugated π system of the porphyrin ring.9,23,24 Metal-based
oxidation should result in rather large g anisotropy that is
well detectable at X-band frequency and giso > 2 for a 4d5
(RuIII) configuration because of the high SO coupling
constant of RuIII;21 thus, the formation of ruthenium(III)
during the first oxidation can be excluded. No EPR signal
was observed during the further oxidation to the (3+) ion,
which would be compatible with a singlet or rapidly-relaxing
triplet species.
The EPR spectra of the obtained reduced species [(Por)-
Ru(NO)(X)]0 (Figure 4, bottom; Table 4) show typically11
invariant EPR characteristics (g factors g1 > 2, g2 ≈ 2.0, g3
< 2; A2(14N) ≈ 3.4 mT) of {RuNO}7 species that have been
previously observed for a large number of very different
complexes containing RuNO where the spin resides mainly
(ca. 70%) on the NO ligand.11 Figure 5 shows the difference
between spin densities calculated for oxidized [(TPP)-
Ru(NO)(H2O)]2+ (spin density on porphyrin) and reduced
[(TPP)Ru(NO)(py)] (spin density of 0.68 on the NO ligand).
The DFT-calculated EPR parameters listed in Table 4 agree
satisfactorily with the experimental data; the calculations
confirm either porphyrin- or NO-centered processes in the
course of the first oxidation or reduction, respectively (Tables
5 and 6).
DFT Calculations. The compositions of the DFT-calculated
frontier orbitals of [(TPP)Ru(NO)(H2O)]+ and [(TPP)Ru-
(NO)(py)]+ complexes are listed in Tables 5 and 6. The set of
two almost-degenerate lowest-unoccupied molecular orbitals
(LUMO and LUMO+1) is mainly formed by π* orbitals of
the NO ligand with contributing 4d (Ru) orbitals. Two closely
lying highest-occupied orbitals (HOMO and HOMO-1) are
mainly composed of π (porphyrin) orbitals. Pyridine and H2O
do not contribute substantially to the frontier orbitals; the most
Figure 5. Representation of spin densities of [(TPP)Ru(NO)(H2O)]2+ (top)
and [(TPP)Ru(NO)(py)] (bottom).
Table 5. DFT G03/PBE0-Calculated One-Electron Energies and
Compositions of Selected Highest-Occupied and Lowest-Unoccupied
Molecular Orbitals of the [(TPP)Ru(NO)(H2O)]+ Complex Expressed in
Terms of Composing Fragments
MO
E (eV) prevailing character Ru NO porph H2O
unoccupied
LUMO+3 -4.91
LUMO+2 -4.93
LUMO+1 -5.37
π* por + Ru
π* por + Ru
π* NO + Ru
π* NO + Ru
14
14
15
15
21
21
45
47
65
64
40
38
0
0
0
0
LUMO
-5.40
occupied
π por
HOMO
-8.17
1
0
0
0
0
0
0
0
98
100
100
100
1
0
0
0
HOMO-1 -8.45
HOMO-2 -9.27
HOMO-3 -9.30
π por
π por
π por
significant of such an effect concerns the LUMO of [(TPP)-
Ru(NO)(py)]+ with a 4% contribution from pyridine. Accord-
ingly, the stabilization of the reduced forms has been achieved
with pyridine axial ligands.
Following theoretical and experimental results, especially
the combined EPR and IR spectroelectrochemical measure-
ments, we have thus established the sequence of oxidation
state combinations, as shown in Scheme 1.
UV-Vis Spectroelectrochemistry. As known from a
great amount of work on porphyrins and their metal
(23) (a) Wayland, B. B.; Newman, A. R. Inorg. Chem. 1981, 20, 3093. (b)
Renner, M. W.; Cheng, R. J.; Chang, C. K.; Fajer, J. J. Phys. Chem.
1990, 94, 8508. (c) Tokita, Y.; Yamaguchi, K.; Watanabe, Y.;
Morishima, I. Inorg. Chem. 1993, 32, 329. (d) Bhaskar, G.; Krishnan,
V. Inorg. Chem. 1985, 24, 3253.
(24) Fuhrhop, J. H.; Kadish, K. M.; Davis, D. G. J. Am. Chem. Soc. 1973,
95, 5140.
Inorganic Chemistry, Vol. 47, No. 16, 2008 7111

Singh et al.
Table 6. DFT G03/PBE0-Calculated One-Electron Energies and
Compositions of Selected Highest-Occupied and Lowest-Unoccupied
Molecular Orbitals of the [(TPP)Ru(NO)(py)]+ Complex Expressed in
Terms of Composing Fragments
MO
E (eV) prevailing character Ru NO porph py
unoccupied
LUMO+3
LUMO+2
LUMO+1
LUMO
-4.80
-4.82
-5.30
-5.32
π* por + Ru
π* por + Ru
π* NO + Ru
π* NO + Ru
11
10
17
17
15
12
52
53
73
77
30
25
0
2
1
4
occupied
π por
HOMO
-7.98
1
0
0
0
0
0
0
0
98
100
100
100
1
0
0
0
HOMO-1 -8.27
HOMO-2 -9.17
HOMO-3 -9.18
π por
π por
π por
Scheme 1
Figure 6. UV-vis spectroelectrochemical response for the conversion
[(TPP)Ru(NO)(H2O)](+)f(2+) in CH2Cl2/0.1 M n-Bu4NClO4.
Figure 7. UV-vis spectroelectrochemical response for the conversion
[(OEP)Ru(NO)(py)](+)f(0) in CH2Cl2/0.1 M n-Bu4NClO4.
the spectra of the OEP complex presumably arises from
perturbed orbital degeneracy when the ligand arrangement
induces a lower-than-D4h symmetry (e.g., through different
axial ligands); such Soret and Q-band splitting has been
observed before.25
In contrast, the UV-vis spectroelectrochemical reduc-
tion experiments for [(Por)Ru(NO)(py)]+ (Por ) OEP, Figure
7; Por ) TPP, Figure S5) show a nitrosyl-based reduc-
tion7,11,21 accompanied by the appearance of new bands in
the visible region (Table 7); these could be due to metal-
to-ligand charge transfer (MLCT) transitions d(Ru) f
π*(NO•) and to ligand-to-ligand charge transfer processes
π*(NO•) f π*(Por).
complexes,5,9 the UV-vis absorption spectra of ruthenium
nitrosyl porphyrins show blue-shifted Q bands and a very
sharp Soret band (Table 7). As illustrated in Figure 6, the
spectrum of oxidized [(TPP)Ru(NO)(H2O)](+)f(2+) displays
a decrease in the intensity of both the Soret and the Q band.
Whereas the Soret band is only partially diminished, the Q
band at 558 nm seems to disappear completely at the cost
of new broad bands at 616, 658, and 742 nm. Such change
in absorption spectra is typical5,9 for the formation of
porphyrin π-radical complexes. During the second oxidation,
there is a noticeable splitting of the Soret band in the UV
region, and the transitions in the visible are diminished
(Figure S4).
The assignments are in agreement with the calculated
molecular orbital scheme (Table 6). In the reduced complex,
an excitation of the unpaired electron from an orbital with
The splitting of the Soret band observed particularly after
the two-electron oxidation of [(TPP)Ru(NO)(H2O)]+ and in
Table 7. Absorption Data and TD-DFT-Calculated Lowest-Lying Transitions for Porphyrin Complexes
a
compounds
λ/nm (ε/M-1 ·cm-1
)
[(TPP)Ru(NO)(H2O)]+ (exp) (TD-DFT-calculated)
410 (78 000), 558 (12 000) 391 (1.248), 391 (1.228), 482 (0.065),
483 (0.068), 610 (0.026), 621 (0.029)
408 (84 000), 496 (sh), 616 (17 000), 658 (15 700), 742(5900)
278, 324, 400, 422, 492, 540, 573, 620 326 (0.187), 396 (1.197),
397 (1.125), 501 (0.056), 503 (0.055), 674 (0.016), 679 (0.016)
279, 310, 394, 430, 538, 574
370 (40 600), 397 (42 400), 478 (11 000)
488 (11 500), 574 (89 800), 642 (62 400)
266, 359, 484, 507, 573
[(TPP)Ru(NO)(H2O)]2+
[(TPP)Ru(NO)(py)]+ (TD-DFT-calculated)
[(TPP)Ru(NO)(py)]0
[(OEP)Ru(NO)(H2O)]+
[(OEP)Ru(NO)(H2O)]2+
[(OEP)Ru(NO)(py)]+
[(OEP)Ru(NO)(py)]0
319, 362, 524, 557
a Calculated values: λ in nanometers (oscillator strengths).
7112 Inorganic Chemistry, Vol. 47, No. 16, 2008

Redox Properties of Ruthenium Nitrosyl Porphyrin Complexes
prevailing contributions from Ru and NO (labeled LUMO
or LUMO+1 in Table 6) is considered to lead to a mainly
π*(porphyrin)-based level (LUMO+2 or LUMO+3).
as established spectroelectrochemically by EPR and IR and
as noted similarly for many other {RuNO}6,7 systems.11
Thus, we have shown that EPR gives clear answers for the
spin location, except for the EPR-silent and still puzzling
tricationic form, that IR spectroelectrochemistry reveals
charge effects at NO and in some cases at the porphyrins,
and that the characterized oxidation state distributions cause
electronic absorption features that may be useful for future
studies. The fascinating metal/NO interaction relevant to
biochemistry,26 organic chemistry,27 and catalysis28 can thus
be probed via an array of spectroelectrochemical methods,
including EPR.
Conclusions
We have observed the reversible electron-transfer pro-
cesses for ruthenium nitrosyl porphyrins by varying the axial
ligand. Except for the redox potentials, the differences among
complexes involving different pyridine ligands have been
small. The complexes of the type [(Por)Ru(NO)(X)]BF4
containing X as a neutral O-donor aqua ligand show a two-
step oxidation, and those with X as a neutral N-donor
pyridine ligand show a reversible reduction. The sites of all
processes have been assessed by means of IR and UV-
vis-NIR spectroelectrochemical measurements and through
theoretical support. The first oxidation of [(Por)Ru(NO)-
(H2O)]+ occurs on the porphyrin ring, as documented now
completely by EPR, IR, and UV-vis-NIR spectroelectro-
chemistry. This is then followed by a metal involving second
oxidation according to the observed ∆ν (NO) shift. That
species is EPR silent, and its further calculations and, if
possible, NMR spectroscopic investigation, are warranted.
The reduction of [(Por)Ru(NO)(Py)]+ is mainly NO-centered,
Acknowledgment. This work has been supported by the
Deutsche Forschungsgemeinschaft (Graduate College “Mag-
netic Resonance” and project Ka 618/27-1), by the European
Union (COST D35), by the Grant Agency of the Academy
of Sciences of the Czech Republic (KAN100400702) and
the Ministry of Education of the Czech Republic (OC 139),
and by Argentinian grants PICT 03/14418 and X104 from
the ANPCYT and the UBA, respectively.
Supporting Information Available: X-ray crystallographic file
for [(TPP)Ru(NO)(H2O)]BF4; Experimental and calculated structure
parameters for [(TPP)Ru(NO)(H2O)]+); IR spectroelectrochemical
response of [(TPP)Ru(NO)(H2O)]BF4 with 10-fold excess of
pyridine; cyclic voltammogram of [(TPP)Ru(NO)(H2O)]BF4; EPR
spectrum of [(TPP)Ru(NO)(H2O)]2+; UV-vis spectroelectrochemi-
(25) (a) Jansen, G.; Noort, M. Spectrochim. Acta 1976, 32A, 747. (b)
Canters, C. W.; van der Waals, J. H.; High Resolution Zeeman
Spectroscopy of Metalloporphyrins. In The Porphyrins; Dolphin, D.,
Ed.; Academic Press: New York, 1978; Vol. 3, p. 531. (c) Suisalu,
A.; Mauring, K.; Kikas, J.; Herenyi, L.; Fidy, J. Biophys. J. 2001, 80,
498.
cal response for the conversion [(OEP)Ru(NO)(H2O)](2+)f(3+)
;
(26) Ghosh, A. Acc. Chem. Res. 2005, 38, 943.
UV-vis spectroelectrochemical response for the conversion
[(TPP)Ru(NO)(Py)](+)f(0); and frontier orbitals of [(OMP)Ru-
(NO)(H2O)]3+). This material is available free of charge via the
Internet at http://pubs.acs.org.
(27) (a) Di Salvo, F.; Escola, N.; Scherlis, D. A.; Estrin, D. A.; Bond´ıa,
C.; Murgida, D.; Ramallo-Lo´pez, J. M.; Requejo, F. G.; Shimon, L.;
Doctorovich, F. Chem.sEur. J. 2007, 13, 8428. (b) Doctorovich, F.;
Di Salvo, F. Acc. Chem. Res. 2007, 40, 985.
(28) Hong, S.; Rahman, T. S.; Jacobi, K.; Ertl, G. J. Phys. Chem. C 2007,
111, 12361.
IC702371T
Inorganic Chemistry, Vol. 47, No. 16, 2008 7113