Single- and double-linkage isomerism in a six-coordinate iron porphyrin containing nitrosyl and nitro ligands

Article abstract of DOI:10.1021/ja0488986

Density Functional theoretical calculations confirm the experimental observation that the low-temperature photolysis of (TPP)Fe(NO)(NO₂) (as a KBr pellet) results in the generation of linkage isomers involving the axial NO and NO₂ groups and suggest the possible formation of the double linkage isomer (TPP)Fe(ON)(ONO). The energy difference between the ground state (porphine)Fe(NO)(NO₂) and the double-linkage isomer (porphine)Fe(ON)(ONO) is 1.57 eV, which is comparable to the 1.59 eV calculated previously for the nitrosyl-to-isonitrosyl linkage isomerism in the five-coordinate (porphine)Fe(NO) analogue. Copyright

Full text of DOI:10.1021/ja0488986

Published on Web 05/22/2004
Single- and Double-Linkage Isomerism in a Six-Coordinate Iron Porphyrin
Containing Nitrosyl and Nitro Ligands
Jonghyuk Lee,† Andrey Yu. Kovalevsky,‡ Irina V. Novozhilova,‡ Kimberly A. Bagley,*,§
Philip Coppens,*,‡ and George B. Richter-Addo*,†
Department of Chemistry and Biochemistry, UniVersity of Oklahoma, 620 Parrington OVal,
Norman, Oklahoma 73019, Department of Chemistry, State UniVersity of New York at Buffalo,
Buffalo, New York 14260-3000, and Department of Chemistry, State UniVersity College of New York at Buffalo,
Buffalo, New York 14222
Received February 26, 2004; E-mail: coppens@acsu.buffalo.edu, grichteraddo@ou.edu
Metals play significant roles in the chemistry of nitrogen oxides
the IR spectral features, implying that the isonitrosyl linkage is
stable at 50 K. However, warming the sample to 200 K results in
the disappearance of the 1699 cm-1 band but not the nitrito bands.
Furthermore, warming the sample to 295 K shows the complete
restoration of the bands due to GS, indicative of the reversibility
of both the nitrofnitrito and nitrosylfisonitrosyl linkage isomer-
ism.
-
in the environment. Nitrite reductase enzymes convert nitrite (NO2
)
to nitric oxide (NO) in bacterial denitrification.1 Although many
binding modes of the nitrite group in coordination compounds have
been established,2 the relative energies of the observed binding
modes in metalloproteins or their bioinorganic model compounds
are not always known. NO is a signaling molecule that binds to
the iron center of many heme proteins.3 Information regarding
ground states and metastable states of metal-NOx groups may help
shed light on possible intermediates involved in NOx association
and/or dissociation from heme centers. Linkage isomerism of
metal-NOx compounds has been reviewed recently.4
Density Functional Theory calculations on the model compound
(porphine)Fe(NO)(NO2) and its isomers were performed to further
investigate the experimental observation of both the nitrofnitrito
and nitrosylfisonitrosyl linkage isomerism in the same compound.7
The compound (porphine)Fe(NO)(NO2) belongs to the {FeNO}6
class according to the Enemark-Feltham notation.8 Two locally
stable ground-state (GS) conformations are found, which differ with
respect to the relative orientation of the axial ligand planes; GS//
represents parallel FeNO and Fe-NO2 planes, and GS represents
perpendicular planes.9 GS// is the minimum energy structure (∆E
|GS// - GS | ) 0.03 eV) and exhibits an FeNO bond angle of
157.4° with an off-axis tilt of 8.2° of the axial N(O) atom. The
equatorial Fe-Np bonds are longer in the direction of the N(O) tilt
and shorter in the direction away from the N(O) tilt, consistent
with that observed previously for another {FeNO}6 compound
containing a bent NO group, namely, (OEP)Fe(NO)(p-C6H4F).10
However, this Fe-Np bond length asymmetry in (porphine)Fe(NO)-
(NO2) is opposite to that observed for the five- and six-coordinate
{FeNO}7 porphyrins.11-13
Interestingly, the DFT calculations reveal that not only the
nitrosyl-nitrito (MSa) and the isonitrosyl-nitro (TPP)Fe(ON)(NO2)
(MSb) but also the isonitrosyl-nitrito (TPP)Fe(ON)(ONO) (MSc)
linkage isomers correspond to minima on the ground-state potential
energy surface.14 To the best of our knowledge, the latter result
represents the first example of double-linkage isomerism. As is
observed with the ground-state and MSa structures, the energy
differences between the // and forms of both MSb and MSc are
small (∼0.03 eV) and at room-temperature comparable with kT.15
Calculated geometrical parameters for the linkage isomers of
(porphine)Fe(NO)(NO2) are collected in the Supporting Information,
and the relative energies and representative structures are shown
in Figure 1. Importantly, the Fe-Np bond length asymmetry, off-
axis tilts of the nitrosyl N-atoms and isonitrosyl O-atoms, and the
bending of the FeNO and FeON groups are general features of all
the minimum energy structures of the GS, MSa, MSb, and MSc
linkage isomers. The structures of MSaL and MScL contain linear
FeNO or FeON moieties, respectively, and display nitrito ligand
conformations that place the terminal nitrito O-atom closer to the
molecular axes; however, these structures correspond to higher
energy conformations for MSa and MSc (Figure 1 and Table S2).
We reported that the five-coordinate nitrosyl porphyrins (por)-
Fe(NO) (por ) TTP, OEP; TTP ) tetratolylporphyrinato dianion,
OEP ) octaethylporphyrinato dianion) undergo photoinduced NO
linkage isomerism at 25 K to generate the isonitrosyls (por)Fe-
(ON).5 We were thus interested in investigating related photo-
isomerizations in six-coordinate iron nitrosyl porphyrins. In this
communication, we report that the photolysis of the ground-state
nitrosyl-nitro complex (TPP)Fe(NO)(NO2) (TPP ) tetraphenyl-
porphyrinato dianion) generates both the nitrito and isonitrosyl
linkage isomers.
Irradiation of (TPP)Fe(NO)(NO2)6 (KBr pellet; 300 < λ < 500
nm; 300 W xenon lamp) at 200 K for 10 min results in the
appearance of new N-isotope-sensitive bands in the difference IR
spectrum at 1507 and 934 cm-1, with a concomitant decrease in
the bands at 1464, 1302, and 806 cm-1. These IR spectral changes
suggest a change in the binding mode of the NO2 group from nitro
to nitrito4 and a conversion of the nitrosyl nitro complex (TPP)-
Fe(NO)(NO2) (GS) to the first metastable state (TPP)Fe(NO)(ONO)
(MSa). The new IR bands remain after the illumination is ceased,
indicating that complex MSa is stable at 200 K. However,
continuous irradiation at 250 K is required to maintain spectro-
scopically detectable amounts of MSa, indicative of its decay at
this temperature.
The nitrofnitrito linkage isomerization also occurs at 11 K as
determined from the IR difference spectral analysis of the irradiated
sample (Figure S1 in the Supporting Information). However, in
addition to the bands due to the nitrofnitrito conversion, a new
N-isotope-sensitive band at 1699 cm-1 is also observed in the IR
spectrum. The ∆υNO value of -184 cm-1 is consistent with a
nitrosylfisonitrosyl linkage isomerization,4 suggesting the pos-
sibility of an unprecedented double-linkage isomerism. Warming
the irradiated sample to 50 K does not change the appearance of
† University of Oklahoma.
‡ State University of New York at Buffalo.
§ State University College of New York at Buffalo.
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J. AM. CHEM. SOC. 2004, 126, 7180-7181
10.1021/ja0488986 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
as indeed suggested by the DFT calculations, cannot be ruled out,
though its concentration may be too low to be easily detectable.
The fact that this involves Fe-NOx species opens up new avenues
for further investigations into NO and NO2 combination and
recombination with iron porphyrins.
Acknowledgment. Support for this work by the National
Institutes of Health (GM 64476 to G.B.R.-A.) and the National
Science Foundation (CHE0236317 to P.C.) is gratefully acknowl-
edged.
Supporting Information Available: Difference infrared spectra
and experimental IR frequencies for (TPP)Fe(NO)(NO2) and optimized
structures for all linkage isomers of (porphine)Fe(NO)(NO2) (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) Wasser, I. M.; de Vries, S.; Moenne-Loccoz, P.; Schroder, I.; Karlin, K.
D. Chem. ReV. 2002, 102, 1201.
(2) Hitchman, M. A.; Rowbottom, G. L. Coord. Chem. ReV. 1982, 42, 55.
(3) Cheng, L.; Richter-Addo, G. B. In The Porphyrin Handbook; Guilard,
R., Smith, K., Kadish, K. M., Eds.; Academic Press: New York, 2000;
Vol. 4, Chapter 33.
Figure 1. Calculated energies and representative structures for the linkage
isomers of (porphine)Fe(NO)(NO2). The axial nitrogen and oxygen atoms
are colored blue and red, respectively. // ) ligand planes are coplanar;
) ligand planes are mutually perpendicular. MSaL and MScL are the
isomers displaying linear FeNO and FeON groups, respectively.
(4) Coppens, P.; Novozhilova, I.; Kovalevsky, A. Chem. ReV. 2002, 102, 861
and references therein.
(5) Cheng, L.; Novozhilova, I.; Kim, C.; Kovalevsky, A.; Bagley, K. A.;
Coppens, P.; Richter-Addo, G. B. J. Am. Chem. Soc. 2000, 122, 7142.
(6) Yoshimura, T. Inorg. Chim. Acta 1984, 83, 17-21.
(7) Details on the DFT calculations are given in the Supporting Information.
(8) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339.
(9) The crystal structure of (TPP)Fe(NO)(NO2) has been reported, but severe
disorder involving the axial ligands limits the accuracy of the geometrical
parameters. Related porphyrin derivatives yielded more accurate structures.
Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 1999, 38,
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(10) Richter-Addo, G. B.; Wheeler, R. A.; Hixon, C. A.; Chen, L.; Khan, M.
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(11) Rath, S. P.; Koerner, R.; Olmstead, M. M.; Balch, A. L. J. Am. Chem.
Soc. 2003, 125, 11798.
The DFT calculations confirm that the double-linkage isomerism
is an achievable experimental event; the lowest energy conformation
of the double linkage isomer (i.e., MSc//) is 1.57 eV higher in
energy than the ground-state GS//, and this ∆E value is very similar
to the 1.59 eV value calculated previously for the nitrosylfisonitrosyl
conversion for the five-coordinate (porphine)Fe(NO).5
There are several additional interesting points to note. First,
photolysis of (TPP)Fe(NO)(NO2) as a KBr pellet at low temperature
results in linkage isomerism without loss of NO or NO2. This
contrasts with the observed competitive NO/NO2 dissociation during
photolysis of the compound in toluene solution.16 Second, it is
tempting to speculate that the MSa isomers could be generated as
transient intermediates during the reactions of iron porphyrins with
NOx species to give (por)Fe(NO)(NO2). This hypothesis is supported
by the fact that both O-bonded and N-bonded NO2 groups are ob-
served as disordered components in a crystal form of [(TpivPP)-
Fe(NO)(NO2)]-.15
(12) Wyllie, G. R. A.; Schultz, C. E.; Scheidt, W. R. Inorg. Chem. 2003, 42,
5722.
(13) Wyllie, G. R. A.; Scheidt, W. R. Chem. ReV. 2002, 102, 1067.
(14) We have also calculated vibrational frequencies. The results fully confirm
our experimental IR spectral assignments for the axial ligands in the
linkage isomers of (TPP)Fe(NO)(NO2) and (TPP)Fe(15NO)(15NO2). A
small number of negative frequencies were identified as being associated
with porphyrin ring deformations.
(15) A crystal form of the anion [(TpivPP)Fe(NO)(NO2)]- (TpivPP ) picket
fence porphyrinato dianion) containing a bent NO ligand displays parallel
axial ligand orientations, whereas a second crystal form displays perpen-
dicular axial ligand orientations. Nasri, H.; Ellison, M. K.; Chen, S.;
Huynh, B. H.; Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 6274.
(16) Lim, M. D.; Lorkovic, I. M.; Wedeking, K.; Zanella, A. W.; Works, C.
F.; Massick, S. M.; Ford, P. C. J. Am. Chem. Soc. 2002, 124, 9737.
In summary, we have observed linkage isomerism involving two
ligands in the same complex by IR spectroscopy and performed
parallel DFT calculations. The possibility that in addition to the
two separate linkage isomers a double-linkage isomer is formed,
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