Iron complexes of N-confused pyriporphyrin: NMR studies

Article abstract of DOI:10.1021/ic060906r

Insertion of iron(II) into 6,11,16,21-tetraaryl-3-aza-m-benziporphyrin (N-confused pyriporphyrin, (PyPH)H) yielded the high-spin iron(II) complex, (PyPH)Fe^IIBr. The coordination of iron(II) to the perimeter nitrogen atom of (PyPH)Fe^IIBr resulted in the formation of the diiron species. Oxidation and oxygenation of (PyPH)Fe^IIBr were followed by ¹H NMR spectroscopy. The addition of Br₂ to the solution of (PyPH)Fe^IIBr in the absence of dioxygen results in a one-electron oxidation yielding the high-spin iron(III) N-confused pyriporphyrin [(PyPH)Fe^IIIBr]⁺ which preserves the side-on interaction between the inverted pyridine ring and metal ion. The reaction of (PyPH)Fe ^IIBr with dioxygen ends up with the formation of a five-coordinate species (PyPO)Fe^IIIBr] ((PyPOH)H = 3-aza-22-hydroxy-m-benziporphyrin, PyPO = the corresponding dianion) which is formed by oxygenation at the C(22) position. Coordination of a metal ion by 3-aza-22-hydroxy-benziporphyrin imposes a steric constraint on the geometry of the ligand. The halide ligand of (PyPO)Fe^IIIBr coordinates on one of the two inequivalent faces of the macrocycle, leading to two distinct species: syn and anti. The ¹H NMR spectra of paramagnetic iron(II) and iron(III) N-confused pyriporphyrin complexes have been examined. The characteristic patterns of pyrrole and pyridine resonances have been found to be diagnostic of the ground electronic state of iron and the donor nature of the C(22)H and N(3) centers. The enormous downfield H(22) paramagnetic shift, determined for the iron(II) N-confused pyriporphyrin, provides a distinct resonance in a peculiar spectroscopic window (350-800 ppm) for a series of axial ligands which can be considered as a diagnostic sign of an agostic Fe^II...{C(22)-H} interaction. Coordination of the pyridine moiety via the perimeter N(3) atom is reflected unambiguously by the H(2/4) resonance at 201 ppm.

Full text of DOI:10.1021/ic060906r

Inorg. Chem. 2006, 45, 7828−7834
Iron Complexes of N-Confused Pyriporphyrin: NMR Studies
Radomir Mys´liborski, Krystyna Rachlewicz, and Lechosław Latos-Graz3yn´ski*
Department of Chemistry, UniVersity of Wrocław, 14 F. Joliot-Curie Street,
Wrocław 50 383, Poland
Received May 24, 2006
Insertion of iron(II) into 6,11,16,21-tetraaryl-3-aza-m-benziporphyrin (N-confused pyriporphyrin, (PyPH)H) yielded
the high-spin iron(II) complex, (PyPH)FeIIBr. The coordination of iron(II) to the perimeter nitrogen atom of (PyPH)-
FeIIBr resulted in the formation of the diiron species. Oxidation and oxygenation of (PyPH)FeIIBr were followed by
1H NMR spectroscopy. The addition of Br2 to the solution of (PyPH)FeIIBr in the absence of dioxygen results in a
one-electron oxidation yielding the high-spin iron(III) N-confused pyriporphyrin [(PyPH)FeIIIBr]+ which preserves the
side-on interaction between the inverted pyridine ring and metal ion. The reaction of (PyPH)FeIIBr with dioxygen
ends up with the formation of a five-coordinate species (PyPO)FeIIIBr] ((PyPOH)H
benziporphyrin, PyPO the corresponding dianion) which is formed by oxygenation at the C(22) position. Coordination
) 3-aza-22-hydroxy-m-
)
of a metal ion by 3-aza-22-hydroxy-benziporphyrin imposes a steric constraint on the geometry of the ligand. The
halide ligand of (PyPO)FeIIIBr coordinates on one of the two inequivalent faces of the macrocycle, leading to two
1
distinct species: syn and anti. The H NMR spectra of paramagnetic iron(II) and iron(III) N-confused pyriporphyrin
complexes have been examined. The characteristic patterns of pyrrole and pyridine resonances have been found
to be diagnostic of the ground electronic state of iron and the donor nature of the C(22)H and N(3) centers. The
enormous downfield H(22) paramagnetic shift, determined for the iron(II) N-confused pyriporphyrin, provides a
distinct resonance in a peculiar spectroscopic window (350
considered as a diagnostic sign of an agostic FeII‚‚‚{C(22)
the perimeter N(3) atom is reflected unambiguously by the H(2/4) resonance at 201 ppm.
−800 ppm) for a series of axial ligands which can be
−H} interaction. Coordination of the pyridine moiety via
Introduction
N-confused pyriporphyrin 1 contains the (CNNN) coordina-
tion core of the carbaporphyrinoids.4-12
N-confused pyriporphyrin (6,11,16,21-tetraaryl-3-aza-m-
benziporphyrin) (PyPH)H 1 is the simplest homologue of
N-confused 5,10,15,20-tetraarylporphyrin 3 with the supple-
mentary carbon inserted into a pyrrolic CR-N bond.1 Because
it is readily seen, by analogy to the porphyrin-N-confused
porphyrin couple, the 3-aza-m-benziporphyrin 1 can be
related to the regular pyriporphyrin (22-aza-m-benziporphy-
rin) 2 by application of the N-confusion concept.2 Formally,
1 can be also constructed by replacement of one of the
pyrrole rings of 5,10,15,20-tetraarylporphyrin with a pyridine
moiety, linked to the macrocycle at the 3,5-position. Alter-
natively, the molecule can be treated as a derivative of
6,11,16,21-tetraaryl-m-benziporphyrin 4,3 where the 3-CH
fragment has been replaced with a nitrogen atom (Chart 1).
In general, carbaporphyrinoids are versatile ligands, which
offer a means to study metal-carbon bonding in a macro-
cyclic environment.3,12-17 The coordination brings the metal
(4) Lash, T. D. Synlett 1999, 279.
(5) Latos-Graz˘yn´ski, L. Core Modified Heteroanalogues of Porphyrins
and Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000;
pp 361-416.
(6) Pawlicki, M.; Latos-Graz˘yn´ski, L. Chem. Rec. 2006, 6, 64.
(7) Furuta, H.; Maeda, H.; Osuka, A. Chem. Commun. 2002, 1795.
(8) Harvey, J. D.; Ziegler, C. J. Coord. Chem. ReV. 2003, 247, 1.
(9) Ghosh, A. Angew. Chem., Int. Ed. Engl. 2004, 43, 1918.
(10) Srinivasan, A.; Furuta, H. Acc. Chem. Res. 2005, 38, 10.
(11) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Coord. Chem. ReV. 2005, 249,
2510.
(12) Stepien´, M.; Latos-Graz˘yn´ski, L. Acc. Chem. Res. 2005, 38, 88.
c
(13) Stepien´, M.; Latos-Graz˘yn´ski, L.; Lash, T. D.; Szterenberg, L. Inorg.
c
* To whom correspondence should be addressed. E-mail: LLG@
wchuwr.chem.uni.wroc.pl.
(1) Mys´liborski, R.; Latos-Graz˘yn´ski, L. Eur. J. Org. Chem. 2005, 5039.
(2) Mys´liborski, R.; Latos-Graz˘yn´ski, L.; Szterenberg, L. Eur. J. Org.
Chem. 2006, 3064.
Chem. 2001, 40, 6892.
(14) Stepien´, M.; Latos-Graz˘yn´ski, L. J. Am. Chem. Soc. 2002, 124, 3838.
c
(15) Stepien´, M.; Latos-Graz˘yn´ski, L.; Szterenberg, L.; Panek, J.; Latajka,
Z.cJ. Am. Chem. Soc. 2004, 126, 4566.
(16) Stepien´, M.; Latos-Graz˘yn´ski, L.; Szterenberg, L. Inorg. Chem. 2004,
c
(3) Ste¸pien´, M.; Latos-Graz˘yn´ski, L. Chem.sEur. J. 2001, 7, 5113.
43, 6654.
7828 Inorganic Chemistry, Vol. 45, No. 19, 2006
10.1021/ic060906r CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/18/2006

Iron Complexes of N-Confused Pyriporphyrin
Chart 1. Selected Carbaporphyrinoids and Pyriporphyrin
m-phenylene ring is held proximate to the metal ion without
actually forming a covalent bond.3,12,15-17
Because the present studies are focused on the iron
complexes of N-confused porphyrin, the relevant studies
concerning iron carbaporphyrinoid chemistry have been
briefly overviewed. Iron(II) inverted porphyrin complexes
3-FeBr and 3-Fe(SC7H7) present the conformation with the
iron in a side-on position with respect to the inverted pyrrole
plane.29 The agostic iron(II)-inverted pyrrole ring interaction
1
(FeII‚‚‚C(21)-H) is reflected in the H NMR studies by an
unprecedented isotropic shift of the engaged H(21).30
A
similar coordination mode has been determined for iron(II)
m-benziporphyrin where the FeII‚‚‚C(22)-H interaction was
also reflected by a considerable paramagnetic shift of H(22).17
A dimeric iron(II) N-confused porphyrin, [3-Fe]2, was
obtained from the anaerobic reaction of 3-FeBr with NaSePh.
Under aerobic conditions, a µ-hydroxo-bridged iron(III)
dimer was obtained with a sodium ion bridging the outer-N
atoms. Oxygenation occurred at the inner-core pyrrolic
carbon to form a novel porphyrinic ring.21 Oxidation and
ion into the vicinity of chosen carbocycles or heterocycles,
leading to activation of C-H bonds, followed by metal coor-
dination or resulting in weak interactions, which can be spec-
troscopically observed.15 A special role in this line of investi-
gations has been played by the first discovered carbapor-
phyrinoid-inverted (N-confused) porphyrin.18,19 The inverted
porphyrin and its derivatives revealed a remarkable tendency
to stabilize peculiar organometallic compounds.5,7,8,10,11 Com-
plex dimeric or oligomeric structures, involving N-confused
porphyrins, linked by external coordination using the N(2)
donor are of particular interest.10,20-26 An iron(III)-N-con-
fused porphyrin complex assembled using axially coordinated
phenol and the perimeter nitrogen of the macrocycle has been
recently reported.27
The ability of carbaporphyrinoids to coordinate metal ions
and to form metal-carbon bonds extends beyond the family
of N-confused porphyrins.3,5,7,13,14,28 For this paper, the
coordination properties of m-benziporphyrin and closely
related molecules are of particular interest. To date, m-benzi-
porphyrin has revealed two coordination modes. In the first
one, a metal ion is bound in the macrocyclic cavity by three
pyrrolic nitrogens and a trigonal carbon of the benzene
ring.13,28 The macrocyclic effect is not always sufficient to
enforce the metalation of benzene. In such a case, the
1
oxygenation of 3-FeBr was also investigated using H and
2H NMR spectroscopy.31 Two modes of coordination of
iron(III) to the N-confused pyrrole ring (side-on and via the
trigonally hybridized carbon atom) were determined. In the
presence of oxygen, the formation of the C(22)O carbonyl
group was established; it was involved in a direct interaction
between the iron center and the π-electron density on the
carbonyl group in an η2 mode.
1H NMR spectroscopy provides a uniquely useful probe
for studying the structure of iron porphyrin complexes in
solution.32 The hyperfine shift patterns are particularly
sensitive to the ligation, oxidation, and spin state of the metal
ion. For instance, the iron(III) 2-hydroxy-5,10,15,20-tet-
raphenylporphyrin oligomerization yielded a 1H NMR spec-
troscopic pattern that is unequivocally explainable in terms
of a cyclic trimeric structure.33,34 In this contribution, our
investigations have been focused on iron complexes of
N-confused pyriporphyrin. Three inequivalent pyrrole and
two pyridyl resonances of iron complexes provided a direct
probe of the spin density around the porphyrin macrocycle
and allowed the detection of a diiron(II) structure. The
perimeter coordination can be examined by monitoring some
1
reactions directly by H NMR spectroscopy without the
problems inherent in separation and isolation of products.
(17) Hung, C.-H.; Chang, F.-C.; Lin, C.-Y.; Rachlewicz, K.; Stepien´, M.;
Latos-Graz˘yn´ski, L.; Lee, G.-H.; Peng, S.-M. Inorg. Chem.c2004, 43,
4118.
Results and Discussion
(18) Chmielewski, P. J.; Latos-Graz˘yn´ski, L.; Rachlewicz, K.; Głowiak,
Formation and Characterization of Iron(II) Complexes.
Insertion of iron(II) into 6,11,16,21-tetraaryl-3-aza-m-ben-
T. Angew. Chem., Int. Ed. Engl. 1994, 33, 779.
(19) Furuta, H.; Asano, T.; Ogawa, T. J. Am. Chem. Soc. 1994, 116, 767.
(20) Furuta, H.; Youfu, K.; Maeda, H.; Osuka, A. Angew. Chem., Int. Ed.
2003, 42, 2186.
(21) Hung, C.-H.; Chen, W.-C.; Lee, G.-H.; Peng, S.-M. Chem. Commun.
2002, 1516.
(22) Furuta, H.; Ishizuka, T.; Osuka, A. J. Am. Chem. Soc. 2002, 124, 5622.
(23) Chmielewski, P. J.; Schmidt, I. Inorg. Chem. 2004, 43, 1885.
(24) Chmielewski, P. J. Angew. Chem., Int. Ed. 2005, 44, 6417.
(25) Maeda, H.; Furuta, H. J. Porphyrins Phthalocyanines 2004, 8, 67.
(26) Furuta, H.; Morimoto, T.; Osuka, A. Inorg. Chem. 2004, 43, 1618.
(27) Hung, C.-H.; Chang, C.-H.; Ching, W.-M.; Chuanga, C.-H. Chem.
Commun. 2006, 1866.
(29) Chen, W.-C.; Hung, C.-H. Inorg. Chem. 2001, 40, 5070.
(30) Rachlewicz, K.; Wang, S.-L.; Peng, C.-H.; Hung, C.-H.; Latos-
Graz˘yn´ski, L. Inorg. Chem. 2003, 42, 7348.
(31) Rachlewicz, K.; Wang, S.-L.; Ko, J.-L.; Hung, C.-H.; Latos-Graz˘yn´ski,
L. J. Am. Chem. Soc. 2004, 126, 4420.
(32) Walker, F. A. Proton NMR and EPR Spectroscopy of Paramagnetic
Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; pp
81-183.
(33) Wojaczyn´ski, J.; Latos-Graz˘yn´ski, L. Inorg. Chem. 1995, 34, 1044.
(34) Wojaczyn´ski, J.; Latos-Graz˘yn´ski, L.; Olmstead, M. M.; Balch, A. L.
Inorg. Chem. 1997, 36, 4548.
(28) Venkatraman, S.; Anand, V. G.; Pushpan, S. K.; Sankar, J.; Chan-
drashekar, T. K. Chem. Commun. 2002, 462.
Inorganic Chemistry, Vol. 45, No. 19, 2006 7829

Mys´liborski et al.
Scheme 1. Synthesis of Iron(II) N-confused Pyriporphyrin
Complexes
Figure 1. Electronic spectrum of 5-Br (black line, in toluene), 6-Br (red
line, in dichloromethane), and 7-Br (blue line, in dichloromethane).
ziporphyrin 1 has been achieved by addition of iron(II)
bromide to a THF solution of the ligand under strictly anaero-
bic conditions. Iron(II) bromide (free of iron(II)) was pre-
pared by stirring anhydrous iron(II) bromide and an excess
of iron powder in THF. This reduction step is essential for
metalation as the residual iron(III) can be involved in side
reactions with the products of insertion. The insertion yield
is quantitative with respect to 1. Actually, two products have
been identified using the 1H NMR spectroscopy. The marked
difference of their solubility in toluene allowed their straight-
forward separation. The resulting complexes will be denoted
5-Br and 6-Br (Scheme 1). A similar procedure yielded 5-Cl
as a single product once hydrated FeCl2 was applied.
The ESI mass spectrum of 5-Br shows the small peak
corresponding to a parent species (PyPH)FeIIBr 5-Br at m/z
) 872.5. The most intense peak at m/z ) 793.5 is from the
dissociation of a bromide ligand from (PyPH)FeIIBr forming
a cation [(PyPH)FeII]+. The ESI spectrum of 6-Br, when
dissolved in chloroform/methanol, yielded the most intense
peak at m/z ) 1074.8 which corresponds to the formula
{[FeIIBr2(CH3OH)]-[(PyPH)FeII(CH3OH)]}+, 6-(CH3OH)+.
From 6-Br in Scheme 1, this observation is consistent with
a substitution of the bromide ligand of [(PyPH)FeIIBr] with
methanol (L ) CH3OH). The tetrahedral coordination
geometry around the external iron(II) is suggested.
The electronic spectrum of 5-Br resembles that of iron-
(II) benziporphyrin (Figure 1).17 The coordination of iron-
(II) to the external nitrogen of 5-Br is reflected in particular
by a new band at 649 nm. The reaction of 5-Br and 6-Br
with dioxygen, which yielded 7-Br, has been followed using
UV-vis spectroscopy. The relatively slow reaction of 5-Br
with dioxygen is in contrast to the fast oxygenation of iron-
(II) N-confused porphyrin.31
NMR Studies of Iron(II) N-Confused Pyriporphyrin.
The molecular structure of 5-Br is expected to be analogous
to those of the X-ray characterized metallobenziporphyrins15-17
because the external nitrogen atom is expected to play a
minor role in the structural preferences (Figure 2). The
Figure 2. Drawing of the 5-Br structure obtained from the molecular
mechanics calculations.
coordinating environment of the metal ion forms a trigonal
bipyramid, with the N(24) atom, anionic ligand, and the
C(22)-H bond occupying the equatorial positions. In the
absence of X-ray data, molecular mechanics calculations
have been used to visualize the structure of 5-Br in the
minimization procedure using the standard MM+ param-
etrization with exception of the iron coordination surround-
ings. Namely, constraints have been imposed which reflect
iron-nitrogen bond lengths typical for high-spin state iron-
(II) porphyrinoids.17,29,35
1
The H NMR spectrum of 5-Br, corresponding to the
effective Cs symmetry of the molecule, is consistent with
the structure shown in Figure 2. The mirror plane goes
through the Br, Fe, and N(24) atoms. There are three distinct
â-H pyrrole positions, two pyridine H(2/4) and H(22)
positions, and two different meso-aryls.
The pertinent spectral parameters have been gathered in
Table 1. Resonance assignments, which are given above
selected peaks, have been made on the basis of relative
intensities, line widths, and pyrrole specific deuteration. The
line widths were compared for the spectra collected at 233
K to avoid any problems resulting from the feasible
exchange-related broadening. The observed pattern of chemi-
cal shifts of 5-X is quite typical for high-spin iron(II)
porphyrins and porphyrin analogues with three â-pyrrole
resonances in the region from 60 to -2 ppm and the meso-
aryl peaks in the 15 to -5 ppm range.30-32,36-38 The relatively
large line widths of meso-phenyl resonances as compared
(35) Latos-Graz˘yn´ski, L.; Lisowski, J.; Olmstead, M. M.; Balch, A. L. Inorg.
Chem. 1989, 28, 1183.
7830 Inorganic Chemistry, Vol. 45, No. 19, 2006

Iron Complexes of N-Confused Pyriporphyrin
Table 1. 1H NMR Chemical Shiftsa of Iron N-confused Pyriporphyrins
Scheme 2. Equilibrium between 5-L and 6-L (L ) Cl or solvent) in
Coordinating Solvent
complex
5-Br
5-Cl
solvent
H(22)b
H(2/4)
pyrrole
C7D8
CDCl3
424.6 (4600)
416.6 (4760)
54.3
52.4
64.3
70.1
63.5
201.3
50.8 24.5 -1.2
48.7 22.9 -1.0
44.0 25.0 -1.5
46.0 23.4 -1.0
5-(THF-d8) THF-d8 468.8 (4560)
5-(CD3CN) CD3CN 502.2 (4030)
5-(CD3OD) CD3OD 758 (11700)
53.6 45.0
57.2 53.6
c
0.7
6-Br
CD2Cl2
375.3 (2500)
a All data in ppm at 298 K. b Line widths (Hz) in parentheses. c Not
identified.
because of the replacement of bromide ligand by methanol
has been noticed.
Titration with Paramagnetic Metal(II) Ions. To probe
the coordination ability of the perimeter nitrogen to bind
iron(II), systematic titration of (PyPH)FeIICl, 5-L, in aceto-
nitrile-d3 or THF-d8 has been carried out. In the course of
titration of (PyPH)FeIICl with FeIICl2, smooth changes of
the chemical shifts of the pyridine moiety were noticed,
indicating that it was directly engaged in coordination. Under
these experimental conditions, the reaction involves solvated
forms such as [(PyPH)FeIIL]+, 5-L, 6-L, and FeIILn, respec-
tively (Scheme 2).
The detected chemical shifts are the time-averaged values,
which reflect relative populations of the mono- and diiron
species (5-L-6-L), which are in fast exchange. In the final
stage of titration, determined by the solubility of iron(II) in
acetonitrile-d3, the H(2/4) and H(22) resonances have been
located at 123 and 489 ppm, respectively, which is a
remarkable relocation with respect to the starting values (70
ppm for H(2/4) and 502 ppm for H(22)). Smaller changes
following the same directions were determined in THF-d8.
An analogous titration of 5-L with cobalt(II) chloride in
acetonitrile-d3 resulted in a pattern similar to that described
for iron(II) in the same solvent. All titrations were carried
out in anaerobic conditions. Once dioxygen was added to
the sample containing 6-L, the spectroscopic changes initially
reflected the reconversion to 5-L. Presumably, the oxidation
of the external iron(II) to iron(III) resulted in its dissociation
from 6-L. Under these experimental conditions, the affinity
of iron(III) to N(3) of pyridine seems to be smaller than that
of iron(II).
Figure 3. NMR spectra: (A) 5-Br (1H NMR, toluene-d8, 298 K); (B)
5-Br-d6 (2H NMR, toluene, 298 K). The inset (A′) presents the H(22) peak.
to â-H ones reflect the rotation of meso-aryls (Figure 3).32
The hydrogens on the m-pyridyl ring have been identified
by default. Interestingly, ∼420 ppm shift of the inner
C(22)-H proton is similar to that of iron(II) m-benzipor-
phyrin17 but much smaller than the corresponding H(21) shift
determined for iron(II) N-confused porphyrin (812 ppm, 298
K).30 A similar value has been found value for nickel(II)
m-benziporphyrin (386 ppm, 298 K).15 An agostic mecha-
nism of spin-density transfer, described previously for nickel-
(II) m-benziporphyrin, explains the shifts of 5-X as resulting
from electron donation from the CH bond to the metal. The
addition of small amount of acid to 5-Br in toluene-d8 gave
rise to the strongly upfield resonances at -72.7 ppm (298
K) assigned to NH hydrogen of protonated pyridine. The
plots of the temperature dependence of the chemical shifts
(Figure 3S, Supporting Information) demonstrated the fea-
tures typical for high-spin iron(II) porphyrinoids.30
Diiron Species 6. The titration experiment set some limits
1
the H NMR spectroscopic range expected for the diiron
(PyPH)FeIIBr, 5-Br, once dissolved in coordinating sol-
vents, yields the high-spin five-coordinate species [(PyPH)-
FeIIL]+, 5-L (L ) THF-d8, acetonitrile-d3, methanol-d4). The
solvent coordination produces visible changes in positions
of the selected resonances (Table 1), but the overall 1H NMR
features assigned to high-spin iron(II) N-confused pyripor-
phyrin are preserved. In particular, the dramatic relocation
of the H(22) resonance from 424.6 to 758 ppm (298 K)
species 6 in the slow exchange limits (i.e., under conditions
created in a chlorinated solvent). The 1H NMR spectrum of
the detected diiron compound 6-Br is presented in Figure 4.
The spectroscopic pattern of 6-Br resembles that of the
monoiron species 5-L with one but crucial addition. Con-
sequently, the H(2/4) hydrogen yields a remarkably shifted
resonance at 201 ppm (298 K) (Figure 4). Actually, this
resonance is located in the region which is consistent with
the titration data presented previously. The 1H NMR spectra
of pyridine coordinated to a high-spin metal ion with half-
(36) Balch, A. L.; Chan, Y.-W.; La Mar, G. N.; Latos-Graz˘yn´ski, L.;
Renner, M. W. Inorg. Chem. 1985, 24, 1437.
(37) Pawlicki, M.; Latos-Graz˘yn´ski, L. Inorg. Chem. 2002, 41, 5866.
(38) Pawlicki, M.; Latos-Graz˘yn´ski, L. Inorg. Chem. 2004, 43, 5564.
2
occupied dz , which may serve as the reference point to assign
H(2/4) are difficult to generate because of the intrinsic lability
Inorganic Chemistry, Vol. 45, No. 19, 2006 7831

Mys´liborski et al.
Figure 4. Composite 1H NMR spectrum of 6-Br (dichloromethane-d2,
298 K). Labels as in Chart 1. This figure was composed from three spectra
collected over the following regions: 160 to -40, 245 to 45, and 380 to
180 ppm. The frequency domain spectra were subsequently added,
preserving the relative intensities of the resonances.
Figure 6. NMR spectra: (A) 7a-Br (black labels) and 7b-Br (gray labels)
formed by the addition of dioxygen to 5-Br (1H NMR, toluene-d8, 298 K)
and (B) 7a-Br-d6 and 7b-Br-d6 (2H NMR, toluene, 298 K).
of the formed compounds and the rather large shifts and line
widths expected for the R-H-pyridine resonances. Previously
it was determined that pyridine, axially coordinated to nickel-
(II) thiaporphyrin, revealed the following pattern: H(2)
156.3, H(3) 68.4, H(4) 17.2 ppm (203 K, dichloromethane-
d2).39 The downfield position of H(2/4) resonance for 6-Br
is consistent with coordination to the high-spin iron(II),
assuming that the spin delocalization pathways are identical
with that determined for nickel(II) thiaporphyrin.
6-Br can be easily converted in the derivative of 5-X via
the addition of competing ligands or external iron(II) directed
oxidation. The addition of pyridine-d5 to the solution of 6-Br
1
in dichloromethane-d2, as followed by H NMR, removed
its H(2/4) resonance from the spectrum because of the
formation of 5-Br and the iron(II)-pyridine-d5 complex. The
1H NMR titration of 6-Br with bromine is shown in Figure
5. The selected region covers H(2/4) resonance of 6-Br and
H(22) resonances of 5-Br and 6-Br. The overlapping pyrrole
and H(2/4) resonances of 5-Br and 6-Br are shown in insets
(traces B, C, and D). Throughout the entire titration, a single
set of resonances has been detected for all â-H positions,
which reflects the fact that 5-Br and 6-Br are in fast exchange
in this particular spectroscopic window. At the same time,
two separate H(22) resonances have been detected as the
chemical shift difference are markedly large for 5-Br and
6-Br for these particular hydrogens. Actually, the individual
H(2/4) resonances assigned for 5-Br and 6-Br have been
clearly identified throughout the whole titration. Despite the
enormous line widths, the far downfield-shifted resonances
provide the very valuable analytical pattern, which is unique
for the N-confused pyriporphyrin diiron species 6-Br.
Figure 5. Composite 1H NMR spectra showing a conversion of 6-Br
(dichloromethane-d2, 298 K) into 5-Br in the course of titration with Br2 (5
µL/200 µL solution in dichloromethane-d2). Trace A shows the spectrum
of 6-Br separated from the insertion products by extraction of 5-Br with
toluene. Trace B shows the mixture of 5-Br and 6-Br as obtained directly
in synthesis. Traces C and D show the effects of additions of 4 µL (C) and
6 µL (D) of the Br2 solution to the original sample shown in B. To generate
the figure, the frequency domain spectra were subsequently added,
preserving the relative intensities of the resonances. The insets present the
70 to -20 ppm region of the appropriate spectrum.
(39) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Inorg. Chem. 1992, 31, 5231.
7832 Inorganic Chemistry, Vol. 45, No. 19, 2006

Iron Complexes of N-Confused Pyriporphyrin
Scheme 3. Oxidation and Oxygenation of 5-Br
Chart 2. Structure of 9-Br
expected to be positioned at an angle to the macrocyclic
plane. Thus, in the ferric 7-Br complexes, the halide ligand
can be coordinated on one of the two inequivalent faces of
the macrocycle, leading to two distinct species (syn and anti).
A similar type of isomerism was previously observed for
the 22-hydroxybenziporphyrin complexes and asymmetric
metalloporphyrin derivatives.41-43 The shift pattern demon-
strated by the pyridinolato moiety is of particular signifi-
cance, revealing the downfield positions of H(2/4) for 7a
1
and 7b. This H NMR position is very characteristic for a
phenoxide ligand coordinated to iron(III) porphyrins and
porphyrinoids.44
Oxidation and Oxygenation of 5-Br. The reaction of
(PyPH)FeIIBr, 5-Br, with dioxygen (72 h in toluene-d8)
results in the formation of a five-coordinate species
(PyPO)FeIIIBr, 7-Br ((PyPOH)H 3-aza-22-hydroxy-m-ben-
ziporphyrin, PyPO - the corresponding dianion). 7-Br is
formed by oxygenation at the C(22) position (Scheme 3).
A careful titration of (PyPH)FeIIBr, 5-Br, with Br2 has been
carried out in dichloromethane-d2. The characteristic set of
three pyrrole resonances at 122.4, 79.6, and 70.4 ppm
(dichloromethane-d2, 298 K) assigned to [(PyPH)FeIIIBr]+,
8-Br (i.e., to the product of one-electron metal-centered
oxidation of (PyPH)FeIIBr, 5-Br). Significantly, the fast
conversion of [(PyPH)FeIIIBr]+, 8-Br, into (PyPO)FeIIIBr,
7-Br. in the presence of dioxygen takes place.
In the course of this process, the new type of core-modified
porphyrin was identified. The macrocycle incorporates a
4-hydroxypyridine unit into the porphyrin-like structure. 7-Br
is stable and was purified by the standard chromatographic
procedure. Actually, the 1H NMR spectroscopic features of
7-Br reproduce those of the iron(III) core modified porphyrin
incorporating a phenolate donor 940 (Chart 2), with the H(3)
resonance missing from the spectrum as the corresponding
CH unit was replaced in 7-Br by nitrogen. The compound
7-Br is actually a mixture of two isomers (Scheme 3). They
will be denoted 7a-Br and 7b-Br with isomer 7a-Br being
more abundant. Two sets resonances in the downfield region
of the spectrum, each composed of three pyrrole and one
2/4-H resonances of the pyridinolato fragment, allowed their
identification. Relative amounts of the two isomers were
estimated by careful deconvolution of the downfield region
of the spectrum. The concentration ratio [7a-Br]/[7b-Br]
equals 3.5 (298 K, toluene-d8).
Conclusions. This study broadens our understanding of
the coordination of iron in a porphyrin and in porphyrin-
like environments. Essential similarities of iron N-confused
porphyrin and iron N-confused pyriporphyrin complexes
have been established. There is a considerable analogy in
1
their H NMR properties. Significantly, the nitrogen atom
at the macrocyclic perimeter may provide a promising means
to control overall macrocyclic properties, for instance, via
protonation or alkylation. Potentially, an external N-coor-
dination of pyridine offers a route to construct polymetallic
arrays where the pyridine moiety will serve as a unique
bridge, which links two metal ions, involving a simultaneous
(41) Balch, A. L.; Latos-Graz˘yn´ski, L.; Noll, B. C.; Olmstead, M. M.;
Zovinka, E. P. Inorg. Chem. 1992, 31, 2248.
Coordination of a metal ion by 3-aza-22-hydroxybenzi-
porphyrin imposes a steric constraint on the geometry of the
ligand. In analogy to 9,40 the pyridinolato moiety of 7-Br is
(42) Kalish, H.; Camp, J. E.; Stepien´, M.; Latos-Graz˘yn´ski, L.; Olmstead,
M. M.; Balch, A. L. Inorgc. Chem. 2002, 41, 989.
(43) Tung, J.-Y.; Jiang, J.-I.; Lin, C.-C.; Chen, J.-H.; Hwang, L.-P. Inorg.
Chem. 2000, 39, 1.
(44) Arasasingham, R. D.; Balch, A. L.; Hart, R. H.; Latos-Graz˘yn´ski, L.
(40) Stepien´, M.; Latos-Graz˘yn´ski, L. Inorg. Chem. 2003, 42, 6183.
J. Am. Chem. Soc. 1990, 112, 7566.
c
Inorganic Chemistry, Vol. 45, No. 19, 2006 7833

Mys´liborski et al.
The solid 6-Br was filtered off. The toluene solution was dried
under vacuum to obtain species 5-Br.
coordination by the C (CH or CO) and N ends of pyridine.
Actually, the design of discrete ordered patterns of porphyrins
and metalloporphyrins has attracted much interest recently
leading to the synthetic methodology with a “building block”
approach.45-50 The oligomeric porphyrins and metallopor-
phyrins of various structures are under intensive investigation
because of their potential applications in molecular electron-
ics and as new photonic materials.51,52 Previously, several
polymeric or oligomeric metalloporphyrin structures, ob-
tained by a self-assembly approach, were constructed em-
ploying a meso-pyridyl moiety coordination.47,48,53,54 In this
context, the coordinating properties of N-confused pyripor-
phyrin, containing a pyridyl moiety directly built into the
porphyrin frame, are of further interest.
5-Br. HRMS (ESI, m/z): [M - Br]+ 793.2894 (793.2994 calcd
for C53H45N456Fe+). UV-vis (λmax, nm): 335, 373, 436, 881.
6-Br. Dissolved in CH2Cl2/CH3OH. MS (ESI, m/z): 1073.8
(calcd 1073.5 for [C55H53N4O279Br81Br56Fe2]+ which corresponds
to {[FeIIBr2(CH3OH)]-[(PyPH)FeII(CH3OH)]}+). UV-vis (λmax
,
nm): 330, 385, 385, 649, 875.
7-Br. A dichloromethane or toluene solution of 5-Br (3 mg in 1
mL of solvent) was purged with dioxygen for 15 min. Subsequently,
7-Br was separated using column chromatography on silica. The
fraction eluted with acetone, containing 7-Br, was collected and
evaporated. HRMS (ESI, m/z): [M - Br]+ 808.2882 (calcd
808.2866 for [C53H44N4O56Fe]+). UV-vis (λmax, nm): 412, 832,
978.
1H NMR Studies. The solution of 5-Br or 6-Br in a deuterated
solvent was prepared under purified nitrogen in a glovebox.
Typically a sample of ∼1-2 mg of 5-Br or 6-Br was dissolved in
0.5 mL of deuterated solvent. The solution was directly placed into
an 1H NMR tube and sealed with a septum cap. In titration
experiments, a solution of the pyridine-d5 or oxidizing reagent in
the deuterated, deoxygenated solvent was gradually added to the
sample through a 10 µL microsyringe. Usually an appropriate mass
of the applied reagent was dissolved to give an approximately 0.25
M solution. It typically required 2-5 µL of the solution to be added
to the NMR tube containing iron N-confused pyriporphyrin to cause
a detectable conversion. The addition of dioxygen was carried out
by purging of oxygen through the sample.
Experimental Section
Materials. N-confused pyriporphyrin was obtained by previously
described methods.1 The deuterated derivatives were synthesized
using pyrrole-d5 and trifluoroacetic anhydride/D2O in a condensation
to give a sample of N-confused pyriporphyrin which was shown
1
by H NMR spectroscopy to be maximally 50% deuterated at the
pyrrolic positions.
Toluene-d8 (distilled under nitrogen from sodium), tetrahydro-
furane-d8, dichloromethane-d2, chloroform-d, and acetonitrile-d3
were degassed by the freezing-pumping-thawing method and
stored in a dry glovebox.
Iron(II) Insertion into N-Confused Pyriporphyrin: 5-Br and
6-Br. Iron(II) bromide (free of iron(III)) was prepared by stirring
under N2 (in a glovebox) 0.043 g (0.2 mmol, 5 equiv) of anhydrous
iron(II) bromide and excess (∼0.07 g) iron powder in ∼4 mL THF
(distilled from sodium under N2) until the solution became colorless.
Iron was filtered off, and the colorless solution of iron(II) bromide
was added to a dark green THF solution of 1 (0.03 g, 0.04 mmol).
An aliquot of ∼5 µL of collidine was added. The resulting brownish
solution was heated and stirred at 55 °C for 3 h. The iron insertion
was quantitative, as confirmed by 1H NMR. The solution was
evaporated and dried in a glovebox. The residue was redissolved
in a minimum volume of toluene. The marked solubility difference
in toluene allowed straightforward separation of 5-Br and 6-Br.
2
2H NMR Studies. The samples used in H NMR experiments
1
have been prepared similarly as those for H NMR starting from
5-Br-d6 or 6-Br-d6 using regular solvents.
1
Instrumentation. H NMR 500 MHz spectra were measured
on Bruker Avance 500 spectrometers. The peaks were referenced
2
against the residual resonances of the deuterated solvents. The H
NMR spectra were collected using a Bruker Avance 500 instrument
operating at 76.77 MHz. A spectral width of 200 ppm was typical
and 32 K points were used. A pulse delay of 50 or 100 ms was
applied. The residual 2H NMR resonances of the solvents were used
as a secondary reference.
Molecular Mechanics Calculation. Molecular mechanics
calculations were performed using the HyperChem software
(Autodesk). The standard MM+ force field, with the constraints
set on the coordination bonds to achieve a high-spin iron(II)
porphyrin geometry, were used as described in the text.
(45) Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. Chem. ReV.
2001, 101, 2751.
(46) Burrell, A. K.; Wasielewski, M. R. J. Porphyrins Phthalocyanines
2000, 4, 401.
(47) Imamura, T.; Fukushima, K. Coord. Chem. ReV. 2001, 198, 133.
(48) Wojaczyn´ski, J.; Latos-Graz˘yn´ski, L. Coord. Chem. ReV. 2000, 204,
113.
(49) Prodi, A.; Indelli, M. T.; Kleverlaan, C. J.; Alessio, E.; Scandola, F.
Coord.Chem. ReV. 2002, 229, 51.
(50) Iengo, E.; Zangrando, E.; Bellini, M.; Alessio, E.; Prodi, A.; Chiorboli,
C.; Scandola, F. Inorg. Chem. 2005, 44, 9752.
(51) Benniston, A. C. Chem. Soc. ReV. 2004, 33, 573.
(52) Drain, C. M.; Goldberg, I.; Sylvain, I.; Falber, A. Top. Curr. Chem.
2005, 245, 55.
(53) Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas, J. D. Angew. Chem.,
Int. Ed. Engl. 1998, 37, 2344.
Acknowledgment. This work was supported by the
Ministry of Scientific Research and Information Technology
of Poland under Grant 3 T09A 162 28.
Supporting Information Available: MS spectra for 5+, 6+,
and 7+ and VT NMR data for 5-Br and 6-Br. This material is
available free of charge via the Internet at http://pubs.acs.org.
(54) Drain, C. M.; Lehn, J. J. Chem. Soc., Chem. Commun. 1995, 503.
IC060906R
7834 Inorganic Chemistry, Vol. 45, No. 19, 2006