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-
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Inorganic Chemistry, Vol. 45, No. 19, 2006 7829