Dimeric iron N-confused porphyrin complexes

Article abstract of DOI:10.1039/b202679a

A dimeric iron N-confused porphyrin, [Fe(NCTPP)]₂ was obtained from the anaerobic reaction of Fe(NCTPP)Br with NaSePh while under aerobic conditions a hydroxo bridged iron dimer with Na bridging the outer-N atoms was obtained and oxygenation occurred on the inner core pyrrolic carbon to form a novel ONCTPP porphyrinic ring.

Full text of DOI:10.1039/b202679a

Dimeric iron n-confused porphyrin complexes†
Chen-Hsiung Hung,*a Wan-Chin Chen,a Gene-Hsiang Leeb and Shie-Ming Pengc
a Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan.
E-mail: chhung@cc.ncue.edu.tw
b Instrumentation Center, National Taiwan University, Taipei 10764, Taiwan
c
Department of Chemistry, National Taiwan University, Taipei 10764, Taiwan
Received (in Cambridge, UK) 18th March 2002, Accepted 25th May 2002
First published as an Advance Article on the web 13th June 2002
A dimeric iron N-confused porphyrin, [Fe(NCTPP)]2 was
obtained from the anaerobic reaction of Fe(NCTPP)Br with
NaSePh while under aerobic conditions a hydroxo bridged
iron dimer with Na bridging the outer-N atoms was obtained
and oxygenation occurred on the inner core pyrrolic carbon
to form a novel ONCTPP porphyrinic ring.
coordinating ability of selenate is essential to the formation of
the dimeric complex. The UV-vis spectrum of the dimeric iron
NCP complex 6 has a Soret band at 459 nm, a broad Q band at
755 nm and a shoulder around 660 nm. The energy of the Soret
band in 6 is comparable to that for Fe(NCTPP)Br but the Q
bands are slightly red-shifted. The similar absorption spectra
suggest identical tautomeric forms for 6 and Fe(NCTPP)Br (3,
M = Fe) with a dianionic NCP ring from the deprotonation of
the inner core amino nitrogen and a peripheral amino nitrogen.
SQUID measurement of dimer 6 gives a magnetic moment of
4.31 mB at 300 K for each Fe. The magnetic moment decreases
with a decrease in temperature to 1.64 mB at 6 K. Preliminary
fitting of the SQUID data suggested weak antiferromagnetic
coupling between the two iron centers. However, fitting the data
using magnetic susceptibility equations for binuclear com-
plexes employing Heisenberg intercluster magnetic exchange
with a combination of different spin quantum numbers8 has not
yet given a reliable result due to either partial oxidation of the
extremely air sensitive 6 during data collection or because of a
complicated electronic structure.
N-Confused porphyrin (NCP), an isomeric porphyrin, has an
inverted pyrrole ring joined to the porphyrinic conjugated
system through a b-carbon, and exhibits a peripheral pyrrolic
nitrogen and an inner core carbon. Recent studies on the N-
confused porphyrin demonstrated that this molecule displays
distinct physical properties and coordination chemistry.1 N-
Confused porphyrins were reported to stabilize metal centres
with unusual oxidation states through a planar 32 or 22
charged porphyrin ring (1, 2).2,3 We and others also showed that
NCP can coordinate to divalent metals through a tridentate
coordination mode to form a tilted inverted pyrrole ring (3).4,5
In addition to mononuclear complexes, a binuclear Pd(II
double-decker complex (4)6 with metals bridging two NCP
rings, or a bis-Rh(
) NCP complex (5)7 with metals simply
)
I
The geometry of the dimeric compound was determined by
single crystal structure analysis.‡ Compound 6 crystallized in
an orthorhombic Pbca cell leading to an independent
Fe(NCTPP) and a unit of Fe(NCTPP) generated from symmetry
(Fig. 1). The dimeric structure assembles by a direct coordina-
tion of iron to the neighboring peripheral nitrogen on the other
NCP ring. The inverted pyrrole rings cant significantly from the
mean porphyrin plane with an angle of 137° between the Fe–C
bond and the inverted pyrrole ring. The distance of iron to the
inner carbon in the inverted pyrrole ring is 2.341(5) Å which is
close to the distance of 2.361(10) Å in Fe(NCTPP)Br. The bond
distances and bond angles around the porphyrin core resemble
those in Fe(NCTPP)Br and suggest that the tautomer form
remains unchanged from the monomeric compound to the
dimer. The average Fe–Np distance of 2.075(5) Å is within the
range 2.072–2.096 Å for the corresponding distance in five-
coordinate high spin iron(II) complexes.9 Interestingly, in order
to release steric constraint between two NCP rings, one of the
phenyl rings rotates significantly resulting in a dihedral angle of
44.6° between this phenyl ring and a neighboring pyrrole
plane.
coordinated to one NCP ring are observed.
Based on our recent progress on iron NCP complexes,4 herein
we report that the reaction of Fe(NCTPP)Br with NaSePh gave
a dimeric NCP complex with the metals retained in the NCP
rings. Interestingly, when the dimeric iron complex was
exposed to air, a hydroxo bridged dimeric iron porphyrin was
isolated. Surprisingly, the NCP rings underwent oxygenations
to form oxoporphyrin type NCP rings and a sodium ion was
coordinated to both peripheral nitrogens.
The dimeric iron complex 6 oxidizes readily in aerobic
conditions. In the solution state in an anaerobic dry box,
compound 6 still picks up oxygen from the environment to give
a product with a UV-vis spectrum identical to compound 7 after
several weeks. The mass spectrum of the oxidation product
Treatment of Fe(NCTPP)Br4 with an excess (1.25 equiv.) of
sodium benzeneselenate, NaSePh, in THF under anaerobic
condition yields 85% of dark green crystalline [Fe(NCTPP)]2
(6) after recrystallization (Scheme 1). The relatively weak
† Electonic supplementary information (ESI) available: general informa-
tion; preparation and crystal data for 6 and 7; Fig. S1: absorption spectra for
6 and 7; Figs. S2 and S3: magnetic susceptibility data for 6 and 7. See http:
//www.rsc.org/suppdata/cc/b2/b202679a/
Scheme 1
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This journal is © The Royal Society of Chemistry 2002

mainly on meso carbons. Compound 7 represents a new
porphyrin analogue with a hydroxy group on the inner core
pyrrolic carbon. The separation of 1.336(6) Å between oxygen
(O(2)) and the inner core carbon is much longer than the average
bond distance of 1.227(9) Å for CNO found in thallium
dioxoporphodimethene,12 but is close to that of 1.356(22) Å in
the oxophlorin complex NiII(OEPOH).13 Although the hydroxy
form is observed in 7, a tautomerization, as in oxophlorin,
between the hydroxy and keto form for the free base of
ONCTPP is expected. The short distance of 1.957(4) Å between
iron and the oxo group on the ONCTPP ring suggests a
coordination of oxygen to the iron center. Furthermore, the
distance of 2.296(5) Å between iron and the inner core carbon
is much closer in comparison with the corresponding distance
for iron or manganese NCTPP complexes4 and indicates a much
stronger Fe…C interaction. The distance of 1.945(2) Å between
iron and oxygen on the m-hydroxo group is comparable to that
of 1.924(2) Å in {[Fe(OEP)]2(OH)}+.14 In contrast with mostly
linear Fe–O–Fe bond in iron(III) porphyrin m-oxo dimers, the
angle of 142.9(3) for Fe–OH–Fe further confirms the hydroxo
bridging in 7.15 Finally, the distances of 2.414(6) and 2.315(6)
Å for Na–N and Na–O are similar to literature values.16 Overall,
neglecting the potential electron delocalization, the ONCTPP
cores are trianionic charged from the deprotonation of the
peripheral nitrogen, the inner core amino nitrogen and the inner
core hydroxy group. Both irons are in 3+ oxidation state and the
negative charge of bridging hydroxo group is balanced by the
sodium cation. The demetallation and purification of ONCTPP
is under active study.
Fig. 1 The ORTEP diagram (35% ellipsoids) of [Fe(NCTPP)]2 6.
suggested the insertion of an oxygen atom into the NCP core.
The absorption spectrum of 7 isolated from the reaction shown
in Scheme 2 gives an unusually broad Soret band at 389 nm
without a clearly identified Q band and suggests perturbation of
the N-confused porphyrin p-conjugated system. SQUID mag-
netic susceptibility measurement gives a magnetic moment of
3.40 mB at room temperature for each metal center and suggests
a stronger antiferromagnetic coupling between the two iron
centers compared to 6.
Notes and references
‡
Crystallographic data: for [Fe(NCTPP)]2 6: C96H72Fe2N8O2, T = 293
K, M = 1481.32, orthorhombic, space group Pbca, a = 15.867(5), b =
20.697(6), c = 22.397(7) Å, V = 7355(4) Å3, Z = 4, Dc = 1.338 Mg m23
,
l = 0.71073 Å, m = 0.454 mm21, F(000) = 3088. The structure was
solved by direct methods and refined by least squares against F2 to R1 =
0.0756 (wR2 = 0.1546) and Sgof = 0.983.
For [Fe(ONCTPP)]2(OH)·Na(THF)2 7: C108H95Fe2N8NaO8, T = 293 K,
M
= 1767.61, monoclinic, space group C2/c, a = 28.473(3), b =
18.7702(18), c = 16.8963(16) Å, b = 104.922(2)°, V = 8725.6(14) Å 3, Z
= 4, Dc = 1.346 Mg m23, l = 0.71073 Å, m = 0.404 mm21, F(000) =
3704. The structure was solved by direct methods and refined by least
squares against F2 to R1 = 0.0702 (wR2 = 0.1815) and Sgof = 0.924.
CCDC reference numbers 184398 and 184399. See http://www.rsc.org/
suppdata/cc/b2/b202679a/ for crystallographic data in CIF or other
electronic format.
Scheme 2
The crystal structure of 7 (Fig. 2)‡ reveals unexpected
chemistry and is important in understanding the NCP oxidation.
Instability to oxidation of iron NCTPP complexes was observed
in our previous report.4 Recently, Furuta and Osuka10 isolated a
tripyrrinone complex from the aerobic preparation of
Cu(NCTPP) complex and suggested mechanisms for NCP ring
opening reactions. However, the structure of 7 suggests that the
oxidation reaction of the iron NCP complex proceeds via a
different pathway. According to the crystal structure, in addition
to a normal m-hydroxo iron dimer, a novel oxo N-confused
porphyrin (ONCP) is clearly observed. The oxygenation of
porphyrin to oxophlorin or dioxoporphodimethene is well-
known in the literature.11 However, the oxygenation sites are
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Fig. 2 The ORTEP diagram (35% ellipsoids) of [Fe(ONCTPP)]2(OH)·
Na(THF)2 7.
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