High affinity of 'arbor' iron porphyrins for dioxygen

Article abstract of DOI:10.1039/a805357j

The equilibrium rates of dioxygen and carbon monoxide binding have been measured for a series of capped iron porphyrins called 'arbor'. The affinity for dioxygen of these models is 100-fold higher than the highest previously reported values.

Full text of DOI:10.1039/a805357j

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High affinity of “arborÏ iron porphyrins for dioxygen¤
David Ricard, Bruno Andrioletti, Bernard Boitrel and Roger Guilard*
L aboratoire de Synthese et dÏElectrosynthese Organometalliques (L SEO, CNRS UMR 5632),
Faculte des Sciences Gabriel, Universite Dijon, 6, boulevard Gabriel, 21004, Dijon cedex, France
L e t t e r
The equilibrium rates of dioxygen and carbon monoxide binding have been measured for a series of capped iron
porphyrins called “arborÏ. The affinity for dioxygen of these models is 100-fold higher than the highest
previously reported values.
In order to deÐne the di†erent structural features that could
be involved in the binding and discrimination of gaseous mol-
shifted signals.° Gas binding studies were performed by
adding increasing quantities of diluted dioxygen in nitrogen to
ecules such as CO or O , di†erent types of dioxygen-binding
heme models have been described over the past two decades.1
a solution of the Ðve-coordinate species (k \ 444 nm) in a
2
max
known-volume tonometer,7 the reversibility of the binding
This biomimetic approach Ðrst allowed the design and synthe-
sis of molecules possessing some properties of the protein sur-
rounding the natural heme and mainly the reversibility of the
molecular oxygen coordination.2 Stability of the oxygenated
complex and discrimination of the carbon monoxide coordi-
nation are also two signiÐcant factors. It is now well-
established that the same distal histidine residue is responsible
being monitored before and after the measure. Indeed, nitro-
gen bubbling in the tonometer leads to the starting Ðve-
coordinate material.
for both stabilization of O via a hydrogen bond and destab-
2
ilization of CO. Momenteau and coworkers were the Ðrst to
report the existence of such a hydrogen bond3 involving the
NH amide group located on a lateral position of the model.
More recently, Reed and coworkers described picket-fence-
derived models possessing a potential H-bonding group in a
more apical4 position than the usual amide group linked to
the meso aromatic group of the porphyrin.
Herein, we report a preliminary study of O and CO
2
coordination with TREN [tris(2-aminoethylamine)]-capped
porphyrins, named “arborÏ. These two di†erent porphyrins
have exactly the same distal superstructure tethered at two
di†erent distances by four linkers.5 For such models, two sec-
ondary amines are eventually able to engage a H bond with
the dioxygen bound in the distal cage created by the TREN
moiety. The synthesis of the iron(II) Ðve-coordinate complexes
is depicted in Scheme 1. The atropisomer aaaa of the meso-
(tetra-o-aminophenyl)porphyrin is functionalized by either
chloroacetyl chloride or acryloyl chloride leading, respectively,
to the picket porphyrins 1 or 2. Then, iron insertion is carried
out by heating the porphyrin in THF with iron bromide and
2,6-lutidine.” The TREN molecule is grafted at 55 ¡C in
MeOH for 14 h, after which an excess of nitrogenous base is
added to stabilize the iron(II) as a Ðve-coordinate complex.
For this purpose, we chose 2-methylimidazole.6 The NMR
spectrum of 4 in pyridine-d clearly shows that the solvent
5
cannot coordinate inside the distal pocket, hence the obser-
vation of a Ðve-coordinate complex with typically downÐeld-
¤ Non-SI units employed: 1 Torr B 133 Pa.
” To control that no atropisomerization has occurred, the proton
NMR spectrum of the iron picket porphyrin 2 was recorded in the
Scheme 1 Reagents and conditions: (i) chloroacetyl chloride or acry-
loyl chloride in dry THFÈNEt ; (ii) FeBr ÈTHFÈ2,6-lutidine under
coordinating solvent pyridine-d , in which the porphyrin yields a dia-
5
3
2
magnetic six-coordinate complex. As expected for a complex pos-
nitrogen; (iii) TREN in THF or MeOH at 55 ¡C; (iv) 1000 equiv. 2-
sessing a C geometry, the NMR spectrum exhibits two triplets and
methylimidazole.
2v
two doublets corresponding to the meso aromatic protons. 1H NMR
500 MHz (pyridine-d , 323 K) of 2(Pyd5) : d 8.82 (d, J \ 7.0 Hz,
5
2
4H ); 8.74 (s, 8H, b-pyr.); 8.23 (large s, 4H, wNHCO); 7.98 (d,
° The NMR spectrum of 4(Pyd5) clearly shows four resonances corre-
sponding to the b-pyrrolic protons at 51.07, 48.85, 46.96 and 46.22.
HR-MS (LSIMS) m/z calcd: 1091.4132 [M [ H]` for
ar
J \ 7.0 Hz, 4H); 7.77 (t, J \ 7.5 Hz, 4H ); 7.47 (t, J \ 7.5 Hz, 4H );
ar
ar
5.90 (d, J \ 17.0 Hz, 4H, wCHx); 5.17 (m, 4H, xCH ); 5.06 (m, 4H,
2
xCH ).
C
H
FeN O ; found: 1091.4160 for 4 without any axial ligand.
2
62 59
12 4
New J. Chem., 1998, Pages 1331È1332
1331

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Table 1 Selected equilibrium parameters for O and CO binding (P in Torr), in toluene at 25 ¡C, except for Mb and Hb (H O, pH 7, 25 ¡C)
2
1@2
2
P
(O )
P
(CO)
M
Ref.
1@2
2
1@2
Mb
5 ] 10~1
3 ] 10~1
2 ] 10~2
1.4 ] 10~3
3 ] 10~1
8.9 ] 10~3
1.9 ] 10~1
2.9
2 ] 10~1
3 ] 10~4
3.1 ] 10~4
25
8
9
9
10
11
12
13
14
Hb (human R)
Hb (human T)
170
133
4270
8 ] 10~2
8 ] 10~1
105
100
0.3
40
38
1.6 ] 10~2
2.3
21
Fe(picket-fence)(1,2-DiMeIm)
FeG2(1,2-DiMeIm)
FeTACN(1,2-DiMeIm)
FeArC2Py
FeSFH-C7(1-MeIm)a
Fe4C2(2-MeIm), 4
Fe4Cl(2-MeIm), 3b
3 ] 10~2
9.8 ] 10~5
This work
This work
\10~5
\10~5
È
a The complex remains mostly Ðve-coordinate at the concentration of nitrogeneous base used by the authors. b To obtain a precise value, higher
dilutions than 105 would have been necessary; this is the reason why only limiting values are reported.
Our results are gathered in Table 1 together with the data
observed for natural proteins and the landmark picket-fence
References
1 M. Momenteau and C. A. Reed, Chem. Rev., 1994, 94, 659 and
references herein.
2 J. P. Collman, R. R. Gagne, T. R. Halbert, J.-C. Marchon and
C. A. Reed, J. Am. Chem. Soc., 1973, 95, 7868.
model. In this Table are reported both the partial pressure at
half-saturation for O and CO binding and the partition coef-
2
Ðcient M that characterizes the preference for CO versus O
2
binding. It is deÐned as the ratio of P (O ) over P (CO), or
the ratio of K (CO) over K (O ): the lowest M gives the best
discrimination against CO. For example, FeTACN(1,2-
3 (a) J. Mispelter, M. Momenteau, D. Lavalette and J.-M. Lhoste, J.
Am. Chem. Soc., 1983, 105, 5165. (b) D. Lavalette, C. Tetreau, J.
Mispelter, M. Momenteau and J.-M. Lhoste, Eur. J. Biochem.,
1984, 145, 555.
1@2
2
1@2
eq
eq
2
DiMeIm) recently reported by Collman et al.12 (Table 1) has
4 G. E. Wuenschell, C. Tetreau, D. Lavalette and C. A. Reed, J. Am.
Chem. Soc., 1992, 114, 3346.
the same affinity for O and CO, therefore an M value equal
2
to ca. 1. In our case, it is striking that the porphyrin Fe4C2
5 B. Andrioletti, B. Boitrel and R. Guilard, J. Org. Chem., 1998, 63,
(2-MeIm), 4, has an affinity for O 100 times greater than the
1312.
2
highest affinity already reported for synthetic models such as
6 J. P. Collman and C. A. Reed, J. Am. Chem. Soc., 1973, 95, 2048.
7 (a) T. J. Beugelsdijk and R. S. Drago, J. Am. Chem. Soc., 1975, 97,
6466. (b) J. P. Collman, J. I. Braumann, K. M. Doxsee, T. R.
Halbert and K. S. Suslick, Proc. Natl. Acad. Sci. USA, 1978, 75,
564. (c) J. R. Long and R. S. Drago, J. Chem. Educ., 1982, 59, 1037.
8 F. Antonini and M. Brunori, Hemoglobin and Myoglobin and
T heir Reactions with L igands, North Holland Publishing,
Amsterdam, 1971.
9 C. A. Sawicki and Q. H. Gibson, J. Biol. Chem., 1977, 252, 7538.
10 J. P. Collman, J. I. Brauman, B. L. Iverson, J. L. Sessler, R. M.
Morris and Q. H. Gibson, J. Am. Chem. Soc., 1983, 105, 3052.
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1997, 193.
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Eubanks, B. Boitrel, P. Hayoz, X. Zhang, J. I. Brauman and V. W.
Day, J. Am. Chem. Soc., 1997, 119, 3481.
13 E. Rose, B. Boitrel, M. Quelquejeu and A. Kossanyi, T etrahedron
L ett., 1993, 34, 7267.
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Schae†er, C. Huel and J.-M. Lhoste, J. Chem. Soc., Perkin T rans.
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FeSFH-C7(1-MeIm) or FeG2(1,2-DiMeIm). It is very tempt-
ing to explain the very high affinity of our models by a
favored distal H-bonding stabilization due to the presence of
two secondary apical amines. The slightly better affinity of
porphyrin 3 could be explained by a stronger H bond
resulting from a shorter distance between the amino group
and dioxygen, illustrating that in our models, the e†ective dis-
crimination comes from stabilization of O (presumably
2
through hydrogen bonding) rather than CO destabilization by
steric e†ects.15 On the other hand, the observed CO affinity is
also very high and quite comparable to the FeSFH-C7
(1-MeIm) of Momenteau et al.14 (see Table 1). It is also worth
noting that these two observations are consistent with the
pocket shaped by the TREN that is apparently not congested
enough to a†ect signiÐcantly the CO affinity in terms of steric
e†ects.
In conclusion, “arborÏ porphyrins are the Ðrst models of
myoglobin with speciÐc features such as a polar-capped struc-
ture, including secondary amines in an almost apical position,
as potential H-bonding groups. Our preliminary studies show
that these models are both very stable in oxygenated solution
and show reversible binding. The two di†erent linkers,
employed to graft the TREN, lead to a signiÐcant discrimi-
nation against CO, the M values being close to 0.1. Further
studies are in progress, Ðrst to conÐrm the role of the capping
structure and the potential H-bonding secondary amines, and
secondly to determine the kinetic rates.
Received in Strasbourg, France, 8th July 1998;
L etter 8/05357J
1332
New J. Chem., 1998, Pages 1331È1332