O2 and CO binding to tetraaza-tripodal-capped iron(II) porphyrins

Article abstract of DOI:10.1021/ic0514703

A series of tris(2-aminoethylamine) (tren) capped iron(II) porphyrins has been synthesized and characterized and their affinities for dioxygen and carbon monoxide measured. The X-ray structure of the basic scaffold with nickel inserted in the porphyrin is also reported. All the ligands differ by the nature of the group(s) attached to the secondary amine functions of the cap. These various substitutions were introduced to probe if a hydrogen bond with these secondary amine groups acting as the donor could rationalize the high affinity of these myoglobin models. This work clearly indicates that the cage structure of the tren predominates over all the other appended groups with the exception of p-nitrophenol.

Full text of DOI:10.1021/ic0514703

Inorg. Chem. 2006, 45, 1338−1348
O2 and CO Binding to Tetraaza-Tripodal-Capped Iron(II) Porphyrins
Christian Ruzie´,† Pascale Even,† David Ricard,† Thierry Roisnel,‡ and Bernard Boitrel*,†
UniVersite´ de Rennes 1, Institut de Chimie, UMR CNRS 6509, Organome´talliques et Catalyse,
Chimie et Electrochimie Mole´culaire, 35042 Rennes Cedex, France and UniVersite´ de Rennes 11,
Institut de Chimie, UMR CNRS 6511, Laboratoire de Chimie du Solide et Inorganique
Mole´culaire, 35042 Rennes Cedex, France
Received August 27, 2005
A series of tris(2-aminoethylamine) (tren) capped iron(II) porphyrins has been synthesized and characterized and
their affinities for dioxygen and carbon monoxide measured. The X-ray structure of the basic scaffold with nickel
inserted in the porphyrin is also reported. All the ligands differ by the nature of the group(s) attached to the
secondary amine functions of the cap. These various substitutions were introduced to probe if a hydrogen bond
with these secondary amine groups acting as the donor could rationalize the high affinity of these myoglobin
models. This work clearly indicates that the cage structure of the tren predominates over all the other appended
groups with the exception of p-nitrophenol.
Introduction
On the other hand, this same histidine residue could be also
responsible for steric interactions with heme-bound carbon
monoxide, which binds in an upright fashion. Indeed, the
close proximity of His E7 to the ligand binding site led to
the proposal that this residue could serve to hinder sterically
the binding of CO and thus reduce its affinity for myoglobin
and hemoglobin (Collman hypothesis).6 This hypothesis of
steric hindrance explains how nature has solvedspartially
at leaststhe problem of carbon monoxide toxicity without
steric constraint on O2 binding as the latter binds in a bent
orientation.7 Indeed, the natural affinity of the heme pros-
thetic group for CO is about 2 × 104 higher than that for
O2. This ratio is reduced around 200 for the proteins.
Actually, the discrimination against CO allows the protein
to decrease the CO affinity by a factor 100. Undoubtedly,
the biomimetic studies initiated some 30 years ago have
permitted a better comprehension of these observations.
During this period of time iron complexes of various
superstructured porphyrins have been elaborated in order to
mimic the microenvironment of the prosthetic groups of
dioxygen carrier hemoproteins.
Dioxygen transport, storage, and activation are the three
critical steps of all aerobic life and performed by hemopro-
teins in vertebrates.1 For the latter reaction the hemoprotein
coordination site is a copper hemoprotein (cytochrome c
oxidase) and the bound dioxygen is presumably interacting
with copper(I) above the iron porphyrin in a light off-centered
location.2 In the case of dioxygen storage and transport
(myoglobin (Mb)3 and hemoglobin (Hb))4 the coordination
site is composed of a single heme buried in a hydrophobic
pocket. In this instance the bound dioxygen does not interact
with a second metal but with a histidine residue E7 positioned
over the iron atom to be able to establish a hydrogen bond
with the superoxo complex (Pauling hypothesis).5 This
hydrogen bond is considered to stabilize the dioxygen adduct.
* To whom correspondence should be addressed. Phone: +33(2)2323
5856. Fax: +33(2)2323 5637. E-mail: Bernard.Boitrel@univ-rennes1.fr.
† UMR CNRS 6509.
‡ UMR CNRS 6511.
(1) (a) Zubay, G. Biochemistry; Addison-Wesley Publishing Co.: New
York, 1983. (b) Stryer, L. In Biochemistry, 4th ed.; Freeman: 1995;
pp 147-180.
(2) (a) Tsukihara, T.; Aoyama, H.; Yamashita, E.; Tomizaki, T.; Yamagu-
chi, H.; Shinzawa-Itoh, K.; Nakashima, R.; Yaono, R.; Yoshikawa,
S. Science 1995, 269, 1069-1074. (b) Iwata, S.; Ostermeier, C.;
Ludwig, B.; Michel, H. Nature 1995, 376, 660-669. (c) Yoshikawa,
S.; Shinzawa-Itoh, K.; Nakashima, R.; Yaono, R.; Yamashita, E.;
Inoue, N.; Yao, M.; Fei, M. J.; Libeu, C. P.; Mizushima, T.;
Yamaguchi, H.; Tomizaki, T.; Tsukihara, T. Science 1998, 280, 1723-
1729.
(3) Kendrew, J. C.; Dickerson, R. E.; Strandberg, B. E.; Hart, R. G.;
Phillips, D. C.; Shore, V. C. Nature 1960, 185, 422-427.
(4) Perutz, M. F.; Rossmann, M. G.; Cullis, A. F.; Muirhead, H.; Will,
G.; North, A. C. T. Nature 1960, 185, 416-422.
The very first milestone is undeniably represented by the
“picket-fence” porphyrin which is still the only model of
(5) Pauling, L. Nature 1964, 203, 182-183.
(6) Collman, J. P.; Brauman, J. I.; Halbert, T. R.; Suslick, K. S. Proc.
Natl. Acad. Sci. U.S.A. 1976, 73, 3333-3337.
(7) (a) Li, X.-Y.; Spiro, T. G. J. Am. Chem. Soc. 1988, 110, 6024-6033.
(b) Ray, G. B.; Li, X. Y.; Ibers, J. A.; Sessler, J. L.; Spiro, T. G. J.
Am. Chem. Soc. 1994, 116, 162-176. (c) Spiro, T. G.; Kozlowski, P.
M. Acc. Chem. Res. 2001, 34, 137-144.
1338 Inorganic Chemistry, Vol. 45, No. 3, 2006
10.1021/ic0514703 CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/06/2006

Tetraaza-Tripodal-Capped Iron(II) Porphyrins
Table 1. Dioxygen and Carbon Monoxide Affinities of Models 1, 3-9, and Related Compounds from the Literaturea
solvent
P
1/2(O2) (Torr)
P1/2(CO) (Torr)
M P1/2(O2)/P1/2(CO)
Mb34
H2O, pH 7
H2O, pH 7
0.37-1
2 × 10-3
38
1.4-2.5 × 10-2
20-40
0.02
4280
270
Ascaris17
0.1
Fe(picket fence)(1,2-Me2Im)11
FeMedPoc(1-MeIm)11
Fe-aBHP(C3PyC3)(C12)12c,13 b
1 (2-MeIm)19
3 (2-MeIm)
toluene, 25 °C
toluene, 25 °C
toluene, 20 °C
toluene, 25 °C
toluene, 25 °C
toluene, 25 °C
toluene, 25 °C
CH2Cl2, 25 °C
toluene, 25 °C
toluene, 25 °C
toluene, 25 °C
toluene, 25 °C
8.9 × 10-3
6.5 × 10-4
9 × 10-5
3.1 × 10-4
<10-7
0.36
2
2 × 104
0.32
na
9.8 × 10-5
2.4 × 10-5
2.7 × 10-4
0.18
4 (1,2-Me2Im)
5 (2-MeIm)
<10-7
nd
na
nd
5 (2-MeIm)
0.67
7 × 10-3
8.8 × 10-6
1.7 × 10-6
7 × 10-7
<10-7
96.4
0.46
0.71
1.43
na
6 (2-MeIm)19
7 (2-MeIm)
4.0 × 10-6
1.2 × 10-6
10-6
8 (2-MeIm)
9b
1.6 × 10-5
a na: not accessible. nd: not determined. b Models bearing an internal fifth ligand.
Figure 1. Chemical structure of three myoglobin models reported by Reed et al.15 with the different X-H dipoles indicated: (a) phenylurea, (b) 3-phenol,
and (c) 2-phenol; L ) 1,2-Me2Im.
oxy-myoglobin isolated in the solid state.8 With this molecule
it was demonstrated that similar affinities to those exhibited
by the natural proteins could be reached and that the fence
concept was efficient to prevent formation of the µ-ox-
odimer.9 Since then so many other models have been
proposed that only the reading of overviews can summarize
the different approaches.10 However, among them different
specific ligands or families of ligands are worth being
detailed. The family of “pocket porphyrins” provided sup-
porting evidence for the steric influence on CO binding.
Indeed, additional steric hindrance around the binding site
resulted in unaltered dioxygen affinity but decreased carbon
monoxide affinity.11 In another study Momenteau et al.
clearly demonstrated with two basket-handle porphyrins
differing only by the linkage of the strapsseither ether or
amidesthat the iron(II) complex bearing the straps via an
amide linkage, one of them delivering a built-in nitrogen
base, exhibited a dioxygen affinity 10 times greater than the
ether-linked strapped porphyrin (Table 1, entry 5).12 This
10-fold increase in O2 affinity was attributed to a hydrogen
bond between the terminal oxygen atom of the iron(II)-bound
dioxygen and the NH of the meso-aromatic amide linkage.13
In another report the size of the cavity from a “picnic-basket”
porphyrin was systematically varied by changing the length
of the linker between the two straps. It was shown that as
the picnic-basket cavity becomes smaller, the H-bond or
dipole-dipole interactions and the dioxygen affinity in-
crease.14 In 1995 systematic variation of one picket from
the picket fence was described by Reed et al.15 Interestingly,
they reported a 10-fold increase of dioxygen affinity for the
compound bearing either a phenyl urea-linked picket (Figure
1a) or a m-phenol amido-linked picket (Figure 1b). It has to
be underlined that with these particular examples the location
of the hydrogen-bond donor is changed and shifted toward
(8) (a) Collman, J. P.; Gagne, R. R.; Halbert, T. R.; Marchon, J.-C.; Reed,
C. A. J. Am. Chem. Soc. 1973, 95, 7868-7870. (b) Collman, J. P.;
Gagne, R. R.; Reed, C. A.; Halbert, T. R.; Lang, G.; Robinson, W. T.
J. Am. Chem. Soc. 1975, 97, 1427-1439. (c) Jameson, G. B.; Rodley,
G. A.; Robinson, W. T.; Gagne, R. R.; Reed, C. A.; Collman, J. P.
Inorg. Chem. 1978, 17, 850-857. (d) Jameson, G. B.; Molinaro, F.
S.; Ibers, J. A.; Collman, J. P.; Brauman, J. I.; Rose, E.; Suslick, K.
S. J. Am. Chem. Soc. 1980, 102, 3224-3237.
(9) (a) Chin, D.-H.; DelGaudio, J.; LaMar, G. N.; Balch, A. L. J. Am.
Chem. Soc. 1977, 99, 5486-5488. (b) Chin, D. H.; Mar, G. N. L.;
Balch, A. L. J. Am. Chem. Soc. 1980, 102, 4344-4349.
(10) (a) Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659-698. (b)
Collman, J. P. Inorg. Chem. 1997, 36, 5145-5155. (c) Collman, J.
P.; Fu, L. Acc. Chem. Res. 1999, 32, 455-463. (d) Collman, J. P.;
Boulatov, R.; Sunderland, C. J.; Fu, L. Chem. ReV. 2004, 104, 561-
588.
(12) (a) Momenteau, M.; Mispelter, J.; Loock, B.; Bisagni, E. J. Chem.
Soc., Perkin Trans. 1 1983, 189-196. (b) Momenteau, M.; Mispelter,
J.; Loock, B.; Lhoste, J.-M. J. Chem. Soc., Perkin Trans. 1 1985, 61-
70. (c) Momenteau, M.; Mispelter, J.; Loock, B.; Lhoste, J.-M. J.
Chem. Soc., Perkin Trans. 1 1985, 221-231. (d) Momenteau, M.;
Loock, B.; Huel, C.; Lhoste, J.-M. J. Chem. Soc., Perkin Trans. 1
1988, 283-295.
(13) Mispelter, J.; Momenteau, M.; Lavalette, D.; Lhoste, J.-M. J. Am.
Chem. Soc. 1983, 105, 5165-5166.
(11) (a) Collman, J. P.; Brauman, J. I.; Collins, T. J.; Iverson, B. L.; Lang,
G.; Pettman, R. B.; Sessler, J. L.; Walter, M. A. J. Am. Chem. Soc.
1983, 105, 3038-3052. (b) Collman, J. P.; Brauman, J. I.; Iverson,
B. L.; Sessler, J. L.; Morris, R. M.; Gibson, Q. H. J. Am. Chem. Soc.
1983, 105, 3052-3064.
(14) Collman, J. P.; Zhang, X. M.; Wong, K.; Brauman, J. I. J. Am. Chem.
Soc. 1994, 116, 6245-6251.
(15) Wuenschell, G. E.; Te´treau, C.; Lavalette, D.; Reed, C. A. J. Am.
Chem. Soc. 1992, 114, 3346-3355.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1339

Ruzie´ et al.
TBI probe or a Bruker Avance 300 spectrometer with a BBO probe.
Spectra were referenced with residual solvent protons. UV-vis
spectra were recorded on an Uvikon XL spectrometer.
a more apical position. On the other hand, for the o-phenol
amido-linked picket (Figure 1c) no effect was detected
relative to the picket-fence reference compound. Almost 10
years later an unprecedented noncoordination of carbon
monoxide at atmospheric pressure was reported with a
cyclam-capped porphyrin, demonstrating the effect of steric
interactions on porphyrin CO affinities.16 It is worth noting
that the latter was synthesized starting from a cyclic
tetraamine with a resulting distance between the mean
porphyrin plane and the N4-cyclam plane of 3.91 Å. Finally,
a very high O2 affinity, close to the Hb from Ascaris,17 has
been reported with dendritic porphyrins.18 Some time later
we reported a very high O2 affinity as well,19 obtained with
the first generation of tren-capped iron(II) porphyrins lacking
a built-in fifth ligand.20 As this type of Mb model bears
secondary amino functions in an apical position relative to
the meso-aromatic amide linkage, we explained this unex-
pected O2 affinity by the possible hydrogen bond with these
specific NH groups. Indeed, these hydrogen-bond donors
could be located in a position more favorable to a strong
stabilization of the dioxygen adduct. In this paper, owing to
the investigation of a series of tren-capped iron(II) porphy-
rins, differing by various substitutions on one or two amino
functions, and the X-ray structure of such a tren-capped
porphyrin, we show that the effect of the tripodal cage
remains predominant versus the peripheral groups around
the distal pocket. However, we also demonstrate that a group
such as a nitrophenol, when appropriately attached at the
periphery of the distal cage, can dramatically effect the
dioxygen affinity by a factor up to 104, and so electrostatic
repulsions can strongly destabilize the oxygenated complex
without having a significant influence on the CO complex,
which is more sensitive to steric effects.
Equilibrium Measurements. The O2, CO affinities were
measured spectrophotometrically under equilibrium conditions with
an Uvikon XL spectrometer. Measurements were performed with
toluene solutions thermostated in the spectrometer to 25.0 ( 0.3
°C with a Lauda RL6 constant-temperature bath and circulator.
The equilibrium between the metalloporphyrin (MP) with an
excess of axial base (B)sor an intramolecular fifth ligandsand
O2 can be expressed as follows
K
MP‚B + O2 y
\
z MP‚B‚O2
The equilibrium constant K is defined by
K ) [MP‚B‚O2]/[MP‚B]p(O2)
If C0 is the total concentration of (MP‚B), then at t ) 0 (without
dioxygen) C0 ) [MP‚B]. At a t instant with a dioxygen partial
pressure (p(O2)), C0 ) [MP‚B] + [MP‚B‚O2]. Then
K ) [MP‚B‚O2]/((C0 - [MP‚B‚O2])‚p(O2))
Only (MP‚B) and (MP‚B‚O2) have absorbance in the UV-vis. The
Beer-Lambert law can be written as follows.
When t ) 0
Ao ) ꢀMP‚B‚Co ) ꢀMP‚B[MP‚B] + ꢀMP‚B[MP‚B‚O2]
and at any t
A ) ꢀMP‚B[MP‚B] + ꢀMP‚B‚O2[MP‚B‚O2]
Then
A - Ao ) (eMP‚B‚O - ꢀMP‚B)[MP‚B‚O2] or ∆A ) ∆ꢀ[MP‚B‚O2]
2
The equilibrium constant K could be defined by
Experimental Section
K ) ∆A/((C0∆ꢀ - ∆A)‚p(O2)) or K-1 ) ((C0∆ꢀ/∆A)‚p(O2)) -
p(O2)
Materials and Methods. All commercial chemicals (Aldrich,
Acros) and solvents (ACS for analysis) were used without further
purification unless otherwise stated. Solvents used in an inert
atmosphere (drybox; [O2] < 1 ppm) were freshly distilled and
deoxygenated by three “freeze-pump” cycles. Benzene, toluene,
and THF were distilled over potassium benzophenone ketyl;
methylene chloride, pyridine, triethylamine, and pentane were
distilled over CaH2; and methanol was distilled over magnesium
turnings. Column chromatographies were performed on SiO2
(Merck TLC-Kieselgel 60H, 15 µm). Mass spectra (EI, Varian
MAT 33 spectrometer; ESI, Micromass MS/MS ZABSpec TOFF
spectrometer) were performed at the C.R.M.P.O. (University of
The partial pressure of dioxygen (p(O2)) could be defined as p(O2)
) ((C0∆ꢀ/∆A)‚p(O2)) - K-1
.
This is the equation of a straight line with a slope of C0∆ꢀ and
an intercept of -1/K. When the metalloporphyrin is half-oxygen-
ated, [MP‚B] ) [MP‚B‚O2], then 1/K ) P1/2(O2). The partial
pressure of oxygen at which 50% of metalloporphyrin sites are
bound with O2 is defined as P1/2(O2).
The reversibility of gas binding was monitored afterward by
bubbling argon through the tonometer and recording several UV-
vis spectra. However, since the affinity for dioxygen is very high,
it was difficult to obtain the original absorption of the five-
coordinate UV-vis spectrum due to solvent evaporation. As the
reaction is very slow, the UV-vis spectrum of the dioxygen adduct
was recorded after at least a week to verify that the absorption was
not modified and hence that the equilibrium was actually reached.
Synthesis and Characterization. meso-R-5,10,15,20-Tetrakis-
(2-acryloylamino)phenylporphyrin (R,R,R,R Michael acceptor) was
prepared as described in the literature.23
1
Rennes 1). H and 13C NMR spectra were recorded on a Bruker
Avance 500 or Bruker Avance 300 spectrometer equipped with a
(16) Collman, J. P.; Herrmann, P. C.; Fu, L.; Eberspacher, T. A.; Eubanks,
M.; Boitrel, B.; Hayoz, P.; Zhang, X. M.; Brauman, J. I.; Day, V. W.
J. Am. Chem. Soc. 1997, 119, 3481-3489.
(17) Golberg, D. E. Chem. ReV. 1999, 99, 3371-3378.
(18) (a) Collman, J. P.; Fu, L.; Zingg, A.; Diederich, F. Chem. Commun.
1997, 193-194. (b) Weyermann, P.; Diederich, F. J. Chem. Soc.,
Perkin Trans. 1 2000, 4231-4233. (c) Zingg, A.; Felber, B.; Gramlich,
V.; Fu, L.; Collman, J. P.; Diederich, F. HelV. Chim. Acta 2002, 85,
333-351.
(19) Ricard, D.; Andrioletti, B.; Boitrel, B.; Guilard, R. New J. Chem. 1998,
22, 1331-1332.
(20) Andrioletti, B.; Boitrel, B.; Guilard, R. J. Org. Chem. 1998, 63, 1312-
1314.
meso-R-5,10,15,20-Tetrakis{2-[3,3′,3′′3′′′-{N,N,N′,N′′-tris(2-
aminoethyl)amino}tetrapropionylamino]phenyl}porphyrin, 1H2.
To a solution of R,R,R,R Michael acceptor (2.5 g, 2.8 mmol) in
chloroform/methanol (1/5) mixture (600 mL) degassed with Ar at
1340 Inorganic Chemistry, Vol. 45, No. 3, 2006

Tetraaza-Tripodal-Capped Iron(II) Porphyrins
65 °C for 1 h was added tris(2-aminoethyl)amine (465 µL, 3.1
mmol). The mixture was stirred for 2 days; then solvent was
removed under vacuum. The resulting powder was dissolved in
CH2Cl2/NH3(g) and directly loaded on a silica gel chromatography
column. The expected compound eluted with 4% MeOH/CH2Cl2/
NH3(g) was obtained in 68% yield (1.96 g). 1H NMR (pyridine-d5,
323 K): δ 11.42 (s, 2H, -NHCO); 9.17 (s, 2H, -NHCO), 9.08
(d, 2H, J ) 4.5 Hz, Hâpyr); 9.06 (m, 2H, Haro); 9.02 (d, 2H, J ) 5.0
Hz, Hâpyr); 8.96 (s, 2H, Hâpyr); 8.81 (m, 4H, Haro + Hâpyr); 8.09 (d,
2H, J ) 6.5 Hz, Haro); 7.87 (t, 2H, J ) 7.5 Hz, Haro); 7.83 (t, 2H,
J ) 7.5 Hz, Haro); 7.77 (d, 2H, J ) 7.5 Hz, Haro); 7.55 (t, 2H, J )
7.0 Hz, Haro); 7.46 (t, 2H, J ) 7.0 Hz, Haro); 2.35-2.12 (m, 10H);
2.09-1.99 (m, 4H); 1.86-1.80 (m, 2H); 1.09 (m, 2H); 0.84 (m,
2H); 0.70 (m, 2H); 0.47 (m, 2H); 0.01 (m, 2H); -0.13 (m, 2H);
-1.57 (m, 2H); -2.29 (s, 2H, -NHpyr). HR-MS (LSIMS-MS):
calcd m/z ) 1037.4939 for C62H61N12O4 [M + H]+, found
1037.4929. UV-vis (CH2Cl2) λ, nm (10-3 ꢀ, dm3 mol-1 cm-1):
421 (180.4); 515 (9.0); 548 (3.1); 588 (3.4); 644 (1.8). Anal. Calcd
for C62H60N12O4‚2H2O: C, 69.38; H, 6.01; N, 15.66. Found: C,
68.81; H, 5.54; N, 15.79.
directly loaded on a silica gel chromatography column. The
expected compound was eluted with 2% MeOH/CHCl3 and obtained
in 64% yield (151 mg). H NMR (CDCl3, 323 K): δ 9.76 (s, 2H,
1
-NHCO); 8.92 (d, 2H, J ) 5.0 Hz, Hâpyr); 8.89 (d, 2H, J ) 4.7
Hz, Hâpyr); 8.84 (s, 2H, Hâpyr); 8.70 (s, 2H, Hâpyr); 8.66 (d, 2H, J )
8.5 Hz, Haro); 8.57 (d, 2H, J ) 8.5 Hz, Haro); 8.07 (s, 2H, -NHCO);
3
4
3
8.02 (dt, 2H, J ) 7.5 Hz and J ) 1.0 Hz, Haro); 7.86 (td, 2H, J
) 8.0 Hz and 4J ) 1.5 Hz, Haro); 7.81 (td, 2H, 3J ) 8.0 Hz and 4J
) 1.5 Hz, Haro); 7.52 (m, 4H, Haro); 7.37 (td, 2H, 3J ) 7.5 Hz and
4J ) 1.0 Hz, Haro); 3.92 (q, 4H, J ) 7.0 Hz, -CH2 ester); 2.58 (d,
2H, J ) 16.5 Hz, -CH2); 2.51 (m, 2H); 2.37 (m, 2H); 2.12 (m,
4H); 2.01 (m, 2H); 1.86 (m, 2H); 1.70 (m, 4H); 1.10 (t, 6H, J )
7,5 Hz, -CH3); 0.91 (m, 2H); 0.66 (m, 4H); 0.55 (m, 4H); -1.72
(m, 4H); -2.55 (s, 2H, -NHpyr). HR-MS (ESI-MS): calcd m/z )
1209.5674 for C70H73N12O8 [M + H]+, found 1209.5678. UV-vis
(CH2Cl2) λ, nm (10-3 ꢀ, dm3 mol-1 cm-1): 421 (271.1); 514 (16.7);
547 (5.1); 588 (6.0); 645 (2.2). Anal. Calcd for C70H72N12O4: C,
69.52; H, 6.00; N, 13.90. Found: C, 69.21; H, 6.07; N, 13.81.
meso-R-5,10,15,20-Tetrakis{2-[3,3′,3′′3′′′-{N,N,N′,N′′-tris(2-
aminoethyl)amino}-(N′,N′′-bis-3-ethoxycarbonyl-propionylamino)-
tetrapropionylamino]phenyl}porphyrin, 4H2. In a round-bottom
flask equipped with a stir bar and gas inlet porphyrin 1H2 (100
mg, 0.09 mmol) was charged with THF (50 mL) and Et3N (200
µL, 1.4 mmol). Ethyl succinyl chloride (55 µL, 0.38 mmol) was
added dropwise. The solution was stirred for 12 h; then solvent
was removed under vacuum. The resulting powder was dissolved
in CHCl3 and directly loaded on a silica gel chromatography
column. The expected compound was eluted with 1.5% MeOH/
CHCl3 and obtained in 88% yield (100 mg). 1H NMR (DMSO-d6,
343 K): δ 9.67 (s, 2H, -NHCO); 8.77 (s, 2H, -NHCO); 8.69 (d,
2H, J ) 4,5 Hz, Hâpyr); 8.62 (d, 2H, J ) 4,5 Hz, Hâpyr); 8.52 (s,
2H, Hâpyr); 8.49 (s, 2H, Hâpyr); 8.31 (d, 2H, J ) 4.8 Hz, Haro); 8.12
(d, 2H, J ) 7.3 Hz, Haro); 7.85 (m, 5H, Haro); 7.75 (d, 1H, J ) 7.3
Hz, Haro); 7.59 (m, 4H, Haro); 7.49 (t, 2H, J ) 7.1 Hz, Haro); 4.07
(q, 4H, J ) 7,1 Hz, -CH2CH3); 3.21 (m, 4H); 2.60 (m, 4H); 2.39
(m, 6H); 2.20 (m, 2H); 2.07 (m, 2H); 1.64 (br s, 4H); 1.48 (m,
6H); 1.25 (t, 6H, J ) 7.5 Hz, -CH2CH3); 0.47 (t, 2H, J ) 9,3
Hz); -1.16 (br s, 2H); -1.34 (br s, 2H); -2.75 (s, 2H, -NHpyr);
-2.86 (br s, 2H). HR-MS (ESI-MS): calcd m/z ) 1293.5885 for
C74H77N12O10 [M + H]+, found 1293.5888. UV-vis (CH2Cl2) λ,
nm (10-3 ꢀ, dm3 mol-1 cm-1): 420.0 (291.3); 512.0 (13.2); 545.5
(4.7); 586.0 (4.7); 622 (1.4).
Nickel(II) meso-R-5,10,15,20-Tetrakis{2-[3,3′,3′′3′′′-{N,N,N′,N′′-
tris(2-aminoethyl)amino}tetrapropionylamino]phenyl}-
porphyrin, 2. To a solution of porphyrin 1H2 (0.539 mmol, 500
mg) in DMF (40 mL) was added nickel chloride (2.70 mmol, 350
mg). The mixture was stirred 3 h at 80 °C; then solvent was
removed under vacuum. The resulting powder was dissolved in
CHCl3/NH3(g) and directly loaded on a silica gel chromatography
column. The expected compound was eluted with 0.8% MeOH/
CHCl3/NH3(g) and obtained in 70% yield (412 mg). 1H NMR
(500.13 MHz, DMSO-d6, 333 K): δ 11.11 (br s, 2H, -NHCO);
9.10 (s, 2H, -NHCO); 8.71 (s, 4H, Hâpyr); 8.65 (s, 2H, Hâpyr); 8.61
(d, 2H, 3J ) 8.2 Hz, Haro); 8.49 (s, 2H, Hâpyr); 8.26 (d, 2H, J ) 8.2
3
4
Hz, Haro); 7.81 (m, 4H, Haro); 7.73 (td, 2H, J ) 8.7 Hz and J )
1.5 Hz, Haro); 7.51 (t, 2H, J ) 7.4 Hz, Haro); 7.39 (d, 2H, J ) 7.4
3
4
Hz, Haro); 7.33 (td, 2H, J ) 7.4 Hz and J ) 0.8 Hz, Haro); 2.14
(br m, 2H, -CH2-); 2.04 (br m, 2H, -CH2-); 1.98 (br m, 4H,
-CH2-); 1.82 (br m, 4H, -CH2-); 1.67 (br m, 2H, -CH2-); 0.95
(br s, 2H, -CH2-); 0.71 (br s, 2H, -CH2-); 0.61 (br m, 4H,
-CH2-); 0.38 (br s, 2H, -CH2-); 0.16 (br m, 2H, -CH2-); -0.90
(br s, 2H, -CH2-). HR-MS (ESI-MS): calcd m/z ) 1093.4135
for C62H59N12O458Ni [M + H]+, found 1093.4128 (1 ppm). UV-
vis (CH2Cl2) λ, nm (10-3 ꢀ, dm3 mol-1 cm-1): 415.0 (288.6); 526.5
(23.4); 559.5 (7.5).
meso-R-5,10,15,20-Tetrakis{2-[3,3′,3′′3′′′-{N,N,N′,N′′-tris(2-
aminoethyl)amino}-(N′-2-hydroxy-5-nitro-benzyl)tetra-
propionylamino]phenyl}porphyrin, 5H2. In a 100 mL round-
bottom flask under argon, 200 mg (0.193 mmol) of 1H2, 2-bro-
momethyl-4-nitrophenol (53.6 mg, 0.231 mmol), and 0.2 mL of
Et3N were dissolved in 50 mL of freshly distilled THF. The solution
was heated at 65 °C overnight and then dried under vacuum. The
product was dissolved in chloroform and poured onto a silica gel
column. The disubstituted compound was eluted first with 5.6%
MeOH/CHCl3 in 16.5% yield (38 mg) followed by compound 5H2
eluting with 6.4% MeOH/CHCl3 in 66% yield (168 mg). 1H NMR
(DMSO-d6, 363 K): δ 10.93 (br s, 1H, NH); 9.42 (br s, 1H); 9.24
(br s, 1H); 8.90 (d, 1H, J ) 5.0 Hz, Hâpyr); 8.89 (d, 1H, J ) 4.5
Hz, Hâpyr); 8.79 (d, 1H, J ) 5.0 Hz, Hâpyr); 8.76 (d, 1H, J ) 5.0
Hz, Hâpyr); 8.69 (br s, 1H); 8.67 (d, 1H, J ) 4.5 Hz, Hâpyr); 8.63
(d, 1H, J ) 4.5 Hz, Hâpyr); 8.55 (d, 1H, J ) 4.5 Hz, Hâpyr); 8.51 (d,
1H, J ) 4.5 Hz, Hâpyr); 8.24 (dd, 1H, 3J ) 7.5 Hz and 4J ) 1.5 Hz,
meso-R-5,10,15,20-Tetrakis{2-[3,3′,3′′3′′′-{N,N,N′,N′′-tris(2-
aminoethyl)amino}-(N′,N′′-bis-ethoxycarbonylmethyl)-
tetrapropionylamino]phenyl}porphyrin, 3H2. In a two-neck
round-bottom flask equipped with a stir bar and gas inlet porphyrin
1H2 (202 mg, 0.195 mmol) was charged with THF (80 mL) and
Et3N (275 µL, 1.9 mmol). The reaction mixture was heated to 65
°C; then ethyl bromoacetate (194 µL, 1.75 mmol) was added
dropwise. The solution was stirred for 2 days, and solvent was
removed under vacuum. The resulting oil was precipitated with a
THF/pentane (1/5) mixture and then dissolved with CHCl3 and
evaporated. The resulting powder was dissolved in CHCl3 and
(21) (a) Larsen, N. G.; Boyd, P. D. W.; Rodgers, S. J.; Wuenschell, G. E.;
Koch, C. A.; Rasmussen, S.; Tate, J. R.; Erler, B. S.; Reed, C. A. J.
Am. Chem. Soc. 1986, 108, 6950-6960. (b) Collman, J. P.; Bro¨ring,
M.; Fu, L.; Rapta, M.; Schwenninger, R.; Straumanis, A. J. Org. Chem.
1998, 63, 8082-8083.
(22) (a) Ricard, D.; Andrioletti, B.; L’Her, M.; Boitrel, B. Chem. Commun.
1999, 1523-1524. (b) Ricard, D.; L’Her, M.; Richard, P.; Boitrel, B.
Chem. Eur. J. 2001, 7, 3291-3297.
(23) Collman, J. P.; Zhang, X.; Herrmann, P. C.; Uffelman, E. S.; Boitrel,
B.; Straumanis, A.; Brauman, J. I. J. Am. Chem. Soc. 1994, 116, 2681-
2682.
H
aro); 8.01 (d, 1H, J ) 8.0 Hz, Haro); 7.97 (d, 1H, J ) 8.0 Hz,
3
4
Haro); 7.90 (dd, 1H, J ) 9.0 Hz and J ) 3.0 Hz, H5); 7.87 (td,
3
4
1H, J ) 7.5 Hz and J ) 1.5 Hz, Haro); 7.84 (d, 1H, J ) 3.0 Hz,
3
H3); 7.83-7.75 (overlapping m, 6H, Haro); 7.70 (td, 1H, J ) 7.5
Inorganic Chemistry, Vol. 45, No. 3, 2006 1341

Ruzie´ et al.
Hz and 4J ) 1.5 Hz, Haro); 7.64 (td, 1H, 3J ) 7.5 Hz and 4J ) 1.5
Hz, Haro); 7.71 (d, 1H, J ) 6.5 Hz, Haro); 7.48 (td, 1H, 3J ) 7.5 Hz
and 4J ) 1.5 Hz, Haro); 7.45 (td, 1H, 3J ) 7.5 Hz and 4J ) 1.5 Hz,
vacuum. The residue was dissolved in 30 mL of CH2Cl2, and 3
mL of trifluoroacetic acid was added. After 2 h the mixture was
washed twice with 10 mL of aqueous NaOH (5%). The organic
phase was concentrated by rotary evaporation and poured onto a
15 µm silica gel column (3 × 10 cm). The resulting tris-
acrylamidophenyl-monoaminophenyl picket porphyrin was eluted
with 0.6% MeOH/CH2Cl2. After evaporation to dryness, 330 mg
was collected (yield ) 50%). Then, in a 50 mL round-bottom flask
under argon, 140 mg of picket porphyrin, 2-hydroxybenzaldehyde
(150 mg, 1.2 mmol) and trifluoroacetic acid (55 µL), was dissolved
in acetonitrile. After it was stirred overnight at room temperature,
NaBH3CN was added and the solution was stirred for an additional
4 h. Then the mixture was neutralized by NH3(g) and dried under
vacuum. The resulting powder dissolved in CH2Cl2 was loaded on
a silica gel chromatography column. The desired product was eluted
with 10% acetone/CH2Cl2, and after evaporation to dryness it was
allowed to react with tren according to the same procedure of 6H2.
The desired product was dissolved in CH2Cl2, poured onto a 15
µm silica gel column, and eluted with 5-10% MeOH/CH2Cl2/NH3-
(g). After evaporation to dryness, 7H2 was collected (yield ) 26%).
1H NMR (500 MHz, DMSO-d6, 333 K): δ 10.95 (br s, 1H, -NH),
10.63 (br s, 2H, -NH), 8.78 (br s, 4H, Hâpyr), 8.73 (br s, 4H, Hâpyr),
8.65 (br s, 2H, Haro), 8.42 (d, 2H, J ) 8.5 Hz, Haro), 8.02 (br s, 1H,
H3), 7.82 (td, 3H, Jo ) 7.5 Hz, Jm ) 1.5 Hz, Haro), 7.79 (td, 1H, 3J
) 7.5 Hz, 4J ) 1.5 Hz, Haro), 7.74 (m, 1H, H5), 7.77-7.64 (m, 5H,
3
4
H
aro); 7.41 (td, 1H, J ) 7.5 Hz and J ) 1.5 Hz, Haro); 7.64 (td,
3
4
1H, J ) 7.5 Hz and J ) 1.5 Hz, Haro); 6.77 (d, 2H, J ) 9.0 Hz,
H6); 3.18 (s, 2H, CH2benz); 2.43 (m, 1H, -CH2-); 2.24 (m, 1H,
-CH2-); 2.10-1.75 (overlapping m, 7H, -CH2-); 1.62 (m, 4H,
-CH2-); 1.18 (m, 2H, -CH2-); 0.87 (m, 2H, -CH2-); 0.72 (m,
1H, -CH2-); 0.5 (m, 1H, -CH2-); -0.08 (m, 1H, -CH2-);
-0.22 (m, 1H, -CH2-); -0.48 (m, 2H, -CH2-); -0.58 (m, 1H,
-CH2-); -1.14 (m, 1H, -CH2-); -1.20 (m, 1H, -CH2-); -1.63
(m, 1H, -CH2-); -2.45 (m, 1H, -CH2-); -2.57 (s, 2H, -NHpyr).
HR-MS (ESI-MS): calcd m/z ) 1188.5208 for C69H66N13O7 [M +
H]+, found 1188.5195. UV-vis (CHCl3/MeOH 10%) λ, nm (10-3ꢀ,
dm3 mol-1 cm-1): 423 (301.2); 517 (15.4); 550 (4.2); 589 (4.9);
645 (2.0).
meso-R-5,10,15-Tris{2-(3,3′,3′′-[N,N′,N′′-tris(2-aminoeth-
ylamino)propionylamino]triphenyl}-R-20-(2-methylamino-
phenyl)porphyrin, 6H2. The atropisomer R,R,R,R of meso-5,10,-
15,20-tetrakis(2-amino)phenylporphyrin (TAPP) (1 g, 1.48 mmol)
was singly acetylated by slow addition of acetyl chloride over 1 h
(115 µL, 1.1 mmol) in dry THF (500 mL) at 0 °C in the presence
of NEt3 (210 µL, 3.0 mmol). Stirring was maintained 1 h after the
addition, and the solution was dried by rotary evaporation. The
resulting powder was dissolved in methylene chloride and washed
twice with 50 mL of aqueous NaOH (5%). The organic phase was
concentrated and poured on a 15 µm silica gel column (6 × 10
cm) prepared with methylene chloride. The desired product was
eluted with a mixture of 1.25% MeOH/CH2Cl2 (490 mg, yield )
46%). The reaction of the three remaining amino groups with
acryloyl chloride in dry THF, according to the original method,
led to the corresponding picket porphyrin. This picket porphyrin
was allowed to react with tren according to the same procedure
described for 1H2. The desired product 6H2 was eluted with 4%
MeOH/CH2Cl2. After evaporation to dryness, 234 mg of a purple
powder was collected (yield ) 37%). 1H NMR (500 MHz, CDCl3,
323 K): δ 11.31 (s, 1H, -NHCO), 10.78 (s, 2H, -NHCO), 8.93
(d, 3J ) 7.5 Hz, 4J ) 1.2 Hz, 2H, Haro), 8.91 (d, 3J ) 5.0 Hz, 2H,
3
3
H
H
H
aro), 7.48 (t, 3H, J ) 7.0 Hz, Haro), 7.42 (t, 2H, J ) 6.5 Hz,
aro), 7.22 (d, 2H, J ) 8.5 Hz, Haro), 7.11 (t, 2H, J ) 7.5 Hz,
aro), 6.33 (br s, 2H, H6), 4.19 (s, 2H, CH2), 2.00-1.84 (m, 12H,
3
-CH2-), 0.21 (br s, 2H, -CH2-), -0.17 (br s, 2H, -CH2-),
-0.43 (br s, 2H, -CH2-), -0.81 (br s, 2H, -CH2-), -1.05 (br
s, 2H, -CH2-), -1.59 (br s, 2H, -CH2-), -2.67 (s, 2H, -NHpyr).
HR-MS (ESI-MS): calcd m/z ) 1134.5102 for C66H64N13O6 [M +
H]+, found 1134.5114. UV-vis (CHCl3/MeOH 10%) λ, nm (10-3
ꢀ, dm3 mol-1 cm-1): 421 (261.4); 517 (16.4); 551 (3.7); 590
(4.9);646 (1.5).
meso-R-5,10,15-Tris{2-(3,3′,3′′-[N,N′,N′′-tris(2-amino-ethyl-
amino)propionylamino]triphenyl}-R-20-(2-hydroxy-5-nitro-
benzoylamino-phenyl)porphyrin, 8H2. In a 100 mL round-bottom
flask under argon, 80 mg (0.096 mmol) of meso-R-5,10,15-tris(2-
acryloylamino)phenyl-R-20-(2-amino)phenylporphyrin described for
7H2, DCC (39 mg, 0.191 mmol), DMAP (1 mg, 0.01 mmol), and
2-hydroxy-5-nitrobenzoic acid (35 mg, 0,191 mmol) were dissolved
in 10 mL of freshly distilled pyridine. Stirring was maintained
overnight, and then the mixture was dried under vacuum. The
product was dissolved in chloroform and poured onto a 15 µm silica
gel column. The desired product was eluted with 0.7% MeOH/
CHCl3. After evaporation to dryness, 50 mg was collected (yield
) 52%) and then allowed to react with tren according to the same
procedure described for 1H2. The desired product was dissolved in
CH2Cl2, poured onto a 15 µm silica gel column, and eluted with
5-10% MeOH/CH2Cl2/NH3g. After evaporation to dryness, 50 mg
of 8H2 was collected (yield ) 49%). 1H NMR (500 MHz, DMSO-
d6, 363 K): δ 11.03 (br s, 3H, NH), 8.75 (d, 4H, J ) 4.5 Hz,
Hâpyr), 8.73 (d, 2H, J ) 4.8 Hz, Hâpyr), 8.71 (d, 2H, J ) 4.8 Hz,
Hâpyr), 8.67 (broad d, 2H, Haro), 8.60 (d, 2H, J ) 8.0 Hz, Haro),
8.56 (d, 1H, J ) 3.2 Hz, H3), 7.83-7.45 (m, 4H, Haro), 7.73 (dd,
1H, 3J ) 7.0 Hz, 4J ) 1.5 Hz, Haro), 7.52 (dd, 2H, 3J ) 7.5 Hz, 4J
) 1.8 Hz, Haro), 7.43 (td, 2H, 3J ) 7.5 Hz, 4J ) 1.8 Hz, Haro), 7.37
3
3
Hâ-pyr), 8.88 (d, J ) 5.0 Hz, 2H, Hâ-pyr), 8.86 (d, J ) 8.0 Hz,
2H, Haro), 8.85 (d, 3J ) 5.0 Hz, 2H, Hâ-pyr), 8.76 (br s, 2H, Hâ-pyr),
7.89 (m, 1H, Haro), 7.83-7.79 (m, 5H, Haro), 7.67 (d, 3J ) 7.7 Hz,
4J ) 1.5 Hz, 2H, Haro), 7.51 (m, 1H, Haro), 7.41-7.35 (m, 3H,
Haro), 7.27 (s, 1H, -NHCO), 2.12-1.95 (m, 12H, -CH2), 1.30 (s,
3H, -CH3), 0.95 (m, 2H, -NH), 0.71 (m, 2H, -CH2), 0.62 (m,
2H, -CH2), 0.24 (m, 2H, -CH2), -0.16 (m, 2H, -CH2), -1.04
(m, 2H, -CH2), -1.25 (m, 3H, -CH2 + -NH), -2.53 (s, 2H,
NHpyr). 13C NMR (125 MHz, CDCl3, 323K): δ 171.9, 171.8, 163.2,
139.8, 139.0, 137.2, 136.7, 135.8, 132.2, 131.6, 131.0, 130.3, 130.0,
122.9, 122.7, 122.2, 121.8, 117.5, 117.2, 115.5, 49.6, 47.2, 43.9,
43.3, 42.5, 41.2, 35.4, 34.8, 30.1. UV-vis (CH2Cl2) λ, nm (10-3
ꢀ; dm3 mol-1 cm-1): 421 (221.7); 515 (11.8); 552 (3.0); 589 (3.2);
646 (1.3). HR-MS (LSIMS): calcd m/z ) 1025.4939 for C61H61N12O4
[M + H]+, found 1025.4915. Anal. Calcd for C61H60N12O4‚H2O:
C, 70.22, H, 5.99, N, 16.12. Found: C, 69.96, H, 5.89, N, 16.44.
meso-R-5,10,15-Tris{2-(3,3′,3′′-[N,N′,N′′-tris(2-amino-ethyl-
amino)propionylamino]triphenyl}-R-20-(2-hydroxy-5-nitro-
benzylamino-phenyl)porphyrin, 7H2. In a 500 mL round-bottom
flask under argon, 650 mg (0.708 mmol) of meso-5,10,15-tris(2-
amino)phenyl-20-(2-trityl-amino)phenylporphyrin (4.0-TrTAPP)21
and 1 mL of Et3N were dissolved in 250 mL of freshly distilled
THF; then acryloyl chloride (208 µL, 2.55 mmol) dissolved in 20
mL of THF was added dropwise at -50 °C over 10 min. Stirring
was maintained for 10 min, and then the mixture was dried under
3
4
3
(td, 2H, J ) 7.5 Hz, J ) 1.8 Hz, Haro), 7.35 (dd, 1H, J ) 9.0
4
Hz, J ) 3.2 Hz, H5), 5.38 (d, 2H, J ) 9.0 Hz, H6), 2.06 (m, 4H,
CH2), 1.88 (m, 2H, CH2), 1.80 (m, 4H, CH2), 1.66 (m, 2H, CH2),
0.62 (t, 2H, J ) 9.0 Hz, CH2), -0.06 (t, 2H, J ) 9.0 Hz, CH2),
-0.20 (m, 2H, CH2), -0.36 (t, 2H, J ) 9.0 Hz, CH2), -1.23 (m,
1342 Inorganic Chemistry, Vol. 45, No. 3, 2006

Tetraaza-Tripodal-Capped Iron(II) Porphyrins
2H, CH2), -1.46 (m, 2H, CH2), -2.52 (s, 2H, -NHpyr). HR-MS
(ESI-MS): calcd m/z ) 1148.4895 for C66H62N13O7 [M + H]+,
found 1148.4877. UV-vis (CHCl3/MeOH 10%) λ, nm (10-3 ꢀ,
dm3 mol-1 cm-1): 423 (323.4); 519 (16.8); 550 (3.9); 590 (4.7);
646 (1.9).
After filtration and evaporation the resulting powder was dissolved
in chloroform and poured onto a silica gel column. The disubstituted
compound was eluted first with 8-12% MeOH/CHCl3 followed
by compound 5 eluted with 16% MeOH/CHCl3 in 66% yield (53
mg). In a drybox, to a solution of porphyrin 5 (20 mg, 0.016 mmol)
in CHCl3 (4 mL) was added a solution of Na2S2O4. The mixture
was stirred 30 min at room temperature; then the organic layer was
separated and dried over MgSO4. After filtration and evaporation
the resulting powder was dissolved in chloroform, and a large excess
of 2-methylimidazole was added. The crude product was precipi-
tated by addition of pentane, providing iron(II)-metalated 5. HR-
MS (ESI-MS): calcd m/z ) 1277.4092 for C69H64N13O756Fe35Cl
[M + H]+, found 1277.4086. UV-vis (toluene, 1000 equiv of
2-methylimidazole): λmax 444.5 (Soret), 564 nm; +O2 425 (Soret),
510 nm.
Iron(II)-Metalated 6. The synthesis is identical to that of 1.
HR-MS (LSIMS): calcd m/z ) 1079.4132 for C61H59N12O4Fe [M
+ H]+, found 1079.4133.
Iron(II)-Metalated 7. The insertion of the iron(II) is carried out
according to the procedure described for 5. HR-MS (ESI-MS): calcd
m/z ) 1223.3984 for C66H62N13O656Fe35Cl [M + H]+, found
1223.4019. UV-vis (toluene, 1000 equiv of 2-methylimidazole):
λmax 444 (Soret), 564 nm; +O2 425 (Soret), 510 nm.
Iron(II)-Metalated 8. The insertion of the iron(II) is carried out
according to the procedure described for 5. HR-MS (ESI-MS): calcd
m/z ) 1237.3779 for C69H65N13O756Fe35Cl [M + H]+, found
1237.3783. UV-vis (toluene, 1000 equiv of 2-methylimidazole):
λmax 443 (Soret), 564 nm; +O2 422 (Soret), 510 nm.
Iron(II)-Metalated 9. In a drybox, to a solution of porphyrin
9H2 (10 mg, 0.009 mmol) in THF (6 mL) was added iron(II)
bromide. The mixture was stirred overnight at 65 °C; then solvent
was removed under vacuum. The resulting powder was dissolved
in benzene/MeOH (5/1) and directly loaded on a silica gel
chromatography column. The expected compound was eluted with
a gradient of methanol in benzene with triethylamine. HR-MS
(ESI-MS): calcd m/z ) 1198.4741 for C68H66N14O456Fe, found
1198.4745 [M]+•. UV-vis (toluene): λmax 439 (Soret); + O2 428
nm.
meso-R-5,10,15-Tris{2-(3,3′,3′′-[N,N′,N′′-tris(2-amino-ethyl-
amino)propionylamino]triphenyl}-â-20-(2-{3-[(pyridin-
3-ylmethyl)amino]propionylamino}phenyl)porphyrin, 9H2. In a
100 mL round-bottom flask under argon atmosphere meso-R-5,-
10,15-â-20-tetrakis(2-acryloylamino)phenylporphyrin (R,R,R,â
Michael acceptor) was prepared according to the usual method.
The latter (1 g, 1.12 mmol) was dissolved in 600 mL of a CHCl3/
MeOH (1/5) mixture degassed for 2 h. The solution was heated at
55 °C, and then tris(2-aminoethyl)amine (170 µL, 1.13 mmol) was
added in one portion with a syringe. The reaction mixture was
stirred for 24 h, and 5 equiv of 3-aminomethylpyridine (305 µL,
5.65 mmol) was added in one portion. Stirring was maintained for
24 h, and then the mixture was dried under vacuum. The resulting
powder was dissolved in dichloromethane and loaded on a silica
gel chromatography column. The expected compound eluted with
4% MeOH/CHCl3/NH3(g) was obtained in 12% yield (160 mg).
1H NMR (500 MHz, CDCl3, 323 K): δ 11.27 (s, 1H), 10.96 (s,
2H), 9.31 (s, 1H), 8.95 (d, 2H, J ) 8.5 Hz), 8.92 (d, 2H, J ) 4.5
3
Hz), 8.86 (m, 8H), 8.68 (d, 1H, J ) 8.5 Hz), 7.95 (dd, 1H, J )
7.5 Hz, 4J ) 1.5 Hz), 7.89 (td, 2H, 3J ) 1.5 Hz, 4J ) 7.5 Hz), 7.83
3
(t, 3H, J ) 7.5 Hz), 7.67 (d, 1H, J ) 5.0 Hz), 7.59 (td, 3H, J )
4
1.5 Hz, J ) 7.5 Hz), 7.40 (t, 3H, 7.5 Hz), 6.35 (s, 1H), 5.83 (dd,
3
4
1H, J ) 4.5 Hz, J ) 7.5 Hz), 4.50 (d, 1H, J ) 7.5 Hz), 2.15-
1.90 (m, 10H), 1.79 (t, 2H, J ) 5.5 Hz), 1.69 (t, 2H, J ) 5.5 Hz),
1.33 (m, 2H), 0.91 (t, 1H), 0.66 (m, 2H), 0.55 (br s, 2H), 0.37 (s,
2H), 0.21 (br s, 2H), -0.19 (br s, 2H), -0.13 (br s, 2H), -1.31(br
s, 2H), -1.63 (br s, 2H), -2.67 (s, 2H). HR-MS (ESI): calcd m/z
) 1145.5626 for C68H69N14O4 [M + H]+, found 1145.5632. UV-
vis (CHCl3) λ, nm (10-3 ꢀ, dm3 mol-1 cm-1): 425 (453.7); 517
(18.6); 552 (4.9); 592 (5.5); 648 (1.8).
Iron(II)-Metalated 1. In a drybox, to a solution of porphyrin
1H2 (10 mg, 0.01 mmol) in THF (6 mL) was added iron(II) bromide
(20 mg, 0.1 mmol). The mixture was stirred overnight at 65 °C;
then solvent was removed under vacuum. The resulting powder
was dissolved in benzene/methanol (5/1) and directly loaded on a
silica gel chromatography column. The expected compound was
eluted with a gradient of methanol in benzene. HR-MS (LSIMS-
MS): calcd m/z ) 1091.4132 for C62H59N12O456Fe [M + H]+, found
1091.4160. UV-vis (toluene, 1000 equiv of 2-methylimidazole):
λmax 442 nm (Soret); +O2 420 nm.
Iron(II)-Metalated 3. The insertion of the iron(II) is carried out
according to the procedure described for 1 in a drybox. HR-MS
(ESI-MS): calcd m/z ) 1285.4686 for C70H70N12O856FeNa [M +
Na]+, found 1285.4671. UV-vis (toluene, 1000 equiv of 2-meth-
ylimidazole): λmax 443 (Soret), 564 nm; +O2 424 nm.
Iron(II)-Metalated 4. The insertion of the iron(II) is carried out
according to the procedure described for 1 in a drybox. HR-MS
(ESI-MS): calcd m/z ) 1400.5082 for C75H77N12O11Na56Fe [M -
OMe + Na]+, found 1400.5082. UV-vis (toluene, 1000 equiv of
2-methylimidazole): λmax 439 (Soret), 564 nm; +O2 422 (Soret),
510 nm.
Iron(II)-Metalated 5. In a 100 mL round-bottom flask under
argon, 86 mg (0.04 mmol) of iron(III) µ-oxodimer of 1, 2-bromo-
methyl-4-nitrophenol (21.8 mg, 0.094 mmol), and 0.1 mL of Et3N
were dissolved in 20 mL of freshly distilled THF. The solution
was heated at 65 °C overnight and then dried under vacuum. The
resulting powder was dissolved in chloroform, washed with HCl
(2 M), neutralized with saturated NaHCO3, and dried over MgSO4.
Results and Discussion
Although the series of porphyrins 1 and 3-9 were initially
synthesized as bimetallic chelators able to hold a copper
cation above iron in the porphyrin,22 they were studied in
this work as iron complexes, the tripodal cap remaining free
of any metal. Indeed, owing to Collman congruent Michael
addition,23 we were able to obtain two different tren-capped
porphyrins related to 1 and 6 (Figure 2).20 In these two
ligands the tren is attached to the porphyrin core by either
four or three linkers, respectively. As previously reported
for aza-crown-capped porphyrins,16 the iron complexes of
these tetraaza-tripodal ligands do not form a six-coordinate
complex but remain five coordinate with a nitrogen base as
pyridine or N-methyl imidazole. This observation is of
paramount importance as five-coordinate porphyrin com-
plexes are required for equilibrium measurements. In Figure
1
3 is represented the typical H NMR spectrum of a S ) 2
paramagnetic complex with the characteristic â-pyrrolic
signals around 50-55 ppm.
In a preliminary study we noticed that 1 and 6 exhibited
high affinities for dioxygen with P1/2 values around 10-5
Torr.19 It should be mentioned that such affinities for
Inorganic Chemistry, Vol. 45, No. 3, 2006 1343

Ruzie´ et al.
Figure 2. Chemical structure of the various compounds referred to in the text: L ) 2-MeIm or 1,2-Me2Im.
affinities as well as the resulting M values. Two striking
observations have to be made. First, whatever the number
of linkers between the tripodal cap and the porphyrin, the
dioxygen affinities are about 102 higher than those reported
for dendritic porphyrins.18a For the latter, a hydrogen bond
engaged with an apical amide groupsfive atoms away from
the meso-phenyl ringshas been proposed. As in our tren-
capped complexes the secondary amines atom are located
four atoms away from the meso-phenyl ring, they could be
implied to be in a strong hydrogen bond with the terminal
oxygen atom of the bound dioxygen. On the other hand, if
these amine functions were the single factor able to influence
the dioxygen affinity, the latter should not be as high as it is
in the case of porphyrin 3 or 4. Indeed, in these two last
complexes the two secondary amines have been substituted
by either an acetate or an ethyl succinate group for 3 or 4,
respectively. In both cases, the resulting tertiary amines or
the secondary amides are not able to act as a donor in a
hydrogen bond. Nevertheless, the measured P1/2 are 2.4 ×
10-5 and 2.7 × 10-4 Torr for 3 and 4, respectively.
Although the affinity of the acyl-substituted compound 4
is 10-fold decreased, these results suggest that the dioxygen
affinity is predominantly determined by the nature and
structure of the cavity. In our case, the tetraaza-tripodal/
tetrapodal structure is likely to determine the high dioxygen
affinity.
Figure 3. 1H NMR spectra of O2-free complex of compound 1 in pyridine-
d5.
dioxygen are by far too high to yield biomimetic dioxygen
carriers. Consequently, it is crucial to determine the various
features that could be able to modulate this affinity. Obvi-
ously, we have explained these affinities by a possible
hydrogen bond with the secondary amines of the cap. This
result has motivated the substitution of at least one amino
function of the cap by various groups to obtain the family
of eight iron porphyrins depicted in Figure 2. One can
distinguish two distinct groups of porphyrins. The first one
related to 1 in which the tetraaza-tripod is tethered by four
links is composed of 3, 4, and 5 (first line, Figure 2). The
second group related to 6 consists of various substitutions
on one meso-aromatic ring of the porphyrin, as the tripod is
attached by only three links in this group (second line, Figure
2). The dioxygen affinities have been measured as the P1/2,
according to the method exhaustively reported by Collman,14
and are specified in Table 1 along with the carbon monoxide
The predominance of the distal cap is also significant in
regard to the proximal base. Indeed, by comparing iron
porphyrins 6 and 9swhich bears an internal nitrogen base
and for which we have reported a one-pot two-step
synthesis24swe note that the affinities for dioxygen are not
very different. This second comparison is definitely in
(24) Even, P.; Ruzie´, C.; Ricard, D.; Boitrel, B. Org. Lett. 2005, 7, 4325-
4328.
1344 Inorganic Chemistry, Vol. 45, No. 3, 2006

Tetraaza-Tripodal-Capped Iron(II) Porphyrins
facing the tren cap with the hydroxy group oriented inside
the cavity, instead of being linked on it as in 5. Additionally,
we studied the conformation in solution of 7 and 8.25 On
the ROESY spectrum of 8 (see Supporting Information, S19)
in which the phenol is linked via an amide linkage it appears
obvious that the OH group is indeed blocked between two
lateral tren linkers. This orientation generates significant NOE
on the corresponding methylene signals of either the linkers
or the tren. The observation of the ROESY spectrum of 7 in
which the phenol is linked via a benzyl linkage led to the
conclusion that this specific orientation does not exist in 7
and that the nitrophenol hydroxy function is not oriented
toward the center of the cavity. Thus, in porphyrins with
the tren being tris-linked and when oriented toward the inside
of the distal cage as in 8, it is plausible that the phenol cannot
be located close enough to the superoxo complex to
destabilize it. Indeed, in this case the phenolic OH could be
maintained away from the cavity by the presence of the tren
itself. Therefore, the only predominant effect is the influence
of the tripodal cap. On the other hand, in porphyrin 5, in
which the phenol is attached to the cap with the possibility
to “dive” inside the distal cavity, the effect of the cap
becomes negligible.
Figure 4. Typical data obtained for determination of P1/2(O2) for 5 with
2-MeIm in CH2Cl2 at 25 °C: (a) under 1 atm of nitrogen; (b) under O2
partial pressure of 8.27 Torr. Intermediate partial pressures of O2: 0.059,
0.353, 1.235, and 3.291 Torr.
agreement with what was observed with the triaza-crown-
capped porphyrins of Collman et al.16 Incidentally, the
binding of dioxygen on 9 led to a diamagnetic superoxo
complex, as probed by its proton NMR spectrum ranging
from 11 to 0 ppm, with poorly resolved signals.
On the other hand, a third comparison including porphyrins
5, 7, and 8 with porphyrin 1 results in a new conclusion.
Indeed, it is striking that the P1/2 ) 0.675 Torr value obtained
for 5 (Figure 4) is not at all in the same range as all the
others (Table 1). This measurement was carried out in
methylene chloride and toluene but is much more difficult
to carry out in toluene because the complex tends to
precipitate in this solvent. Nevertheless, measurements of
P1/2(O2) in these two solvents show that they do not
significantly effect the affinity of the complex for dioxygen.
However, it is known from the work of Reed et al.15 that
a phenol group, when located around the distal cage of a
heme, should enhance its affinity for dioxygen (Figure 1,
model b). One of the two secondary amines of porphyrin 5
has been alkylated by a benzyl nitrophenol moiety. The
proton of the p-nitrophenol group is known to be acidic as
the pKa of the acido-basic couple phenol/phenate is around
7.5. In contrast to a phenol group, a possible explanation
could be that the nitrophenol group would partially exist as
the nitrophenate and thereby strongly destabilize the iron
superoxo complex. Indeed, in complex 5 the “lateral arm”
is expected to be flexible and the O atom of the phenol could
point toward the inside of the cavity. Our molecular
mechanics modeling show that this particular orientation of
the hydroxy group inside the pocket is possible without any
distortion of either the porphyrin core or the tripodal cap.
This destabilization of the superoxo complex could explain
the 104-fold decrease of the dioxygen affinity of iron complex
5 relative to iron complex 1. So, why are the affinities of 7
and 8 not affected by the same factor as both of them also
bear the same nitrophenol motif ? The main structural
difference between 7-8 and 5 is the point of attachment of
the phenol. Indeed, in 7 and 8 the phenol is grafted on an
ortho position of the fourth meso-aromatic cycle. This
particular geometry implies that the phenol is somehow
Obviously, the tetraaza composition of the cap has to be
important for the gas binding interactions. However, our
compounds do not behave as the aza-crown-capped porphy-
rins of Collman do toward the coordination of O2 or CO23
as, for instance, the cyclam-capped iron(II) porphyrin does
not bind carbon monoxide at atmospheric pressure. This
prompted us to also investigate the shape of the cap itself.
As already mentioned above, by proton NMR spectroscopy
we studied our capped tripodal porphyrins as free-base
molecules in solution. We were able to perform the full
assignment of the protons using a combination of standard
2D experiments such as COSY, TOCSY, heteronuclear H-C
HSQC. The first comment concerns the high symmetry of
the molecule as shown by the equivalence of the two
branches of the tren surrounding the substituted phenol in
8, for instance. Also remarkable is the strong high-field shift
of all the resonances of the methylene protons of the tren.25
This shielding must be related to a large movement of the
tren toward the porphyrin plane. These conclusions were also
confirmed by a single-crystal X-ray structural analysis of 2,
the nickel counterpart of 1. Although nickel(II) cation is
smaller than iron(II) cation26 and is known to remain four
coordinate in a square-planar geometry, the structure of 2
should be close to the structure of the six-coordinate
oxygenated iron(II) complex 1. Indeed, the nickel atom is
expected to be located in the mean plane of the porphyrin
as the iron atom in the case of a diamagnetic six-coordinate
complex.27 Nevertheless, due to O2 or CO binding in the
distal pocket, it should be mentioned that the cap itself can
(25) Gueyrard, D.; Didier, A.; Ruzie´, C.; Bondon, A.; Boitrel, B. Synlett
2004, 1158-1162.
(26) (a) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr. 1969, B25, 925.
(b) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767.
(27) (a) Bytheway, I.; Hall, M. B. Chem. ReV. 1994, 94, 639-658. (b)
Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry;
University Science Books: Mill Valley, 1994.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1345

Ruzie´ et al.
Figure 5. Stick drawing of the solid-state structure of compound 2. Note
that two of the four spacers are disordered over two positions.
adopt a different conformation in the six-coordinate iron(II)
complex. X-ray-quality crystals of 2 were obtained by slow
diffusion at room temperature of a mixture of acetonitrile/
ethanol in an ethylenediamine solution of 2. The nickel
complex crystallized with both an ethanol molecule and an
acetonitrile molecule surrounding it in the lattice.
Observing the stick drawing of 2 (Figure 5), it is evident
that the tren cap is linked by four linkers as predicted by the
proton NMR analysis. It is also clear that two diametrically
opposed linkers are disordered. Structural refinement evi-
denced a light disorder in carbon and/or oxygen atoms in
two of the four “O‚‚‚N” pathways. One of them concerns
only two carbon atoms (C47 and C48), where the second
one is extended over four atoms (O66 oxygen atom and C66,
C67, and C68 carbon atoms). Use of PART keyword in the
SHELXL28 refinement program allowed us to correctly
describe this disorder (labeled as ‘a’ and ‘b’ pathways) and
get realistic anisotropic displacement parameter values for
these parts of the molecular structure. Attempts to refine the
ratio of the different pathways has been realized but clearly
indicated a 50-50 proportion for each of them. Final
refinement has been performed with fixed and equal values
of ‘a’ and ‘b’ pathways. Additionally, disorder has also been
introduced in the modeling of the ethanol solvent molecule
present in the asymmetric unit cell. The tetraaza-tripod is
attached through its three primary amines to four pickets of
the porphyrin. Thus, one nitrogen atom (N11) is linked to
two pickets, where the two other nitrogen atoms (N9 and
N10) are linked to only one picket each. We have taken
advantage of this possible structure of the tren to obtain two
different geometries of tren-capped porphyrins represented
by 1 and 6. To obtain the three linked series, different
synthetic routes have been explored. The first one that we
have chosen consists of opposing tren with a tris-acrylamide
picket porphyrin to prepare 6.29 The second possibility is
Figure 6. Thermal ellipsoid views (set to the 10% probability level) of
the solid-state structure of compound 2 (top: apical view; bottom: side
view). Solvent molecules are not represented.
illustrated by the reaction of a tris-alkylated tren on the
tetrakis-acrylamide picket porphyrin.30 Thus, when the tren
is attached by four spacers, the resulting distal pocket is
actually composed, on one hand, of two tertiary nitrogen
atoms (N11 and N12), one of them being the tripodal
nitrogen atom, and, on the other hand, of two secondary
nitrogen atoms (N9 and N10). The latter are midway between
a lateral location as are N5, N6, N7, and N8 and an apical
site as is N12 (Figure 6). It is worth mentioning that in this
solid-state structure all the N-H dipoles are oriented toward
the inside of the cavity but N7, incorporated in one of the
four meso-amide linkages.
Thus, these N-H dipoles are potential candidates as
hydrogen-bond donors. Incidentally, the two secondary
nitrogen atoms N9 and N10 from the tripod are each included
in a short hydrogen bond with N5 and N6, respectively, but
in these hydrogen bonds N9 and N10 act as acceptors with
an acceptor to donor distance between d(N10-N6) ) 2.741
(28) Sheldrick, G. M. SHELXS and SHELXL, Programs for Crystal Structure
Analysis (Release 97-2); Institut fu¨r Anorganische Chemie der
Universita¨t: Go¨ttingen, Germany, 1998.
(30) (a) Baeg, J. O.; Holm, R. H. Chem. Commun. 1998, 571-572. (b)
Collman, J. P.; Fu, L.; Herrmann, P. C.; Wang, Z.; Rapta, M.; Bro¨ring,
M.; Schwenninger, R.; Boitrel, B. Angew. Chem., Int. Ed. Engl. 1998,
37, 3397-3400.
(29) Didier, A.; L’Her, M.; Boitrel, B. Org. Biomol. Chem. 2003, 1, 1274-
1276.
1346 Inorganic Chemistry, Vol. 45, No. 3, 2006

Tetraaza-Tripodal-Capped Iron(II) Porphyrins
Å and d(N9-N5) ) 2.776 Å. Apparently, in solution this
observation implies that the N-H dipoles from N9 and N10
could also act as a hydrogen-bond donor with the oxygenated
complex. A third weaker intermolecular hydrogen bond exists
between the amide N7 and the amide O46 with a distance
of 2.996 Å. The three secondary nitrogen atoms of the tripod
form a plane with an angle of 4.53° with the least-squares
mean plane of the porphyrin. These two planes are separated
by a distance of 3.53 Å, and the tertiary nitrogen atom N12
is located 0.85 Å above the N9,N10,N11 plane and, so, 4.38
Å above the mean porphyrin plane. These distances can be
compared with those reported for the unique X-ray structure
of a cyclic triaza-crown-capped porphyrin.31 For this triaza-
cyclononane-capped zinc(II) porphyrin the distance between
the mean porphyrinic plane and the triazacycle is 3.97 Å
and the angle between the two planes is 5.75°. Although
the distance between the two planes is smaller in our system,
the tren is much more flexible with distances between the
nitrogen atoms also greater than in the case of the triazacy-
clononane. Additionally, the nitrogen atom located at the top
of the cap is not tightly bound in a rigid cyclic structure.
Figure 7. Distances of the 24 atoms from the mean plane of nickel
porphyrin 2.
atom cannot induce any steric hindrance to the linear bound
CO.
This may explain why these tren-capped iron(II) porphy-
rins, from a general point of view, do not discriminate carbon
monoxide versus dioxygen in the range that could be
expected. However, while observing the P1/2(CO) column,
a trend tends to emerge from Table 1. Definitely, the three-
linked tren-capped iron(II) complexes 6-9 exhibit affinities
for CO which are at least 102 higher than those measured
for complexes 1 and 3-5 in which the cap is four-linked to
the porphyrin. This observation is consistent with a stronger
steric hindrance of the tripodal cap in the complexes in which
the center of the tripod is almost in an apical position to the
metal in the porphyrin (Figure 6, top view). On the other
hand, in complexes 6-9 the tren motif being only three-
linked to the porphyrin is clearly expected to be off-centered
relative to the apical position to the iron and hence should
not exhibit a strong influence on CO binding. Furthermore,
one can verify that the affinity of 5 for CO is 10-fold lower
than it is for 1, for instance. This led to a discrimination
factor M around 100 instead of 1 for the other complexes of
this study. However, it should be emphasized that this
increase of M is mostly due to the decrease of affinity for
O2 binding. Nevertheless, this small decrease of the CO
affinity is explainable either by an additional and direct steric
effect of the nitrophenol residue on the bound CO or to a
change of conformation of the tren cap due to substitution
by a nitrophenol group. However, it is difficult to choose
between these two different hypotheses in the absence of an
X-ray structure of ligand 5H2 or one of its metalated
derivatives. In summary, it is worth noting that these tren-
capped iron(II) porphyrins exhibit high affinities for both
dioxygen and CO. Indeed, in only one example for which
the dioxygen adduct is destabilized due to the nitrophenol
residue the M value reaches 100. This can be explained as
the X-ray structure of 2 indicates that the tripodal nitrogen
Surprisingly, the porphyrinic core of 2 is not as distorted
as it is expected to be. Indeed, nickel(II) porphyrins are
usually deformed in a ruffled geometry.32 The average Ni-N
bond length is 1.945 Å and compares with other severely
ruffled nickel(II) porphyrins for which this distance is 1.921
Å.33,34 On the other hand, the torsion angle between two
opposite pyrroles is very often an accurate criterion to
evaluate the degree of distortion in ruffled porphyrins. For
instance, it can rise in value up to 37° in bis-strapped
porphyrins. However, in the X-ray structure of 2 this torsion
(C2, C3, C12, C13) is only 2.46°. In fact, the distortion
observed in 2 is more related to a saddle shape than a ruffle
shape as it should be.
In the saddle shape, and as depicted in Figure 7, the two
pairs of opposite pyrroles are located on each side of the
porphyrin: N1-C1-C2-C3-C4 and N3-C11-C12-
C13-C14 below the mean porphyrin plane where N2-C6-
C7-C8-C9 and N4-C16-C17-C18-C19 are above the
latter. Again, it appears that the predominance of the cap
influences not only the affinity of dioxygen or carbon
monoxide, but also the distortion of the porphyrin plane itself.
Conclusion
From this work we showed that arbor porphyrins capped
with a tetraaza tripod exhibit remarkable affinities for
dioxygen. To know if these unusual dioxygen affinities are
due to the presence of secondary amine group donors of
(32) (a) Sparks, L. D.; Medforth, C. J.; Park, M. S.; Chamberlain, J. R.;
Ondrias, M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am.
Chem. Soc. 1993, 115, 581-592. (b) Jentzen, W.; Simpson, M. C.;
Hobbs, J. D.; Song, X.; Ema, T.; Nelson, N. Y.; Medforth, C. J.; Smith,
K. M.; Veyrat, M.; Mazzanti, M.; Ramasseul, R.; Marchon, J. C.;
Takeuchi, T.; Goddard, W. A.; Shelnutt, J. A. J. Am. Chem. Soc. 1995,
117, 11085-11097.
(33) Richard, P.; Rose, E.; Boitrel, B. Inorg. Chem. 1998, 37, 6532-6534.
(34) Tani, F.; Matsu-ura, M.; Ariyama, K.; Setoyama, T.; Shimada, T.;
Kobayashi, S.; Hayashi, T.; Matsuo, T.; Hisaeda, Y.; Naruta, Y. Chem.
Eur. J. 2003, 9, 862-870.
(31) Collman, J. P.; Schwenninger, R.; Rapta, M.; Bro¨ring, M.; Fu, L. Chem.
Commun. 1999, 137-138.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1347

Ruzie´ et al.
hydrogen bond in the distal cage itself these amine functions
were substituted in a systematic way by various groups. The
latter were shown not to have a significant effect on the
affinity of the iron complex for dioxygen except for only
one model. Indeed, if one of the secondary amine functions
of the tripod carries an electron-withdrawing group as a
p-nitrophenol, the stability of the oxygenated complex is
decreased by a factor 104. We propose, to explain this
remarkable effect, that the phenol is in fact in the form of
phenate due to the acidity of the phenolic proton. Thus, the
stability of the phenate group reinforced by the nitro group
in the para position destabilizes in a clear way the oxygenated
complex by means of electrostatic repulsion.
On the other hand, if this same group is attached to one
of the aromatic cycles in a meso position, it remains without
effect on the stability of the oxygenated complex. We explain
this observation by the fact that the phenate in this particular
spatial arrangement is facing the tripod and hence cannot
approach the superoxo complex sufficiently to interact with
it. In summary, this series of tren-capped iron(II) porphyrins
represents the first example of a synthetic dioxygen carrier
for which the very high affinity for dioxygen can be
moderated by associating a low-pKa phenol group with the
cap. Concomitantly, as the tripod does not induce any steric
hindrance close enough to the linear bound CO, no significant
discrimination for the latter is observed, thereby confirming
the crucial role of the steric factors for CO discrimination.
Acknowledgment. The authors thank the CNRS for
financial support as well as the MENRT. C.R. is indebted
to Re´gion Bretagne for his grant. B.B. particularly acknowl-
edges Re´gion Bretagne for his significant financial support.
D.R. thanks the MENRT for his Ph.D. grant.
Supporting Information Available: Spectroscopic data, in-
cluding 1D, 2D NMR, electrospray HR-MS spectra, as well as
detailed data for the P1/2 measurements of the various new
compounds and X-ray data for the crystal structure deposited at
the Cambridge Crystallographic Data Center and allocated deposi-
tion number CCDC 273334. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC0514703
1348 Inorganic Chemistry, Vol. 45, No. 3, 2006