Asymmetry and steric hindrance in tripodal ligands: Reaching the limit for octahedral geometry with the newly synthesized [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine tripod in FeCl2 complexes

Article abstract of DOI:10.1016/j.ica.2011.04.015

We have synthesized the fully unsymmetrical [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine tripod FBrTPA. The synthesis involves preparation of the already known [(6-bromo 2-pyridylmethyl) (2-pyridylmethyl)] amine BrDPA, which is obtained either by classical condensation of α-substituted pyridine carboxaldehyde with aminomethyl pyridine, or by a pathway involving the protection/deprotection sequence of this primary amine. This second way is useful for syntheses of monosubstituted DPAs in the cases where α-substituted pyridine carboxaldehydes are unavailable. The crystal structure of the FeCl₂ complex shows that the ligand binds in a κ⁴-N fashion, with however relatively long ligand-to-metal distances. The spectroscopic and electrochemical studies support decoordination of the bromopyridyl substituent in solution, with κ³-N coordination mode of the tripod within a complex displaying a trigonal bipyramidal geometry at the metal centre. This complex reacts easily with dry oxygen to yield an unsymmetrical μ-oxo diferric complex, the structure of which is also reported.

Full text of DOI:10.1016/j.ica.2011.04.015

Inorganica Chimica Acta 373 (2011) 195–200
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Asymmetry and steric hindrance in tripodal ligands: Reaching the limit for
octahedral geometry with the newly synthesized [(6-bromo 2-pyridylmethyl)
(6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine tripod in FeCl2 complexes
Laila Benhamou a,b, Hassen Jaafar a, Aurore Thibon a, Mohammed Lachkar b, Dominique Mandon a,
a Laboratoire de Chimie Biomimétique des Métaux de Transition, UMR CNRS n° 7177, Institut de Chimie et Université de Strasbourg, Bâtiment Le Bel, 4 rue
⇑
Blaise Pascal, F-67070 Strasbourg cedex, France
b Laboratoire d’Ingénierie des Matériaux Organométalliques et Moléculaires, Unité associée au CNRST URAC 19, Université Sidi Mohamed Ben Abdellah, Faculté des Sciences, BP
1796 (Atlas), 30000 Fez, Morocco
a r t i c l e i n f o
a b s t r a c t
Article history:
We have synthesized the fully unsymmetrical [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl)
(2-pyridylmethyl)] amine tripod FBrTPA. The synthesis involves preparation of the already known
[(6-bromo 2-pyridylmethyl) (2-pyridylmethyl)] amine BrDPA, which is obtained either by classical con-
Received 28 January 2011
Received in revised form 5 April 2011
Accepted 9 April 2011
densation of
ing the protection/deprotection sequence of this primary amine. This second way is useful for syntheses
of monosubstituted DPAs in the cases where -substituted pyridine carboxaldehydes are unavailable.
The crystal structure of the FeCl2 complex shows that the ligand binds in a
4-N fashion, with however
relatively long ligand-to-metal distances. The spectroscopic and electrochemical studies support decoor-
dination of the bromopyridyl substituent in solution, with
3-N coordination mode of the tripod within a
complex displaying a trigonal bipyramidal geometry at the metal centre. This complex reacts easily with
a-substituted pyridine carboxaldehyde with aminomethyl pyridine, or by a pathway involv-
Available online 16 April 2011
a
Keywords:
j
Halogen-substituted tris(2-
pyridylmethyl)amine ligands
Iron complexes
Tripodal tetramines
Asymmetry
j
dry oxygen to yield an unsymmetrical
l-oxo diferric complex, the structure of which is also reported.
Dioxygen activation
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
other perspective, asymmetrical substitution of all the a position
of the pyridyl units would represent a more straightforward mod-
ification of the reactivity site, i.e. it would directly affect the coor-
dination sphere. When all the pyridyl arms are substituted with
identical groups, the ligand ideally exhibits the C3 symmetry;
The design of simple multidentate ligands in coordination
chemistry is an exponentially growing area: a computer-assisted
search with ‘‘tripodal ligands’’ entered as keywords resulted in find-
ing more than 2000 references over the last two decades, 2/3 of
them being published only during the last 10 years [1]. When re-
stricted to ‘‘asymmetrical tripodal ligands’’ keywords, the total num-
ber of references dropped to less than 100 over the last 20 years
[2]. Indeed, it appears that there is an increasing demand for
easy-to-prepare small ligands providing upon coordination with
transition metals thermally stable complexes. Tetradentate tripods
derived from the tris(2-pyridylmethyl)amine (TPA) are among the
most thoroughly studied with a broad range of potential applica-
tions [3–7]. The introduction of asymmetric centres by substitution
at the methylene carbon has been reported for several years, allow-
ing extension of the chemistry of these simple ligands to various
areas (asymmetric recognition of amino acids, study of chiroptical-
or luminescence properties to cite only a few of them) [7–13]. In an
mono- or di-homosubstituted tripods exhibit in principle a C
r
symmetry: there are numerous examples of such a situation.
Tris-hetero -substituted TPA-type ligands should be completely
a
asymmetric: to our knowledge, the preparation of ligands of this
type has never been reported. Additionally, the selective introduc-
tion of substituents of different nature is highly desirable since it
should allow fine modulation of both steric and electronic proper-
ties of the ligand.
In the course of our investigations on the reactivity of Fe(II)
complexes versus molecular oxygen, we reported the structures
of the FeCl2 complexes with the bis
F2TPA and Br2TPA. Whereas the former ligand binds in a four-coor-
dinate (
4-N) fashion, the second one, more bulky, is three-coordi-
nate (
3-N) in the complex, providing a trigonal bipyramidal
geometry around the metal leading to enhanced reaction kinetics.
Thus, a tripod containing both an -bromo and an -fluoro pyri-
a-halosubstituted TPA tripods
j
j
a
a
dine might induce a borderline geometry in such complexes. Keep-
ing in mind that the presence of a vacant coordination site is a key
point with respect to reactivity studies, and that coordination of
⇑
Corresponding author. Tel.: +33 (0) 368 851 537; fax: +33 (0) 368 851 438.
E-mail address: mandon@chimie.u-strasbg.fr (D. Mandon).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.04.015

196
L. Benhamou et al. / Inorganica Chimica Acta 373 (2011) 195–200
2.2. Syntheses
2.2.1. Protection of the 2-picolylamine: synthesis of tert-butyl pyridin-
2-ylmethylcarbamate
To a solution of 6 g (55 mmol) of 2-picolylamine in 300 ml of
CH2Cl2 were added 35 g (160 mmol) of di-tert-butyl dicarbonate,
followed by the addition of 50 cm3 of 7 N NaOH. The medium
was stirred at room temperature for 5 h. The medium was washed
several times with water, and the organic phase dried over MgSO4.
The concentrated CH2Cl2 phase was deposited at the top of a silica
gel chromatography column mounted with CH2Cl2. Elution with a
mixture CH2Cl2/ethyl acetate (95/5) afforded the desired com-
pound as a pale yellow oil. About 8 g were obtained, corresponding
to a 69% yield.
1H NMR (CDCl3), d, ppm: 8.30 (d, 1H); 7.42 (t, 1H); 7.08 (d, 1H),
6.94 (t, 1H), 6.09 (br, 1H), 4.24 (s, 1H), 4.23 (s, 2H), 1.26 (s, 9H).
Scheme 1. The ligand FBrTPA.
2.2.2. BrDPA
molecular oxygen should proceed smoothly in order to be fully
under control [14–17], we decided to prepare the fully
unsymmetrical FBrTPA ligand, [(6-bromo 2-pyridylmethyl) (6-flu-
oro 2-pyridylmethyl) (2-pyridylmethyl)] amine depicted in
Scheme 1, and to characterize the corresponding FeCl2 complex
with hope that modulation of the steric hindrance of the ligand
would enhance the flexibility at the metal site.
We report in this communication the preparation of the asym-
metric FBrTPA ligand, the full characterization of the FeCl2 complex
showing that our goal has been reached: whereas tetradentate
coordination of the tripod is observed in solid state, solution stud-
ies indicate that the complex in solution displays a bipyramidal tri-
gonal geometry. In addition, we report the crystal structure of the
This is a known compound, which could be prepared according
to the procedure published in reference [18]. The following method
represents an alternate synthetic way of monosubstitued DPAs in
the cases where 6-substituted pyridine carboxaldehydes (or 6-
substituted picolylamines) are unavailable.
To a solution of 1.0 g (4.8 mmol) of tert-butyl pyridin-2-ylmeth-
ylcarbamate in dry DMF were added 211 mg (5.3 mmol) of a sus-
pension of 60% sodium hydride in mineral oil. The medium was
stirred at room temperature for 1 h. To this mixture was added
1.3 g (5.3 mmol) of 2-bromo-6-bromomethylpyridine. The result-
ing medium was stirred at room temperature for 9 h. The reaction
solvent was then evaporated and the solid was dissolved in CH2Cl2.
This solution was washed three times with water, and the organic
phase dried over MgSO4, filtered and evaporated to complete
dryness.
end product of oxygenation of FBrTPAFeCl2, the
plex FBrTPAFeClOFeCl3.
l-oxo diferric com-
The sticky solid (1.65 g) was then dissolved in 200 cm3 of diox-
ane, and 150 cm3 of 6 N HCl were added to this solution. This mix-
ture was stirred at room temperature for 3 h. The pH of the mixture
was then raised to pH 11 by addition of aqueous NaOH and CH2Cl2
was added. The mixture was washed with water and the aqueous
phase was discarded. The organic phase was then extracted with
an aqueous solution of HCl and the CH2Cl2 phase was discarded.
The pH was then raised with a 10% aqueous solution of Na2CO3
and the desired compound was extracted with CH2Cl2. The organic
phase was washed with water, dried over MgSO4 and the solvent
was removed by rotary evaporation. BrDPA could be obtained as
a clean compound by chromatographic separation of the main frac-
tion on a silica gel column mounted with diethyl ether. Elution
with a mixture diethyl ether/methanol 85/15 containing 1% of
aqueous ammonia afforded 734 mg of a white solid (54%).
The 1H and 13C NMR spectra are identical to those already men-
tioned in reference [18].
2. Experimental
2.1. General considerations and physical methods
All solvents used during the metallation reactions and work-up
were distilled and dried according to published procedures [28]
and degassed shortly before use. 2-Aminomethyl pyridine and
6-bromo 2-pyridine carboxaldehyde were purchased from Al-
drich; di-tert-butyl dicarbonate was purchased from Fluka. These
chemicals were used as received. 2-Bromo-6-bromomethylpyri-
dine and 2-fluoro-6-bromomethylpyridine were prepared as pre-
viously reported [7,14]. Analytical anhydrous FeCl2 was obtained
as a white powder by reacting iron powder (ACS grade) with
hydrochloric acid in the presence of methanol under an argon
atmosphere. UV–Vis spectra were recorded on a Varian Cary 05e
UV–Vis-NIR spectrophotometer at room temperature in CH3CN.
1H NMR data were collected in CD3CN solutions (or CDCl3 for
1H NMR (CDCl3), d, ppm: 8.53 (d, 1H), 7.65 (m, 1H), 7.45 (t, 1H),
7.33 (m, 3H), 7.12 (m, 1H), 3.94 (s, 2H), 3.93 (s, 2H), 2.45 (br, NH).
13C NMR (CDCl3): d, ppm: 54.2, 54.6, 121.0, 122.0, 122.3, 126.1,
136.4, 138.9, 141.7, 149.3 159.6, 161.8.
the free ligands) on
a Bruker AC 300 spectrometer at
300.1300 MHz using the residual signal of CD2HCN (CHCl3 for li-
gand) as a reference for calibration. 13C data were collected on
the same spectrometer at 75.4867 MHz, using the same calibra-
tion procedure. 19F data were also collected on the same spec-
trometer at 282.4037 MHz, and calibrated with CFCl3. Cyclic
voltammetry measurements were obtained from a PAR 273A
potentiostat in a 0.1 M acetonitrile solution of TBAPF6 (supporting
electrolyte), using platinum electrodes and a saturated calomel
electrode as reference. At the end of each measurement, the po-
tential was checked by addition of a small amount of ferrocene
(Fc/Fc+ = 0.380 V/SCE) in the cell. Elemental analyses were carried
out by the Service Central d’Analyses de l’Institut de Chimie de
Strasbourg.
2.2.3. FBrTPA
To a solution of 400 mg (1.43 mmol) of BrDPA in 150 cm3 of eth-
anol were added 300 mg (1.58 mmol) of 2-fluoro-6-bromomethyl-
pyridine and 360 mg (4.30 mmol) of sodium hydrogenocarbonate.
This mixture was refluxed for 16 h. The solvent was removed by
rotary evaporation, and the solid extracted with CH2Cl2. This or-
ganic phase was washed three times with water, and dried over
MgSO4. After filtration, the solvent was removed by rotary evapo-
ration. The yellow-brownish sticky oil was extracted five times
with pentane. The pentane extracts were combined, the resulting
solution was concentrated, and kept overnight at ꢀ20 °C. A white

L. Benhamou et al. / Inorganica Chimica Acta 373 (2011) 195–200
197
solid precipitated, and was washed with cold pentane. About
180 mg of a white, analytically pure solid could be recovered. At
this moment, the calculated yield was 32%. Further concentration
of the pentane afforded an other 80 mg crop, spectroscopically
clean, giving an overall yield of 46%. The pentane liquors still con-
tained some ligand for which additional purification was needed.
1H NMR (CDCl3), d, ppm: 8.54 (m, 1H), 7.65 (m, 1H), 7.45 (t, 1H),
7.33 (m, 3H), 7.12 (m, 1H), 3.90 (s, 2H), 3.88 (s, 2H), 3.83 (s, 2H).
13C NMR (CDCl3), d, ppm: 169.9, 158.8, 149.1, 141.4, 141.3,
139.8, 136.5, 122.2, 121.6, 120.2, 120.1, 107.9, 107.4, 126.3,
123.1, 60.1, 59.4, 59.2.
solved acetonitrile, and the UV–Vis data of the solution were found
to be identical to those obtained from the powder samples.
2.3. Crystal structures
Single crystals were obtained by slow diffusion of diethyl ether
in a tube containing FBrTPAFeCl2 (sealed tube under argon in that
case) and FBrTPAFeClOFeCl3 in solution in CH3CN.
Selected single crystals were mounted on a Nonius Kappa-CCD
area detector diffractometer (Mo Ka k = 0.71073 Å). The complete
conditions of data collection (Denzo software [29]) and structure
refinements are given below. The cell parameters were determined
from reflections taken from one set of 10 frames (1.0° steps in phi
angle), each at 20 s exposure. The structures were solved using di-
rect methods (SHELXS97) and refined against F2 using the SHELXL97
software [30]. No absorption correction was applied. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were generated according to stereochemistry and refined using a
riding model in SHELXL97. The R2 value refers to all data; the R1 va-
lue refers to observed data.
Elemental analysis for C18H16N4FBr: Anal. Calc: C, 55.8; H, 4.1;
N, 14.5. Found: C, 55.6, H: 4.2; N, 14.1%.
2.2.4. FBrTPAFeCl2
Preparation and handling of this complex was performed under
argon atmosphere by using Schlenk technique following standard
procedures [21].
About 120 mg (0.31 mmol) of free FBrTPA were dissolved in a
schlenk tube containing 20 cm3 of dry and degassed THF. About
36 mg (0.28 mmol) of anhydrous FeCl2 was dissolved in a second
schlenk tube containing 10 cm3 of dry and degassed THF. The solu-
tion of FeCl2 was transferred under argon in the schlenk containing
the ligand, and the medium was stirred overnight. The solvent was
then evaporated to dryness, and the compound was extracted with
dry and degassed CH3CN, filtered under inert atmosphere and con-
centrated. Addition of diethyl ether afforded a yellow solid, which
was thoroughly washed with this solvent, prior to be dried under
vacuum. About 120 mg (83%) of FBrTPAFeCl2 with good analytical
and spectroscopic data could be obtained.
FBrTPAFe(II)Cl2 = ‘‘C18 H16 Br Cl2
F
Fe N4’’, T = 173 K,
MW = 514.01. Monoclinic, Cc, a = 9.3228(6) Å, b = 15.5947(13) Å,
c = 14.5296(10) Å, b = 105.400(4)°, V = 2036.6(3) Å3. Dc = 1.676
g cmꢀ3
, Z = 4, R1 = 0.0650, wR2 = 0.1742 for 3354 reflections,
hmax = 27.35°. Rint = 0.0975, total number of reflexions: 6108.
FBrTPAFeClOFeCl3ꢁ2CH3CN = ‘‘C18 H16 Br Cl4 F Fe2 N4 O, 2 (C2
H3 N)’’, T = 173 K, MW = 738.86. Monoclinic, P21/c, a =
10.1944(6) Å, b = 16.3289(7) Å, c = 20.1980(11) Å, b = 118.211(3)°,
V = 2962.62(8) Å3. Dc = 1.656 g cmꢀ3
, Z = 4, R1 = 0.0913, wR2 =
0.3051 for 6740 reflections, hmax = 27°. Rint = 0.0882, total number
of reflexions: 17 742.
Single crystals were obtained by slow diffusion of diethyl ether
in a sealed tube containing FBrTPAFeCl2 in solution in CH3CN. The
crystals obtained by this way were crushed and dissolved in dry
and degassed acetonitrile, and the spectroscopic data (UV–Vis
and 1H NMR) of the solution were found to be identical to those ob-
tained from the powder samples.
3. Results and discussion
The key step in the synthesis of FBrTPA is the preparation of the
[(2-bromo 6-pyridylmethyl) (2-pyridylmethyl)] amine] ligand
BrDPA, which was already reported in the past [18]. We tried to ob-
tain this compound from direct equimolar reaction of 2-picolyl-
amine with 2-bromo-6-bromomethyl pyridine, as it was
originally reported for DPA [19]. In that case however we were un-
able to recover anything else than the ligand Br2TPA and some
unreacted amine, as the 2-bromo-6-bromomethyl pyridine reac-
tant reacted faster with the newly formed BrDPA than with the
starting 2-picolylamine. Thus, the enhanced reactivity of electron
deficient pyridines makes it essential to follow either the pyridine
carboxaldehyde route, or the protection procedure of the picolyl-
amine, as outlined in Scheme 2.
UV–Vis, CH3CN, RT, k, nm (103 molꢀ1 L cmꢀ1): 262 (9.5); 386
(0.7).
1H NMR, CD3CN, d, ppm, (Dm1/2, Hz): 76, weak
a
, (>1000); 66, 34,
22, CH2 (500, 460, 480); 55, 53, 52, 31, b, b0subst.Py (153, 90,
170,130); 9.3, dangling pyridine (190), 7.6, (38).
c
19F NMR, CD3CN, d, ppm: 78, Dm1/2 = 550 Hz.
All spectroscopic data are given in Supplementary material.
Elemental analysis for C18H16N4FBrFeCl2: Anal. Calc. C, 42.0; H,
3.1; N, 10.9. Found: C, 42.5; H, 3.3; N, 10.5%.
Cyclic voltammetry, CH3CN TBAPF6 0.1 M, C = 3 mM.
Scan rate 200 mV/s: E1/2Fe(II)Fe(III) = 29 mV/SCE. DEc/Ea = 109 mV.
Ipc/Ipa = 1.09.
Scan rate 50 mV/s: E1/2Fe(II)Fe(III) = 25 mV/SCE. DEc/Ea = 115 mV.
Ipc/Ipa = 0.95.
From BrDPA, FBrTPA could easily been obtained following a
standard procedure [14,19,20] as a stable white solid and in a clean
way, with a 46% yield from BrDPA. The most noticeable spectro-
scopic feature of this compound is the splitting of the CH2 signals
in 1H NMR into three distinct signals at d = 3.90, 3.89 and
3.84 ppm, supporting asymmetry at the central amine site. Inter-
estingly, the irreversible oxidation wave observed in cyclic voltam-
metry was measured at Ea = 1.210 V/SCE (TBAPF6 0.1 M CH3CN,
200 mV/s), i.e. slightly above that found with Br2TPA,
Ea = 1.163 V/SCE, and not far from the Ea = 1.236 V/SCE value al-
ready reported for F2TPA, all data being obtained in the same con-
centration conditions (4 mM).
The data are given in Supplementary material.
2.2.5. FBrTPAFeClOFeCl3
About 75 mg (0.14 mmol) of FBrTPAFeCl2 were dissolved under
argon atmosphere in 10 cm3 of dry and degassed acetonitrile. Dry
oxygen was bubbled into the medium for 5 s. A brown colour
developed immediately, and the medium was stirred 24 h. A
brown solid was formed and bulky precipitation occurred upon
addition of 2–3 cm3 of diethyl ether. This precipitate was filtered,
carefully washed with diethyl ether and dried. Obtained: 40 mg
(87%). This compound was moderately soluble in acetonitrile.
UV–Vis, CH3CN, RT, k, nm (103 molꢀ1 L cmꢀ1): 263 (11.0); 339
(2.8).
The reaction of FBrTPA with 1 equiv of FeCl2 afforded the corre-
sponding FBrTPAFeCl2 complex with a 83% yield. This yellow com-
pound turned out to be oxygen-sensitive in solution. In UV–Vis, the
Single crystals were obtained by slow diffusion of diethyl ether
in a sealed tube containing a saturated solution of the compound in
CH3CN. The crystals obtained by this way were crushed and dis-
ligand-centered
= 9559 molꢀ1 L cmꢀ1). The MLCT signal was measured at
k = 386 nm with
= 698 molꢀ1 L cmꢀ1. In FeCl2 complexes, we
absorption
appeared
at
k = 262 nm
(e
e

198
L. Benhamou et al. / Inorganica Chimica Acta 373 (2011) 195–200
Scheme 2. Synthetic procedure for the preparation of FBrTPA.
found the intensity of the MLCT band to be indicative of the coor-
dination mode of the tripod [14,21,22]. In the present case, the va-
lue is comparable to that found in the Br2TPAFeCl2 complex [21]
and does not stand higher than the maximum one obtained within
the series of complexes with a hypodentate coordination mode of
the ligand, i.e.
e
= 750 molꢀ1 L cmꢀ1 in the compound with a bis
(2,3-dimethoxy phenyl)substituted tripod [16]. The 1H NMR spec-
trum is dominated by the presence of broad signals, even if one
considers the spectral window which is widely open. In the same
fashion as with UV–Vis spectroscopy, we found in previous studies
that 1H NMR can also be an interesting tool to predict the geometry
of the complexes in solution. The spectrum exhibited broad signals
at d = 66 ppm
(Dm1/2 = 500 Hz), 34 ppm (Dm1/2 = 460 Hz) and
Scheme 3. The proposed structure of FBrTPAFeCl2 in the solid state and in solution.
Fig. 2. ORTEP diagram of FBrTPAFeClOFeCl3 showing a partial numbering scheme
(50% probability level). The fluorine and bromine atoms are disordered: the F1 label
corresponds to a 70%F/30%Br occupation rate, while the Br1 corresponds to a 30%F/
70%Br occupation rate. Selected bond distances (Å) and angles (°): Cl1–Fe1:
2.329(3), Cl2–Fe2: 2.223(3), Cl3–Fe2: 2.229(3), Cl4–Fe2: 2.235(3), Fe1–O1:
1.765(7), Fe1–N2: 2.150(8), Fe1–N3: 2.219(7), Fe1–N1: 2.226(7), Fe1–N4:
2.253(7), Fe2–O1: 1.765(6). \O1Fe1N2: 94.8(3), \O1Fe1N3: 103.8(3), \N2Fe1N3:
81.1(3), \O1Fe1N1: 173.1(3), \N2Fe1N1: 78.3(3), \N3Fe1N1: 74.6(3), \O1Fe1N4:
106.4(3), \N2Fe1N4: 85.8(3), \N3Fe1N4: 147.9(3), \N1Fe1N4: 74.1(3),
\O1Fe1Cl1: 97.5(2), \N2Fe1Cl1: 167.2(2), \N3Fe1Cl1: 92.5(2), \N1Fe1Cl1:
89.4(2), \N4Fe1Cl1: 94.2(2), \O1Fe2Cl2: 108.9(3), \O1Fe2Cl3: 113.4(2),
\Cl2Fe2Cl3: 108.90(13), \O1Fe2Cl4: 111.6(2), \Cl2Fe2Cl4: 110.01(11),
\Cl3Fe2Cl4: 103.91(12), \Fe2O1Fe1: 163.7(5). Inter-metallic distance: Fe1–Fe2:
3.494 (7).
Fig. 1. ORTEP diagram of FBrTPAFeCl2 showing a partial numbering scheme (50%
probability level). Selected bond distances (Å) and angles (°): N1–Fe1: 2.280(7), N2–
Fe1: 2.146(7), N3–Fe1: 2.403(7), N4–Fe1: 2.398(7), Cl1–Fe1: 2.403(2), Cl2–Fe1:
2.333(2). \N2Fe1N1: 77.4(3), \N2Fe1Cl2: 96.82(19), \N1Fe1Cl2: 170.63(16),
\N2Fe1N4: 82.6(3), \N1Fe1N4: 71.9(2), \Cl2Fe1N4: 115.04(19), \N2Fe1Cl1:
167.01(19), \N1Fe1Cl1: 89.91(18), \Cl2Fe1Cl1: 96.12(8). \N4Fe1Cl1: 90.85(18),
\N2Fe1N3: 81.5(3), \N1Fe1N3: 72.2(3), \Cl2Fe1N3 : 99.8(2), \N4Fe1N3 : 143.1(3),
\Cl1Fe1N3 : 97.5(2). By contrast to the second structure reported in this paper, no
disorder was observed here, both Br1 and F1 atoms having a 100% occupation rate
at their respective positions.

L. Benhamou et al. / Inorganica Chimica Acta 373 (2011) 195–200
199
22 ppm (Dm1/2 = 480 Hz), assigned to the methylene protons, and
sharper resonances assigned to the
and b0-protons, at
d = 55 ppm (Dm1/2 = 153 Hz), 52.7 ppm (Dm1/2 = 90 Hz), 52.0 ppm
Dm1/2 = 170 Hz) and 31 ppm (Dm1/2 = 130 Hz). A sharp signal ob-
served at d = 7.6 ppm (Dm1/2 = 38 Hz) was assigned to the protons
adopts the regular
j
4-N coordination mode [16,24,25]. On the
b
other hand, hypodentate coordination of the tripod generally af-
fords lower values (60 < E1/2 < 10 mV/SCE) of the potentials [16,24].
In the present case the Fe(II)/Fe(III) couple of FBrTPAFeCl2 was
measured at E1/2 = 29 mv/SCE (scan rate 200 mV/s, NBu4PF6 0.1 M
(
c
of the coordinated pyridyl arms. Qualitatively, the spectrum differs
Pt electrodes, SCE as reference). This value definitely supports j3
-
from the well-defined ones obtained from FeCl2 complexes in
N binding of FBrTPA in solution in the FeCl2 complex. The data
which the ligand coordinates in
a tetradentate way [15–
are displayed in Supplementary content.
17,21,23–25]. Quantitatively, the measured values of the mid-
intensity width (Dm1/2) of the b-protons stand above those gener-
ally observed for complexes with distorted octahedral geometry
(40 < Dm1/2 < 130 Hz): this strongly supports the occurrence of a
trigonal bipyramidal geometry around the metal centre
[14,21,23,24]. The observation of a broad signal within the diamag-
netic region at d = 9.3 ppm (Dm1/2 = 190 Hz) supports the presence
of a dangling (uncoordinated) pyridyl arm. In addition, a very
broad signal is detected at 76 ppm (Dm1/2 > 1000 Hz), that might
Finally and taken together, all these results converge and indi-
cate that the solution structure of FBrTPAFeCl2 can be described
as indicated in Scheme 3.
Single crystals of FBrTPAFeCl2 could be obtained, and the corre-
sponding ORTEP diagram is displayed in Fig. 1. In solid state the li-
gand coordinates in a tetradentate fashion around the metal, and
the two chloride ions complete the coordination sphere.
The substituted pyridines lie in trans position to each other,
with Fe–Nsubstituted distances close to 2.4 Å. The trans-equatorial
correspond to the
a
proton of the coordinated unsubstituted pyri-
distortion parameter
cator within series of FeCl2 complexes [15] (see the Supporting
Information for details). In the present case,
= 67°2, which is a
q was defined in the past as a distortion indi-
dine. Thus either the bromo- or fluoro-substituted pyridine might
decoordinate in solution. For obvious reasons of steric hindrance,
we believe that the decoordinated pyridine is the bromo-substi-
tuted one. Additional experimental data support this hypothesis:
q
moderate distortion, in line with elongated metal to ligand dis-
tances. These values are reminiscent of those measured in Me2TPA-
FeCl2 [25]. In this latter complex the effect of the distance
elongation was to weaken the coordination polyhedron, leading
to ligand dissociation in strongly polar medium. These observa-
tions bring additional support to a decoordination of the most hin-
in 19F NMR a broad signal is measured at d = 78 ppm (Dm
1/
2 = 550 Hz), in line with coordination of the fluoropyridine to the
iron [14].
We already reported that cyclic voltammetry may be a useful
technique to address the question of the coordination mode of
the ligand within such complexes: in cases where the Fe(II)/Fe(III)
couple (generally reversible waves) is observed at moderate posi-
tive potential (150 < E1/2 < 300 mV/SCE), the ligand in LFeCl2
dered pyridyl unit of the ligand (affording in that case a
j
3-N
tridentate coordination mode) in the complex in solution. As
shown in Fig. 3 (A), and as usually seen within this class of com-
plexes, intramolecular H-bonding occurs between the axial
Fig. 3. Intramolecular interactions in FBrTPAFeCl2 (A) and FBrTPAFeClOFeCl3 (B). Left: hydrogen bonds. Right: interatomic distances between the halogen atoms and the
equatorial ligand (Cl2 and O1, respectively).

200
L. Benhamou et al. / Inorganica Chimica Acta 373 (2011) 195–200
chloride ion Cl1 and the down-pointing H atoms from the methy-
lene groups, with dCl1–H13B = 2.75 Å and dCl1H7B = 2.82 Å. The
Acknowledgements
equatorial Cl2 interacts with the
as seen by the distance dCl2–H6 = 2.73 Å. Also, the separations
dCl2–Br1 = 3.60 Å and Cl2–F1 3.21 Å fall within the expected range
of distances between an equatorial chloride and an
pyridyl heteroatom. Finally, and from a more general point of view,
the crystal packing – shown in Supplementary content – does not
lead to any particular comment.
The addition of dioxygen into a solution of FBrTPAFeCl2 re-
sulted in immediate darkening of the solution. A dark brown solid
could be isolated, and its UV–Vis spectrum was reminiscent of that
obtained upon oxygenation of the F2TPAFeCl2 complex [15], with
a-H atom of the axial pyridine
We thank the CNRS, the Université Louis Pasteur and the Uni-
versité de Strasbourg. The Institut de Chimie de Strasbourg is also
gratefully acknowledged. We also thank all the colleagues from the
Service de Diffraction des Rayons X, the Service de Microanalyse,
and the Service de Spectrométrie de Masse of the Institut de Chi-
mie de Strasbourg. This work was originally supported as a collab-
orative action between CNRS (project No. 18552 – France) and
CNRST (project No. chimie 08/06 and 08/07 – Morocco). Both orga-
nizations are gratefully acknowledged for their support.
a-substituted
Appendix A. Supplementary material
absorptions at k = 263 nm
k = 339 nm with
= 2800 molꢀ1 L cmꢀ1, suggesting the presence
of a -oxo diferric segment. This dark brown compound was
dissolved in acetonitrile, and layering the solution with diethyl
(
e
= 11 000 molꢀ1 L cmꢀ1
)
and
e
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2011.04.015.
l
ether afforded single crystals of the unsymmetrical
FBrTPAFeClOFeCl3.
l-oxo
References
[1] Scifinder computer-assisted search with ‘‘tripodal ligands’’ entered as
keywords. Scores: 1955–1990: 277; 1990–2000: 758; 2000–2011 (January),
1977.
[2] Scifinder computer-assisted search with ‘‘asymmetric(al) tripodal ligands’’
entered as keywords. Scores: 1955–1990: 1; 1990–2000: 19; 2000–2011
(January), 42.
[3] G. Anderegg, F. Wenk, Helv. Chim. Acta 50 (1967) 2330.
[4] A.G. Blackman, Eur. J. Inorg. Chem. (2008) 2633.
[5] A.G. Blackman, Polyhedron 24 (2005) 1.
[6] L.M. Berreau, Comments Inorg. Chem. 28 (2007) 123.
[7] Z. Dai, J.W. Canary, New J. Chem. 31 (2007) 1708.
[8] J.W. Canary, C.S. Allen, J.M. Castagnetto, Y.H. Chiu, P.J. Toscano, Y. Wang, Inorg.
Chem. 37 (1998) 6255.
[9] T. Yamada, S. Shinoda, J.-I. Uenishi, H. Tsukube, Tetrahedron Lett. 42 (2001)
9031.
[10] T. Yamada, S. Shinoda, H. Sugimoto, J.-I. Uenishi, H. Tsukube, Inorg. Chem. 42
(2003) 7932.
[11] Z. Dai, X. Xu, J.W. Canary, Chirality 17 (2005) S227.
[12] X. Xu, K.J. Maresca, D. Das, S. Zahn, J. Zubieta, J.W. Canary, Chem. Eur. J. 8
(2002) 5679.
[13] S. Zahn, J.W. Canary, J. Am. Chem. Soc. 124 (2002) 9204.
[14] A. Machkour, D. Mandon, M. Lachkar, R. Welter, Inorg. Chem. 43 (2004) 1545.
[15] N.K. Thallaj, O. Rotthaus, L. Benhamou, N. Humbert, M. Elhabiri, M. Lachkar, R.
Welter, A.-M. Albrecht-Gary, D. Mandon, Chem. Eur. J. 14 (2008) 6742.
[16] L. Benhamou, A. Machkour, O. Rotthaus, M. Lachkar, R. Welter, D. Mandon,
Inorg. Chem. 48 (2009) 4777.
[17] A. Wane, N.K. Thallaj, D. Mandon, Chem. Eur. J. 15 (2009) 10593.
[18] M. Merkel, D. Schnieders, S.M. Baldeau, B. Krebs, Eur. J. Inorg. Chem. (2004)
783.
[19] M.S. Nelson, J. Rodgers, J. Chem. Soc. A (1968) 272.
[20] C.L. Chuang, O. Dos Santos, X. Xu, J.W. Canary, Inorg. Chem. 36 (1997) 1967.
[21] D. Mandon, A. Machkour, S. Goetz, R. Welter, Inorg. Chem. 41 (2002) 5363.
[22] A. Machkour, D. Mandon, M. Lachkar, R. Welter, Inorg. Chim. Acta 358 (2005)
339.
[23] H. Jaafar, B. Vileno, A. Thibon, D. Mandon, Dalton Trans. 40 (2011) 92.
[24] H. Jaafar, R. Louis, D. Mandon, Inorg. Chim. Acta 366 (2011) 147.
[25] L. Benhamou, M. Lachkar, D. Mandon, R. Welter, Dalton Trans. (2008) 6996.
[26] A. Hazell, K.B. Jensen, C.J. McKenzie, H. Toftlund, Inorg. Chem. 33 (1994) 3127.
[27] T. Kojima, R.A. Leising, S. Yan, L. Que Jr., J. Am. Chem. Soc. 115 (1993) 11328.
[28] W.L.F. Armarego, D.D. Perrin, Purification of Laboratory Chemicals, fourth ed.,
Pergamon Press, Oxford, 1997.
[29] Kappa CCD Operation Manual, B.V. Nonius, Delft, The Netherlands, 1997.
[30] G.M. Sheldrick, SHELX97, Program for the Refinement of Crystal Structures,
University of Göttingen, Germany, 1997.
The ORTEP diagram is displayed in Fig. 2. Simple l-oxo diferric
compounds in which the metals are bridged by one oxygen atom
only are well known with TPA ligands [15,26,27]. In the symmetri-
cal [
cent pyridines of distinct TPA ligands stabilize the structure. We
have already shown that single substitution of the ligand in posi-
tion by a fluorine atom results in decoordination of the substituted
pyridine, affording the [
3-N (FTPAFeCl2)2-O] neutral species [15].
In the reaction of oxygen with Fe(II) complexes, the formation of
the stable -oxo segment corresponds to the driving force of the
reaction, with a huge gain in stability of the product [15]. We reach
here the situation where steric factors due to -substitution pre-
clude coordination of one ligand per metal within a very stable
Fe–O–Fe -oxo segment. This leads to decoordination of a second
ligand, which has also been observed in related -oxo dimers with
j p interaction between two adja-
4-N (TPAFeCl)2-O]2+ dication,
a
j
l
a
l
l
F3TPA and F2TPA tripods [14,15]. In the end product and in this
kind of unsymmetrical structure, the oxygen atom interacts with
the
a-H atom of the axial pyridine, with dO1H6 = 2.64 Å, as seen
in Fig. 3. The steric interaction between the halides atoms of the
tripod and the bridging oxygen atom remains moderate, making
the ligand coordinating in the
j
4-N tetradentate fashion. For this
structure also, the crystal packing is displayed in Supplementary
content.
In summary, we have reported the synthesis of a compound
which is, to the best of our knowledge, one of the first (if not
the first) tripod within the series of simple tris(2-pyridyl-
methyl)amine ligands, in which a full asymmetry is brought by
different
a-substituents of the pyridyl arms. Whereas the solid
state structure of the FeCl2 complex is close to that of F2TPAFeCl2,
the solution behaviour supports a structure being very similar to
that of Br2TPAFeCl2. The facility by which those kind of tripods
can be obtained opens new perspectives in fields requiring a fine
modulation of the steric and electronic properties of such tripo-
dal ligands.