Tridentate coordination of monosubstituted derivatives of the tris(2-pyridylmethyl)amine ligand to FeCl3

Article abstract of DOI:10.1021/ic001339p

In trichloroferric complexes with tris(22-pyridylmethyl) amine ligands α-substituted by bulky groups, tridentate coordination of the tripode is observed. The X-ray crystal structures of the complexes ((2-bromopyridyl)methyl)bis(2-pyridylmethyl)amine Fe

Full text of DOI:10.1021/ic001339p

Inorg. Chem. 2001, 40, 4803-4806
4803
Notes
Tridentate Coordination of Monosubstituted
Derivatives of the Tris(2-pyridylmethyl)amine
Ligand to FeCl3: Structures and Spectroscopic
Properties of ((2-Bromopyridyl)methyl)bis-
(2-pyridylmethyl)amine FeIIICl3 and (((2-p-
Methoxyphenyl)pyridyl)methyl)Bis(2-pyridyl-
methyl)]amine FeIIICl3 and Comparison with the
Bis(2-pyridylmethyl)]amine FeIIICl3 Complex
Dominique Mandon,* Agathe Nopper,
Thomas Litrol, and Sandrine Goetz
Figure 1. Ligands and complexes reported in this study.
in view of future developments of TPA-based iron systems with
functional groups, we have prepared and studied robust chloro-
ferric complexes in which the metal is coordinated to a TPA
ligand monosubstituted in the R position of one pyridyl arm by
a bulky bromine atom or a methoxyphenyl ring. We have found
that the steric hindrance provided by the R-substituted arm
pushes it away from the coordination site, leaving it free and
potentially reactive. We report herein, together with their
spectroscopic properties, the crystal structures of the complexes
((2-bromopyridyl)methyl)bis(2-pyridylmethyl)]amine FeIIICl3,
L1FeIIICl3, (((2-p-methoxyphenyl)pyridyl)methyl)bis(2-pyridyl-
methyl)]amine FeIIICl3, L2FeIIICl3, and a comparaison of both
spectroscopic and structural features with those of the parent
complex bis(2-pyridylmethyl)]amine FeIIICl3, L3FeIIICl3.
Laboratoire de Chimie Organome´tallique et de Catalyse,
UMR CNRS No. 7513, Universite´ Louis Pasteur, Institut Le
Bel, 4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France
ReceiVed NoVember 28, 2000
Introduction
Small nitrogen tripodal ligands based on alkylpyridyl motifs
are known to form stable complexes with a large number of
transition metals.1 Among them, the tris(2-pyridylmethyl)amine
ligand (TPA) has been extensively studied because, in part, of
the ability of some of its iron complexes to carry out catalytic
oxidation reactions similar to those observed with some non-
heme metalloenzymes.2 The rich bioinspired iron chemistry
developed over the past years involves the use of precursors
which are in general simple ferric derivatives, alkyl substituted
or not.3-7 These are compounds for which the standard
tetradentate coordination mode of the TPA ligand is observed.8,9
Substituted TPA are known, although at present very little
information has been reported about stable ferric monomers of
functionalized ligands in that family.10-13 We believe that the
introduction of functional groups might induce noticeable change
in the structure and reactivity of the iron complexes.4,14 Thus,
Experimental Section
The UV-vis spectra were recorded on a Varian Cary 05 E UV-vis
1
NIR spectrophotometer. H NMR data were recorded in CD3CN at
ambient temperature on a Bruker AC 300 spectrometer at 300 MHz
using the residual signal of CD2HCN as a reference for calibration.
Ligands L1 and L3 were prepared according to published methods.10
L2 was prepared from L1 by adaptation of a published procedure using
the Suzuki cross-coupling procedure.10 1H NMR, CDCl3, δ, ppm,
TMS: 8.53, 2H d; 7.95, 2H d; 7.65-7.13 9H m; 6.98, 2H d; 3.96, 4H,
s; 3.94, 2H, s; 3.85, 3H, s. Mass spectrum: impact mode, m/z ) 396.20.
Anal. Calcd for L2 (C25H24ON4): C 75.66, H 6.05. Found: C 75.37, H
6.02.
Typical Metalation Experiment. Anhydrous FeCl3 (0.9 equiv) in
dry diethyl ether was added to a solution of ligand Ln in a diethyl ether.
Precipitation occurred immediately, and the reaction medium was
allowed to stir for 2 h. The yellow-orange solid was filtered, washed
with cold acetonitrile and diethyl ether, and dried under vacuum. Further
purification was achieved by diethyl ether crystallization from an
acetonitrile solution. The yields are quantitative, and all compounds,
air and thermally stable, gave satisfactory elemental analyses.
X-ray Analysis. Quantitative data were obtained at room temperature
for L1FeIIICl3 and L3FeIIICl3 and at -100 °C for L2FeIIICl3. All
experimental parameters used are given in the Supporting Information.
The resulting dataset was transferred to a DEC Alpha workstation, and
for all subsequent calculations the Enraf-Nonius OpenMoleN package18
was used.
(1) Toftlund, H. Coord. Chem. ReV. 1989, 94, 67-108.
(2) Que, L., Jr.; Dong, Y. Acc. Chem. Res. 1996, 29, 190-196
(3) Dong, H.; Fujii, H.; Hendrich, M. P.; Leising, R. A.; Pan, G.; Randall,
C. R.; Wilkinson, E. C.; Zang, Y.; Que, L., Jr..; Fax, B. G.; Kauffmann,
K.; Mu¨nck, E. J. Am. Chem. Soc. 1995, 117, 2778-2792.
(4) Lange, S. J.; Miyake, H.; Que, L., Jr. J. Am. Chem. Soc. 1999, 121,
6330-6331.
(5) Zheng, H.; Yoo, S. J.; Mu¨nck, E.; Que, L., Jr. J. Am. Chem. Soc.
2000, 122, 3789-3790.
(6) Ho, R. Y. N.; Roelfes, G.; Hermant, R.; Hage, R.; Feringa, B. L.;
Que, L., Jr. J. Chem. Soc., Chem. Commun. 1999, 2161-2162.
(7) Simaan, A. J.; Boillot, M. L.; Rivie`re, E.; Boussac, A.; Girerd, J.-J.
Angew. Chem., Int. Ed. 2000, 39, 196-198.
(8) Hazell, A.; Jensen, K. B.; McKenzie, C. J.; Toftlund, H. Inorg. Chem.
1994, 33, 3127-3134.
(9) Kojima, T.; Leising, R. A.; Yan, S.; Que, L., Jr. J. Am. Chem. Soc.
1993, 115, 11328-11335.
(10) Chuang, C. L.; Dos Santos, O.; Xu, X.; Canary, J. W Inorg. Chem.
1997, 36, 1967-1972.
(11) Xu, J.; Chuang, C. L.; Canary, J. W. Inorg. Chim. Acta 1997, 256,
125-128.
(15) Sugimoto, H.; Sasaki, Y. Chem. Lett. 1997, 541-542.
(16) Xu, L.; Sasaki, Y.; Abe, M. Chem. Lett. 1999, 163-164
(17) Jang, H. G.; Cox, D. D.; Que, L., Jr. J. Am. Chem. Soc. 1991, 113,
9200-9204.
(18) 1 OpenMoleN, Interactive Structure Solution, Nonius B.V., Delft, The
Netherlands, 1997.
(12) Zahn, S.; Canary, J. W. Angew. Chem., Int. Ed. 1998, 37, 305-306.
(13) Harata, M.; Jitsukawa, K.; Masuda, H.; Einaga, H. Chem. Lett. 1995,
61-62.
(14) Zang, Y.; Kim, J.; Dong, Y.; Wilkinson, E. C.; Appelman, E.; Que,
L., Jr. J. Am. Chem. Soc. 1997, 119, 4197-4205.
10.1021/ic001339p CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/03/2001

4804 Inorganic Chemistry, Vol. 40, No. 18, 2001
Notes
Figure 2. ORTEP diagrams of the LnFeIIICl3 complexes.
The structure was solved using direct methods. After refinement of
the heavy atoms, a difference Fourier map revealed maximas of residual
electronic density close to the positions expected for hydrogen atoms;
they were introduced as fixed contributors in structure factor calculations
by their computed coordinates (C-H ) 0.95 Å) and isotropic
temperature factors such as B(H) ) 1.3Beqv(C) Å19 but not refined. Full
least-squares refinements on /F/. A final difference map revealed no
significant maxima. The scattering factor coefficients and anomalous
dispersion coefficients come respectively froms ref 19a and 19b.
Crystal data for L1FeIIICl3. Yellow crystals, crystal dimensions 0.40
× 0.40 × 0.40 mm3, C18H17N4Cl3FeBr, M ) 531.5, monoclinic, space
group C12/c1, a ) 20.602 (1), b ) 14.5887(6), and c ) 14.2907(9) Å,
â ) 106.227(4)°, V ) 4124.0(7) Å3, Z ) 8, Dc ) 1.71 g cm-3, µ(Mo
KR) ) 3.054 mm-1. A total of 4486 reflections were used, 2.5° < θ
< 26.29°. A total of 2920 independent reflections having I > 3σ(I);
244 parameters. Final results: R(F) ) 0.033, Rw(F) ) 0.048, GOF )
Slow diffusion of diethyl ether in acetonitrile solutions of
the complexes yielded in each case crystals suitable for X-ray
diffraction analysis. As shown in Figure 2, for each complex
tridentate coordination is observed, with the substituted pyridine
dangling away from the metal for L1 and L2. Consequently, all
compounds reported in this paper are neutral molecules.
From a structural point of view, similar features are observed
for each compound, i.e., a distorded octahedral geometry with
trans ligand angles below 180°, typically for L1FeIIICl3, values
of 163.02(7)° for Cl2-Fe-N2, 167.56(7)° for Cl1-Fe-
N1, and 166.28(7)° for Cl3-Fe-N3. The angles between the
chloride ligands are significantly larger than 90°, with values
of 97.13(4)° for Cl1-Fe-Cl2, 96.52(4)° for Cl1-Fe-Cl3,
and 98.67(3)° for Cl2-Fe-Cl3. The flexibility of the ligand
is expressed by the small angles between the nitrogen atoms,
with 73.65(9)° for N2-Fe-N3, 77.39(9)° for N1-Fe-N2,
and 83.8(1)° for N1-Fe-N3. As is often the case for other
already reported FeIIITPA complexes, the Fe-N(pyridine) bonds
(average 2.19 Å) are shorter than the Fe-N(amine) bond (2.281-
(2) Å). These bond lengths, listed in Table 1, are consistent
with the high spin state of the metal in the complex.
The ferric derivative [TPAFeCl2](ClO4) has already been
structurally characterized, and the ligand displays the usual,
tetradentate coordination mode.8,9 In our case, tridentate coor-
dination is observed. However, it is likely that the use of other
anions, smaller or weakly coordinating, would result in tet-
radentate coordination of the substituted ligands. Precedents of
tridentate coordination with the TPA ligand are known with
rhenium and molybdenum complexes and justified by thermo-
dynamic patterns such as trans effects [(η-3)TPAReVOL]+ or
decreased π-acceptor capability of the TPA ligand [(η-3)-
TPAMo0(CO)3].14,16 In this study, the choice of chloride ions,
which in organic solvents are strongly coordinating, was dictated
by the need to obtain complexes displaying excellent criteria
of stability in various media. Indeed, the high affinity of the σ
donor chloride ions for the iron stabilizes the LFeIIICl3 form in
which the need for an octahedral geometry forces the substituted
pyridine to decoordinate. Interestingly, this geometry is retained
in solution, as shown by NMR measurements (vide infra).
The UV-visible spectra of the compounds are shown in
Figure 3, and their absorption maxima are listed in Table 2.
The intense peak in the UV range is broader than and slightly
blue-shifted from that in the free ligand and assigned to a ligand-
centered π-π* transition. Upon metalation of the ligands, two
broad and weak bands appear that we assign as charge-transfer
1.004, maximum residual electronic density ) 0.985 e Å-3
.
Crystal Data for L2FeIIICl3. Yellow crystals, crystal dimensions
0.23 × 0.16 × 0.09 mm3, C25H24N4OCl3Fe, M ) 558.7, monoclinic,
space group P121/c1, a ) 21.9206(6), b ) 7.9500(2), and c ) 15.2009-
(4) Å, â ) 109.39(1)°, V ) 2498.8(4) Å3, Z ) 4, Dc ) 1.48 g cm-3
,
µ(Mo KR) ) 0.951 mm-1. A total of 22038 reflections were used,
2.5° < θ < 29.55°. A total of 3369 independent reflections having I >
3σ(I).;307 parameters. Final results: R(F) ) 0.039, Rw(F) ) 0.055,
GOF ) 1.082, maximum residual electronic density ) 0.365 e Å-3
.
Crystal data for L3FeIIICl3. Yellow crystals, crystal dimensions 0.30
× 0.30 × 0.30 mm3, C12H13N3Cl3Fe, M ) 361.4, orthorhombic, space
group Pna21, a ) 15.6038(8), b ) 8.4742(7), and c ) 22.751(1) Å, V
) 3008.4(6) Å3, Z ) 8, Dc ) 1.60 g cm-3, µ(Mo KR) ) 1.528 mm-1
.
A total of 3483 reflections were used, 2.5° < θ < 26.29°. A total of
2203 independent reflections having I > 3σ(I); 342 parameters. Final
results: R(F) ) 0.028, Rw(F) ) 0.033, GOF ) 1.022, maximum residual
electronic density ) 0.228 e Å-3
.
Results and Discussion
The metalation of the ligands is straightforward and requires,
as displayed in Figure 1, the stoichiometric addition of an ether
solution of anhydrous FeCl3 to the TPA derivative Ln dissolved
in ether (L1 ) ((2-bromopyridyl)methyl)bis(2-pyridylmethyl)]-
amine, BrTPA; L2 ) (((2-p-methoxyphenyl)pyridyl)methyl)-
bis(2-pyridylmethyl)]amine, MeOPhTPA; L3 ) bis(2-pyridyl-
methyl)]amine, DPA). Yellow to orange-yellow compounds are
thus quantitatively obtained as LnFeIIICl3 complexes, which are
air stable in the solid state as well as in solution.
(19) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography, 1974, Vol. IV; Kynoch Press: Birmingham: (a) Table 2.2b,
(b) Table 2.3.1.

Notes
Inorganic Chemistry, Vol. 40, No. 18, 2001 4805
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the LnFeIIICl3 Complexes
L1FeIIICl3
L2FeIIICl3
L3FeIIICl3
Fe-Cl1
2.293(1)
2.2880(9)
2.2867(9)
2.166(3)
2.281(2)
2.220(3)
97.13(4)
96.52(4))
167.56(7)
90.19(7)
92.20(8)
98.67(3)
94.70(8))
163.02(7)
90.71(7)
85.47(8)
95.68(7)
166.28(7)
77.39(9)
83.8(1)
Fe-Cl1
2.313(2)
2.279(2)
2.288(2)
2.191(4)
2.211(5)
2.177(5)
97.81(7)
98.90(7)
87.2(1)
Fe-Cl1
2.3076(9)
2.286(1)
2.296(1)
2.185(3)
2.300(3)
2.176(3)
98.07(4)
97.92(4)
87.61(7)
90.41(7)
164.81(8)
97.65(4)
93.58(8)
164.73(8)
93.11(8)
166.63(8)
93.71(8)
90.71(8)
74.0(1)
Fe-Cl2
Fe-Cl2
Fe-Cl2
Fe-Cl3
Fe-Cl3
Fe-Cl3
Fe-N1
Fe-N1
Fe-N1
Fe-N2
Fe-N2
Fe-N2
Fe-N3
Fe-N3
Fe-N3
Cl1-Fe-Cl2
Cl1-Fe-Cl3
Cl1-Fe-N1
Cl1-Fe-N2
Cl1-Fe-N3
Cl2-Fe-Cl3
Cl2-Fe-N1
Cl2-Fe-N2
Cl2-Fe-N3
Cl3-Fe-N1
Cl3-Fe-N2
Cl3-Fe-N3
N1-Fe-N2
N1-Fe-N3
N2-Fe-N3
Cl1-Fe-Cl2
Cl1-Fe-Cl3
Cl1-Fe-N1
Cl1-Fe-N2
Cl1-Fe-N3
Cl2-Fe-Cl3
Cl2-Fe-N1
Cl2-Fe-N2
Cl2-Fe-N3
Cl3-Fe-N1
Cl3-Fe-N2
Cl3-Fe-N3
N1-Fe-N2
N1-Fe-N3
N2-Fe-N3
Cl1-Fe-Cl2
Cl1-Fe-Cl3
Cl1-Fe-N1
Cl1-Fe-N2
Cl1-Fe-N3
Cl2-Fe-Cl3
Cl2-Fe-N1
Cl2-Fe-N2
Cl2-Fe-N3
Cl3-Fe-N1
Cl3-Fe-N2
Cl3-Fe-N3
N1-Fe-N2
N1-Fe-N3
N2-Fe-N3
90.4(1)
162.8(1)
98.88(7)
94.2(1)
167.5(1)
93.3(1)
164.6(1)
89.2(1)
92.2(1)
76.6(2)
78.9(2)
76.7(2)
81.4(1)
76.5(1)
73.65(9)
Figure 3. Electronic absorption spectra of the LnFeIIICl3 complexes in acetonitrile (concentration: L1FeIIICl3, 0.282 mmol; L2FeIIICl3, 0.236 mmol;
L3FeIIICl3, 0.299 mmol).
Table 2. Absorption Maxima for the LnFe((iii)Cl3 Complexes
complex
wavelength, λmax, nm (ꢀ, 10-3 mmol-1 cm2), CH3CN, rt
Table 3. 1H NMR Chemical Shifts and T1 Relaxation Times for the
LnFeIIICl3 Complexesa
L1FeIIICl3 257 (14.70)
301 (7.14)
308 (sh.)
295 (6.64)
390 (4.05)
385 (3.69)
378 (4.82)
L2FeIIICl3 261 (20.40), 287 (19.40)
L3FeIIICl3 253 (13.40)
transitions from the coordinated chloride to the metal. In the
visible range, the values of 378.0 nm (L3FeIIICl3), 384.5 nm
(L2FeIIICl3), and 389.5 nm (L1FeIIICl3) are not far from that
reported for the TPAFeIII+ cation (λmax ) 382 nm), thus
indicating for all complexes a similar ligand field around the
metal.9
1
The H NMR spectra of the LnFeIIICl3 complexes look very
a â, â′, R, γ: common features. âu, γu (dark gray): uncoordinated
pyridyl arm. O-;m-phen, OCH3 (light gray): methoxyphenyl substituent.
similar and differ from those of the already reported TPA ferric
monomers in which the tripod acts as a tetradentate ligand.8,9,17
The spectra are dominated in the paramagnetic region by three
very broad resonances, listed in Table 3. For L2FeIIICl3 for
instance, as shown in Figure 4, they appear at δ ) 105, 91, and
52 ppm. On the basis of the integrals and T1 relaxation times,
we assign the downfield signals to the â and â′ protons of the
coordinated pyridines. The middle field shifted signal is
extremely broad and, although less shifted than the â protons,
is assigned to the R protons of the coordinated arms with the
smallest observed relaxation time. In the diamagnetic region,
the sharp resonance at δ ) 11.5 ppm is assigned to the γ-pyridyl
protons of the coordinated pyridine. A comparison of NMR data
of TPA derivatives L1FeIIICl3 and L2FeIIICl3 with that of L3-
FeIIICl3 indicates that the only difference is the presence in the
former two complexes of extra signals in the diamagnetic region,
with two of them found at 14.0 (14.6 for L2FeIIICl3) and 4.6
(5.0 for L2FeIIICl3) ppm. We assign these signals to the â- and
γ-pyridyl protons of the pyridyl arm obviously missing in L3-
FeIIICl3. Thus: (i) all compounds display a similar environment
at the paramagnetically shifted sites, indicating an identical
symmetry and coordination mode; (ii) because of the small shifts

4806 Inorganic Chemistry, Vol. 40, No. 18, 2001
Notes
In the paramagnetic region the T1 relaxation times are very
short, ranging from 60 to 120 µs. This supports a high spin
5
1
state, S ) /2, for the iron in these complexes. H NMR data
have already been reported on the ferric high spin monomers
FeCl2(TPA)+, ClO4- and Fe(DBC)(TPA)+, BPh4- 9,17 In these
.
compounds, the resonances appear at lower fields (245, 165,
137, 112, 90 ppm and 196, 116, 92, 90 ppm, respectively) but
the T1 relaxation times range from 100 to 1000 µs. The
R-protons signal also display the shortest relaxation times but,
in contrast to our compounds, are more shifted than the â-ones.
Another difference in our compounds is the absence of any
signals, either broad and shifted, in the diamagnetic region
assigned to the methylene protons. The reasons for these
differences are as yet unclear. In our compounds, which are
neutral molecules with only two coordinated pyridines, the
nuclear relaxation times are short, which might reflect a higher
spin density delocalized over the ligand. Also likely is that the
broadness of the bands might arise from a longer electronic
spin-lattice relaxation time. Extensive spectroscopic studies
addressing this point will be reported in a forthcoming article.
In conclusion, we have shown that R-monofunctionalization
of TPA ligands with bulky groups induces tridentate coordina-
tion in FeCl3 derivatives. The uncoordinated arm is the
substituted pyridine, and comparison of NMR data with those
of the bis(pyridylmethyl)amine parent complex, whose crystal
structure is also reported, indicates that this geometry is retained
in solution. These high spin compounds are very stable, and
the presence of potentially reactive pyridine at their periphery,
together with a metal protecting the tertiary amine, opens the
way to further use of these complexes in synthetic strategies.
Figure 4. 1H NMR spectrum of L2FeIIICl3, CD3CN, room tempera-
ture: trace A, paramagnetic region; trace B, diamagnetic region.
Acknowledgment. The authors are indebted to Prof. J.
Fischer, Dr. A. De Cian, Mrs. N. Gruber, and the Service
Commun de Rayons X, Faculte´ de Chimie, Universite´ louis
Pasteur, for the X-ray structural determinations. Financial
support by the CNRS and the ULP is gratefully acknowledged.
and longer relaxation times, the two above-mentioned extra
signals indicate that the substituted pyridyl arm remains
uncoordinated. With L2FeIIICl3, additional evidence of the
noncoordination of the substituted pyridyl arm arises from the
observation of resonances of the phenyl and methoxy groups
at δ ) 8.1, 7.0, and 3.9 ppm with long relaxation times. Thus,
we demonstrate that the structure as observed from the solid
state is retained in solution.
Supporting Information Available: Crystallographic files in CIF
format for the three reported structures. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC001339P