Synthesis and characterization of iron(III) complexes of a new ligand containing a potentially bridging carboxylate; structural characterization of a helical tetranuclear iron complex

Article abstract of DOI:10.1039/a809888c

Reaction of the new polydentate ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (LH) with Fe(ClO₄)₃ followed by addition of chloroacetic acid leads to the formation of the tetranuclear complex [{Fe₂OL(ClCH₂-CO₂)₂}₂](ClO₄)₄, the crystal structure of which reveals that it consists of two Fe(II)₂(μ-O)(μ-RCO₂)₂ cores that are linked via the two L ligands in a helical structure, with the carboxylate moieties of the two ligands forming a hydrogen-bonded pair at the center of the helix.

Full text of DOI:10.1039/a809888c

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Synthesis and characterization of iron(iii) complexes of a new ligand
containing a potentially bridging carboxylate; structural characterization of a
helical tetranuclear iron complex
Vladimir M. Trukhan,a Cortlandt G. Pierpont,b Kenneth B. Jensen,c Ebbe Nordlanderd and Albert A.
Shteinmana
a Institute of Chemical Physics, 142432 Chernogolovka, Russia
b Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA
c Department of Chemistry, Odense University, DK-5230 Odense M, Denmark
d Inorganic Chemistry 1, Chemical Center, Lund University, Box 124, S-221 00 Lund, Sweden.
E-mail: Ebbe.Nordlander@inorg.lu.se
Received (in Basel) 21st December 1998, Accepted 9th April 1999
Reaction of the new polydentate ligand 2,6-bis{3-[N,N-di(2-
pyridylmethyl)amino]propoxy}benzoic acid (LH) with
Fe(ClO4)3 followed by addition of chloroacetic acid leads to
the formation of the tetranuclear complex [{Fe2OL(ClCH2-
CO2)2}2](ClO4)4, the crystal structure of which reveals that it
consists of two FeII2(m-O)(m-RCO2)2 cores that are linked via
the two L ligands in a helical structure, with the carboxylate
moieties of the two ligands forming a hydrogen-bonded pair
at the center of the helix.
The assignments of the 1H NMR spectra of 1 and 2 are based on
their similarities to the spectra of oxo-bridged diiron complexes
of related ligands, viz. {PyCH2}2N(CH2)nCO22 (n = 1,2).11 All
features in the NMR spectra of 1 and 2 appear in the range d
0–28, indicative of relatively strong antiferromagnetic coupling
between the iron atoms. There are broad features above d 20
(PyCH2 and o-Py protons), sharper peaks at d 15–18 (m-Py
protons), and a very sharp peak at d ca. 6 (p-Py protons). In
addition, there are sharp peaks of the spacer polymethylene
chain at d 3–9. However, the mass spectrum of 2 (vide infra)
and the analytical data suggest that it is a tetranuclear complex
(a ‘dimer of dimers’) rather than a simple diiron complex.
It was possible to grow crystals of 2 from a MeOH–MeCN
solution. In order to confirm the proposed tetranuclear structure
and to assess whether the carboxylate moiety of 2 acts as a
bridge between the metals, the crystal structure of 2 was
determined.¶ The molecular structure of 2 is shown in Fig. 1.
The Fe–O–Fe moieties are bridged by two chloroacetates while
the non-bridging facial positions are filled by two di(picolyl)-
amine moieties from two molecules of L so that the two L
ligands link the two diiron–m-oxo units. The average Fe–m–oxo
distance is 1.79 Å and the average Fe–O–Fe angle is 121.8°; the
intra-dimer Fe-Fe distances are 3.102(4) Å [Fe(1)–Fe(2)] and
3.138(4) Å [Fe(3)–Fe(4)] while the interdimer Fe–Fe distances
vary from ca. 10.5 Å to 10.8 Å . Complex 2 is structurally and,
to some extent, chemically related to previously structurally
characterized tetranuclear bis-Fe2O species12 but there are
several features of the molecular structure of 2 that render it
unique. The conformation of the linking ligands is such that the
whole molecule acquires a helical shape; the dihedral angle
between the Fe–O–Fe planes in 2 is 42.5°. The center of the
helix is held together through the central carboxylates of the two
A number of Fe2O complexes have been synthesized to model
the spectroscopic and structural properties of m-oxo-m-carbox-
ylato diiron sites in proteins such as soluble methane mono-
oxygenase (sMMO), ribonucleotide reductase (RNR) and
hemerythrin.1 In many cases, such biomimetic complexes have
been found to be relatively unstable.1,2 Efforts have therefore
been made to prepare framework polydentate ligands that may
stabilize complexes of this type,3 examples include ligands in
which multidentate nitrogen bases that bind to metals in a
capping manner are linked to alkoxo or phenoxo groups capable
of bridging two metals.4 Binucleating polydentate ligands in
which nitrogen donors are linked to a carboxylate moiety should
be even more relevant for the modelling of m-oxo-m-carboxylate
diiron sites because the carboxylate bridges in these enzymes
are part of the protein.5,6 Two successful7,8 and two un-
successful9,10 attempts have been reported for the preparation of
dinuclear iron complexes containing similar ligands; in these
ligands, the carboxylate is one of the ‘peripheral’ donors at the
terminus of a polydentate ligand. Here we report the synthesis of
a new ligand containing a potentially bridging carboxylate
moiety which is a part of the spacer that links two di(picolyl)a-
mine groups. Reaction of this ligand with an iron(iii) salt
followed by addition of chloroacetic acid leads to the formation
of a tetranuclear complex which consists of two linked FeIII2(m-
O) dimers.
The ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]-
propoxy}benzoic acid (LH) has been prepared in a three-step
synthesis with a general overall yield of ca. 60% (Scheme 1).
The products LMe and LH (Scheme 1) have been identified by
1H NMR spectroscopy, microanalysis and, in the case of LMe,
FAB mass spectrometry. The 1H NMR spectra of LMe and LH
are identical except for the methyl resonance at d 3.61 that is
present in the spectrum of LMe.†
Reaction of LH with 2 equiv. of Fe(ClO4)3 in methanol leads
to the formation of a green–yellow product, which has
tentatively been assigned the formula [Fe2OL(H2O)2](ClO4)3 1
on the basis of UV–VIS, IR, 1H NMR spectroscopy and
microanalysis.‡ Addition of 10 equiv. of chloroacetic acid to a
solution of 1 in ethanol results in the formation of a brown
Scheme 1 Synthetic route to LMe and LH. Reagents and conditions: i,
Me2CO, K2CO3, 72 h, 94% yield; ii, Et3N, 96 h, 77% yield; iii, KOH–
EtOH–H2O, 12 h, 84% yield.
1
complex 2. The UV–VIS and H NMR spectra§ of complex 2
indicate that it contains a m-oxo-bis-m-carboxylate diiron core.
Chem. Commun., 1999, 1193–1194
1193

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Professor Fred Menger and Prof. Jean-Pierre Tuchagues for
useful synthetic information and Maria Johansson for assistance
with the crystallographic data collection.
Notes and references
† LMe: 1H NMR (CDCl3, 25 °C): d 8.50 (d, 4H, py-Ha), 7.58 (t, 4H, py-Hb),
7.46 (d, 4H, py-Hd), 7.19 (t, 1H, Ar-H,p), 7.11 (t, 4H, py-Hh), 6.46 (d, 2H,
Ar-H,m), 3.98 (t, 4H, OCH2), 3.82 (s, 8H, pyCH2N), 3.61 (s, 3H, OCH3),
2.70 (t, 4H, NCH2), 1.96 (t, 4H, CH2). FAB-MS, m/z (rel. intensity, %): 647
(M, 100), 554 (M–CH2py, 40), 448 (M 2 2CH2py, 45). Anal. C38H42N6O4.
Calc.: C, 70.56; H, 6.55; N, 13.00. Found: C, 69.9; H, 6.85; N, 13.6%.
LH: 1H NMR (CDCl3, 25 °C): 8.50 (d, 4H, py-Ha), 7.58 (t, 4H, py-Hb),
7.46 (d, 4H, py-Hd), 7.19 (t, 1H, Ar-H,p), 7.11 (t, 4H, py-Hh), 6.46 (d, 2H,
Ar-H,m), 3.98 (t, 4H, OCH2), 3.82 (s, 8H, pyCH2N), 2.70 (t, 4H, NCH2),
1.96 (t, 4H, CH2). Anal. C37H40N6O4. Calc.: C, 70.23; H, 6.37; N, 13.28;
Found: C, 70.57; H, 6.54; N, 12.99%.
‡ [Fe2OL(H2O)2](ClO4)3 1: UV–VIS [MeCN; lmax/nm (eM(Fe)dm3 mol21
cm21)]: 220 (35000), 259 (20000), 330 (4000), 474 (659), 520sh (535), 724
(97). IR (KBr), n/cm21:1608, 1541 (CO2, as), 1463, 1419 (CO2, s), 537 (Fe–
O–Fe, s), 473. 1H NMR (CD3CN, 25 oC): d 26 (o-py), 15.4, 14.0 (pyCH2N),
11.6, 11.3 (m-py), 8.9, 8.6 (m-Ph), 8.0 (p-Ph), 7.3 (p-py), 6.7, 4.4, 3.9, 3.6
(CH2). Anal. C37H43Cl3Fe2N6O19. Calc.: C, 40.63; H, 3.96; N, 7.68; Cl,
9.72; Found: C, 40.47, H, 4.10, N, 7.69; Cl, 10.20%.
§ [{Fe2OL(ClCH2CO2)2}2](ClO4)4 2: UV–VIS [MeCN; lmax/ nm (eM(Fe)/
dm3 mol21 cm21): 211 (23000), 245sh (15000), 344 (4200), 380sh (3100),
472 (840), 510 (730), 555sh (220), 721 (155). IR (KBr), n/cm21: 1687
(CO2H), 1607 (py), 1570 (CO2, as), 1463, 1418 (CO2, s), 536 (Fe–O–Fe).
1H NMR (CD3CN, 25 °C): d 28 (o-py), 15.6 (pyCH2N), 12.0 (m-py), 11.5
(O2CCHCl), 8.5 (m-Ph), 7.8 (p-Ph), 7.4 (p-py), 6.7, 5.4, 4.1 (CH2).
¶ Crystal data: C84H80Cl8Fe4N12O36.75, brown, crystal dimensions 0.23 3
0.21 3 0.18 mm, M = 2352.60, triclinic, space group P1 (no. 2), a =
16.4575(3), b = 17.8346(2), c = 19.3790(3) Å, a = 77.521(1), b =
89.245(1), g = 86.432(1)°, U = 5542.9(2) Å3, Z = 2, Dc = 1.410 Mg m23
,
m = 0.788 mm21, F(000) = 2404, 45710 reflections collected (1.1 < q <
31.8°) at 293(2) K, 31848 independent reflections used in the structure
refinement [Fo > 2s(Fo)], R1 (F) = 0.0973, wR2 (F2) = 0.2047, goodness-
of-fit (F2) = 0.854. Data were collected with a Siemens SMART CCD area
detector, using graphite-monochromated Mo-Ka radiation (l = 0.71069 Å)
from a Rigaku rotating anode X-ray generator. The intensity measurements
were corrected for Lorentz, polarization and absorption effects. The
positions of the metal atoms were found by direct methods,13 and all the
non-hydrogen atoms were located from difference Fourier syntheses. The
hydrogen atoms were placed in calculated positions. A disorder in the
orientations of the CH2Cl groups of the chloroacetates containing Cl3A and
Cl1A was detected and was successfully modelled; the conformations
shown in Fig. 1 are favoured. The final refinement was carried out by
full-matrix least-squares calculations13 on F2 and with anisotropic
thermal parameters for all non-hydrogen atoms. CCDC 182/1215. See
http://www.rsc.org/suppdata/cc/1999/1193/ for crystallographic files in .cif
format.
Fig. 1 (a) XP13 and (b) space-filling drawings of the molecular structure of
2, showing the atom labeling scheme. Thermal ellipsoids are drawn at the
20% level. Selected distances (Å) and angles (°): Fe(1)–O(1) 1.776(6),
Fe(2)–O(1) 1.787(6), Fe(3)–O(2) 1.794(6), Fe(4)–O(2) 1.786(6), Fe(1)–
N(1) 2.196(8), Fe(1)–N(2) 2.215(9), Fe(1)–N(3) 2.100(10), Fe(1)–O(3)
2.028(8), Fe(1)–O(5) 2.013(7), Fe(1)–O(1)-Fe(2) 121.1(4), Fe(3)–O(2)–
Fe(4)122.5(4).
L ligands which form a hydrogen-bonded pair, the O(11)–O(15)
and O(12)–O(16) distances are 2.61 and 2.60 Å, respectively. It
is usually observed that the tertiary nitrogen atoms of
di(picolyl)amines coordinate trans to the m-oxo ligand in this
type of diiron–oxo species but in 2, the tertiary amines are
coordinated cis to the oxo ligand and trans to one of the bridging
carboxylates so that the apices by the bridging oxygens of the
Fe2O units point towards each other. It is likely that the ‘girdle’
that is formed by the hydrogen-bonded pair of central
carboxylates prevents the two ligands from binding to the Fe2O
unit in the favoured coordination mode, instead they bind in the
observed ‘inverted’ coordination mode.
1 D. M. Kurtz, Jr., Chem. Rev., 1990, 90, 585.
2 S. Ménage, E. C. Wilkinson, L. Que, Jr. and M. Fontecave, Angew.
Chem., Int. Ed. Engl., 1995, 34, 203.
3 L. Berchet, M. N. Collomb-Dunand-Sauthier, P. Dubordeaux, W.
Moneta, A. Deronzier and J.-M. Latour, Inorg. Chim. Acta, 1998, 282,
243.
4 D. E. Fenton and H. Okawa, Chem. Ber./Recl. Trav. Chim. Pays-Bas,
1997, 130, 433.
5 D. M. Kurtz, Jr., JBIC, 1997, 2, 159.
6 A. A. Shteinman, FEBS Lett., 1995, 362, 5.
7 C. Hemmert, M. Verelst and J.-P. Tuchagues, Chem. Commun., 1996,
617.
8 V. M. Trukhan and A. A. Shteinman, Russ. Chem. Bull., 1997, 46,
202.
9 A. Hazell, K. B. Jensen, K. J. McKenzie and H. Toftlund, J. Chem. Soc.,
Dalton Trans., 1993, 3249.
10 V. M. Trukhan, I. L. Eremenko, N. S. Ovanesyan, A. A. Pasynskii, I. A.
Petrunenko, V. V. Strelets and A. A. Shteinman, Russ. Chem. Bull.,
1996, 45, 1981.
Repeated attempts to detect the molecular ion of 2 by
electrospray mass spectrometry (ESMS) have thus far proven
unsuccessful. However, we do detect prominent peaks at m/z
952, 601 and 426; these peaks are consistent with the molecular
x+
formulation(s) {[Fe4L2(ClCH2CO2)2](ClO4)42x
}
(x = 2–4,
respectively). Furthermore, we have prepared a number of other
carboxylate derivatives of 1. It appears that in solution, these
iron–oxo carboxylate species are in equilibrium with other
complexes of similar composition, the nature of which is
currently under investigation. We are currently preparing a
number of derivatives of LH containing other nitrogen donors
and are attempting to use these ligands to synthesize structural
and functional model complexes for the active sites of sMMO
and RNR R2.
This research was supported by grants from the Russian
Foundation for Basic Research (no. 97-03-32253), INTAS (no.
97-1289), the Swedish Natural Science Research Council
(NFR) and the Royal Swedish Academy of Sciences. C. G. P.
thanks the STINT foundation for financial support. We thank
11 S. Ménage and L. Que, Jr., New J. Chem., 1991, 15, 431.
12 Cf. N. Arulsamy, J. Glerup and D. J. Hodgson, Inorg. Chem., 1994, 33,
3043 and references therein.
13 Structure determination, refinement and data presentation were carried
out using programs contained in the SHELXTL Crystal Structure
Determination Package, V; Siemens Industrial Automation, Inc.;
Madison, WI, 1995.
Communication 8/09888C
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Chem. Commun., 1999, 1193–1194