The coordination chemistry of amine triphenolate tripod ligands with iron(III). Old organic compounds but new tripod ligands

Article abstract of DOI:10.1039/a802725k

The coordination chemistry of new C₃ symmetric triphenolate amine tripod ligands has been demonstrated with Fe^III.

Full text of DOI:10.1039/a802725k

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The coordination chemistry of amine triphenolate tripod ligands with iron(III).
Old organic compounds but new tripod ligands
JungWon Hwang, Kumar Govindaswamy and Stephen A. Koch*†
Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400, USA
The coordination chemistry of new C3 symmetric tripheno-
The reaction of the trilithium salt of 1 with FeCl3 in the
presence of 3 equiv. of 1-methylimidazole (1-Meim) in MeOH
gave dark red crystals of [Fe{N(CH2-o-C6H4O)3}(1-Meim)] (6)
in 65% yield. The structure of 6, which was determined by
X-ray crystallographic analysis§ (Fig. 1), consists of a trigonal
bipyramidal structure with an N1–Fe–N2 angle of 173.4(1)°.
The Fe atom is 0.045 Å out of the plane of the three oxygen
donors toward the imidazole nitrogen. The FeO3 plane shows
small deviations from trigonal symmetry with O–Fe–O angles
of 117.4(1) to 126.3(1)°. The average Fe–O bond distance
[1.876(15) Å] is intermediate between the Fe–O in tetrahedral
[FeIII(OC6HMe4-2,3,5,6)4]2 [1.847(13) Å] and in octahedral Fe
tris-catecholate complexes (2.02 Å).11,12 The analogous com-
pound, [Fe{N[CH2-o-C6H2(OMe)2-3,5]3}(1-Meim)] (7), has
also been characterized with ligand 2.
late amine tripod ligands has been demonstrated with
FeIII
.
A key feature of the coordination chemistry of tripod ligands is
their ability to divide the metal–ligand coordination sphere into
non-labile sites and reactive sites. Saconni and the Florence
school first demonstrated the rich coordination chemistry of
tripod ligands in the 1960s and 1970s.1,2 In the 1990s the
chemistry of tripod ligands has undergone a renaissance.3–5 We
report the coordination chemistry of new polyphenolate amine
tripod ligands: tris(2-hydroxybenzyl)amine (1) and tris(2-
hydroxy-3,5-dimethylbenzyl)amine (2). Phenolate containing
ligands are valuable in modeling the active sites of metal–
tyrosine centers in metalloproteins6 and homogeneous and
heterogeneous catalysts.7 Historically, the hydrochloride salt of
1 was the subject of its first and only report in 1922,8 while the
synthesis of 2 appeared in the literature in 1949.9 The
coordination chemistry of compounds 1 and 2 has never been
previously reported.
Our route to 1, which differs from the original synthesis, is
outlined in Scheme 1. The reaction of 2 equiv. of 2-methoxy-
benzyl bromide10 (3) with commercially available 2-methoxy-
benzylamine (4) in refluxing CH3CN with added K2CO3 gives
tris(2-methoxybenzyl)amine (5) in 80–85% yield. The methyl
protecting groups are removed by refluxing 5 in toluene with 5
equiv. of AlCl3 to give 1 in 70% yield. We were able to
reproduce the synthesis of 2 by the one step Mannich reaction of
2,4-dimethylphenol with hexamethylenetetramine.9 Since both
1 and 2‡ were reported before the advent of modern spectro-
scopic techniques, the congruence of our compounds with the
literature compounds was established by melting point com-
parisons.
The tripod ligand coordinates to the metal to generate a chiral
C3 conformation13 with both enantiomeric structures present in
the centric crystal lattice. These tripod ligands 1 and 2 differ
from the well studied N(CH2CH2OH)3 and N(CH2CO2H)3
ligands in the phenolate versus alkoxide and carboxylate donors
and by having six- versus five-membered metal ligand chelate
rings. The aminetriphenolate ligand N(o-C6H4O)3 and its
coordination chemistry with Al have been reported.14
The thiol analog of 1 and its [FeIII{N(CH2-o-C6H4S)3}(1-
Meim)] complex has recently been reported.15 The 0.43 Å
change in Fe–O versus Fe–S bond distance reflects the
difference in the ionic radii of S and O. The larger average Fe–
O–C angle [134(1)°] in 6 versus the Fe–S–C [110(1)°] angle is
OCH3
OCH3
NH2
Br
+
2
3
4
OH
OCH3
N
N
3
3
1
5
OH
OH
N
N
N
+
N
N
Fig. 1 Structural diagram for [Fe{N(CH2-o-C6H4O)3}(1-Meim)] (6).
Selected distances (Å) and angles (°) Fe1–O1 1.881(3); Fe1–O2 1.888(3);
Fe1–O3 1.859(3); Fe1–N1 2.191(3); Fe1–N2 2.090(4); O1–Fe1–
O2–126.3(1); O1–Fe1–O3–117.4(1); O2–Fe1–O3 116.2(1); O1–Fe1–N1
87.1(1); O1–Fe1–N2 87.6(1); O2–Fe1–N1 88.9(1); O2–Fe1–N2 91.1(1);
O3–Fe1–N1 89.9(1); O3–Fe1–N2 96.0(1); N1–Fe1–N2 173.4(1); Fe1–
O–Cavg 134(1).
3
2
Scheme 1
Chem. Commun., 1998
1667

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Notes and References
† E-mail: stephen.koch@sunysb.edu
‡ Compound 1: 1H NMR ([2H6]DMSO): d 4.25 (s; 6 H; CH2), 6.82 (t; 3 H;
3-H), 6.93 (d; 3 H; 1-H), 7.24 (m; 6 H; 3,4-H), 8.40 [s(br); 1 H; NH+], 10.5
(s; 3 H; OH). FAB-MS: 335 [M+]. Compound 2: 1H NMR (CDCl3): d 2.23
(s; 18 H; CH3), 3.61 (s; 6 H; CH2), 6.52 [s(br); 3 H; OH], 6.72 (s; 3 H; 6-H),
6.84 (s; 3 H; 4-H). FAB-MS: 419 [M+].
§ Crystal data for 6 (crystallized from DMF–isopropanol): FeC25H24N3O3,
M = 469.85, monoclinic, space group P21/c, a = 14.837(3), b = 9.410(8),
c = 17.162(3) Å, b = 110.836(9)°, U = 2239(1) Å3, Z = 4, Mo-Ka
radiation (l = 0.71073 Å), m = 7.016 cm21. The structure was solved and
refined using standard crystallographic techniques with R(Rw)
0.040(0.039) for 1993 observed reflections I 3s(I). For 7: Fe-
31H36O3N3, M = 554.49, monoclinic, space group P21/n, a = 16.409(4),
=
>
C
b = 10.841(1), c = 17.082(4) Å, b = 110.37(1)°, U = 2849(1) Å3, Z = 4,
m(Mo-Ka) = 7.127 cm21; R(Rw) = 0.053(0.025) for 1733 reflections I >
3s(I). For 8: FeC24H25N2O4, M = 449.3, monoclinic, space group P21/c, a
= 14.670(3), b = 9.413(9), c = 16.807(3) Å, b = 108.054(8)°, U =
2206(1) Å3, Z = 4, m(Mo-Ka) = 5.65 cm21, R(Rw) = 0.039(0.047) for
2888 observed reflections I > 3s(I). For 9 (crystallized from DMF–
MeOH): FeC33H26N3O3, M = 568.44, monoclinic, space group P21/c, a =
12.042(3), b = 18.356(3), c = 12.713(4) Å, b = 110.02(1)°, U = 2640(2)
Å3, Z = 4, m(Mo-Ka) = 6.087 cm21, R(Rw) = 0.050(0.044) for 2888
observed reflections I > 3s(I). CCDC 182/921.
Fig. 2 Structural diagram for [Fe{N(CH2-o-C6H4O)3}(phen)] (9). Selected
distances (Å) and angles (°): Fe1–O1 1.897(6); Fe1–O2 1.926(6); Fe1–O3
1.893(7); Fe1–N1 2.225(8); Fe1–N2 2.158(7); Fe1–N3 2.340(7); N1–Fe–
O3 91.5(3); O3–Fe–N2 87.7(3); N2–Fe–N3 73.9(3); N3–Fe–N1 107.0(3);
O1–Fe–O2 164.1(3); N–Fe–O2avg 88.4(3); N–Fe–O1avg 87.5(3); Fe–
O–Cavg 128.5(7).
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four coordinate complex, the reaction of Li3N(CH2-o-C6H4O)3]
with FeCl3 was repeated in the absence of 1-Meim. However the
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X-ray
structure§
of
the
product,
[Fe{N(CH2-
o-C6H4O)3}(DMF)] (8), revealed that a molecule of solvent was
coordinated to the iron. The crystal of 8 is isomorphous and is
essentially isostructural with 6. The atomic positions of the non-
hydrogen atoms of the DMF molecule are structurally equiva-
lent to five of the atoms of the 1-Meim in 6. The reaction of 8
with 1,10-phenanthroline in DMF followed by the addition of
MeOH gave dark red crystals of [Fe{N(CH2-
o-C6H4O)3}(phen)] (9) in 50% yield. The structure of 9§ (Fig.
2) demonstrates that the tripod ligands can support an
octahedral coordination center. The phen ligand has a distinctly
asymmetric coordination with Fe–N2 2.158(7) Å and Fe–N3
2.340(7) Å distances. The distortion results from a short contact
between a benzyl proton and the hydrogen atom ortho to N3.
Compounds 6–9 have magnetic moments indicative of high
spin Fe3+ and have an intense phenolate to Fe charge transfer
transition at 399 nm (e = 7960 dm3 mol21 cm21), 422 (8320),
405 (9700), and 399 (7751) respectively.18 A quasireversible
FeIII–FeII redox couple was observed in the cyclic voltammo-
gram of 7 occurring at E1/2 = 20.78 V (DE = 107 mV) versus
Ag/AgCl in DMF solution. Only irreversible oxidation proc-
esses were observed for all the compounds.19
19 A. Sokolowski, J. Müller, T. Weyhermüller, R. Schnepf, P. Hildebrandt,
K. Hildenbrand, E. Bothe and K. Wieghardt, J. Am. Chem. Soc., 1997,
119, 8889.
The chemistry of these tripod ligands with other metal ions is
under investigation. We thank the National Institutes of Health
for support.
Received in Bloomington, IN, USA, 14th April 1998; 8/02725K
1668
Chem. Commun., 1998