Preparation and separation of asymmetric tripodal iron complexes of tren and selected aldehydes

Article abstract of DOI:10.1016/j.ica.2004.07.044

The reaction of iron salts with the Schiff bases formed from the condensation of tren and mixtures of 2-pyridinecarboxaldehyde and 6-methyl-2-pyridinecarboxaldehyde, salicylaldehyde and 2-hydroxy-5- methylbenzaldehyde, salicylaldehyde and pyrrole-2-carboxaldehyde, or salicylaldehyde and 4-methyl-5-imidazolecarboxaldehyde affords a mixture of complexes that can be represented as [FetrenL_xL′_y] ^z (x +y = 3). In the case of salicylaldehyde and 4-methyl-5- imidazolecarboxaldehyde it is possible to separate the mixture with ion exchange chromatography. Reaction of iron salts with the tripodal ligands formed from the condensation of tris-(2-aminoethyl)amine (tren) with the following mixtures of aldehydes, 2-pyridinecarboxaldehyde (py) and 6-methyl-2- pyridinecarboxaldehyde (6-CH₃py), salicylaldehyde (sal) and 2-hydroxy-5-methylbenzaldehyde (5-CH₃sal), salicylaldehyde and pyrrole-2-carboxaldehyde (pyr), and 4-methyl-5-imidazolecarboxaldehyde (4-CH₃ImH) and salicylaldehyde yielded mixtures of all four possible iron complexes, [Fe(tren)L_xL′_y]^z (x +y = 3), where L and L′ represent different aldehydes. Preliminary indication of mixtures of products was provided by IR and UV-Vis spectroscopy. Conclusive proof of the existence of all four components (x = 0-3) in each of the reaction mixtures is provided by mass spectroscopy. Separation of the [Fetren(sal) _x(4-CH₃ImH)_y](ClO₄)_y mixture can be achieved using a Sephadex ion exchange column. Treatment of the two observed fractions with base yields greatly enriched [Fetren(sal) ₂(4-CH₃Im)] and [Fetren(sal)(4-CH₃Im) ₂], which were identified by mass spectroscopy.

Full text of DOI:10.1016/j.ica.2004.07.044

Inorganica Chimica Acta 358 (2005) 239–245
www.elsevier.com/locate/ica
Note
Preparation and separation of asymmetric tripodal iron
complexes of tren and selected aldehydes
*
Greg Brewer , Charles Luckett
Department of Chemistry, The Catholic University of America, 620 Michigan Avenue, Washington, DC 20064, USA
Received 4 June 2004; accepted 23 July 2004
Available online 9 September 2004
Abstract
Reaction of iron salts with the tripodal ligands formed from the condensation of tris-(2-aminoethyl)amine (tren) with the follow-
ing mixtures of aldehydes, 2-pyridinecarboxaldehyde (py) and 6-methyl-2-pyridinecarboxaldehyde (6-CH3py), salicylaldehyde (sal)
and 2-hydroxy-5-methylbenzaldehyde (5-CH3sal), salicylaldehyde and pyrrole-2-carboxaldehyde (pyr), and 4-methyl-5-imidazole-
carboxaldehyde (4-CH3ImH) and salicylaldehyde yielded mixtures of all four possible iron complexes, [Fe(tren)LxL0y]z
(x +y = 3), where L and L0 represent different aldehydes. Preliminary indication of mixtures of products was provided by IR and
UV–Vis spectroscopy. Conclusive proof of the existence of all four components (x = 0–3) in each of the reaction mixtures is pro-
vided by mass spectroscopy. Separation of the [Fetren(sal)x(4-CH3ImH)y](ClO4)y mixture can be achieved using a Sephadex ion
exchange column. Treatment of the two observed fractions with base yields greatly enriched [Fetren(sal)2(4-CH3Im)] and [Fet-
ren(sal)(4-CH3Im)2], which were identified by mass spectroscopy.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Tripodal complexes; Asymmetric complexes; Ion exchange separation; Mass spectroscopy
1. Introduction
In both these cases the ligand, H3L, is deprotonated
on bonding to the iron(III). Replacement of either of
the above carboxaldehydes with 4-imidazolecarboxalde-
hyde (4-ImH) or 4-methyl-5-imidazole carboxaldehyde
(4-CH3ImH) gives ligand systems tren(4-ImH)3 and
tren(4-CH3ImH)3, respectively. These can bind to iron
with or without ionization of the imidazole. Fetren(4-
CH3Im)3 [3] and Fetren(4-Im)3 [4] are both LS while
their protonated counterparts [4], [Fetren(4-ImH)3]3+
and [Fetren(4-CH3ImH)3]3+, are spin crossover (SC)
and HS, respectively, illustrating the effect of protona-
tion state on spin state selection.
The effect of ligand control over oxidation state is
illustrated with the tren Schiff base condensates of 2-
imidazolecarboxaldehyde [4] (tren(2-ImH)3), 1-methy-
limidazole-2-carboxaldehyde [5] (tren(N-CH3Im)3),
2-methyl-4-imidazole carboxaldehyde [6] (tren(2-CH3-
4-ImH)3), and 2-pyridinecarboxaldehyde [5] (tren(py)3).
Reaction of iron salts with the ligands formed from
the Schiff base condensation of tris-(2-aminoethyl)amine
(tren) with various aldehydes (see Fig. 1) results in octa-
hedral complexes in which the center nitrogen atom of
tren is unbound. Both the spin state and oxidation state
of the iron complexes are determined by the aldehyde
as the following examples illustrate. The reaction of
iron(III) with the Schiff base condensate of tren with
salicylaldehyde (tren(salH)3) [1] and with pyrrole-2-
carboxaldehyde (tren(pyrH)3) [2] gives the high spin
(HS), Fetren(sal)3 (N3O3), and low spin (LS), Fet-
ren(pyr)3 (N6), complexes, respectively, illustrating the
dependence of spin state on donor set in this system.
*
Corresponding author. Tel.: +1 2023195385; fax: +1 2023195381.
E-mail address: brewer@cua.edu (G. Brewer).
0020-1693/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.07.044

240
G. Brewer, C. Luckett / Inorganica Chimica Acta 358 (2005) 239–245
2. Experimental
H
H
N
CH2CH2N
Mass spectral analyses were obtained from the Mass
Spectrometry Center, University of Massachusetts at
Amherst. Tris-(2-aminoethyl)amine (tren), 2-pyridine-
N
CH2CH2N
HO
N
H
3
3
carboxaldehyde,
6-methyl-2-pyridinecarboxaldehyde,
4-methyl-5-imidazolecarboxaldehyde, salicylaldehyde,
2-hydroxy-5-methylbenzaldehyde, pyrrole-2-carboxal-
dehyde, Sephadex, Alumina neutral Brockman I, and
anhydrous ferric chloride were obtained from Aldrich.
Sodium perchlorate monohydrate and ferrous chloride
were obtained from Fisher. All solvents were of reagent
grade and used without further purification.
tren(salH)3
tren(pyrH)3
H
H
CH3
N
N
CH2CH2N
N
CH2CH2N
NH
N
H
N
3
3
2.1. Spectra
-
tren(4
tren( 4-
ImH)3
CH3ImH)3
IR spectra were obtained as KBr pellets on a Perkin–
Elmer 1600 FT IR spectrometer. UV–Vis spectra were
obtained on a Perkin–Elmer Lambda 4 spectrometer.
H
N
H
N
CH2CH2N
N
CH2CH2N
N
2.2. Syntheses
NH
HN
3
3
CH3
Caution! Perchlorate salts of metal complexes with
organic ligands are potentially explosive and should be
handled with care.
tren(2-
ImH)3
tren(2-
-4-
ImH)3
CH3
2.2.1. Synthesis of [Fetren(py)x(6-CH3py)y](ClO4)2
(x + y = 3)
H
N
H
N
CH3
N
CH2CH2N
N
CH2CH2N
The synthesis of [Fetren(py)2(6-CH3py)](ClO4)2 was
done as described in the literature [7] using a 2:1 ratio
of py/6-CH3py. As will be described in Section 3 the prod-
uct of this reaction was not [Fetren(py)2(6-CH3py)]-
ClO4)2 but instead [Fetren(py)x(6-CH3py)y](ClO4)2.
N
3
3
tren(N-
CH3 Im)3
tren(py)3
2.2.2. Synthesisof[Fetren)(sal)x(5-CH3sal)y](x + y = 3)
2-Hydroxy-5-methylbenzaldehyde (5-CH3sal) (0.272
g, 2 mmol) was added to a solution of tren (0.299 ml,
0.292 g, 2 mmol) in methanol (50 ml). The solution
was stirred for 20 min resulting in a yellow solution. Sal-
icylaldehyde (0.426 ml, 0.488 g, 4 mmol) was added and
stirring was continued for another 20 min resulting in no
apparent change in solution. Anhydrous iron(III)chlo-
ride (0.324 g, 2 mmol) in methanol (10 ml) was added
to the solution and the reaction mixture was stirred
for 5 min giving a deep purple solution. Addition of
KC2H3O2 (0.589 g, 6 mmol) turned the solution dark
red. The solution was reduced in volume to 20 ml and
then chilled in an ice-bath for 5 min. The purple crystals
were filtered off leaving a dark red filtrate. The crystals
were washed with chilled methanol and air dried to give
0.731 g of product.
Fig. 1. Line drawings of tren ligands.
Reaction of iron(III) salts with these ligands results
exclusively in the isolation of the iron(II) complexes.
Clearly the identity of the aldehyde in the Schiff base ad-
ducts with tren determines the oxidation and spin state
of iron in its complexes with these ligands.
In an effort to begin the process of greater fine tun-
ing of the redox and spin state control of these sys-
tems the synthesis of mixed ligand complexes,
[FetrenLxL0y]z (x + y = 3), where L and L0 are differ-
ent aldehydes was examined. Several mixed ligand
complexes were prepared and characterized by MS,
and in one case partial separation of a mixture was
achieved using ion exchange chromotography. The
procedure reported here will be refined and employed
in future work to produce complexes with specific re-
dox and spin state properties that would not be avail-
able using only symmetric ligands.
2.2.3. [Fetren(sal)x(pyr)y] (x + y = 3)
Pyrrole-2-carboxaldehyde (287 mg, 3.02 mmol) was
added to a solution of tren (293 mg, 2.00 mmol) in meth-
anol (25 ml). The mixture was stirred for ꢀ20 min to

G. Brewer, C. Luckett / Inorganica Chimica Acta 358 (2005) 239–245
241
give a straw-colored solution. Salicylaldehyde (367 mg,
3.02 mmol) was added and the mixture immediately
turned yellow. It was stirred for an additional 20 min.
FeCl3 (328 mg, 2.02 mmol) in methanol was added
and the reaction mixture immediately turned purple.
The mixture was stirred for ꢀ5 min before sodium
hydroxide (490 mg, 12 mmol) was added. During the
following 15–20 min, the reaction mixture gradually
turned red. Concentration of the reaction mixture gave
0.887 g of a red powder. The red solid was dissolved
in CH2Cl2 (45 ml) and the solution was filtered to re-
move a whitish-tan solid (90 mg). The filtrate was evap-
orated to give a red powder.
the organic layer. The organic layer was removed and
concentrated to yield a purple solid (16 mg, termed
SP1). Elution of the column with more concentrated so-
dium chloride did not result in movement of the dark
band at the top of the column. This band was isolated
by transferring the top portion of the column into a
separatory funnel containing dichloromethane (30 ml).
Aqueous sodium hydroxide was added dropwise until
the purple color moved into the organic phase. This
was drawn off and concentrated to yield a few mg of
purple solid termed SP2.
3. Results and discussion
2.2.4. [Fetren(sal)x(4-CH3ImH)y](ClO4)y (x + y = 3)
4-Methyl-5-imidazolecarboxaldehyde (330 mg, 3.00
mmol) was added to a solution of tren (293 mg, 2.00
mmol) in methanol (25 ml). The mixture was stirred
and warmed for ꢀ20 min, during which time the solu-
tion became pale yellow. Salicylaldehyde (367 mg, 3.02
mmol) was added and the mixture immediately turned
a darker yellow. The reaction mixture was stirred and
warmed for 20 min more. Iron(III)chloride (331 mg,
2.04 mmol) in methanol (5 ml) was added and the reac-
tion mixture immediately turned purple. After 5 min of
stirring while maintaining heating, potassium acetate
(296 mg, 3.02 mmol) in methanol (5 ml) was added
and the mixture was stirred and warmed for 15 min. So-
dium perchlorate (847 mg, 6.0 mmol) in methanol (5–10
ml) was added and the reaction mixture was set aside to
concentrate. A purple powder (1.5 g) was isolated by
filtration.
3.1. Synthesis
The synthesis of asymmetric ligands from the Schiff
base condensation of symmetric polyamines with differ-
ent aldehydes is a challenge. Examples of structurally
verified successes that come about as a result of a spe-
cific condition (solubility, choice of carbonyl compound
or metal, or stoichiometry) include complexes contain-
ing 1,2-diaminobenzene [8], 1,3-diaminopropane [9]
and 1,2-diaminoethane [10]. The chemistry of asymmet-
ric copper imidazole complexes, and their use as build-
ing blocks for larger assemblies, has recently been
reviewed [11]. Specifically with tren there is a report of
a structurally characterized asymmetric complex with
lanthanum(III) in which only one of the tren arms forms
a Schiff base condensate with salicylaldehyde [12]. How-
ever, this method is not of general applicability to tran-
sition metal ions or in fact to other lanthanide ions and
must be regarded as a special case governed by the size
and solubility of the ion selected. With iron there are re-
ports of the syntheses of [Fetren(py)2(6-CH3py)]X2 and
2.2.5. [Fetren(sal)x(4-CH3Im)y] (x + y = 3)
Application of [Fetren(sal)x(4-CH3ImH)y](ClO4)y
(100 mg) as a solid to the top of a dry packed alumina
column followed by elution with methanol/dichloro-
methane (50/50) resulted in elution of a focused dark
blue band. Concentration afforded 46 mg of a dark pur-
ple solid.
[Fetren(py)(6-CH3py)2]X2 ðX ¼ ClO4 or PF6ꢁÞ that
ꢁ
rely on stoichiometric control. Specifically the authors
stated that these complexes could be prepared by simply
controlling the stoichiometric amount of aldehyde used
and the order of addition of the aldehydes [7]. The pre-
sumed resultant asymmetric complexes were character-
ized by bulk methods including elemental analysis, IR,
and Mo¨ssbauer data. It should be pointed out that these
methods would fail to conclusively distinguish a true
asymmetric complex such as [Fetren(py)2(6-CH3py)]X2
from a bulk mixture, [Fetren(py)x(6-CH3py)y]X2, in
which the ratio of py/6-CH3py was 2:1. Such distinction
could only be made on the basis of more specific meth-
ods such as mass spectroscopy or X-ray crystallography
that would give a singular result for each molecule in
question. Since a method existed in the literature for
the preparation of iron complexes of tren and mixed
py/6-CH3py complexes it was decided to follow this
methodology to determine if it worked in that case
and if so, to extend it to other mixed aldehyde cases.
2.2.6. Separation of [Fetren(sal)x(4-CH3ImH)y]-
(ClO4)y (x + y = 3)
A solution of [Fetren(sal)x(4-CH3ImH)y](ClO4)y (75
mg) in water to which a small amount of methanol
was added was applied to the top of a Sephadex SP
G25 ion exchange column (3 · 75) that had been al-
lowed to swell for several days prior to use in aqueous
sodium chloride. The dark purple solution bound tightly
to the top of the column and was not eluted by aqueous
NaCl solutions below 0.3 M. A dark purple band eluted
with 1.0 M aqueous sodium chloride leaving another
dark band at the top of the column. This fraction was
added to a separatory funnel which contained dichloro-
methane (20 ml). Aqueous sodium hydroxide (0.10 M)
was added dropwise until the purple color moved into

242
G. Brewer, C. Luckett / Inorganica Chimica Acta 358 (2005) 239–245
Table 1
Mass spectral analysis of reaction mixtures
reaction should have yielded only [Fetren(sal)2(5-
CH3sal)]. Mass spectral examination by two different
methods shows again that all four possible products
are produced in amounts which are in very close agree-
ment to one another and not that different from statisti-
cal predictions. A third reaction mixture of sal/pyr in 1:1
ratio was examined. Here again all four products are
produced. The observed amounts deviate from the sta-
tistical amounts (12.5%:37.5%:37.5%:12.5%) which
may indicate a difference in reactivity between the alde-
hydes or some ionization suppression of one species over
the others. The issue of ionization suppression is a seri-
ous one in MS analysis and the method of ionization
needs to be considered carefully in the analysis of mix-
tures to demonstrate that one component does not sup-
press the ionization of the others. An example of this
was observed in a preliminary analysis of a simple 1:1
binary mixture of [Fetren(pyr)3] and [Fetren(sal)3] by
ESI. With ESI only [Fetren(sal)3] (m/e = 512) was ob-
served in a 1:1 mixture even though [Fetren(pyr)3]
(m/e = 431) was observed when it was analyzed by itself.
Either problem (chemical reactivity or ion suppression)
is possible in this case as the chemical differences be-
tween the two aldehydes is great relative to the py/6-
CH3py and sal/5-CH3sal mixtures.
Reaction
mixture
Ionization m/e
method
Assignment
(% observed)
Py/6-CH3Py
ESI
469 (24)
483 (48)
498 (23)
512 (5)
[Fetren(py)3]+
[Fetren(py)2(6-CH3-py)]+
[Fetren(py)(6-CH3-py)2]+
[Fetren(6-CH3py)3]+
Sal/5-CH3sal FAB
511 (27)
525 (40)
539 (26)
553 (7)
[Fetren(sal)3]+
[Fetren(sal)2(5-CH3sal)]+
[Fetren(sal)(5-CH3sal)2]+
[Fetren(5-CH3sal)3]+
EI
511 (25)
525 (42)
535 (27)
553 (6)
[Fetren(sal)3]+
[Fetren(sal)2(5-CH3sal)]+
[Fetren(sal)(5-CH3sal)2]+
[Fetren(5-CH3sal)3]+
Sal/pyr
EI
430 (26)
457 (35)
484 (23)
511 (16)
[Fetren(pyr)3]+
[Fetren(pyr)2(sal)]+
[Fetren(pyr)(sal)2]+
[Fetren(sal)3]+
3.2. Mass spectral characterization of [Fet-
ren(L)x(L0)y]z reaction mixtures
Table 1 shows the results of mass spectral character-
ization of the first three reaction mixtures described in
Section 2. The py/6-CH3py mixture was carried out as
described in the literature [7] and should have afforded
exclusively [Fetren(py)2(6-CH3py)](ClO4)2. Inspection
of the table indicates that in fact all four possible prod-
ucts are found in amounts close to what would be statis-
3.3. Separation of the [Fetren(sal)x(4-CH3ImH)y]-
(ClO4)y reaction mixture
The results shown in Table 1 clearly indicate that sim-
ple stoichiometric control over the reaction does not re-
sult in the isolation of a single product. Instead a
mixture of all four possible products results as would
be expected given the equilibrium nature of the Schiff
base condensation. However, it is possible in principle
tically predicted for
a 2:1 ratio of py/6-CH3py
(29.6%:44.4%:22.2%:3.7%). In a method analogous to
the above procedure a 2:1 ratio of sal/5-CH3sal was used
in a reaction with tren and iron(III)chloride so that the
y+
N
H
N
N
H
N
O
sephadex
NaOH
Fe
CH3
NH
x
y
x + y = 3
N
N
H
H
N
N
H
N
N
N
H
N
N
O
N
O
+
Fe
Fe
CH3
CH3
2
2
Fig. 2. Separation of [Fetren(sal)x(4-CH3ImH)y](ClO4)y reaction mixture.

G. Brewer, C. Luckett / Inorganica Chimica Acta 358 (2005) 239–245
243
to enrich the mixture in a particular component through
separation methodology. The py/6-CH3py or sal/5-
CH3sal mixtures are bad candidates for separation due
to the close chemical similarity of the aldehydes involved
and the fact that the charges on the molecules do not
vary within the mixture, +2 for py/6-CH3py and zero
Fig. 3. ESI MS of iron complexes of tren and sal/4-CH3ImH series. Top spectra is [Fetren(sal)x(4-CH3ImH)y](ClO4)y reaction mixture with peaks at
477 [Fetren(4-CH3Im)(4-CH3ImH)2]+, 488[Fetren(sal)(4-CH3Im)(4-CH3ImH)]+, 500 [Fetren(sal)2(4-CH3ImH)]+, and 512 [Fetren(sal)3H]+. Middle
spectra is SP1 with dominant peak at 488 [Fetren(sal)(4-CH3Im)(4-CH3ImH)]+. Bottom spectra is SP2 with dominant peak at 500 [Fetren(sal)2(4-
CH3ImH)]+.

244
G. Brewer, C. Luckett / Inorganica Chimica Acta 358 (2005) 239–245
for sal/5-CH3sal mixtures. There is greater chemical dif-
ference between the aldehydes in the sal/pyr mixture but
again the charge is the same, zero, for all four species. Of
the reaction mixtures examined the [Fetren(sal)x(4-
CH3ImH)y](ClO4)y mixture seems best suited to separa-
tion as the charge on the four components of the
mixture varies from zero through positive three, making
it ideally suited for ion exchange chromatography and
the chemical differences between the aldehydes is great.
Separation of the [Fetren(sal)x(4-CH3ImH)y](ClO4)y
reaction mixture on Sephadex as described in Section 2
(see Fig. 2) results in significant enrichment of [Fet-
ren(sal)(4-CH3Im)2] in the fraction labeled SP1 and
enrichment of [Fetren(sal)2(4-CH3Im)] in SP2 relative
to the reaction mixture. This is clearly shown in
Fig. 3, which gives the MS results for the original mix-
Fetren(sal)3
[Fetren(4MeImH)3](ClO4)3
%T
[Fetren(sal)x(4MeImH)y](ClO4)y
4400.0
4000
3000
2000
1500
1000
450.0
cm-1
Fetren(sal)3
Fetren(4MeIm)3
%T
Fetren(sal)x(4MeIm)y
4400.0
4000
3000
2000
1500
1000
450.0
-1
cm
Fetren(sal)3
Fetren(4MeIm)3
%T
SP1 Fetren(sal)(4MeIm)2
SP2 Fetren(sal)2(4MeIm)
4400.0
4000
3000
2000
1500
1000
450.0
cm-1
Fig. 4. Comparison of IR spectra of iron complexes of tren and sal/4-CH3ImH series.

G. Brewer, C. Luckett / Inorganica Chimica Acta 358 (2005) 239–245
245
ture and for SP1 and SP2. Both fractions were converted
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ESI MS analysis. It should be pointed out that the re-
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Acknowledgements
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