Inducing spin crossover in amphiphilic iron(III) complexes

Article abstract of DOI:10.1002/ejic.200901183

Thermal spin crossover (SCO) was induced in a high-spin Fe^III complex by alkylation of the polyamino ligand backbone. The SCO profile was responsive to chain length with partial crossover observed with C₆ alkylation and full transiti

Full text of DOI:10.1002/ejic.200901183

SHORT COMMUNICATION
DOI: 10.1002/ejic.200901183
Inducing Spin Crossover in Amphiphilic Iron(III) Complexes
[a]
[b]
[a]
[c]
Paulo N. Martinho, Charles J. Harding, Helge Müller-Bunz, Martin Albrecht, and
Grace G. Morgan*[
a]
Keywords: Amphiphiles / Spin crossover / Iron / Molecular switching / Magnetic properties
Thermal spin crossover (SCO) was induced in a high-spin
Fe complex by alkylation of the polyamino ligand back-
bone. The SCO profile was responsive to chain length with
tion upon lengthening to C12. No crossover was observed
when the backbone was unfunctionalised, and all three com-
plexes were obtained as unsolvated tetrafluoroborate salts.
III
6
partial crossover observed with C alkylation and full transi-
+
Introduction
[Fe(sal)
2
trien] has emerged in recent years as a proto-
III
[7]
typical Fe spin crossover (SCO) ion and its ease of syn-
The ability of certain first-row transition-metal ions to thesis renders it an attractive candidate for further function-
switch the electronic state with thermal or optical stimulus alisation for surface deposition or solution processing. The
has long been cited as having excellent potential for data solid-state ion, however, shows a complex and unpredict-
[
1]
storage, and in recent years the emphasis has moved from able range of magnetic responses to thermal perturbation
[
8–11]
synthesis of new examples towards suitable engineering of depending on the contents of the unit cell.
A compre-
[2]
[3a–3e]
existing complexes into polymeric, amphiphilic
or hensive report by Halcrow compared magnetic and struc-
[
3f,3g]
nanocrystalline
environments. Most of the published tural data of new and known examples of solvated and un-
examples focus on iron(II),[ but iron(III) is also a prom- solvated salts of [Fe(sal) trien] , and examined aspects of
4]
[5]
+
2
ising candidate and has the advantage of improved redox the crystal packing that might explain the variety of ther-
[
6]
[12]
stability.
mal responses.
In particular the influence of the change
Scheme 1. General method for synthesis of complexes 1–3.
in dihedral angle between the phenoxy rings during spin
[
a] School of Chemistry & Chemical Biology and the SFI-Strategic conversion was considered.
Research Cluster in Solar Energy Conversion, University Col-
lege Dublin
We have recently examined the solution and solid-state
+
thermal response of the PF salt of [Fe(sal) trien] and
three derivatives obtained by substituting C , C or C
alkyl chains of the phenolate rings.
of the phenolate rings was observed to quench SCO in the
solid state, there was a marked enhancement in the sharp-
ness of the transition in solution compared to the non-
alkylated complex. Analysis of the crystalline packing in
6
2
Belfield, Dublin 4, Dublin, Ireland
Fax: +353-1-716-1178
6 8 18
E-mail: grace.morgan@ucd.ie
[13]
Although alkylation
[
[
b] Department of Chemistry, The Open University
Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
c] Department of Chemistry, University of Fribourg
Ch. du Musée 9, 1700 Fribourg, Switzerland
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200901183.
Eur. J. Inorg. Chem. 2010, 675–679
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
675

G. G. Morgan et al.
SHORT COMMUNICATION
the C amphiphile revealed a cis orientation for the lipo-
8
philic groups and a layering of the potentially spin-labile
[
13]
cations.
We have therefore expanded our studies to
change the morphology of the amphiphile in the solid state
by moving the position of the alkylation to the secondary
amino groups (Scheme 1). We now report the advantageous
effect of this repositioning of the aliphatic chains on the
magnetic switching properties.
Results and Discussion
+
Functionalised [Fe(sal) trien] complexes were obtained
2
from condensation of trien with salicylaldehyde, followed
by alkylation and subsequent metallation with iron(II) tet-
rafluoroborate to yield the iron(III) complex as a purple
crystalline solid on standing in air (Scheme 1). Plots of
magnetic moment vs. temperature for solid samples of the
unappended complex 1 and functionalised complexes 2 and
3
1
(Figure 1) show that, although the unappended complex
is high-spin (HS) down to 10 K, both the C - and C -
6
12
alkylated complexes show SCO over this range but with
markedly different profiles (see Supporting Information for
full magnetic data).
Figure 2. Variable-temperature EPR spectra of (a) [FeLC12]BF
and (b) [FeLC6]BF (2).
4
(3)
4
Crystals of 1 and 2 were grown by slow concentration of
solutions of each, and single-crystal diffraction data were
collected on both at temperatures between 100 and 293 K
(Table 1). The structure of complex 1 at 293 K (Figure 3)
contains one cation and one disordered anion with bond
III
lengths typical for HS Fe . The local geometry around the
coordinated HS ion is highly distorted at 293 K with elon-
6
gated bond lengths as expected for the A state where both
antibonding orbitals are occupied. The degree of distortion
II
III
in Fe and Fe is known to be highly sensitive to the spin
[
14]
[15]
Figure 1. Variable-temperature magnetic moment of [FeLC0]BF
squares), [FeLC6]BF (circles), and [FeLC12]BF (triangles).
4
state,
and for 1 the degree of both angular (Σ)
and
[
15]
(
4
4
trigonal (Θ) distortion is high as expected (Table 2).
H 4
Table 1. Representative bond lengths [Å] of [FeL ]BF (1) at 293 K
With the C -appended ligand the crossover to HS ap-
proaches completion, switching from 1.9 µB at 10 K to
.0 µB at room temp. with a gentle sigmoidal profile over
1
2
4
and of [FeLC6]BF (2) at 250 and 100 K.
Fe–donor
1 (293 K)
2 (250 K)
2 (100 K)
5
8
5 K and T1/2 = 125 K. This is corroborated by the X-band Fe(1)–O(2)
1.914(2)
1.916(2)
2.114(2)
2.114(2)
2.157(2)
2.185(2)
–
–
–
–
–
–
1.902(4)
1.910(3)
2.096(4)
2.107(4)
2.229(4)
2.233(4)
1.910(3)
1.896(3)
2.053(4)
2.070(4)
2.165(4)
2.176(4)
1.911(2)
1.919(2)
2.093(2)
2.103(3)
2.215(2)
2.227(2)
1.884(2)
1.884(2)
1.944(2)
1.948(2)
2.047(2)
2.058(2)
III
Fe(1)–O(1)
Fe(1)–N(1)
Fe(1)–N(4)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(2)–O(4)
Fe(2)–O(3)
Fe(2)–N(5)
Fe(2)–N(8)
EPR spectra with typical low-spin (LS) axial Fe signals
of g = 2.1 and g = 2.0 at 113 K, which diminish on warming
to room temperature with concomitant growth of a high-
spin g = 5.2 feature typical for anisotropic S = 5/2 Fe
Figure 2a).
III
(
With the shorter C amphiphile the crossover is incom-
6
plete at both ends of the measured range (Figure 1), with
the high-spin fraction falling from 82% at room temp. to Fe(2)–N(7)
III
4
1% at 15 K (assuming µ = 2.00 µB for LS Fe with a Fe(2)–N(6)
small orbital contribution and spin-only µ = 5.92 µ for HS
B
III
Fe ). This temperature dependence of the HS/LS ratio is
Single-crystal data for complex 2, collected at 100 and
also reflected in the EPR spectra (Figure 2b), with charac- 250 K, reveal two independent cations in the unit cell at
teristic signals of both spin states at room temp. and at both temperatures (Figure 4) with a unique response to
113 K.
cooling for each, which explains the partial crossover. The
676
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 675–679

Inducing Spin Crossover in Amphiphilic Iron(III) Complexes
Figure 3. View of the X-ray crystal structure of [FeL
93 K. Thermal ellipsoids are drawn on the 15% probability level.
The counterion was omitted for clarity.
H 4
]BF (1) at
2
Table 2. Σ, Θ and α of [FeL
H
]BF
4
at room temp. and [FeLC6]BF
4
at 100 and 250 K.
Σ/°
Θ /° α /° Spin behavior
1
2
[FeL
H
]BF
4
at 293 K
95.67
156.55 76.0(1)
HS
[FeLC6]BF
4
at 250 K Fe1 81.78
4
at 100 K 79.18
126.50 85.1(2)
124.41 82.4(1)
HS
HS
[FeLC6]BF
2
[FeLC6]BF
4
at 250 K Fe2 67.22
4
at 100 K 65.02
113.34 86.3(2)
103.33 86.3(1)
SCO
SCO
[FeLC6]BF
Fe(1) site clearly remains HS down to 100 K with no signifi-
cant change in bond lengths, whereas the depopulation of
antibonding orbitals, which accompanies spin crossover in
Fe(2), is unambiguously reflected in shorter bonds on cool- The counterion was omitted for clarity.
ing (Table 1). This is in line with the magnetic and EPR
Figure 4. View of the X-ray crystal structure of [FeLC6]BF
4
(2) at
100 K. Thermal ellipsoids are drawn on the 50% probability level.
data, which show a partial crossover with approximately
half the molecules undergoing thermally induced spin tran-
and SCO behaviour, α being very similar for both HS Fe1
and SCO Fe2 in complex 2 and remaining unchanged upon
SCO (Table 2).
sition between 50 and 150 K. The difference in thermal spin
response between the two crystallographically independent
iron centers in 2 is also reflected by the significantly more
pronounced distortion in HS ion Fe1 compared to Fe2 (cf.
Σ and Θ; Table 2).
The focus of our structural investigation was a compari-
son of the packing between 1 and 2 in the search for a
coherent connection between intermolecular interactions
and magnetic response (see Supporting Information for
packing diagrams). This is particularly pertinent for our
system where other packing effects such as ligand sphere,
The magnetic behaviour of (saltrien)FeIII derivatives is
therefore unpredictable. An apparent lack of correlation
with any single geometric parameter indicates more com-
plex influences, which are not evident from the available
data.
phenolate substitution, counterion and lattice solvent are Conclusions
kept constant. Possible intermolecular interactions includ-
ing van der Waals distances, hydrogen bonding, π–π and
anion–π interactions were all investigated, but no corre- HS tetrafluoroborate salt of [Fe(sal) trien] switches on
We have shown that appropriate functionalisation of the
+
2
lation between packing and thermal spin response emerged SCO in this ion. Alkylation with short C chains induces a
6
(see Supporting Information for comparison of close con- partial SCO, and increasing the chain length to C12 brings
tacts in HS and SCO sites in 2). the SCO to completion over 10–293 K. Work is now un-
The dihedral angle α between the two phenoxy rings was derway to investigate the effects of longer-chain substitution
also analysed but, unlike earlier reports,[ we did not ob- on thermal magnetic response in this ion in both the solid
serve any correlation between changes in this parameter state and solution and soft matter environments.
12]
Eur. J. Inorg. Chem. 2010, 675–679
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
677

G. G. Morgan et al.
SHORT COMMUNICATION
based on redundant reflections was performed by the program SA-
DABS.[ The structures were solved by direct methods by using
Experimental Section
16]
Preparation of Ligands: H
2
LC6, and H
2
L
C12 were prepared by using
[17]
2
SHELXS-97
all data by using SHELXL-97.
and refined by full-matrix least squares on F for
inert and dry conditions. Dry acetonitrile was used for the synthesis
of the ligands. The complexation was carried out under aerobic
conditions.
[17]
Hydrogen atoms were added at
calculated positions and refined by using a riding model. Their
isotropic temperature factors were fixed to 1.2 times (1.5 times for
methyl groups) the equivalent isotropic displacement parameters of
the parent carbon atom. Anisotropic thermal displacement param-
eters were used for all non-hydrogen atoms. Data for 1 (293 K):
Complex 1: To a solution of triethylenetetramine (0.292 g, 2 mmol)
in methanol (25 mL), salicylaldehyde (0.488 g, 4 mmol) was added.
After 15 min, solid Fe(BF
give a deep purple solution that was stirred at room temp. for
0 min. The solvent was evaporated at room temp., and after 3 d
black crystals were obtained. Yield: 430 mg (43%). IR (KBr): ν˜ =
858 (vw), 3743 (w), 3472 (m), 3408 (m), 3281 (m), 3122 (w), 2932
4 2 2
) ·6H O (0.676 g, 2 mmol) was added to
–
3
C
20
H
24BF
4
FeN
4
O
2
(Mr = 495.09, ρ = 1.484 gcm ), λ = 0.71073 Å,
/c (#14),
.178(1) Å, b = 25.282(3) Å, c = 9.654(1) Å, β = 98.374(2)°, V =
crystal system: monoclinic, space group: P2
1
a =
3
9
2
6
3
216.3(4) Å , Z = 4. Data for 2 (250 K): C32
H
48BF
4
FeN
63.40, ρ = 1.325 gcm ), λ = 0.71073 Å, crystal system: ortho-
(#29), a = 31.1434(19) Å, b =
4 2
O (Mr =
3
–
3
(w), 2361 (m), 1623 (s), 1540 (s), 1450 (s), 1302 (s), 1203 (m), 1079
rhombic, space group: Pca2
19.2970(11) Å, c = 11.1027(7) Å, V = 6672.4(7) Å , Z = 8. Data
1
(
2
BF
s), 895 (m), 773 (s), 606 (m), 521 (m), 467 (w), 393 (m), 327 (w),
3
–1
23 (s) cm . HRMS (ESI): calcd. for C20
H
24FeN [M –
4
O
2
–
3
+
for 2 (100 K): C32
4 4 2
H48BF FeN O (Mr = 663.40, ρ = 1.363 gcm ),
4
]
409.1327, found 409.1313. C20
H24BF
4
FeN O (495.03):
4 2
λ = 0.71073 Å, crystal system: orthorhombic, space group: Pca2
1
calcd. C 48.48, H 4.89, N 11.31; found C 48.23, H 4.82, N 11.14.
(
6
#29), a = 30.654(3) Å, b = 19.1482(18) Å, c = 11.0156(10) Å, V =
465.8(10) Å and Z = 8. CCDC-746613 (1), -746611 (2, 100 K)
Complex 2: To a solution of triethylenetetramine (0.292 g, 2 mmol)
in dry acetonitrile (50 mL), salicylaldehyde (0.488 g, 4 mmol) was
3
and -746612 (2, 250 K) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
added. After 15 min, molecular sieves and anhydrous K
0.552 g, 4 mmol) were added. The reaction mixture was stirred
for 10 min, after which 1-bromohexane (0.660 g, 4 mmol) and KI
0.664 g, 4 mmol) were added, and the suspension was refluxed for
8 h. The reaction mixture was filtered and the solvent removed
2 3
CO
(
(
4
Supporting Information (see footnote on the first page of this arti-
cle): Tables and graphs of magnetic data for complexes 1–3, tables
of short contacts for complex 2 and packing diagrams for com-
plexes 1 and 2.
under reduced pressure. The resulting oil was dissolved in chloro-
form and washed with brine, and the organic fraction was dried
with anhydrous MgSO
ligand (0.551 g, 1 mmol) was dissolved in methanol (20 mL), and
solid Fe(BF ·6H O (0.338 g, 1 mmol) was added togive a deep
4 2
to obtain H LC6 as a bright yellow oil. The
4
)
2
2
Acknowledgments
purple solution that was stirred at room temp. for 30 min. The sol-
vent was evaporated, and the resulting purple oil was dissolved in
Funding of this work by the Irish Research Council for Science,
Engineering and Technology and the Swiss National Science Foun-
dation under the ERA-Chemistry programme is gratefully ac-
knowledged. The award of a travel grant by University College
Dublin to complete the magnetic measurements and an equipment
grant to fund the EPR is also gratefully acknowledged. M. A. is
very grateful for an Alfred Werner assistant professorship.
2
chloroform (10 mL) and filtered through a bed of SiO by using
chloroform to elute the product. The solvent was removed under
reduced pressure, and the residue was dissolved in tetrahydrofuran
2 3
(10 mL) and purified on a column of Al O by using as eluent first
tetrahydrofuran and then ethanol. The ethanol was removed under
reduced pressure to yield a purple oil that was recrystallised from
methanol. The solvent was evaporated slowly at room temp., and
after 2 d black crystals were obtained. Yield: 100 mg (15%). IR
(
2
1
KBr): ν˜ = 3858 (vw), 3744 (w), 3619 (w), 3442 (vw), 3059 (w),
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926 (m), 2861 (m), 2361 (m), 1620 (s), 1540 (s), 1464 (s), 1302 (s),
199 (m), 1053 (s), 897 (m), 759 (s), 602 (m), 517 (w), 397 (m), 324
[
–
1
(
w), 224 (s) cm . HRMS (ESI): calcd. for C32
H
48FeN
4 2
O (663.22):
4
O
2
[M –
+
BF
4
]
576.3127, found 576.3130. C32
H48BF
4
FeN
calcd. C 57.90, H 7.28, N 8.45; found C 57.01, H 7.32, N 7.81.
Complex 3: The method of preparation of 3 was similar to that for
2
2
29–257; e) M. Seredyuk, A. Gaspar, M. Muñoz, M. Verdag-
, but 1-bromododecane (0.997 g, 4 mmol) was used instead of 1-
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bromohexane. After purification and recrystallisation, a purple wax
was obtained. The wax was dissolved in chloroform and layered
with n-hexane. After 5 d, small black crystals were obtained. Yield:
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125 mg (15%). IR (KBr): ν˜ = 3858 (vw), 3744 (w), 3649 (vw), 3059
(vw), 2922 (m), 2851 (m), 2361 (m), 1624 (m), 1542 (m), 1465 (m),
1
4
C
C
307 (m), 1198 (w), 1059 (m), 898 (w), 755 (m), 607 (w), 516 (vw),
–1
01 (w), 328 (vw), 222 (s) cm . HRMS (ESI): calcd. for
+
44
H
72FeN [M
4
O
2
–
BF
4
]
744.5005, found 744.5026.
44
H
72BF FeN O (831.40): calcd. C 63.51, H 8.73, N 6.74; found
4
4 2
C 63.69, H 9.01, N 5.94.
2
008, 47, 6433–6437; g) F. Volatron, L. Catala, E. Rivière, A.
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detector diffractometer. A full sphere of reciprocal space was
scanned by φ-ω scans. Pseudo-empirical absorption correction
6
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678
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 675–679

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Published Online: January 5, 2010
Eur. J. Inorg. Chem. 2010, 675–679
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
679