A homochiral 2D copper(II) coordination framework

Article abstract of DOI:10.1002/ejic.200801098

Reaction of copper(II) salts with (S)-phenylethylaminodiacetate [(S)-peadaa] results in the 2D coordination network (S)-[Cu(peadaa)]n (space group P2₁), which displays an unusual Cu...Cu connectivity mode. Solid-state circular dichroism (CD) in

Full text of DOI:10.1002/ejic.200801098

SHORT COMMUNICATION
DOI: 10.1002/ejic.200801098
A Homochiral 2D Copper(II) Coordination Framework
John Fielden,[a] Kyle Quasdorf,[a] Arkady Ellern,[b] and Paul Kögerler*[a,c]
Keywords: Coordination modes / N,O ligands / Ligand design / Chirality / Magnetic properties
Reaction of copper(II) salts with (S)-phenylethylaminodiacet-
ate [(S)-peadaa] results in the 2D coordination network (S)-
[Cu(peadaa)]n (space group P21), which displays an unusual
Cu···Cu connectivity mode. Solid-state circular dichroism
(CD) indicates a strong interaction between the ligand chiral
centre and the metal centres, whereas magnetic measure-
ments show both antiferromagnetic coupling and strong li-
gand field effects.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
short Cu···Cu connections, and through its CD spectrum,
the peadaa ligand demonstrates its ability to transfer chiral-
ity to coordinated metal centres.
Introduction
Chiral coordination compounds are interesting both for
their fundamental properties and potential applications in
optical, electro-optical and magneto-optical communica-
tion and data processing technologies.[1–5] In addition to
their well-known ferroelectric[4] and nonlinear optical prop-
erties,[5] such systems can show unusual magneto-optical
cross effects such as magnetochiral dichroism.[6] This has
led to an increasing interest in chiral magnetic materials
with asymmetry being introduced by enantiomerically
pure organic ligands including nitroxides,[1] amines,[2,7,8]
amino acids,[3,5] carboxylates[9] and hydroxamates.[10] In the
solid state, a common orientation of ligand chiral centres
should enhance the optical activity of such materials. Con-
sequently, we are interested in coordination networks based
on reasonably rigid, chiral ligands capable of directing the
assembly of extended framework structures.
With this in mind, we synthesised the novel, chiral amino
bis(carboxylate) ligand (S)-phenylethylamino-N,N-diacetic
acid [(S)-peadaa, Scheme 1]. Coordination to a metal centre
through the N,O,O-tris-chelating unit locks the rotation of
the carboxylate arms, and the rigid (S)-[M(peadaa)] unit
presents two orthogonal carboxylate chelating groups to al-
low connection into a network. Furthermore, the rigidity
and steric demand of the ligand phenyl group is likely to
help impose the chirality of the ligand over the entire struc-
ture. Herein, we present preliminary studies of the coordi-
nation chemistry of this ligand with CuII, resulting in the
synthesis of a thermally stable, homochiral 44 copper(II)
network. This structure shows a unique pattern of long and
Scheme 1. The (S)-peadaa ligand in its protonated form, (S)-
H2peadaa.
Results and Discussion
Synthesis of the proligand (S)-Na0.4H1.6peadaa (1) can
be achieved in high yields (90%) by reaction of the parent
amine (S)-phenylethylamine with iodoacetic acid and
NaOH in water. The coordination network (S)-[Cu-
(peadaa)]n (2) was accessed as a microcrystalline material in
29% yield by aqueous reaction of copper(II) nitrate with an
equimolar amount of 1 and 0.5 equivalents of KOH under
hydrothermal conditions. Small single crystals suitable for
X-ray diffraction were obtained in a lower yield (20%) by
reaction under analogous conditions without base.[11] The
synthesis of 2 was achieved in comparable yields through
the reaction with copper(II) perchlorate at room tempera-
ture, indicating that a preferred coordination mode exists in
2. In addition to single-crystal X-ray diffraction, compound
2 was characterised by XRPD, elemental analysis, TGA,
FTIR, solid state UV and CD and magnetic measurements.
The copper centre in 2 adopts a highly distorted octahe-
dral geometry, with an (S)-peadaa ligand binding in a mer
fashion to the equatorial coordination sites with its single N
and two carboxylate O donors (Figure 1). The other three
coordination sites allow interlinking of the (S)-[Cu(peadaa)]
monomers into the network: the remaining equatorial posi-
tion and one axial site are chelated by a carboxylate group
from one adjacent monomer, whereas the second axial site
[a] Ames Laboratory, Iowa State University,
Ames, IA 50011, USA
[b] Department of Chemistry, Iowa State University,
Ames, IA 50011, USA
[c] Institut für Anorganische Chemie, RWTH Aachen University,
52074 Aachen, Germany
E-mail: paul.koegerler@ac.rwth-aachen.de
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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J. Fielden, K. Quasdorf, A. Ellern, P. Kögerler
SHORT COMMUNICATION
is occupied by the exo oxygen atom from another. Copper
bond lengths indicate considerable Jahn–Teller distortion;
the Cu–O distances in the equatorial plane average
1.965(3) Å, the equatorial Cu–N distance is 2.053(3) Å, and
the two elongated, markedly different axial Cu–O distances
are 2.208(3) and 2.686(3) Å. Bond angles also deviate sub-
stantially from 90°, varying between 53.72(8) and
111.73(12)°. The noncentrosymmetry of the metal environ-
ment is also indicated by the considerable displacement (ca.
0.4 Å) of the Cu centre from the centroid described by the
six donor atoms.
Figure 2. The (S)-[Cu(peadaa)]n network (2). View along the crys-
tallographic c axis (left); view along the crystallographic ab plane
showing the interpenetrating phenyl groups (right). Colour scheme
as in Figure 1.
Figure 1. Section of (S)-[Cu(peadaa)] (2) highlighting the copper
coordination mode. Besides tris-coordination of a peadaa group,
two additional carboxylate groups (semitransparent) bind to Cu
(violet). C, black; O, red; N, blue; H, omitted.
Scheme 2. Representation of the 2D network in 2 showing long
[grey, 5.521(1) Å] and short [black, 4.597(1) Å] Cu···Cu connectiv-
ity modes.
Assembly of the monomers into a network results in
rhombic 44 (S)-[Cu(peadaa)]n layers propagating along the
ab plane, with the structure displaying only two different of 2. This thermal stability, along with the apparent lack
orientations for the ligand chiral centre (Figure 2). Each of temperature or anion dependence in its formation, is an
Cu4 rhombus has two long and two short sides described indicator that network structure 2 represents a thermo-
by Cu···Cu distances of 4.597(1) and 5.521(1) Å and angles dynamic stability maximum.
of 80.60(1), 88.48(1) and 101.93(1)° (Scheme 2). As a result,
Because of the insolubility of 2, UV/Vis and circular di-
each Cu centre shares two short and two long connections chroism (CD) measurements were recorded on the solid
with its nearest neighbours: these are composed of two material as KBr discs. The UV/Vis spectrum of 2 (see Sup-
short Cu–µ2-O–Cu bridges and two long Cu–O–C–O–Cu porting Information) shows one very broad absorption at
bridges. The resulting linear chains of alternating long and 823 nm and a stronger peak at 258 nm (which may partly be
short Cu···Cu connections intersect the a and b axes at the associated with absorption by the disc). CD measurements
origin. Consequently, the network in 2 can be broken into indicate a positive Cotton effect at ≈310 nm when corrected
1D zigzag chains of Cu–µ2-O–Cu linkages, which run paral- for the KBr disc, with a weaker, negative signal at ≈250 nm
lel to the b axis. These chains are interconnected by the also being observed (Figure 3). Unfortunately, the lower en-
longer Cu–O–C–O–Cu bridges. To the best of our knowl- ergy absorption observed in the UV/Vis spectrum was be-
edge, this particular connectivity is unprecedented in transi- yond the measurement range of the CD spectrophotometer.
tion-metal carboxylate chemistry.[12]
Note that ligand 1 shows no Cotton effects at either of these
The layers themselves are perfectly stacked along the c wavelengths, indicating that these CD absorptions are asso-
axis and appear to assemble through C–H···π interactions ciated with the CuII centre, and also establishing the ability
(Figure 2, right), with carbon-to-ring centroid distances of of the (S)-peadaa ligand to transfer chirality to the metal
ca. 3.670 and 3.767 Å. This combination of strong copper environment.
carboxylate coordination bonds with the dense interlayer
Discrete Cu4 carboxylate squares have been seen to show
packing explains the insolubility of 2 in all common sol- global ferri-,[14] ferro-[15] and antiferromagnetic behav-
vents. Thermogravimetric measurements show no signifi- iour,[16] often with both competing ferro- and antiferromag-
cant mass loss up to 270 °C, confirming the absence of in- netic couplings present in the same molecule.[17] Magnetic
cluded solvent indicated by the calculated accessible void susceptibility measurements (Figure 4) show relatively weak
space of zero.[13] There follows a rapid mass loss between antiferromagnetic coupling between the s = 1/2 Cu2+
270 and 330 °C, corresponding to thermal decomposition centres in 2. This is in line with other CuII systems showing
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A Homochiral 2D Copper(II) Coordination Framework
ments (Dq = 100 cm–1, Dt = 500 cm–1, κ = –1).[22] Note that
the effects of solely the tetragonal ligand field on the CuII
centres, minus the Cu–Cu exchange interactions, are shown
in Figure 4 as the dark-grey line.
Conclusions
In summary, reaction of the dicarboxylate ligand (S)-
peadaa with copper(II) demonstrates the ability of this chiral
ligand system to direct formation of coordination networks.
The resulting 44 copper(II) coordination framework is ther-
mally stable and also shows a highly unusual Cu···Cu con-
nectivity that mediates moderate antiferromagnetic coup-
ling. The network forms in a range of reaction conditions
(temperature, anion and pH), suggesting it represents a
structural archetype that can be achieved for a range of
other transition-metal ions. Furthermore, CD spectroscopy
indicates that peadaa strongly transfers chirality to coordi-
nated metal centres, making it a promising, readily access-
ible building block for multiproperty chiral coordination
compounds.
Figure 3. Solid-state (KBr) CD spectra of the uncomplexed ligand
(S)-Na0.4H1.6peadaa (1) and coordination network (S)-[Cu-
(peadaa)]n (2).
similar carboxylate/oxo bridging modes, which tend to exhi-
bit weak ferro- or antiferromagnetic coupling.[18] The ob-
served monotonous decrease in χmolT with decreasing tem-
perature, with a clear drop below 25 K, is not only due to
antiferromagnetic exchange coupling but also an effect of
orbital momentum contributions. Thus, we simulated[19]
both local CuII site (ligand field, spin-orbit coupling) effects
and Cu–Cu coupling by using the molecular field model
Experimental Section
k
approximation χ–1 = χ–1(ζ,Bq ) – λMF, where χm designates
(S)-Na0.4H1.6peadaa (1): (S)-(–)-1-Phenylethylamine (1.05 g,
8.40 mmol) was added to a stirred, ice-cooled solution of iodoacetic
acid (6.564 g, 17.6 mmol) in water (10 mL), resulting in a cloudy
the single-ion susceptibility and λMF the molecular field pa-
rameter (positive and negative values of λ indicate domi-
nant ferromagnetic and antiferromagnetic interactions,
respectively),[20] ζ represents the spin-orbit coupling energy
white suspension.
A solution of sodium hydroxide (1.410 g,
35.3 mmol) in water (10 mL) was added dropwise, and after stirring
for ca. 1 h, the mixture was warmed to room temperature before
heating to 50 °C for 5 d. The resulting solution (pH ≈ 9.3) was then
acidified to pH ≈ 5.5 by using 2.0  aqueous HCl, and the water
was removed in vacuo. Ethanol (50 mL) was added, and the mix-
ture was stirred and sonicated and then filtered to remove NaCl.
The ethanol was then evaporated in vacuo, and the dissolution/
filtration/evaporation cycle repeated until addition of ethanol re-
sulted in a completely clear solution. At this point, the solvent was
evaporated one last time, yielding 1 as an off-white crystalline solid
(2.126 g, 7.60 mmol, 90% yield based on the formula
1·0.4EtOH·0.2H2O·0.2NaCl). This material, although pure enough
for further reaction, contained trace amounts of unreacted
iodoacetic acid. 1H NMR (400 MHz, [D6]DMSO): δ = 7.35–7.19
(m, 5 H, 5ArH), 3.98 (q, 1 H, PhCHMeN), 3.28 (m, 4 H, 2NCH2),
1.27 (d, 3 H, CH3) ppm. 13C NMR (100 MHz, D2O): δ = 175.32
(acid, C), 144.14 (Ar, C), 128.21 (Ar, CH), 126.98 (Ar, CH), 126.78
(Ar, CH), 60.15 (CH), 57.37 (CH2), 19.50 (CH3) ppm. MS
k
and Bq are the ligand-field parameters in Wybourne nota-
tion.[21] The λMF and Dqk values, for a Jahn–Teller-distorted
tetragonal ligand field, were determined by least-squares fit-
ting, with the best fit (SQ = 0.9%) yielding λMF
=
=
2
4
–0.754ϫ106 molcm–3 and B0
=
2900 cm–1, B0
4
–4866 cm–1, B4 = 4324 cm–1, which are in good agreement
with published standard data for tetragonal CuIIL6 environ-
(APCI+): m/z = 238.3 [M + H]+. FTIR (KBr disc): ν = 3418 (s),
˜
3026 (m), 2968 (m), 1605 (vs), 1351 (w), 1329 (m), 1213 (m), 1157
(w), 1122 (w), 1090 (w), 1064 (w), 1029 (w), 980 (w), 926 (w), 909
(m), 885 (w), 850 (w), 780 (m) cm–1. A small, iodoacetic acid free
sample was prepared for elemental analysis by esterification of 1 in
dry methanol (under an atmosphere of dry N2) and 98% H2SO4,
followed by chromatographic purification of the resulting methyl
ester and base hydrolysis to recover 1. C12.8H17.4NO4.6Na0.6Cl0.2
(1·0.4EtOH·0.2H2O·0.2NaCl, 279.76): calcd. C 54.95, H 6.27, N
5.01, Na 5.2, Cl 2.54; found C 55.18, H 6.72, N 4.91, Na 4.93, Cl
2.21.
Figure 4. Temperature dependence of χmolT for (S)-[Cu(peadaa)]n
(2) at 0.1 Tesla (black squares). Light-grey line: best fit resulting
for a model expression of ligand field and exchange coupling ef-
fects; dark-grey line: simulated single-ion behaviour for isolated
Jahn–Teller distorted tetragonal CuL6 environments (see text).
(S)-[Cu(peadaa)]n (2):
A solution of (S)-Na0.4H1.6peadaa (1;
0.071 g, 0.25 mmol) and KOH (0.0072 g, 0.13 mmol) in water
(3 mL) was mixed with Cu(NO3)2·2.5H2O (0.060 g, 0.26 mmol) in
Eur. J. Inorg. Chem. 2009, 717–720
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J. Fielden, K. Quasdorf, A. Ellern, P. Kögerler
SHORT COMMUNICATION
H. Zeng, B. Wang, X.-Y. Wang, W.-X. Zhang, X.-M. Chen, S.
Gao, Inorg. Chem. 2006, 45, 7069–7076; c) P. Gerbier, N. Do-
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water (3 mL), resulting in a cloudy blue-green suspension. This was
filtered, transferred to a 23-mL PTFE Parr acid digestion bomb
liner and purged with bubbling argon for 5–10 min, before sealing
the bomb and heating to 110 °C for 72 h. After slow cooling, 2 was
recovered as
a bright-blue microcrystalline solid (0.021 g,
0.07 mmol, 29%). Small single crystals suitable for X-ray diffrac-
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[10] C. M. Zaleski, E. C. Depperman, J. W. Kampf, M. L. Kirk,
V. L. Pecoraro, Inorg. Chem. 2006, 45, 10022–10024.
[11] Crystal data for [Cu(peadaa)]n (2): monoclinic, P21,
298.77 gmol–1, a = 7.090(2) Å, b = 7.142(2) Å, c = 11.224(2) Å,
β = 90.145(3)°, V = 568.4(2) Å3, Z = 2, ρcalcd. = 1.746 gcm–3,
µ(Mo-Kα) = 1.927 mm–1, 5092 reflections measured, 2583 [Rint
= 0.0503] unique, which were used in all calculations. GooF on
F2 = 1.002, final R1 = 0.0387 and wR2 = 0.0848 (all data).
Flack absolute structure parameter x = 0.039(18). Data were
measured at 193(2) K with a Bruker CCD-1000 diffractometer
[λ(Mo-Kα) = 0.71073 Å] with a graphite monochromator.
CCDC-704969 contains 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.
(0.015 g, 0.05 mmol, 20%). FTIR (KBr disc): ν = 3442 (w), 3024
˜
(vw), 2981 (w), 2962 (w) 2938 (w), 1650 (s), 1600 (vs), 1573 (s),
1492 (w), 1450 (w), 1435 (w), 1421 (s) 1398 (w), 1380 (w), 1362 (w),
1335 (m), 1295 (m), 1267 (m), 1218 (w), 1198 (m), 1148 (m), 1124
(w), 1103 (m), 1080 (m), 1045 (w), 1015 (m), 972 (w), 944 (w), 917
(m), 813 (m), 790 (w), 755 (m), 739 (s), 710 (m), 652 (m), 634 (m),
614 (w), 576 (w), 536 (m), 508 (w), 488 (w), 454 (w), 417 (m) cm–1.
C12H13NO4Cu (298.77): calcd. C 48.24, H 4.39, N 4.81; found C
48.10, H 4.32, N 4.98.
Supporting Information (see footnote on the first page of this arti-
cle): Full experimental section, crystallographic data, thermogravi-
metric data, UV/Vis and IR spectra.
[12] A search of the CSD (v7.29, November 07+1 update) for car-
boxylate-bridged TM tetramers found 701 structures. Only two
were 44 nets, and neither showed the pattern of long and short
M···M connections observed in 2.
[13] Calculated using PLATON: A. L. Spek, J. Appl. Crystallogr.
2003, 36, 7–13.
[14] R. Acevedo-Chávez, M. E. Costas, S. Bernès, G. Medina, L.
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[16] S. Wang, S. J. Trepanier, J.-C. Zheng, Z. Pang, M. J. Wagner,
Inorg. Chem. 1992, 31, 2118–2127.
Acknowledgments
We thank Steve Veysey of Iowa State University for valuable help
with CD measurements and CHN analysis. Ames Laboratory is
operated for the U.S. Department of Energy by Iowa State Univer-
sity under Contract No. DEACD2 07CH11358.
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Published Online: January 16, 2009
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Eur. J. Inorg. Chem. 2009, 717–720