Pseudopeptidic ligands: Exploring the self-assembly of isophthaloylbisglycine (H2IBG) and divalent metal ions

Article abstract of DOI:10.1039/c2dt31383a

We present a systematic study of the complexation of the new pseudopeptidic ligand isophthaloylbisglycine (H₂IBG) with divalent metal ions of varying ionic radius. This work represents the initial employment of H ₂IBG in the coordina

Full text of DOI:10.1039/c2dt31383a

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PAPER
Pseudopeptidic ligands: exploring the self-assembly of isophthaloylbisglycine
(H2IBG) and divalent metal ions†
Vassiliki N. Dokorou,a,b Constantinos J. Milios,c Athanassios C. Tsipis,d Matti Haukka,e Peter G. Weidler,f
Annie K. Powella,g and George E. Kostakis*a
Received 27th June 2012, Accepted 10th August 2012
DOI: 10.1039/c2dt31383a
We present a systematic study of the complexation of the new pseudopeptidic ligand
isophthaloylbisglycine (H2IBG) with divalent metal ions of varying ionic radius. This work represents the
initial employment of H2IBG in the coordination chemistry of alkaline earth, 3d transition, Zn(II) and
Cd(II) metal elements. Infrared, NMR, thermal, magnetic, adsorption and theoretical studies of these
compounds are also discussed.
Introduction
In recent years, a number of basic inorganic principles have pro-
vided the framework and inspiration for chemists in developing
coordination chemistry with a view to synthesizing materials
with potential applications.1 For example, peptides coordinating
transition metal elements have been used to obtain “bioinspired”
microporous coordination polymers with an emphasis on cataly-
sis.2 In addition, coordination polymers based on amino acid
backbones have gained widespread interest due to their intri-
guing properties such as asymmetric catalysis, chiral sorption,
etc.3
A pseudopeptidic ligand can be described as a molecule
belonging to the class of compounds where amino acids or
oligo-peptides are attached to scaffolds which may be aromatic
or non-aromatic. Several research groups are using various types
of pseudopeptidic ligands for the synthesis of macrocycles or
coordination polymers.4–6 Up to now, we have focused on the
investigation of the coordination abilities of a specific type of
pseudopeptidic ligand drawn in Scheme 1, for the synthesis of
Scheme 1 The general motif of the pseudopeptidic ligand.
zero or multidimensional compounds.6 These multidentate
ligands have a semi- or fully flexible backbone offering several
positions for coordination and hydrogen bonding interactions.
These are important features since the nature of the amino acid
or oligo-peptide (R2, Scheme 1) and that of the scaffold (R1,
Scheme 1) can vary the solubility and the coordination abilities
of the ligand. In this way, the introduction of different R groups
(R1 or R2, Scheme 1) allows for the possibility, for example, of
obtaining chiral compounds or water-friendly “green” nano-
porous products easily, suggesting that pseudopeptidic ligands
represent a class of compounds which deserve attention.
Up to now, we have studied the complexation abilities of
terephthaloylbisglycinate (TBG2−, R1 = p-C6H4 R2 = CH2,
Scheme 1) with metal ions at room temperature which resulted
in the formation of infinite non-porous coordination polymers
with fascinating topologies.6a,c Furthermore, it was found that
the compound [Cu(TBG)(μ-H2O)(H2O)2]·2(H2O) represents an
efficient heterogeneous catalyst for the oxidation of 3,5-di-tert-
butylcatechol (DTBC), and where the Cu–Cu distance of ca.
4 Å, similar to that found in tyrosinase, may be an important
factor. The next logical step in our study was to extend the
flexible arms of the H2TBG ligand by one methylene group and
thus we studied the complexation abilities of terephthaloyl-bis-
β-alaninate (TBbA2−, R1 = p-C6H4, R2 = CH2CH2, Scheme 1)
with metal ions at room temperature which resulted in the iso-
lation of polymeric compounds with interesting topological and
crystal engineering features.6d As an extension of this project,
we present herein our investigation on the influence of
aInstitute of Nanotechnology, Karlsruhe Institute of Technology,
Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen,
Germany. E-mail: george.kostakis@kit.edu
bX-Ray Unit, Department of Chemistry, University of Ioannina,
45110 Ioannina, Greece
cDepartment of Chemistry, The University of Crete, Voutes, 71003,
Herakleion, Greece
dLaboratory of Inorganic and General Chemistry, Department of
Chemistry, University of Ioannina, 451 10 Ioannina, Greece
eUniversity of Jyväskyla, Department of Chemistry, P.O. Box 35,
FI-40014 University of Jyväskyla, Finland
fInstitute of Functional Interfaces, Karlsruhe Institute of Technology,
Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen,
Germany
gInstitute of Inorganic Chemistry, Karlsruhe Institute of Technology,
Engesserstrasse 15, 76131 Karlsruhe, Germany
†Electronic supplementary information (ESI) available: Fig. S1–S13.
CCDC 886200–886207. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt31383a
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substituents on the aromatic ring. Thus, we have chosen to syn-
thesize the new pseudopeptidic ligand isophthaloylbisglycine
(H2IBG, R1 = m-C6H4, R2 = CH2, Scheme 1) (1). A further
motivation for this work was that whilst 3d transition metal
elements7 have been widely used for the synthesis of multi-
dimensional coordination polymers the use of alkaline earth
elements is much rarer8 and we thought it of interest to compare
the coordination behaviour of these two classes of compounds,
especially given the important role of alkaline earths in bio-
inorganic chemistry.
Thus, nine new compounds formulated as [Mg(H2O)6]-
[(IBG)]·5.5(H2O) (2), [Ca(IBG)(H2O)2] (3), [Sr(IBG)(H2O)3]·
2(H2O) (4), [Mn(IBG)(H2O)4]·2(H2O) (5), [Co(IBG)(μ-H2O)-
(H2O)2]·2(H2O) (6), [Ni(IBG)(μ-H2O)(H2O)2]·2(H2O) (7), [Cu-
(IBG)(μ-H2O)(H2O)2]·2(H2O) (8), [Zn(IBG)(H2O)3]·2(H2O) (9),
[Cd(IBG)(H2O)3]·2(H2O) (10) have been synthesized and
characterized by elemental analysis, IR spectroscopy, powder
X-ray diffraction and single-crystal X-ray diffraction measure-
ments. Infrared, NMR, thermal, magnetic, adsorption and theo-
retical studies of those compounds are also discussed.
calcd for C12H12N2O6 (found): C, 51.43 (51.22); H, 4.32 (4.20);
N, 9.99 (9.80)%. H-NMR (ppm): 3.96 (d, 4H, J = 5.8 Hz),
1
7.61 (t, 1H, J = 7.8 Hz), 8.02 (dd, 2H, J1 = 7.8, J2 = 1.70), 8.38
(s, 1H), 8.97 (t, 2H, J = 5.90 Hz), 12.68 (s, 2H). 13C-NMR
(ppm): 41.24, 126.42, 128.52, 129.95, 134.09, 166.02, 171.20.
Selected IR data (KBr disc, cm−1): 3287, 3063, 2976, 2867,
2560, 1713, 1643, 1536, 1434, 1408, 1340, 1325, 1295, 1269,
1243, 1072, 1026, 1002, 919, 895, 819, 738, 673, 644.
Synthesis of [Mg(H2O)6][(IBG)]·5.5(H2O) (2). The same pro-
cedure was followed for the preparation of all compounds, so
only the synthesis of 2 is described in detail. H2IBG (140 mg,
0.500 mmol) and K2CO3 (69 mg, 0.500 mmol) were dissolved
in water (25 mL) with stirring. Mg(NO3)2·3H2O (128 mg,
0.500 mmol) was added to the solution. The solution was stirred
at room temperature for five minutes and then filtered and left
undisturbed. Slow evaporation yielded colourless crystals after
almost 2 months. The crystals were collected by filtration and
washed with water. Yield: 215 mg, 85% based on Mg. Anal.
calcd for C24H66Mg2N4O35 (found): C, 28.27 (28.03); H, 6.52
(6.78); N, 5.50 (5.79)%. Selected IR data (KBr disc, cm−1):
3325, 2937, 2400, 1632, 1579, 1544, 1445, 1399, 1303, 1280,
1090, 1065, 1021, 1003, 936, 912, 824, 745, 686.
Experimental
Materials and methods
Synthesis of [Ca(IBG)(H2O)2] (3). The metal salt used was
Ca(NO3)2·4H2O. The product was obtained as colourless
needles. Yield: 129 mg, 73% based on Ca. Anal. calcd for
C12H14CaN2O8 (found): C, 40.67 (40.43); H, 3.98 (3.99);
N, 7.90 (7.86)%. Selected IR data (KBr disc, cm−1): 3533, 3398,
3268, 1633, 1598, 1573, 1481, 1437, 1396, 1336, 1299, 1280,
1247, 1191, 1076, 1021, 1001, 949, 928, 821, 737, 690, 659,
629.
All chemicals and solvents used for synthesis were obtained
from commercial sources and used as received without further
purification. All reactions were carried out under aerobic con-
1
ditions. The H-NMR and 13C-NMR spectra were recorded on a
Bruker AMX 500 MHz spectrometer. All spectra were recorded
using commercially available d6-DMSO or D2O (Aldrich) of
99.6% isotopic purity or better and referenced to a residual
solvent. The elemental analyses (C, H and N) were carried out at
the Institute of Nanotechnology, Karlsruhe Institute of Techno-
logy, using an Elementar Vario EL analyzer. Fourier transform
IR spectra (4000 to 400 cm−1) were measured on a Perkin-Elmer
Spectrum GX spectrometer with samples prepared as KBr discs.
UV-Vis diffuse reflectance spectra (DRS) of 6 and its dehydrated
and rehydrated analogues were recorded on a Perkin-Elmer UV/
vis/NIR Lambda 900 spectrophotometer. Variable-temperature,
solid-state direct current (dc) magnetic susceptibility data down
to 5 K were collected on a Quantum Design MPMS-XL SQUID
magnetometer (University of Crete) equipped with a 7 T dc
magnet. Diamagnetic corrections were applied to the observed
paramagnetic susceptibilities using Pascal’s constants.
Synthesis of [Sr(IBG)(H2O)3]·2(H2O) (4). The metal salt used
was Sr(NO3)2·6H2O. The product was obtained as colourless
needles. Yield: 201 mg, 88% based on Sr. Anal. calcd for
C12H20N2O11Sr (found): C, 31.61 (31.83); H, 4.42 (4.27);
N, 6.14 (6.23)%. Selected IR data (KBr disc, cm−1): 3401, 1646,
1576, 1478, 1435, 1400, 1315, 1298, 1020, 931, 814, 748, 721.
Synthesis of [Mn(IBG)(H2O)4]·2(H2O) (5). The metal salt
used was Mn(OAc)2·4H2O. The product was obtained as colour-
less needles. Yield: 129 mg, 62% based on Mn. Anal. calcd for
C14H20MnN2O9 (found): C, 40.49 (40.63); H, 4.86 (4.67);
N, 6.74 (6.73)%. Selected IR data (KBr disc, cm−1): 3333, 1634,
1580, 1544, 1439, 1395, 1302, 1280, 1063, 1020, 1002, 911,
822, 741, 685.
Synthesis of [Co(IBG)(μ-H2O)(H2O)2]·2(H2O) (6). The metal
salt used was Co(NO3)2·6H2O. The product was obtained as col-
ourless needles. Yield: 171 mg, 80% based on Co. Anal. calcd
for C12H20CoN2O11 (found): C, 33.74 (33.20); H, 4.71 (4.76);
N, 6.56 (6.40)%. Selected IR data (KBr disc, cm−1): 3350, 2961,
2927, 1651, 1564, 1483, 1439, 1406, 1323, 1267, 1196, 1069,
982, 907, 746, 693.
Synthetic part
Synthesis of isophthaloyl-bis-β-glycine (IBGH2) (1). Glycine
(6.006 g, 0.080 mol) and NaOH (4.80 g, 0.120 mol) were dis-
solved in water (50 mL) with stirring in a round-bottom flask in
an ice bath. After the solution cooled to below 10 °C, a solution
of isophthaloylchloride (8.12 g, 0.040 mol) dissolved in toluene
(50 mL) was added dropwise. The reaction mixture was stirred
for one hour, after which the aqueous phase was separated from
the organic phase and collected. The aqueous phase was
acidified with 35% HCl until the pH was approximately 1. The
white product was collected by vacuum filtration and dried in a
vacuum oven overnight at 50 °C. Yield: 11.84 g, 100%. Anal.
Synthesis of [Ni(IBG)(μ-H2O)(H2O)2]·2(H2O) (7). The metal
salt used was Ni(NO3)2·6H2O. The product was obtained as col-
ourless needles. Yield: 173 mg, 81% based on Ni. Anal. calcd
for C12H20CoN2O11 (found): C, 33.75 (33.12); H, 4.72 (4.76);
N, 6.56 (6.40)%. Selected IR data (KBr disc, cm−1): 3343, 2963,
12502 | Dalton Trans., 2012, 41, 12501–12513
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2928, 2248, 1653, 1563, 1483, 1441, 1408, 1325, 1303, 1267,
1195, 1135, 1068, 1009, 916, 834, 746, 692.
better than 0.001 Å and 0.1°, respectively. The computed elec-
tronic energies were corrected for zero point energy (ZPE) differ-
1
ences. H and 13C NMR shielding tensors have been computed
Synthesis of [Cu(IBG)(μ-H2O)(H2O)2]·2(H2O) (8). The metal
salt used was Cu(NO3)2·3H2O. The product was obtained as col-
ourless needles. Yield: 162 mg, 75% based on Cu. Anal. calcd
for C12H20CuN2O11 (found): C, 33.38 (33.22); H, 4.69 (4.46);
N, 6.49 (6.40)%. Selected IR data (KBr disc, cm−1): 3364, 2928,
1654, 1586, 1561, 1482, 1431, 1397, 1385, 1313, 1263, 1197,
1068, 1009, 924, 746, 717, 689.
with the GIAO (gauge-including atomic orbitals) DFT
method10k,l as implemented in the GAUSSIAN03 series of pro-
grams9 employing the PBE0 functional. The potential energy
surfaces for the adiabatic rotation of the glycine ligand around
the M–O (M = Mn, Cu) bond were calculated at the
DSD-PBEP86/Def2-TZVP level of theory. The reduced density
gradient (RDG) plots were obtained employing the Multiwfn
software version 2.2.1.10m
Synthesis of [Zn(IBG)(H2O)3]·2(H2O) (9). The metal salt used
was Zn(NO3)2·4H2O. The product was obtained as colourless
needles. Yield: 141 mg, 65% based on Zn. Anal. calcd for
C12H20N2O11Zn (found): C, 33.23 (33.03); H, 4.65 (4.47);
N, 6.46 (6.33)%. Selected IR data (KBr disc, cm−1): 3348.35,
1630, 1590, 1544, 1478, 1438, 1397, 1307, 1260, 1002, 929,
824, 688.
Single crystal X-ray data collection and structure determination
Data for 1–10 were collected at 180 K on a Stoe IPDS II area
detector diffractometer using graphite-monochromated Mo-Kα
radiation. Semi-empirical absorption corrections were applied
using XPREP in SHELXTL.11 The structures were solved using
direct methods, followed by a full-matrix least-squares refine-
ment against F2 (all data) using SHELXTL.11 Anisotropic refine-
ment was used for all ordered non-hydrogen atoms; organic
hydrogen atoms were placed in calculated positions, while co-
ordinates of hydroxo hydrogen and amine hydrogen atoms were
either placed in calculated positions or located from the differ-
ence Fourier map and then constrained to ride on their parent
atom with Uiso = 1.5Ueq(parent atom). The crystallographic and
structure refinement data for all compounds are given in Table 1.
The crystals of 4 and 8 were refined as a twin using the twin
matrix [1 0 0.147 0 −1 0 0 0 −1] and [1 0 0.140 0 −1 0 0 0 −1]
and the BASF were refined to 0.12164, 0.26067, respectively.
The crystal structure of 5 was a solved monoclinic Cc space
group. All efforts to solve this structure in a higher symmetry
space group such as Ama2 suggested by Platon were proved
impossible. CCDC 886200–886207.
Synthesis of [Cd(IBG)(H2O)3]·2(H2O) (10). The metal salt
used was Cd(NO3)2·4H2O. The product was obtained as colour-
less needles. Yield: 180 mg, 75% based on Cd. Anal. calcd for
C12H20CdN2O11 (found): C, 29.99 (30.23); H, 4.19 (4.07);
N, 5.83 (5.73)%. Selected IR data (KBr disc, cm−1): 3348.35,
1630, 1590, 1544, 1478, 1438, 1397, 1307, 1260, 1002, 929,
824, 688.
Thermal studies
Thermogravimetric analysis (TGA) curves were measured using
a Netzsch STA 409C Thermal Analyzer under nitrogen flow
(30 mL min−1) at a scan rate of 5 °C min−1 from 25 to 800 °C.
The reversibility of water loss of compounds 6, 7 and 8 was
confirmed by heating a small sample of each in a vacuum oven
at 50 °C overnight. The dehydrated product was characterized by
IR, powder XRD, UV/Vis DRS and elemental analysis. The
dehydrated product was then rehydrated by suspending the
sample in water. The dehydration/rehydration procedure was
repeated on the same sample and characterized by IR to confirm
that the water loss is reversible.
Results
Synthesis
All compounds were prepared under normal atmospheric con-
ditions in aqueous solutions. Isomorphous compounds were
obtained using several bases such as sodium carbonate, lithium
hydroxide, or sodium hydroxide instead of potassium carbonate.
Using different metal salts including nitrate, chloride, acetate,
and sulphate did not result in any change to the motif of the final
products. Besides, reactions with an excess either of the metal
salt or the ligand, with different metal-to-ligand ratios (3 : 1 to
1 : 3), also result in the formation of the same isomorphous pro-
ducts demonstrating the stability of the compounds which also
retain their crystallinity on exposure to air. The phase purity of
the products arising from all the employed reaction conditions
was checked for all compounds by comparing their powder
XRD patterns with those simulated from the single crystal
studies (see Fig. S1–S6†).
Computational details
All calculations were performed using the Gaussian03 suite of
programs9 employing the recently developed DSD-PBEP86
(including spin-component scaled MP2 and dispersion correc-
tions)10a functional combined with the Def2-TZVP basis set,10b
for the model complexes chosen and the nonlocal hybrid gener-
alized gradient approximation (GGA) PBE0,10c–h combined with
the Pople’s 6-311+G(d,p) basis set for the H2IBG and IBG2−
ligands. Full geometry optimization was performed for each
structure using Schlegel’s analytical gradient method10i and the
attainment of the energy minimum was verified by calculating
the vibrational frequencies that result in the absence of imaginary
eigenvalues. The harmonic vibrational frequencies were scaled
employing the fundamental vibration frequency scale factor
0.9594.10j This was achieved with the SCF convergence on
the density matrix of at least 10−9 and the rms force less than
10−4 au. All bond lengths and bond angles were optimized to
Ligand conformation
Before going into a detailed description of the crystal structures,
it is important to establish a nomenclature in order to clarify the
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Table 1 Crystallographic and structure refinement data for compounds 1–10
1
2
3
4
5
Formula
Mr
Crystal system
Space group
C12H12N2O6
280.24
Monoclinic
C2/c
C24H64Mg2N4O35
1017.41
C12H14CaN2O8
354.33
Monoclinic
C2/m
C12H20N2O11Sr
455.92
Monoclinic
P2/c
C12H18MnN2O12
437.22
Monoclinic
Cc
Triclinic
ˉ
P1
T (K)
a (Å)
b (Å)
c (Å)
α (°)
β (°)
180(2)
180(2)
180(2)
180(2)
200(2)
6.4778(14)
7.3896(11)
26.049(5)
90
95.345(16)
90
7.2573(10)
12.1847(16)
14.7493(18)
106.389(10)
102.151(9)
100.772(9)
1180.3(3)
1
6.6529(11)
25.696(6)
8.8244(14)
90
105.668(12)
90
15.246(3)
6.8008(9)
8.0856(13)
90
92.234(14)
90
6.9305(12)
7.8963(13)
30.874(6)
90
90.20(3)
90
γ (°)
V (Å3)
1241.5(4)
4
1452.5(5)
4
837.7(2)
2
1689.6(5)
4
Z
Reflections collected
Unique reflections
Rint
R1 (I > 2σ(I))
wR2 (all data)
4153
1301
0.0716
0.0383
0.0970
7070
4356
0.0234
0.0432
3404
1375
0.1602
0.0532
4170
4170
0.0000
0.0601
0.1666
5075
2951
0.1016
0.0829
0.2445
0.1272
0.1255
6
7
8
9
10
Formula
Mr
Crystal system
Space group
C12H20CuN2O11
431.84
Monoclinic
P2/c
C12H20N2O11Zn
433.67
Orthorhombic
C2221
C12H20CdN2O11
480.70
Orthorhombic
C2221
Monoclinic
Monoclinic
T (K)
a (Å)
b (Å)
c (Å)
180(2)
180(2)
180(2)
180(2)
6.8874(7)
7.6896(8)
29.879(5)
90
180(2)
6.9402(7)
7.9330(7)
29.988(4)
90
15.2970(25)
6.9180(12)
7.9090(18)
90
15.3163(22)
6.9200(12)
7.8249(19)
90
15.211(3)
6.8469(14)
8.1434(16)
90
α (°)
β (°)
90.200(14)
90
90.198(13)
90
92.16(3)
90
847.5(3)
2
90
90
1582.5(3)
4
90
90
1651.0(3)
4
γ (°)
V (Å3)
Z
Reflections collected
Unique reflections
Rint
R1 (I > 2σ(I))
wR2 (all data)
4578
4578
0.0000
0.0741
0.2099
2949
1589
0.0251
0.0226
0.0514
3396
1545
0.0391
0.0295
0.0590
below (b) the plane of the aromatic ring, the first amide group is
trans (t), the carbonyl groups are in a syn-form (s), the second
amide group is trans (t) and the second glycine group is above
(a) the plane of the aromatic ring (Scheme 2).
Crystal structures description
Scheme 2 The H2IBG ligand possessing
conformation.
a
hypothetical btsta
H2IBG (1) has a 2-fold rotation symmetry and consequently the
torsion angles of its two halves are equal but with opposite
signs. The two glycine residues are characterized by the torsion
angles (C1C2N1C3) and (O2C1C2N1), the values of which are
close to those found in the polyglycine II structure (61.6° and
155.5°, respectively).13 The molecular conformation is also
characterized by the N1C3C4C7 torsion angle of 150.70 (13)°,
which clearly deviates from 180°. Thus, a displacement of the
planar amide group out of the plane of the benzene ring (29.5°)
is produced. Adopting the nomenclature described above the
conformation of the ligand can be described as atstb. As
expected, compound 1 is extended to three dimensions through
two strong hydrogen bonds; the first contains the nitrogen atom
(N1) of the amide group and the oxygen atom (O3) of the amide
group and the second contains the oxygen atom of the carboxylic
different conformations that this type of ligand can adopt which
results from the flexible character of the arms of this organic
molecule. This flexibility has been shown to lead to different
conformations in previous examples,6d and the nomenclature
also simplifies the crystal structure description. For this purpose,
we take into account (a) the positions of the two carbonyl
groups of the two amides (syn–anti),12 (b) the conformation of
the amide (trans–cis) and (c) the position of the aliphatic (R2,
Scheme 1) residues below or above the plane of the aromatic
ring. Thus, starting from the left to right (Scheme 2) an abbrevi-
ation such as btsta (below, trans, syn, trans, above) corresponds
to a hypothetical conformation where the first glycine group is
12504 | Dalton Trans., 2012, 41, 12501–12513
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Fig. 1 Crystal structure of 1 possessing an atstb conformation.
Scheme 3 The three coordination modes of IBG found in 3–10.
ligand adopts an atsta conformation, similar to that one observed
in 2, while the two carboxylate groups show as chelate-bridging
modes (mode I, Scheme 3). There are two different metal–metal
distances; the short CaII⋯CaII distance is 4.049(4) Å and each
pair of adjacent CaII centres in the chain is bridged by a chelate-
bridging carboxylate, with the chains propagating parallel to the
a axis in the crystal (Fig. 3). The chains are further linked by
IBG ligands (CaII⋯CaII distances 10.539 Å, 13.272 Å,
15.157 Å) parallel to the b axis forming an infinite 2D pattern
(Fig. 3 centre), the topology of which gives a sql net.16 More-
over, the adjacent 2D grids are themselves connected by medium
strength stacking interactions between the aryl rings of the IBG
ligands to form a supramolecular 3D structure. The distances
between the centroids of the aromatic rings are 3.428(3) Å,
3.563(3) Å and 4.081(3) Å, with projection distances of 3.318(2)
and 3.321(2) Å and offsets from the vertical of 0.860 Å and
1.292 Å, respectively.14 The solvent-accessible volume has been
calculated using Platon to be 5.2% of the unit cell (Fig. 3 lower).
Similarly to 3, compound 4 consists of SrII chains linked
through the IBG ligand to a 2D layer. The asymmetric unit con-
tains a half SrII centre, half an IBG ligand, one and half ligated
and one lattice water molecules. In 4, Sr(1) is ligated to nine
oxygen atoms, forming a polyhedron which can be best
described as a tricapped trigonal prism.15a In contrast to 2 and 3,
the ligand adopts a btsta conformation, while the two carboxy-
late groups show chelate-bridging (mode I, Scheme 3). There are
two different metal–metal distances; the short SrII⋯SrII distance
is 4.044(3) Å and each pair of adjacent SrII centres in the chain
is bridged by a chelate-bridging carboxylate, with the chains pro-
pagating parallel to the c axis in the crystal (Fig. 4 upper). The
chains are further linked by IBG ligands (SrII⋯SrII distances
15.246, 15.925, 17.533 Å) forming an infinite 2D grid parallel
to the a0c plane. As for 3, the topological evaluation of this
net results in a sql topology.16 However, due to the different
Fig. 2 The asymmetric unit of 2. Mg green, O red, N blue, C grey.
Lattice water molecules are omitted for clarity.
group (O1) and the oxygen atoms of the carbonyl group (O2). It
is worth noting that no stacking interaction14 can be found in the
crystal structure of 1 (Fig. 1).
The asymmetric unit of compound 2 consists of one MgII, one
doubly deprotonated IBG organic dianion, six aqua ligands and
5.5 lattice water molecules. Surprisingly, the ligand is not coordi-
nated to the Mg metal ion and interestingly the organic moiety
adopts an atsta conformation (Fig. 2).
Before describing the crystal structures of compounds 3–10,
we note that upon ligand coordination to the metal centres the
CH2NHCO(m-C6H4)CONHCH2 residue is flattened. In com-
pounds 3–10, the ligand coordinates to the metal centres through
three different coordination modes which are depicted in
Scheme 3.
The crystal structure of 3 (Fig. 3) reveals one dimensional
(1D) CaII chains which are linked by IBG ligands into an infinite
two dimensional (2D) network. The asymmetric unit of com-
pound 3 comprises a half CaII centre, half a ligand and one
ligated water molecule. Each CaII centre has a coordination
number 8 with a geometry not close really to that described by
ideal polyhedra such as a dodecahedron or a square antiprism.15a
The geometry of CaII in
3 is very close to that of
Ca(NO3)2(MeOH)4,15b and the best fit is to a dodecahedron in
which the bidentate ligands occupy an edge in opposite trape-
zoids. Each CaII centre is ligated to two chelate carboxylates,
two water molecules and two bridging oxygen atoms belonging
to two different carboxylates (Fig. 3 upper). Interestingly, the
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Fig. 4 (up) The asymmetric unit of 4. Sr green, O red, N blue, C grey.
Lattice water molecules are omitted for clarity. (down) A sphere packing
diagram along the c axis indicating the difference in the packing
diagram of 3.
Fig. 5 The asymmetric unit of 5. Mn light blue, O red, N blue, C grey.
Lattice water molecules are omitted for clarity.
Fig. 3 (upper) The asymmetric unit of 3. Ca green, O red, N blue, C
grey. Lattice water molecules are omitted for clarity. (middle) A projec-
tion of the 2D network. (lower) A space filling diagram along the a axis
indicating the small pores.
indicates that the two carboxylates behave differently; the first
retains the single and double bond character with C(1)–O(1) and
C(1)–O(2) being 1.228(18) and 1.285(18) Å respectively, while
in the second a delocalization of the carboxylate group occurs
with the C(12)–O(5) and C(12)–O(6) bonds being 1.25(2) and
1.253(19) Å, respectively (Fig. 5).
A combination of single crystal and powder diffraction
measurements established that compounds 6–8 are isostructural
(Table 1 and Fig. S3–S5†). Compounds 6 and 7 consistently
gave crystals of rather poor quality; therefore their structures
could not be refined. However, we were able to identify their
unit cell (Table 1) and by recording their powder patterns
(Fig. S4 and S5†) we identified that they are isostructural with
compound 8 (Fig. 6).
X-Ray single-crystal analysis reveals that 8 is composed of
2-D layers. The asymmetric unit comprises a half CuII centre,
half an IBG ligand, one and half ligated and one lattice water
molecules. Each CuII centre is coordinated, in a rather regular
octahedron to two oxygen atoms belonging to two monodentate
carboxylates of two different IBG ligands, two bridging and two
terminal water molecules, in trans positions. The bridging water
molecules occupy the axial positions of the octahedron and their
bond distances of 2.298(2) Å indicate large Jahn–Teller distor-
tion. The ligand adopts an atstb conformation similar to 5 while
coordination environment of the metal centre, there is a differ-
ence in the stacking interactions motif. In 4, the distance
between the centroids of the aromatic rings is 4.081(3) Å, with
a projection distance of 3.4489(19) Å and a high offset of
2.181 Å, indicating a weak stacking interaction (Fig. 4 lower).14
X-Ray single-crystal analysis reveals that 5 is composed of
1-D chains. The structure comprises a MnII centre, one IBG
ligand, four ligated and two lattice water molecules, forming zig-
zag [Mn(IBG)(H2O)4]n 1D chains running along the c direction.
Each MnII centre is situated in the center of a slightly distorted
octahedron and ligated to four water molecules and to two
oxygen atoms of two different carboxylates. All the Mn–O
bonds are different and their length varies from 2.140(10) to
2.241(11) Å. The angle O(2)–Mn(1)–O(6) at 92.5(4)° exhibits a
cis configuration of the two different IBG ligands. A similar
coordination behavior was observed in the compound [Mn-
(TBG)(H2O)4]n·2(H2O), where TBG is the terephthaloylbisglyci-
nate dianion.6c In contrast to 2–4 the ligand adopts an atstb
conformation and possesses the coordination mode II depicted in
Scheme 3. A detailed study of the bond lengths and distances
12506 | Dalton Trans., 2012, 41, 12501–12513
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Fig. 7 The asymmetric unit of 9. Zn light blue, O red, N blue, C grey.
Lattice water molecules are omitted for clarity.
right handed helical chains running along the c direction (Fig. 7
upper). The refined Flack parameter of −0.002(13) for 9 and
0.02(4) for 10 shows that the crystals examined have the same
handedness throughout its structure, rather than being racemi-
cally-twinned with different domains containing right and left
handed helices. Within a batch of crystals there will be compar-
able amounts of crystals showing either a right or left handed
enantiomer.18 The Zn(1) atom and water oxygen O(5) lie on a
two-fold axis and the IBG ligand lies about an inversion centre.
Each ZnII centre in 9 has a trigonal bipyramidal environment,
coordination number 5, with one terminal water molecule and
two oxygens belonging to two monodentate carboxylates of two
different IBG ligands in the equatorial plane, while two terminal
water molecules occupy the axial positions. Analysis of the
shape determining angles using the approach of Addison,
Reedijk et al.19 yields a value for the trigonality index, τ, of 0.85
for 9 suggesting trigonal bipyramidal geometry about Zn in both
compounds (τ = 0 and 1 for perfect square planar and trigonal
bipyramidal geometries respectively). As for 8, the ligand adopts
an atstb conformation and shows a bis-monodentate coordination
mode which is depicted in Scheme 3 (mode III). In addition, the
ZnII⋯ZnII separation within the helical chain for 9 is 15.179 Å.
IR studies
The IR spectra of compounds 2–10 are consistent with what can
be expected from the crystallographic studies. All the spectra
show a very strong band in the region of 3340–3400 cm−1 indi-
cating protonation of the nitrogen atom of the amide group,
while the peaks around 1640 cm−1 indicate that the carbonyl
group of the amide is not coordinated. The absence of any peak
in the region of 1710 cm−1 and the existence of peaks around
1580 cm−1 indicate that the carboxylic groups are deprotonated.
In Table 2 are depicted some characteristic bands of all com-
pounds, including the potassium salt of the ligand (1K), in order
to assign the coordination mode of the carboxylate group accord-
ing to the literature.20 The difference (Δ) of the asymmetric
ν(CO2)as and symmetric ν(CO2)s bands of 1K is 173 cm−1, thus
as anticipated for compounds 5–10 which show monodentate
mode, Δ is higher than that of 1K (Table 2). Interestingly, the
difference in the values 177 cm−1 and 192 cm−1 found for 8 and
9 (Table 2) can be attributed to the slight difference in the bond
distance of the carbonyl group of the carboxylate (1.237(7) in 8
and 1.229(3) in 9) which indicates that the carbonyl group of the
carboxylate in 9 has greater double bond character than in 8. The
Δ values of compounds 3 and 4 (chelate bridging mode, mode I,
Scheme 3) are close to those of 1K, while the Δ value of the
ionic compound 2 is higher than in 1K.
Fig. 6 (upper) The asymmetric unit of 8. Cu light blue, O red, N blue,
C grey. Lattice water molecules are omitted for clarity. (middle) A pro-
jection of the 2D network. (lower) A sphere packing diagram along the
a axis indicating the interaction of the aromatic rings of different layers.
the X-ray structure shows that the ligand adopts the coordination
mode III which is depicted in Scheme 3. The structure of 8 is
constructed from 1D chains interlinked through bridging water
and IBG groups into a 2D structure. The structure propagates
along the c axis through the water bridge with a CuII⋯CuII dis-
tance of 4.072 Å, a value which is very close to previously
reported examples,6a,17 while the IBG bridges propagate alter-
nately parallel to the a0c plane (CuII⋯CuII 15.598 Å). As for 3
and 4, the topological evaluation of this net results in a sql
topology.16 It is worth noting that the stacking interactions motif
presented in 8 is similar to that found in 4, indicating a weak
strength stacking interaction.14 In 8, the distance between the
centroids of the aromatic rings is 4.101(3) Å, with a projection
distance of 3.468(2) Å and a high offset of 2.190 Å.
Single crystal diffraction studies showed that compounds 9
and 10 are isostructural; thus only compound 9 will be further
described. Compound 9 is composed of 1D [Zn(IBG)(H2O)3]n
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Table 2 Diagnostic IR bands of compounds 1–10 and the potassium salt of the ligand (1K)
Compound
ν(NH)-CONH
ν(CO2)as
ν(CvO)-CONH
ν(CO2)s
Δ (cm−1
)
1
1K
2
3
4
5
6
7
8
3287
3356
3324
3398
3402
3333
3350
3343
3364
3348
3348
1713
1583
1578
1573
1576
1580
1582
1584
1584
1589
1589
1643
1638
1636
1633
1646
1634
1651
1653
1654
1630
1630
1320
1410
1398
1396
1400
1395
1406
1408
1407
1397
1397
173
180
177
176
185
176
176
177
192
192
9
10
Table 3 1H and 13C NMR chemical shifts
1
1Na
3
7
10
H(OH)
H(NH)
H(CH2)
C(CH2)
(CvO)amide
(CvO)carb
12.08
8.97
3.95, 3.96
41.24
166.02
171.20
—
8.64
3.95
43.23, 43.34
169.75, 169.82
175.71
—
8.58
3.90, 3.91
43.77, 43.89
169.66, 169.72
176.59
—
8.62
3.85
43.95
169.94
175.09
—
8.58
3.89, 3.90
43.75, 43.87
169.73
176.74
NMR studies
Thermal studies
In order to investigate the solution behavior of the aforemen-
tioned compounds, NMR spectra were also recorded at pH 5 and
in an M(II) : ligand molar ratio 1 : 1. The 1H- and 13C-NMR
chemical shifts of the sodium salt of the ligand (1Na) and the
compounds (3, 7, 10) were recorded in D2O solution and the
ligand in DMSO solution (Table 3).
The powder XRD patterns of the bulk material of all compounds
(Fig. S1–S6†) show that the previously crystallographically
described compounds are the only products of these reactions.
Thermogravimetric analysis (TGA) was carried out on com-
pounds 3, 6 and 10 (Fig. S8†). All water molecules were
successfully removed from all compounds, as indicated by the
lack of a mass loss around 100 °C. The decomposition of
compounds 3, 6 and 10 begins at 275 °C, 284 °C and 274 °C
respectively, resulting in metal oxide as the final residue. The
removal and reabsorption of the lattice and ligated water
molecules of compounds 6–8 is successful (Fig. S4–S6†). Upon
removal of the water molecules in 6, each Co(II) centre adopts
a tetrahedral geometry and when the rehydration procedure is
followed it readopts an octahedral geometry (Fig. S9†).
1
The downfield chemical shift for NH in the H-NMR spec-
trum of the ligand indicates that this H-atom is involved in
H-bonding. The crystal structure of H2IBG (1) shows the pres-
ence of an intermolecular H-bond between the NH group and
the carbonyl O-atom of the amide group of another molecule.
For compounds, 1Na, 3, 7 and 10, the absence of any peak in
the area of 12 ppm in the 1H-NMR spectra indicates the deproto-
nation of the carboxylic group. The downfield chemical shift for
1
NH in the H-NMR spectrum of 1Na, 3, 7 and 10 indicates that
this H-atom is involved in stronger H-bonding. In compounds 3,
7 and 10 the H-atoms of the two methylene groups are shifted to
lower chemical shifts due to the ligation of the ligand, while in
the 13C-NMR spectrum the peaks of the methylene groups are
shifted at higher values. The involvement of the carboxylic
groups in bonding to the metal centres in 3, 7 and 10 was also
confirmed by the chemical shift of the carbon atom of the car-
boxylate groups which were shifted to higher values. In order to
obtain a better understanding of the synthesis of those com-
pounds a 113Cd NMR spectrum was recorded for compound 10
at pH 5. This (Fig. S7†) indicates that the ligand coordinates
with the Cd centre in a chelated fashion; the geometry of
each Cd centre in compound 10 is closely related to that of
Cd(NO3)2·4H2O.21 The crystalline solid which precipitated from
the solution could be identified as compound 10 based on the
powder XRD pattern.
Gas sorption studies
N2 gas sorption experiments were carried out for compounds 3
and 8 with a Quantachrome Autosorb 1-MP. Prior to measure-
ment the samples were outgassed at 95 °C overnight. The
isotherms were run from partial pressure (p/po where po is the
saturation pressure) from 10−5 to 1 p/po (Fig. S10†). The N2
cross section was assumed to be 0.162 nm2. Specific surface
area (SSA) was calculated according to the BET equation22a and
found to be 4.5 m2 g−1 for 3 and 12.0 m2 g−1 for 8. The pore
size distribution was determined following the NL-DFT
method22b using a slit pore geometry and N2 sorption at 77 K
on carbon with an NLDFT equilibrium model resulting
in 0.05 cm3 g−1 for 3 and 0.09 cm3 g−1 for 8 (pore volume
calculated for a maximum pore size of 52 nm). Comparison of
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the fitting error of different NL-DFT models, i.e. slit, cylindrical
or a mixture of both, revealed that the pure slit geometry yielded
the best agreement with the data.
antiferromagnetic interactions arises from a Curie–Weiss analysis
of the high temperature (40–300 K) magnetic susceptibility data
with θ = −43.8 K (Fig. 8 down).
For compound 7, the room temperature χMT value of
2.22 cm3 K mol−1 is slightly lower than the expected value of
2.35 cm3 K mol−1 for two non-interacting NiII ions (with g =
2.17). Upon cooling the value of χMT decreases to a minimum
value of 0.03 cm3 K mol−1 at 5 K, suggesting the presence of a
relatively strong antiferromagnetic interaction. We were able to
simulate the magnetic susceptibility data successfully using a
1 − J model and Hamiltonian (1) that assumes one exchange
interaction between neighboring NiII centres by using the
program MAGPACK25 and employing the Hamiltonian in
eqn (1).
Magnetic studies
Variable temperature dc magnetic susceptibility data were col-
lected for compounds 6 and 7 in the temperature range
5.0–300 K under an applied field of 0.2 T. These are plotted as
χMT versus T in Fig. 8 up. For 6, the room temperature χMT
value of 12.17 cm3 K mol−1 corresponds to two non-interacting
CoII ions with S = 3/2 and g = 2.67. Upon cooling the χMT
value decreases to a minimum value of 0.59 cm3 K mol−1 at
5 K. Given the presence of octahedral CoII ions with a T1g
4
ground term that splits into a doublet ground-state at low temp-
erature when in a distorted environment due to spin–orbit coup-
ling,23 it is very laborious, extremely difficult and risky to apply
an exact theoretical model for fitting the magnetic susceptibility
data.24 In addition, it should be noted that Co(II) ions in octa-
hedral symmetry may be treated as pseudo- “seff = 1/2” systems
at low temperature due to the splitting of the Kramers doublets.
Nevertheless, evidence supporting the presence of dominant
Ĥ ¼ ꢀ2JðŜ1 ꢁ Ŝ2Þ
ð1Þ
This afforded the parameters J = −15.25 cm−1 and g = 2.18.
The antiferromagnetic nature of J may be attributed to the large
Ni–Owater–Ni angle found in 7.
Theoretical studies. Theoretical studies of the “free-standing”
H2IBG and IBG2− molecules
First we optimized the molecular geometries of the “free-stand-
ing” H2IBG (1) ligand and its deprotonated dianionic conjugate
base IBG2− at the PBE0/6-311+G(d,p) level of theory. The equi-
librium geometries along with selected structural parameters are
shown in Fig. S11.†
It can be seen that contrary to 2-fold rotation symmetry of the
crystal structure of the H2IBG (1) ligand the DFT calculations
predict a non-centrosymmetric structure for the “free-standing”
molecule in the gas phase. One of the glycine residues is charac-
terized by the torsion angles C4–C3–N2–C1 and O5–C4–C3–
N2 of 170.6° and −1.9° respectively, while the other by the
torsion angles C4–C3–N2–C1 and O5–C4–C3–N2 of −93.4°
and −179.7° respectively. The molecule is also characterized by
the O–C1–C7–O torsion angle of 4.5°. On the other hand, the
two glycine residues in the IBG2− ligand are characterized by
the torsion angles C4–C3–N2–C1 and O5–C4–C3–N2 of 176.0°
and 0.8° respectively, while the O–C1–C7–O torsion angle
is 0.0°. The carboxylato moieties of the glycine residues in
IBG2− are found in the syn conformation with one of the
oxygen atoms forming a hydrogen bond with the N–H hydrogen
atom.
In the IR spectra of the H2IBG species the diagnostic
νstr(N–H), νas(OvCvO), νstr(CvO)amide, νsym(OvCvO) and
νbend(OvCvO) bands absorb at 3492–3523 cm−1
,
1773–1783 cm−1, 1695–1712 cm−1, 1329–1379 cm−1, and
596–602 cm−1 respectively. In the IR spectrum of IBG2− species
the aforementioned diagnostic bands absorb at 3357–3358 cm−1
,
1654–1657 cm−1, 1661–1669 cm−1, 1335, 1336 cm−1 and
638–653 cm−1 respectively. The IR spectra of the “free-stand-
ing” H2IBG and IBG2− species are in good agreement with the
experimentally obtained IR spectra of 1 and 1K molecules.
A noteworthy point is the excellent agreement of the 1H
and 13C NMR chemical shifts (δ, ppm) calculated at the
GIAO-PBE0/6-311+G(d,p) level listed in Fig. S12† (compare
the values given in Table 3 with the calculated ones).
Fig. 8 (up) Plot of χMT vs. T for complexes 6 (green triangles) and 7
(black squares). The solid line represents a fit of the data in the tempera-
ture range 5–300 K (see text for details) (down) Curie–Weiss plot for
complex 6 for the 40–300 K range.
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adopting the anti configuration three hydrogen bonds are formed
and this could be the reason for the preferred syn configuration
of the carbonyl groups of the coordinated glycine ligands.
The hydrogen bonds formed in the Mn(glycine)2(H2O)4 and
Cu(glycine)2(H2O)4 complexes are clearly shown in the 3D plots
of the reduced density gradient (RDG) shown in Fig. S13.†
The optimized geometry of the Cu(glycine)2(H2O)4 complex
corresponds to a rather regular centro-symmetric octahedron with
large Jahn–Teller distortion. This structure closely resembles the
crystal structure of complex 8. Two of the coordinated water
molecules occupy the axial positions having Cu–O bond lengths
of 2.423 Å, the other two occupy trans equatorial positions with
Cu–O bond lengths of 2.054 Å. The two glycine ligands are
coordinated to the CuII metal centre in a monodentate fashion,
their carbonyl groups adopting the anti configuration. The co-
ordinated carboxylato groups retain the single C–O and double
CvO bond character with bond lengths of 1.267 Å and 1.224 Å
respectively. All attempts to locate on the potential energy sur-
faces (PES) the structure involving the two glycine ligands
having their carbonyl groups in syn configuration starting from
such geometry were unsuccessful due to convergence problems
for a variety of functionals and basis sets tested. Finally, it is
worth noting that the spin density is almost totally located on the
central Mn and Cu metal atoms (Fig. 9).
Fig. 9 Equilibrium geometries of the Mn(glycine)2(H2O)4 and Cu(gly-
cine)2(H2O)4 complexes calculated at the DSD-PBEP86/Def2-TZVP
level.
Theoretical studies of the model Mn(glycine)2(H2O)4 and
Cu(glycine)2(H2O)4 complexes
To explain the preferred syn and anti configurations of the carbo-
nyl groups of the two trans coordinated IBG ligands in the
[Mn(IBG)2(H2O)4]n (5) and [Cu(IBG)(μ-H2O)(H2O)2]n (8) com-
plexes respectively, we performed DFT calculations on the
model Mn(glycine)2(H2O)4 and Cu(glycine)2(H2O)4 complexes.
The equilibrium geometries of the Mn(glycine)2(H2O)4 and
Cu(glycine)2(H2O)4 complexes calculated at the DSD-PBEP86/
Def2-TZVP level of theory are shown in Fig. 9. The 3D plots of
the total spin density are also shown in Fig. 9. DSD-PBEP86 is
a recently developed functional that includes spin-component
scaled MP2 and dispersion corrections.10a
Perusal of Fig. 9 reveals that for the Mn(glycine)2(H2O)4
complex two different low energy structures are obtained at the
DSD-PBEP86/Def2-TZVP level depending on the starting geo-
metry chosen. Structure A results from a starting geometry
resembling the crystal structure of 5, while structure B results
from a starting geometry involving the two coordinated glycine
ligands with their carbonyl groups in anti configuration. Struc-
ture B is slightly more stable (by 0.8 kcal mol−1) than structure
A. Both structures are non-centrosymmetric with the two carbo-
nyl groups of the coordinated glycine ligands adopting the syn
configuration closely resembling the crystal structure of 5. All
the Mn–O bonds are different with the bond lengths found in the
range 2.075–2.365 in excellent agreement with the Mn–O bond
lengths in the crystal structure of 5. As in the case of complex 5
the two carboxylato moieties have different structural para-
meters, the first retains the double CvO and single C–O bond
character with the C(6)–O(7) and C(6)–O(5) bond lengths in A
(B) being 1.245(1.240) Å and 1.272(1.277) Å respectively,
while in the second carboxylato group both C–O bonds acquire
a partially double bond character with the C(3)–O(4) and C(3)–
To unveil the reason for the preferred configurations of the
coordinated glycine ligands in the Mn(glycine)2(H2O)4 and
Cu(glycine)2(H2O)4 complexes we calculated the potential
energy surfaces (PES) for the adiabatic rotation of one of the
glycine ligands around the Mn–O and Cu–O bonds respectively
(Fig. 10).
Perusal of Fig. 10 reveals that the PES of the Mn(glycine)2-
(H2O)4 complex is characterized by three minima corresponding
to structures with torsion angles φ of 30°, 140° and 220°. The
global minimum corresponds to the structure with φ = 220°
involving the two trans coordinated glycine ligands with the car-
bonyl groups in syn configuration closely resembling the struc-
ture of complex 5. The lowest energy structure involving the two
trans coordinated glycine ligands with the carbonyl groups in
anti configuration corresponds to the structure with φ = 30°
which is predicted to be 23.8 kcal mol−1 higher in energy than
the global minimum structure. Here it is noteworthy that in the
global minimum structure four hydrogen bonds are formed while
in the structure with φ = 30° only one hydrogen bond shown in
Fig. 10 with an O⋯H distance of 1.625 Å is formed. The higher
number of hydrogen bonds formed in the global minimum struc-
ture relative to the structure with φ = 30° should be the reason
for stabilizing the observed structure with the two trans coordi-
nated glycine ligands with the carbonyl groups preferring the
syn configuration. Also, in the low energy structure occurring at
φ = 140° which is found to be 21.1 kcal mol−1 higher in energy
than the global minimum structure one hydrogen bond is
formed. The global maximum (saddle point at φ = 80°) is found
to be 85.9 kcal mol−1 higher in energy than the global
minimum. It is worth noting that the predicted high rotational
barrier prohibits any free rotation of the glycine ligand and there-
fore only the global minimum structure would exist.
O(2) bond lengths in A(B) being 1.255(1.258)
Å and
1.264(1.261) Å respectively.
It is noteworthy that four hydrogen bonds are present in struc-
tures A and B stabilizing the complexes. In a structure involving
the trans coordinated glycine ligands with their carbonyl groups
The PES for the adiabatic rotation of one of the glycine
ligands around the Cu–O bond in the Cu(glycine)2(H2O)4
complex (Fig. S12†) is characterized by four minima and four
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Fig. 10 Potential energy surface for the adiabatic rotation of one of the glycine ligands around the Mn–O and Cu–O bonds in the Mn(glycine)2-
(H2O)4 and Cu(glycine)2(H2O)4 complexes respectively calculated at the DSD-PBEP86/Def2-TZVP level.
Table 4 Summary of herein reported results
Compound
Ligand conformation
Coordination mode
Coordination number
Coordination geometry
Dimension
2
3
4
5
6
7
8
9
atsta
atsta
btsta
atstb
atstb
atstb
atstb
atstb
atstb
None
6
8
9
6
6
6
6
5
5
Octahedron
0D
2D
2D
1D
2D
2D
2D
1D
1D
Chelate-bridging
Chelate-bridging
Monodentate
Monodentate
Monodentate
Monodentate
Monodentate
Monodentate
Dodecahedron/square antiprism
Tricapped trigonal prism
cis-Octahedron
trans-Octahedron
trans-Octahedron
trans-Octahedron
Trigonal bipyramidal
Trigonal bipyramidal
10
saddle points. The global minimum corresponds to the structure
with φ = 160° involving the two trans coordinated glycine
ligands with the carbonyl groups in anti configuration closely
resembling the structure of complex 8. The structure involving
the two trans coordinated glycine ligands with the carbonyl
groups in syn configuration corresponds to the structure
with φ = 10° which is predicted to be the global maximum
15.6 kcal mol−1 higher in energy than the global minimum struc-
ture. It is clear that the global minimum structure is the preferred
structure for the Cu(glycine)2(H2O)4 complex rather than the
structure with φ = 10° involving the two trans coordinated
glycine ligands with the carbonyl groups in syn configuration.
However, the relatively low rotational barrier of 15.6 kcal mol−1
allows rotation of the glycine ligands. Therefore low energy
structures of the Cu(glycine)2(H2O)4 complex corresponding to
minima at 60°, 210° and 320° could also be isolated. In this case
it can be noted that there are two hydrogen bonds in the global
minimum structure but only one in the global maximum struc-
ture, thus explaining the preferred anti configuration of the
carbonyl groups of the two trans coordinated glycine ligands in
the Cu(glycine)2(H2O)4 complex in line with the experimentally
determined structure of complex 8.
Discussion
The results presented herein are summarized in Table 4. In the
present study the coordination abilities of H2IBG along with
alkaline earth, transition metal elements, Zn(II) and Cd(II) were
investigated. The different coordination behaviour of the ligand
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Dalton Trans., 2012, 41, 12501–12513 | 12511

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observed in compounds 2, 3 and 4 can be attributed to the differ-
ent radii of the alkaline earth element. In compound 2 the ligand
is isolated as a simple anion; this may be attributed to the fact
that the radius of Mg(II) (coordination number 6) is rather small
to accommodate the ligand and therefore is coordinated by six
water molecules. In contrast, compounds 3 and 4 are 2D coordi-
nation polymers; the ligand shows similar coordination bis-
chelating-bridging behaviour but different conformations, while
Ca(II) and Sr(II) have coordination numbers 8 and 9 in 3 and 4,
respectively. A further increase in the radius of the alkaline earth
by using Ba(II) (ligand conformation btsta, coordination number
9, geometry capped square antiprism, two different coordination
modes; chelating-bridging and bidentate) does not increase the
dimensionality of the final motif which is also a 2D layer struc-
ture motif although the structure is not reported here due to the
poor quality of the crystals.
The ligand shows similar coordination mode (bis-monoden-
tate) and conformation in compounds 5–10, however there are
basic differences in the shape of the final motif. A comparison
with our previous studies with H2TBG6a,c (where R1 = p-C6H4
and R2 = CH2, Scheme 1) along with a literature survey for the
coordination abilities of hippuric acid26 (where R1 = C6H5, R2 =
CH2, Scheme 1), a ligand which can be considered as the
“parent” of H2TBG and H2IBG, shows that there is a systematic
preference of ligation to the 3d transition, Zn(II) and Cd(II) metal
elements. When Fe(II), Co(II), Ni(II) or Cu(II) metal centres are
ligated to the aforementioned ligands, then each metal centre
adopts a trans-octahedron geometry, while the axial positions of
the metal centre are occupied by bridging water molecules. In
contrast, when Mn(II) is ligated to this type of ligand a cis octa-
hedral geometry is adopted. Theoretical studies were carried out
for compounds 5 and 8 indicating that the difference in the
coordination geometry of the metal centres in 5 and 8 is the
result of the different number of hydrogen bonds formed within
the crystal structures. Finally, Zn(II) and Cd(II) prefer a trigonal
pyramidal geometry (coordination number 5) which is in agree-
ment with previous results.6c
have presented a detailed study of the coordination abilities of
H2IBG towards alkaline earth, 3d transition, Zn(II) and Cd(II)
metal elements. It appears that apart from the coordination
number of the metal centre, there are several factors affecting the
network dimensionality for the room temperature reactions.
Having gained these insights into the coordination behaviour of
this ligand, our following efforts will be focused on increasing
the dimensionality and porosity of the system using pillars such
as 4,4-bipyridine.
Acknowledgements
The authors would like to thank Sven Stahl from the Institute of
Nanotechnology at the Karlsruhe Institute of Technology for
performing the elemental analysis, thermogravimetric analysis,
NMR measurements and differential thermal analysis
experiments.
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HCH2COO) unit on the aromatic scaffold from para (where
R1 = p-C6H4, R2 = CH2, Scheme 1) to meta (where R1
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