A hydrogen bonded Co(ii) coordination complex and a triply interpenetrating 3-D manganese(ii) coordination polymer from diaza crown ether with dibenzoate sidearms

Article abstract of DOI:10.1039/c6ce00360e

The diaza crown ether dicarboxylate ligand, 4,4′-((1,7-dioxa-4,10-diazacyclododecane-4,10-diyl)bis(methylene)dibenzoate), L, forms a monomeric coordination complex of Co(ii) ions, CoL(H₂O)₂·2H₂O, 1, and a coordination polymer (MnL·H₂O)_n, 2, with Mn(ii) ions under hydrothermal conditions. The monomeric coordination complex (structure 1) is polar with mirror symmetry and crystallizes in the non-centrosymmetric space group Cm. The mirror plane bisects the complex at the six-coordinate Co(ii) ion, the two oxygen atoms of the crown moiety and the two oxygen atoms from coordinated water molecules. The coordinated water molecules take part in strong, linear hydrogen bonds with carboxylate oxygens provided by neighboring Co(ii)-crown complexes resulting in a three-dimensional (1 : 1)_n polar network in which the topology of the underlying 8-coordinated net is sqc3. The structure of 2 crystallizes in the orthorhombic Fdd2 space group as an infinite, polar triply interpenetrating three-dimensional network. The eight-coordinate Mn(ii) ion coordinates two oxygen and two nitrogen atoms of the crown moiety, as well as single carboxylate O atom from each of two neighboring ligands. In both structures, the ligand assumes a flexed-wing bird shape, with the two benzoate sidearms of the crown moiety locked in a syn orientation. The metal ion elevated above the plane of the diaza-crown oxygen and nitrogen atoms. The diaza-crown moiety with its two benzoate sidearms has the peculiar property of forming an oriented crystal structure where the metal-crown vectors are oriented parallel to each other in the crystal. However, the structures are achiral, but the rigid crystal lattice prevents re-orientation of the structure through inversion. The coordination polymer (MnL·H₂O)_n, 2, is a 3-center uninodal net with ths (ThSi₂) topology. Computational results using the density functional theory (DFT) calculations explain why the flexed-wing bird shape is the preferred and most stable ligand conformation for metal ion binding. Both structures demonstrate magnetic properties that are characteristics of the respective non-interacting isolated paramagnetic transition metal ions that are present.

Full text of DOI:10.1039/c6ce00360e

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DOI: 10.1039/C6CE00360E
A Hydrogen Bonded Co(II) Coordination Complex and a Triply Interpenetrating 3-D
Manganese(II) Coordination Polymer from Diaza Crown Ether with Dibenzoate Sidearms
Liang Liao§, Conrad W. Ingram§*, John Bacsa†, Z. John Zhang‡, Tandabany Dinadayalane 
§ Center for Functional Nanoscale Materials, Department of Chemistry, Clark Atlanta
University, 223 James P. Brawley Drive, Atlanta, GA 30314, †Department of Chemistry, Emory
University, Atlanta, GA 30332, ‡ School of Chemistry and Biochemistry, Georgia Institute of
Technology, Atlanta, GA 30322
*Corresponding author: Tel.: +1 404-880-6898; E-mail address:cingram@cau.edu
ABSTRACT
The diaza crown ether dicarboxylate ligand, 4,4'-((1,7-dioxa-4,10-diazacyclododecane-
4,10-diyl)bis(methylene))dibenzoate), L, forms a monomeric coordination complex of Co(II)
ions, CoL(H2O)2•2H2O, 1, and a coordination polymer (MnL•H2O)n, 2, with Mn(II) ions under
hydrothermal conditions. The monomeric coordination complex (Structure 1) is polar with
mirror symmetry and crystallizes in the non-centrosymmetric space group Cm. The mirror plane
bisects the complex at the six-coordinate Co(II) ion, the two oxygen atoms of the crown moiety
and the two oxygen atoms from coordinated water molecules. The coordinated water molecules
take part in strong, linear hydrogen bonds with carboxylate oxygens provided by neighboring
Co(II)-crown complexes resulting in a three-dimensional (1:1)n polar network in which the
topology of the underlying 8-coordinated net is sqc3. The structure of 2 crystallizes in the
orthorhombic Fdd2 space group as an infinite, polar triply interpenetrating three-dimensional
network. The eight-coordinate Mn(II) ion coordinates two oxygen and two nitrogen atoms of
the crown moiety, as well as single carboxylate O atom from each of two neighboring ligands.
In both structures, the ligand assumes a flexed-wing bird shape, with the two benzoate sidearms
of the crown moiety locked in a syn orientation. The metal ion elevated above the plane of the
diaza-crown oxygen and nitrogen atoms. The diaza-crown moiety with its two benzoate
sidearms has the peculiar property of forming an oriented crystal structure where the metal-
crown vectors are oriented parallel to each other in the crystal. However, the structures are
achiral, but the rigid crystal lattice prevents re-orientation of the structure through inversion.
The coordination polymer (MnL•H2O)n, 2, is a 3-center uninodal net with ths (ThSi2) topology.
Density Functional Theory calculations explain why the flexed-wing bird shape is the preferred
and most stable ligand conformation for metal ion binding. Both structures demonstrate

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magnetic properties that are characteristic of the respective non-interacting isolated
paramagnetic transition metal ions that are present.
INTRODUCTION
Coordination polymers (CPs) and metal-organic frameworks (MOFs), whose 1-D, 2-D
or 3-D structures are based on extended networks of coordinatively bonded metal ions and
multidentate organic linkers, are of great interest for applications in heterogeneous catalysis,1
adsorption,2 and molecular separation3 among many others. The overwhelming approach to the
assembly of the structures involves coordinating the extended functional groups of the organic
linkers with individual metal ions or metal clusters. Much less explored is the positioning of
additional metal ions within the center of the organic linkers, thus generating a metalloligand.
This approach can lead to an increase in the number and types of metal coordination sites, and
also to increased diversity in the resulting CP and MOF structures.
The use of metalloligand-based linkers to synthesize CPs and MOFs have been scarcely
reported, 4 examples of which include, Ag-tris(dipyrrinato), 5 Mn metalloporphyrins, 6 Cu-
pyridine-2,4-dicarboxylate and Cu-sulfosalicylideneglycylglycine,7 Fe, Ru, Os porphyrinato and
8
phthalocyaninato macrocycles,
4,4’-biphenyldicarboxylate–Mn(II) chiral Schiff base
9
10
11
metalloligand, Pt2+ bypyridyl carboxylate
and Pd organometallic complexes
.
A
macrocyclic polyamine tetraacetic acid was recently used to synthesize heterometallic MOFs.12
Ligands containing a crown ether core are potentially useful platforms for isolating the
metal ions in center of the linker. Batten and coworkers synthesized 1-D and 2-D coordination
polymers using N,N'-bis(4-pyridyl-methyl) diaza-18-crown-6 in which the 4-pyridyl-methyl
side arms coordinates Co2+, Fe2+ and Ag+ metal ions, and with the crown moiety simultaneously
coordinating the s-block metal ions, K+, Ba2+ or Ca2+.13 We have been investigating the use
diaza crown ether polycarboxylates as platforms to coordinate and isolate transition metal ions
within their crown moieties, while simultaneously directing the self-assembly of the CP and
MOFs structures.1416 It was observed that the dimensionality and connectivity of the CP
structures obtained were influenced by the number and position of carboxylate groups present.
We observed that the presence of four carboxylates groups (two on each side of the diaza crown
moiety and in meta positions on each benzene ring) resulted in a triply interpenetrating 3-D
2

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structure when the ligand combines with Co (II) ions. With two carboxylates (one on each side
of the diaza crown moiety and in meta position on each benzene ring), 2-D layered and 1-D
chain structures were obtained when the ligand combines with Co (II) and Zn (II) ions.
We herein modified the diaza crown ether dibenzoate ligand by placing the carboxylate
groups in para position on each benzene ring to generate 4,4'-((1,7-dioxa-4,10-
diazacyclododecane-4,10-diyl)bis(methylene))dibenzoate), LH2.
LH2
The reaction of LH2 with Co (II) ions yields a coordination complex, 1, while the
reaction of the ligand and Mn(II) ions yields a triply interpenetrating 3-D coordination polymer,
2. Both 1 and 2 have their respective metal ions encapsulated within the ligand’s diaza crown-4
moiety. Structure 1 is composed of CoL.H2O units that are interconnected into a 3-D (1:1)n
polymer-like network through hydrogen bonding. The ligand’s carboxylate O atoms form
strong hydrogen bonds with the H atoms of water molecules that are coordinated to Co(II) ions
that are encapsulated in the center of neighboring ligands. For 2, the 3-D connectivity is
achieved as the encapsulated Mn(II) ion in one ligand unit is further coordinated by a single
carboxylate O atom from each of two neighboring ligands. In both structures, the ligand
assumes a flexed-wing bird shape configuration with the two benzoate side-arms of the crown
moiety locked in syn orientation. DFT calculations show this to be the thermodynamically
preferred conformation of this ligand.
EXPERIMENTAL SECTION
Materials: Chemicals and solvents are commercially available and were purchased and used
without further purification.
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Synthesis of dimethyl 4,4'-((1,7-dioxa-4,10-diazacyclododecane-4,10-diyl)bis(methylene))
dibenzoate (A)
Methyl 3-(bromomethyl)benzoate (0.378g, 2.17 mmol) and sodium carbonate (0.68 g,
6.42 mmol) were added to a solution of 1,7-diaza-12-crown-4 (0.189 g, 1.09 mmol) in
acetonitrile (15 mL) and the mixture was refluxed with stirring for 1 d. The concentrated filtrate
of the reaction mixture produced title product after crystallization in acetonitrile. Pale yellow
crystals (0.435 g, yield 85 %). Chemical formula: C24H30N2O6 (FW: 442.51). 1H NMR (400
MHz, CDCl3) : 2.73 (8H, t, J = 4.0 Hz, OCH2CH2N), 3.57 (8H, t, J = 4.0 Hz, OCH2CH2N),
3.69(4H, s NCH2Ar), 3.90( 6H, s, OCH3), 7.50 (4H, d, J = 7.5 Hz, ArH), 7.99 (4H, d, J = 7.5
13
Hz, ArH). C NMR (100 MHz, CDCl3) d: 52.1(OCH3), 55.3 (NCH2CH2O), 60.8 (NCH2Ar),
69.7(NCH2CH2O), 128.8(Ar-C), 128.9(Ar-CCO2), 129.7(Ar-C), 145.5(Ar-CCH2N),167.2
(CO2CH3).
Synthesis of 4,4'-((1,7-dioxa-4,10-diazacyclododecane-4,10-diyl)bis(methylene)) dibenzoic acid,
LH2
Compound A (0.2303 g, 0.489 mmol) was reacted with HCl (aq. 6 N, 2.56 mL) at 90 °C
for 6 h with stirring. The white crystals were collected and then recrystallized in water, filtered
and dried in air at ambient temperature for 1 d to produce LH2 as its chloride salt (0.193 g, yield
89 %) with chemical formula, C24H30N2O6∙2HCl∙nH2O. Elem. anal.∙ calcd.∙%: C, 47.61; H, 6.99;
1
N, 4.63. Found %: C, 47.44; H, 6.80; N, 4.57. H NMR (400 MHz, D2O): 3.35-3.55 (m, 4 H),
3.55-3.68 (m, 4 H), 3.68-3.77 (m, 4 H), 3.77-3.91 (m, 4 H), 4.58 (s, 4 H, ArCH2-N), 7.71 (d,
J=8.0 Hz, 4H, ArH3,5), 8.12 (d, J=8.0 Hz, 4H, ArH2,6); 1H NMR (400 MHz, DMSO-d6): 2.80-
3.73 (multi, 16 H, N-CH2CH2-O), 4.53 (br., 4H, ArCH2-N), 7.58-8.10 (multi, 8H, Ar-H), 10.35
(2H, COOH). 13C NMR (100 MHz, D2O)): 55.5 (N-CH2CH2-O), 59.7 (N-CH2-Ar), 64.1(N-
CH2CH2-O), 130.2(C3 and C5 of Ar), 132.4(C2 and C6 of Ar), 132.0(C1 of ArCOOH), 132.7(C4
of ArCOOH), 170.0(COOH). 13C NMR (100 MHz, DMSO-d6): 52.93 (N-CH2CH2-O), 59.05
(N-CH2-Ar), 64. 35 (N-CH2CH2-O), 129.30 (C3 and C5 of Ar), 131.93 (C2 and C6 of Ar),
133.05 (C2 and C6 of Ar), 133.68 (C1 of ArCOOH), 166.78 (COOH). FTIR (cm-1): 426(w),
454(w), 498(m), 585(w), 636(w), 705(m), 727(m), 768(w), 784(m), 808(w), 857(w), 878(w),
902(w), 955(m), 969(w), 1021(w), 1054(m), 1079(m), 1109(s), 1131(s), 1159(w), 1183(m),
4

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1227(m), 1273(m), 1322(w), 1355(w), 1374(m), 1418(m), 1452(w), 1513(w), 1577(w), 1615(w),
1641(w), 1702(s), 2500-3150(broad, COOH), 3305.
Synthesis of coordination complex 1
A mixture of ligand, LH2, (7.1 mg, 0.016 mmol), Co(AcO)2 4H2O(14.7 mg, 0.059 mmol),
pyridine (16 µL, 0.19 mmol) and water (10 mL, 0.56 mol) was sealed in a Teflon lined
autoclave, heated at 125 °C for 3 d in an oven and then cooled to ambient temperature. The red
crystals were collected and washed with water to yield structure 1 (5.8 mg, 80.2 % yield based
on LH2) with formula C24H32CoN2O8. Elem. anal. C24H28CoN2O6•2(H2O) (FW535.44). calcd.
(wt %): C, 53.83; H, 6.02; N, 5.23; Found: C, 52.60; H, 5.85; N, 5.02. FTIR (cm-1): 408(w),
458(w), 513(w), 566(w),591(w), 717(s), 741(m), 774(m), 797(m), 851(m), 944(m), 980(w),
1011(w), 1033(w),1046(w), 1075(w), 1092(m),1104(m), 1175(w), 1375(s), 1456(w), 1547(m),
1595(m),2356(w), 2908(w), 2949(w), 2993(w).
Synthesis of coordination polymer 2
A mixture of LH2 chloride salt (9.8 mg, 0.018 mmol), Mn(AcO)2∙4H2O (29.4 mg, 0.12
mmol), pyridine (80 µL, 0.94 mmol) and H2O (15 mL, 0.83 mol) was sealed in a Teflon lined
autoclave, heated at 125 °C for 3 d in an oven and then cooled to ambient temperature. The
yellowish crystals were collected and washed with water to give 2 (9.4 mg, 95 % yield based on
LH2) with formula C24H28MnN2O6•2(H2O) (FW 531.45); Elem. anal. calcd. (wt %): C, 54.24; H,
6.07; N, 5.27; Found: C, 53.42; H,5.82; N, 5.21. FTIR (cm-1): 401(m), 465(m), 513(m), 530(m),
587(m), 611(w), 720(m), 740(m), 783(s), 814(m), 838(w), 852(w), 887(w), 932(w), 959(m),
1017(w), 1027(m), 1051(w), 1081(s), 1076(s), 1104(w), 1137(w), 1156(w), 1182(w), 1257(w),
1286(w), 1394(w), 1380(s), 1441(w), 1470(w), 1481(w), 1554(m), 1598(s), 1667(m), 2600-
3100, 3475(m, broad).
Characterization:
13
1H and C NMR spectra were acquired on a Bruker AVANCE400 spectrometer at 400
MHz and 100 MHz, respectively (peak pattern: d-doublet, m-multilet, q-quartet, s-singlet, t-
triplet). Infrared measurements were recorded on a Bruker Alpha-P FTIR spectrophotometer
(intensive pattern: m-medium, s-strong, w-weak). Single crystal X-ray data were collected on a
Bruker APEX2 diffractometer with 1.6 kW graphite monochromated Mo radiation (fine-focus
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sealed tube, 45 kV, 35 mA).. The frames were integrated with the SAINT v7.68a.17 The structure
18
was solved and refined with Olex2
and SHELX. 19 Structure refinement and other
experimental details are presented in Table S1. Powder X-Ray diffraction pattern was recorded
on a Panalytical Emperian Series II diffractometer with CuKα radiation source at 45kV and 40
mA, time per step 2s, and step size of 0.04. Thermogravimetric analysis was conducted on a TA
Instrument Q50 thermal analyzer at heating rate of 5 ºC/min from ambient temperature to 700
ºC under air flow. Temperature dependent magnetization measurements were carried out on a
superconducting quantum interference device (SQUID) magnetometer (Quantum Design
MPMS-5S). All samples for the magnetic measurements were prepared by fully dispersing an
appropriate amount of dry powder nanocrystals in eicosane.
RESULTS AND DISCUSSION
Ligand LH2 was synthesized through reaction of 1,7-diaza-12-crown-4 with
methyl 3-(bromomethyl)benzoate in the presence of sodium carbonate in acetonitrile under
reflux, followed by treatment with aqueous hydrochloric acid. Upon HCl treatment, the ligand
crystallized as the chloride salt in orthorhombic space group, Pca21, and with unit cell
dimensions, a = 10.6102(10) Å, b = 21.855(2) Å, c = 22.183(2) Å. Other crystallographic and
structure refinement data are presented in Table 1 (and Electronic Supplementary Information
ESI Tables S12-S17). The ligand assumes a flexed-wing bird shape configuration, the
orientation of which appears to be stabilized by the electrostatic bonding of the chloride ions to
its diaza crown moiety (ESI Figure S1). The configuration of the ligand is further explored
below in coordination complex 1 and coordination polymer 2, which were formed from the
thermal reaction of the ligand with cobalt (II) acetate and manganese (II) acetate respectively, in
the presence of pyridine and water, and both of which have the ligand’s crown moiety
coordinating metal ions instead chloride.
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Table 1. Crystallographic data and structure refinement for LH2, 1 and 2.
Ligand-LH2chloride
1
2
Empirical formula
Formula weight
C24H34Cl2N2O7
533.22
C24H32CoN2O8
535.44
C24H32MnN2O8
531.45
Temperature/K
100(2)
110(2)
173(2)
Crystal system
Space group
orthorhombic
Pca21
monoclinic
Cm
orthorhombic
Fdd2
a/Å
b/Å
c/Å
α/°
10.6102(10)
21.855(2)
22.183(2)
90
9.7472(9)
22.777(2)
7.0903(7)
90
33.046(4)
9.0309(10)
16.4592(18)
90
β/°
90
90
5143.9(8)
8
131.453(6)
90
1179.8(2)
2
90
90
4912.0(9)
8
γ/°
Vol/Å3
Z
Z’
2
Density(calcd)mg/mm3
Absorption coefficient(mm-1)
F(000)
1.378
0.299
2256
1.507
1.437
0.781
0.589
562
2232
Crystal size/mm3
Θ range for data collection
Index ranges
0.427 × 0.203 × 0.062
2.465 to 29.569°
0.888 × 0.619 × 0.318
1.79 to 37.78°
0.27 x 0.14 x 0.13
1.864 to 27.484°
-13 ≤ h ≤ 14, -29 ≤ k ≤ 29,
-30 ≤ l ≤ 30
-8 ≤ h ≤ 14, -33 ≤ k ≤ 24, - -56 ≤ h ≤ 40, -15 ≤ k ≤
10 ≤ l ≤ 10
4866
2617[R(int) = 0.0368]
2617/5/173
1.020
R1 = 0.0462, wR2 = 0.0940 R1=0.0489,wR2= 0.1259
R1 = 0.0539, wR2 = 0.0975 R1=0.0523,wR2= 0.1299
1.01/-0.50
0.032(18)
13, -22 ≤ l ≤ 25
15648
4964[R(int) = 0.0335]
4964/14/186
1.060
36384
11495[R(int) = 0.0877]
8549/8/650
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å-3
Flack parameter
1.028
R1 = 0.0641, wR2 = 0.1327
R1 = 0.0970, wR2 = 0.1515
1.202/-0.348
1.03/-0.33
0.01(2)
0.28(9)
Crystal structure of 1
Compound 1 crystallizes in the monoclinic space group Cm as a polymer-like (1:1)n
polar but achiral complex. Crystallography data are presented in Table 1. Bond angles, bond
lengths and other crystallographic data are presented in ESI Tables S1-S6. The powdered X-
Ray diffraction pattern of 1 matches that of its simulated pattern from single crystal diffraction
analysis, indicating that the structure crystallizes as a pure phase (ESI Figure S2a). The
coordination complex is comprised of the deprotonated ligand, L, and a single Co (II) ion that is
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coordinated by the four donor atoms (two N and two O atoms) of the 1,7-diaza-12-crown-4
moiety (Figure 1). In order to satisfy its six coordinate octahedral environment, the Co(II)
elevates above the crown to form two bonds to water molecules. The four ligating atoms of the
crown distort the octahedral geometry. The Co-N(1) bond length is 2.242(3) Å and Co-O bond
lengths ranged from 2.114 (3) to 2.118(3) Å. The crown based angle between oxygen atoms
(O1-Co1-O2) is 87.1(2)º The angle of the coordinated oxygen atom and the Co-N axis (O-Co-
N) is 101.86(8)° and 99.40(8)° for the second water molecule.
Figure 1. Structure unit of 1.
The two benzoate sidearms of the L are tethered in a syn conformation, and therefore the
overall metalloligand with the encapsulated metal ion, assumes a flexed-winged bird shape
configuration, similar to that observed in the chloride salt of the pure ligand, described above.
The angle between the benzene ring and the crown at the methylene group (angle N1-C5-C6) is
115.0(3)°. There are multiple conformers that can be accommodated by this angle as the
molecular modelling studies below show. However, it is this ligand conformation that is
observed here, in 2 discussed below, and in other 1-D, 2-D, and 3-D coordination polymers with
a similar ligand L but containing different number and locations of the carboxylate groups that
we previously reported.14-16 It is also observed in a K-diaza-18-crown-6-ether coordination
complex containing partially-fluorinated benzyl sidearms,20 and seems to suggest that this is
thermodynamically the most preferred conformation of the ligand. However, further
stabilization of this conformer by coordination to Co(II) ions is quite likely. The N(1)-Co(1)-
N(1) angle is 147.17(17)º and the Co(II) ion, is elevated above the crown, presumably due to the
small size of the crown. However the cobalt atom remains below the methylene carbons of the
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sidearms. This is also observed in our prior reported 1-D, 2-D and 3-D coordination network
structures with meta located carboxylates.
Structure 1 crystallizes as a coordination compound instead of a covalently bonded
network for unclear reasons, but is most likely due to a delicate balance between the para
location of the carboxylate groups and the size, electronic environment and elevation of Co(II)
ion. However, the extended hydrogen bonded network, with its very strong and linear hydrogen
bonds between the carboxylate groups and the coordinated water does resemble a coordination
polymer. This arrangement makes sense from a steric and bond valence perspective. The
coordinated water forms bonds of 0.5 v.u. to each Co(II) and two external hydrogen bonds of
0.25 v.u. to each oxygen of the carboxylate group. The bond valence sum of the two hydrogen
bonds is what can be expected for a Co-O bond in an arrangement with bridging Co(II) atoms
without intervening water molecules.
The structure 1 consists of infinite chains of hydrogen bonded [CoL(H2O)2] units that lie
on mirror planes perpendicular to the ab-plane. The ligand’s carboxylate O atoms accept strong
hydrogen bonds form water molecules that are coordinated to Co(II) ions encapsulated in the
center of neighboring ligands. Each carboxylate oxygen atom accepts a hydrogen bond from a
unique [CoL(H2O)2] unit. There are four acceptor oxygen atoms and four donor hydrogen
atoms in the [CoL(H2O)2] unit which therefore behaves as 8-coordinate node in the underlying
uni-nodal net. Thus the hydrogen bonding extends the [CoL(H2O)2] units into a three
dimensional (1:1)n network via hydrogen bonding with H atoms of the coordinated water
molecules and O atoms of the carboxylate groups. The position of coordinated water molecules
in the midpoint of the ligand and to O atoms of the carboxylate groups at either end, has an
interesting effect on the stacking of CoL(H2O)2 units and the overall topology of the network
(Figures 2a and 2b). Visually, the structure appears like a flock of birds formation along the c-
direction. The chains are not linear but instead we have a zig-zag arrangement in which the
each [CoL(H2O)2] unit is displaced 11.388(2) Å along the b-direction. In this arrangement, the
H atoms of the water molecules are in ideal positions for donating strong hydrogen bonds to
carboxylate O atoms. This repeat distance is exactly half the length of the b-axis (22.777(2) Å).
The O…H bonds are very short (1.641(7) Å and 1.617(7) Å) and linear (as expected for strong
hydrogen bonds), compared to 1.8 Å commonly reported for normal hydrogen bonds in water
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and in ice. In other polymers the Co atoms are directly bridged by carboxylate O atoms (for
example catena-poly[[diaquacobalt(II)]-l-oxalato]). 21 However, in this arrangement, the
hydrogen bonds are taking the role of bridging Co atoms, and the Co atoms are bridged
indirectly via the water molecules. Thus the shortened bond is a reflection of the significant
strength of the Co-O bond. This suggests that the O-H bonds are weakened considerably to
compensate, and that these protons are acidic. Topological analysis of the underlying net shows
that it is an 8-connected body centered cubic net with symbol sqc3.22
Figure 2a. 3-D hydrogen bonded network of 1 (viewed along c)
Figure 2b. 3-D hydrogen bonded network of 1 (viewed a)
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Crystal structure of 2
Compound 2 is stable in air and in common organic solvents such as methanol, ethanol,
acetonitrile, acetone, tetrahydrofuran, and dimethylformamide. Single-crystal X-ray
crystallography analysis reveals that it is an infinite, polar 3-D coordination network. The
infinite chains lie on twofold symmetry axes parallel to the c-axis in the orthorhombic space
group Fdd2. There are three interpenetrating nets present. Crystallography data for 2 are
presented in Table 1. Bond angles, bond lengths and other crystallographic data are presented in
ESI Table S1and Tables S7-S10. The powdered X-Ray diffraction pattern of 2 matches that of
its simulated pattern from single crystal diffraction analysis, indicating that the structure
crystallizes as a pure phase (ESI Figure S2b).
Figure 3. Representation of the partial structure unit of 2.
Mn(II) coordination environment: The Mn(II) ion is eight coordinate and forms a highly
irregular polyhedron with surrounding atoms (Figure 3). Four bonds are with two N and two O
atoms of the 1,7-diaza-12-crown-4 moiety. The remaining four bonds are with both carboxylate
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O atoms from each of two neighboring ligands. The carboxylate binds asymmetrically; one Mn-
O bond (2.816(3) Å) is significantly longer than the other (2.128(2) Å). It is not clear why this
group forms such an irregular coordination environment but perhaps the resulting tilt is
necessary to form a close packed network. Rather, it could be the result of steric crowding
about the Mn(II) ion since the ideal coordination number is close to six. The Mn-O1, Mn-O3,
Mn-N1 bond lengths are 2.1278(18), 2.3203(19) and 2.393(2) Å, respectively and within the
range reported for Mn-O and Mn-N in coordination polymers.23 The O11-Mn-O1, O31-Mn-O3,
and N11-Mn-N1 bond angles are 99.63(11) o, 110.22(12) o and 120.94(11) o respectively.
Ligand coordination: The conformation of L with the encapsulated Mn(II) ions is shown
in Figures 4. The ligand is octadentate and each ligand complexes three Mn(II) atoms, two
through two terminal carboxylates, and one through four donor atoms from the crown. The
deprotonated ligand encapsulates Mn(II) ions in its diaza-crown moiety and its two benzoate
sidearms further coordinate Mn(II) ions in neighboring ligands. The ligand conformation is
similar to 1, and to those we have reported for Co(II) and Zn(II) based 1-D, 2-D and 3-D
structures in which the carboxylates were in meta positions on the ligand’s benzene rings.14-16
In order to satisfy its eight coordinate environment the Mn(II) elevates above the crown to
accommodate two carboxylate groups and the four ligating atoms of the crown distort the
geometry. As in structure 1, the two benzoate sidearms of L are locked in a syn conformation,
and therefore the overall metalloligand with the encapsulated metal ion in its crown, assumes
the flexed-winged bird shape configuration (Figure 4). Each methylene group (C5) between the
benzene ring and the crown shows a C-C-N angle of 115.4º, hence the degree of curvature of
the benzoate sidearm in 2 is similar to that in 1. The C(8)-N(1)-Mn(1) angle of 108º is due to a
higher elevation of the Mn(II) above the crown in comparison Co(II), and is consistent with the
longer Mn-O and Mn-N bond lengths present.
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.
Figure 4. Packing diagram of 2 viewed along b-axis showing ligand sidearms in syn
configuration (blue-green Mn, red O, grey C, blue N, white H).
.
Figure 5. A portion from one of three 3-D networks of structure 2.
Three dimensional connectivity is achieved by the carboxylate oxygens of one ligand
coordinating Mn(II) ions in crown of two adjacent ligands in bis-bidentate mode (Figures 4 and
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5). Also, each Mn(II) is coordinated by carboxylate oxygens from each of the two neighboring
ligands. A lattice water molecule is present and engages in hydrogen bonding with the
uncoordinated oxygen atoms from the carboxylates of these neighboring ligands, thus further
stabilizing the structure.
The Mn centers can be regarded a 3-connection nodes in the structure. Similarly, the
ligands can be regarded as 3-connection nodes by connecting to one Mn atom through the
crown, and to two Mn atoms through the carboxylate groups. This is a 3-center uninodal net
with ths (ThSi2) topology (Figure 6a). In the figure below the Mn are shown in blue and the 3-
connection nodes of the ligands are shown in red. This net can be further simplified: each
Mn(II) is linked to 4 other Mn(II) ions through the ligands. Thus each Mn can be considered to
be a 4-coordinate node and the ligands as edges of the network. This simplification gives a 4-
connected diamond net with dia topology and 4/6/c1; sqc6 code symbol. However, the 3-center
uninodal net with ths topology is a better description because it contains the polar structure. The
topological analysis with TOPOS22 indicates that structure 2 consist of three interpenetration
vectors (Figure 6b). This is commonly observed in coordination polymers and MOFs.
Figure 6a. The underlying 3-center uninodal net with ths (ThSi2) topology in structure 2 viewed
along the b-axis.
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(i)
(ii)
Figure 6b. Topological representation of the triply interpenetrating 3-D nets of 2 in (i) 2D and
(ii) 3-D projection.
Computational investigation of Co-complex and ligand conformations
The shape of the ligand conformation observed in 1 and 2 as well as in the previously
reported work encouraged us to conduct the computational investigation. Single unit of ligand
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with Co metal ion was studied using the density functional theory (DFT) level. Truhlar’s
Minnesota density functional of M06-2X with the triple- basis set of 6-311+G(d,p) was used
for the geometry optimizations of the complex as well as the conformers of the neutral ligand.24,
25 All the calculations were carried out using Gaussian 09 program package.26 Restricted open-
shell (RO) formalism was used for optimization of the complex. The optimized structure of the
complex with selected geometrical parameters is depicted in Figure 7. The computed bond
distances are in close agreement with the experimental values except for CoN distance.
However, both computation and experiment show that two CoN distances are identical.
Figure 7. Selected bond distances (in Å) and bond angle (in degrees) obtained from the
ROM06-2X/6-311+G(d,p) optimized geometry for the single-unit of cobalt complex 1. The
experimental values obtained by X-ray crystallography are given in parentheses with bold.
Seventeen (17) possible conformers were prepared for the neutral ligand structure (LH2) and
they were initially optimized using the M06-2X with 6-31G(d) basis set. The full geometry
optimizations of the conformers were also carried out at the M06-2X/6-311+G(d,p) level in
order to obtain very reliable results. The optimized structures of all the conformers are given in
the ESI Figure S3 and Table S11. As depicted in Figure 8, the relative energies of the
conformers show similar trend at the two levels and the difference in E between these two
levels is very small or negligible. The conformers of the neutral ligand structure (LH2) contain
two dicarboxylic acid groups that form hydrogen bond. We have also observed additional C-
…
…
…
H O and C-H N hydrogen bonds in some of the conformers. Weak intramolecular C-H 
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interactions in addition to the above-mentioned hydrogen bonds were also observed in few
cases.
M06-2X/6-31G(d)
M06-2X/6-311+G(d,p)
30
25
20
15
10
5
0
p
m
o
C
-
2
LH
Figure 8: The relative energy (E, in kcal/mol) of different conformers computed at the M06-
2X/6-31G(d) and M06-2X/6-311+G(d,p) levels.
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LH2-Comp
LH2-C4
LH2-C5
LH2-C9
L-Comp
L-C5
Figure 9. Structures of selected conformers of the neutral ligand (LH2) and the dicarboxylate
anion optimized at the M06-2X/6-311+G(d,p) level.
Our computational investigation shows that LH2-C5 is the lowest energy structure
among all the conformers explored and it is isoenergetic to LH2-C4. As shown in Figure 9,
…
…
…
these two conformers possess multiple OH O, CH O and CH N hydrogen bonds along
…
with intramolecular weak CH  interactions because of the existence of two six-membered
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rings in close contact to each other. However, the neutral conformer of ligand structure that
resembles the shape in the complex, LH2-Comp is around 7 kcal/mol less stable than the lowest
energy conformer of LH2-C5. It is important to note that the relative energy of the conformer
LH2-C9, in which the basic framework of diaza crown ether unit was altered and with no
…
OH O hydrogen bond within COOH groups, is significantly higher (about 24 kcal/mol)
compared to LH2-C5. Computational results reveal that the low-energy conformers, which
exhibit very similar shape of diaza crown ether framework of the complex, could be
experimentally accessible. After thorough computational exploration of the conformers of the
neutral ligand structure, we selected only the lowest energy conformer and also the conformer
which looks the same in the complex to form dicarboxylate anions and run DFT calculations.
In case of dianions, the ligand conformer in the complex (L-Comp) is more stable than L-C5,
which is the optimized geometry by removing two hydrogens from the lowest energy LH2-C5.
This observed trend is reverse to the neutral conformers. Thus, the dicarboxylate anion prefers
to be in the flexed-wing bird shape for metal ion binding.
Fourier Transform Infrared spectral analysis (LH2, and 2)(cm-1)
The C=O stretching of LH2 is observed at 1702 cm-1 while that of 2 is split into 1598
1-
and 1554 cm-1 ((CO2)) due to the formation of the bonds between Mn(II) and CO2 . The C-O
stretching of LH2 is observed at 1272 and 1227 cm-1, and that of 2 shows a strong absorption at
1380 cm-1. The stretching bands of CH2-O-CH2 and CH2-N-CH2 of the diaza crown moiety are
observed at 1109 and 1131 cm-1 for LH2, respectively. The formation of coordination bonds
with Mn(II) in 2 shift these bands to 1076 and 1081 cm-1, respectively.
Thermal Analysis
The thermal properties of 1 and 2 were assessed from their respective TGA curves
(Figure S4). For structure 1, weight loss event between 170 ºC to 235 ºC of 7.4 % corresponds
to the loss of two coordinated water molecules (calculated 6.7 wt %). Decomposition of the
ligand corresponds to weight loss event between 240 ºC and 443 C, leaving final residue of 13.6
wt % that is attributed to CoO (calculated 14.0 wt %). For structure 2, weight loss event up to
125 ºC of 6.5 wt. % is attributed to loss of hydrogen bonded lattice water molecules. Weight
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loss event between 360 ºC and 460 ºC is attributed to the decomposition the ligand, and the final
residue of 17.1 wt % (calc. 16.4 %) is attributed to manganese dioxide (MnO2).
Magnetic properties
Temperature dependent magnetic susceptibilities were measured between 5 K to 320 K
for 1 and 2. The χ and χT values versus temperature are both plotted, where χ is the molar
magnetic susceptibility per molecular unit (Figure 10a). The χMT remains constant (inset) at ~
0.25 cm3 mol-1K over the temperature range measured.27 The curve obeys the Curie-Weiss law
with a Curie constant of 2.8 for translating into three unpaired electrons. The smaller value of
χMT at room temperature is larger than the expected spin only value of 3.75 cm3 mol-1K for
single independent Co (II) ions with spin S =3/2, and is attributed to spin orbit coupling.
The temperature dependent magnetic susceptibility graph profile of structure 2 is similar
to 1 (Figure 10b). The flat line of the χMT vs temperature plot (inset) indicates that the sample
obeys the Curie-Weiss law, and with a Curie-Weiss constant of 0.4 cm3 mol-1K over the
temperature range investigated. This behavior is typical of isolated paramagnetic Mn (II) ions
with five unpaired d electrons.21, 28
a) Structure 1
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.
b) Structure 2
Figure 10. Magnetic susceptibility vs temperature plots for 1 (a) and 2 (b).
Conclusions
Diaza-12-crown-4 with carboxylate groups located at para positions on its dibenzoate
sidearms, forms a polymer-like (1:1)n polar coordination complex with Co(II) ions, and a 3-D
coordination polymer with Mn(II) ions. The 6-coordinate Co(II) ions are coordinated by the
four ligating atoms of the crown and by the O atoms from water molecules for 1, whereas the 8-
coordinate Mn(II) utilizes carboxylate groups of neighboring ligands, instead of water
molecules for 2. In both structures, the metal ion positioned above the plane of the crown
moiety and the metalloligand assumes a flexed-wing bird shape, with the two benzoate sidearms
of the crown moiety locked in syn conformation. DFT calculations showed this to be the most
preferred and most stable ligand conformation of the deprotonated ligand, even in the absence
of the metal ion, and therefore, the ligand will assume this predictable conformation in its role
as a secondary building unit in synthesis of coordination complex, CPs and MOFs with a
transition metal ion located in its center. Though the ligand conformation is common to both
structures, it remains unclear why 1 crystallizes as a hydrogen bonded coordination compound,
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while 2 is an infinite polar triply interpenetrating 3-D coordination network. Both structures
demonstrate magnetic properties that are characteristic of the respective non-interacting isolated
paramagnetic transition metal ions present.
Crystallographic data for the structural analysis have been deposited with the Cambridge
Crystallographic Data Center as follows: CCDC 1452096 for LH2 chloride salt, CCDC
1419712 for structure 1 and CCDC 1419711 for structure 2. These data can be obtained online
free of charge via http://www.ccdc.cam.ac.uk/pages/Home.aspx (or from Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK, or
www.deposit@ccdc.cam.ac.uk).
Acknowledgement
This work was supported by United States National Science Foundation Grants Nos.
HRD-0630456 and HRD-1305041, United States National Nuclear Security Administration
Grant No. NA0000979, and, United States Department of Energy Grant No. DE-FE0022952.
We thank Daniel Sabo at Georgia Institute of Technology for conducting the magnetic
susceptibility measurements.
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Graphical Abstract
Hydrogen bonded (top) and coordination polymeric (bottom) 3-D structures from diaza 12 crown
4 ether based dicarboxylate ligand and a transition metal ion. The ligand assumes a flexed-
winged bird shape with the benzoate sidearms of the crown moiety locked in a syn orientation.