Co3+-based building blocks with appended phenol and catechol groups: Examples of placing hydrogen-bond donors and acceptors in a single molecule

Article abstract of DOI:10.1021/cg201369g

This work shows the syntheses, structures, and hydrogen-bonding based self-assembly of Co³⁺ coordination complexes containing appended phenol and catechol groups. Placement of hydrogen-bond donors (in O-H groups of phenol and catechol) and acce

Full text of DOI:10.1021/cg201369g

Article
pubs.acs.org/crystal
Co3+-Based Building Blocks with Appended Phenol and Catechol
Groups: Examples of Placing Hydrogen-Bond Donors and Acceptors
in a Single Molecule
Afsar Ali,† Geeta Hundal,‡ and Rajeev Gupta*,†
†Department of Chemistry, University of Delhi, Delhi −110 007 India
‡Department of Chemistry, Guru Nanak Dev University, Amritsar −143 005, Punjab, India
S
* Supporting Information
ABSTRACT: This work shows the syntheses, structures, and hydrogen-bonding based self-assembly of Co3+ coordination
complexes containing appended phenol and catechol groups. Placement of hydrogen-bond donors (in O−H groups of phenol
and catechol) and acceptors (in COamide groups) in a single molecule results in a complementary yet unique self-assembly.
Nangia,11 Suslick,12 Baruah,13 Aoyama and Kobayashi,14 and
INTRODUCTION
The rational design of molecular solids has attracted
■
others.15 In particular, Tominaga et al.16 have shown the
generation of predictable networks due to the mechanically
considerable interest in the area of designed and functional
materials.1,2 Such solids hold promise as microporous materials
robust and conformationally well-defined nature of a few
adamantane-based phenol derivatives in conjunction with
with applications in gas storage, separation, sensing, and
catalysis.3,4 A good number of examples are available utilizing
cooperative interactions.
Our research has been focusing on developing coordination
complexes as the building blocks for the construction of
the concept of spontaneous self-assembly. Such a self-assembly
has been achieved taking advantage of either metal−ligand
ordered structures.17 A coordination complex as the building
block offers structural rigidity, and such an induced rigidity has
the ability to place the auxiliary functional groups into a
preorganized conformation. Such auxiliary functional groups
could then be utilized to either coordinate a secondary metal
ion or be involved in the self-assembly. The first synthetic
strategy has allowed us to prepare the {Mn+−Co3+−Mn+}
heterobimetallic complexes (Mn+ = Zn2+, Cd2+, Hg2+, and
Cu+)17a−c and {Co3+−Zn2+}17e or {Co3+−Cd2+}17f networks of
a highly ordered nature (Scheme 1). In the aforementioned
systems, selection of the secondary metal was targeted to
harness the Lewis acidic property in catalysis and that was
indeed shown by several organic transformations.17,18 The
second strategy was to introduce H-bond based functional
coordinate bond formation3a,b or weak but directional
interactions, such as hydrogen bonding (H-bonding).5 The
hydrogen bond (H-bond), an important noncovalent inter-
action with directionality and moderate bond strength, is a
valuable tool in crystal engineering.5 Phenol and phenolic
group-based building blocks are known to afford a variety of
supramolecular networks.6 The stability and the utility of these
networks depend on both the quality and the quantity of such
interactions that constitute the resultant supramolecular
architecture. Multiple intermolecular interactions between
participating molecules give rise to cooperative stabilization of
the final architecture. In this category, considerable work has
been done with bis-, tris-, tetrakis-, and multikis-phenols to
understand their self-assembly process.6 Notably, these phenol-
based synthons are known to afford a variety of supramolecular
networks, where the phenolic group plays a crucial role in the
formation of characteristic H-bonded motifs.7 Noteworthy are
the examples from the groups of Desiraju,8 Wuest,9 Toda,10
Received: October 14, 2011
Revised: January 2, 2012
Published: January 4, 2012
© 2012 American Chemical Society
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Scheme 1. Pyridine-Based Building Blocks and Their Zero-Dimensional Hetero-Bimetallic Complexes and Two-Dimensional
Networks (A Schematic Representation Only)
Scheme 2. Phenol and Catechol-Based Building Blocks 1−3 Discussed in This Work
groups that have the propensity to self-assemble, and we chose
phenol and catechol groups for that purpose. The present work
explores the self-assembly of Co3+-based building blocks
appended with phenol and catechol groups. An important
aspect of this work is that the titled complexes offer both H-
bond donors (in O−H group) as well as acceptors (in O
Camide group) in a single molecule (Scheme 2). Thus, these
functional groups are expected to operate in a highly
complementary manner where the O−H groups will function
as the H-bond donors and the Oamide groups will act as the
complementary H-bond acceptors. Interestingly, the systematic
presence and/or absence of such complementary functional
groups may help in understanding the rational design of the
resultant supramolecular architecture.
catechol-based molecule 3 has more numbers of H-bond
donors than acceptors, and it would be interesting to see the
effect of this mismatch on the self-assembly (Scheme 2).
EXPERIMENTAL SECTION
■
General Procedures. Unless otherwise noted, chemicals and
reagents were obtained from commercial sources and used without
further purification. Prior to use, solvents were purified following the
standard procedures.19
Syntheses of Ligands. 2,6-Bis(4-methoxy-benzene-
carbamoyl)pyridine (H2Lp‑OMe). To a solution of pyridine-2,6-
dicarboxylic acid (2.0 g, 0.011 mol) in pyridine (20 mL) was added
4-methoxy aniline (2.94 g, 0.023 mol) and triphenylphosphite (7.4 g,
0.023 mol) while stirring. The resulting mixture was stirred at 110 °C
for 6 h. After the solution was cooled, excess pyridine was removed
under reduced pressure. The resulting thick oil was dissolved in
dichloromethane (50 mL), and the product was extracted in aqueous
hydrochloric acid (150 mL, 1:1 v/v). The acidic aqueous extract was
neutralized by the addition of solid sodium bicarbonate. This resulted
in the precipitation of a white solid that was filtered, washed with
water, and air-dried. Recrystallization of the crude product from a
saturated solution of methanol−water (2:1 v/v) afforded the title
compound. Yield: 3.3 g (75%). Anal. Calcd for C21H19N3O4: C, 66.83;
H, 5.07; N, 11.13. found: C, 66.70; H, 4.97; N, 11.16. FT-IR (KBr,
cm−1): ν 3280 (NH), 1652, 1667 (CO). 1H NMR (300 MHz,
DMSO-d6): δ 3.76 (s, 6H, OCH3), 6.99 (d, J = 7.5 Hz, 4H, H10 and
H12), 7.77 (d, J = 9.1 Hz, 4H, H9 and H13), 8.28 (t, J = 7.8 Hz, 2H,
H4), 8.35 (d, J = 7.3 Hz, 1H, H3 and H5), 10.92 (s, 2H, NH−CO).
13C NMR (300 MHz, DMSO-d6): δ 140.13 (C4), 125 (C3 and C5),
In particular, we have designed complexes [Co(Lp‑OH)2]− (1)
and [Co(Lm‑OH)2]− (2) (where H2Lp‑OH: N,N′-bis(4-
hydroxyphenyl)pyridine-2,6-dicarboxamide) and H2Lm‑OH
:
N,N′-bis(3-hydroxyphenyl)pyridine-2,6-dicarboxamide) which
differ by the systematic placement of an −OH group at the
para or meta position of the arene ring, respectively. Further, if
two rings containing p- and m-phenol are merged together, it
would give rise to a catechol ring. Thus, a catechol-based
complex [Co(Lcat)2]− (3) (where H2Lcat: N,N′-bis(3,4-
dihydroxyphenyl)pyridine-2,6-dicarboxamide) was also de-
signed to systematically understand the network creation due
to the H-bond donors and H-bond acceptors. Of course, the
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148.83 (C2 and C6), 160.74 (C7), 131.06 (C8), 122.90 (C9 and C13),
113.89 (C10 and C12), 156.01 (C11), 55.22 (OCH3). MS (EI+, m/z):
calcd 377.14, found 378.02 [M + H]+.
298 K): ΛM = 65 Ω−1 cm2 mol−1. MS (EI+, m/z): calcd 832.17, found
833.04 [M + H]+.
(H3O+)[Co(Lmd)2] (3P). This complex was again synthesized in a
similar manner as discussed for complex 1P with following reagents:
H2Lmd: 0.100 g (0.246 mmol); NaH: 0.011 g (0.493 mmol),
[Co(H2O)6](ClO4)2 0.045 g (0.123 mmol). Yield: 0.155 g (75%).
Anal. Calcd for H3O+·C42H26CoN6O12·2H2O: C, 54.79; H, 3.61; N,
9.13. found: C, 54.94; H, 3.19; N, 9.47. 1H NMR (300 MHz, DMSO-
d6): δ 5.85 (s, 4H, −CH2−), 6.09 (d, J = 8.0 Hz, 2H, H12), 6.17 (s, 2H,
H9), 6.47 (d, J = 8.1 Hz, 2H, H13), 7.65 (d, J = 7.7 Hz, 2H, H3 and
H5), 8.03 (t, J = 7.6 Hz, 1H, H4). 13C NMR (300 MHz, DMSO-d6): δ
100.58 (OCH2), 100.93 (C9), 107.31 (C12), 118.37 (C13), 122.98 (C5
and C3), 139.18 (C8), 139.89 (C4), 143.14 (C11), 146.43 (C10), 156.20
(C2 and C6), 166.83 (C7). FT-IR (KBr, cm−1): ν 2923, 2855 (C−H),
1588, 1502 (CO). Absorption spectrum (DMF, λmax, nm (ε, M−1
cm−1): 636 (12), 463 (sh, 450), 317 (sh, 2640). Conductivity (DMF,
∼1 mM solution at 298 K): ΛM = 60 Ω−1 cm2 mol−1. MS (EI+, m/z):
calcd 888.08, found 888.90 [M + Na]+.
2,6-Bis(3-methoxy-benzene-carbamoyl)pyridine (H2Lm‑OMe). This
ligand was synthesized in a similar manner as discussed for H2Lp‑OMe
with following reagents: pyridine-2,6-dicarboxylic acid (2.0 g, 0.011 mol),
3-methoxy aniline (2.94 g, 0.023 mol), and triphenylphosphite (7.4 g,
0.023 mol). Yield: 3.25 g (72%). Anal. Calcd for C21H19N3O4: C,
66.83; H, 5.07; N, 11.13. found: C, 67.06; H, 4.88; N, 11.67. FT-IR
(KBr, cm−1): ν 3294 (NH), 1665, 1679 (CO). 1H NMR (300 MHz,
DMSO-d6): δ 3.79 (s, 6H, -OCH3), 6.77 (d, J = 8.24 Hz, 2H, H11),
7.35 (t, J = 8.34 Hz, 2H, H13), 7.47 (d, J = 7.9 Hz, 2H, H12), 7.62 (s,
2H, H9), 8.31 (t, J = 7.34 Hz, 1H, H4), 8.39 (d, J = 7.32 Hz, 1H, H5
and H3), 10.99 (s, 2H, NH-CO). 13C NMR (300 MHz, DMSO-d6):
δ 139.23 (C4), 125.42 (C3 C5), 148.83 (C2 C6), 161.69 (C7), 140.03
(C8), 106.83 (C9), 159.54 (C10), 109.76 (C13), 129.63 (C12), 113.32
(C11), 55.11 (OCH3). MS (EI+, m/z): calcd 377.14, found 377.82
[M + H]+.
Synthesis of Co3+ Complexes with Deprotected Phenol and
Catechol Groups. (H3O+)[Co(Lp‑OH)2] (1). The compound Na[Co-
(Lp‑OMe)2] (1P) (0.5 g, 0.6 mmol) was dissolved in 40 mL of dry and
degassed CH2Cl2 under magnetic stirring. BBr3 (0.28 mL, 3.0 mmol)
was carefully introduced to the aforementioned solution under the
dinitrogen atmosphere. The resulting red-brown slurry was stirred for
4 h at −70 °C. The excess BBr3 was slowly quenched with the addition
of MeOH. Evaporation of the solvent under vacuum afforded a brown
solid which was further recrystallized from the methanol. Yield: 0.26 g
(55%). Anal. Calcd. for H3O+·C38H26CoN6O8·6H2O: C, 51.82; H,
4.69; N, 9.54. Found: C, 51.53; H, 4.59; N, 9.67. 1H NMR (300 MHz,
DMSO-d6): δ 6.20 (d, J = 8.0 Hz, 4H, H10 and H12), 6.39 (d, J = 9.5 Hz,
4H, H9 and H13), 7.45 (d, J = 8.0 Hz, 2H, H3 and H5), 7.87 (t, J = 7.7
Hz, 1H, H4). 13C NMR (300 MHz, DMSO-d6): δ 114.21 (C10 and
C12), 122.24 (C3 and C5), 126.44 (C9 and C13), 137.27 (C4), 138.36
(C8), 153.04 (C6), 156.46 (C11), 166.67 (C7). FT-IR (KBr, cm−1):
ν 3400 (O−H), 1664, 1570 (CO). Absorption spectrum (DMF,
λmax, nm (ε, M−1 cm−1): 642 (45), 473 (sh, 1830), 315 (sh, 920).
Conductivity (DMF, ∼1 mM solution at 298 K): ΛM = 55 Ω−1 cm2
mol−1. MS (EI+, m/z): calcd 776.10, found 777.07 [M + Na]+.
(H3O+)[Co(Lm‑OH)2] (2). This complex was synthesized in a similar
manner with an identical scale as discussed for compound 1. Yield:
0.24 g (52%). Anal. Calcd for H3O+·C38H26CoN6O8·6H2O: C, 51.90;
2,6-Bis(3,4-methylinedioxy-benzene-carbamoyl)-pyridine
(H2Lmd). This ligand was also synthesized in a similar manner as
mentioned for H2Lp‑OMe with following reagents: pyridine-2,6-
dicarboxylic acid (2.0 g, 0.011 mol), 3,4-methylenedioxy aniline
(3.28 g, 0.023 mol), triphenylphosphite (7.4 g, 0.023 mol). Yield: 3.8 g
(78%). Anal. Calcd for C21H15N3O6: C, 62.22; H, 3.73; N, 10.73.
found: C, 61.93; H, 4.17; N, 10.36. FT-IR (KBr, cm−1): ν 3274 (N−H),
1
1666, 1650 (CO). H NMR (300 MHz, DMSO-d6): δ 6.00 (s, 4H,
OCH2), 6.84 (d, J = 6.3 Hz, 2H, H12), 7.30 (d, J = 6.3 Hz, 2H, H13),
7.56 (s, 2H, H9), 8.14 (t, J = 5.8 Hz, 1H, H4), 8.42 (d, J = 5.8 Hz, 2H,
H3 and H5). 13C NMR (300 MHz, DMSO-d6): δ 101.23 (OCH2),
103.18 (C9), 108.07 (C12), 114.48 (C13), 125.14 (C3 and C5), 132.23
(C8), 139.93 (C4), 143.88 (C11), 147.15 (C10), 148.78 (C2 and C6),
161.34 (C7). MS (EI+, m/z): calcd 405.10, found 406.10 [M + H]+.
Synthesis of Co3+ Complexes with Protected Phenol and
Catechol Groups. Na[Co(Lp‑OMe)2].2CH3OH (1P). The ligand
H2Lp‑OMe (0.100 g, 0.264 mmol) was dissolved in DMF (10 mL)
and deprotonated with the solid NaH (0.013 g, 0.529 mmol) under
magnetic stirring. The mixture was stirred for 15 min. Solid
[Co(H2O)6](ClO4)2 (0.048 g, 0.132 mmol) was added to the above
said mixture under magnetic stirring. After 30 min of stirring, dry O2
was purged to the solution for 2 min. The solution was finally stirred
for additional 1 h. The reaction mixture was filtered followed by the
removal of solvent under reduced pressure. The crude product was
isolated after washing with diethyl ether. The crude product thus
obtained was redissolved in DMF and subjected to vapor diffusion of
diethyl ether. This resulted in a highly crystalline product within a day.
Yield: 0.154 g (70%). Anal. Calcd. for C42H34CoN6NaO8·2CH3OH: C,
1
H, 5.01; N, 9.08. found: C, 52.05; H, 5.12; N, 8.73. H NMR (300
MHz, DMSO-d6): δ 6.08 (d, J = 7.56 Hz, 2H, H11), 6.14 (s, 2 H, H9),
6.23 (d, J = 8.0 Hz, 2 H, H13), 6.59 (t, J = 7.9 Hz, 2 H, H12), 7.49 (d,
J = 7.7 Hz, 2 H, H3 and H5), 7.88 (t, J = 7.3 Hz, 1 H, H4). 13C NMR
(300 MHz, DMSO-d6): δ 111.44 (C9), 113.29 (C13), 117.25 (C11),
123.47 (C3 and C5), 128.84 (C12), 139.53 (C8), 146.61 (C4), 156.46
(C2 and C6), 157.03 (C10), 166.95 (C7). FT-IR (KBr, cm−1): ν 3417
(OH), 1602 (CO). Absorption spectrum (DMF, λmax, nm (ε, M−1
cm−1): 642 (15), 469 (sh, 270), 304 (sh, 220). Conductivity (DMF,
∼1 mM solution at 298 K): ΛM = 60 Ω−1 cm2 mol−1. MS (EI+, m/z):
calcd 776.10, found 777.05 [M + Na]+.
1
58.93; H, 4.72; N, 9.37. found: C, 59.19; H, 4.70; N, 9.80. H NMR
(300 MHz, DMSO-d6): δ 3.56 (s, 12H, OCH3), 6.44 (d, J = 8.7 Hz,
8H, H10 and H12), 6.57 (d, J = 9.0 Hz, 8H, H9 and H13), 7.50 (d, J =
7.8 Hz, 4H, H3 and H5), 7.90 (t, J = 7.8 Hz, 1H and H4). 13C NMR
(300 MHz, DMSO-d6): δ 138.74 (C8), 122.59 (C3 and C5), 155.29
(C6), 166.73 (C7), 138.64 (C4), 126.60 (C9 and C13), 112.99 (C10 and
C12), 156.33 (C11), 54.99 (OCH3). FT-IR (KBr, cm−1): ν 3073, 2934,
(H3O+)[Co(Lcat)2] (3). The complex 3 was also synthesized using an
identical procedure with similar scale as discussed for compound 1.
Yield: 0.28 g (60%). Anal. Calcd for H3O+·C38H26CoN6O8·4H2O: C,
2834 (C−H), 1676, 1585 (CO). Absorption spectrum (DMF, λmax
,
nm (ε, M−1 cm−1): 627 (15), 464 (sh, 200), 299 (sh, 1412).
Conductivity (DMF, ∼1 mM solution, 298 K): ΛM = 60 Ω−1 cm2
mol−1. MS (EI+, m/z): calcd 832.17, found 833.09 [M + H]+.
Na[Co(Lm‑OMe)2] (2P). This complex was synthesized in a similar
manner with an identical scale as discussed for complex 1P. Yield:
0.158 g (72%). Anal. Calcd for C42H34CoN6NaO8·CH3OH·3H2O: C,
1
50.23; H, 4.10; N, 9.25. Found: C, 50.43; H, 4.31; N, 9.34. H NMR
(300 MHz, DMSO-d6): δ 5.98 (d, J = 7.9 Hz, 2H, H12), 6.23 (d, J = 7.9
Hz, 2H, H13), 6.27 (s, 2H, H9), 7.66 (d, J = 7.6 Hz, 2H, H3 and H5),
7.98 (t, J = 7.8 Hz, 1H, H4). 13C NMR (300 MHz, DMSO-d6): δ
113.51 (C9), 114.72 (C12), 116.55 (C13), 123.17 (C5 and C3), 137.02
(C8), 141.89 (C4), 144.52 (C11 and C10), 156.35 (C2 and C6), 166.89
(C7). FT-IR (KBr, cm−1): ν 3420 (O−H), 1563, 1517 (CO).
Absorption spectrum (DMF, λmax, nm (ε, M−1 cm−1): 638 (15), 456
(sh, 105), 330 (sh, 400). Conductivity (DMF, ∼1 mM solution at 298
K): ΛM = 58 Ω−1 cm2 mol−1. MS (EI+, m/z): calcd 840.08, found
841.09 [M + Na]+.
1
56.21; H, 4.83; N, 9.15. found: C, 56.58; H, 4.39; N, 9.39. H NMR
(300 MHz, DMSO-d6): δ 3.48 (s, 6H, OCH3), 6.21 (s, 2H, H9), 6.26
(d, J = 8.0 Hz, 2H, H11), 6.37 (d, J = 7.7 Hz, 2H, H13), 6.47 (t, J = 8.0
Hz, 2H, H12), 7.48 (d, J = 8.0 Hz, 2H, H3 & H5), 7.86 (t, J = 7.8 Hz,
1H, H4). 13C NMR (300 MHz, DMSO-d6): δ 109.92 (C9), 110.82
(C13), 118.65 (C11), 122.80 (C3 and C5), 128.30 (C12), 138.98 (C8),
147.20 (C4), 156.39 (C2 and C6), 158.78 (C10), 166.49 (C7), 54.55
(OCH3). FT-IR (KBr, cm−1): ν 2930 (C−H) 1663, 1605 (CO).
Absorption spectrum (DMF, λmax, nm (ε, M−1 cm−1): 635 (10),
470 (sh, 102), 299 (6220). Conductivity (DMF, ∼1 mM solution at
Physical Methods. The FT-IR spectra (KBr disk, 4000−400 cm−1,
0.12 cm−1 resolution) were recorded on a Perkin-Elmer FTIR 2000
spectrometer. Absorption spectra were obtained from the Perkin-
Elmer Lambda-25 spectrophotometer. ESI-MS mass spectra were
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measured with the LC-TOF (KC-455) mass spectrometer of Waters.
1H NMR and 13C NMR spectra were recorded either on Avance
Bruker (300 MHz) or Jeol (400 MHz) instrument with TMS as the
internal standard. Microanalytical data were obtained from the
Elementar Analysen Systeme GmbH Vario EL-III instrument.
Thermogravimetric analysis (TGA) and differential scanning calori-
metric (DSC) studies were performed on DTG − 60 SHIMADZU
and TA DSC Q 200 instruments, respectively, at 5 °C/min heating
rate under the dinitrogen atmosphere. X-ray powder diffraction
(XRPD) patterns were recorded using a Bruker AXS D8 Discover
(Cu−Kα radiation, λ = 1.54184 Å) diffractometer. The samples were
ground and subjected to the range of 2θ−50θ with a step size of 0.02°
in 2θ angle. The nitrogen sorption isotherms were measured with an
ASAP 2020 V3.00H Micromeritices, USA instrument in a standard
volumetric technique at 177 K. Under continuous adsorption
conditions, prior to measurements, samples were heated at 150 °C
for 12 h with helium gas used for flushing purposes.
deprotection using BBr3 (Scheme 3). The complexes 1−3 were
isolated as the deep red-brown crystalline solids, and their DMF
solutions display absorption maxima at ca. 650 nm (Figures
S1−S3, Supporting Information). Complexes 1−3 are mono-
anionic in nature and the charge is balanced by the protonated
form of the water molecule (i.e., hydronium ion, H3O+). The
evidence for the presence of H3O+ has come from the
conductivity data (that shows a 1:1 electrolytic nature) and pH
measurements (the pKa was observed below 4 for all three
complexes).25 The protonation that resulted in the generation
of H3O+ ion has most probably occurred during the
deprotection step involving BBr3 (the reaction generates
HBr). The deprotected complexes 1−3 show broad signals at
3402, 3368, and 3234 cm−1, respectively, which we assign to
νO−H of the phenolic group26 (Figure S4, Supporting
Information). Notably such stretches were absent in their
precursor protected molecules 1P−3P. In addition, the proton
NMR spectra displayed a broad feature between 3−4 ppm for
the presence of a phenolic −OH group which disappeared on
the addition of D2O (Figures S5−S10, Supporting Informa-
tion). All complexes were also characterized by thermogravi-
metric analysis (TGA) and differential thermal analysis (DTA).
The TGA profiles of complexes 1 and 2, respectively, show ca.
15.2% and 13.7% weight loss between 25−115 °C that fits
nicely with the calculated weight loss of ca. 14.4% for the
elimination of six water molecules and one hydronium ion27
(Figure S11, Supporting Information). Notably, the micro-
analytical data for both complexes 1 and 2 support the presence
of six water molecules and one hydronium ion. Furthermore,
the crystallographic studies also suggest the presence of
unaccountable electron density which is most likely due to
the disordered water molecules. Similarly, for complex 3, the
initial weight loss (obs. 9.2%) describes the release of four
water molecules and one hydronium ion27 that match well with
the calculated value of 10.02%. The presence of four water
molecules and one hydronium ion in complex 3 is again
supported by the microanalytical results. Complex 1P shows the
weight loss (obs. 6.7%) due to the release of two methanol
molecules that nicely matches with the calculated value of
7.14%. For complex 2P, the observed weight loss of 9.6%
describes the release of one methanol and three water
molecules that fits well with the calculated value of 9.3%.
The observation of voids in the crystal structure of complex 3P
is strongly justified by the thermal weight loss due to one
hydronium ion27 and four water molecules, although the
microanalytical data suggest the presence of only two water
molecules in addition to a hydronium ion.
Crystallography. Single crystals suitable for the X-ray diffraction
for complexes 1P, 3P, 1, 2, and 3 were grown by the slow diffusion of
diethyl ether or isopropyl ether to a methanol solution of compound at
room temperature. X-ray diffraction data were collected on a Bruker
AXS SMART Apex CCD diffractometer using MoKα radiation (λ =
0.71073 Å).20a,b The intensity data were corrected for the Lorentz and
polarization effects.20c The multiscan absorption correction was also
applied.20c The structures were solved by the direct methods using
SIR-9721 and refined by full-matrix least-squares method on F2
(SHELXL-97).22 All calculations were carried out using the WinGX
crystallographic package.23 The non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were fixed geometrically with Uiso
values of 1.2, 1.2, and 1.5 times the Uiso values of methylene,
phenylene, and methyl carbon atoms, respectively. Notably, the data
for complexes 2 and 3 were measured in nonstandard space group
I222. The refinement of complex 1 showed an oxygen atom with the
site occupancy factor (SOF) of 0.25, which was modeled as the
hydronium ion (H3O+); however, the hydrogen atoms of this ion
could not be located because of the partial occupancy. These hydrogen
atoms were included in the molecular formula, however. Apart from
this hydronium ion, there was still some unaccountable electron
density which is most likely due to a few disordered solvent molecules,
but it was too diffused to be modeled accurately. Therefore, it was
removed by using the SQUEEZE routine of the PLATON program.24
Such a treatment improved the model tremendously and the R factor
decreased from ca. 0.09 to ca. 0.05. The structure also showed solvent
accessible voids (SAV) of volume 1282 Å3. The latter were present
even before squeezing out the disordered solvent molecules, however,
with a much smaller volume of 181 Å3. Both complexes 2 and 3 also
showed disordered water molecules apart from the complex anion.
These oxygen atoms were refined with the SOF of 0.25 which
significantly improved their thermal parameters in comparison to their
refinement with full site occupancy of 1. Such oxygen atoms were
modeled as the hydronium ions whose hydrogen atoms could not be
located. Their molecular formulas include these hydrogen atoms. The
crystal structures of both complexes 2 and 3 contain SAVs of volumes
277 and 317 Å3, respectively, due to the crystal packing. The
refinement of complex 3P showed an oxygen atom with an SOF of 0.5.
As this O atom was on special position and two hydrogen atoms could
be located easily, one of them was refined with an SOF of 1 while the
other one with an SOF of 0.5. This generates an H3O+ ion with one of
its hydrogen atoms being disordered over two positions with 50% site
occupancy. For this complex, the model again shows SAVs of volume
274 Å3 due to packing interactions. The crystallographic data
collection and structure refinement parameters for all complexes are
summarized in Table 1.
The thermal studies were followed by X-ray powder
diffraction (XRPD) studies to understand the effect of solvent
removal (guest water molecules) on the H-bonded network’s
stability. For all three deprotected complexes, it was observed
that the H-bonded networks are stable up to 75 °C; however,
when the temperature was raised to 150 °C, crystallinity was
partially lost as evidenced by broad features in the XRPD
patterns (Figures S12−S14, Supporting Information). The
deprotected complexes 1, 2, and 3 were also analyzed for their
gas sorption properties owing to their crystallographically
observed void structures (cf. crystal structures). The N2
sorption studies indicated poorly porous structures with BET
surface area of 1.23, 2.53, and 3.67 m2/g for complexes 1, 2, and
3, respectively (Figures S15−S17, Supporting Information).
The observed low porosity suggest that most of the available
space is being occupied by the building blocks themselves, thus
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The complexes 1−3
were synthesized starting with the ligands containing protected
phenol (H2Lp‑OMe or H2Lm‑OMe) or catechol (H2Lmd) groups,
preparing their Co3+ complexes (1P−3P), and subsequent
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Table 1. Data Collection and Structure Solution Parameters for Complexes Na[Co(Lp‑OMe)2]·2CH3OH (1P), H3O+[Co(Lmd)2]
(3P), H3O+[Co(Lp‑OH)2] (1), H3O+[Co(Lm‑OH)2] (2), and H3O+[Co(Lcat)2] (3)
1P
3P
1
2
3
formula
fw
C44H42CoN6NaO10
896.76
C42H29CoN6O13
784.64
C38H29CoN6O9
772.60
C38H29CoN6O9
772.60
C38H29CoN6O13
836.60
T (K)
crystal system
space group
a (Å)
298(2)
298(2)
293(2)
293(2)
293(2)
monoclinic
P21/c
trigonal
tetragonal
orthorhombic
I222
orthorhombic
I222
R3c
̅
I4
̅
13.9491(7)
18.6397(7)
16.798(1)
90
25.186(2)
25.186(2)
33.184(3)
90
13.948(5)
13.948(5)
13.771(5)
90
10.039(4)
15.446(5)
14.872(5)
90
10.3708(3)
15.3526(5)
15.0072(6)
90
b (Å)
c (Å)
α (°)
β (°)
101.633(7)
90
90
90
90
90
γ (°)
V (Å3)
Z
120
90
90
90
4277.9(4)
4
18230(2)
18
2679(2)
2
2306.1(14)
2
2389.43(14)
2
−3
d (g cm )
1.392
1.450
0.958
1.113
1.163
F(000)
goodness of fit (F2)
1864
8172
796
796
860
0.990
1.273
1.002
1.110
1.026
R1, wR2 [I >2 (I)]
R1 = 0.0681
wR2 = 0.1811
R1 = 0.1025
wR2 = 0.1976
R1 = 0.0806
wR2 = 0.2024
R1 = 0.0845
wR2 = 0.2052
R1 = 0.0515
wR2 = 0.1281
R1 = 0.0684
wR2 = 0.1350
R1 = 0.1120
wR2 = 0.2871
R1 = 0.1239
wR2 = 0.3059
R1 = 0.0783
wR2 = 0.2198
R1 = 0.0978
wR2 = 0.2277
R1, wR2[ all data]
a
Scheme 3. Preparative Routes for the Protected Complexes 1P−3P and Their Deprotected Complexes 1−3
a
Conditions: (a) 2 equiv of NaH, DMF, N2; (b) 0.5 equiv of [Co(H2O)6](ClO4)2; (c) excess O2; (d) 5 equiv of BBr3, CH2Cl2, −70 °C.
significantly reducing the sorption properties. Our earlier
{Co3+−Zn2+} and {Co3+−Cd2+} networks have also shown a
similar moderate porous behavior due to the space occupying
nature of the building blocks.17e,f
characterized. Table 1 contains the data collection and structure
refinement parameters, whereas Tables 2−4 display the
bonding and hydrogen bonding parameters, respectively.
Subsequent sections discuss the molecular structures and
their organization through various H-bonding and other
noncovalent interactions.
Crystal Structures. Protected complexes 1P and 3P and the
deprotected complexes 1, 2, and 3 were crystallographically
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and two methanol molecules. The molecular structure of
complex anion shows that the Co3+ ion is coordinated by two
protected tridentate ligands in their dianionic form, [Lp‑OMe]2−.
Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for
a
Complexes 1P and 3P
Na[Co(Lp‑OMe)2]·2CH3OH (1P)
Around Co3+ Ion
1.842(2) Co1−N1
H3O+[Co(LMD)2] (3P)
The cobalt ion is bound to four Namide atoms (avg. Co−Namide
:
1.962(2) Å) in the basal plane and two Npyridine atoms (avg.
Co−Npyridine: 1.848(2) Å) in the axial position (Figure 1a). This
results in an octahedral geometry around the Co(III) ion.
Complex 1P shows interesting solid-state packing where three
Oamide groups are coordinated to three symmetry related Na+
ions (avg. Na···Oamide: 2.271(3) Å), while the fourth Oamide
group forms a H-bond with the methanol molecule
(Omethanol···Oamide: 2.699(6)Å) (Figure 1b). Notably, each Na+
ion acts as the nodal point and connects three different anionic
molecules together (Figure 1c). The fourth coordination of the
sodium ion comes from the oxygen atom of the second
methanol molecule (Na···OMeOH: 2.285(3) Å). Thus, the
geometry around the Na+ ion is tetrahedral (Figure 1c). The
overall bonding between Na+ ions and complex anions result in
the formation of a zigzag chain propagating along the c-axis.
Two such chains are further connected to each other via an H-
bond between two methanol molecules (O10···O9ii: 2.709(6) Å)
(Figure 1d,e, Table 4). Thus, an array of H-bonds involving
Oamide (O4), Omethanol (O9), Omethanol (O10), and Na+ connect
two chains together (Table 4). In addition, several weak
C−H···O H-bonding interactions exist involving Oamide (O2
and O3) and Omethoxy (O7) and hydrogen atoms connected to
pyridyl, methoxy, and anisidyl groups (see Table S1,
Supporting Information).
Co1−N1
1.855(3)
1.972(3)
1.955(3)
Co1−N2
1.956(3)
Co1−N2
Co1−N3
1.959(3)
Co1−N3
Co1−N4
Co1−N5
Co1−N6
1.853(2)
N1−Co1−N1i
N1−Co1−N3
N1−Co1−N3i
N3−Co1−N3i
N1−Co1−N2i
N1i−Co1−N2i
N2i−Co1−N3i
N2−Co1−N3i
N2−Co1−N2i
174.1(2)
1.964(2)
81.00(14)
94.93(13)
93.10(12)
102.20(15)
81.94(14)
162.87(14)
89.81(14)
92.32(19)
1.968(2)
N1−Co1−N4
N1−Co1−N2
N4−Co1−N2
N4−Co1−N5
N4−Co1−N3
N1−Co1−N5
N1−Co1−N6
N4−Co1−N6
N3−Co1−N6
N5−Co1−N6
177.62(11)
82.07(10)
95.60(11)
81.61(10)
100.70(11)
97.93(10)
98.64(10)
81.83(10)
92.65(10)
163.43(10)
Around Na+ Ion
Na−O1
Na−O2
Na−O3
Na−O10
2.231(3)
2.311(2)
2.271(3)
2.285(3)
a
(i) Symmetry transformations used to generate equivalent atoms for
complex 3P: x − y + 1/3, −y + 2/3, −z + 13/6.
Complex 3P. Complex 3P crystallized in the trigonal cell
Complex 1P. The crystal structure of complex 1P is shown
in Figure 1, whereas Table 2 displays the selected bond
distances and bond angles. Complex 1P crystallized in the
monoclinic cell system and the space group P21/c. The
asymmetric unit contains one complex anion, one Na+ cation,
system with R3c space group. The charge on the anionic metal
̅
complex is balanced by a hydronium ion (i.e., H3O+). The
molecular structure of complex anion shows that the Co3+ ion is
coordinated by four Namide atoms (avg. Co−Namide: 1.963(3) Å) in
the basal plane and two Npyridine atoms (avg. Co−Npyridine
:
a
Table 3. Selected Bond Lengths (Å) and Bond Angles (°) for Complexes 1−3
H3O+[Co(Lp‑OH)2] (1)
H3O+[Co(Lm‑OH)2] (2)
H3O+[Co(Lcat)2] (3)
Co1−N1
Co1−N2
1.846(4)
1.952(2)
180
1.828(9)
1.939(7)
180
1.853(4)
1.966(4)
180
N1−Co1−N1i(1),ii(2),iii(3)
N1−Co1−N2
81.96(6)
98.04(6)
91.12(16)
163.92(11)
91.12(16)
81.26(17)
98.74(17)
90.0(4)
81.64(9)
98.36(9)
94.4(2)
163.27(18)
88.1(2)
N1−Co1−N2i(1),ii(2),iv(3)
v(1)N2−Co1−N2i(1),ii(2),iii(3)
v(1)N2−Co1−N2
N2−Co1−N2
162.50(3)
92.7(4)
vi(2),vii(3)
viii(1),vii(2),iv(3)
a
Symmetry transformations used to generate equivalent atoms: (i) −x + 1/2, y + 1/2, −z + 3/2; (ii) x, −y + 2, −z; (iii) −x, −y, z; (iv) x, −y, −z;
(v) −x + 1, −y, z; (vi) −x, −y + 2, z; (vii) −x, y, −z; (viii) x − 1/2, −y + 1/2, −z + 3/2.
a
Table 4. Some Important H-Bond Parameters for Complexes 1P, 3P, and 1−3
complex
D−H···A
d(H···A) (Å)
d(D···A) (Å)
∠(DHA) (°)
Na[Co(Lp‑OMe)3]·2CH3OH (1P)
O9−H9 ···O4i
2.06
1.74
1.73
1.67
1.88
1.81
2.23
1.84
1.85
2.699(6)
2.709(6)
2.769(4)
2.789(7)
2.689(4)
2.610(11)
2.688(5)
2.585(5)
2.664(5)
135
174
167
169
171
164
114
148
163
O10−H10···O9ii
O7−H71···O1iii
O7−H72···O2iv
O2−H2A···O1v
O2−H2A···O1vi
O2−H21···O3i
O2−H21···O1vii
O3−H31···O2viii
H3O+[Co(LMD)3] (3P)
H3O+[Co(Lp‑OH)3] (1)
H3O+[Co(Lm‑OH)3] (2)
H3O+[Co(Lcat)3] (3)
a
Symmetry transformations used to generate equivalent atoms: (i) x, y, z; (ii) −x, +y + 1/2, −z + 1/2 + 1; (iii) y, +x, −z + 1/2 + 1; (iv) y − 1/3, −x
+ y + 1/3, −z + 1/3 + 1; (v) −x + 1/2, −y + 1/2, +z + 1/2; (vi) −x + 1/2, −y + 1/2 + 1, +z − 1/2; (vii) −x + 1/2, +y − 1/2, −z − 1/2; (viii) x + 1/
2, −y − 1/2, −z − 1/2.
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Figure 1. (a) Molecular structure of the anionic part of complex 1P (thermal ellipsoids are drawn at the 40% probability level; only methoxy
hydrogen atoms are shown for clarity); (b) shows the arrangement of different components in the unit cell; (c) shows the coordination environment
of Na+ ion; (d) shows interaction between different components that results in the generation of a 2D network (view along the b axis); (e) shows a
small portion of panel (d) depicting hydrogen bonding involving Oamide (O4) and two methanol molecules (with atoms O9 and O10). See text for
detail.
1.855(3) Å) in the axial position (Figure 2a) resulting in an
octahedral geometry around the metal. The hydronium ion,
present to balance the charge of anionic complex, takes part in
unique H-bonding with the complex anions. Notably, due to
the site occupancy factor and the special position of oxygen
atom, the hydronium ion was modeled with one of the
hydrogen atoms disordered over two positions with 50%
probability; as a result it (O7) was making H-bonds with four
different complex anions (Figure 2b). These H-bonds were
formed between Oamide atoms (O1 and O2 and their symmetry
related atoms) and the hydrogen atoms attached to the
Ohydronium ion. The heteroatom separations O7···O1iii and
O7···O2iv were observed to be 2.769(4) and 2.789(7) Å,
respectively (Table 4). A c-axis view shows that 18 complex
anions in conjunction with equal number of hydronium ions
were creating a hexagonal unit (Figure 2c). These hexagonal
units were further connected to each other to generate a
honey−comb like network (Figure 2d). The said network
further get strengthened by the presence of C−H···O H-bonding
interactions between the oxygen atom of methylenedioxy group
(O3) and the hydrogen atom attached to the pyridine ring
(C10) (see Table S1, Supporting Information).
Crystal Structures and Self-Assembly of Complexes
1−3. The molecular structures for all three deprotected
complexes (1−3) are quite similar and show that the central
Co3+ ion is meridionally coordinated by two tridentate ligands.
The octahedral cobalt(III) ion is bonded by four Namide atoms
in the basal plane and two Npyridine atoms in the axial position
(Figures 3a−5a). The Co−Namide and Co−Npyridine distances
were found to be in the range of 1.939(7)−1.966(4) Å and
1.828(9)−1.853(4) Å, respectively (Table 3). Two axial
pyridine rings were trans to each other, whereas the diagonal
Namide groups make angle with the central Co(III) ion nearly
163° in all three cases (Table 3). Notably, these molecules are
equipped with the peripheral H-bonding donor −OH groups
and have the ability to self-assemble considering the fact that
there are H-bonding acceptors in Oamide groups. Thus, all three
molecules self-assemble into complementary supramolecular
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Figure 2. (a) Molecular structure of the anionic part of complex 3P (thermal ellipsoids are drawn at 40% probability level); (b) molecular
architecture formed by O−H···O interactions between the hydrogen atoms of hydronium ion and Oamide atoms from complex anions; (c)
arrangement of complex anions and hydronium ions that result in the generation of a hexagonal structure (view along the c-axis); and (d) additional
H-bonding interactions between hexagonal units generates a honeycomb structure (view along the c-axis). See text for detail.
networks via intermolecular H-bonding between OH and
COamide groups.
with the heteroatom separation between any such pair being
2.689(4) Å, while the O−H···Oamide angle was found to be
171°. In this compound, the negative charge on the complex
anion was balanced by the presence of a hydronium ion
(H3O+). Notably, the hydronium ion which is present within
the H−bonded network was found without significant
interactions. We believe that the highly symmetrical array of
H-bonds between phenolic OH and Oamide groups has
prevented these functional groups from further interacting
with the hydronium ion. In fact, a somewhat different situation
was noticed in complex 2 with the m-phenol appended group
where the hydronium ion showed significant interactions with
Oamide groups (cf. crystal structure of complex 2). The space
filling diagram of complex 1 shows solvent accessible voids
(SAVs) of 1282 Å3 (Figure 3d) that were filled with disordered
Complex 1 crystallized in a highly symmetrical tetragonal cell
system with I4 space group (Figure 3). In this molecule, the
̅
intermolecular H-bonding takes place between p-phenolic OH
and COamide groups in a highly symmetrical fashion.
Interestingly, due to the octahedral geometry of the metal,
the H−bonding donor (OH) and H−bonding acceptor
(Oamide) groups are placed above and below in an alternate
manner. Thus, the central molecule is able to interact
simultaneously with eight neighboring molecules via
OH···COamide H-bonds (Figure 3b,c, shown by two shades
of green color). The phenolic O−H group (O2−H2A)
interacts with the complementary COamide oxygen atom
O1, forming the O2−H2A···O1v motif (Figure 3b,c; Table 4)
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Figure 3. (a) Molecular structure of the anionic part of complex 1 (thermal ellipsoids are drawn at 40% probability level whereas only phenolic
hydrogen atoms are shown for clarity); (b, c) different views of H-bonded networks; (d) space filling model of the packing diagram (view along the
c-axis). Hydronium ion has been omitted for clarity in panels b, c, and d. See text for detail.
solvent molecules which could not be assigned and were thus
squeezed.
These interactions result in the generation of a chain running in
the ac plane (shown in purple color; Figure 4c). The purple
chain further connects with two alternate chains (green chains
from top and bottom without containing the hydronium ions)
that result in the generation of a 2D network (Figure 4c). The
SAVs are clearly visible in the space filling diagram of complex
2; however, with much smaller volume of 277 Å3 (Figure 4d)
than that of complex 1.
The catechol-based building block (complex 3) also
crystallized in the orthorhombic cell and I222 space group
(Figure 5). Complex 3 has four peripheral catechol units and
thus offers eight H-bond donors in OH and four H-bond
acceptors in Oamide groups. Clearly, there is a mismatch in the
number of donors and acceptors. Consequently, two types of
intermolecular H-bonds were detected: first between OH and
COamide groups and second between OH and OH groups
(Figure 5b,c). In this complex, the first O−H group of the
catechol ring (O2 at meta position) forms an intermolecular H−
bond (OH···COamide) with the COamide oxygen atom O1
(O2···O1vii: 2.585(5)Å, Table 4). The O−H···Oamide angle was
The complex 2 crystallized in the orthorhombic cell and I222
space group. As observed for the p-phenol isomer (complex 1),
in this case also, the intermolecular OH···COamide H-bonding
was noticed (Figure 4). The m-phenolic group O2−H2A forms
the O2−H2A···O1vi motif (Figure 4, Table 4) with the C
Oamide oxygen atom O1 in a complementary manner. The
heteroatom separation between any such interacting pair is
2.610(11) Å, while the O−H···Oamide angle was 164° (Figure
4b; Table 4). Notably, for the m-phenol (complex 2), the
OH···COamide H-bond is ca. 0.08 Å shorter than the p-phenol
(complex 1). In compound 2, the hydronium ion (which is
required to balance the charge) was found to interact with the
complex anion. Although the hydrogen atoms of the hydro-
nium ion could not be located but from the short interatomic
contacts between O1···O3 (2.78(2) Å) and O3···O3a (2.89(3)
Å, where a = 1 − x, y, −z), it may be inferred that the
hydronium ion is involved in H-bonding interactions with the
complex anion and its own symmetry related counterpart.
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Figure 4. (a) Molecular structure of the anionic part of complex 2 (thermal ellipsoids are drawn at 40% probability level whereas only
phenolic hydrogen atoms are shown for clarity); (b) a view of H-bonded network; (c) a view of H-bonded network including the hydronium
ion; (d) space filling model of the packing diagram (view along the c-axis whereas the hydronium ion has been omitted for clarity). See text
for detail.
found to be 148°. Interestingly, atom O2 (at meta position)
also forms an intramolecular H-bond with the para-positioned
OH group O3 with O2···O3i separation of 2.688(5) Å. Further,
the second O−H group of the catechol ring (O3 at para
position) donates a H-bond to O2 (at meta position) forming
an intermolecular H-bond O3−H31...O2viii with the O3···O2viii
separation and O−H···O angle being 2.664(5) Å and 163°,
respectively (Table 4). Thus, each of the O−H groups of the
catechol ring serves both to donate and accept an H-bond. In
complex 3, the hydronium ion (i.e., H3O+) was found to make
an H-bond with the COamide group with the heteroatom
(O4···O1) separation of 2.86(2) Å (Figure 5c). A similar
situation was noted for the complex 2 (vide supra). The space
filling model of complex 3 clearly shows cavities (with volume
of 371 Å3) which were partially occupied by the hydronium
ions (Figure 5d).
may be considered as isomorphous in nature.28 The
hydronium ion was found to make an H-bond with the
Oamide atom in both complexes 2 (2.78(2) Å) and 3 (2.86(2)
Å), whereas it was completely uncoordinated in complex 1
(O···COamide separation > 4.5 Å). We relate this
observation to a similar but little offset self-assembly in
complexes 2 and 3, whereas a highly symmetrical self-
assembly in complex 1 has restricted the Oamide groups to
interact with the hydronium ion. This can be understood in
terms of relative twisting of the phenol-containing arene
rings in complexes 1 and 2. In complex 1, two p-phenol rings
make an angle of 70.19(2)° with each other, whereas this
angle increases to 84.28(2)° for two m-phenol rings in
complex 2. This increase of ca. 14° is significant and tilts the
m-phenol rings in such a way that the COamide groups are
nicely positioned to interact with the hydronium ion.
Further, the relative twisting of the phenol rings in complex
2 also makes enough room so that the hydronium ions are
comfortably accommodated (Figure 4c). A similar situation
was observed for the catechol rings in the case of complex 3
(Figure 5c).
There are several noteworthy structural similarities and
differences between three deprotected complexes. For
example, compounds 2 and 3 have identical space groups
and very similar unit cell parameters. Further, both
complexes 2 and 3 display a similar interaction of hydronium
ion present in the crystal lattice. Thus, these two complexes
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Figure 5. (a) Molecular structure of the anionic part of complex 3 (thermal ellipsoids are drawn at 40% probability level, whereas only catechol
hydrogen atoms are shown for clarity); (b, c) views of H-bonded network including the hydronium ion; (d) space filling model of the packing
diagram (view along the c-axis whereas the hydronium ion has been omitted for clarity). See text for detail.
spectra; TGA-DTA analysis; XRPD patterns; and N2 sorption
studies; and a table for hydrogen bonding metrics. This material
is available free of charge via the Internet at http://pubs.acs.org.
CONCLUSIONS
■
To summarize, we have shown that the position of the hydroxyl
group on the arene ring (para versus meta) is critical in
generating an unique supramolecular architecture. Similarly, the
number of hydroxyl groups on the arene ring (phenol versus
catechol) is equally significant in determining the outcome of
the self-assembly. Once, a perfect symmetry and orientation
match is achieved, the result is the generation of a highly
symmetrical and complementary network as noted in case of p-
phenol complex 1. We also show that a metal ion can induce
structural rigidity and thus places the appended functional
groups to a fixed conformation which has only been achieved
with synthetically challenging rigid organic molecules. Studies
are underway to generalize the geometrical role of metal ion
and thus orientation of the appended phenol/catechol rings on
the self-assembly process and to investigate the self-assembly in
the presence of guest molecules having complementary H-bond
donors and acceptors.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rgupta@chemistry.du.ac.in. Fax: +91-11-2766 6605.
Tel: +91-11-2766 6646 Ext. 172.
ACKNOWLEDGMENTS
■
R.G. gratefully acknowledges the financial support from the
Department of Science & Technology (DST), New Delhi, and
crystallographic data collection from IIT−Bombay, IIT−
Roorkee, and University of Hyderabad. Authors thank CIF-
USIC center of this university for instrumental facilities and
Nethi Vamsidhar (IISER − Kolkata) for technical assistance
during his summer project. A.A. thanks UGC for the SRF
fellowship.
ASSOCIATED CONTENT
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REFERENCES
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S
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Crystal Growth & Design
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