Insight into the crystallization process: Relationships between crystal structures and properties of copper(ii) coordination polymers containing dimethylbis(4-pyridyl)silane

Article abstract of DOI:10.1039/c3ce40257f

A systematic investigation into the favorable reaction conditions for construction of copper(ii) coordination polymers was carried out. Various reactions of copper(ii) bromide with dimethylbis(4-pyridyl)silane (L) produce two kinds of skeletal structures: the 1D loop-chain and the square-tubular helix. The fast-formation condition affords the 1D loop-chain, whereas the slow-formation condition yields the helical structure. The formation of significant condition-dependent structures can be partially attributed to the nature of the bromide anion and angle strain around the copper(ii) cation. The relationships between structures and physicochemical properties such as anion exchangeability, catalytic oxidation of 3,5-di-tert-butylcatechol, and thermal properties are discussed in these pages.

Full text of DOI:10.1039/c3ce40257f

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PAPER
Insight into the crystallization process: relationships
between crystal structures and properties of copper(II)
coordination polymers containing dimethylbis(4-
pyridyl)silane3
Cite this: CrystEngComm, 2013, 15,
3854
So Yun Moon, Tae Hwan Noh and Ok-Sang Jung*
A
systematic investigation into the favorable reaction conditions for construction of copper(II)
coordination polymers was carried out. Various reactions of copper(II) bromide with dimethylbis(4-
pyridyl)silane (L) produce two kinds of skeletal structures: the 1D loop-chain and the square-tubular helix.
The fast-formation condition affords the 1D loop-chain, whereas the slow-formation condition yields the
helical structure. The formation of significant condition-dependent structures can be partially attributed to
the nature of the bromide anion and angle strain around the copper(II) cation. The relationships between
structures and physicochemical properties such as anion exchangeability, catalytic oxidation of 3,5-di-tert-
butylcatechol, and thermal properties are discussed in these pages.
Received 7th February 2013,
Accepted 6th March 2013
DOI: 10.1039/c3ce40257f
www.rsc.org/crystengcomm
dimethylbis(4-pyridyl)silane (L) based on the change of various
reaction conditions were carried out. Our original goals were
Introduction
The prevalence of kinetic or thermodynamic control via the
change of competing pathways or reaction conditions in
numerous chemical reactions determines molecular structures
and compositions.1–3 Of course, the design and construction
of coordination polymers is highly intricate and is sometimes
achieved serendipitously due to the presence of numerous
factors such as the flexibility of organic ligands, the coordina-
tion geometry of metal cations, counteranions, mole ratios,
solvents, and unpredictable weak interactions, in addition to
kinetic and thermodynamic control.4–9 Among the various
coordination polymers, particularly, induction into desirable
helical structures is a hot issue, since there are valuable
practical implications in multidisciplinary areas such as
precursor templating, molecular memory devices, biomimetic
chemistry, chiral separation, DNA mimics, and structural
molecular biology.10–16 Thus, several methodologies applic-
able to helical skeletons have been attempted,17–20 though the
change of crystallization processes has yet to be explored in
any significant depth. Thus, in order to elucidate the effects of
crystallization processes, crystallizations of CuBr2 with
the construction and functionalization of an ideal square-helix
using the concepts of a skewed conformational L with non-
rigid inter-annular dihedral angles; however, the treatments
produced either the 1D loop-chain coordination polymeric or
helical coordination polymeric skeleton. The relationships
between the coordination polymeric structures and physico-
chemical properties including anion exchangeability, thermal
behavior, and oxidation of catechol were systematically
investigated. Numerous organic ligands have been used for
molecular helical building blocks,16,20 though exploitation of
dimethylbis(4-pyridyl)silane as a helical component was, until
only recently, unprecedented.21 The ligand can possess stable
skewed conformers with non-rigid inter-annular dihedral
angles between two pyridyl groups. The copper(II) ion can be
employed as an angular directional metal cation for the
formation of some helical units.17–19
Experimental
Materials and measurements
CuBr2 was purchased from Aldrich, and used as received
without purification. Dimethylbis(4-pyridyl)silane (L) was
prepared according to the procedures outlined in the
literature.22 Elemental microanalyses (C, H, N) were performed
on crystalline samples at the KBSI Pusan Center using a Vario-
EL III analyzer. Thermal analyses were undertaken under N2 at
a scan rate of 10 uC min21 using a Labsys TGA-DSC 1600. The
infrared spectra were obtained on a Nicolet 380 FT-IR
Department of Chemistry, Pusan National University, Pusan 609-735, Korea.
E-mail: oksjung@pusan.ac.kr; Fax: +82 51 516 7421; Tel: +82 51 510 2591
Electronic supplementary information (ESI) available: SEM EDX data for
3
[Cu(L)2Br]Br, [Cu(L)2Br]BF4, [Cu(L)2Br]ClO4, and [Cu(L)2Br]NO3, TGA and DSC
overlay of [Cu(L)2Br][Br?2H2O]?4CH3OH. CCDC 923406–923409
([Cu(L)2Br][Br?2H2O]?4CH3OH, [Cu(L)2Br]Br?2Me2SO, M-
[Cu(L)Br2(O-Me2SO)]?Me2SO, and P-[Cu(L)Br2(O-Me2SO)]?Me2SO). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3ce40257f
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1
spectrophotometer using samples prepared as KBr pellets. H
(300 MHz) and 13C (75 MHz) NMR spectra for ligand
characterization were recorded on a Varian Mercury Plus
300. The absorption spectra were recorded on a UV-vis
spectrophotometer S-3150. Scanning electron microscope
(SEM) images were obtained on a Tescan VEGA 3.
Recrystallization of crude solid via vapor diffusion:
conglomerate crystals of mixture of P- and M-
[Cu(L)Br2(O-Me2SO)]?Me2SO
The above filtrate was left undisturbed over 7 days to yield two
kinds of green conglomerate crystals of a mixture of P- and M-
[Cu(L)Br2(O-Me2SO)]?Me2SO, where P denoted right-handed
helices and M denotes left-handed helices. Yield, 34%.
Preparation of crude solid
A methanol solution (5 mL) of CuBr2 (55.8 mg, 0.25 mmol) was
added to an ethanol solution (5 mL) of L (107 mg, 0.5 mmol)
under vigorous stirring for 5 min, and the resulting crude solid
was filtered and dried in a vacuum. Yield, 151 mg (95%). IR
(KBr pellet, cm21): 436(w), 515(w), 656(w), 810(s), 1128(s),
1256(w), 1414(s), 1603(m).
Recrystallization of crude solid via vapor diffusion at 70 uC:
conglomerate crystals of mixture of P- and M-
[Cu(L)Br2(O-Me2SO)]?Me2SO
Vapor diffusion of ethyl acetate into the Me2SO solution of the
crude solid at 70 uC produced only the conglomerate crystals of
a mixture of P- and M-[Cu(L)Br2(O-Me2SO)]?Me2SO. Yield, 75%.
mp 215 uC (dec.). Found: C, 31.80; H, 4.38; N, 4.50. Calc. for
Crystallization via self-assembly: [Cu(L)2Br][Br?2H2O]?4CH3OH
C
16H26N2O2S2SiBr2Cu: C, 32.35; H, 4.41; N, 4.72%. IR (KBr
A methanol solution (3 mL) of CuBr2 (2.3 mg, 0.01 mmol) was
layered onto an ethanol solution (3 mL) of L (4.3 mg, 0.02
mmol), resulting in the formation of blue crystals suitable for
X-ray crystallography. Yield, 5.5 mg (67%). mp 189 uC (dec.).
Found: C, 41.00; H, 5.87; N, 6.90. Calc. for
C28H48N4O6Si2Br2Cu: C, 41.20; H, 5.93; N, 6.86%. IR (KBr
pellet, cm21): 656(w), 787(w), 808(m), 833(m), 1128(s), 1227(w),
1254(w), 1323(w), 1417(m), 1604(m).
pellet, cm21): 430(s), 519(s), 785(s), 812(s), 957(s), 1007(s),
1126(s), 1411(s), 1600(s).
Crystallographic structure determination
All X-ray data were collected on a Bruker SMART automatic
diffractometer with graphite-monochromated Mo Ka radiation
(l = 0.71073 Å) and a CCD detector at 173 K. Thirty-six frames
of two-dimensional diffraction images were collected and
processed to obtain the cell parameters and orientation
matrix. The data were corrected for Lorentz and polarization
effects. The absorption effects were corrected by the multi-
scan method (SADABS). The structures were solved by the
direct method (SHELXS 97) and refined by full-matrix least
squares techniques (SHELXL 97).23 The non-hydrogen atoms
were refined anisotropically, and the hydrogen atoms were
placed in calculated positions and refined using a riding
model. For [Cu(L)2Br][Br?2H2O]?4CH3OH, methyl carbons of
the solvated methanol molecules, owing to their high state of
disorder, were restrained using DFIX and ISOR. The crystal
parameters and procedural information corresponding to the
data collection and structure refinement are listed in Table 1.
Recrystallization of crude solid via vapor diffusion:
[Cu(L)2Br]Br?2Me2SO
Ethyl acetate was vapor-diffused into the Me2SO solution (1
mL) of the above-noted crude solid (44 mg) to obtain, in 2
days, blue single crystals suitable for X-ray crystallography.
Yield, 40%. mp 188 uC (dec.). Found: C, 41.50; H, 5.03; N, 6.80.
Calc. for C28H40N4O2S2Si2Br2Cu: C, 41.61; H, 4.99; N, 6.93%. IR
(KBr pellet, cm21): 426(m), 492(s), 810(s), 1126(s), 1257(w),
1419(m), 1602(m).
Table 1 Crystal data and structure refinement for [Cu(L)2Br][Br?2H2O]?4CH3OH, [Cu(L)2Br]Br?2Me2SO, M-[Cu(L)Br2(O-Me2SO)]?Me2SO, and P-
[Cu(L)Br2(O-Me2SO)]?Me2SO
[Cu(L)2Br][Br?2H2O]?4CH3OH
28H48Br2CuN4O6Si2
[Cu(L)2Br]Br?2Me2SO
M-[Cu(L)Br2(O-Me2SO)]?Me2SO
P-[Cu(L)Br2(O-Me2SO)]?Me2SO
Formula
Mw
C
C28H40Br2CuN4O2S2Si2
808.30
C16H26Br2CuN2O2S2Si
593.96
C16H26Br2CuN2O2S2Si
593.96
816.24
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
C2/c
Monoclinic
P21/n
Tetragonal
P43
11.9663(1)
11.9663(1)
16.2923(3)
90
2332.93(5)
4
1.691
Tetragonal
P41
11.9456(1)
11.9456(1)
16.2905(4)
90
2324.61(6)
4
1.697
18.3107(4)
16.6058(4)
11.7604(3)
99.501(2)
3526.9(2)
4
12.1935(2)
20.2551(3)
15.1134(2)
97.944(1)
3696.9(1)
4
b (u)
V (Å3)
Z
s (g cm23
)
1.537
2.997
1.452
2.960
m (mm21
)
4.605
4.622
Rint
0.1117
1.052
0.0859
0.2887
0.1251
1.013
0.0457
0.1033
0.0512
1.010
0.0374
0.0760
0.0986
1.042
0.0585
0.1613
GoF on F2
a
R1
b
wR2
x
0.01(1)
20.01(2)
a
b
2
R1 = g||Fo| 2 |Fc||/g|Fo|. wR2 = {g[w(Fo 2 Fc2)2]/g[w(Fo2)2]}1/2
.
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Crystal structures
The X-ray crystallographic characterizations revealed that the
skeletal structures of [Cu(L)2Br][Br?2H2O]?4CH3OH and
[Cu(L)2Br]Br?2Me2SO are, as depicted in Fig. 1, 1D loop-chain
coordination polymers consisting of 20-membered rhom-
buses. Their selected bond lengths and angles are listed in
Table 2. For the 20-membered rhombus unit, the dihedral
angles between two opposite pyridyl groups are 0.0(3)u and
0.0(3)u for [Cu(L)2Br][Br?2H2O]?4CH3OH, and 1.6(3)u and
Scheme 1 Overall synthetic process.
Results and discussion
Synthesis
The overall synthetic process is summarized in Scheme 1. The
fast reaction of CuBr2 in methanol with L in ethanol in a 1 : 2
molar ratio in relatively high concentration at room tempera-
ture produces a crude solid product in an almost quantitative
yield (see ESI ). By contrast, slow self-assembly of CuBr in
3
2
methanol with L in ethanol in a somewhat dilute solution (see
the Experimental section) forms blue crystals of
[Cu(L)2Br][Br?2H2O]?4CH3OH in a 67% yield. Vapor diffusion
of ethyl acetate into the Me2SO solution of the crude
precipitate for
[Cu(L)2Br]Br?2Me2SO in
2
days affords the blue crystals of
40% yield. After days, the
a
7
remaining solution yields green crystals of a mixture of P-
and M-[Cu(L)Br2(O-Me2SO)]?Me2SO in a 34% yield. The same
diffusion at 70 uC produces only the green crystals of a mixture
of P- and M-[Cu(L)Br2(O-Me2SO)]?Me2SO without the formation
of the blue crystals. Furthermore, the recrystallization via
vapor diffusion of ethyl acetate into the more dilute Me2SO
solution (20 mg of the crude precipitate in an 1 mL of Me2SO)
even at room temperature yields only the green crystals
without formation of the blue crystals.
Thus, both the crystallization method and the concentration
strongly affect product formation. In the present study,
crystals were characterized by microanalyses, IR, thermal
analyses, and X-ray single crystallography, as will be explained
in detail. The crystal structure of [Cu(L)Br2(O-Me2SO)]?Me2SO
was solved several times to confirm the conglomerate crystals.
The products are insoluble in common organic solvents such
as chloroform, acetone, tetrahydrofuran, diethyl ether, n-hex-
ane, and ethyl acetate, but are decomposed in dimethyl
sulfoxide and N,N-dimethylformamide.
Fig. 1 ORTEP drawings of [Cu(L)2Br][Br?2H2O]?4CH3OH (a), [Cu(L)2Br]Br?2Me2SO
(b), M-[Cu(L)Br2(O-Me2SO)]?Me2SO (c), and P-[Cu(L)Br2(O-Me2SO)]?Me2SO (d)
showing thermal ellipsoid at 30% probability. The hydrogen atoms and solvated
molecules (except for the water molecules in (a)) were omitted for clarity.
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Table 2 Selected bond lengths (Å) and angles (u) for [Cu(L)2Br][Br?2H2O]?4CH3OH, [Cu(L)2Br]Br?2Me2SO, M-[Cu(L)Br2(O-Me2SO)]?Me2SO and P-
[Cu(L)Br2(O-Me2SO)]?Me2SOa
[Cu(L)2Br][Br?2H2O]?4CH3OH
[Cu(L)2Br]Br?2Me2SO
M-[Cu(L)Br2(O-Me2SO)]?Me2SO
P-[Cu(L)Br2(O-Me2SO)]?Me2SO
Cu(1)–N(1)
2.043(5)
2.045(5)
2.043(5)
2.043(5)
2.587(1)
4.348(4)
Cu(1)–N(1)
Cu(1)–N(2)#4
Cu(1)–N(3)
Cu(1)–N(4)#4
Cu(1)–Br(1)
2.038(3)
2.044(3)
2.023(3)
2.029(3)
2.6921(6)
4.2642(6)
Cu(1)–N(1)
Cu(1)–N(2)#5
Cu(1)–Br(1)
Cu(1)–Br(2)
Cu(1)–O(1)
2.040(5)
2.025(5)
2.448(1)
2.428(1)
2.219(5)
Cu(1)–N(1)
Cu(1)–N(2)#6
Cu(1)–Br(1)
Cu(1)–Br(2)
Cu(1)–O(1)
2.034(7)
2.006(7)
2.448(1)
2.428(1)
2.209(6)
Cu(1)–N(1)#1
Cu(1)–N(2)#2
Cu(1)–N(2)#3
Cu(1)–Br(1)
…
…
Cu(1) Br(2)
Cu(1) Br(2)
N(1)–Cu(1)–N(2)#5
Br(1)–Cu(1)–Br(2)
Br(1)–Cu(1)–O(1)
Br(2)–Cu(1)–O(1)
Br(1)–Cu(1)–N(1)
Br(1)–Cu(1)–N(2)#5
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)#5
py–Si–py
176.5(2)
141.84(5)
105.2(1)
112.9(1)
89.7(2)
91.4(2)
87.2(2)
N(1)–Cu(1)–N(2)#6
Br(1)–Cu(1)–Br(2)
Br(1)–Cu(1)–O(1)
Br(2)–Cu(1)–O(1)
Br(1)–Cu(1)–N(1)
Br(1)–Cu(1)–N(2)#6
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)#6
py–Si–py
176.6(3)
141.40(6)
105.3(2)
113.2(2)
89.5(2)
91.4(2)
87.3(3)
N(1)–Cu(1)–N(2)#3
N(1)#1–Cu(1)–N(2)#2
N(1)–Cu(1)–N(1)#1
N(2)#2–Cu(1)–N(2)#3
N(1)–Cu(1)–Br(1)
N(1)#1–Cu(1)–Br(1)
N(2)#2–Cu(1)–Br(1)
N(2)#3–Cu(1)–Br(1)
py–Si–py
87.9(2)
87.9(2)
163.3(3)
160.5(3)
98.4(2)
98.4(2)
99.8(2)
99.8 (2)
105.0(3)
N(1)–Cu(1)–N(3)
86.8(1)
N(2)#4–Cu(1)–N(4)#4 87.0(1)
N(1)–Cu(1)–N(2)#4
N(3)–Cu(1)–N(4)#4
N(1)–Cu(1)–Br(1)
N(2)#4–Cu(1)–Br(1)
N(3)–Cu(1)–Br(1)
N(4)#4–Cu(1)–Br(1)
py–Si–py
164.3(1)
164.5(1)
97.4(1)
98.3(1)
97.2(1)
98.3(1)
104.1(2), 106.0(2)
89.3(2)
105.2(3)
89.3(3)
105.0(4)
a #12x, y, 2z 2 1/2, #2x 2 1/2, 2y + 1/2, z 2 1/2, #32x + 1/2, 2y + 1/2, 2z, #42x + 1/2, y 2 1/2, 2z + 1/2, #52x + 1, y, z + 1/4, #62x, y, z 2 1/4.
1.9(3)u for [Cu(L)2Br]Br?2Me2SO. The L connects two copper(II)
ions to form a molecular rhombus with Cu Cu (10.3015(5) Å
the same. The helical pitch was 16.2923(3) Å for M-helix and
16.2905(4) Å for P-helix. One of the Me2SO molecules is
coordinated to the copper(II) ion (Cu–O = 2.219(5) for M-helix
and 2.209(6) Å for P-helix). The local geometry around the
copper(II) ion is a typical trigonal bipyramid with two bromide
atoms (Cu–Br = 2.428(1) and 2.448(1) Å for M-helix; 2.428(1)
and 2.448(1) Å for P-helix), along with the one oxygen of the
coordinating Me2SO in a basal plane and two nitrogen atoms
…
for [Cu(L)2Br][Br?2H2O]?4CH3OH; 10.3115(2) Å for [Cu(L)2Br]
…
Br?2Me2SO) and Si Si (8.668(4) Å for [Cu(L)2Br][Br?2H2O]?
4CH3OH; 8.662(2) Å for [Cu(L)2Br]Br?2Me2SO) separations. The
local geometry around the copper(II) ion approximates to a
distorted square pyramidal arrangement with four nitrogen
atoms from four L (Cu–N = 2.043(5)–2.045(5) Å for [Cu(L)2Br]
[Br?2H2O]?4CH3OH; 2.023(3)–2.044(3) Å for [Cu(L)2Br]Br?2Me2SO) in
a basal plane and a bromide atom (Cu–Br = 2.587(1) Å for
[Cu(L)2Br][Br?2H2O]?4CH3OH; 2.6921(6) Å for [Cu(L)2Br]Br?2Me2SO)
in an apical position (Fig. 2). The distortion may be ascribed to the
deviated copper(II) cation from a coplanar Cu(II)–N4 (0.322 Å for
[Cu(L)2Br][Br?2H2O]?4CH3OH; 0.277 Å for [Cu(L)2Br]Br?2Me2SO),
presumably owing to repulsion between the bulky bromide anion
and the pyridyl moieties. Another bromide anion acts as a
…
counteranion rather than a coordinating coligand (the Cu Br
distance = 4.348(4) Å for [Cu(L)2Br][Br?2H2O]?4CH3OH; 4.2642(6) Å
for [Cu(L)2Br]Br?2Me2SO). The interesting feature is that the
counteranionic bromide of [Cu(L)2Br][Br?2H2O]?4CH3OH was hydro-
…
gen-bonded with two solvate water molecules (Br H2O: 1.78, 2.07 Å)
to produce an unusual water adduct of bromide in the solid state.
The methyl proton attached to the silicon atom interacts with the
…
pyridyl moiety of the adjacent loop-chain via –CH p interactions
(2.88 Å for [Cu(L)2Br][Br?2H2O]?4CH3OH; 2.58 Å for [Cu(L)2Br]
Br?2Me2SO). Four solvate methanol molecules for [Cu(L)2Br]
[Br?2H2O]?4CH3OH and two solvate Me2SO molecules for
[Cu(L)2Br]Br?2Me2SO stay in the vacancy in the unit cell (Fig. 3).
As regards the green crystals from both the filtrate at room
temperature and the vapor diffusion at 70 uC, their crystal
structures were determined several times, resulting in a
mixture of crystals of P- and M-helices consisting of
[Cu(L)Br2(O-Me2SO)]?Me2SO. Their skeletal structures are
infinite square-tubular helices, and there are four of these
units in each turn as shown in Fig. 4. The most interesting
result is that a crystal consists only of right-handed (P) helices
and that the remaining half-crystals consist only of left-handed
(M) helices. Their structures, except for the helicity, are exactly
Fig. 2 ORTEP drawings and space-filling diagrams of local geometry around
copper(II) ion for [Cu(L)2Br][Br?2H2O]?4CH3OH (a) and [Cu(L)2Br]Br?2Me2SO (b),
designating bromide hydrogen-bonded with solvate water molecules (a) and
free bromide (b).
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Fig. 3 Packing diagrams of [Cu(L)2Br][Br?2H2O]?4CH3OH (top) and
[Cu(L)2Br]Br?2Me2SO (bottom). Solvate molecules are represented in the space-
filling model.
from the two L in an axial position (Cu–N = 2.025(5) and
2.040(5) Å for M-helix; 2.006(7) and 2.034(7) Å for P-helix; N–
Cu–N = 176.5(2)u for M-helix; 176.6(3)u for P-helix). Another
solvate Me2SO molecule stays in the vacancy in the unit cell.
Mechanistic aspect of construction
Our original strategy was the formation of a molecular helix by
means of the silicon-containing pyridyl ligands. These are
useful for construction of various metal–organic coordination
polymers, since they are adjustable in bridging ability and
length, possess flexible angles around silicon, and are
conformationally non-rigid. For the construction of helical
coordination polymers, the skewed (h) conformational
ligand,16 as coordinated to the directional metal cation, is a
key tectonic element. For the present diffusion crystallization
at room temperature, removal of the blue crystals of
[Cu(L)2Br]Br?2Me2SO from the mother liquor subsequently
gave rise to the green crystals of [Cu(L)Br2(O-Me2SO)]?Me2SO,
indicating that [Cu(L)2Br]Br?2Me2SO is a kinetic product and
that [Cu(L)Br2(O-Me2SO)]?Me2SO is a thermodynamic product.
If the reaction was maintained without filtration of blue
crystals, the growth of green crystals was relatively slow,
indicating that the formation of blue versus green crystals in
the solution is competitive. Even though the same compo-
nents of the copper(II) ion and L were treated, two quite
different coordination polymers formed. How can this be
explained? The difference in angle strain between the
distorted square pyramid and the more stable trigonal
bipyramidal geometry might be a factor.24 Also, a good leaving
bromide compared with chloride might be a result of the
formation of significant condition-dependent structures. In
the present system, a high concentration predominantly
produces the blue crystals of the loop-chain, whereas a dilute
concentration yields the green crystals of helices. Thus
concentration seems to play an important role in the
Fig. 4 Top and side views of M-[Cu(L)Br2(O-Me2SO)]?Me2SO (left) and P-
[Cu(L)Br2(O-Me2SO)]?Me2SO (right). The hydrogen atoms were omitted for
clarity. From the space-filling diagrams, the hydrogen atoms and solvated
Me2SO molecules also were omitted.
formation of coordination polymers. Formation of only green
crystals via vapor diffusion at 70 uC can be attributed to the
preferred stable conformation of L at high temperatures.
Indeed, a high-temperature solution is equivalent to a dilute
concentration. In polymerization, fast formation produces the
loop-chain structure, whereas slow formation tends to yield
the helical structure. The present concentration results were
consistent with this fact.
How is the helical coordination polymer thermodynamically
constructed? The combined effects of the skewed conformer of
L and the linear N–Cu(II)–N bonds might be the driving force
behind the formation of such ideal helices. In particular, the
slow diffusion of diethyl ether into a Me2SO solution of crude
solid affords single conglomerate crystals (P + M crystals) in
high yields instead of racemic crystals.16 In our results, the
construction of the helical structure was not significantly
affected by the change of the mole ratio, indicating that the
helix is a thermodynamically favorable species.
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Fig. 5 IR spectra for [Cu(L)2Br]Br?2Me2SO (a) and the exchanged species
[Cu(L)2Br]BF4 (b) and [Cu(L)2Br]ClO4 (c) after anion exchange with three
equivalents of NaBF4 and NaClO4 after 3 h. The triangle and square denote the
Fig. 6 IR spectra during anion exchange of [Cu(L)2Br]Br?2Me2SO with NaNO3 (0
2
BF4 and ClO42, respectively.
equiv. (a), 1 equiv. (b), 2 equiv. (c), and 3 equiv. (d)) after 10 min. The arrow
2
peaks designate NO3
.
Anion exchange and thermal analyses
governed by the mole ratio and ionic character of bromide
anions. For the conglomerate crystals of a mixture of P- and
M-helices, anion exchange was not easily accomplished under
similar conditions.
Thermal analyses have been used to establish the relation-
ships between structural and physicochemical properties.7 The
thermal stabilities of the present coordination polymers were
found to be dependent upon the respective structures (Fig. 7).
The decomposition temperatures of [Cu(L)2Br][Br?2H2O]?
4CH3OH (189 uC) and [Cu(L)2Br]Br?2Me2SO (188 uC) were
lower than that of a mixture of P- and M-helices [Cu(L)Br2
(O-Me2SO)]?Me2SO (215 uC). The thermal properties of
Although the loop-chain of [Cu(L)2Br]Br species had both
coordinated bromide and counteranionic bromide, the differ-
ence between them was not established. In order to investigate
such different behavior between the two kinds of bromide
anions, anion exchange was conducted in the aqueous
suspension state.16,25 The initial evaluation revealed that the
anion exchange occurred smoothly at room temperature. The
2
2
anion exchange of [Cu(L)2Br]Br with BF4 or ClO4 was
2
2
confirmed by the appearance of the BF4 or ClO4 band at
1063 cm21 or 1092 cm21, respectively, after 3 h (Fig. 5). An
interesting feature is that the counteranionic bromide was
easily exchanged, but the coordinated bromide was not
exchanged, resulting in the formation of [Cu(L)2Br]X species
(X2 = BF42 or ClO42). The elemental analysis results (found: C,
46.10; H, 4.69; N, 7.88. Calc.: C, 47.17; H, 4.81; N, 7.86% for
[Cu(L)2Br]BF4; found: C, 45.50; H, 4.44; N, 7.79. Calc.: C, 46.34;
[Cu(L) Br][Br?2H O]?4CH OH (see ESI ) were similar to those
3
2
2
3
of [Cu(L)2Br]Br?2Me2SO, except for the easy evaporation of
solvate methanol molecules at room temperature. For both
loop-chain structures, the weight losses at 188 uC and 189 uC
each seemed to correspond to one ligand loss. For
[Cu(L)2Br]Br?2Me2SO, solvate Me2SO molecules evaporated
within the 102–134 uC range (found: 16.0%; calc.: 19.3%).
For conglomerate [Cu(L)Br2(O-Me2SO)]?Me2SO, the solvate
Me2SO molecules evaporated within the 129–178 uC range
(found: 24.0%; calc.: 26.3%). For [Cu(L)2Br]Br?2Me2SO, after a
weight loss corresponding to one ligand at around 188 uC
occurred, its decomposition temperature (219 uC) and DSC
pattern were very similar to those (215 uC) of
[Cu(L)2Br]Br?2Me2SO, indicating that one ligand-loss trans-
formed the compound consisting of the helical composition
(eqn (1)–(3)).
H, 4.72; N, 7.72% for [Cu(L) Br]ClO ) and SEM-EDX (see ESI )
3
2
4
of the partially exchanged species were satisfactory.
Accordingly, the peak intensity of the exchanged anions at
1063 cm21 or 1092 cm21 was relatively weak compared with
that of the fully exchanged anions.16,23 The other peaks of the
IR spectra remained virtually unchanged, suggesting post-
anion-exchange retention of the skeleton. The exchange-
dependence on the mole ratio of anions was determined by
the anion exchange of [Cu(L)2Br]Br species with NO32 (Fig. 6).
The intensity of the nitrate band around 1385 cm21 increased
with the increase of the mole ratio of the nitrate anions. When
the exchange of [Cu(L)2Br]Br species with at least three moles
2
of NO3 was accomplished, the exchanges species afforded
124 0C
:
½Cu(L)2BrꢀBr 2Me2SO
½Cu(L)2BrꢀBr
(1)
[Cu(L)2Br]NO3. Seemingly therefore, anion exchangeability is
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Fig. 7 TGA (red) and DSC (blue) curves of [Cu(L)2Br]Br?2Me2SO (solid line) and
[Cu(L)Br2(O-Me2SO)]?Me2SO (dotted line).
188 0C
½Cu(L)2BrꢀBr
½Cu(L)Br2ꢀzL:
(2)
(3)
219 0C
½Cu(L)Br2ꢀ
CuO
Fig. 8 UV spectra (in methanol) showing oxidation of 3,5-di-tert-butyl-catechol
(3,5-DBCat) with 0.05 mol% [Cu(L)Br2(O-Me2SO)]?Me2SO (a) and
[Cu(L)2Br]Br?2Me2SO (b) catalysts.
Catalytic oxidation of 3,5-DBCat
3,5-Di-tert-butylcatechol (3,5-DBCat) typically is selected as the
substrate for oxidation catalysis because its low redox potential
oxidizes easily and the bulky tert-butyl groups prevent further
side reactions such as ring-opening.26 The oxidation species
3,5-di-tert-butylquinone (3,5-DBBQ) is a useful intramolecular
electron-transfer ligand,27,28 exhibiting a maximum absorp-
tion around 390 nm in methanol.29 The catalytic effects of
[Cu(L)2Br]Br?2Me2SO and [Cu(L)Br2(O-Me2SO)]?Me2SO were
compared: each compound was treated with 3,5-DBCat in
the mole ratio of 1 : 20 under aerobic conditions in 20 mL of
methanol at room temperature. After the addition of the
substrate to the methanol suspension of the catalysts, the
absorption band at 390 nm, which is indicative of oxidation
from 3,5-DBCat to 3,5-DBBQ, increased in intensity. Both
compounds showed significant catecholase activity compared
with the inactive Cu(ClO4)2?6H2O,29 as shown in Fig. 8. The
reactions were monitored with reference to the increase in the
absorbance at 390 nm as a function of time. The oxidation
reaction using the [Cu(L)Br2(O-Me2SO)]?Me2SO catalyst fin-
ished within almost 50 min, whereas the reaction using the
[Cu(L)2Br]Br?2Me2SO catalyst continued to the 60 min mark.
That is, [Cu(L)Br2(O-Me2SO)]?Me2SO showed slightly higher
catecholase activity than that of [Cu(L)2Br]Br?2Me2SO. This
dependence on the substrate concentration indicates that
catalyst–substrate binding is the initial step in the catalytic
mechanism. An investigation into such a mechanistic aspect
is now underway.
Conclusions
We scrutinized various crystallization processes in an unusual
system that affords 1D loop-chains and square-tubular helices.
The crystallization rate and the concentration are significant
factors in the formation of coordination polymers. Both the
nature of the bromide anion and the angle strain around the
copper(II) cation seem to contribute to the changeable skeletal
structures. The relationships between structures and physico-
chemical properties such as anion exchangeability, catalytic
oxidation of 3,5-di-tert-butylcatechol, and thermal properties
have been well characterized. Although the mechanism of the
structural transformation is manifestly unpredictable, under-
standing the features that can be used to direct key factors can
offer clues to the design and construction of elaborate
materials exhibiting desirable properties.
…
mechanism. Notably, the Cu Cu distances of the present
Acknowledgements
…
system are very long compared with the short Cu Cu
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korean Government
[MEST] (2010-0026167).
distances of the complex with higher catecholase activity,29
which indicates that oxidation occurred in
a different
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15 A. Klug, Angew. Chem., Int. Ed. Engl., 1983, 22, 565.
16 O.-S. Jung, Y. J. Kim, Y.-A. Lee, J. K. Park and H. K. Chae, J.
Am. Chem. Soc., 2000, 122, 9921.
17 L. Han and M. Hong, Inorg. Chem. Commun., 2005, 8, 406.
18 M. Albrecht, Chem. Rev., 2001, 101, 3457.
19 C. Piguet, G. Bernardinelli and G. Hopfgartner, Chem. Rev.,
1997, 97, 2005.
20 E. C. Constable, Chem. Soc. Rev., 2013, 42, 1637.
21 J. W. Lee, E. A. Kim, Y. J. Kim, Y.-A. Lee, Y. Pak and O.-
S. Jung, Inorg. Chem., 2005, 44, 3151.
Notes and references
1 P. Laszlo, in Organic Reactions: Simplicity & Logics, Wiley,
1995, p. 271.
2 W. S. Steed and J. L. Atwood, in Supramolecular Chemistry,
Wiley, 2000, p. 113.
3 G. B. Desiraju, J. J. Vittal and A. Ramanan, in Crystal
Engineering: A Textbook, World Scientific Publishing Co.,
2011, p. 86.
4 M. Du, X.-H. Bu, Z. Huang, S.-T. Chen and Y.-M. Guo, Inorg.
Chem., 2003, 42, 552.
5 H. Gudbjartson, K. Biradha, K. M. Poirier and M.
J. Zaworotko, J. Am. Chem. Soc., 1999, 121, 2599.
6 S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen
and I. D. Williams, Science, 1999, 283, 1148.
7 O.-S. Jung, S. H. Park, D. C. Kim and K. M. Kim, Inorg.
Chem., 1998, 37, 610.
8 O.-S. Jung, S. H. Park, K. M. Kim and H. G. Jang, Inorg.
Chem., 1998, 37, 5781.
9 M. Du, X.-H. Bu, Y.-M. Cuo, J. Ribas and C. Diaz, Chem.
Commun., 2002, 2550.
22 M. Schmitz, S. Leninger, J. Fan, A. M. Arif and P. J. Stang,
Organometallics, 1999, 18, 4817.
23 (a) G. M. Sheldrick, SHELXS-97, Program for solution of
¨
crystal structures, University of Gottingen, Germany, 1997;
(b) G. M. Sheldrick, SHELXL-97, Program for refinement of
¨
crystal structures, University of Gottingen, Germany, 1997.
24 B. W. Clare and D. L. Kepert, in Encyclopedia of Inorganic
Chemistry, ed. R. B. King, John Wiley & Sons Ltd., 2nd edn,
2005, vol. 2, p. 1055.
25 O.-S. Jung, Y. J. Kim, Y.-A. Lee, K.-M. Park and S. S. Lee,
Inorg. Chem., 2003, 42, 844.
26 J. Mukherjee and R. Mukherjee, Inorg. Chim. Acta, 2002,
337, 429.
27 C. G. Pierpont, Coord. Chem. Rev., 2001, 216–217, 99.
28 O.-S. Jung, D. H. Jo, Y.-A. Lee, B. J. Conklin and C.
G. Pierpont, Inorg. Chem., 1997, 36, 19.
10 E. Yashima, K. Maeda and Y. Okamoto, Nature, 1999, 399,
449.
11 D. B. Cordes, C. V. K. Sharma and R. D. Rogers, Cryst.
Growth Des., 2007, 7, 1943.
12 G. W. Orr, L. J. Barbour and J. L. Atwood, Science, 1999, 285,
1049.
29 A. Biswas, L. K. Das, M. G. B. Drew, C. Diaz and A. Ghosh,
Inorg. Chem., 2012, 51, 10111.
13 S.-N. Wang, H. Xing, Y.-Z. Li, J. Bai, M. Scheer, Y. Pan and
X.-Z. You, Chem. Commun., 2007, 2293.
14 R. Chalmers, A. Guhathakurta, H. Benjamin and N. Kleckner,
Cell, 1998, 93, 897.
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