PH-controlled crystal growth of copper/gemini surfactant complexes with bipyridine groups

Article abstract of DOI:10.1039/c7ce01251a

The pH-controlled crystal growth of two complexes with different coordination modes, derived from gemini surfactant molecules with a bipyridyl spacer (12Bpy) and metal copper ions (Cu²⁺) is presented in this work. Such crystalline forms obtained in appropriate pH ranges exhibit dissimilar morphologies, colors and crystalline structures. Under weak acidic conditions with a slightly higher pH (>4.3), blue grain crystals, made from dihydroxo-bridged binuclear complexes with a square pyramidal coordination mode, are formed, whereas under slightly stronger acidic conditions (pH <3.8 in this work), green crystals with mononuclear complexes with a distorted trigonal bipyramidal geometry are readily fabricated, and meanwhile the blue crystals are completely inhibited. In particular, these two crystals concomitantly existed in an intermediate pH range of 3.8-4.3. We suggest a fivefold coordinated Cu(ii)/12Bpy complex with a 1 : 1 metal-ligand ratio and three hydrated water ligands and a pH-controlled crystal growth mechanism on the basis of UV-vis spectra and density functional theory (DFT) calculations. Our findings indicate that pH adjustment is a straightforward and efficient way to control the crystal growth, having potential application in the preparation of smart and multifunctional materials.

Full text of DOI:10.1039/c7ce01251a

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pH-Controlled Crystal Growth of Copper/Gemini Surfactant
Received 00th
January 20xx,
Complexes with Bipyridine Groups †
Junyao Yao, Qibin Chen*, Yujie Sheng, Aiting Kai and Honglai Liu
A pH-controlled crystal growth of two complexes with different coordination modes, derived from Gemini surfactant
2
+
molecules with a bipyridyl spacer (12Bpy) and metal copper ions (Cu ) is presented in this work. Such two crystalline
forms obtained in appropriate pH ranges exhibit dissimilar morphologies, colors and crystalline structures. Under weak
acidic conditions with a slightly higher pH (> 4.3), blue grain crystals, made from dihydroxo-bridged binuclear complexes
with a square pyramidal coordination mode, are formed, whereas under a little stronger acidic conditions (pH < 3.8 in this
work), green crystals with mononuclear complexes with a distorted trigonal bipyramidal geometry are readily fabricated,
and meanwhile the blue crystals are completely inhibited. Specially, such two sets of crystals concomitantly existed in an
intermediate pH range of 3.8~4.3. We suggest a fivefold coordinated Cu(II)/12Bpy complex with a 1:1 metal-ligand ratio
and three hydrated water ligands and a pH-controlled crystal growth mechanism on the basis of UV-Vis spectra and
density functional theory (DFT) calculations. Our findings indicate the pH adjustment is a straightforward and efficient way
to control the crystal growth, having a potential application in the preparation of smart and multifunctional materials.
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
to prepare a variety of metal-ligand complexes with fairly
predictable architectures. This suggests that it is feasible to
judiciously manipulate the structures and the resulting
Introduction
It has been successfully demonstrated that metal complexes
could be assembled into different supramolecular structures
and architectures in artificial and/or natural self-assembling
systems, resulting in a number of fantastic functions and
properties. For example, copper complexes widely exist in
organisms, identified as metal proteins and enzymes,
mediating such processes as electron transfer, dioxygen
transport, dioxygen activation, reduction of nitrogen oxides to
elemental nitrogen, and the disproportionation of superoxide
performances of such complexes via tuning some key factors.
Among these factors, solvent plays an important role in the
synthesis of metal complexes and their subsequent crystal
growths, resulted from the significant difference in the
synthesis condition (e.g., solvothermal synthesis), the binding
capability of solvents to metal ions, the solvophobicity and/or
6,7
solvophilicity of the ligands, and so on. Up to date, a lot of
studies on chelating ligands have involved pyridine, carboxylic
acid, and amine groups, which can be protonated or
1
ion. Generally, the structural diversity in metal complexes
8
,9
deprotonated relying on different pH values of solvent. In
this context, adjusting pH of solvent, acting as an efficiently
mediated strategy, had consequently been used to manipulate
the protonation or deprotonation forms of relevant functional
groups and even to separately encourage and interrupt the
results from variations in their building blocks, involving the
choice of metal ion centers and ligands and the manipulation
of coordination modes. In particular, the versatility in
coordination modes from linear geometries to trigonal/square
planar, tetrahedral, trigonal bipyramidal and octahedral
geometries has allowed a great number of structures and
topologies for such metal complexes, thereby leading to
diversely specific performances of such materials, such as
1
0,11
formation of hydrogen bonds.
In addition, besides the
intermolecular interactions, pH in solutions can also influence
the covalent bond formation, like disulfide bond through
1
2
2
-4
oxidation of thiols.
redox, catalysis and magnetic properties. As a result, some
Recently, binuclear copper (II) complexes have been an
active research subject and many of these complexes have
been of great interest for quite a long time mainly due to their
specific structures, catalytic and magnetic properties and the
capability to act as models for copper proteins. In this regard,
the bis(oxo)dicopper species, an ubiquitous structural type in
non-heme redox metalloenzymes, is very common in copper-
oxo complexes and is particularly thermodynamically favorable
designable strategies, such as click chemistry and in situ ligand
5
synthesis, were attempted to yield modified ligands, and thus
1
3-15
with the formation of oxygen-bridged bonds.
On the other
hand, the hydroxo-bridged Cu(II) binuclear complexes is
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another example, as a simplest model of magnetic interaction purification. Milli-Q water (18.2MΩ·cm) was used in all
1
involving only two unpaired electrons. Actually, the majority experiments. The morphology of crystals was observed with an
6
1
of such hydroxo-bridged dinuclear metal complexes are optical microscopy on Nikon Eclipse 50iPol. H NMR was
generated in the alkaline environment, as reported in recorded on an Avance III 400 MHz NMR spectrometer
7-20
1
literatures,
whereas examples of the hydroxo-bridged (Bruker, Germany), and chemical shifts are reported in ppm (δ)
complexes are fairly rare under acidic conditions. Recently, we using the signal of tetramethylsilane (TMS) as an internal
reported the crystallization of a bis(μ-hydroxo)dicopper(II) standard. Elemental analyses (EA) were carried out with a
2
+
complex and its crystal structure, originated from the Cu ion Vario EL III analyzer (Elementar Germany). Powder X-ray
and the Gemini surfactant-based ligand, 2,2’-bipyridyl-5,5’- diffraction (PXRD) patterns were collected by Rigaku D/max
dimethylene-bis(N,N’-dimethyldodecylammonium
bromide) 2550 VB/PC powder diffractometer (Japan). Single-crystal X-
2
12Bpy, in the form of Br ), in weak acid solution. Gemini ray diffraction (SC-XRD) experiments were performed on a
1
(
L
2
surfactants are made up of two amphiphilic moieties Smart Apex CCD area detector diffractometer (Bruker,
connected by a spacer group at or near the head groups, which Germany), with a Cu Kα (λ = 1.54178 Å). UV-vis spectra were
have aroused many interests because of their unique obtained using a Shimadzu UV-2550 spectrophotometer
properties. Due to the presence of
a hydrophilic or (Japan) at 25 °C using a cuvette with 1 cm path length. All
hydrophobic and rigid or flexible spacer group and two measurements were performed under ambient laboratory
identical or different amphiphiles in a Gemini molecule, it is conditions. The electronic structure calculations of
2
+
possible to synthesize the dimeric surfactants with an [Cu(Bpy)(H
2 n
O) ] (n = 3,4) complexes were performed with the
2
8
enormous variety of chemical structures resulting in a great Gaussian 09 quantum chemistry suite of programs.
difference in their properties. Several quaternary ammonium Geometry optimization calculations were carried out by means
Gemini surfactants were synthesized and investigated in our of a DFT functional at PBE0/6-311+g(d) level of theory.
previous works, involving not only in the organization of
insoluble monolayers or complex monolayers at the air/water Chart 1. The complex structures of
2,23
1
and 2.
2
6
+
interface,
but also in the preparation of new functional
2
1,24,25
materials, such as porous molecular crystals.
N
N
More intriguingly, as for the copper ion and 12Bpy complex
system, the pH of the solution was found to reduce gradually
as crystals grew, implying that the growth process was likely
accompanied with the generation of protons or the depletion
of hydroxyl groups. These unexpected findings led us to look
more closely at the influence of the pH in solution on crystal
structures and coordination modes of Cu(II)/12Bpy complexes.
We were also motivated to carry out this investigation because
we believe that the use of varying pH of solution to optimize
the functional behaviour of resulting crystals has been largely
overlooked. In addition, there is little definitive evidence to
directly unravel the assembly process of many synthetic metal-
C12H25
C12H25
N
HO
N
N
OH2
Cu
Cu
OH
N
H O
2
C
H
C H
12 25
12
25
N
N
6
Br — 8H O
2
1
5+
2
6,27
N
N
ligand complexes in detail.
In the present work, we
C12H25
C12H25
reported the control over the crystal structure and the
coordination mode of the Cu(II)/12Bpy complex with the help
of varying the pH level in solution. Cu(II)-12Bpy complex
solution afforded blue crystals with a dihydroxo-bridged
N
N
N
Cu
Br
N
C12H25
C12H25
binuclear structure, [Cu
2
(L)
previously reported, at pH > 4.3 and green crystals with a
mononuclear coordination form, [Cu(L) Br]Br ·5H O ( ) at pH <
.8, respectively (Chart 1). Especially, such two sets of crystals
2 2 2 2 6 2
(μ-OH) (H O) ]Br ·8H O (1), as
N
N
2
1
5Br — 5H O
2
2
5
2
2
2
3
coexisted at pH = 3.8~4.3.
Synthesis of 12Bpy
1
2Bpy was synthesized in two steps using methods previously
1
2
Experimental Section
described.
dipyridyl
In brief, bromination of 5,5’-dimethyl-2,2’-
with N-bromosuccinimide afforded 5,5’-
Materials and general methods
bis(bromomethyl)-2,2’-bipyridine, 3. Subsequent quaterisation
reaction of with N,N-dimethyldodecylamine afforded 12Bpy,
All commercially available reagents and solvents were
separately purchased from Alfa Aesar, Adamas and General-
reagent. The reagents used were of reagent grade, and the
solvents used were of analytical grade unless otherwise stated.
All reagents and solvents were directly used without further
3
1
4
1
4
3
(Scheme 1). H NMR (400 MHz, CDCl ), δ = 0.88 (t, 6H), 1.24–
.35 (m, 36H), 1.74 (s, 4H), 3.30 (s, 12H), 3.68 (s, 4H), 5.71 (s,
H), 8.34 (s, 4H), 9.01 (s, 2H).
2
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ARTICLE
NBS
isothermal crystallization. It was found that the pH value of the
−1
AIBN
N
N
primary mixed 12Bpy/CuBr solution (0.45 mmol·L for each
2
Br
Br
N
N
3
component) was about 5.5, which was mainly attributed to the
hydrolysis of CuBr
the pure 12Bpy solution was almost neutral. A most striking
phenomenon was that the pH value in 12Bpy/CuBr solution
2
, but was scarcely relevant to 12Bpy since
C
12
H
3 2
25N(CH )
2
N
Br
gradually decreased, as expected. When the pH of the solution
was decreased down to around 4.8, blue grain crystals began
to form and grow at the air/liquid interface, having the same
N
2
1
structure as previously reported, shown in Figure S1A. Such
growth continued until pH reaches approximately 4.3. A new
kind of green long striped crystal was then formed and the
concomitance of blue and green crystals occurred when pH
was in a relatively narrow range of 3.9~4.3, as shown in Figure
S1B. Finally, subsequent growth of green crystals could take
place, whereas blue crystals stopped growing, when the pH
was further down to 3.8 and almost remained unchanged
afterwards. As a matter of fact, the hydroxo-bridged
complexes were generally synthesized in an alkaline
N
Br
N
4
Scheme 1. Synthesis of 12Bpy
Preparation of complex crystals
1
7
18
19
environment by additionally adding NaOH, KOH, Na CO3
2
A slurry of 12Bpy (34.60mg, 0.045mmol) in 50 mL Milli-Q water
2
0
−1
or K CO . In 12Bpy/CuBr mixture system, however, blue
2 3 2
at a concentration of 0.9 mmol·L was heated to 60 °C for
more than 30 minutes to make sure that 12Bpy was
precipitates could immediately be obtained after adding a little
quantity of NaOH solution, which might be commingled with
completely dissolved. An aqueous solution (50mL) of CuBr
10.05mg, 0.045mmol) at the same concentration was added
2
some insoluble Cu(OH) , because of the slight solubility of
2
(
1
2Bpy ligands and their metal complexes in aqueous solution.
to the above solution. The mixture was stirred for a while at
room temperature and then filtered through a 0.45-mm mixed
cellulose esters (MCE) membrane syringe filter. The filtrated
mother liquor was placed into a glass vessel at room
Taken together, our results and these facts motivated us to
explore the possibility that the crystal structures or the
coordination modes could be tuned via directly varying pH of
solution.
temperature of 25.0
± 1.0 °C to grow complex crystals at the
air/water interface by a slow solvent evaporation. It should be
noted that the initial pH is about 5.5 when mixing 12Bpy and
CuBr
adjusted in three ranges, i.e., pH > 4.3, pH = 3.8~4.3 and pH <
.8, using 1.0 mol/L hydrogen chloride. Blue grain and green
2
solutions. Here, pH values in such mixed solution were
3
long striped crystals were obtained at pH > 4.3 and pH < 3.8,
respectively, while both two types of crystals were
simultaneously obtained at pH = 3.8~4.3. Crystalline forms
were generally characterized by PXRD and SC-XRD. As for blue
crystals, analysis calculated for C80
038.7): C, 47.13%; H, 8.21%; N, 5.50%. Found: C, 47.36%; H,
.22%; N, 5.51%. As for green crystals, analysis calculated for
CuN = 1851.1): C, 51.91%; H, 8.39%; N, 6.05%.
6 2 8 12 r
H166Br Cu N O (M =
2
8
C
80
H154Br
6
8 5 r
O (M
Found: C, 51.73%; H, 8.40%; N, 6.03%.
Figure 1. pH-controlled crystallization of 12Bpy/CuBr2 at
different pH: (A) pH = 5.0, (B) pH = 4.0 and (C) pH = 3.0. D and
E are enlarged optical microscopic images of blue and green
crystals, respectively.
Results and discussion
Crystal growth at various pH
The dihydroxo-bridged structures between two copper centers
2
1
in blue crystal were found in our previous work. Such bridged
hydroxyls were a likely consequence of the dissociation of
water molecules, since we did not offer any other oxygen
sources except water in the process of the crystallization. If
In the present work, we sought to determine the
dependence of solvent pH on the formation of complex
crystals. With this purpose in mind, we chose three pH values,
i.e., 5.0, 4.0 and 3.0, adjusted by addition of dilute
hydrochloric acid or sodium hydroxide, as target points. At
such three pH, different complex crystals could be obtained at
the air/water interface due to having the amphiphilic nature of
+
this was the case, a certain amount of H ions would be
released as depleting hydroxyl ions, which definitely led to a
reduction in pH value of solution. Thus, we tracked and
monitored the change in pH values of such solution during the
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2
9
4,
shown in Figure 1. As expected, only blue grain crystals both crystals from the pH-controlled crystallization was
and the green long striped crystals were obtained in solution A consistent with that from the spontaneous crystallization. The
and C, respectively, while such two kinds of crystals coexisted powder XRD measurement results as well as the simulated
at the air/water interface in solution B. The latter case is single crystalline XRD data of these two crystals are given in
similar to the spontaneous crystallization of 12Bpy/CuBr
2
Figure S2. The observed experimental patterns for both blue
without adding dilute hydrochloric acid when pH is in the and green crystals are in good agreement with those
range of 3.8~4.3 (Fig S1B). More interestingly, at lower pH (pH calculated results, indicating that either of these two crystals
<
3.8), green crystals can be formed alone. This suggests that has the same structure, in spite of the different preparing
blue or green crystals may be prepared exclusively via simply methods, i.e., pH-controlled or spontaneous crystallization.
adjusting pH of solution. A primary structural analysis of the green crystal shows the
Our experimental results demonstrate that the pH of formation
solution plays a decisive role in control over the formation of [Cu(L) Br]Br
of
the
mononuclear
neutral
complex
2
5
·5H
2
O with a metal-ligand stoichiometry of 1:2,
complex crystals, that is, the increase of acidity indeed which is distinguished from that of the dihydroxo-bridged
encourages the production of the green crystal, and binuclear Cu(II) complex in the blue crystal (1:1). The crystal
2
+
meanwhile inhibits the growth of the blue crystal. Recently, structure reveals that one Cu ion is located between two
some methodologies were reported to control over the 12Bpy ligands and coordinated with both 2,2’-bipyridines to
polymorphism, size and structure of crystals or assemblies by form a coordination complex
3
2
, as a building block. Here, each
0-32
means of pH modification.
Here, the complex crystal with of two 2,2’-bipyridines adopts nearly coplanar conformations
different shapes and colors is a likely consequence of the with N-C-C-N torsion angles of 2.1° and 12.1°, respectively,
dissimilar microscopic or molecular organization, including the while both of them reflect a distorted structure with a ~50.4°
coordination form between metal ions and ligands. Therefore, dihedral angle between two bipyridine planes containing N(3),
in the present work we determined the structure of the green Cu(1), N(4) and N(1), Cu(1), N(2), as shown in Figure S3. The
crystal and the binding mode between Cu(II) and 12Bpy in coordination and atom-numbering scheme are shown in Figure
aqueous solution, in order to understand the pH-controlled 2. The stereochemistry of the Cu(II) center is best described as
growth process and mechanism of these two complex crystals. a distorted trigonal bipyramid with its Addison constant τ =
0.75, [τ = |α-β|/60°], where α and β are the two largest angles
Table 1. Summary of crystallographic references of the blue around the central atom (175.77(26)° for N(1)-Cu(1)-N(3) and
and green crystal.
130.54(26)° for N(2)-Cu(1)-N(4)). The parameter τ can be
estimated as an index of the distortion degree from trigonal
bipyramid to square pyramid according to Addison’s
a
Blue Crystal
Green Crystal
Formula
Fw (g/mol)
C H Br Cu N O
2
C H Br CuN O
80 154 6 8 5
1851.11
triclinic
8
0
166
6
8
12
33
definition. In the ideal trigonal bipyramid, τ= 1, and in the
2038.74
Crystal system monoclinic
ideal square pyramid, τ= 0. The coordination bond of Cu-N
lengths from 1.973(6) Å to 2.068(7) Å and the Cu(1)–Br(1)
bond distance is 2.580(1) Å, similar to those reported Cu(II)
ꢀ
Space group
Color
Habit
P21/c (14)
blue
grain
P1(2)
green
3
3-36
needle
a=9.4175(5)Å
complexes of 2,2’-bipyridine or its derivatives.
Particularly,
a=30.2435(5)Å
b=9.4252(2)Å
c=18.2742(3)Å
α=90°
β=107.5450(10)°
γ=90°
4966.75(16)
Cu(1) also shows a very weak coordination to Br(6) with a
distance Cu(1)-Br(6) = 4.025(1)Å. However, this copper is
better described as a 5-coordinate rather than a 6-coordinate
system, because, apart from the weakness of the sixth contact,
the axial trans angle, Br(1)-Cu(1)-Br(6), is distorted at
b=17.1528(9)Å
c=28.9801(16)Å
α=79.223(3)°
β=89.723(4)°
γ=82.329(3)°
4556.6(4)
Unit cell
dimensions
37
3
Volume(Å )
165.34(5)°. Here, the difference in the distance between two
-
Br irons and Cu(II) center is likely consequence of the steric
Z
2
2
a
21
restraint that probably results from four quaternary
ammonium head groups of 12Bpy orientated towards the
same side. In addition, each building block sticks to five
molecules of crystalline water, which is consistent with the
Data taken from Reference
Crystal structures of green crystals
The crystal structure of the green crystal, made from the result of thermogravimetric analysis, as shown in Figure S4.
spontaneous crystallization process without adding Here, one very important point that deserves comment is
hydrochloric acid or sodium hydroxide, can be confirmed by there are two crystalline water molecules, O(1) and O(4),
single crystal X-ray diffraction and crystal details are connected to the Br(1) and Br(6) bromine ions, via the
summarized in Table 1. More details of the structure interaction of O-H
refinement references are shown in Table S1. For comparison, Such double O-H
data of the blue crystal are also compiled in Table 1 and Table complexes
…
Br hydrogen bonds, as shown in Figure S5.
…Br hydrogen bonds between neighboring
2
, are favorable for forming one-dimensional
S1. In particular, the power XRD characterizations were carried supramolecular architecture, [Cu(L)
out to characterize structures of green and blue crystals made axis.
from the pH-controlled crystallization via adding HCl and
2 2 2 ∞
Br(H O) Br] , along the a
NaOH, which allowed us to evaluate whether the structure of
4
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intermolecular interactions, i.e., van der Waals force, as an
unneglectable cohesive force along the c axis. It should be
emphasized that the hydrophobic interaction between alkyl
chains would also function in complex solution during the
2
9
crystal growth process. More intriguingly, the orientation of
these tightly stacked alkyl chains is almost identical (Figure
3
A), and also C-C-C planes of the alkyl chains in all-trans
conformation tend to be parallel to each other (Figure 3B). The
weak intermolecular interactions between complexes along
2
the a axis and b axis are mainly dominated by the moieties in
the vicinity of headgroups, including the spacer groups and
quaternary ammonium ions of 12Bpy Gemini surfactant, as
showed in Figure 3C. The packing diagram indicates there are
two types of strong π-π stacking interactions, indicated by
brown dash lines in Figure 3C, between the adjacent bipyridyl
Figure 2. Coordination diagram of the green crystal with the rings with center-to-center separation distances (d ) 4.964 Å
c-c
atom-numbering scheme. H atoms and alkyl chains are and 4.351 Å. The details of π-π stacking interactions, including
4
0
omitted for clarity.
their distances and various crossing angles, are listed in Table
. Meanwhile, one of the quaternary ammonium groups of a
Figure 3 illustrates the molecular packing mode in a unit cell 12Bpy molecule always points to the center of a pyridyl ring of
and multiple weak interactions between complexes . Each an adjacent molecule, facilitated the cation-π interactions,
2
2
unit cell contains eight alkyl chains, as shown in Figure 3A, and indicated by green dash lines in Figure 3C. Two typical cation-π
the occupied area of each alkyl chain is estimated to about interactions are observed in the crystal structure with
2
0Å on the aob projection plane, which is consistent with the distances of 5.422 Å (I and III) and 6.072 Å (III and IV),
2
molecular limiting area of the hydrocarbon chain tightly respectively. Besides, electrostatic attractions between
3
8,39
arranged at the air/water interface.
Carful examination of quaternary ammonium head groups and bromide counter ions
the extended structure shows that 12Bpy almost presents a provide additional cohesive forces on the aob projection plane
fully stretched configuration and the extended alkyl chains of to enhance the stability of crystal structure, with distances of
12Bpy molecules partially interdigitate. These partially- N-Br between 4.059(7) Å and 5.374(7) Å, as shown in Figure
interdigitated tails with a tightly contact facilitate S6.
Figure 3. Single-crystal structure of green crystal: packing diagram observed along the a-axis (A) and the b-axis (B), respectively.
C) The π-π stacking interactions (shown by brown dash lines) and the cation-π interactions (shown by green dash lines) between
(
complex molecules. For convenient demonstration, I, II, III and IV are referred to as the adjacent complexes interacted with each
other. Colour for atoms: C, grey; N, blue; O, red; Br, olive; Cu, turquoise. H atoms in all schematics and alkyl chains in (C) are
omitted for clarity.
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Table 2. Parameters Obtained from the Analysis of the π-π Stacking Interactions
CrystEngComm
a
π-π interaction type
d
π-π (Å)
d
c-c (Å)
α (deg)
β (deg)
γ (deg)
b
c
pyridyl ring 1- pyridyl ring 2 (I-II)
3.238
3.456
4.964
4.351
16.52
4.98
57.41
38.08
41.61
37.60
pyridyl ring 3- pyridyl ring 4 (II-IV)
a
Stacking parameters taken into account are dπ-π (perpendicular distance between the bipyridyl planes), dc-c (center to center
distance of the pyridine rings), and α (dihedral angle between the two pyridine planes). β and γ are the angle of the centroid-
b
centroid distance (dc-c) vector with the normal to the pyridine ring plane and its alternate interior angle, respectively. Here the
rings 1, 2, 3 and 4 represent the six-membered ring connected by N(1)-C(1)-C(2)-C(3)-C(4)-C(5), N(2)-C(6)-C(7)-C(8)-C(9)-C(10),
c
N(3)-C(11)-C(12)-C(13)-C(14)-C(15) and N(4)-C(16)-C(17)-C(18)-C(19)-C(20) atoms, respectively. In the dπ-π, the planes used to
calculate the perpendicular distances refer to the least-squares fitting planes of the 2,2’-bipyridine groups, and as such the two
fitting planes between molecule I and molecule II are parallel (the same with II and IV).
As is apparent, the structure of green crystals is remarkably held the coordination mode constant or varied in the
2
1
different from that of blue crystals reported previously. Such crystallizing process, if ever, and further to measure the
differences in morphologies, crystal structures as well as association constant (or called binding constant) between
intermolecular weak interactions lends support for a pH metal and ligand in solution. With this purpose in mind, we
manipulation on the forming crystals and the coordination studied the complexation of Cu(II) with 12Bpy using titration
4
1,42
modes of metal/ligand complexes.
curves based on UV-Vis spectroscopic analysis.
UV-Vis
absorption spectra were recorded upon the addition of metal
salts stepwise to the solution of the ligand, as shown in Figure
Complexation behaviors in solution
Crystallographic data illustrated that the two solid-state
4A. The titration curves obtained enabled the study of the
structures of metal complexes
different coordination modes, depended on the pH in
2Bpy/CuBr solution. Thus, this promoted us to evaluate
whether the coordination mode in crystal structures was
similar to that in solution in both cases, that is, 12Bpy/CuBr
1 and 2 originated from
binding process and provided a means to quantify complex
stability. Metal-ligand stoichiometry was determined through
43,44
1
2
continuous variation plots (JOB plots),
B.
as shown in Figure
4
2
Figure 4. (A) Titration curves of 12Bpy solution in aqueous at 298K upon addition of CuBr , where the concentration of 12Bpy
2
-1
was kept constant at 0.05 mmol·L . (B) JOB plot for 12Bpy and CuBr in aqueous solution, λ = 316.7nm, where the total
2
obs
-1
concentration of 12Bpy and metal salts was kept constant at 0.2 mmol·L . (C) The absorption spectrums of the blue and green
crystal as well as pure 12Bpy (uncoordinated) in chloroform.
In Figure 4A, the UV-Vis spectrum of pure 12Bpy in the ratio of 0.5 was obtained, indicative of a 1:1 metal-ligand
2
+
aqueous solution displayed absorption maxima at 288nm, stoichiometry between 12Bpy and Cu . According to the JOB
which refers to the characteristic peak of bipyridine group plot data and clear isosbestic points, we speculate that only
mainly attributed to its π-π* excitation. Upon the addition of three species exist: 12Bpy ligand, metal copper ion, and the
copper salts to the solution of 12Bpy, the main absorption of metal-ligand complex in the titration process. From a titration
ligand at 288 nm decreased, and meanwhile, two new of 12Bpy against CuBr
2
(monitored at 316.7 nm), the binding
5
absorptions generated at 304 nm and 316.7 nm respectively, constant was estimated to be 1.16×10 M using the nonlinear
-1
4
1
which implies metal complexes derived from Cu(II) and 12Bpy least-squares regression method, as shown in Figure S7.
are formed. It is worth noting that two clear isosbestic points Although this binding constant measured in the present work
were observed at 264 nm and 297 nm in the titration curves, is about an order of magnitude smaller than the one of a
4
5
indicating the formation of a single new complex. A JOB plot similar system reported in a literature, which maybe results
was then constructed by monitoring the absorbance at 316.7 from the steric hindrance and the charge repulsion of
nm for aqueous mixtures of 12Bpy and CuBr
2
with different quaternary ammonium head groups. Such a binding constant
compositions (Figure 4B). A maximum absorption at the molar indicates that there appears to be a strong association
6
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ARTICLE
2 2
interaction between 12Bpy and CuBr . In 12Bpy/CuBr mixture energy structures for the studied threefold hydrated and
-1
system with a 0.45mmol·L concentration and a 1:1 mole ratio, fourfold hydrated complexes. As for the latter, intermolecular
there should be about 87 percent of 12Bpy ligands that form hydrogen bond formed in the second hydration shell stabilizes
complexes in solution, based on this association constant. If the complex more than axial coordination of water, resulting
+
-4
-1
2+
only the complex 1 is formed, H ions with a ~3.9×10 mol·L
2 3 2
in the form of [Cu(Bpy)(H O) ] (H O) (Figure 5B) with square
concentration would be released. Thus, this would lead to the pyramidal coordination geometry, which is much similar to
2
+
fact that the resulting pH value would be apparently less than that of the threefold hydrated complex, [Cu(Bpy)(H
2 3
O) ]
4
in the corresponding 12Bpy/CuBr solution, which (Figure 5A). It is likely that, in the hydrated environment, the
2
remarkably conflicts with what we have observed. Therefore, relative stability of the fivefold coordinated Cu(II) ion is better
this fact suggests that the coordination mode with the than the one of the sixfold coordination, supported by the fact
1
2Bpy/CuBr
from that in the blue crystal, i.e., the complex
The UV-Vis spectra of blue and green crystals as well as pure here, similar to the conclusion reported in literatures.
2Bpy were also characterized in dehydrated chloroform our case, one 12Bpy molecule may, accordingly, occupy two
2
mole ratio of 1:1 in solution is definitely different that the Cu(II) ion adopts fivefold coordination geometry in
1.
both blue crystals (see Figure S9) and green crystals present
5
0-52
In
1
2
+
solution based on the fact that metal complexes generally hold sites of Cu ion with fivefold coordination to form the 1:1
coordination modes unchanged in common organic solvent, as Cu(II)/12Bpy complex in solution through its bipyridine ligand,
shown in Figure 4C. The characteristic absorption peak of in which the other three sites are coordinated by water
12Bpy was observed at 287.5 nm, which is similar to the one in molecules, as shown in Chart 2.
aqueous solution, while as for the spectrum of the green
crystal, the maximum absorption band appeared at 304 nm,
which may be ascribed to the coordination mode in complex
2.
Interestingly, in the spectrum of the blue crystal, the
absorption maximum with a degree of red-shift was presented
at ~311 nm, which can be assigned to the partial contribution
of O-Cu(II) charge transfer (CT) transition, stemmed from the
form of dihydroxo-bridged structure in the complex
1, as
1
,46
reported in the literatures. Hence, UV-Vis results in aqueous
solution may imply that the absorption peak generated at 304
nm is a likely consequence of the coordination of Cu(II) ion
with 12Bpy, while the band presented at 316.7 nm may result
from the combination of Cu(II) ion with water, i.e., the
Figure 5. The structure optimization for threefold and fourfold
2
hydrated Cu(II)/Bpy complex in form of [Cu(Bpy)(H O) ] (A, τ
+
2
3
2
+
=
0.38) and [Cu(Bpy)(H O) ] (H O) (B, τ = 0.13), respectively.
2 3 2
Chart 2. The existence form of Cu(II)/12Bpy complex in
aqueous solution.
4
7
hydration of Cu(II) ion. In addition, the visible spectrum of
4
+
OH2
Cu
blue crystals displayed a broad d–d transition band centred at
H O
OH2
2
660 nm, while green crystals showed a broad d–d band at 780
nm, as showed in Figure S8. Here, the former band of 660 nm
can be ascribed to the square pyramidal or distorted square
pyramidal structures of complexes, usually showing a band in
the 550-660 nm range. In contrast, the latter spectral feature
N
N
N
1
N
1
11
is typical for trigonal bipyramidal geometries which generally pH-controlled crystallization mechanism
4
8
exhibit a maximum at around 800 nm. Thus, such spectral
features were strongly supported by geometries of complexes
observed from X-ray crystallography.
Taken together, crystal structures, pH-controlled crystallization
and complexation behaviors in aqueous solution, suggest a
forming processes of the corresponding blue and green
crystals. A Cu(II)/12Bpy complex is firstly formed in aqueous
solution with a 1:1 binding stoichiometry and a copper(II)
As we know, the hydrated Cu(II) ion definitely exists in
aqueous solution, though a controversial finding was found,
namely that the expected Jahn-Teller distorted octahedral
hydration shell of the cation, formed by six coordinating water
molecules, was not the most probable aqua-ion structure, but
this was instead a five-fold coordinated species of non-
octahedral geometry. Bowron et al. suggested that an average
centre coordinated to three terminal water ligands, assumed
4+
to Cu(L)(H O) . In the early stage of crystallization at pH > 4.3,
2 3
the blue crystal with the dihydroxo-bridged structure is
generated via the intermolecular dehydration and
deprotonation of the bound water ligands in equatorial
coordination sites, as shown in Equation 1. Thus, this results in
a gradual reduction in pH of solution in the forming process of
the blue crystal due to the proton release. Especially, the axial
2
+
49
coordination for each Cu ion was 4.5
± 0.6 water molecules.
Therefore, in order to gain insight into the probable structural
form of the hydrated 1:1 Cu(II)/12Bpy mononuclear complex,
electronic structure calculations were performed with the G09
quantum chemistry suite of programs. Geometries were
optimized by means of a DFT functional at PBE0/6-311+g(d)
level of theory, and 2,2’-bipyridine group (Bpy) was chosen to
represent 12Bpy. Figure 5 shows the optimized minimum
coordinated water is retained to maintain
a
fivefold
coordinated square pyramidal geometry.
ꢊꢊ
ꢊꢊꢊꢋꢌ
ꢁꢂꢃꢄꢅꢃꢆ ꢈꢅꢉ
ꢊ
ꢊꢊꢋꢌ
ꢌ
2
→ ꢁꢂ ꢃꢄꢅ ꢃꢍ ꢈꢆꢅ ꢃꢆ ꢈꢅ
ꢎ 2ꢆ ꢈ (1)
ꢇ
ꢇ
ꢇ
ꢇ
ꢇ
ꢇ
ꢉ
ꢊꢊ
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J. Name., 2013, 00, 1-3 | 7
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ARTICLE
CrystEngComm
When the pH of solution is decreased down to 4.3, the
Conclusions
reaction (1) would be significantly inhibited due to a great
+
number of the releasing H , and then the green crystal was
In summary, altering the pH of solution has been
demonstrated to be a simple and straightforward method to
control the crystal growth of two kinds of Cu(II)/12Bpy
complexes. Under weak acidic conditions (here, pH > 4.3), the
blue crystals with a bis(hydroxo)dicopper coordinated mode
are formed, whereas under the slightly stronger acidic
conditions (pH < 3.8 in this work), the green crystals with a
mononuclear coordination mode of a distorted trigonal
bipyramidal geometry are readily fabricated, and meanwhile
the blue crystals are completely inhibited. In the modest acidic
environment, such two sets of crystals concomitantly exist in
solution. We propose a pH-controlled growth process and
forming mechanism of such two kinds of complex crystals on
the basis of our results. Our findings demonstrate that the pH
adjustment would not only simplify and control the
preparation of crystals, but should also allow for the ready
fabrication of high-surface-area analogues (e.g., porous
molecular crystals and metal-organic frameworks) for practical
applications.
fabricated. In such green crystals, two striking features were
5
+
observed: a new complex Cu(L)
2
Br , derived from the
replacement of the other two equatorial coordinated water of
4
+
the complex Cu(L)(H
2
O)
3
with the uncoordinated 12Bpy
monomer, are presented on the basis of crystallographic
results; the axial water ligand is simultaneously replaced by a
bromide ion, as shown in Equation 2. Our rationale was based
on the consideration of the association constant, that is, a
certain level of the uncoordinated 12Bpy monomers should
exist in the solution. At the same time, the acidity is generally
3
7,53
in favor of the formation of Cu(II) halide complexes.
Here, a
non-coplanar distorted structure in the two bipyridine
moieties together with Cu(II) centre is likely a result of the
severe steric interference between the hydrogen atoms on 6-
and 6’-carbon in pyridine ring. The coordination geometry of
complex
2 is, thus, distorted from square pyramid to trigonal
bipyramid. In addition, another bromide ion, showing a very
weak coordination to Cu(II) centre, can be described as the
counter ion of the metal ion due to the electrostatic attraction.
ꢊꢊꢊꢋꢌ
ꢎ ꢄꢇꢌ ꢎ ꢏꢐꢑ → ꢁꢂꢃꢄꢅ ꢏꢐ ꢎ 3ꢆ ꢈ
ꢊ
ꢒꢌ
ꢁ
ꢂꢃꢄꢅꢃꢆ ꢈꢅ
(2) Acknowledgements
ꢇ
ꢉ
ꢇ
ꢇ
One final point that deserves comment is that the pH of This work is supported by the National Natural Science
solution presents a slight decrease in the concomitant process Foundation of China (21576079, 91334203), the 111 Project of
of these two kinds of crystals, which means that a few of blue China (No.B08021), and the Fundamental Research Funds for
crystals is still formed in this stage. Anyhow, our findings do the Central Universities of China.
demonstrate that the reduction in pH of solution, resulted
from the deprotonation of hydrated Cu(II)/12Bpy complexes,
remarkably inhibits the formation of the blue crystal, and Notes and references
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2
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Table of Contents
A pH-controlled crystal growth of two copper complexes with different coordination modes is
successfully manipulated by means of pH adjustment
.
1