Synthesis, structures, and DFT study of CuBr based coordination polymers via in situ reduction of copper(II)

Article abstract of DOI:10.1021/cg5013236

This paper describes the one-pot synthesis of two CuBr based coordination polymers, {[Cu(??₂-L¹)Br]?·1.87H₂O}_n (1) and {[Cu(??₂-L²)Br]?·C₄H₁₀O}_n (2), where

Full text of DOI:10.1021/cg5013236

Article
pubs.acs.org/crystal
Synthesis, Structures, and DFT Study of CuBr Based Coordination
Polymers via in Situ Reduction of Copper(II)
Subrata Jana, Klaus Harms, Antonio Bauza, Antonio Frontera,* and Shouvik Chattopadhyay*
́
†
‡
§
,§
,†
†
‡
§
Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700 032, India
FachbereichChemie, Philipps-Universitat Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany
Departament de Química, Universitat de les Illes Balears, Cra. de Valldemossa, km 7.5, 07122 Palma, Illes Balears, Spain
S Supporting Information
̈
*
ABSTRACT: This paper describes the one-pot synthesis of two CuBr
1
based coordination polymers, {[Cu(μ -L )Br]·1.87H O} (1) and {[Cu(μ -
2
2
n
2
2
1
L )Br]·C H O} (2), where L = 2,3-dihydro-5,6-bis(4-methoxyphenyl)-
4
10
n
2
pyrazine and L = 5,6-diphenyl-2,3-dihydropyrazine, upon reduction of
copper(II) at ambient conditions. The structures have been confirmed by
single crystal X-ray diffraction analysis. Both complexes are found to be
highly inert toward oxidation. Finally, a density functional theory (DFT)
study of the energetic features of several noncovalent interactions observed
in the solid state has been analyzed and characterized using Bader’s theory
of “atoms in molecules” and the cuprophilic interactions in complex 2 using
natural bond orbital methodology.
2
5−28
INTRODUCTION
in a hydrothermal condition.
In our previous paper, we
■
reported for the first time the formation of a CuSCN based
coordination polymer starting from a copper(II) salt in a
nonhydrothermal reaction condition at room temperature. In
the present work, we have extended the concept to form CuBr
based coordination polymers starting from copper(II). Herein,
we report the synthesis and characterization of these two
copper(I) complexes, {[Cu(μ
{[Cu(μ -L )Br]·C H10O} (2), where L = 2,3-dihydro-5,6-
bis(4-methoxyphenyl)pyrazine and L = 5,6-diphenyl-2,3-dihy-
dropyrazine. Complex 1 contains only terminal bromides and
forms a one-dimensional (1D) chain, whereas complex 2 forms a
three-dimensional (3D) architecture with Cu Br cores. Both
complexes show fluorescence. The presence of methoxy groups
in L that are crucial in the crystal growth in one direction by
promoting the intermolecular interaction of the infinite 1D
chains of complex 1 by establishing CH ···π interactions. In
addition, the presence of lattice water molecules in 1 is also
crucial for the final 3D architecture of this complex by connecting
the infinite 1D chains by means of O−H···Br interactions. These
interactions have been analyzed by means of density functional
theoretical calculations.
The chemistry of the coordination polymers has advanced
extensively recently, affording fascinating structures with
potentials in applications.
variety of molecular building blocks with different interactions
among them. High-dimensional coordination polymers often
exhibit important functionalities which are absent in low-
dimensional structures. Coordination polymers contain two
essential components, viz., nodes and linkers. Linkers give a
29
1
−4
They are constructed from a
1
5
2
-L )Br]·1.87H
2
O}
n
(1) and
2
1
6
2 4 n
2
wide variety of linking sites with adjusted binding strength and
directionality. On the other hand, transition-metal ions are
frequently employed as nodes. Instead of a bare metal ion,
7,8
6
6
suitable metal−ligand complexes are also used as nodes. In the
present case, we have used CuBr clusters as the nodes and
pyrazine derivatives as the linkers to form the coordination
polymers.
1
10
3
Multinuclear d cuprous halide clusters are important because
of their interesting photochemical and photophysical properties,
with potential applications as light-emitting diodes, luminescent
9
−14
probes, and photovoltaics.
They have also received
considerable attention in coordination chemistry because of
the exciting structural features of cuprous halide clusters acting as
nodes in the construction of multidimensional coordination
1
5−18
EXPERIMENTAL SECTION
networks.
Reactions involving various structural motifs of
■
copper(I) halides and diverse multidentate N-donor ligands lead
to the formation of various coordination networks with
intriguing structures and functional properties.
Materials. All chemicals used were purchased from Sigma-Aldrich
and were of reagent grade. They were used without further purification.
19−24
Cu(I) coordination polymers are usually prepared by reaction
of Cu(I) with appropriate ligands or by the reaction of copper
metal with an appropriate reagent in appropriate solvents, usually
Received: September 4, 2014
Revised: November 19, 2014
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/cg5013236 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Physical Measurements. Elemental analyses (carbon, hydrogen,
to the surface) and d (the distance to the nearest nucleus internal to the
i
nitrogen, and metal) were carried out using a PerkinElmer 2400 II
surface), are defined. The normalized contact distance (dnorm) based on
d and d is given by
−1
elemental analyzer. IR spectra in KBr (4000−400 cm ) were recorded
using a PerkinElmer Spectrum Two Fourier transform infrared (FTIR)
spectrophotometer. The UV−visible spectrum for complex 1 was
recorded on a PerkinElmer lambda 35 spectrophotometer at 298 K in
acetonitrile. The UV−visible diffuse reflectance spectrum for complex 2
was recorded on a Shimadzu UV 2401PC with an integrating sphere
e
i
vdW
vdW
di − ri
ri
de − re
re
dnorm
=
+
vdW
vdW
where rivdW and revdW are the van der Waals radii of the atoms. The value
of dnorm is negative or positive depending on intermolecular contacts
being shorter or longer than the van der Waals separations. The
parameter dnorm displays a surface with a red−white−blue color scheme,
where bright red spots highlight shorter contacts, white areas represent
contacts around the van der Waals separation, and blue regions are
devoid of close contacts. For a given crystal structure and set of spherical
attachment. BaSO was used as the background standard. The steady
4
state emission spectra were recorded on a PerkinElmer LS 55
luminescence spectrometer at room temperature. The topological
analysis of the complexes was produced using the TOPOS
30−32
program.
X-ray Crystallography. Single crystals of the complexes, having
suitable dimensions, were used for data collection using a Bruker D8
QUEST area detector diffractometer for 1 and STOE IPDS2T
diffractometer for 2, equipped with graphite-monochromated Mo Kα
radiation (λ = 0.710 73 Å) at 100 K. The molecular structures were
46
atomic electron densities, the Hirshfeld surface is unique and thus it
suggests the possibility of gaining additional insight into the
intermolecular interaction of molecular crystals.
Theoretical Methods. The geometries of the complexes included in
this study were computed at the BP86-D3/def2-TZVP level of theory
using the crystallographic coordinates within the TURBOMOLE
33
solved using the SHELX-2014 package. Non-hydrogen atoms were
refined with anisotropic thermal parameters. Hydrogen atoms were
placed in their geometrically idealized positions and constrained to ride
on their parent atoms. For 1, multiscan empirical absorption corrections
4
7
program. This level of theory that includes the latest available
dispersion correction (D3) is adequate for studying noncovalent
interactions dominated by dispersion effects such as π-stacking. The
basis set superposition error for the calculation of interaction energies
34
were applied to the data using the program SADABS. Two of the three
independent solvent water molecules showed occupation factors less
than 1 (0.35 and 0.53, respectively). For 2, integration absorption
corrections were applied to the data using indexed faces. Programs used
4
8
has been corrected using the counterpoise method. The “atoms-in-
4
9
molecules” (AIM) analysis of the electron density has been performed
35
50
were STOE X-AREA and STOE X-RED/X-SHAPE. For 2, the
at the same level of theory using the AIMAll program.
1
occupation factors of Cu(2) and Br(2) were less than 1 and refined to
Syntheses. Synthesis of Complex {[Cu(μ -L )Br]·1.87H O} (1).
2
2
n
36
37
The cyclic Schiff base ligand, L1 (2,3-dihydro-5,6-bis(4-
methoxyphenyl)pyrazine), was prepared by refluxing 1,2-diamino-
ethane (2 mmol, 0.14 mL) with 4,4′-dimethoxybenzoin (2 mmol,
0
.42. The figures were prepared using ORTEP and DIAMOND. A
summary of the crystallographic data is given in Table 1. Selected bond
0
.540 g) in methanol for ca. 1 h. The ligand was not isolated and used
Table 1. Crystal Data and Refinement Details of Complexes 1
directly for the synthesis of the complex. A methanol solution of
copper(II) perchlorate hexahydrate (2 mmol, 0.740 g) was added to the
methanol solution of the ligand (1 mmol) and stirred for 30 min to
develop a deep red precipitate. It was collected by filtration and
dissolved in acetonitrile. A methanol/water (2:1) solution (5 mL)
containing potassium bromide (2 mmol, 0.238 g) was added to it. The
mixture was stirred for ca. 5 h. Diffraction quality single crystals were
obtained by slow diffusion of diethyl ether into the mother liquor after a
few days.
and 2
complex
1
2
formula
C H Br Cu N O
C H Br5.42Cu5.42N O
3
6
39.75
2
2
4
5.87
52 52
6
formula wt
cryst size (mm)
temp (K)
cryst syst
909.33
1554.49
0.11 × 0.22 × 0.44
100
0.17 × 0.16 × 0.08
100
monoclinic
monoclinic
space group
a (Å)
P2 /c
P21
1
Yield: 0.45 g (49%). Anal. calcd for C H Br Cu N O (FW
36
39.75
2
2
4
5.87
10.0229(6)
24.6728(14)
15.0813(8)
90
12.9750(4)
9
09.33): C, 47.55; H, 4.41; N, 6.16; found: C, 47.5; H, 4.5; N, 6.3. IR
−
1
−1
−1
b (Å)
16.1618(7)
13.1070(5)
90
(KBr, cm ): 1605 (υ
), UV−vis, λ (nm) [εmax (L mol cm )]
CN
max
c (Å)
(acetonitrile): 290 (14 591), 370 (770). Magnetic moment: diamag-
α (deg)
netic.
2
Synthesis of Complex {[Cu(μ -L )Br]·C H O} (2). The cyclic Schiff
β (deg)
98.3976(19)
90
94.886(3)
90
2
4
10
n
2
base ligand, L (5,6-diphenyl-2,3-dihydropyrazine), was prepared in a
similar way using benzoin (2 mmol, 0.424 g) instead of 4,4′-
dimethoxybenzoin. The ligand was not isolated and used directly for
the synthesis of the complex. A methanol solution of copper(II)
perchlorate hexahydrate (2 mmol, 0.740 g) was added to the methanol
solution of the ligand (1 mmol) and stirred for 15 min to develop a deep
red precipitate. It was collected by filtration and dissolved in acetonitrile.
A methanol/water (2:1) solution (5 mL) containing potassium bromide
(2 mmol, 0.238 g) was added to it. The mixture was stirred for ca. 5 h.
Diffraction quality single crystals were obtained by slow diffusion of
diethyl ether into the mother liquor after a few days.
γ (deg)
Z
4
2
dcalc (g cm−3)
μ (mm−1)
F(000)
1.637
1.885
3.368
6.066
1835
1522
total reflns
unique reflns
obsd data [I > 2σ(I)]
no. of params
R(int)
113 986
8500
14 932
9766
7317
8483
480
644
0.049
0.0373
0.0416, 0.0689
0.0331, 0.0664
Yield: 0.60 g (39%). Anal. calcd for C H Br5.42Cu5.42N O (FW
52
52
6
R1, wR2 (all data)
R1, wR2 [I > 2σ(I)]
0.0338, 0.0554
0.0242, 0.0520
1
(
554.49): C, 40.18; H, 3.37; N, 5.41; found: C, 40.1; H, 3.2; N, 5.5%. IR
−1
KBr, cm ): 1605 (υ
). Magnetic moment: diamagnetic.
CN
lengths and bond angles are given in Table 2. Cambridge Crystallo-
graphic Data Centre (CCDC) reference numbers are 1013217 (for
complex 1) and 1021117 (for complex 2).
RESULTS AND DISCUSSION
■
Synthesis. Copper(II) is reduced to copper(I) by the
38−40
benzoin (or 4,4′-dimethoxybenzoin), which is thereby oxidized
to benzil (or 4,4′-dimethoxybenzil). The 1:1 condensation of
benzil (or 4,4′-dimethoxybenzil) with 1,2-diaminoethane
produces the six-membered cyclic Schiff bases 2,3-dihydro-5,6-
Hirshfeld Surface Analysis. Hirshfeld surfaces
and the
plots were calculated
using Crystal Explorer, with bond lengths to hydrogen atoms set to
41−43
associated two-dimensional (2D) fingerprint
44
45
standard values. For each point on the Hirshfeld isosurface, two
1
distances, d (the distance from the point to the nearest nucleus external
bis(4-methoxyphenyl)pyrazine (L ) and 5,6-diphenyl-2,3-dihy-
e
B
dx.doi.org/10.1021/cg5013236 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for Complexes 1 and 2
1
2
1
2
Br(1)−Cu(1)
Br(1)−Cu(2)
Br(1)−Cu(5)
Br(2)−Cu(2)
Br(3)−Cu(3)
Br(3)−Cu(2)
Br(4)−Cu(3)
Br(4)−Cu(4)
2.3436(5)
2.4344(12)
2.424(2)
2.3830(12)
2.314(3)
2.3620(13)
2.414(3)
2.3656(13)
2.5284(12)
2.3802(12)
2.3499(14)
108.34(18)
−
Br(6)−Cu(5)
Br(6)−Cu(6)
Cu(1)−N(1)
Cu(1)−N(19)
Cu(1)−N(23)
Cu(2)−N(4)
Cu(2)−N(26)
Cu(4)−N(37)
Cu(4)−N(22)
Cu(6)−N(4)
Cu(6)−N(40)
Br(3)−Cu(3)−Br(5)
Br(4)−Cu(3)−Br(5)
Br(4)−Cu(4)−N(37)
−
−
2.3810(12)
2.4486(11)
1.957(6)
1.946(6)
−
−
−
1.9750(15)
2.3565(5)
−
−
−
−
−
−
−
1.9635(15)
2.0088(15)
−
−
a
b
d
1.9628(15)
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
1.974(5)
1.975(6)
1.957(6)
1.993(6)
117.00(5)
123.45(5)
106.63(18)
109.28(16)
137.4(2)
119.57(5)
115.30(5)
125.11(5)
123.03(17)
106.84(17)
127.3(2)
Br(5)−Cu(3)
Br(5)−Cu(5)
c
Br(1)−Cu(1)−N(1)
Br(1)−Cu(1)−N(23)
N(1)−Cu(1)−N(23)
N(1)−Cu(1)−N(19)
Br(1)−Cu(1)−N(19)
Br(1)−Cu(2)−Br(2)
Br(1)−Cu(2)−Br(3)
Br(2)−Cu(2)−Br(3)
Br(2)−Cu(2)−N(4)
Br(2)−Cu(2)−N(26)
120.94(5)
130.01(5)
109.05(6)
−
−
−
−
−
131.6(2)
119.42(17)
115.52(11)
120.18(9)
124.30(10)
−
b
Br(4)−Cu(4)−N(22)
d
b
N(37) −Cu(4)−N(22)
Br(5)−Cu(5)−Br(6)
Br(6)−Cu(5)−Br(1)
Br(5)−Cu(5)−Br(1)
−
111.52(4)
139.89(5)
108.49(6)
a
c
−
−
Br(6)−Cu(6)−N(4)
a
d
N(4)−Cu(2)−N(26)
Br(3)−Cu(3)−Br(4)
Br(6)−Cu(6)−N(40)
c
d
−
118.71(5)
N(4) −Cu(6)−N(40)
a
b
c
Symmetry transformation = −1 + x, y, z. Symmetry transformation = −x + 1, y − 1/2, −z + 2. Symmetry transformation = −x + 1, y − 1/2, −z +
. Symmetry transformation = x + 1, y, z.
d
1
Scheme 1. Syntheses of Complexes 1 and 2
2
29
dropyrazine (L ), following the literature method. The extra
stability of the six-membered ring may be the driving force for the
formation of the ligands. Addition of a methanol:water solution
of KBr in the methanol solution of copper(I) and ligands
C
dx.doi.org/10.1021/cg5013236 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Table 3. All Available References of One-Dimensional Copper(I) Complexes with Pyrazines (pyz) and Substituted Pyrazines
complex
[Cu(μ -L )Br]·1.87H O} (1)
ref
1
{
this work
51
2
2
n
1
1
[
{
{
{
{
{
{
{
{
Cu(μ -L )I] , L = 5,6-diphenyl-2,3-dihydropyrazine
2 n
[Cu(μ-pyz)(pyz)(PPh3)][BF ]·CH Cl }
2 ∞
56
4
2
[Cu(μ-pyz)(pyz)(PPh )][BF ]·THF}
∞
56
3
4
[Cu(μ-pyz)(pyz)(PPh )][ClO ]·CH Cl }
56
3
4
2
2
∞
[Cu(μ-pyz)(pyz)(PPh )][PF ]·THF}
∞
56
3
6
[Cu(μ-pyz)(PPh ) ][BF ]·CHCl }
3 ∞
56
3
2
4
[Cu(μ-pyz)1.5(PPh )][ClO ]·THF}
∞
56
3
4
[Cu(μ-pyz)(pyz)(PPh )][PF ]·CHCl }
56
3
6
3
∞
[Cu(μ-pyz)(PPh )(OClO )]·CHCl }
56
3
3
3
∞
[
(CuI) (2,3-dimethylpyrazine) ]
57
2
3
catena[tri(μ -chloro)bis(μ -2,3-dimethylpyrazine-N,N′)tricopper(I)]
54
2
2
Figure 1. Perspective view of one-dimensional chain of complex 1 with selective atom numbering scheme. Water molecules are not shown for clarity.
produces complexes 1 and 2 (see Scheme 1). The charge of
copper(I) is balanced by bromide. The ligand coordinates
copper(I) in a bidentate fashion in complex 1 and in a
monodentate fashion in complex 2. The great affinity of Cu(I)
to electron-deficient azines, such as pyrazine/dihydropyrazine,
due to the pronounced back-bonding effects, may also contribute
to stabilizing Cu(I) in the (dihydro)pyrazine environment.
It is interesting to note here that we have previously used these
two ligands to prepare copper(I) complexes in the presence of
in the literature, although complexes having Cu Br cores with
3 3
other coligands could be found in the literature (Cambridge
Structural Database (CSD) Search, version 5.35 updates
5
8,59
(November 2013)).
Description of the Structures. {[Cu(μ -L )Br]·1.87H O}
n
1
2
2
(1). Complex 1 crystallizes in the monoclinic space group P2 /c.
1
Two crystallographically independent trigonal planar copper(I)
sites, Cu(1) and Cu(2), are present in the asymmetric unit. A
perspective view of the one-dimensional chain of complex 1 is
shown in Figure 1. Each copper(I) center is bonded to two donor
nitrogen atoms from two different Schiff base molecules {N(1)
5
1
1
iodide. It was observed that L formed a dinuclear copper(I)
−
complex with I . Similar dinuclear copper(I) iodide complexes
a
with pyrazine based ligands structures are huge in the
and N(23) for Cu(1); N(4) and N(26) for Cu(2)} (a = −1 + x,
5
1−55
2
literature.
On the other hand, L formed a one-dimensional
y, z) and one bromide {Br(1) for Cu(1); Br(2) for Cu(2)}. The
1
copper(I) complex with iodide, where halides behaved as
cyclic Schiff base L is acting as a bridging ligand to form a zigzag
2
terminal ligands and L behaved as a bridging bidentate ligand
chain (Figure 1). Sum of the different angles around each
copper(I) center is very close to 360° {360° for Cu(1) and
359.9° for Cu(2)}, indicating trigonal planar geometry. The
deviation of Cu(1) from the mean plane passing through the
coordinating atoms Br(1), N(1), and N(23) is 0.0045(5) Å.
Similarly, the deviation of Cu(2) from the mean plane passing
to connect copper(I) centers. The structure of this complex is
very similar to that of complex 1. Similar one-dimensional
copper(I) complexes with pyrazine based ligands are also
56,57
reported by several groups.
Table 3 gathers all available
references of such one-dimensional copper(I) complexes with
substituted pyrazines. It is of immense importance that similar
copper(I) complexes with several pseudohalides are also
a
through the coordinating atoms N(4), Br(2), and N(26) is
−0.0996(5) Å. Selected bond lengths and bond angles are given
in Table 2.
2
9,51
reported in the literature.
By contrast, the formation of complex 2 is unique in the sense
that no such complexes with pyrazine based ligands are reported
The hydrogen atoms H(1SA) and H(1SB) attached to O(1S)
are involved in hydrogen bonding interactions with bromide ion
D
dx.doi.org/10.1021/cg5013236 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Figure 2. Two-dimensional architecture of complex 1 via hydrogen bonding interactions (hybrid water bromide tetramer is highlighted). Hydrogen
bonds are shown by dotted lines.
b
Br(1) and the symmetry related bromide ion, Br(2) (b = x, 1/2
−
bonding interactions form a three-dimensional network
structure.
y, 1/2 + z), respectively (Figure 2). Similarly, hydrogen atoms
H(1S) and H(2S) attached to O(2S) are involved in hydrogen
bonding interactions with bromide ion Br(1) and the symmetry
2
{[Cu(μ -L )Br]·C H O} (2). Complex 2 crystallizes in the
2
4
10
n
monoclinic space group P2 . It features a three-dimensional
1
b
related bromide ion, Br(2) (b = x, 1/2 − y, 1/2 + z),
CuBr based coordination polymer with Cu Br core. Six
6
6
respectively, to form a bromide−water hybrid tetramer. This
type of bromide−water hybrid tetramer can be found in the
crystallographically independent Cu(I) centers {Cu(1), Cu(2)
with site occupation factor 0.42, Cu(3), Cu(4), Cu(5), and
Cu(6)} and six bromide centers {Br(1), Br(2) with site
occupation factor 0.42, Br(3), Br(4), Br(5), and Br(6)} are
present in the asymmetric unit (see Figure 5). Among them,
three copper centers {Cu(2), Cu(3), and Cu(5)} and three
bromide centers {Br(1), Br(3), and Br(5)} form a six-membered
ring, which assumes a screw-boat conformation with puckering
6
0
literature. These bromide−water hybrid tetramers are
connected via the copper(I) complex to form a 2D layer (Figure
2
). Details of the hydrogen bonding interaction are given in
Table 4. Topological analysis of the hydrogen-bonded structure
of the complex reveals a 3-connected uninodal net with 6 -hcb
3
topology (Figure 3).
61
parameters q(2) = 0.9901(12)Å and ϕ(2) = 198.86(8)°. Cu(3)
and Cu(5) are bonded to Br(4) and Br(6), respectively, which
are in turn bonded respectively with Cu(4) and Cu(6) to form a
Cu Br core. Three Cu(I) centers, Cu(1), Cu(4), and Cu(6), are
Table 4. Hydrogen Bond Distances (Å) and Angles (deg) of
Complex 2
6
6
D−H···A
D−H
H···A
D···A
∠D−H···A
bonded to two donor nitrogen atoms of the Schiff base ligand
O(1S)−H(1SA)···Br(1)
O(1S)−H(1SB)···Br(2)
O(2S)−H(2S)···Br(1)
O(2S)−H(1S)···Br(2)
0.87
0.87
0.84
0.84
2.6500
2.8300
2.55(4)
2.63(3)
3.484(3)
3.689(3)
3.394(2)
3.461(2)
160.00
171.00
177(3)
176(3)
b
{
N(1) and N(19) for Cu(1); N(22) and N(37) for Cu(4);
c
d
N(4) and N(40) for Cu(6)} (c = 1 − x, 1/2 + y, 2 − z; d = 1 + x,
y, z). This pattern continues to form a 3D infinite structure
(
Figure 6). The Cu(I)−Cu(I) distances present in the complex
are within the range 2.80−2.89 Å, which is very close to the sum
of their van der Waals radii (2.80 Å). The attractive interactions
between these types of closed-shell d metal ions are receiving
great attention nowadays because of their interesting structural,
optical, and electronic properties.
Two methylene hydrogen atoms, H(3B), attached to C(3) and
H(27B) attached to C(27), are involved in intermolecular C−
H···π interactions with the symmetry related (−1 + x, y, z)
phenyl ring C(37)−C(38)−C(39)−C(40)−C(41)−C(42) and
1 + x, y, z) phenyl ring C(7)−C(8)−C(9)−C(10)−C(11)−
C(12). One methyl hydrogen atom, H(36C), attached to C(36)
10
62−64
This behavior has been
(
frequently observed in gold, with the term “aurophilicity” being
6
5−67
coined to describe Au(I)−Au(I) interactions.
A better
is also involved in C−H/π interaction with the symmetry related
insight into the nature of the 3D framework of the complex can
be achieved by the application of a topological approach. The 3D
skeleton can be symbolized as a 3,3,3,3,4,4-connected six-nodal
(
2 − x, −y, 1 − z) phenyl ring C(37)−C(38)−C(39)−C(40)−
C(41)−C(42) to form a two-dimensional sheet, as shown in
Figure 4. Geometric features of the C−H/π interactions are
given in Table 5. Combinations of C−H/π and hydrogen
2
3
3
2
2
2
net with a point symbol (10·12 )(10 )(3·10 ·11 )(3·12 )(3 ·4·
2
10·11·12)(3 ·4) (Figure 6). There are no hydrogen bonding
E
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Crystal Growth & Design
Article
3
Figure 3. The 6 -hcb topology of complex 1 via hydrogen bonding interactions.
Figure 4. Two-dimensional architecture of complex 1 via C−H···π interactions.
interactions present in the complex. The hydrogen atom H(16)
attached to C(16) is involved in C−H/π interaction with the
symmetry related (−1 + x, y, z) phenyl ring C(25)−C(26)−
C(27)−C(28)−C(29)−C(30), as shown in Figure S1 (Support-
ing Information). Geometric features of the C−H/π interactions
are given in Table 4.
IR Spectra, Electronic Spectra, and Fluorescence
Spectra. In the IR spectra of both complexes, distinct bands
due to the azomethine (CN) groups were observed around
−1
68
−1
1
600 cm . Complex 1 shows a broad band at 3478 cm due to
O−H stretching vibration. The UV−vis spectrum of complex 1
was recorded in acetonitrile. It shows absorption bands in the UV
region due to charge transfer transitions (Figure S2, Supporting
F
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Crystal Growth & Design
Article
a
Table 5. Geometric Features of the C−H/π Interactions of Complexes 1 and 2
complex
C−H···Cg(ring)
H···Cg (Å)
C−H···Cg (deg)
C···Cg (Å)
sym transformation
1
C(3)−H(3B)···Cg(6)
C(27)−H(27B)···Cg(3)
C(36) −H(36C)···Cg(6)
C(16)−H(16)···Cg(8)
2.63
2.70
2.80
2.59
145
144
148
162
3.484(2)
3.5475(19)
3.671(2)
3.504(8)
−1 + x, y, z
1 + x, y, z
2 − x, −y, 1 − z
−1 + x, y, z
2
a
For complex 1, Cg(3) = center of gravity of six-membered ring C(7)−C(8)−C(9)−C(10)−C(11)−C(12) and Cg(6) = center of gravity of six-
membered ring C(37)−C(38)−C(39)−C(40)−C(41)−C(42). For complex 2, Cg(8) = center of gravity of six-membered ring C(25)−C(26)−
C(27)−C(28)−C(29)−C(30).
Figure 5. Perspective view of Cu Br core of complex 2.
6
6
Figure 6. The 3,3,3,3,4,4-connected six-nodal net of complex 2.
Information). The UV−vis spectrum of complex 2 was recorded
in the solid state, as it is insoluble in acetonitrile and in other
solvents as well. The solid state diffuse reflectance UV−vis
spectrum of complex 2 shows a broad absorption band between
300 and 800 nm, which may be assigned as copper(I) to ligand
charge transfer transitions (Figure S3, Supporting Information).
Complex 1 shows a fluorescence emission band at 396 nm upon
excitation at 290 nm in acetonitrile solution (Figure S2,
G
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Crystal Growth & Design
Article
Figure 7. (a) Hirshfeld surface mapped over dnorm; (b) 2D fingerprint plots; (c) 2D fingerprint plots with Br···H/H···Br interactions highlighted in color
for complex 1.
Figure 8. (a) Hirshfeld surface mapped over dnorm; (b) 2D fingerprint plots; (c) 2D fingerprint plots with Br···H/H···Br interactions highlighted in color
for complex 2.
Figure 9. Theoretical models used to analyze the noncovalent interactions in complex 1.
Supporting Information). Complex 2 shows a broad emission
band at 443 nm upon excitation at 340 nm in the solid state at
room temperature (Figure S3, Supporting Information).
surfaces correspond to H···H contacts. The small extent of area
and light color on the surface indicates weaker and longer contact
other than hydrogen bonds.
Hirshfeld Surface Analysis. The Hirshfeld surfaces mapped
with dnorm for complexes 1 and 2 are illustrated in Figures 7 and 8,
respectively. The dominant interactions between O···H, Br···H,
and N···H atoms in both complexes can be seen in the Hirshfeld
surface as the bright red areas. Other visible spots in the Hirshfeld
Two-dimensional fingerprint plots complement these surfa-
ces, quantitatively summarizing the nature and type of
intermolecular contacts experienced by the molecules in the
crystal. The Br···H/H···Br intermolecular interactions appear as
distinct spikes in the 2D fingerprint plots of both complexes. The
H
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Crystal Growth & Design
Article
Figure 10. Distribution of bond paths and critical points. The bond and ring critical points are shown in red and yellow, respectively. The bond paths are
represented by solid lines apart from those characterizing noncovalent interactions that are represented by dashed lines.
fingerprint plots can be decomposed to highlight particular atom
or destabilizing interaction. To further analyze the importance on
this interaction (de)stabilizing the dimer, we have computed the
binding energy of the dimer in a new theoretical model where
one methyl group has been replaced by a hydrogen atom (see
Figure 9C) and consequently the C−H···H−C interaction is not
established. As a result, the interaction energy is significantly
69
pair close contacts. This decomposition enables separation of
contributions from different interaction types, which overlap in
the full fingerprint. The amount of Br···H/H···Br interactions
comprises 13.8 and 21.3% of the Hirshfeld surfaces for each
molecule of complex 1 and complex 2, respectively.
Theoretical Study. We have divided the theoretical study
into two parts. In the first part we have analyzed the interesting
C−H/π interactions observed in complex 1 that are crucial for
understanding the final 3D architecture of this complex (see
Figure 4), connecting the infinite polymeric 1D chains. In the
second part we have analyzed the cuprophilic Cu···Cu
interactions observed in complex 2 using the natural bond
orbital scheme and examining the Cu donor−acceptor orbitals
involved. As previously mentioned, the p-methoxy groups of the
ligand in complex 1 establish a variety of self-complementary C−
H/π interactions in the solid state interconnecting the polymeric
chains. We have studied this interaction using a monomeric
model (see Figure 9A) to keep the size of the system
computationally approachable. We have computed the inter-
action energy of the self-complementary dimer, and the resulting
reduced to ΔE = −5.4 kcal/mol because only one bifurcated
3
CH ···π interaction is evaluated. The contribution of the C−H···
3
2
H−C interaction can be evaluated as ΔE
= ΔE /2 −
C−H···H−C
2
ΔE = +0.5 kcal/mol. Therefore, in the dimer of complex 2 this
3
contribution is slightly repulsive. A likely explanation is that the
repulsive electrostatic term (dipole−dipole interaction) is higher
in absolute value than the sum of the other favorable
contributions to the interaction. This is due to the inductive
effect of the oxygen atom of the methoxy group that increases the
positive charge of the interacting hydrogen atoms (in
comparison to alkanes). Since the C−H···H−C interaction
energy is very small (within the accuracy of the density functional
theory (DFT) method), we have computed the ΔE
C−H···H−C
using a higher level of theory (RI-MP2/def2-TZVP) and the
resulting value is also repulsive (1 kcal/mol), giving reliability to
the DFT method.
interaction energy is ΔE = −10.0 kcal/mol, which is large
1
because it accounts for four weak C−H/π interactions. In order
to examine whether the coordinated Cu atoms have a long-range
effect on the strength of the interaction, we have also computed a
theoretical model (see Figure 9B) where the coordinated Cu(I)
atoms have been eliminated. The resulting interaction energy is
We have further analyzed the dimer shown in Figure 9 using
Bader’s theory of atoms in molecules. The distribution of critical
points and bond paths is shown in Figure 10. The intermolecular
C−H/π interactions are characterized by the presence of a bond
critical point that connects one hydrogen atom of the methyl
group with the closest carbon atom of the ring. The C−H···H−C
interaction is confirmed by the AIM analysis, and it is also
characterized by the presence of a bond critical point that
connects both hydrogen atoms. It should be mentioned that the
existence of a bond critical point is a confirmation of interaction;
however, it does not imply that the interaction is energetically
favorable. The distribution of critical points shown in Figure 10
also shows the existence of an interesting C−H···Cu agostic
almost unchanged (ΔE = −9.9 kcal/mol), indicating that the
2
effect of Cu is negligible. In addition to the C−H/π bonding
network observed in the dimer, a weak C−H···H−C interaction
is also observed (see red dashed lines in Figure 9). Recently, a
combined computational and CSD study has demonstrated the
70
importance of these weak interactions in the solid state. In fact,
71
the binding energy of the dimer of dodecahedrane, which is
stabilized only by C−H···H−C interactions, is approximately 3
kcal/mol. This interaction is governed by dispersion and orbital
effects, since the electrostatic contribution is repulsive. A delicate
balance between these terms can result in a very weak stabilizing
72
interaction that is characterized by a bond critical point that
connects the Cu atom with the C−H bond critical point. The
value of the Laplacian of the electron density computed at the
I
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Crystal Growth & Design
Article
aforementioned bond critical points is positive, as is common in
closed-shell interactions.
one Cu into an unfilled valence non-Lewis-type lone pair (LP*)
of the other Cu and vice versa is not observed for the other two
interactions due to their longer Cu−Cu distances (see Figure
11).
The second part of the theoretical study is devoted to the
analysis of the Cu···Cu interactions observed in the intricate Cu−
Br polymeric skeleton of complex 2. Interestingly, several
Cu(I)−Cu(I) distances present in 2 are within the range 2.80−
SUMMARY
■
2
.89 Å (close to the sum of their van der Waals radii, 2.80 Å). We
This paper describes the synthesis of two CuBr based
coordination polymers containing cyclic Schiff bases upon
reduction of copper(II) at ambient condition. Complex 1
10
have analyzed the interaction between these closed-shell d
metal ions from an orbital point of view. We have performed
natural bond orbital (NBO) calculations in the theoretical model
complex shown in Figure 11, where the N-donor ligand has been
3
forms an 1D chain with 6 -hcb topology, whereas complex 2
forms a 3D architecture. We have analyzed the interesting C−H/
π and C−H···H−C noncovalent interactions in complex 1 both
energetically and using Bader’s theory of atoms in molecules.
While the C−H···H−C interaction is very weak and repulsive,
the C−H/π interactions play an important role in the crystal
packing and they are responsible for the different solid state
architecture of complex 1 compared to 2. In the latter complex,
10
we have analyzed the closed-shell d Cu···Cu interaction from an
orbital point of view. We have performed natural bond orbital
(
NBO) analysis, and we have found a favorable orbital
contribution that is rationalized by means of donor−acceptor
orbital interactions (LP → LP* electron donation).
ASSOCIATED CONTENT
■
Figure 11. Theoretical model used for the NBO analysis of complex 2.
Distances in Å.
*
S
Supporting Information
Figures S1−S3 showing highlighted C−H/π interactions in
complex 2 and UV−vis fluorescence spectra of complexes 1 and
substituted by HCN, focusing our attention on the second order
2
. X-ray crystallographic information files (CIF) are available for
perturbation analysis that is very useful to study donor−acceptor
7
3
both complexes. This material is available free of charge via the
Internet at http://pubs.acs.org.
interactions. This analysis of the Fock matrix in NBO basis
reveals that the strongest contributions to the Cu(I)−Cu(I)
interactions derive from donation of electron orbital occupancy
from valence Lewis-type lone pairs (LP) of one Cu into an
unfilled valence non-Lewis-type lone pair (LP*) of the other Cu
ion and vice versa (see Table 6). In the Cu(1)−Cu(2) interaction
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: toni.frontera@uib.es.
E-mail: Shouvik.chem@gmail.com.
*
Table 6. Second Order NBO Analysis of the Model Complex
Shown in Figure 11
Notes
a
The authors declare no competing financial interest.
donor NBO
i)
acceptor NBO
E(2)
E(j) − E
F(i,j)
(i)
(
(j)
(kcal/mol)
(au)
(au)
ACKNOWLEDGMENTS
■
CR(3)Cu1
CR(3)Cu2
LP(5)Cu1
LP(5)Cu2
LP(3)Cu3
LP(5)Cu4
LP(3)Cu5
LP(5)Cu6
LP*(7)Cu2
LP*(6)Cu1
LP*(7)Cu2
LP*(6)Cu1
LP*(6)Cu4
LP*(6)Cu3
LP*(6)Cu6
LP*(6)Cu5
0.68
0.63
3.62
6.92
1.22
1.79
0.51
2.07
5.62
6.01
0.33
0.02
0.42
0.32
0.42
0.34
0.056
0.056
0.031
0.023
0.020
0.021
0.013
0.024
This work is supported by CSIR, India (Sanction No. 01(2730)/
13/EMR-II dated April 18, 2013). S.J. thanks the Council of
Scientific and Industrial Research, India, for a senior research
fellowship (Sanction No. 09/096(0659)/2010-EMR-I). A.B. and
A.F. thank DGICYT of Spain (CTQ2011-27512/BQU and
CONSOLIDER INGENIO CSD2010-00065, FEDER funds)
and the Direccio General de Recerca i Innovacio del Govern
́ ́
Balear (Project 23/2011, FEDER funds) for funding. We thank
the CTI (UIB) for free allocation of computer time.
a
Abbreviations: CR = core; LP = Lewis-type lone pair; LP* = unfilled
lone pair.
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