Synthesis of chelating complexes through solid-state dehydrochlorination reactions via second-sphere-coordination interaction with metal chlorides: A combined experimental-molecular modeling study

Article abstract of DOI:10.1021/ic5007583

We have applied crystal engineering as a tool to study the solid-state transformation from molecular salts to coordination complexes via mechanochemical dehydrochlorination reactions. The -(CH₂) _n- (n = 2, 3) alkyl chains were introduced into the bibenzylamine moiety to form the two nitrogen bases N,N,N′,N′- tetrabenzylethylenediamine (L¹) and N,N,N′,N′- tetrabenzylpropydiamine (L²), which were self-assembled with tetrachlorometalates to form a series of supramolecular salts through second-sphere coordination. Single crystals of salts [L¹]2H ⁺·[CuCl₄]^2-·solvent (1) and [L²]2H⁺·[XCl₄] ^2-·solvent (2-4; X = Cu, Hg, Zn) were obtained and their structures determined by single-crystal X-ray diffraction. The effect of different alkyl chains (two and three -CH₂- units) on the solid-state reactivity showed that the chelating complexes resulting from the mechanochemical dehydrohalogenation reaction depend on the formation of quasi-chelating hydrogen-bonding salts. Quantum-mechanical calculations have been used to gain insight in this mechanochemical dehydrohalogenation reaction, demonstrating that not only is size matching between reactants is important but also conformational energies, intermolecular interactions, and the symmetry of frontier molecular orbitals play an important role.

Full text of DOI:10.1021/ic5007583

Article
pubs.acs.org/IC
Synthesis of Chelating Complexes through Solid-State
Dehydrochlorination Reactions via Second-Sphere-Coordination
Interaction with Metal Chlorides: A Combined Experimental−
Molecular Modeling Study
Hong-yu Guan,† Zhen Wang,† Antonino Famulari,‡ Xu Wang,† Fang Guo,*,† and Javier Martí-Rujas*,§
†College of Chemistry, Liaoning University, Shenyang 110036, China
‡Dipartimento di Chimica Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Via L. Mancinelli 7, 20131 Milan,
Italy
§Center for Nano Science and Technology, Istituto Italiano di Tecnologia (ITT@Polimi), Via Pascoli 70/3, 20133 Milano, Italy
S
* Supporting Information
ABSTRACT: We have applied crystal engineering as a tool to study
the solid-state transformation from molecular salts to coordination
complexes via mechanochemical dehydrochlorination reactions. The
−(CH2)n− (n = 2, 3) alkyl chains were introduced into the
bibenzylamine moiety to form the two nitrogen bases N,N,N′,N′-
tetrabenzylethylenediamine (L1) and N,N,N′,N′-tetrabenzylpropydi-
amine (L2), which were self-assembled with tetrachlorometalates to
form a series of supramolecular salts through second-sphere
coordination. Single crystals of salts [L1]2H+·[CuCl4]2−·solvent (1)
and [L2]2H+·[XCl4]2−·solvent (2−4; X = Cu, Hg, Zn) were obtained
and their structures determined by single-crystal X-ray diffraction. The
effect of different alkyl chains (two and three −CH2− units) on the
solid-state reactivity showed that the chelating complexes resulting
from the mechanochemical dehydrohalogenation reaction depend on
the formation of quasi-chelating hydrogen-bonding salts. Quantum-mechanical calculations have been used to gain insight in this
mechanochemical dehydrohalogenation reaction, demonstrating that not only is size matching between reactants is important
but also conformational energies, intermolecular interactions, and the symmetry of frontier molecular orbitals play an important
role.
voids/spaces for inclusion of guest molecules was already
mentioned by Stoddart et al. in the early 1980s6 and, more
recently, has been exploited by Loeb et al.7 The construction of
second-sphere coordination refers to any intermolecular
interactions with the ligands directly bound to the primary
coordination sphere of a metal ion. On the basis of this strategy,
some urea(amido)-containing ligands have been applied as
extractants for highly selective extraction of [PtCl6]2−,
[ZnCl4]2−, and [CoCl4]2− and, more recently, using α-
cyclodextrin for gold recovery.8,9 Halogen bonds10 have also
been used recently to construct second-sphere coordination
isostructural (6,3)-networks able to form adducts with a
mixture of mononegative tetrahedral oxyanions.11
INTRODUCTION
■
Controlling supramolecular self-assembly through crystal
engineering is one of the most important issues in modern
solid-state chemistry. Weak interactions are crucial in the
molecular self-assembling outcome, and its control is hard to
predict.1 Crystals are, in fact, the result of a fine balance of
intramolecular and intermolecular subtle effects. Crystal
engineering of various molecular components, either organic
or mixed metal−organic assembled only by weak intermolec-
ular forces, has led to significant advances in areas like
improved pharmaceutical properties,2 gas adsorption,3 semi-
conductors,4 and molecular transport.5 A crucial aspect in many
solid-state reactions is the orientation and distance between
reactants, which often means that the crystalline environment
can induce a high degree of regio- and stereoselectivity control.
The separation distance, mutual orientation, and space
symmetry of reactive functional groups are crucial in solid-
state chemistry.
We recently reported the solid-state mechanochemical
dehydrohalogenation reaction starting from salts assembled
by second-sphere coordination.12 However, the solid-state
chemistry of second-sphere-coordination networks is still a
The use of organic cations and metal ions via second-sphere
Received: April 2, 2014
coordination to construct host frameworks with accessible
© XXXX American Chemical Society
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diffraction data for L2, 1′, and 2 were recorded with a Bruker X8
Prospector APEX-II/CCD diffractometer equipped with a focusing
mirror (Cu Kα radiation; λ = 1.54056 Å). The structures were
determined using direct methods and refined (based on F2 using all
independent data) by full-matrix least-squares methods (SHELXTL
97).16 All non-H atoms were located from difference Fourier maps and
refined with anisotropic displacement parameters. Powder XRD
(PXRD) was recorded on a D2 Bruker diffractometer (λ = 1.54056
Å) in reflection mode.
research field that remains relatively unexplored, particularly if
compared with that of metal−organic frameworks.13 Reports
studying fundamental solid-state reactions involving second-
sphere-coordination complexes are important because they can
help in the development of such hybrid metal organic adducts
toward new functional crystalline materials.14
Herein, we report a series of supramolecular structures
obtained using second-sphere coordination and their solid-state
chelation upon mechanochemical dehydrochlorination.15 We
have synthesized two bidentate ligands (L1 and L2) by
introducing −(CH2)n− alkyl chains on the bibenzylamine
ligand at the N atom through covalent bonds (Scheme 1a). We
Preparation of Ligands L1 and L2. Preparation of L1. A total of
7 mL of ethylenediamine was slowly added to a solution of 8 g of
NaOH and 20 mL of distilled water. A total of 30 mL of benzyl
chloride (2−3 drops) was then continuously added to the mixture
solution. Then, the reaction was heated to 95 °C and stirred for 4 h.
The mixture was cooled to room temperature. The white reaction
product was separated from diethyl ether and then washed with
distilled water. Recrystallization using anhydrous ethanol (EtOH) and
drying in vacuo produced white crystals. Yield: 12.43 g, 60.0%. Mp:
Scheme 1. Cartoon Showing the Ligands L1 and L2 (a) and
Their Mechanochemical Dehydrochlorination Reactions
with [CuCl4]2− (b and c)
1
98−100 °C. H NMR (CDCl3, 300 MHz): δ 2.60 (2H, s, CH2), 3.50
(4H, s, CH2), 7.23−7.27 (10H, m, ArH).
Preparation of L2. A total of 4.2 mL of 1,3-propanediamine was
slowly added to a solution of 8 g of NaOH and 18 mL of anhydrous
EtOH. A total of 27.5 mL of benzyl chloride (2−3 drops) was then
continuously added to the mixture solution. Then, the reaction was
heated to 80 °C and stirred for 4 h. The mixture was cooled to room
temperature. The white reaction product was separated from EtOH
and then washed with distilled water several times. White needle
crystals were obtained upon recrystallization using anhydrous EtOH
upon mild heating and drying in vacuo. Yield: 14.06 g, 64.6%. Mp:
1
50.6−51.3 °C. H NMR (CDCl3, 300 MHz): δ 1.71 (2H, s, CH2),
2.38−2.42 (4H, t, CH2), 3.48 (8H, s, CH2), 7.21−7.28 (20H, m,
ArH).
Preparation of Crystal 1. A total of 350 mg (0.8 mmol) of L1 and
10 mL of EtOH were placed in a 50 mL Erlenmeyer flask, then 190 mg
of CuCl2·2H2O (11 mmol) and 1 mL of concentrated hydrochloric
acid were slowly added, and the flask was shaken until the contents
were dissolved. The flask was allowed to stand for about 8−10 days at
room temperature, giving rise to orange crystals of 1 suitable for
crystallographic studies.
demonstrate that the chelating reaction upon dehydrochlori-
nation depends on the length of the chelating backbone in
ligands L1 and L2 (Scheme 1b,c). Moreover, using structural
models obtained from single-crystal X-ray diffraction (XRD),
quantum-mechanical (QM) calculations also including meth-
ods specific for solid phases were carried out to gain insight into
molecular conformations, intermolecular interactions, and the
electronic density distribution of frontier molecular orbitals
(FMOs) in ligands L1 and L2. To our best knowledge, the
successful application of crystal engineering to the formation of
chelating complexes by mechanochemical solid-state reactions
through second-sphere coordination has not been reported, and
the effect of −(CH2)n− spacers on the formation of various
second-sphere complexes has not been investigated using such
large and flexible organic cations. Additionally, we combined
experimental and molecular modeling to gain insight into these
uncommon mechanochemical transformations. The approach
used herein can pave the way to understanding mechanochem-
ical reactions and the possible mechanisms behind it in this area
of coordination chemistry.
Preparation of Crystals 2−4. A total of 100 mg (0.2 mmol) of L2
and 20 mL of EtOH were placed in a 50 mL Erlenmeyer flask, then
190 mg of CuCl2·2H2O (1.1 mmol) and 1 mL of concentrated
hydrochloric acid were slowly added, and the flask was shaken until the
contents were dissolved. The flask was allowed to stand for nearly 1
month at room temperature, giving rise to red block crystals (2)
suitable for crystallographic studies.
A total of 10 mg of L2 (0. 02 mmol) and 8 mL of methanol
(MeOH) were placed in a 25 mL Erlenmeyer flask, then 10 g of HgCl2
(0.04 mmol) and 0.05 mL of concentrated hydrochloric acid were
slowly added, and the flask was shaken until the contents were
dissolved. The flask was allowed to stand for over 1 week at room
temperature, giving rise to colorless crystals (3) suitable for
crystallographic studies.
A total of 10 mg of L2 (0. 02 mmol) and 8 mL of MeOH were
placed in a 25 mL Erlenmeyer flask, then 10 mg of ZnCl2 (0. 07
mmol) and 0.05 mL of concentrated hydrochloric acid were slowly
added, and the flask was shaken until the contents were dissolved. The
flask was allowed to stand for 2−3 days at room temperature, giving
rise to colorless crystals (4) suitable for crystallographic studies.
Solid-State Reaction. Liquid-assisted grinding (LAG) of 21 mg
(0.033 mmol) of 1 and 3.7 mg (0.066 mmol) of KOH in a 1:2 molar
ratio, with the addition of 30 μL of MeOH, was carried out. Upon
grinding, a color change from orange to dark green was observed
within 5 min.
EXPERIMENTAL SECTION
■
Materials and Methods. All chemicals were obtained from
commercial sources and used without further purification. IR spectra
were obtained with a PerkinElmer 100 FT-IR spectrometer using KBr
pellets. 1H NMR spectra were recorded on a Mercury-Plus 300
spectrometer (Varian, 300 MHz) at 25 °C with tetramethylsilane as
the internal reference.
Synthesis of Crystal 1′. The ligand L1 was disolved in 5 mL of
EtOH, which was slowly added to a solution of CuCl2·2H2O (42.5 g,
0.25 mmol) in 5 mL of EtOH. The system became turbid and a
precipitate was observed. The solution continued to stir for 30 min,
and then stood overnight at room temperature. After filtration, the
Single-crystal data collection of 1, 3, and 4 was performed on a
Bruker P4 diffractometer with Mo Kα radiation (λ = 0.71073 Å). The
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Figure 1. Crystal 1 formed by L1 and [CuCl4]2−. (a) Quasi-chelating hydrogen-bonding interaction between the dianion and dication. (b) Packing of
salt 1 viewed along the a axis. Guest molecules and H atoms are omitted for clarity.
synthesized17 and self-assembled with [CuCl4]2− using second-
sphere coordination. We note that the N−N distance in the
crystalline structure of L1 is 3.766 Å.18 The diffusion of a
solution of CuCl2·2H2O in EtOH into a HCl/EtOH solution of
L1 resulted in the salt [L1]2H+·[CuCl4]2−·EtOH (1). In 1, there
are two [CuCl4]2− dianions, two doubly protonated L1, and one
EtOH molecule in each asymmetric unit.19 Clearly, a specific
ionic recognition between the doubly protonated L1 and the
[CuCl4]2− anion occurs. The two N protons in one
independent L1 are linked with the same [CuCl4]2− anion
through two charge-assisted N−H···Cl hydrogen bonds i and ii
[i, N1−H1···Cl2 with distance dN1···Cl2 = 3.166(7) Å and angle
N1−H41···Cl2 = 156.8(2)°; ii, N2−H2···Cl1 with distance
dN2···Cl1 = 3.151(6) Å and angle N2−H2···Cl1 = 172.0(4)°] in
one cation−anion couple. For the second cation−anion couple,
the other independent L1 forms the charge-assisted hydrogen
residue was washed with EtOH and dried at 50−55 °C to give large
dark-green crystals of 1′. Yield: 128.5 mg, 87%. Mp: 106−108 °C.
Crystallography. Single-crystal data for L2: monoclinic, space
group P21/c, a = 19.043(2) Å, b = 5.7815(7) Å, c = 23.610(2) Å, β =
109.678(6)°, V = 2447.6(5) Å3, Dc = 1.179 g cm−3, Rint = 0.0451,
F(000) = 936, 13410 total reflections, 4130 unique reflections, and
2887 observed reflections with I > 2.0 σ(I), R1 = 0.0458, wR2 = 0.188,
R2 (all data) = 0.1213, S = 1.121, T = 150 K, CCDC 994116.
Single-crystal data for 1: triclinic, space group P1, a = 9.318(4) Å,
̅
b = 15.815(7) Å, c = 21.419(10) Å, α = 92.167(9)°, β = 92.094(9)°, γ
= 90.933(9)°, V = 3151(2) Å3, Dc = 1.353 g cm−3, Rint = 0.0705,
F(000) = 1328, 16022 total reflections, 10844 unique reflections, and
3978 observed reflections with I > 2.0 σ(I), R1 = 0. 0785, wR2 =
0.2130, R2 (all data) = 0.2296, S = 0.943, T = 293 K, CCDC 994117.
Single-crystal data for 1′: orthorhombic, space group Pbcn, a =
9.9271(13) Å, b = 13.0258(17) Å, c = 21.325(3) Å, V = 2757.5(7) Å3,
Dc = 1.337 g cm−3, Rint = 0.127, F(000) = 1156, 8537 total reflections,
2322 unique reflections, and 1112 observed reflections with I > 2.0
σ(I), R1 = 0.073, wR2 = 0.2223, R2 (all data) = 0.1236, S = 0.863, T =
298 K, CCDC 994118.
bonds iii and iv [iii, N3−H3···Cl7 with distance dN3···Cl7
=
3.166(7) Å and angle N3−H3··Cl7 = 157.7(4)°; iv, N4−
H101···Cl5 with distance dN4···Cl5 = 3.130(7) Å and angle N4−
H101···Cl5 = 171.4(4)°]. The above interactions form a quasi-
chelating20 interaction motif (Figure 1a).21 The quasi-chelating
units are connected to adjacent dications through three C−H···
Cl interactions, forming a hydrogen-bonded chain along the a
axis (Figures S2 and S3 in the Supporting Information, SI). The
two neighboring hydrogen-bonded chains accommodate EtOH
molecules. The guest molecules do not form continuous
channels but confined voids.
Single-crystal data for 2: monoclinic, space group P21/n, a =
13.4266(17) Å, b = 18.462(2) Å, c = 13.5988(18) Å, β = 91.929(6)°, V
= 3368.99(8) Å3, Dc = 1.345 g cm−3, Rint = 0.0346, F(000) = 1412,
18599 total reflections, 5700 unique reflections, and 5031 observed
reflections with I > 2.0 σ(I), R1 = 0.0453, wR2 = 0.1325, R2 (all data)
= 0.0505, S = 1.064, T = 150 K, CCDC 994119.
Single-crystal data for 3: monoclinic, space group P21/c, a =
9.0210(18) Å, b = 11.826(2) Å, c = 30.945(6) Å, β = 91.46(3)°, V =
3300.1(11) Å3, Dc = 1.600 g cm−3, Rint =0.1071, F(000) = 1572, 22889
total reflections, 7553 unique reflections, and 5456 observed
reflections with I > 2.0 σ(I), R1 = 0.0685, wR2 =0.1835, R2 (all
data) = 0.0938, S = 1.021, T = 113 K, CCDC 994114.
Single-crystal data for 4: monoclinic, C2/c, a = 10.976(18) Å, b =
16.39(3) Å, c = 19.97(4) Å, β = 97.652(17)°, V = 3560(11) Å3, Dc =
1.317 g cm−3, Rint = 0.0296, F(000) = 1448, 7041 total reflections,
3048 unique reflections, and 2330 observed reflections with I > 2.0
σ(I), R1 = 0.0630, wR2 = 0.2023, R2 (all data) = 0.0791, S = 1.111, T
= 296 K, CCDC 994115.
Dehydrochlorination reactions consisting of the removal of
HCl from crystalline salts have been rarely investigated in
coordination molecular complexes.22,23 The quasi-chelating salt
1 can provide an opportunity to study the transformation from
a hydrogen-bond salt network to a coordination complex via
dehydrochlorination. That is, it is possible to study whether the
orientation of the reactants (i.e., cations and anions) is
favorable to form a chelated system.
In the doubly protonated L1, the two methylene groups
[(CH2)n (n = 2)] inserted between two dibenzylamine moieties
result in the two N donors separated approximately by 3.683 Å,
and such a distance (i.e., about 0.1 Å), shorter than the
corresponding distance in a neutral ligand, seems appropriate
for chelation to occur. LAG24 of powders of 1 and KOH in a
1:2 molar ratio rapidly showed a color change (i.e., from yellow
to green) within 5 min (Figure 2). PXRD of the powder
RESULTS AND DISCUSSION
■
Quasi-Chelating Hydrogen-Bonding Motif in 1 and Its
Transformation to the Coordination Complex 1′ in the
Solid State. In order to study a new mechanochemical
dehydrochlorination reaction using a large and flexible
molecule, the ligand L1 with a N−(CH2)2−N backbone
between the two bulky dibenzylamine moiety chains was
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whether dehydrochlorination followed by chelation in 1 takes
place upon heating. As observed by PXRD (see Figure S6 in the
SI), complex 1′ was not formed even upon heating to 200 °C.
Structural Diversity of Hydrogen-Bonding Motifs in L2
Salts and Their Mechanochemical Reactivity in the Solid
State. In order to gain insight into how the distances and
orientation of the reacting partners (i.e., cations and anions)
affect the mechanochemical dehydrochlorination reaction, we
synthesized a new ligand (L2) with a longer −N−(CH2)3−N−
backbone by inserting an extra methylene spacer into L1.25
Single-crystal X-ray structure determination of L2 showed that
the N−N distance in the propylenediamine backbone is ca.
5.113 Å. Under the same crystallization conditions as those
used to crystallize 1, a new salt (2) was obtained by mixing
flexible L2 and CuCl2·2H2O in EtOH.
Figure 2. Pictures of the actual microcrystals of 1 and 1′ after
dehydrochlorination. Comparison of the PXRD patterns: solution
synthesis of 1′ from EtOH (black), simulated from single crystals
(red), and the crude product including the product phase, KCl
(green).
Salt 2 crystallizes in the monoclinic P21/n space group
comprising one molecule of doubly protonated ligand L2, one
[CuCl4]2− anion, and one molecule of EtOH in the asymmetric
unit. As shown in Figure 4, one [CuCl4]2− is hydrogen-bonded
product indicated that a new phase and byproduct KCl (2θ =
28°) had been formed (Figure 2). Recrystallization of the
product obtained by grinding yields suitable crystals for single-
crystal XRD analysis. The match between the PXRD pattern
obtained by grinding and that simulated from single-crystal
XRD data suggests that the bulk powder has the same structure
as the one obtained from the single crystal.
X-ray structure determination from single crystals confirms
that salt 1 transformed to the neutral metal coordination
complex (CuCl2)(L1) (1′) upon dehydrochlorination. In the
transformation from 1 to 1′, disruption of weaker noncovalent
charge-assisted N−H···Cl hydrogen bonds took place, while
new Cu−N coordination bonds were formed upon chelation
(Figure 3). The space group changed from triclinic (P1) to
̅
orthorhombic (Pbcn) after dehydrochlorination. The main
changes in the lattice parameters correspond to the shortening
of the b axis (from 15.815 to 13.026 Å) and the change in the α
and β angles (from 92.167° to 90° for α and from 92.094° to
90° for β). The hydrogen-bonded chain along the a axis in
crystal 1 was transferred to coordination complex 1′ through
C−H···Cl interactions (Figure 3). No inclusion of guest
molecules in 1′ was observed. It is known that in some cases
Figure 4. View of the charge-assisted hydrogen bonds (i and ii)
between the [CuCl4]2− dianions and dications in 2. Clearly, one
dication interacts with two different dications (L2 and L2 ). H atoms
a
b
are omitted for clarity.
dehydrochlorination can occur upon heating.22 We tested
b
Figure 3. Crystal structure of 1′. (a) Clamp-shape unit formed upon chelation. (b) Hydrogen-bonded chain along the b axis in coordination complex
1′ through C−H···Cl interactions (dashed lines).
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to two different cations (L2 and L2 ) through i and ii charge-
recrystallization of the product from EtOH further confirms the
presence of L2.
a
b
assisted N−H···Cl hydrogen bonds. The shortest interaction is i
[dN2···Cl4 = 3.177(3) Å and angle N2−H102···Cl4 = 167.8(3)°],
while ii [dN3···Cl3 = 3.207(3) Å and angle N3−H101···Cl3 =
171.3(3)°] is slightly longer. These hydrogen bonds, seemingly
divergent, are most likely a result of the longer distance
between the N+ atoms (5.040 Å in salt 2) in L2; therefore, the
quasi-chelating motif is not formed. The other two Cl atoms are
involved in weak C−H···Cl interactions with neighboring
dications. The disordered nature of the trapped EtOH
molecules can be explained by the fact that hydrogen-bonding
interactions are not established with either the [CuCl4]2−
dianion or the protonated N atom in the dication.
To ascertain if the longer L2 ligand chelates upon
dehydrochlorination, we set up a crystallization experiment
using HgCl2 following the same conditions as those used to
crystallize salt 2. Large colorless single crystals (3) crystallizing
in the monoclinic P21/c space group were obtained.17 The
asymmetric unit consists of one molecule of doubly protonated
L2, one [HgCl4]2−anion, and one molecule of MeOH. In 3, the
two protonated N atoms in L2 form one charge-assisted
hydrogen-bonding interaction (i) [dN1···Cl4 = 3.291(6) Å and
angle N1−H1···Cl4 = 165.0(4)°] between one protonated N
atom of L2 and [HgCl4]2− dianions and one N−H···O;
[dN2−H2···O1 = 2.724(5) Å and angle N2−H2···O1 = 154.7(5)°]
interaction (ii) between the other protonated N atom of L2 and
the MeOH molecules (Figure 6a). MeOH acts as a linker
With this type of anion orientation toward the N atoms in L2,
there is no quasi-chelating motif in which one metal center
reacts with one dication upon the release of HCl. Indeed, it
seems improbable that a chelated ligand such as 1′ could be
obtained. To elucidate if the chelated forms, we grinded
powders of 2 (31 mg, 0.048 mmol) in the presence of KOH
(5.4 mg, 0.096 mmol) for 10 min. Clearly, the diffraction
pattern of the product corresponds to a phase that does not
match that of salt 2 (Figure 5). The new diffraction pattern is in
Figure 6. View showing the hydrogen-bonding interactions between
the dication and dianion in crystals 3 (a) and 4 (b). H atoms are
omitted for clarity.
Figure 5. (a) Simulated PXRD pattern of 2 (150 K). (b) Simulated
PXRD pattern of ligand L2 (150 K). (c) PXRD product of 2 grinded in
the presence of KOH (298 K). The shift in the diffraction peaks is due
to the different temperatures at which the data were recorded.
bridging with another [HgCl4]2− dianion through a new
hydrogen-bonding interaction (iii) [dO1···Cl3 = 3.207(5) Å].
agreement with that of ligand L2, confirming that there is no
chelated Cu−N bond formation (Figure 5). We also note that
there is no color change during grinding, which is expected if
chelation occurs. The dehydrochlorination reaction indeed
takes place because KCl has been formed (2θ = 28°), after
dehydrochlorination between L2 and KOH.
Then we also investigated the possible mechanochemical
reaction by grinding L2 and CuCl2·2H2O.17 The PXRD pattern
obtained from grinding clearly indicates that the final solid is a
mechanical mixture containing L2 and CuCl2·2H2O, showing
that no reaction took place (Figure S5 in the SI). Moreover,
Clearly, one dianion interacts with two different dications (L2
a
and L2 ), being in one case (i.e., ii) mediated through a MeOH
b
molecule (Figure 6a).
The quasi-chelating motif observed in 1 is absent in 3
probably because the covalent N+−N+ length (5.015 Å) is
longer than the [HgCl4]2− dianion can accommodate. There-
fore, only one Cl of the three in [HgCl4]2− interacts with the
same dication.
To test whether chelation occurs for ligand L2, we grinded
crystals of 3 in the presence of KOH. The PXRD pattern in
Figure S7 in the SI shows that salt 3 is maintained after 5 min.
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However, after 10 min, an amorphous phase containing a weak
L2 diffraction is formed. Solution experiments mixing L2 and
HgCl2·H2O did not give a chelating complex.
L2 can form complexes with metals like the L1 ligand, which has
a lower estimated proton affinity.
However, after deprotonation of the protonated L1 (that is
after removal of 2H+ from L1), the resulting unrelaxed L1
neutral molecule, which is kept at the geometry of the
protonated L1 but with two H+ removed, has an energy 35 kcal
mol−1 higher with respect to the relaxed L1 neutral molecule.
These outcomes can partially justify the ability of L2 to form a
chelate system, lowering its energy.
Replacing Hg2+ with Zn2+ in the crystallization with L2 yields
a new crystal structure (4), showing a different hydrogen-
bonding interactions between L2 and [ZnCl4]2− anion. One
dianion interacts with two different dications (L2 and L2 )
a
b
using MeOH as the linking bridge through hydrogen bonds (i)
N1−H1N···O1 = 3.068(1) Å and N1−H1N···O1 = 169.8(4)° and
d
(ii) dO1−H10···Cl1 = 3.109(1) Å and N1−H1N···O1 = 155.4(7)°.
In this case, there is no direct cation−anion interaction. As a
result, the quasi-chelation motif is not formed (Figure 6b). The
new interaction mode is different from that observed in 2 and
3, which show the structural diversity for this flexible ligand.
Mechanochemical grinding of salt 4 with KOH did not form a
chelate but L2 as in the other cases (see Figure S8 in the SI).
Density Functional Theory (DFT) Solid-State QM
Calculations. Complementary insights in the field of solid-
state reactions can be obtained by theoretical methods, also
using QM calculations specific for solid phases. In particular,
DFT approaches have been employed herein. The Perdew−
Burke−Ernzerhof (PBE)26 exchange-correlation functions have
been used both for gas and solid phases (i.e., under periodical
conditions) and, for the sake of consistency, all of the
calculations were accomplished by DMOL3 software.27
The combination of a numerical double-ζ quality basis set
[including polarization functions on all atoms, i.e., double-ζ
numerical with polarization (DNP)] and an effective core
potential for the metal atoms was adopted. We assumed
experimental X-ray-determined geometries for heavy atoms,
while the X−H bond lengths were optimized. A similar
computational approach was proven to be adequate in a
number of cases such as large supramolecular complexes,28
systems containing charged particles,29 and crystalline phases of
thiophene-based oligomers and polymers.30 The effects of intra-
and intermolecular interactions have been accounted for by the
Grimme scheme with the dispersion DFT (i.e., DFT-D)
approach.31
In addition, the interactions involved in hydrogen-bonded
crystals have been analyzed by calculations on small clusters of
particles extracted from the X-ray-determined structures. The
interaction energy of the dimer representing the quasi-chelated
motif involving protonated L1 (see Figure 1a) is ca. 316 kcal
mol−1, which is about 158 kcal mol−1 for each of the charge-
assisted hydrogen bonds. The same calculation has been
accomplished to estimate the stability of the two dimers and of
the tetramer involving protonated L2 in Figure 4. The
interaction energies are 272 kcal mol−1 in the case of a single
dimer (involving one hydrogen bond) and 676 kcal mol−1 in
the case of the tetramer (involving four hydrogen bonds),
which is about 169 kcal mol−1 for each of the charge-assisted
hydrogen bonds. We can easily conclude that protonated L2
forms (at least locally) stronger hydrogen-bonding interactions
than protonated L1.
Further analysis of DFT outcomes has shown that the more
reactive electrons (i.e., electrons in FMOs) in the ligand L1 with
the −N−CH2−CH2−N− moiety are essentially localized at the
N atoms. As is observed in Figure 7a, the two N lobes in the
highest occupied molecular orbitals (HOMOs) are pointing
toward the same direction (blue lobes), which we believe is an
optimal geometry for establishing the charge-assisted hydrogen
bonds with the same [CuCl4]2− dianion, giving rise to the quasi-
In order to assess the reliability of the functional adopted for
DFT-D calculations (PBE/DNP plus Grimme contributions),
the stabilities of L1 and L2 ligands in their crystalline forms have
been compared by solid-phase calculations. In accordance with
thermal analysis results, the packing of L1 molecules (mp 98
°C) seemed to be more efficient than that of L2 molecules (mp
51 °C); i.e., the sublimation energy of the former is higher by
ca. 8 kcal mol−1.32
DFT-D calculations at the same level involving isolated
molecules have been performed to investigate conformational
energies and electron distribution of molecules and charged
particles possibly involved in the chelating reaction. First of all,
it should be underlined once more that in L1 and L2 the low-
energy structures have a very different N−N distance, evident
in both the crystal structures and optimized geometries. In
addition, the conformational freedom introduced by an extra
methylene group has a strong influence in the cyclic
intermediate transition states involved in the chelating
reactions.
Figure 7. Structures showing the calculated HOMOs in ligand L2,
protonated L2, and protonated and removed 2H. For ligand L1, the
case of protonated and removed 2H is presented part a. Clearly, in L1,
the orientation of the HOMOs is in the same direction, while in L2,
the orientation of the lobes is in such a way that rotation of one N
group is necessary if a coordination reaction takes place (i.e., opposite
direction). In 2HL2, clearly the HOMOs are displaced to the pendant
phenyl rings.
DFT-D calculations show that the L1 ligand has a lower
proton affinity with respect to that of the L2 molecule; i.e., the
difference in energy between L1 and protonated L1 is about 384
kcal mol−1, while this difference increases to 433 kcal mol−1 in
the case of L2 (see Tables S1 and S2 in the SI). So, in principle,
F
dx.doi.org/10.1021/ic5007583 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
chelated motif in complex 1. Upon grinding of 1 in the
presence of KOH, this optimal orientation facilitates the
formation of a chelated complex via a dehydrochlorination
reaction.
Currently, we are actively working on the design and
synthesis of functional porous hybrid metal organic salts by
combining second-sphere coordination and mechanochemical
dehydrochlorination reactions by means of crystal engineering.
Gas-phase DFT calculations also show that the FMO
distribution is localized around the −N−CH2−CH2−CH2−
N− group in crystals 2−4. The conformations of ligand L2 in
the crystal of pure L2 and of L2 (2) nonchelating are quite
different. Obviously, the nonchelating structure contains the L2
molecules with protonated N atoms. As shown in Figure S9 in
the SI, the different dihedral angles of the pendant phenyl
groups in pure L2 compared to the nonchelating without the
two N protons in ligand L2 is quite considerable. However,
both structures have a N−N distance too large to be
coordinated to form a chelated complex [dN−N = 5.113
(5.040) vs 3.766 (3.683) Å of the −N−CH2−CH2−N−
backbone]. Moreover, the N lone pairs are directed away with
respect to a hypothetical complexing metal approaching the
system.
ASSOCIATED CONTENT
* Supporting Information
Further details on the synthesis and crystallographic
information. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: fguo@lnu.edu.cn.
*E-mail: javier.rujas@iit.it.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
In L2, the HOMO is localized essentially at the N atoms
(Figure 7b), while after N protonation (Figure 7c), the HOMO
is displaced by the phenyl moieties. It should be emphasized
that, even if the HOMOs of L2 (Figure 7b) and L2
nonchelating without H+ (Figure 7d) reside on the N atoms,
the distribution does not allow the formation of a chelate
without a relative rotation of the N groups to orient the
HOMO lobes properly.33 In fact, for symmetry reasons (the
HOMOs have π symmetry), the lobes of the terminal N atoms
have opposite signs, at variance with the case of L1 (Figure 7a).
In conclusion, the number of CH2 groups between N atoms in
stable conformations of L1 and L2 not only influences the
distance between the N atoms but also plays a role in the
symmetry of the corresponding HOMOs. Both of these effects
seem to support the experimental outcomes relating to the
reticent tendency of L2 to chelate upon dehydrochlorination.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the NSFC (Grants 20903052
and 20772054), the program for Liaoning excellent talents in
University (Grant LJQ 2011003), the innovative team project
of education department of Liaoning Province (Grant
LT2011001), and Liaoning University (Grant 2012LDGY06).
A.F. acknowledges CINECA for computational resources
through a Laboratory for Interdisciplinary Advanced Simulation
Initiative 2013 grant (Projects SIMFO and QMC_opt) and
CARIPLO for financial support (Project PLENOS). J.M.-R.
thanks the Italian Institute of Technology for financial support.
REFERENCES
■
CONCLUSIONS
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In summary, we have monitored the solid-state dehydrochlori-
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different backbone chain lengths (i.e., two and three −CH2−
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H
dx.doi.org/10.1021/ic5007583 | Inorg. Chem. XXXX, XXX, XXX−XXX