Supramolecular patterns of cationic and neutral Ni(ii) complexes from the interplay of hydrogen-bonding, stacking interactions and metal-coordination motifs

Article abstract of DOI:10.1039/c2ce25704a

In an effort to explore the way in which the chemical subunits of crystal structures of transition metal complexes are put together in periodic ordered arrays in the solid state via noncovalent interactions, an investigation on a series of 13 nickel(ii) complexes has been carried out. The complexes were isolated from the general Ni^II/X^-/L or HL′ [X ^- = Cl^-, Br^-, I^-, NO ₃^-, NO₂^-, ClO₄ ^-; L = 1-methyl-4,5-diphenylimidazole, and HL′ = 4,5-diphenylimidazole] reaction system. A single-crystal diffraction analysis shows that, independently of the ligand used, most of the complexes contain the rigid square planar [NiL₄]²⁺ (1-5) or [Ni(HL′) ₄]²⁺ (6-10) cation, which seems to have an impact on the self-assembly process by adopting a structure-directing role: the supramolecular assembly is organized around the rigid bulky cations via interactions with the surrounding counterion/solvent clusters in each individual structure. The components of the clusters, i.e. OH^-/H₂O (1), [NiCl ₄]^2-/EtOH (2), Br^-/H₂O (3), NO ₃^-/MeOH (4), ClO₄^-/Me₂CO (5), Cl^-/H₂O (6), Br^-/MeCN (7), I ^-/Me₂CO/H₂O (8), NO₃ ^-/EtOH/H₂O (9), and ClO₄^-/Me ₂CO (10), are held tightly together by strong or weak hydrogen bonding. The structure-directing action of the cations is accomplished via weak C-H...O/Cl/Br^-, π...π and C-H...π interactions (for L-containing complexes) or strong recurring N-H.../Cl^-/Br^-/I^-/O motifs dominating the molecular self-assembly together with weak interactions (for HL′-containing complexes). The variety of the stereochemistries observed among the studied compounds (square planar, 1-10; tetrahedral, 11 and 12; octahedral, 13) seems to advocate the choice of nickel as an interesting candidate in metallosupramolecular research. This journal is

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PAPER
Supramolecular patterns of cationic and neutral Ni(II) complexes from the
interplay of hydrogen-bonding, stacking interactions and metal-coordination
motifs{
Konstantina A. Kounavi,a Eleni E. Moushi,b Manolis J. Manos,b Constantina Papatriantafyllopoulou,a
Anastasios J. Tasiopoulosb and Vassilios Nastopoulos*a
Received 7th May 2012, Accepted 22nd June 2012
DOI: 10.1039/c2ce25704a
In an effort to explore the way in which the chemical subunits of crystal structures of transition metal
complexes are put together in periodic ordered arrays in the solid state via noncovalent interactions,
an investigation on a series of 13 nickel(II) complexes has been carried out. The complexes were
isolated from the general NiII/X2/L or HL9 [X2 = Cl2, Br2, I2, NO32, NO22, ClO42; L = 1-methyl-
4,5-diphenylimidazole, and HL9 = 4,5-diphenylimidazole] reaction system. A single-crystal diffraction
analysis shows that, independently of the ligand used, most of the complexes contain the rigid square
planar [NiL4]2+ (1–5) or [Ni(HL9)4]2+ (6–10) cation, which seems to have an impact on the self-
assembly process by adopting a structure-directing role: the supramolecular assembly is organized
around the rigid bulky cations via interactions with the surrounding counterion/solvent clusters in
each individual structure. The components of the clusters, i.e. OH2/H2O (1), [NiCl4]2–/EtOH (2), Br2/
H2O (3), NO32/MeOH (4), ClO42/Me2CO (5), Cl2/H2O (6), Br2/MeCN (7), I2/Me2CO/H2O (8),
NO32/EtOH/H2O (9), and ClO42/Me2CO (10), are held tightly together by strong or weak hydrogen
2
…
bonding. The structure-directing action of the cations is accomplished via weak C–H O/Cl/Br ,
2
2 2
p and C–H p interactions (for L-containing complexes) or strong recurring N–H /Cl /Br /I /
…
… …
p
O motifs dominating the molecular self-assembly together with weak interactions (for HL9-containing
complexes). The variety of the stereochemistries observed among the studied compounds (square
planar, 1–10; tetrahedral, 11 and 12; octahedral, 13) seems to advocate the choice of nickel as an
interesting candidate in metallosupramolecular research.
high-dimensional supramolecular architectures. The coordina-
Introduction
tion bond as a supramolecular synthon4 has been widely studied
in recent decades (catenanes, rotaxanes, molecular knots,
coordination polymers, metal–organic frameworks, etc). The
hydrogen bonds5 are the most common directing forces on the
self-assembly process of individual molecules due to their relative
The rational design and preparation of supramolecular assem-
blies through the coordination of metal ions with organic ligands
has attracted attention for developing novel crystalline materials
with interesting structural topologies and promising applica-
tions, and has evolved an interesting research field termed as
metallosupramolecular chemistry.1 The metals used in these
complexes can serve as structural components and/or as a source
of properties (e.g., magnetic, catalytic, optoelectronic, etc). The
two most commonly used approaches for engineering the crystal
structure of such complexes employ either coordination bonds2
or supramolecular interactions.3 Both approaches and their
combinations have resulted in the construction of pre-designed
strength, directionality and ability to act synergically. At the
…
same time, the significance of other motifs such as p
p
interactions,6 a class of weak non-directional forces found in
both natural and synthetic systems, should not be under-
estimated in the context of crystal engineering. Studies on such
interactions have mainly focused on organic systems, while
transition metal complex systems are somewhat less explored.
Among other transition metals, nickel has played an important
role in the development of modern inorganic chemistry.7,8 The
chemistry of Ni features a very wide variety of mono- and
polydentate ligands, with metal oxidation states ranging from 0 to
+V, although organometallic Ni2I and Ni2II species have been
reported. The most common oxidation state by far is NiII. The
coordination chemistry of Ni(II) has flourished since 1980,
stimulated by, inter alia, the discovery and mechanism of action
of the enzyme urease, the use of complexes in catalysis and the
aDepartment of Chemistry, University of Patras, Patras, 26504, Greece.
E-mail: nastopoulos@chemistry.upatras.gr
bDepartment of Chemistry, University of Cyprus, Nicosia, 1678, Cyprus
{ Electronic supplementary information (ESI) available: Molecular
overlay figures, additional drawings, IR spectra and tables with
hydrogen-bonding geometries. CCDC reference numbers 881062–
881074. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2ce25704a
6492 | CrystEngComm, 2012, 14, 6492–6502
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booming interest in NiII-containing materials with specific
properties related to magnetism and nonlinear optics.7 By contrast
with the molecular chemistry of Ni(II), the supramolecular
chemistry of this metal ion has received scant attention.
Nickel(II) has certain characteristics that might prove valuable
for the development of its supramolecular chemistry. It forms a
large number of complexes with coordination numbers 3 to 6.9
Being a d8 system, it shows an exceptional preference for
octahedral as opposed to tetrahedral coordination with weak-
field ligands, due to the large Crystal Field Stabilization Energy
(CFSE) for the former geometry.10 Bulky weak-field ligands often
give tetrahedral complexes, whereas medium-field bulky or
strong-field ligands favour square planar coordination; the latter
is a natural consequence of the d8 configuration, since the planar
ligand set causes one of the d orbitals (dx22y2) to be uniquely high
in energy and the eight electrons can occupy the other four d
orbitals leaving this strongly antibonding one vacant. Ni(II) also
has a tendency to add a further ligand to give 5-coordinate
species, both trigonal bipyramidal and square pyramidal
depending on the present and/or the incoming fifth ligand.
Another particular characteristic9 of its chemistry is the
existence of complicated equilibria in solution, commonly
temperature- and concentration-dependent, and this sometimes
gives different structural types (formation of 4-, 5- and
6-coordinate complexes, monomer-polymer formation, planar-
tetrahedral complex formation, unusual isomerism) in the solid
state even with the same set of ligands. All the above mentioned
molecular characteristics of Ni(II) complexes can lead to
atom) enabling the formation of recurring intermolecular units
(supramolecular packing motifs or synthons) that can potentially
assist the molecular self-assembly, and (iii) they have similar
molecular structures, hence comparative studies of the inter-
molecular organization of their complexes is easier. There has
been relatively little work on the coordination chemistry of
heavily substituted imidazoles, and in particular about L14–18
2
and HL9.18,19 The ions X2 (X2 L Cl2, Br2, I2, NO32, NO2
,
ClO42) used were selected on the basis of (i) their tendency to act
as monodentate terminal ligands,16,18 thus avoiding the isolation
of coordination polymers, (ii) their ability to coordinate or act as
counterions:18,20 neutral and cationic complexes were expected,
respectively, and (iii) their size,21 in order to study their effect on
the complex stoichiometry (when coordinated) or their role in
the supramolecular organization process as counterions. Strong
2
basic counterions, e.g. MeCO2 ions, gave strong evidence of
ligand deprotonation in the case of HL9 and formation of
polymeric species.
We report here the results of this approach: a total of 13 new
Ni(II) complexes (1–13) have been prepared and characterized.
The methodology used comprises reactions of simple Ni(II)
salts (NiX2) with the L and HL9 ligands; the parameters
studied include the nature of X2, the reaction solvent, the
molar ratio of reagents, the crystallization solvent, and the
crystallization method. In each group of experiments we tried
to vary one parameter at a time in order to isolate the
maximum number of products for a given reaction system. The
structural analysis indicated the tendency of the Ni(II) centres
to form square planar [NiL4]2+ and [Ni(HL9)4]2+ cations (1–10)
adopting a structure-directing role in the supramolecular
organization of the corresponding complexes, while the
tetrahedral (11, 12) and octahedral (13) stereochemistries also
observed among the prepared compounds seem to advocate the
choice of nickel as an interesting candidate in metallosupra-
molecular research.
interesting supramolecular features, e.g. formation of 1D, 2D
…
or 3D networks formed by hydrogen bonding and/or p
p
stacking interactions and supramolecular isomerism, amongst
others. It is thus obvious that the choice of organic and
inorganic ligands is not only important in the classical
(molecular) coordination of Ni(II), but it is also crucial in the
development of its metallosupramolecular chemistry.
In this context we have designed and prepared a series of NiII
complexes using as ligands two substituted imidazoles, 1-methyl-
4,5-diphenylimidazole (L) and 4,5-diphenylimidazole (HL9),
coordinated to the metal ion via the pyridine-type N3 atom
(Scheme 1). Imidazole and its derivatives are, among others,
particularly interesting ligands in bioinorganic11,12 and metallo-
supramolecular13 chemistry. The ligands employed have the
following features: (i) Both have two phenyl rings which,
combined with the p-excessive character of the 5-membered
Results and discussion
Synthetic comments
A multitude of reactions have been systematically explored with
differing reagent ratios, reaction solvents, hydro(solvo)thermal
techniques, and other conditions aimed at preparing the largest
possible number of Ni(II) complexes of L and HL9 ligands. In
some cases triethylorthoformate (TEOF) was added as a drying
agent. Ni(II) is air-stable, therefore the synthetic work was
conducted in the normal laboratory atmosphere; furthermore it
is located on the right of the Irving–Williams series, ensuring
significant thermodynamic stability for its complexes, a key issue
in crystal engineering. The general synthetic route with the
individual formulae of the complexes is illustrated in Scheme 2.
In each case, the crystalline product was characterized by IR
spectroscopy, microanalysis, and single-crystal X-ray diffraction.
Complexes 1–10 were obtained by reactions with metal-to-ligand
ratio of 1 : 2.5 up to 1 : 4 with a square planar geometry for the
metal centre; however, by changing the ratio to 1 : 1, the
reactions resulted in complexes 11–13 with tetrahedral and
octahedral geometries. Reactions with an excess of NiII did not
affect the product identity.
…
heterocyclic ring, can form p p interactions, (ii) ligand HL9, in
contrast to L, has a hydrogen-bond donor (the pyrrolic-type N1
Scheme 1 The monodentate ligands L (left) and HL9 (right).
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Scheme 2 General synthetic route for the Ni(II) compounds with ligands L and HL9.
to the [NiL4]2+ cations (Fig. 1). Similar p p motifs have been
encountered in a few previously characterized complexes with
L,15–18 supporting its utility as a crystal engineering tool. The
conformation similarity among the [NiL4]2+ cations is high-
lighted in Fig. S1 (ESI{) showing the overlay of the cation pairs.
…
Description of the structures of complexes 1–5
[NiL4]X2 type complexes. Note: To facilitate discussion,
molecular comparison and overlay, the same numbering scheme
has been assigned (where applicable) to the ligand atoms
(Scheme 1), the coordinated ions, and the counterions/solvents
for all compounds presented herein.
As reasonably expected, the largest deviations occur in the region
…
of the phenyl rings not involved in intramolecular p
p
This group comprises complexes with X2 = OH2 (1),
interactions, probably to comply with packing requirements.
K[NiCl4]2– (2), Br2 (3), NO3 (4) and ClO4 (5). Repeated
attempts to isolate the anticipated compound [NiL4]I2?solvent
yielded only an amorphous material making it impossible to
characterize crystallographically. We observe that the structures
of 1 (isolated by hydrothermal conditions) and 2 contain as
counterions OH2 and [NiCl4]2–, respectively, instead of the
expected Cl2 (Scheme 2). A common feature in all five structures
is the presence of the [NiL4]2+ cation (Fig. 1) in which the metal
is coordinated through the pyridine-type nitrogen donor atom
from each of the four ligands, resulting in a NiN4 square planar
geometry. This coordination is in accordance with the principles
of the CFSE theory for Ni(II) complexes, given that L is a
medium-field ligand. The asymmetric unit of compounds 1, 2
and 3 contain half a [NiL4]2+ cation (Z9 = K), the other half
generated by a C2 axis lying on the NiN4 equatorial plane and
passing through the metal centre. Nevertheless, NiII is not in a
perfectly square planar geometry due to lack of D4h (4/mmm)
symmetry in the coordination environment of the nickel ion, as
evidenced by the N–Ni–N values (close to 90u) of the NiN4
group. In all cases (1–5), the cation’s ligands are combined into
two pairs; within each pair the two ligands are mutually
accommodated in a syn fashion and in an ‘‘antiparallel’’ way
2
2
The r.m.s.d. values for the four pairs of cations (1 being the
˚
˚
reference molecule) are: 0.360 A (1, 2), 0.039 A (1–3), 0.466 A (1–
˚
˚
4), and 0.153 A (1–5).
The packing of compounds 1–5 discloses a characteristic mode of
organization, common in all five compounds: the supramolecular
Fig. 1 The [NiL4]2+ cation of compound 1 with the four intramolecular
with their methyl groups pointing at opposite directions. This
…
…
˚
˚
p motifs (dp p = 3.577 A for the blue pairs and 3.498 A for the brown
arrangement favors the formation of two intramolecular p
p
…
p
pairs). Hydrogen atoms have been omitted.
stackings in each pair of ligands, providing considerable rigidity
6494 | CrystEngComm, 2012, 14, 6492–6502
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assembly is essentially organized around the [NiL4]2+ cations via
1–5 and 6–10, respectively, is reflected in the overlays in Fig. S1,
ESI.{ At the supramolecular level, however, the presence of the
N1–H group in ligand HL9, capable of directing strong
hydrogen-bond motifs, provides additional tools for supramo-
lecular self-assembly while increasing system complexity. The
counterion/solvent species of each structure are, similarly to
compounds 1–5, organized into clusters, providing the appro-
priate acceptors (Cl, O, Br, I) to generate strong hydrogen
bonding with the N1–H donors of the rigid [Ni(HL9)4]2+
cations. The resulting 3D structures are further reinforced by
weak interactions, depending on the composition, shape and
size of the cluster in each individual structure. It has been
shown that the inclusion of water in organic and metal–organic
structures is of fundamental and practical importance.23,24 Due
to its very small size and excellent hydrogen-bonding ability, it
can be hosted in many locations and environments. Indeed, the
water molecules in compounds 1, 3, 6, 8 and 9 form in all cases
discrete clusters with the corresponding counterions, thus
contributing to the self-assembly process of the crystalline
structures, rather than being innocuous bystanders within the
crystals.24
2
…
weak C–H X interactions (X = O, Cl, Br ) with the surrounding
counterion/solvent molecules. The latter are grouped in discrete
2
clusters [OH2/H2O (1), [NiCl4]2–/EtOH (2), Br2/H2O (3), NO3
/
MeOH (4) and ClO42/Me2CO (5)] with their components held
firmly together via strong (compounds 1, 2, 3 and 4) or weak
(compound 5) hydrogen bonding. In this way, the [NiL4]2+ cations
adopt a structure-directing role resulting in 3D architectures for
compounds 2, 3, 4 and 5. As a representative example, the structure
of compound 4 is given in Fig. 2. Compound 1 can only form 2D
layers due to a lack of interactions between the [NiL4]2+ and the
OH2 ions of neighbouring layers, probably due to the small size of
the latter ions (Fig. S2, ESI{). A detailed survey of all interactions
and structural motifs that contribute to the supramolecular self-
assembly of the compounds is given in Table 1. Despite the large
number of ligand p-systems in the cations of compounds 1–5, no
…
intermolecular p p interactions have been identified, due to the
buffering action of the intervening clusters. However, weak
22
…
intermolecular C–H p contacts add, in most cases, to the
stability of the resulting crystal structures. The coordination
geometry of NiII in compound 2 is square planar in the [NiL4]2+
cations and tetrahedral in the [NiCl4]2– counterions, the chloride
ions being weak-field ligands in the latter case.
In particular, the asymmetric unit of compound 6, a rather
unexpected product isolated by hydrothermal conditions, also
contains two tetrahedrally coordinated [NiCl2(HL9)2] molecules.
In terms of cluster organization, disordered chloride counterions
and solvent water molecules close to an inversion centre sum up
with their symmetry-related counterparts into a tight Cl2/H2O
cluster. A second chloride counterion acts as a multihydrogen-
bonded acceptor. Thus, donors (N–H) and acceptors (Cl2, O)
Description of the structures of complexes 6–10
[Ni(HL9)4]X2 type complexes. This group includes compounds
2
2
6–10 with X2 = Cl2 (6), Br2 (7), I2 (8), NO3 (9) and ClO4
(10). Interestingly, the [Ni(HL9)4]2+ cation is again a permanent
2
…
component in all structures, with a distorted square planar
…
co-operate via N–H Cl /O hydrogen bonding towards a 3D
coordination and stabilized by the same intramolecular p
p
assembly (geometry details in Table S1, ESI{). To accommodate,
…
2
where appropriate, the formation of the dominant N–H Cl /O
motif as in compounds 1–6. The conformation similarity
between the [NiL4]2+ and [Ni(HL9)4]2+ cations in compounds
patterns and/or minimize steric hindrance, the two [NiCl2(HL9)2]
2
Fig. 2 A slab of the 3D structure of compound 4 showing the self-assembly of the NO3 counterions with the MeOH solvent molecules in discrete
clusters (red dashed lines). The clusters are arranged around the [NiL4]2+ cations via weak C–H O interactions (blue dashed lines). Hydrogen atoms
…
(except those of the MeOH) have been omitted.
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Table 1 Summary of all intra/intermolecular interactions observed in the crystal structures of the studied compounds 1–13
Counterion +
Coordination geometry solvent clusters
Intra Inter
…
…
Compound
Weak H-bonds
Strong H-bonds
p
4
4
4
p
p
p
Network
2D
2(OH2)/H2O
[NiCl4]2+/2EtOH
2Br2/3.4H2O
C–H O(OH2
—
—
—
…
…
…
O(H2O)–H O(OH2
1?H2O
)
)
C–H
p
a
…
…
…
2?2EtOH
C–H Cl
O–H Cl
3D
C–H
O
2
2
…
…
O–H Br
3?3.4H2O
4?2.8MeOH
5?Me2CO
C–H Br
3D
…
C–H
C–H
O
…
…
p
NO32/2MeOH
C–H O(H O)
2
)
3
4
4
—
—
3D
3D
…
O(MeOH)–H O(NO
2
NO32/0.8MeOH
C–H O(NO
2
)
3
…
…
…
C–H
p
2(ClO42)/Me2CO
C–H O(ClO
2
)
—
4
…
C–H O(Me CO)
2
…
C(Me CO)–H O(ClO
2
)
2
…
4
C–H
p
2
b
2 c
Cl2/H2O
N–H Cl
…
N–H O
2
4
1
1
4
2
3D
3D
3D
3D
…
C–H Cl
…
6?1.35H2O
Cl2, 0.35H2O
C–H Cl[NiCl (HL9) ]
d
d
…
2
2
…
…
…
O–H Cl
N–H Br
2
2
2Br2/4.7MeCN
C–H Br
—
—
—
…
…
7?4.7MeCN
C–H N(MeCN)
N–H N(MeCN)
…
C–H
C–H
p
2
2
2I2/2Me2CO/0.8H2O
I
4
4
…
…
N–H I
O(H O)–H
8?2Me2CO?0.8H2O
9?2EtOH?2H2O
2
2
…
I
…
C(Me CO)–H I
C–H O(Me CO)
2
…
…
…
2
2
2(NO32)/2H2O/2EtOH C–H O(H O)
2
)
3
…
N–H O(NO
2
…
O(H O)–H O(NO
C–H O(NO
C–H O(EtOH)
2
3
2
)
3
)
2
…
…
…
…
C–H
p
2(ClO42)/2.8Me2CO
C–H O(ClO
C–H O(Me CO)
C(Me CO)–H O(ClO
2
4
2
)
4
4
—
3D
…
N–H O(ClO
10?2.8Me2CO
)
2
…
2
)
2
…
4
C–H
p
e
…
C–H Cl
11
12
—
—
—
—
2
2
2
3D
3D
…
C–H
p
…
C–H Br
—
…
C–H
p
f
…
…
…
13
—
C–H
C–H
C–H
O
N
p
—
—
—
2D
a
b
c
Tetrahedral coordination for the [NiCl4]2- complex. Tetrahedral coordination for the [NiCl2(HL9)2] complexes. Supramolecular synthons are in
d
e
bold. Intramolecular p p stacking in the [NiCl2(HL9)2] complexes. X = Cl, Br.
f
…
represents the bidentate chelating nitrito ligand.
…
molecules are not, as would be expected, heavily involved in
…
interactions (Table 1). No C–H p interactions have been
detected, owing to the large separation of the [Ni(HL9)4]2+
cations.
p
p stacking, nor do they retain the same conformation. In
…
fact, one intramolecular p p stacking is observed in each
The [Ni(HL9)4]2+ cations of compound 9 are surrounded by
2(NO32)/2H2O/2EtOH rigid clusters located around the C2 axes
[NiCl2(HL9)2], while one of the latter molecules forms dimers
…
with its centrosymmetric partner via two intermolecular p
p
stackings (Table 1). The crystal packing is further supported via
…
of the structure. Its crystal structure is similar to 7; however, the
…
weak intermolecular C–H Cl contacts.
cations are connected in tapes via strong N–H ONO
2
motifs
3
The [Ni(HL9)4]2+ cations of compound 7 are linked by lattice
(Fig. 3, right). The tapes are linked in layers, and the layers are
…
2
Br2 ions into chains along the a axis via strong N–H Br
further expanded in a 3D network via weak C–H O and
…
…
motifs (Fig. 3, left). The chains are then connected by
2Br2/4.7MeCN clusters into sheets parallel to the ab plane via
C–H p interactions. The crystal structure of compound 10
resembles that of 8, i.e. layers are formed by strong
…
2
N–H Br and N–H NMeCN bonding; weak C–H Br and
2
…
…
…
N–H OClO
2
4
motifs and subsequent expansion to 3D is
…
C–H p interactions connect the sheets into the final 3D
achieved via weak intermolecular interactions (Table 1). It
emerges therefore from the structures of 6–10 that the interaction
of the rigid [Ni(HL9)4]2+ building blocks through recurring N–
structure. In compound 8, the large radius of the iodide ions
allow the direct formation of layers (rather than chains as in 7) of
[Ni(HL9)4]2+ cations via N–H
2
2
2
2 patterns (Fig. 4), the involved
H
X (X = Cl , Br , I , O) hydrogen-bonding motifs (acting as
…
…
I
iodides belonging to 2I2/2Me2CO/0.8H2O clusters. The 3D
2
…
supramolecular synthons) is a dominating factor in the process
of their supramolecular organization.
network is completed by C–H
I and acetone-mediated weak
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Fig. 3 (Left) Chains in the crystal structure of compound 7 formed by the [Ni(HL9)4]2+ cations and the lattice Br2 ions via strong N–H Br2 motifs.
…
…
(Right) The cations and the nitrate ions of the surrounding rigid clusters in the structure of 9 are connected in tapes via strong N–H ONO motifs.
2
3
Only species involved in these interactions are shown.
…
Description of the structures of complexes 11–13
in the intramolecular p p pattern. However, small differences in
the molecular structure can result in noticeable changes in the
crystal packing.25 Indeed, the supramolecular organization of
[NiX2L2] type complexes. By changing the metal-to-ligand ratio
to 1 : 1, the reactions resulted in complexes 11–13 with X2 = Cl2
(11), Br2 (12), and NO22 (13). Attempts to isolate the analogous
compounds with HL9 did not yield any diffraction-quality crystals.
The metal centre in complexes 11 and 12 is coordinated by two
pyridine-type nitrogen donor atoms from each ligand and the two
terminal halides, resulting in a distorted tetrahedral N2X2
geometry. The two ligands in both complexes are mutually
arranged in a syn fashion. Each complex is stabilized by two
…
compound 11 is based on weak intermolecular C–H Cl and
…
C–H p interactions forming layers, which are further linked by
…
similar interactions and p p stackings in a 3D structure in the
Pbcn space group (Fig. 5, left). The same type of bonding
…
…
(C–H Br and C–H p) is also present in compound 12 but no
…
intermolecular p p stackings form, resulting in a different crystal
packing (I41cd space group). This difference is likely due to a
combination of several effects, including the longer Ni–Br distance
…
…
intramolecular p p stackings, similar to those observed in
(compared to Ni–Cl) and the different number of C–H X (X =
complexes 1–10, and have a similar conformation, the main
Cl, Br) interactions formed by the coordinated halogeno ions (up
differences being in the orientation of the phenyl rings not involved
to two bonds for compound 11 and up to four for 12).
Fig. 4 Part of the structure of compound 8. The rigid [Ni(HL9)4]2+ cations are surrounded by clusters of iodide counterions and water solvent
2
…
molecules (red dashed lines) and form layers via strong N–H
I motifs (blue dashed lines). The H-atoms and the acetone molecules of the clusters,
linking the layers in a 3D structure, have been omitted. Non-carbon atoms are drawn as spheres of arbitrary radii.
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… …
Fig. 5 (Left) Projection of compound 11 on the ac plane. The intermolecular p p and C–H p interactions connecting the layers (which are parallel
…
to the ab plane) in a 3D structure are coloured violet. (Right) Tapes in the crystal structure of compound 13 formed by weak C–H O, C–H N and
…
…
C–H p interactions (shown in red dashed lines). The octahedral polyhedron around the nickel atoms is coloured yellow.
In compound 13, the two nitrite ions, acting as bidentate
chelating ligands, coordinate to the nickel through the four
oxygen atoms while the two imidazole ligands, through their
pyridine-type nitrogen atoms, act as axial ligands allowing the
nickel cation to acquire its preferred octahedral coordination
components of the clusters, i.e. OH2/H2O (1), [NiCl4]2–
/
EtOH (2), Br2/H2O (3), NO32/MeOH (4), ClO42/Me2CO
2
(5), Cl2/H2O (6), Br2/MeCN (7), I2/Me2CO/H2O (8), NO3
/
EtOH/H2O (9), and ClO42/Me2CO (10), are held tightly
together by strong or weak hydrogen bonds. In particular,
the water molecules within the clusters of 1, 3, 6, 8 and 9 seem
to exert a structural role towards the organization of the
crystalline structures, rather than simply being present as co-
crystallized solvent molecules.
(Fig. 5, right). The trans disposition of the ligands does not
…
favour the formation of the characteristic intramolecular p
p
motif observed so far in all square planar (1–10) and
tetrahedral structures (11, 12). However, this allows the phenyl
rings to rotate so that the structure assembles in a ‘tighter’
supramolecular packing. This is reflected in a higher packing
index26 (71.5%) for 13, as compared to 67.2% for 11 and 67.3%
for 12. In terms of supramolecular organization, the complexes
The ligands selected allowed the implementation of a designed
bonding scheme, including classical and weak hydrogen bonds,
as well as weak p-system interactions. In particular, the assembly
of the [NiL4]2+ cations with the counterion/solvent clusters in
…
are connected via weak C–H O, C–H N and C–H p
…
…
compounds 1–5 is accomplished through a network of weak, yet
2
steadily occurring in all structures, C–H O/Cl/Br , p p and
…
…
interactions into tapes along the a axis which are further
…
…
linked by C–H p interactions into layers parallel to the ab
C–H
p interactions, demonstrating the strength of their
plane.
collaborative action and leading to well-organized 2D and 3D
structures. On the other hand, the system of interactions
responsible for the construction of the crystalline structures with
HL9 (compounds 6–10) is clearly hierarchic. At the first level of
organization, the counterion/solvent clusters provide the appro-
priate acceptors to generate, with the NH donor groups of the
Conclusions
The study of a series of Ni(II) complexes with a variety of
coordination motifs has disclosed interesting features of their 2D
and 3D supramolecular architectures. In terms of molecular
organization, use of the L and HL9 ligands has resulted in square
planar [NiL4]2+ (1–5) and [Ni(HL9)4]2+ (6–10) complexes in
accordance to Crystal Field Theory for medium-field bulky
ligands, in tetrahedral coordination in [NiX2L2] complexes for
weak-field halogeno ligands [X = Cl (11), Br (12)], and
octahedral in the [Ni(NO2)2L2] complex (13) due to the large
CFSE for this geometry.
[Ni(HL9)4]2+ cations, strong N–H Cl /Br /I /O motifs (supra-
molecular synthons) present in all crystalline structures. Then,
similarly to compounds 1–5, weak interactions contribute to the
3D expansion of the structures.
2
2 2
…
Complexes 1–10 have no counterparts in CoII and ZnII
chemistry,18 due to the inability of the latter two metal ions to
form square planar structures. The molecular structures of 11
and 12 are similar to those of [MX2L2] (M = Co, Zn; X = Cl, Br).
We were unable to observe polymorphs of 11, in contrast to
[CoCl2L2] and [ZnCl2L2] where two polymorphs were identified
for each complex. The supramolecular structures of 12,
[CoBr2L2] and [ZnBr2L2]18 are isomorphous crystallizing in the
I41cd space group. Octahedral [M(NO2)2L2] complexes are
known in CoII and ZnII chemistry.18 However, the two L ligands
are mutually cis in contrast to their trans orientation in 13. As a
result, their supramolecular structures differ.
The rigid [NiL4]2+ and [Ni(HL9)4]2+ cations in compounds
1–10, stabilized by a characteristic motif of four intramole-
…
cular p p stacking interactions, seem to have an impact on
the supramolecular organization by adopting a structure-
directing role. Regardless of the ligand (L or HL9) used, each
structure is organized around the rigid and bulky [NiL4]2+ or
[Ni(HL9)4]2+ cations via interactions with surrounding coun-
terion/solvent clusters in each individual structure. The
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We are currently expanding our research efforts to prepare
analogous species with copper(II), a 3d-metal ion whose
coordination preferences differ from those of nickel(II), to
check the impact of the metal coordination on the composition
of the building blocks and their subsequent supramolecular
organization.
1070 m, 1020 m, 986 w, 952 w, 794 w, 772 s, 750 w, 700 s, 648 s,
588 w, 514 wb, 436 w.
Synthesis of [NiL4](NO3)2?2.8MeOH (4?2.8MeOH). A solu-
tion of L (0.293 g, 1.25 mmol) and Ni(NO3)2?6H2O (0.145 g,
0.50 mmol) in MeOH (25 ml) was stirred for 30 min. The
reaction solution was filtered. Upon slow evaporation of the
filtrate, yellow prismatic crystals of 4?2.8MeOH were obtained
after 7 days in a 30% yield (based on L). Anal. Calcd for 4: C,
68.64; H, 5.04; N, 12.51%. Found: C, 68.56; H, 4.91; N, 12.62%.
Selected IR bands (KBr, cm21): 3134 w, 1654 w, 1528 m, 1484
w, 1384 s, 1196 w, 1072 m, 1020 m, 986 w, 724 w, 698 s, 670 w,
514 w.
Experimental
Materials and instruments
Chemicals (reagent grade) were purchased from Merck and Alfa
Aesar. All manipulations were performed under aerobic condi-
tions using materials and solvents as received; water was distilled
in-house. The ligand 1-methyl-4,5-diphenylimidazole (L) was
synthesized as already described in the literature.27 Micro-
analyses (C, H, N) were performed by the University of Ioannina
(Greece) Microanalytical Laboratory using an EA 1108 Carlo
Erba analyzer. IR spectra were recorded on a Perkin-Elmer PC
16 FT-IR spectrometer with samples prepared as KBr pellets
(Fig. S3, ESI{).
Synthesis of [NiL4](ClO4)2?Me2CO (5?Me2CO). A solution of
L (0.375 g, 1.6 mmol) and Ni(ClO4)2?6H2O (0.146 g, 0.40 mmol)
in Me2CO (25 ml) was stirred for 30 min. The reaction solution
was filtered. Upon slow evaporation of the filtrate, yellow
prismatic crystals of 5?Me2CO were obtained after 4 days in a ca.
60% yield [based on nickel(II)]. Anal. Calcd for 5: C, 64.33; H,
4.72; N, 9.37%. Found: C, 64.42; H, 4.86; N, 9.24%. Selected IR
bands (KBr, cm21): 3138 m, 3050 w, 1602 m, 1528 s, 1484 m,
1444 m, 1194 m, 1098 vs, 1018 m, 782 m, 774 m, 746 m, 696 s,
648 m, 624 s.
Safety note: Perchlorate salts are potentially explosive; such
compounds should be synthesized and used in small quantities,
and treated with utmost care at all times.
Synthesis of [NiL4](OH)2?H2O (1?H2O). This compound was
synthesized by a solvothermal reaction of L (0.059 g, 0.25
mmol) and NiCl2?6H2O (0.024 g, 0.10 mmol) in MeCN (8 ml).
The resultant solution was heated at 100 uC in a Teflon-lined
stainless steel autoclave for 3 days. The reaction system was
then slowly cooled (5 uC h21) to room temperature. The
reaction solution was filtered. Upon slow evaporation of the
filtrate, light green prismatic crystals of 1?H2O were obtained
after 11 days in a 60% yield (based on L). Anal. Calcd for
1?H2O: C, 73.35; H, 5.77; N, 10.69%. Found: C, 73.44; H, 5.63;
N, 10.58%. Selected IR bands (KBr, cm21): 3470 sb, 3420 mb,
1602 w, 1527 m, 1504 m, 1483 m, 1441 s, 1365 w, 1248 w, 1194
m, 1157 w, 1068 m, 1020 m, 985 w, 951 w, 793 w, 769 s, 748 w,
701 s, 650 s, 587 w, 517 w, 435 w.
Synthesis of [Ni(HL9)4][NiCl2(HL9)2]2Cl2?1.35H2O
(6?1.35H2O). This compound was synthesized by the solvother-
mal reaction of HL9 (0.165 g, 0.75 mmol) and NiCl2?6H2O
(0.071 g, 0.30 mmol) in MeCN (8 ml). The reaction procedure
was similar to that of 1?H2O. The reaction solution was stored
in a closed flask at room temperature. Blue prismatic crystals
of 6?1.35H2O were obtained after 2 days in a 30% yield (based
on HL9). Anal. Calcd for 6?1.35H2O: C, 66.45; H, 4.55; N,
10.33%. Found: C, 66.57; H, 4.39; N, 10.19%. Selected IR
bands (KBr, cm21): 3134 w, 3064 w, 1508 m, 1458 w, 1398 w,
1374 w, 1072 w, 1026 w, 978 w, 764 s, 724 w, 694 s, 648 m,
494 w.
Synthesis of [Ni(HL9)4]Br2?4.7MeCN (7?4.7MeCN). A solution
of HL9 (0.138 g, 0.63 mmol) and NiBr2?3H2O (0.055 g, 0.25
mmol) in MeCN (20 ml) was stirred for 30 min. The colourless
solution was left to slowly evaporate at low temperature (5 uC).
Yellow prismatic crystals of 7?4.7MeCN were obtained after 3
days in a 40% yield (based on HL9). Anal. Calcd for 7: C,
65.53; H, 4.40; N, 10.19%. Found: C, 65.41; H, 4.29; N,
10.27%. Selected IR bands (KBr, cm21): 3060 mb, 1604 w, 1510
s, 1486 m, 1442 m, 1314 w, 1294 w, 1158 w, 1132 m, 1072 m,
958 w, 764 s.
Synthesis of [NiL4][NiCl4]?2EtOH (2?2EtOH). A solution of L
(0.375 g, 1.60 mmol) and NiCl2?6H2O (0.095 g, 0.40 mmol) in
EtOH/TEOF (15 ml/3 ml) was refluxed for 1 h. The reaction
solution was filtered. Upon slow evaporation of the filtrate, light
green prismatic crystals of 2?2EtOH were obtained after 3 days
in a 40% yield [based on nickel(II)]. Anal. Calcd for 2: C, 64.25;
H, 4.71; N, 9.36%. Found: C, 64.34; H, 4.83; N, 9.29%. Selected
IR bands (KBr, cm21): 3420 sb, 3114 w, 1602 m, 1578 w, 1528 s,
1508 m, 1482 m, 1444 m, 1196 m, 1156 w, 1072 m, 1018 m, 794
m, 774 s, 700 s, 650 m, 516 w, 402 w.
Synthesis of [Ni(HL9)4]I2?2Me2CO?0.8H2O (8?2Me2CO?0.8H2O).
A solution of HL9 (0.275 g, 1.25 mmol) and NiI2?6H2O
(0.210 g, 0.50 mmol) in Me2CO (25 ml) was refluxed for 1 h.
The reaction solution was filtered. The resultant green solution
was layered with n-hexane (50 ml) to produce dark orange
prismatic crystals of 8?2Me2CO?0.8H2O in 6 days; yield ca.
40% (based on HL9). Anal. Calcd for 8?0.8H2O: C, 60.32; H,
4.13; N, 9.37%. Found: C, 60.24; H, 4.01; N, 9.26%. Selected
IR bands (KBr, cm21): 3068 mb, 1604 m, 1590 w, 1508 m,
Synthesis of [NiL4]Br2?3.4H2O (3?3.4H2O). A solution of L
(0.375 g, 1.60 mmol) and NiBr2?3H2O (0.087 g, 0.40 mmol) in
CH2Cl2/TEOF (20 ml/3 ml) was refluxed for 1 h. The reaction
solution was filtered. Upon slow evaporation of the filtrate,
yellow prismatic crystals of 3?3.4H2O were obtained after 1 day
in a 60% yield [based on nickel(II)]. Anal. Calcd for 3?3.4H2O: C,
63.54; H, 5.16; N, 9.26%. Found: C, 63.41; H, 5.02; N, 9.34%.
Selected IR bands (KBr, cm21): 3418 mb, 3112 w, 1600 w, 1528
m, 1506 m, 1482 m, 1442 m, 1366 w, 1250 w, 1194 m, 1158 w,
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1490 m, 1444 m, 1158 w, 1026 w, 982 w, 914 w, 764 s, , 696 s,
648 m, 564 w, 494 w.
1374 w, 1330 w, 1298 w, 1262 w, 1214 s, 1198 sh, 1178 sh, 1126 w,
1020 w, 826 w, 784 m, 774 sh, 692 m, 650 m, 580 w.
X-ray crystallography
Synthesis of [Ni(HL9)4](NO3)2?2EtOH?2H2O (9?2EtOH?2H2O).
A solution of HL9 (0.138 g, 0.63 mmol) and Ni(NO3)2?6H2O
(0.073 g, 0.25 mmol) in EtOH/TEOF (20 ml/3 ml) was refluxed
for 1 h. The reaction solution was filtered. Upon slow
evaporation of the filtrate, yellow prismatic crystals of
9?2EtOH?2H2O were obtained after 1 day in a 40% yield (based
on HL9). Anal. Calcd for 9?2H2O: C, 65.52; H, 4.76; N, 12.73%.
Found: C, 65.43; H, 4.62; N, 12.86%. Selected IR bands (KBr,
cm21): 3112 w, 3064 w, 1604 m, 1510 w, 1488 m, 1444 m, 1384 s,
1340 m, 1318 m, 1156 w, 1134 m, 1072 m, 1046 m, 822 w, 764 s,
694 s, 648 m, 580 m, 402 w.
Single-crystals covered with paratone-N oil were mounted on
the tip of glass fibres or were scooped up in cryo-loops at the
end of a copper pin. X-ray diffraction data were collected (v-
scans) with an Xcalibur-3 and
a SuperNova A Oxford
Diffraction diffractometers under a flow of nitrogen gas at
˚
100(2) K using Mo-Ka radiation (l = 0.7107 A) except for
˚
compounds 5 and 13 where Cu-Ka (l = 1.5418 A) was used.
Data were collected and processed by the CRYSALIS CCD
and RED software,28 respectively, and the reflection intensities
were corrected for absorption by the multiscan method. All
structures were solved using SIR9229 and SHELXS-9730 and
refined by full-matrix least-squares on F2 with SHELXL-97.31
All non-H atoms were refined anisotropically, and carbon-
bound H-atoms were introduced at calculated positions and
allowed to ride on their carrier atoms. Non-routine aspects of
structure refinement are as follows: All imidazole H-atoms on
the pyrrolic type N1 atom of the HL9-containing compounds,
as well as the hydroxyl H-atoms of solvents in compounds
1 (H2O), 4 (MeOH), and 2 and 9 (EtOH) were located in
difference Fourier maps and refined isotropically applying soft
distance restraints (DFIX). The structure of 1 was refined as a
merohedral twin (with an 97 : 3 twin components ratio), the
structure of 5 as a twin by inversion (51 : 49 twin ratio), and the
structures of 10 and 13 as non-merohedral twins (64 : 36 and
70 : 30 twin ratio, respectively). The bromide (in 3 and 7) and
iodide (in 8) counterions are disordered and have been
modelled over two orientations, while the chloride counterion
and a lattice water molecule in structure 6 are disordered over
two sites forming two mixed Cl2/H2O sites as was concluded
after competitive and detailed refinement. The crystal struc-
tures of 1 and 9 contain a small area of disordered solvent
(water), and the structure of 5 a highly disordered perchlorate
counterion.
Synthesis of [Ni(HL9)4](ClO4)2?2.8Me2CO (10?2.8Me2CO). A
solution of HL9 (0.350 g, 1.60 mmol) and Ni(ClO4)2?6H2O
(0.146 g, 0.40 mmol) in Me2CO (20 ml) was stirred for 30 min.
The reaction solution was filtered. The resultant orange
solution was layered with Et2O/n-hexane (20 ml/20 ml) to
produce orange prismatic crystals of 10?2.8Me2CO in 2 days;
yield ca. 60% [based on nickel(II)]. Anal. Calcd for 10: C, 63.28;
H, 4.24; N, 9.84%. Found: C, 63.13; H, 4.11; N, 9.72%. Selected
IR bands (KBr, cm21): 3146 w, 3054 w, 1604 w, 1526 m, 1483
m, 1442 w, 1194 m, 1098 vs, 1016 m, 789 m, 778 w, 742 m, 694 s,
648 m, 628 s.
Synthesis of [NiCl2L2] (11). A solution of L (0.094 g, 0.40
mmol) and NiCl2?6H2O (0.095 g, 0.40 mmol) in EtOH (20 ml)
was stirred for 30 min. The reaction solution was filtered. Upon
slow evaporation of the filtrate, violet prismatic crystals of 11
were obtained after 20 days in a 30% yield (based on L). Anal.
Calcd for 11: C, 64.25; H, 4.71; N, 9.36%. Found: C, 64.38; H,
4.54; N, 9.22%. Selected IR bands (KBr, cm21): 3384 sb, 3134 w,
1636 w, 1602 w, 1544 w, 1516 m, 1480 m, 1442 m, 1366 w,
1318 w, 1192 m, 1072 m, 980 w, 916 w, 792 m, 770 s, 698 s,
648 m, 584 w, 456 w.
Attempts to model them with a chemically reasonable
geometry were unsuccessful; therefore, the SQUEEZE procedure
of PLATON32 was employed to remove the contribution of the
electron density associated with those molecules from the
intensity data. However, the role of the disordered perchlorate
in the supramolecular organization of 5 has been taken into
account using a structural model prior to deletion. Geometric/
crystallographic calculations were carried out using PLATON,32
OLEX2,33 X-Seed34 and WINGX35 packages; molecular/packing
graphics were prepared with DIAMOND36 and MERCURY.37
Crystallographic data collection and refinement parameters are
listed in Table 2.
Synthesis of [NiBr2L2] (12). A solution of L (0.087 g, 0.40
mmol) and NiBr2?3H2O (0.087 g, 0.40 mmol) in CH2Cl2/TEOF
(20 ml/3 ml) was refluxed for 1 h. The reaction procedure was
similar to that of 3?3.4H2O. Dark blue needles of 12 were
obtained after 17 days in a 40% yield (based on L). Anal. Calcd
for 12: C, 55.93; H, 4.11; N, 8.15%. Found: C, 55.71; H, 3.94; N,
8.29%. Selected IR bands (KBr, cm21): 3414 sb, 3114 w, 1602 m,
1576 w, 1528 s, 1482 m, 1444 m, 1420 m, 1404 w, 1326 w, 1196 m,
1154 w, 1072 m, 1044 w, 1018 m, 912 w, 794 s, 774 s, 750 m,
724 m, 702 sh, 694 s, 650 m, 588 w, 436 w.
Synthesis of [Ni(NO2)2L2] (13). Ni(ClO4)2?6H2O (0.110 g,
0.30 mmol) and L (0.176 g, 0.75 mmol) were dissolved in MeOH
(15 ml). To the resulting green solution, NaNO2 (0.052 g,
0.75 mmol) in MeOH (10 ml) was added dropwise. The reaction
solution was filtered. Upon slow evaporation of the filtrate, light
green rods of 13 appeared after 5 days in a 40% yield [based on
nickel(II)]. Anal. Calcd for 13: C, 62.06; H, 4.55; N, 13.57%.
Found: C, 62.21; H, 4.36; N, 13.41%. Selected IR bands (KBr,
cm21): 3060 w, 1602 w, 1524 m, 1508 sh, 1458 w, 1442 w, 1402 w,
Acknowledgements
This work was supported by the Research Committee of the
University of Patras, Greece (K. Caratheodory program, Grant
No. C.585 to VN). AJT thanks the Cyprus Research Promotion
Foundation Grant ‘‘ANABAHMISH/PACIO/0308/12’’ which is
co-funded by the Republic of Cyprus and the European Regional
Development Fund. We also thank graduate student Despoina
Dermitzaki for performing some preliminary experiments.
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