MARIE CURIE ACTIONS
FINAL REPORT
PROJETCT NO: 301507
PROJECT ACCRONYM: SCSCT
PROJECT NAME: Solid State Chameleons: Chemical
Transformations as Single Crystal Transitions
Period covered: from 03/01/2013 to 02/01/2015
Start date of Project: 03/01/2013
Project co-ordinator name: JAYARAMA PRAKASHA REDDY
Project co-ordinator organization name: DURHAM UNIVERSITY
ACHIEVEMENTS DURING THE PERIOD
Publishable Summary
This report briefly summarises the research work that can be published and also covering
wider societal implications of the project.
1. Two polymorphic forms of 4-pyridineboronic acid and Cu(II) complex.
Two polymorphic metal complexes of have been prepared by reacting 4-pyridineboronic
acid (4-pyba) with CuCl2; [4-pybaH]2[CuCl4] (form I) and [(4-pyba)2 (4-pybaH)]2[CuCl4]
(form II). Crystal and molecular structures of the copper (II) complexes were determined by
the single crystal X-ray diffraction method. The complexes were further characterized by
thermogravimetric, differential scanning calorimetry measurements, and the powder X-ray
diffraction method. Herein we report preparation and structural characterization of two metal
complexes (polymorphs), which have been prepared by reacting 4-pyridineboronic acid with
copper chloride in presence of few drops of HCl.
Polymorphism has been receiving an ongoing interest over decades. Different packing
arrangements and/or conformations can lead to significant changes in the physical and
chemical properties of polymorphs, which makes them attractive in various fields. Although
polymorphism has been extensively investigated in the organic field, particularly focused on
drug design, growing attention has been devoted to the polymorphism of transition metal
complexes, and coordination polymers. During the past few years there has been an
increasing interest in the preparation and study of polymorphism in metal complexes because
of their potential applications in catalysis, magnetism, gas adsorption, etc. Though there are
few examples reported in the literature but still the area of research remains uninvestigated.
In terms of crystal engineering approach, double- or higher-bridged functions are required.
One of the most common synthons in hydrogen-bonded architectures is the carboxylic acid
dimer. However, this synthon lacks additional hydrogen atoms required for a self
complementary association to higher dimensions. Two synthons which fulfil this criterion are
the carboxamide and the boronic acid dimers both of which have been employed for the
purposes of crystal engineering. To the best of our knowledge, this is the first example of
polymorphism in boronic acid metal complexes.
A search of the Cambridge Structural Database (CSD) revealed that boronic acids are
indeed capable of forming higher-dimensional hydrogen-bonded assemblies. The appearance
of polymorphs greatly depends on the interplay between kinetic and thermodynamic factors.
Suitable conditions, for example, solution concentration, temperature of crystallization, rate
of evaporation, choice of solvents, or pressure are of crucial importance for their synthesis.
Herein, we present synthetic, crystallographic, and thermal studies of two polymorphs of 4pyridineboronic acid with CuCl2. Intermolecular interactions have also been investigated not
only in the context of different crystal packing but also considering their role in the
supramolecular architecture formation. In addition, slight change occurring in unit cell
parameters in the solid state of two different complexes triggered by heating (thermally) is
reported.
Preparation of form I and form II. All the reagents were purchased from commercial
source. A solution of 4-pyridineboronic acid in water (10 mL) and concentrated HCl (37%,
2/3 drops till 4-pyba dissolves) was added dropwise to a stirring solution of copper chloride
in water (5 mL). The resulting mixture was stirred over a period of 3 h and then allowed to
evaporate at room temperature. After one week, green/yellow crystals suitable for X-ray
diffraction analysis formed were collected by filtration.
Forms I and II have been characterized by single-crystal X-ray diffraction analysis.
The -B(OH) groups adopt the most preferred syn-anti conformation. In the crystal structure of
form I, each two chloride ions are hydrogen bonded to one [4-pybaH]+ cation through +NH…Cl- hydrogen bonding forming tape like structure in 1-D. In addition, these tapes interact
with adjacent tapes through B(O)-H…Cl hydrogen bonding forming a sheet like structure. In
the extended crystal structure, neighbouring antiparallel running tapes are linked through
additional π-π stacking interactions thus forming a layer structure. In form II, boronic acid
molecules form a fused structure with four molecules being a cluster. Each cluster interacts
with Cl- to form +N-H…Cl- and C-H…Cl hydrogen bonding. Thus, in both the forms the
pyridine moieties are involved in hydrogen bonding showing that the protonated nitrogen
plays important role on the formation overall 3-D supramolecular structure.
The analysis of the crystal structures (form I and II) described herein has shown that
pyridineboronic acids can be versatile ditopic building blocks for crystal engineering of
higher frameworks. This is because the pyridine moiety can form either coordinate covalent
bond (N M) or charge assisted +N-H…Cl2-M- hydrogen bonds
2. Phase transformation via single crystal to single crystal transition
Single crystal to single crystal transformations in metal complexes have attracted
considerable attention as an interesting solid state phenomenon and are usually induced by
temperature, pressure, solvent molecules, etc. The systems involving SCSCT can be achieved
in different ways and the most common being use of flexible ligands which are linked to
metal center through coordinate bond. As the ligand is more flexible, this gives more
flexibility around metal center and can exhibit SCSCT. The second one being the use of rigid
ligands, in which case making and breaking of bonds involves hydrogen bonding interactions
in addition to coordinate bond. We report synthesis and characterization of two solid forms of
metal complexes via single crystal to single crystal transformations (SCSCT).
Preparation of complex [Zn(C7H11N2)2Cl4], 1a: To a solution of 4-(dimethylamino)pyridine
in methanol a solution of ZnCl2 in methanol was added with few drops of dilute HCl. The
reaction was stirred and kept for crystallization and single crystals suitable for X-ray
diffraction were collected after 3/4 days. X-ray structural analysis revealed that the structure
contains a distinct Zn+2 metal center in a distorted tetrahedral geometry. The asymmetric unit
consists of 4 chloride ions coordinated to Zn metal center and 2 molecules of protonated 4(dimethylamino)pyridine. The chloride ion interacts with 4-(dimethylamino)pyridine through
protonated hydrogen atom forming H...Cl hydrogen bonding on both directions to form a tape
like structure.
Preparation of complex [Zn(C7H11N2)2Cl4], 1b: Slow insitu heating of single crystals of
complex 1a resulted in the formation of complex 1b. This was carried out step by step
process. Initially, cell parameters were collected at 120K to see whether the material is in the
same phase as of room temperature phase. This was confirmed by unit cell parameters as
there is no change and the slow raising of temperature from 120K till 340K resulted in no
change. Collection of unit cell parameters was performed at every 10 degree raise in
temperature. At 340K, there is abrupt change in the unit cell parameters were observed at
which temperature data was collected. Structure elucidation by single crystal X-ray analysis
collected at this temperature revealed that SCSCT was observed with completely solved
structure. Assymmetric unit retained as observed in the complex 1a but with a much more
closeness towards tetrahedron geometry. This was further confirmed by DSC analysis in
which an endothermic peak was observed over the temperature range 335-345K. The
transformation from 1a to 1b involves a space group change from P-1 (triclinic) to C2/c
(monoclinic). Packing analysis revealed that ZnCl4 unit interacts with protonated 4(dimethylamino)pyridine through H...Cl hydrogen bonding as observed in 1a but in 1b both
chloride ions forms hydrogen bonding. Interestingly, the transformation is reversible and
upon cooling to room temperature 1a phase is obtained. But above 340K, only 1b phase is
observed till 390K and above this temperature crystallinity is lost.
3. Substitution reaction occurring in the solid state by grinding.
Grinding of 4-chloro-3,5-dinitrobenzoic acid has been carried out in presence of base
(ammonia, methyl amine and dimethyl amine) which lead to the formation of corresponding
salt. Further, grinding in presence of few drops of aqueous HCl followed by washing with
water lead to the formation of final product. This is confirmed by both single crystal and
powder X-ray diffraction.
COOH
COO
COOH
NH3
O2N
NO2
NH4
O2N
Cl
COOH
NO2
NO2
COO
COOH
O2N
COOH
NO2
Cl
dil HCl
NO2
O2N
NO2
NHCH3
NHCH3
COO
COOH
(CH3)2NH
O2N
NO2
NH2
CH3NH3
Cl
O2N
NH2
CH3NH2
O2N
dil HCl
(CH3)2NH2
O2N
NO2
N(CH3)2
dil HCl
O2N
NO2
N(CH3)2
The reaction didn’t proceed via single crystal to single crystal fashion but by grinding the
reaction mixture, corresponding products could be obtained.
Outreach Activities
Science awareness in children: Chemistry department and Grey College hosted the Annual
North East Schools-Industry Partnership (NESIP) Project from Sunday 20th July to Friday
25th July 2014. Students from various schools in the North East region accompanied by their
teachers spent a week in the department undertaking research projects supplied by both local
industrial and academic partners. The projects undertaken ranged from organic synthesis,
supramolecular gel chemistry, crystallization of different molecules which are both gel as
well as crystalline material and biochemistry assays. I was involved in monitoring and
explaining to group of children about my project single crystal to single crystal transitions
and supramolecular inorganic chemistry involving preparation and analysis of metal
complexes.
1. Preparation of [Zn(C7H11N2)2Cl4] from 4-(dimethylamino)pyridine and ZnCl2.
First, all children were told about safety procedures to be followed in handling all chemicals
including commonly used solvents. Then, they were asked to follow the procedure given to
them and carefully monitored in the laboratory. After preparation, I showed them, on hot
stage microscope, how single crystal to single crystal transition occurs on heating and
explained what structural changes occur before and after heating of the complex.
2. Preparation of 4-amino-3,5-dinitrobenzoic acid from 4-chloro-3,5-dinitrobenzoic acid
This method was explained to the children how a reaction can be carried out in solid state
without use of any solvents (Figure 1). Also, explained how grinding of reagents would lead
to the formation of pure product which is good from environmental point of view.
Figure 1. Colour change indicates the formation of different product upon grinding.
At the end, children were made to understand and choose science as their career. With respect
to my project, I explained them how the preparation and characterization of different crystal
forms of same molecule represents one of the most active areas of modern solid state
chemistry. Further, they were explained how the investigation of crystal forms and of single
crystal to single crystal transitions impacts all fields of chemistry associated with the
production and commercialization of molecules in the form of crystalline material such as
drugs, food additives, materials and explosives, etc., and is essential to know the science
involved.
Jayarama Prakasha Reddy
Prof. Jonathan W. Steed
Signature Researcher
Signature Scientist in Charge
Date: 02/03/2015
Date: 02/03/2015