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Solid State Chameleons:
Chemical Transformations as Single Crystal Transitions

Final Report Summary - SCSCT (Solid State Chameleons: <br/>Chemical Transformations as Single Crystal Transitions)

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 4-pyridineboronic 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 +N-H…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.
The reaction didn’t proceed via single crystal to single crystal fashion but by grinding the reaction mixture, corresponding products could be obtained.