Insights into simple metal complexes to nanoscale applications

Abstract

Investigations embodied in thesis entitled “INSIGHTS INTO SIMPLE METAL COMPLEXES TO NANOSCALE APPLICATIONS” were initiated in January 2013 in the Discipline of Chemistry, Indian Institute of Technology Indore, Indore, 453552, India. The focal points of thesis work are as follows- 1. Construction of simple metal complexes based on Co(II), Ni(II), Cu(II) and Zn(II) with organic ligands viz. 2-(2- hydroxyethyl)pyridine (hep−H), 2-(2-hydroxymethyl)pyridine (hmp-H) and Benzoic acid (BA). 2. Solid-state structural reactivity of discrete Co(II) complexes with 2-(2-hydroxyethyl)pyridine (hep−H) at ambient conditions was explored. 3. The synthesized metal complexes were further employed as single-source molecular precursors (SSMPs) for the preparation of different metal oxide nanostructures of cobalt, nickel, copper and zinc. 4. To explore the performance of these metal oxide nanostructures in various applications viz. catalysis, photocatalysis, adsorption and sensing. The present thesis comprises of six chapters. Chapter 1 contains the introduction about the transition metal complexes and their applications, Single-Crystal-to-Single-Crystal (SCSC) transformations at discrete level which proceeds via different external stimuli such as solvent, heat and light etc. Further, the discussion on the use of different molecular precursors (metal complexes or coordination polymers or metal-organic frameworks) for the formation of nanostructures has been carried out. Additionally, the brief preface has been given on the use of nanostructures in different applications such as catalysis,photocatalysis, adsorption and separation, magnetic materials, sensor and energy storage. In Chapter 2, monomeric [CoII(hep-H)(H2O)4]SO4,[1]SO4 and [CoII(hep-H)2(H2O)2](NO3)2, [2](NO3)2 have been synthesized from 2-(2-hydroxyethyl)pyridine (hep-H) and CoSO4.7H2O/ Co(NO3)2.6H2O respectively, at 298 K. On exposure to heat(120 °C), the light orange single crystal of [1]SO4 transforms to a pink single crystal corresponding to the neutral sulfato bridged dimeric complex [(CoII(hep-H)(H2O)2(μ2-sulfato-O,O⁄))2], (3). However, the orange single crystal of [2](NO3)2 transforms to the single crystal of monomeric [CoII(hep-H)2(NO3)]NO3, [4]NO3(Orange) on exposure to heat (110 °C) where one of the NO3- counter anions in [2](NO3)2 moves to the coordination sphere. The facile SCSC transformations of [1]SO4(Orange)3(Pink) and [2](NO3)2 (Orange)[4]NO3(Orange) involve intricate multiple bond breaking and bond forming processes without losing the crystallinity. Moreover, on immersion of the pink single crystal of 3 to 1N HCl results in a green single crystal of ionic monomeric [CoII(H2O)6]SO4, [5]SO4 which indeed demonstrates the unprecedented unique two-step SCSC transformations. In Chapter 3, cobalt-based nanocomposite (CoNC) has been prepared from recently reported single source molecular precursor (SSMP) [CoII(hep-H)(H2O)4]SO4,{A=1[SO4]} (hep-H= 2-(2-hydroxylethyl) pyridine). The CoNC was characterized by using various physicochemical techniques such as XRD, SEM, EDAX, TEM and XPS. X-ray diffraction pattern shows weakly crystalline nature of the catalyst. This was also confirmed by SAED pattern obtained from HR-TEM. XPS analysis reveals the formation of metallic cobalt and cobalt oxide (CoO) nanocomposite. CoNC was employed for the facile catalytic hydrogenation of 2-nitrotriptycene (M1) and 2,6,14-trinitrotriptycene (M2) as model substrates underatmospheric reaction conditions, which otherwise takes place either by RANEY® Nickel or Pd/C or SnCl2/HCl catalyst under drastic conditions. The mechanistic pathway reveals that the reduction of M1 proceeds via the intermediacy of azoxytriptycene (III) and N-hydroxylamine triptycene (IV). To the best of our knowledge transformation of M1 and M2 to their respective amines by cobalt based nanocomposite (CoNC) have not been investigated under atmospheric reaction conditions using NaBH4 as reducing agent. Further, the scope of CoNC was tested for other polyaromatic nitro compounds. CoNC with magnetic properties can be considered as a promising synthon for other potential applications. In Chapter 4, two new complexes as single-source molecular precursors (SSMPs) [NiII(hep-H)(H2O)4]SO4, (SSMP-1) and [CuII(μ-hep)(BA)]2, (SSMP-2) were prepared by reaction of NiSO4·6H2O with hep-H in MeOH and Cu(CH3COO)2·H2O, hep-H [(2-(2-hydroxylethyl)pyridine)] and BA(benzoic acid) in acetonitrile at room temperature. Further, SSMP-1 and SSMP-2 were used as the molecular precursors for the preparation of NiO and CuO nanocatalysts. Moreover, two nanocatalysts (NiO or CuO) have been shown to catalyse the oxidative amidation of various organic substrates very efficiently. In Chapter 5, two Co/CoO nanocomposites (NC-1 and NC-2) were synthesized from single-source molecular precursors (SSMPs) [Co(hep-H)(H2O)4]SO4, (1) and [Co(hep-H)2(H2O)2](NO3)2, (2), [hep-H= 2-(2-hydroxylethyl) pyridine], respectively, via wet-chemical reduction method. The synthesis of [Co(hep-H)(H2O)4]SO4 and [Co(hep-H)2(H2O)2](NO3)2 were carried out as per the discussion in Chapter 2. The aim of this work was to study the effect of different counter anions (i.e. SO42- and NO3-) of precursors on the surface properties of synthesized materials. Both the nanocomposites (NC-1 and NC-2) were characterized by powder-XRD, SEM,EDAX, FTIR and BET analyses. NC-1 synthesized from 1, had nanosphere type morphology whereas NC-2 synthesized from 2, had nanoflakes type structures with higher surface area. Due to the good surface area, mesoporous nature and different shapes of NCs such as nanosphere and nanoflakes, we studied adsorption behavior of these NCs for dyes, choosing Congo Red (CR) as the model dye. Detailed study of parameters influencing adsorption efficiency such as pH, adsorbent dosages etc. have been conducted. Further, the study has been extended for the other dyes removal and showed the excellent dye adsorption behavior. This study further encouraged us to explore the use of NC-1 as adsorbent to some other toxic metals (Pb, Ce, Cr, Cu, Fe, Cd, As, Ni and Hg). The good adsorption efficiencies were obtained even after five cycles for dye and three cycles for metal. In Chapter 6, an asymmetric dimer [Zn(hmp-H)2(H2O)(μ-Cl)Zn(μ-Cl)(Cl)3], (A) was obtained by the reaction of ZnCl2 with hmp-H {hmp-H (hmp-H=2-(2-hydroxymethyl)pyridine} in MeOH at room temperature. However, the synthesis of A was reported from our group.The prepared [Zn(hmp-H)2(H2O)(μ-Cl)Zn(μ-Cl)(Cl)3], (A) was used as the molecular precursor for the synthesis of ZnO-Nanoflowers (ZnO-NFs) at facile condition. Further, the prepared ZnO-NFs was systematically probed for its structural, morphological and functional properties. Our primary aim was to utilize ZnO-NFs as a photocatalyst for the degradation of different dyes such as Methyl orange (MO), Congo red (CR), Chicago sky blue (CSB) and Eosin B (EO). Based on the photocatalytic evaluations, it can be concluded that ZnO-NFs could possess versatile photocatalytic properties for remediation of contaminated water. Moreover, we extended our work to synthesize the composite using single-source molecular precursor. Here, one pot in-situ synthesis of rGO/ZnO-NFs composite was carried out by reaction of graphene oxide (GO) and asymmetric Zn(II) complex as single-source molecular precursor (A). The process has occurred in two steps: (i) synthesis of ZnO nanoflowers from precursor (A) and (ii) simultaneous reduction of GO to rGO at 150 ºC. A binder free electrode was fabricated using this rGO/ZnO-NFs as composite on Glassy carbon electrode, (GCE) and has been explored for the electrochemical detection of nitroaromatics such as p-nitro-phenol (p-NP), 2,4-dinitrophenol (2,4-DNP), 2,4-dinitrotoluene (2,4-DNT), and 2,4,6-trinitrophenol (2,4,6-TNP). The fabricated sensor (GCE-rGO/ZnO) has shown the remarkable low detection limit, considerably good sensitivity with high repeatability and stability towards the measured analyte. Thanks are due to my fellow colleagues and friends for their kind co-operation during my research tenure.