Sustainable Chemistry Lab
. . . towards green and environmentally benign innovations based on 3d transition metals
Research Areas
Electrocatalyzed Water Splitting with 3d transition metals
Hydrogen (H2) emerged as a very efficient, clean, carbon neutral alternate energy source that can replace the fossil fuels such as coal, petroleum etc. , thus reducing the emission of greenhouse gases. In addition to this, oxygen is also a viable environment friendly chemical fuel. In nature, hydrogen is produced form water by hydrogenase enzyme in the prokaryotes and eukaryotes and the oxygen is produced in the photosystem II through splitting of water during the process of natural photosynthesis in the green plants. The thermodynamically uphill oxidation of water to oxygen, protons and electrons (2H2O → O2 + 4H+ + 4e−, E° =1.23 V vs. NHE) is catalysed by Mn4Ca cluster within PS II in presence of sunlight. This oxygen evolution reaction (OER) can also be performed artificially using electrochemistry in presence of suitable metal-ligand complexes. Notably, the electrons produced during OER can be used for the reduction of the protons to generate molecular hydrogen as the fuel (2H+ + 2e → H2). Thus, our primary focus lies on the development of some new catalysts based on the first row transition metal that can act as homogeneous water oxidation catalyst and hydrogen evolution catalyst during electrochemical splitting of water.
Development of First row transition metal catalyzed Outer sphere C-H Bond functionalization
A. Electrochemical Approach
Development of any mild and efficient methodologies for direct functionalization of C-H bonds has paramount importance in fundamental research as well as in industrial applications. The activation of the unactivated C(sp3)-H bond is one of the most challenging tasks because of its low polarity and high thermodynamic stability. This challenge has been addressed through the development of bio-mimetic transition metal dependent catalytic system in presence of chemical oxidants in the last two decades. However, these site-selective methodologies for the functionalization aliphatic C-H bond is comprise of several limitations. These drawbacks intrigued our group to ponder for a mild, green, sustainable alternative to chemical procedures. In this quest for a sustainable substitute, we opt for the electrochemical synthetic methodology that uses electrons as a traceless redox reagent as a replacement of toxic and hazardous chemical oxidants. Therefore, our primary focus lies on the development of some bio-inspired catalysts based on first row transition metal that will electrochemically generate high valent putative intermediates. Thereafter, this putative intermediate will help in selective electrochemical aliphatic C-H bond transformation methodologies such as hydroxylation, azidation, fluorination, chlorination, and trifluoromethylation reaction.
B. Chemical Approach
In nature, the C-H bond functionalization is performed by several enzymes such as hydroxylation by non-heme iron based α-ketoglutarate dioxygenases and heme-iron based cytochromes P450 and selective halogenation reaction by heme-iron based chloroperoxidase (CPO), non-heme iron based syringomycin halogenase. This type of naturally occurring C-H functionalization goes via the initial formation of high-valent iron-oxo intermediate. Thereafter the iron-oxo intermediate abstracts the proton from the substrate to form the alkyl radicals. Notably, in case of the halogenating enzymes (CPO and syringomycin halogenase), this alkyl radical species and Fe(III) halide species remain inside the cage of enzyme in such a fashion, so that the radical undergoes rebound with halide selectively to form the halogenated product. Different research groups have utilized various iron complexes having tetradentate and pentadentate ligand to mimic the enzymatic system. However, in most of the cases, the major drawback is the formation of hydroxylation product along with the generation halogenations product. Till date, all the trials made for iron catalyzed C-H bond halogenation were stoichiometric in nature. Thus, in this research area, we want to focus on the development of first-row transition metal based complexes that can catalyze selective sp3 C-H bond halogenation by suppressing the common hydroxylation products. In addition to that, similar metal-ligand complexes will also be utilized for several other catalytic C-H bond functionalization such as azidation, thiocyanation and trifluoromethylation.
Electrochemical heterocycle synthesis
Heterocyclic compounds are one of the largest groups of organic compounds being present in large number of organic materials, pharmaceuticals, agrochemicals and several biologically active molecules. In this modern era of sustainability, the development of an efficient, environment friendly, greener and cost-effective methodology for heterocycle synthesis that can exclude the use of toxic late transition metal catalysts, expensive chemical reagents and harsh conditions, is highly desirable. Electrochemical organic synthesis has become a sustainable, environment benign alternative over the typical organic synthesis using electrons as a traceless redox reagent, thus decreasing the amount of chemical wastes. In this regard, our aim is the development of efficient, greener and operationally simple methodology towards the synthesis of pharmaceutically important heterocyclic compounds by employing electrochemical strategies.