Design of Dipyrrin Dicarboxamide Complexes for Reductive Activation of CO₂ & N₂

Das, Soumadip (2025) Design of Dipyrrin Dicarboxamide Complexes for Reductive Activation of CO₂ & N₂. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Soumadip Das (19RS053)) 19RS053.pdf - Submitted Version Restricted to Repository staff only Download (13MB)

Abstract

Nature efficiently activates small, stable molecules like N₂, CO₂, O₂, H₂, and CO using metalloenzymes under ambient conditions—unlike synthetic systems that often require harsh environments. Processes such as photosynthesis exemplify the effective use of solar energy for H₂O oxidation and CO₂ conversion during the dark phase. However, mimicking such transformations, particularly CO₂ reduction, is very important due to its growing greenhouse effect, and it remains a major chemical challenge due to its thermodynamic stability. Likewise, nitrogenase catalyzes N₂ fixation to NH₃ under mild conditions, in stark contrast to the energy-intensive Haber–Bosch process, and this process is the major contributor to the CO2 production. These natural models inspire the design of synthetic metal complexes for value-added transformations via small molecule activation. Inspired by the strategies employed in small molecule activation by metalloenzymes, this doctoral work centers on the rational design and synthesis of ligands and their corresponding metal complexes for the activation of CO₂ and N₂. A novel class of dipyrrin-dicarboxamide ligands, termed NDPA, was developed with the aim of enabling trianionic N₄ coordination around metal centers—a feature unattainable with the previously reported DADP framework. Consistent with this hypothesis, a CuNDPA complex exhibiting the desired N₄ coordination geometry was synthesized and extensively characterized. Interestingly, a pH-responsive hemilability was observed in the CuNDPA system, which enabled the isolation and structural characterization of alternative coordination species, specifically CuN₃O and CuN₂O₂ complexes, using SC-XRD and other spectroscopic techniques. The photophysical properties of the CuNDPA complex were systematically investigated, revealing promising features that were further utilized in self-sensitized photocatalytic CO₂ reduction, leading to the selective formation of CO with a notably high TON. Additionally, a CoNDPA complex was synthesized and characterized, displaying a rare trianionic N₃O coordination environment. This complex was found to facilitate selective N₂ reduction to N₂H₄, enabled by an unusual bent N₂ binding mode at the reduced metal center, which increases electron density at the terminal Nα position and favors selectivity. Electrochemical CO₂ reduction by this Co complex was also explored in detail, and the proposed catalytic cycle was elucidated using a combination of EPR, spectroelectrochemistry, IR, and HRMS techniques. Furthermore, the catalytic scope of the CoNDPA system was extended to enable electrochemical CO(NH2)2 (Urea) synthesis through the simultaneous reduction of CO₂ and N₂. Building on these findings, efforts are currently underway to develop structurally modified next-generation ligand frameworks for the catalytic conversion of small molecules into other value-added chemical products.

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