Anion and Ion-Pair Directed Self-Assembly of Urea Functionalized Molecules
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The thesis consists of five chapters including introduction and experimental sections. The first experimental work viz Chapter 3 describes the evidence of fluoride induced uptake of atmospheric CO2 and its stabilization as HCO3ˉ anion by a structurally simple acyclic 1,3-bis(4-iodophenyl)urea receptor, L1. The in situ formed HCO3ˉ complex (1a) is stabilized by a concert act of hydrogen and halogen bonding donated by the receptors. To the best of our knowledge, 1,3-bis(4-iodophenyl)urea is the simplest of anion receptors that displays a concert act of hydrogen and halogen bonding to stabilize aerial CO2 as HCO3ˉ. While 1,3-bis(4-bromophenyl)urea, L2 has shown completely lacking if halogen-bonding donor ability throughout its entire results. The second experimental work viz Chapter 4 is in two parts. Part 1 deals with a set of three nitrophenyl functionalized tripodal urea scaffolds, L3, L4 and L5 and demonstrate the effect of positional isomerism in anion coordination. The para-isomer (L3) self-assembles into dimeric (pseudo)molecular capsules as observed in carbonate and terephthalate complexes (3a and 3b). On the other hand, the meta-isomer (L4) self-assembles into dimeric capsules only in the presence of inorganic oxyanions (complex 4a), and assembles into a 2D sheet-like structure in the presence of terephthalate dianion (4b). In contrast to L3 and L4, structural authentication of the ortho-isomer (L5) in the presence of different oxyanions was not fruitful presumably due to the steric effect provided by the −NO2 group at the ortho-position, which hinders the facile inclusion and coordination of an anion due to electrostatic factor, as confirmed by 2D-NOESY NMR analysis of the free receptor. Part 2 of the Chapter 4 deals with the para-isomer (L3), which in association with 18-crown-6-ether self-assembles into an integrated 1D coordination polymer in the presence of K2CO3 (complex 3d). The last Chapter is about the recognition of tetrameric mixed phosphate clusters by L3 in its neutral and charged state. Receptor L3 self-assembles, in the presence of excess n-TBA(H2PO4), into a tetrahedral molecular cage by encapsulation of a tetrameric tetrahedral mixed phosphate cluster (3e). Further in the presence of H3PO4, L3 self-assembled with side-cleft bonded cyclic planar tetrameric anion-acid cluster (3f).
Supervisor: Gopal Das