Abstract
The homologous series of phenyl and pyridyl substituted bis(acrylamido)alkanes have been synthesized with the aim of systematic analysis of their crystal structures and their solid-state [2 + 2] reactivities. The changes in the crystal structures with respect to a small change in the molecular structure, that is by varying alkyl spacers between acrylamides and/or by varying the end groups (phenyl, 2-pyridyl, 3-pyridyl, 4-pyridyl) on the C-terminal of the amide, were analyzed in terms of hydrogen-bonding interference (N-H⋯Npy versus N-H⋯O=C) and network geometries. In this series, a greater tendency towards the formation of N-H⋯O hydrogen bonds (β-sheets and two-dimensional networks) over N-H⋯N hydrogen bonds was observed. Among all the structures seven structures were found to have the required alignments of double bonds for the [2 + 2] reaction such that the formations of single dimer, double dimer and polymer are facilitated. However, only four structures were found to exhibit such a solid-state [2 + 2] reaction to form a single dimer and polymers. The two-dimensional hydrogen-bonding layer via N-H⋯O hydrogen bonds was found to promote solid-state [2 + 2] photo-polymerization in a single-crystal-to-single-crystal manner. Such two-dimensional layers were encountered only when the spacer between acryl amide moieties is butyl. Only four out of the 16 derivatives were found to form hydrates, two each from 2-pyridyl and 4-pyridyl derivatives. The water molecules in these structures govern the hydrogen-bonding networks by the formation of an octameric water cluster and one-dimensional zigzag water chains. The trends in the melting points and densities were also analyzed.
Highlights
Apart from these very few homologous series, in particular molecules containing more than one functional group that are capable of hydrogen bonding, are characterized crystallographically
In these lines we have previously reported our studies on homologous series of N,N0-bis(pyridinecarboxamido)alkanes (Sarkar & Biradha, 2006) and N,N0-bis(pyridyl)alkanediamides (Rajput et al, 2007b) in which amide moieties are separated by alkyl (—(CH2)n—) spacers (Fig. 1)
The comparison of X-ray powder diffraction (XRPD) patterns with the related analogues hinted at their probable crystal structures
Summary
The systematic exploration of solid-state ensembles of organic molecules containing functional groups which possess similar interactive capabilities albeit minor variations laid the foundation for the gigantic rise of the field of crystal engineering (Schmidt, 1971; Desiraju, 1989; Lehn, 1995; Whitesides et al, 1995; Aakeroy et al, 2001; Moulton & Zaworotko, 2001; Zaworotko, 2001; Desiraju, 2002; Biradha, 2003; Vangala et al, 2005; Rajput et al, 2007a; Mukherjee & Biradha, 2011b). Such polymerization reactions are very rare and the only example in the literature is 2,5-distyrylpyrazine reported by Hasegawa et al (Hasegawa, 1983; Hasegawa et al, 1986) In this manuscript the following points will be addressed by analyzing homologous series of crystal structures of 1–4: (1) competition between the O atom of the amide and the N atom of pyridine to form hydrogen bonds with the amide N—H group; (2) similarities or differences with previously published bis-amide analogues; (3) formation of the two-dimensional layer versus the -sheet hydrogen-bond networks (Fig. 3); (4) propensity for the formation of hydrates; (5) trends in their melting points; (6) alignment of double bonds for solid-state [2 + 2] reactions; (7) exploration of their reactivities and characterization of their products wherever possible. Anal.: calc. for C20H22N4O2: C 68.55, H 6.33, N 15.99; found: C 67.74, H 6.63, N 14.96%
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