2 research outputs found
Modulating Supramolecular Reactivity Using Covalent âSwitchesâ on a Pyrazole Platform
Systematic co-crystallizations of halogen- methyl- and
nitro-substituted
pyrazoles with a library of 20 aromatic carboxylic acids have been
carried out using melt and solution-based experiments. The solids
resulting from all reactions were screened using infrared spectroscopy
in order to determine if a reaction (co-crystal or salt) had taken
place. The halogenated pyrazoles, including their dimethyl analogues,
gave a supramolecular yield of 55â70%. Replacing a halogen
atom (R-X, X = Cl, Br, I) with a nitro (R-NO<sub>2</sub>) group drops
the success rate significantly to 10% due to the reduced charge on
the basic nitrogen atom of the pyrazole. Eleven crystal structures
were obtained: six were co-crystals and five were salts (including
one hydrate). In all six co-crystals, dimeric pyrazole···acid
assemblies were constructed via a combination of OâH---NÂ(pyz)
and NâH---O hydrogen bonds corresponding to a 100% supramolecular
yield. A variety of weaker halogen-bonds CN---I, I---I and X---O<sup>â</sup> connect dimers into infinite one-dimensional chains.
In contrast, the salts displayed a variety of stoichiometries and
a much wider range of noncovalent interactions, although a charge-assisted
N<sup>+</sup>-H---O<sup>â</sup> hydrogen bond was present in
all five structures. In general, the salts lack structural and stoichiometric
predictability and stability as compared to the co-crystals. Furthermore,
the overall electrostatic charge on the key binding sites on the pyrazole
backbone can be modulated by using specific covalent switches, which
in turn can increase (or decrease) the success rate for a reaction
Modulating Supramolecular Reactivity Using Covalent âSwitchesâ on a Pyrazole Platform
Systematic co-crystallizations of halogen- methyl- and
nitro-substituted
pyrazoles with a library of 20 aromatic carboxylic acids have been
carried out using melt and solution-based experiments. The solids
resulting from all reactions were screened using infrared spectroscopy
in order to determine if a reaction (co-crystal or salt) had taken
place. The halogenated pyrazoles, including their dimethyl analogues,
gave a supramolecular yield of 55â70%. Replacing a halogen
atom (R-X, X = Cl, Br, I) with a nitro (R-NO<sub>2</sub>) group drops
the success rate significantly to 10% due to the reduced charge on
the basic nitrogen atom of the pyrazole. Eleven crystal structures
were obtained: six were co-crystals and five were salts (including
one hydrate). In all six co-crystals, dimeric pyrazole···acid
assemblies were constructed via a combination of OâH---NÂ(pyz)
and NâH---O hydrogen bonds corresponding to a 100% supramolecular
yield. A variety of weaker halogen-bonds CN---I, I---I and X---O<sup>â</sup> connect dimers into infinite one-dimensional chains.
In contrast, the salts displayed a variety of stoichiometries and
a much wider range of noncovalent interactions, although a charge-assisted
N<sup>+</sup>-H---O<sup>â</sup> hydrogen bond was present in
all five structures. In general, the salts lack structural and stoichiometric
predictability and stability as compared to the co-crystals. Furthermore,
the overall electrostatic charge on the key binding sites on the pyrazole
backbone can be modulated by using specific covalent switches, which
in turn can increase (or decrease) the success rate for a reaction