Different hydrogen-bonding modes in two closely related oximes

Area of Science:

• Organic Chemistry
• Crystallography
• Supramolecular Chemistry

Background:

• Oximes are organic compounds containing the C=N-OH functional group.
• Hydrogen bonding plays a crucial role in determining crystal structures and material properties.
• Piperidinone derivatives are important scaffolds in medicinal chemistry and materials science.

Purpose of the Study:

• To investigate and compare the crystal structures of two closely related oximes: 1-chloroacetyl-3-ethyl-2,6-diphenylpiperidin-4-one oxime (I) and 1-chloroacetyl-2,6-diphenyl-3-(propan-2-yl)piperidin-4-one oxime (II).
• To elucidate the influence of subtle structural modifications on hydrogen bonding patterns and crystal packing.
• To analyze the nature of disorder observed in the oxime group of compound (I).

Main Methods:

• Single-crystal X-ray diffraction analysis was performed on both compounds (I) and (II).
• Analysis of hydrogen bonding interactions, including donor-acceptor distances and angles.
• Investigation of molecular conformation and torsion angles, particularly around the oxime and chloroacetyl groups.
• Variable-temperature X-ray diffraction studies were conducted for compound (I) to study the temperature dependence of disorder.

Main Results:

• Compound (I) forms centrosymmetric dimers linked by oxime-oxime O-H...N hydrogen bonds.
• Compound (II) forms infinite chains connected by O-H...O hydrogen bonds involving the carbonyl oxygen of the chloroacetyl group.
• Despite differing hydrogen bonding schemes, the -OH groups in both compounds adopt anti conformations (C-N-O-H torsion angles ~180°).
• The oxime group in compound (I) exhibits significant disorder, with hydroxy groups occupying two positions, observed even at low temperatures, suggesting a statistical rather than dynamic origin.

Conclusions:

• Subtle differences in the substituents of closely related oximes can lead to fundamentally different hydrogen bonding patterns and crystal architectures.
• The observed disorder in compound (I) is statistically driven and persists across a range of temperatures.
• Understanding these structure-property relationships is vital for the rational design of organic materials with specific crystalline properties.