Direct Observation of Unstable Reaction Intermediates by Acid‐Base Complex Formation

Abstract

The structures of several unstable or metastable reaction intermediates that were photoproduced in crystals were analyzed by using X-ray techniques. The presence of enough void space around the reactive group(s) is an essential factor for the reaction to occur with retention of the single-crystal form. To expand the void space, an acid group (COOH) was substituted onto the reactant molecule and acid-base complex crystals were prepared with several amines, such as dibenzylamine and dicyclohexylamine. Following the formation of such acid-base complexes in crystals, the metastable structures of nitrenes and red species of photochromic salicylideneanilines have been successfully analyzed by using X-ray techniques. Moreover, the structure of a Pt complex anion in the excited state has been analyzed, which formed acid-base complex crystals with various alkylammonium cations. The formation of acid-base complexes will be a powerful tool for directly observing the structure of unstable or metastable reaction intermediates by using X-ray techniques.

Crystal Melting by Light: X-ray Crystal Structure Analysis of an Azo Crystal Showing Photoinduced Crystal-Melt Transition

“…In this type of reaction, initial, final, and possibly even intermediate phases in the reaction process can be directly observed by X-ray crystal structure analysis. 6,7 Reactions reaching a same metastable or new stable phase often lose long-range order in the initial crystalline phase, and it becomes increasingly difficult to track the reaction process by X-ray crystal structure analysis. In those cases, combining X-ray crystallography for the initial phase and spectroscopy and powder X-ray diffraction (PXRD) to track the phase change is effective in obtaining details about the reactions.…”

“…Recently, it was proposed that a photochemical phase transition accompanying a solid-state reaction must begin by formation of a solid solution (dilute mixed crystal of the product in the crystalline phase of the reactant) and ultimately reach either a same stable, a same metastable, a new stable, or an isotropic (melt or glass) phase of product. , The case of reaching a same stable phase is classified as a single crystal-to-single crystal reaction. In this type of reaction, initial, final, and possibly even intermediate phases in the reaction process can be directly observed by X-ray crystal structure analysis. , Reactions reaching a same metastable or new stable phase often lose long-range order in the initial crystalline phase, and it becomes increasingly difficult to track the reaction process by X-ray crystal structure analysis. In those cases, combining X-ray crystallography for the initial phase and spectroscopy and powder X-ray diffraction (PXRD) to track the phase change is effective in obtaining details about the reactions. − Crystals showing amorphization or melting by photoreaction have a tendency to become unsuitable for X-ray crystal structure analysis.…”

Synthesis and Characterization of Binary-Complex Models of Ureas and 1,3-Dicarbonyl Compounds: Deeper Insights into Reaction Mechanisms Using Snap-Shot Structural Analysis

“…To overcome this inevitable challenge, we planned to synthesize thermodynamically stable binary complexes that could mimic the intermediate formed between the catalyst and nucleophile via ternary complex B and evaluate their structural by spectroscopic properties. It was envisaged that a properly constructed motif would enable the direct observation of the labile reaction species . With this in mind, the diphenylacetylene unit was selected as an ideal covalent linker for the thiourea and 1,3-dicarbonyl compounds (complex model D in Figure ) because the 2-amino-2′-carboxyldiphenylacetylenes, which were developed by Kemp, are known to function as β-turn motifs in artificial β-sheet structures, and it was envisaged that two fragments linked by this unit would form the appropriate intramolecular hydrogen bonding interactions between the carbonyl oxygen(s) and the N–H proton(s) .…”

Crystal structure analysis of molecular dynamics using synchrotron X-rays

“…34,35 The first is the transition to an equivalent stable crystalline phase, which has the same periodicity and is formed without crystal deterioration. 36,37 The second is the transition to an equivalent metastable crystalline phase. This phase has the same periodicity, but deterioration from a single crystalline state to a polycrystalline state occurs in this transition process.…”