In Situ Solid‐State Reactions Monitored by X‐ray Absorption Spectroscopy: Temperature‐Induced Proton Transfer Leads to Chemical Shifts

Abstract

Abstract:The dramatic colour and phase alteration with the solidstate, temperature-dependent reaction between squaric acid and 4,4'-bipyridine has been probed in situ with X-ray absorption spectroscopy. The electronic and chemical sensitivity to the local atomic environment through chemical shifts in the near-edge X-ray absorption fine structure (NEXAFS) reveals proton transfer from the acid to the bipyridine base through the change in nitrogen protonation state in the high-temperature form. Direct detection of proton transfer coupled with structural analysis elucidates the nature of the solid-state process, with intermolecular proton transfer occurring along an acid-base chain followed by a domino effect to the subsequent acid-base chains leading to the rapid migration along the length of the crystal. NEXAFS thereby conveys the ability to monitor the nature of solid-state chemical reactions in situ, without the need for a priori information or long-range order.Proton transfer plays an important role in many chemical and biological processes, and has the ability to profoundly impact electronic structure and functional properties of materials. The potential to tune chemical and physical properties of organic materials through varying the degree of proton transfer and molecular interactions has consequently led to advances in crystal engineering for pharmaceuticals, [1,2] energetic materials, [3] organic ferroelectrics, [4] molecular switches [5] and optical materials. [6,7] Solid-state reactions initiated by external factors such as temperature, light or pressure ('chromisms') are also often attributed to intra-or intermolecular proton migration in molecular chemistry. [8][9][10][11] Detection of whether proton transfer has occurred in solidstate products, and therefore study and elucidation of proton transfer processes, is not trivial as it typically involves the position of a single hydrogen atom. This is compounded in the case of temperature-dependent transitions where collection of data from diffraction techniques becomes increasingly complicated (e.g. difficulties of accurately locating hydrogen with X-ray diffraction, minimum crystal size for single crystal neutron diffraction and potential decomposition or loss of crystallinity) or impossible for solutions with the loss of long-range order (e.g. alteration of tautomer populations).A pertinent example of dramatic changes through a solidstate reaction ascribed to proton transfer is the temperatureinduced transition observed for the complex of 4,4'-bipyridine (4BPY) and squaric acid (SQU). [12] Aqueous crystallisation of 4BPY and SQU forms a yellow salt [4BPYH] + [SQU] − , along with an additional orange salt (minor product), with the yellow form undergoing an unusual reversible, solid-state colour and phase change at high-temperatures (Scheme 1). An increase in the transition temperature with deuteration suggested proton transfer as a potential explanation, although single crystal X-ray diffraction was unable to determine the proton positions due to the hi...

The role of solvation in proton transfer reactions: implications for predicting salt/co-crystal formation using the ΔpKa rule

“…7 The characterisation of such systems can be complicated by the inherent difficulties associated with locating H atoms using Xray diffraction data, and in some cases the extent of proton transfer may be temperature dependent. 8,9 Because of this, complementary characterisation techniques, such as XPS, [10][11][12] solid-state NMR 13 or neutron diffraction 14,15 (amongst others 16 ), oen need to be used to locate precisely the position of the acid H atoms. There are also cases where both ionised and non-ionised species of an acid or base are present in the same crystal.…”

Colorful 4,4-Bipyridine–Squaric Acid Multicomponent Complexes with Varying Degrees of Proton Transfer: Exploring the Nature of New Form IV in the Salt Co-Crystal Continuum

“…Previous powder neutron diffraction refinement for form II correlated with a model where both 4BPY nitrogens become protonated, providing indirect evidence of a di-salt. NEXAFS shows that both of the 4BPY nitrogen atoms are protonated, directly revealing that the transition to the high-temperature red form II involves proton transfer and formation of the di-salt 27 The XPS and NEAXFS nitrogen spectra for the new orange form IV are clearly different from that of the other complexes (Figure 5). The signal is predominantly comprised of the C NH + peak, with a much smaller secondary peak in the position for unprotonated CN.…”

“…22,23 The form I → II phase (and color) transformation occurs along the length of the crystal, with sequential proton transfer along the hydrogen-bonded chains in form I as the remaining SQU proton migrates to BPY. 27 In the di-salt form II, the remaining bipyridine nitrogen thus becomes protonated, and the + N−H•••O − interactions lead to hydrogenbonded chains of BPY di-cations and SQU di-anions (N•••O at 2.57 Å, Figure 2). 23 This is accompanied by variation in 4BPY conformation, as the bipyridine rings change from twisted by 24°(torsion angle) in form I to planar in form II; the dark orange form III meanwhile has an intermediate value ca.…”

“…23 Both form I and III are salts [4BPYH] + [SQU] − , with one of the squaric acid protons transferred to one of the bipyridine nitrogens (Figure 1). 22,23 The high-temperature form II was initially indicated to be a disalt [4BPYH 2 ] 2+ [SQU] 2− from powder neutron diffraction refinement, 23 with transfer of the remaining squarate proton to bipyridinium directly revealed using core-level X-ray spectroscopy 27 (Figure 1). While forms I and II have had some focus due to the thermochromic behavior, there is has been little study of form III aside from its initial (brief) reporting.…”

Intermolecular Interactions in Functional Crystalline Materials: From Data to Knowledge

“…Particularly, indexing of the crystal phases becomes of practical interest for nonlinear optics [120], organic photonics [121], piezoelectrics [122][123][124], and organic electronics [125][126][127][128]. Knowledge of functional groups or supramolecular synthons forming crystal faces allowed in some cases to rationalize mechanical effects in dynamic crystals [129][130][131][132] such as self-healing, jumping, bending, twisting at heat, humidity, force or light, and to describe pressure-induced phase transitions (see Refs. [133][134][135] for analysis of such transitions in amino acids).…”