Show Abstract ▼

Abstract: Photoresponsive materials are an important contemporary research area with applications in, for example, energy and catalysis. Mechanistic information on solid-state photochemical reactions has traditionally come from spectroscopy and modelling, with crystallography limited to snapshots of endpoints and long-lived intermediates. Recent advances in X-ray sources and detectors have made it possible to follow solid-state reactions in situ with dynamic single-crystal X-ray diffraction (SCXRD) methods, allowing a full set of atomic positions to be determined over the course of the reaction. These experiments provide valuable structural information that can be used to interpret spectroscopic measurements and to inform materials design and optimisation. Solid-state linkage isomers, where small-molecule ligands such as NO, NO2−, N2 and SO2 show photo-induced changes in binding to a transition metal centre, have played a leading role in the development of dynamic SCXRD methodology, since the movement of whole atoms and the predictable temperature dependence of the excited-state lifetimes make them ideal test systems. The field of “photocrystallography”, pioneered by Coppens in the late 1990s, has developed alongside advances in instrumentation and computing and can now provide the 3D structures of species with lifetimes down to femtoseconds. In this chapter, we will review the development of photocrystallography experiments against linkage isomer systems, from the early identification of metastable species under continuous illumination, through measuring kinetics at low temperature, to recent experiments studying species with sub-second lifetimes. We will discuss the advances in X-ray sources and instrumentation that have made dynamic SCXRD experiments possible, and we will highlight the role of kinetic modelling and complementary spectroscopy in designing experiments. Finally, we will discuss possible directions for future development and identify some of the outstanding challenges that remain to be addressed.

Show Abstract ▼

Abstract: Single crystal X-ray diffraction has been used to study the CO2 absorption sites in a microporous Cu-MOF, [CuI2(py-pzpypz)2(μ-CN)2]n (1) (where py-pzpypz = 4-(4-pyridyl)-2,5-dipyrazyl-pyridine), which features zigzag-shaped channels, at a range of CO2 pressures (1, 5, and 10 bar) and at two temperatures (240 and 298 K). Unlike the acetonitrile molecules in the as-synthesized MOF, 1·MeCN, the CO2 molecules in 1·nCO2 (n = 0.8, 0.7, 0.45) are preferentially centered on the vertices of each zig and zag, which allows for weak (azine) C–H···OCO interactions with the H atoms on the electron-deficient pyrazine and pyridine rings of the MOF.

Open Access

Show Abstract ▼

Abstract: Molecular crystals can be bent elastically by expansion or plastically by delamination into slabs that glide along slip planes. Here we report that upon bending, terephthalic acid crystals can undergo a mechanically induced phase transition without delamination and their overall crystal integrity is retained. Such plastically bent crystals act as bimorphs and their phase uniformity can be recovered thermally by taking the crystal over the phase transition temperature. This recovers the original straight shape and the crystal can be bent by a reverse thermal treatment, resulting in shape memory effects akin of those observed with some metal alloys and polymers. We anticipate that similar memory and restorative effects are common for other molecular crystals having metastable polymorphs. The results demonstrate the advantage of using intermolecular interactions to accomplish mechanically adaptive properties with organic solids that bridge the gap between mesophasic and inorganic materials in the materials property space.

Show Abstract ▼

Abstract: The synthesis of conventional porous crystals involves building a framework using reversible chemical bond formation, which can result in hydrolytic instability. In contrast, porous molecular crystals assemble using only weak intermolecular interactions, which generally do not provide the same environmental stability. Here, we report that the simple co-crystallization of a phthalocyanine derivative and a fullerene (C60 or C70) forms porous molecular crystals with environmental stability towards high temperature and hot aqueous base or acid. Moreover, by using diamond anvil cells and synchrotron single-crystal measurements, stability towards extreme pressure (>4 GPa) is demonstrated, with the stabilizing fullerene held between two phthalocyanines and the hold tightening at high pressure. Access to open metal centres within the porous molecular co-crystal is demonstrated by in situ crystallographic analysis of the chemisorption of pyridine, oxygen and carbon monoxide. This suggests strategies for the formation of highly stable and potentially functional porous materials using only weak van der Waals intermolecular interactions.

Show Abstract ▼

Abstract: [FeFe]-hydrogenases contain strongly electronically coupled diiron [2Fe]H and tetrairon [Fe4–S4]H clusters, and thus much recent effort has focused on the chemistry of diiron-dithiolate biomimics with appended redox-active ligands. Here we report on the synthesis and electrocatalytic activity of Fe2(CO)4(μ-edt)(κ2-bpcd) (2) in which the electron-acceptor 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) acts as a surrogate of the [Fe4–S4]H sub-cluster. The complex is prepared in low yield but has been fully characterised, including a crystallographic study which shows that the diphosphine adopts a basal-apical coordination geometry in the solid state. Cyclic voltammetry shows that 2 undergoes four reduction events with DFT studies confirming that the first reduction is localised on the low-lying π* system of the diphosphine ligand. The addition of the second electron furnishes a triplet dianion that exhibits spin density distributed over the diphosphine and diiron subunits. Protonation at the Fe–Fe bond of the triplet dianion furnishes the corresponding bridging hydride as the thermodynamically favoured species that contains a reduced bpcd ligand. Complex 2 functions as a catalyst for proton-reduction at its second reduction potential, in contrast to the related 2,3-bis(diphenylphosphino)maleic anhydride (bma) complex, Fe2(CO)4(μ-pdt)(κ2-bma) (1), which shows similar electrochemical behaviour but is not catalytically active. The difference in chemical behaviour is attributed to greater stability of the 4-cyclopenten-1,3-dione platform in 2 as compared to the maleic anhydride ring of the bma ligand in 1 following the uptake of the second electron. Thus protonation of the Fe–Fe bond in the 22− affords a species which is stable enough to undergo a further reduction–protonation event, unlike the bma ligand whose maleic anhydride ring undergoes deleterious C–O bond scission upon protonation or reaction with adventitious moisture. DFT studies, however, suggest that electron-transfer from the diphosphine to the diiron centre is not significant, probably due to their poor redox levelling. Thus, while the diphosphine is readily reduced, the added electron is apparently not utilised in proton-reduction and hence cannot truly be considered as an [Fe4–S4]H surrogate.