Thermal and photochemical control of nitro-nitrito linkage isomerism in single-crystals of [Ni(medpt)(NO₂)(η²-ONO)]

The known complex [Ni(medpt)(η1-NO2)(η2-ONO)] 1 (medpt = 3,3′-diamino-N-methyldipropylamine) crystallises in the monoclinic space group P21/m with 1.5 molecules in the asymmetric unit with two different η1-NO2 ligand environments in the crystal structure. At 298 K the molecule (A) sitting in a general crystallographic site displays a mixture of isomers, 78% of the η1-NO2 isomer and 22% of an endo-nitrito–(η1-ONO) form. The molecule (B) sitting on a crystallographic mirror plane adopts the η1-NO2 isomeric form exclusively. However, a variable temperature crystallographic study showed that the two isomers were in equilibrium and upon cooling to 150 K the η1-ONO isomer converted completely to the η1-NO2 isomer, so that both independent molecules in the asymmetric unit were 100% in the η1-NO2 form. A kinetic analysis of the equilibrium afforded values of ΔH = −9.6 (±0.4) kJ mol−1, ΔS = −21.5 (±1.8) J K−1 mol−1 and EA = −1.6 (±0.05) kJ mol−1. Photoirradiation of single crystals of 1 with 400 nm light, at 100 K, resulted in partial isomerisation of the η1-NO2 isomer to the metastable η1-ONO isomer, with 89% for molecule (A), and 32% for molecule (B). The crystallographic space group also reduced in symmetry to P21 with Z′ = 3. The metastable state existed up to a temperature of 150 K above which temperature it reverted to the ground state. An analysis of the crystal packing in the ground and metastable states suggests that hydrogen bonding is responsible for the difference in the conversion between molecules (A) and (B)

Snapshots of a solid-state transformation: Coexistence of three phases trapped in one crystal

Crystal-to-crystal transformations have been crucial in the understanding of solid-state processes, since these may be studied in detail by means of single crystal X-ray diffraction (SCXRD) techniques. The description of the mechanisms and potential intermediates of those processes remains very challenging. In fact, solid-state transient states have rarely been observed, at least to a sufficient level of detail. We have investigated the process of guest extrusion from the non-porous molecular material Fe(bpp)(H2L)](ClO4)2·1.5C3H6O (bpp = 2,6-bis(pyrazol-3-yl)pyridine; H2L = 2,6-bis(5-(2-methoxyphenyl)-pyrazol-3-yl)pyridine; C3H6O = acetone), which occurs through ordered diffusion of acetone in a crystal-to-crystal manner, leading to dramatic structural changes. The slow kinetics of the transition allows thermal trapping of the system at various intermediate stages. The transiting single crystal can be then examined at these points through synchrotron SCXRD, offering a window upon the mechanism of the transformation at the molecular scale. These experiments have unveiled the development of an ordered intermediate phase, distinct from the initial and the final states, coexisting as the process advances with either of these two phases or, at a certain moment with both of them. The new intermediate phase has been structurally characterized in full detail by SCXRD, providing insights into the mechanism of this diffusion triggered solid-state phenomenon. The process has been also followed by calorimetry, optical microscopy, local Raman spectroscopy and powder X-ray diffraction. The discovery and description of an intermediate ordered state in a molecular solid-state transformation is of great interest and will help to understand the mechanistic details and reaction pathways underlying these transformations

Tunable trimers: Using temperature and pressure to control luminescent emission in gold(i) pyrazolate-based trimers

A systematic investigation into the relationship between the solid-state luminescence and the intermolecular AuAu interactions in a series of pyrazolate-based gold(I) trimers; tris(μ2-pyrazolato-N,N')-tri-gold(I) (1), tris(μ2-3,4,5- trimethylpyrazolato-N,N')-tri-gold(I) (2), tris(μ2-3-methyl-5- phenylpyrazolato-N,N')-tri-gold(I) (3) and tris(μ2-3,5-diphenylpyrazolato- N,N')-tri-gold(I) (4) has been carried out using variable temperature and high pressure X-ray crystallography, solid-state emission spectroscopy, Raman spectroscopy and computational techniques. Single-crystal X-ray studies show that there is a significant reduction in the intertrimer AuAu distances both with decreasing temperature and increasing pressure. In the four complexes, the reduction in temperature from 293 to 100 K is accompanied by a reduction in the shortest intermolecular AuAu contacts of between 0.04 and 0.08 The solid-state luminescent emission spectra of 1 and 2 display a red shift with decreasing temperature or increasing pressure. Compound 3 does not emit under ambient conditions but displays increasingly red-shifted luminescence upon cooling or compression. Compound 4 remains emissionless, consistent with the absence of intermolecular AuAu interactions. The largest pressure induced shift in emission is observed in 2 with a red shift of approximately 630 cm-1 per GPa between ambient and 3.80 GPa. The shifts in all the complexes can be correlated with changes in AuAu distance observed by diffraction. © 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim