No abstract

Iron(II) complexes of the [Fe(II)(1-bpp2)](2+) type (1-bpp = 2,6-di(pyrazol-1-yl)pyridine) have been intensively investigated in the context of crystal engineering of switchable materials because their spin-crossover (SCO) properties dramatically depend on the counterions. Here, by means of DFT + U calculations at the molecular and solid state levels we provide a rationale for the different SCO behaviour of the BF4(-) and ClO4(-) salts of the parent complex; the former features Fe(II) complexes with a regular coordination geometry and undergoes a spin transition, whereas the Fe(II) complexes of the latter adopt a distorted structure and remain in the high-spin state at all temperatures. The different SCO behaviour of both salts can be explained on the basis of a combination of thermodynamic and kinetic effects. The shape of the SCO units at high temperature is thermodynamically controlled by the intermolecular interactions between the SCO units and counterions within the crystal. The spin trapping at low temperatures in the ClO4(-) salt, in turn, is traced back to a kinetic effect because our calculations have revealed the existence of a more stable polymorph having SCO units in their low-spin state that feature a regular structure. From the computational point of view, it is the first time that the U parameter is fine-tuned on the basis of CASPT2 calculations, thereby enabling an accurate description of the energetics of the spin transition at both molecular and solid-state levels.

A total of 816 ab initio points at the PUMP4/6-311G(2d) level were used to derive an analytical expression for the lowest 3 AЈ adiabatic potential energy surface ͑PES͒ of the reaction O͑ 3 P͒ϩCS͑X 1 ⌺ ϩ ͒→CO͑X 1 ⌺ ϩ ͒ϩS͑ 3 P͒. Thermal rate constants calculated using the variational transition state theory and semiclassical tunneling correction were used as a tool to determine the optimum analytical surface. This was done by comparing the calculated rate constant at room and lower temperatures with the experimental values. The best analytical surface ͑PES 3͒ reproduces the rate constant at low temperatures well. However, it has not been possible to obtain an analytical PES capable of reproducing both the rate constant at 300 K and the activation energy ͑150-300 K range͒. At higher temperatures, the contribution of the lowest 3 AЉ adiabatic potential energy surface to the rate constant seems to be important to reproduce the experimental data. At present, the PES 3 is the most suitable analytical surface to be used for kinetic and dynamical single surface studies.

The crystal packing of a-nitronyl nitroxide radicals that have dominant ferromagnetic or antiferromagnetic interactions is analyzed in order to test if there are characteristic orientations of their functional groups that can be associated with these magnetic interactions. From a large crystalline structural database of compounds containing anitronyl nitroxide radical units (143 structures), 23 representative cases with dominant intermolecular ferromagnetic interactions, and 24 cases exhibiting dominant antiferromagnetic interac-tions were selected. The spatial distribution of the NÀO´´´OÀN, C(sp 3 )ÀH´´Ó N, and C(sp 2 )ÀH´´´ON contacts whose distance is smaller than 10 was analyzed, with special emphasis on the 0 ± 5 region for the NÀO´´´OÀN contacts and 0 ± 3.8 for the CÀH´´Ó À N contacts. No correspondence is found between the presence of intermolecular ferro-or antiferromagnetic interactions and the geometry of any of the previous isolated contacts. Therefore, there is a need to change the way in which some structure ± magnetism correlations are obtained in a-nitronyl nitroxide crystals. These results also show that the intermolecular magnetic interaction is related to the relative orientation of the nearby molecules as a whole, that is, with the collection of intermolecular contacts made by them.

Dithiazolyl (DTA)-based radicals have furnished many examples of organic spin-transition materials, some of them occurring with hysteresis and some others without. Herein, we present a combined computational and experimental study aimed at deciphering the factors controlling the existence or absence of hysteresis by comparing the phase transitions of 4-cyanobenzo-1,3,2-dithiazolyl and 1,3,5-trithia-2,4,6-triazapentalenyl radicals, which are prototypical examples of non-bistable and bistable spin transitions, respectively. Both materials present low-temperature diamagnetic and high-temperature paramagnetic structures, characterized by dimerized (⋅⋅⋅A-A⋅⋅⋅A-A⋅⋅⋅) and regular (⋅⋅⋅A⋅⋅⋅A⋅⋅⋅A⋅⋅⋅A⋅⋅⋅) π-stacks of radicals, respectively. We show that the regular π-stacks are not potential energy minima but average structures arising from a dynamic inter-conversion between two degenerate dimerized configurations: (⋅⋅⋅A-A⋅⋅⋅A-A⋅⋅⋅) ↔(-A⋅⋅⋅A-A⋅⋅⋅A-) . The emergence of this intra-stack dynamics upon heating gives rise to a second-order phase transition that is responsible for the change in the dominant magnetic interactions of the system. This suggests that the promotion of a (⋅⋅⋅A-A⋅⋅⋅A-A⋅⋅⋅) ↔(-A⋅⋅⋅A-A⋅⋅⋅A-) dynamics is a general mechanism for triggering spin transitions in DTA-based materials. Yet, this intra-stack dynamics does not suffice to generate bistability, which also requires a rearrangement of the intermolecular bonds between the π-stacks via a first-order phase transition.