Isokinetics of Spin-Crossover Complexes

Isokinetic relationships (IKR, sometimes referred to as compensation effect, or, especially for thermodynamic systems, Isoequilibrium Relationship) refer to (neglecting problematic statistical analysis of data) a linear relationship between entropic and enthalpic contributions to both temperature dependent reaction rates and temperature dependent equilibrium constants within a given series of reactions. Such a series can be formed by variation of substituents, variation of solvents, of catalyst, etc... The theory of IKR and its connection to generalised linear free energy relationships can seemingly be applied to systems exhibiting spin crossover both in solid state and in solution. Based an an estabhshed experimental and theoretical background (including a correct statistical analysis of data to avoid artefacts) to the occurrence of the IKR the aim of the present project is to extend our knowledge to include spin-crossover systems. In the last few decades the investigation of low spin H high spin crossover phenomena lead to new compounds and the investigation of new phenomena (gradual and sharp spin transition, hysteresis effects, LIESST, NIESST, etc ... ). Thereby significant differentes have been found between the spin transition in liquid and in the crystalline solid state. For 15 years our group has dealt with spin transition phenomena in liquid systems (solutions) and - additionally - is investigatiog mononuelear and polynuelear complexes in solid state in recent years. Our project will focus an the investigation of iron(II) coordination compounds based an the chelating benzimidazolpyridine figands and the functionafzed 1-alkyl-tetrazole monodentate figands. Based an former work in this field we want to modify the benzimidazolpyridine ligand in such a way, that a bridging bis-tridentate ligand can be obtained. Such a bridging ligand should give rise to a linear chain-type polynuelear structure with a well defined octahedral coordination sphere around the iron(II) centers and the bridging figands as spacer. Due to the rather rigid ligand system cooperativity should be enhanced in solid state. The other type of figands used are functionalized 1alkyl-tetrazoles. We will focus both an mononuclear and polynuelear iron(II) complexes alike. Especially the variation of the terminal or bridging tetrazole figands will yield insight into the parameters governing the structure and dimensionafity of these coordination compounds. For both the mononuclear and the polynuelear complexes very promising prefminary results have been already obtained. The main aim of the project will be the systematic variation of substituents in order to fine-tune the ligand field imposed by the respective ligands thus varying the spin erossover temperature as well as the sharpness of the spin transition. These spin-crossover systems provide all necessary conditions to establish proper IKR series, namely: a) Thermal induced spin state transitions can be exactly followed by several independent macroscopie and microscopic techniques due to the changes in magnetic, spectral and structural properties using temperature dependent magnetic susceptibility, UV-VIS-NIR, MIR and FIR measurements as well as X-ray diffraction, NMR- (Evans-method for solutions) and Moessbauer spectroscopy (solid state). b) Even small variations in substituents, counter-anions, solvents, crystal solvents, and even the preparation (size of the particles, grinding, etc) are known to lead to pronounced effects in terms of the spin crossover parameters like the transition temperature, transition enthalpy and the transition entropy.

The aim of the present project was a detailed investigation of substances which are able to change their magnetic behaviour by varying the temperature and/or through optical switching. This effect is generally called Spin- Crossover and materials that show such effects will be important in the design of new advanced materials. New materials were synthesised systematically by making small changes at the molecular level. These changes included the type of anion, and the length of the organic framework coordinating the iron(II) metal ions etc. A major question of this project was whether an Isoequilibrium relationship could be observed within these families of compounds and whether consequently this information could be used in the development and understanding of Spin-Crossover systems. Isoequilibrium relationships refer to a linear relationship between entropic and enthalpic contributions to a chemical equilibrium or reaction and significant variation in these relationships allow important information to be gleaned. Indeed, such relationships were found and applied to a variety of new and previously characterized Spin-Crossover compounds. This shed light on observable variations of the magnetic behaviour and led us to detect new effects such as `parity effects`; it was found that in one series of compounds the magnetic behaviour depended upon whether there were even or odd numbers of carbon atoms in the organic framework. This work has been successful in not only synthesising new magnetic materials but also, through a very systematic approach to these syntheses, the work has provided molecular information about the energetics involved.