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Switchable catalytic activity of spin crossover materials

Periodic Reporting for period 1 - SWITCHCATALYSIS (Switchable catalytic activity of spin crossover materials)

Reporting period: 2015-06-01 to 2017-05-31

Spin crossover (SCO) complexes form a large group of thermo-, photo-, magneto-, piezo- and chemo-switchable compounds. Recent achievements allowed their implementation into electronic circuits, MEMS, micro-thermometers, chemical sensors, etc. Most of attention has been paid to changes of physical properties that accompany a spin state switch. The change of the chemical and biochemichal activity of SCO complexes may lead to achievements in the field of molecular switches. Taking into account the changes of electronic and structural characteristics of a material during the molecular SCO, the transition should have a pronounced effect on the chemical reactivity of metallic centres and molecular fragments influenced by them. This means that the spin transition should affect the affinity to molecules interacting with the host both in its volume and on the surface.

We aimed to develop catalysts based upon SCO complexes whose activity may be switched by different external stimuli. Understanding the reactivity of transition metal complexes involving multiple spin states is a challenge of experimental and theoretical chemistry. Studies of the catalysis by SCO complexes may give a direct answer about the effect of spin state on the reactivity (catalytic activity) of coordination compounds. This kind of switchable catalysts may be applied when a catalytic reaction is to be started or stopped, a target compound of reaction should be changed from one to another, a stereoselectivity of reaction should be changed, in various biochemical processes.

The scientific key question was how the SCO phenomenon affects the catalytic activity. We aimed to develop materials that can display a spin dependent catalytic activity, to understand the mechanisms of the catalysis and spin state role and catalytic reaction. We wanted to test mechanisms that can have the biggest impact in controlling the catalytic activity of switchable materials.
The spin state of 3d metal centers plays a role in enzymatic and organometallic catalysis. We report on the catalytic activity of a Fe(II) coordination compound that undergoes a cooperative switch between low-spin (LS) and high-spin (HS) states. Between 291 – 318 K a drop of the catalytic activity was observed that is related to a switch between the LS and the HS states. For the thermoswitchable [Fe(NH2trz)3]Br2 complex (Tup = 305 K), the activation energy was lower for the LS (158 kJ mol-1) than for the HS state (305 kJ mol-1). Such spin-dependent activity will be analyzed for other switchable SCO complexes and other types of chemical reactions to derive a broader picture of the effect of spin state on the catalytic metal centers, as photochemical and electrochemical activities may be sensitive to the spin state of the catalyst. Application of chiral switchable complexes may lead to a switchable stereoselectivity in catalysis.
In addition, a highly cooperative spin crossover was observed at 367 and 349 K in heating and cooling modes for the 3D cyanoheterometallic compound [Fe(pz){Au(CN)2}2]. Negative thermal expansion along the c axis was detected below and above activities:
Presentations at the international conferences and workshops (EU funding acknowledged):
- 6th International Workshop on Phase Transition and Dynamical Properties of Spin Transition Materials, 2016, Gandia, Spain.
- 2nd Mediterranean Conference on the Applications of the Mossbauer Effect, 2016, Cavtat, Croatia.
- 42nd International Conference on Coordination the transition temperature. A high temperature spin crossover (Tup = 367 K) was detected in a metal–organic framework [Fe(pz){Ag(CN)2}2]·MeCN (pz = pyrazine). Upon heating, acetonitrile guest molecules were released, which slightly shifted the transition temperature of the complex (Tup = 370 K and Tdown = 356 K).

DisseminationChemistry, 2016, Brest, France
Other presentations:
- Monthly presentations of the intermediate results within the group (5-10 researchers) working in the domain of catalysis.
- Four presentations at the seminars in the institute (November 2015, June 2015, January 2016, December 2016).
Three papers published in peer-reviewed journals (all papers contain references to EU funding):
1) I.A. Gural'skiy, B.O. Golub, S.I. Shylin, V. Ksenofontov, H.J. Shepherd, P.R. Raithby, W. Tremel and I.O. Fritsky. Cooperative High-Temperature Spin Crossover Accompanied by a Highly Anisotropic Structural Distortion. Eur. J. Inorg. Chem. 2016, 2016, 19, 3191-3195.
2) I.A. Gural'skiy, S.I. Shylin, B.O. Golub, V. Ksenofontov, I.O. Fritsky and W. Tremel. High temperature spin crossover in [Fe(pyrazine){Ag(CN)2}2] and its solvate. New J. Chem. 2016, 40, 9012-9016.
3) I.A. Gural'skiy, S.I. Shylin, V. Ksenofontov, W. Tremel. Spin-State Dependent Redox Catalytic Activity of a Switchable Iron(II) Complex. Eur. J. Inorg. Chem. 2017, DOI: 10.1002/ejic.201700454.
Other dissemination activities:
- Project web-page was developed and updated.
- ResearchGate profile of the researcher and his publications was updated.
- Facebook and VK posts on the new publication achieved within the project.
- Kiev University Magazine published an article on receiving a fellowship by the principle PI.
- One newspaper and one TV channel have made an interview to the PI on his fellowship.
Impact on the researcher career:
- 2 years experience in a group with focus in biomimetic catalysis to acquire an expertise in the domain of catalysis by inorganic and coordination materials.
- participation in 3 international conferences helped to integration into a scientific community within the spin-crossover domain of research.
- collaboration with researchers from Mainz University will continue: W. Tremel in biomimetic catalysis and magnetism; V. Ksenofontov in the domain of Mossbauer spectroscopy.
- After finishing a post-doc stage in Mainz University, the PI directly moved to a permanent position as research associate at Taras Shevchenko National University of Kyiv. He will continue a research in the related domain of switchable materials.

Impact on the research domain, industry and society:
- A new type of catalysts (switchable catalyst) was obtained. It displays a unique ability to be switched by temperature and consequently change their catalytic activity. New materials that display such unique properties were developed.
- This results are unique for the research domain. They showed that not only physical, but also chemical properties change with the spin transition in a material.
- The idea can be implemented in industry by combing technically important catalysts with SCO materials.
- A new domain of research around switchable catalysts was launched.

Progress beyond the state of art and results:
To observe the effect of spin state on the catalytic activity of iron complexes, a 1D iron-triazole coordination polymer was studied. Studies were done on [Fe(NH2trz)3]Br2 prepared from water (1a) or ethanol (1b).
1a and 1b displayed a cooperative spin transition around room temperature (1a: Tup = 305 K, Tdown = 296 K; 1b: Tup = 301 K, Tdown = 283 K). [Fe(NH2trz)3](ClO4)2 (2) containing a perchlorate counter anion without a spin transition (LS→HS transition occurs at ~240 K) was used for comparison.
The kinetics for different quantities of 1a show a pronounced catalytic efficiency of the Fe(II)-triazole complex in the solid state.