Understanding The Electronic Structure of Transition Metal Complexes

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Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS Kaan Atak

Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS Kaan Atak Bodrum, September 3, 2019

Outline 1. Introduction 2. Experimental and Theoretical Methods 3. A Recent Application • Electronic

Outline 1. Introduction 2. Experimental and Theoretical Methods 3. A Recent Application • Electronic structure of aqueous [Co(bpy)3]2+ and [Co(bpy)3]3+ 4. Summary | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 2

Cobalt TM-complexes in catalysis Example for ‘Co’ centric systems: electron mediators in a dye

Cobalt TM-complexes in catalysis Example for ‘Co’ centric systems: electron mediators in a dye sensitized solar cell [Co(bpy)3]2+/3+ [Co(bpy-pz)3]2+/3+ W. Xiang, et. al, Energy Environ. Sci. , 2012, 6, 121– 127. . [Co(phen)3]2+/3+ [Co(CN)6]3− [Co(bpy)3]2+ and [Co(bpy)3]3+ Soft X-rays | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 3

A biologically relevant Co TM-complex Example for ‘Co’ centric systems- Cobalamin (B 12) •

A biologically relevant Co TM-complex Example for ‘Co’ centric systems- Cobalamin (B 12) • • The central Co 3+ ion is vital for biological activity. The alpha position (R) can bind to CN–, OH, CH 3, and 5 -deoxyadenosyl ligands. | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 4

Several core-level spectroscopic methods Radiative decay Resonant X-ray absorption (XAS/XANES) Non-radiative decay Resonant inelastic

Several core-level spectroscopic methods Radiative decay Resonant X-ray absorption (XAS/XANES) Non-radiative decay Resonant inelastic X- Resonant Auger-electron ray scattering (RIXS) emission (RPE) Core-level photoemission Non-resonant/X-ray emission Normal Auger-electron emission | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 5

A third generation synchrotron and a soft X-ray beamline A soft X-ray beamline: U

A third generation synchrotron and a soft X-ray beamline A soft X-ray beamline: U 49/2 PGM-1 (70 – 1500 e. V) Radiation Characterized by • High photon flux (1013 photons/s) • 80 X 22 μm 2 • Energy resolution (wavelength dependent) Image: HZB Webpage | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 6

A free electron laser in the soft X-ray regime • based on self-amplified spontaneous

A free electron laser in the soft X-ray regime • based on self-amplified spontaneous emission (SASE) • bunch pattern allows precise pump probe measurements New Journal of Physics, 2010, 12, 035002 https: //flash. desy. de/ | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 7

An endstation capable of XANES/RIXS applied to liquids Sample mounting techniques Liquid jet Flow

An endstation capable of XANES/RIXS applied to liquids Sample mounting techniques Liquid jet Flow cell Journal of Large-scale Research Facilities , 2016, Vol 2, page 80 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 8

First principles computational methods WIREs Comput Mol Sci, 2018, Vol 8, 1759 | Understanding

First principles computational methods WIREs Comput Mol Sci, 2018, Vol 8, 1759 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 9

Octahedral splitting in strong ligand field [Co(bpy)3]2+ | Understanding The Electronic Structure of Transition

Octahedral splitting in strong ligand field [Co(bpy)3]2+ | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 [Co(bpy)3]3+ Page 10

Aqueous [Co(bpy)3]2+ and [Co(bpy)3]3+ [Co(bpy)3]2+ [Co(bpy)3]3+ • [Co(bpy)3]2+ - two spin possibilities for the

Aqueous [Co(bpy)3]2+ and [Co(bpy)3]3+ [Co(bpy)3]2+ [Co(bpy)3]3+ • [Co(bpy)3]2+ - two spin possibilities for the valence d 7 configuration. LS with t 2 g 6 eg 1 and HS with t 2 g 5 eg 2. • The uneven occupation in the eg level causes strong Jahn-Teller distortions for LS case. • [Co(bpy)3]3+ - three different spin possibilities. LS with t 2 g 6 and eg 0 HS with t 2 g 4 and eg 2. Inorganic Chemistry, 2019, Vol 58, pages 4731 -4740 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 11

Co L-edge RIXS of [Co(bpy)3]2+ and[Co(bpy)3]3+ [Co(bpy)3]2+ [Co(bpy)3 ]3+ • Co L-edge RIXS maps

Co L-edge RIXS of [Co(bpy)3]2+ and[Co(bpy)3]3+ [Co(bpy)3]2+ [Co(bpy)3 ]3+ • Co L-edge RIXS maps showing dd and CT emission following a 2 p 3 d excitation. • In the [Co(bpy)3]2+ case, both possible spin states are open shell. • The [Co(bpy)3]3+ case, spin state is not purely LS (zero spin), a HS component must be considered Inorganic Chemistry, 2019, Vol 58, pages 4731 -4740 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 12

Co L-edge PFY-XA spectra [Co(bpy)3]2+ [Co(bpy)3]3+ Partial fluorescence yield mode X-ray absorption (PFY- XA)

Co L-edge PFY-XA spectra [Co(bpy)3]2+ [Co(bpy)3]3+ Partial fluorescence yield mode X-ray absorption (PFY- XA) spectra obtained by integrating the Co L-edge RIXS maps Inorganic Chemistry, 2019, Vol 58, pages 4731 -4740 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 13

Co L-edge PFY-XA spectra comparison with theory [Co(bpy)3]2+ [Co(bpy)3]3+ • [Co(bpy)3]2+ ≈ 43% of

Co L-edge PFY-XA spectra comparison with theory [Co(bpy)3]2+ [Co(bpy)3]3+ • [Co(bpy)3]2+ ≈ 43% of low spin (S 0. 5) and 57% of high spin (S 1. 5) • [Co(bpy)3]3+ ≈ 80% of low spin (S 0) and 20% of high spin (S 2) Inorganic Chemistry, 2019, Vol 58, pages 4731 -4740 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 14

N K-edge RIXS of [Co(bpy)3]2+ and [Co(bpy)3]3+ [Co(bpy)3]2+ • RIXS/X-ray emission after ligand 1

N K-edge RIXS of [Co(bpy)3]2+ and [Co(bpy)3]3+ [Co(bpy)3]2+ • RIXS/X-ray emission after ligand 1 s 2 p excitation. • Main emission from ligand 2 p orbitals. A special feature is seen on the high energy emission side marked by dotted blue line. • Both systems exhibit fluorescence features above B 4 and B 4 [Co(bpy)3]3+ Inorganic Chemistry, 2019, Vol 58, pages 4731 -4740 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 15

N K-edge PFY-XA spectra [Co(bpy)3]3+ [Co(bpy)3]2+ PFY- XA spectra obtained by integrating the N

N K-edge PFY-XA spectra [Co(bpy)3]3+ [Co(bpy)3]2+ PFY- XA spectra obtained by integrating the N K-edge RIXS maps Inorganic Chemistry, 2019, Vol 58, pages 4731 -4740 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 16

N K-edge PFY-XA spectra comparison with theory [Co(bpy)3]2+ [Co(bpy)3]3+ • Sensitivity towards metal spin

N K-edge PFY-XA spectra comparison with theory [Co(bpy)3]2+ [Co(bpy)3]3+ • Sensitivity towards metal spin state is minimum from the ligand side. • B 2 and B 2 – N 1 s→π* characteristic C=N π* unoccupied orbitals. B 5 and B 5 – σ* shape resonance • Inorganic Chemistry, 2019, Vol 58, pages 4731 -4740 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 17

Summary • • • Synchrotrons and FELs are accelerator based photon sources X-rays produced

Summary • • • Synchrotrons and FELs are accelerator based photon sources X-rays produced in these photon sources can probe matter to shed light into its electronic structure TM complexes constitute a large portion of catalytically and biologically relevant molecules XANES and RIXS are photon-in photon-out spectroscopic methods We probed cobalt tris bipyridine complexes using XANES and RIXS It is observed that these complexes exhibit multiple spin state behavior | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 18

Thank you for your attention! Acknowledgements Dr. Bernd Winter Department of Molecular Physics, Fritz-Haber-Institut

Thank you for your attention! Acknowledgements Dr. Bernd Winter Department of Molecular Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin , Germany Dr. Sreeju Sreekantan Nair Lalithambika Freie Universität Berlin, Fachbereich Physik, Berlin, Germany Dr. Ronny Golnak Helmholtz-Zentrum Berlin für Materialien und Energie, Department of Highly Sensitive X-ray Spectroscopy, Berlin, Germany Prof. Simone Techert Deutsches Elektronen-Synchrotron DESY, Structural Dynamics in Chemical Systems, Hamburg, Germany

Contact Deutsches Elektronen-Synchrotron www. desy. de Kaan Atak Structural Dynamics in Chemical Systems kaan.

Contact Deutsches Elektronen-Synchrotron www. desy. de Kaan Atak Structural Dynamics in Chemical Systems kaan. atak@desy. de

N K-edge experimental and theoretical X-ray emission • Experimental XES at 406. 4 e.

N K-edge experimental and theoretical X-ray emission • Experimental XES at 406. 4 e. V compared with calculated XES. • The emission from pink shaded area originates from metal-ligand mixed orbitals. • [Co(bpy)3]2+ with 53. 5% metal 3 d t 2 g [Co(bpy)3]3+ 30% 3 d t 2 g Lalithambika et al. Inorganic Chemistry, 2019, Vol 58, pages 4731 -4740 | Understanding The Electronic Structure of Transition Metal Complexes Through XANES and RIXS | Kaan Atak, 3. 9. 2019 Page 21

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