1. Structures and Spin States of Transition-Metal Cation Complexes with Aromatic Ligands Free Electron Laser IRMPD Spectra Robert C. Dunbar Case Western Reserve University

2. Structures and Spin States of Transition-Metal Cation Complexes with Aromatic Ligands Coauthors: David T. Moore, Jos Oomens FOM Institute for Plasma Physics John R. Eyler Univ. of Florida Gerard Meijer, Gert von Helden Fritz Haber Institute Free Electron Laser IRMPD Spectra Robert C. Dunbar Case Western Reserve University

3. Structures and Spin States of Transition-Metal Cation Complexes with Aromatic Ligands Spectroscopy of metal ion complexes Cr + ligand binding sites Cr + spin states (effect of coordination) Transition metals with acetophenone Rearrangement product ion structure

4. Infrared Spectroscopy of Molecular Ions Low densities of ionic molecules due to Coulombic repulsion: Direct absorption spectroscopy difficult at best Action spectroscopy, such as IR photodissociation Infrared multiple photon photodissociation = IRMPD Free Electron Laser High power Excellent tunability Ion storage devices Trap ions, irradiate for seconds

5. Free electron laser Relativistic electron beam in periodic B field Wavelength determined by electron energy and B field High intensity pulsed output

6. FELIX – Free Electron Laser for Infrared eXperiments Tuning range Macropulse energy Bandwidth : 40 – 2000 cm -1 : 100 mJ : Transform limited

7. Fourier Transform Ion Cyclotron Resonance Mass Spectrometer 4.7 T supercon magnet ~10 ppm homogeneity Open ended ICR cell Ion sources: EI, ESI, laser vaporization Laser ablation metal ion source Vapor phase complex formation Moore, Oomens, van der Meer, von Helden, Meijer, Valle, Marshall & Eyler, Chem. Phys. Chem. 5, 740 (2004) Generate complexes Mass selective isolation Irradiate with FELIX Record MS Plot fragment yield vs. IR wavelength

8. Density Functional Calculations of Complexes Structure, Spin and Spectrum predictions Choice of functional and basis set Comparison of different spin states Empirical scaling of vibrational frequencies BSSE Vibrational zero point energies and thermal energies

9. Binding Site: Cr + (Aniline) Choice of Ring (R) or nitrogen (N) binding sites Cr + (Acetophenone) Choice of Ring (R) or carbonyl oxygen (O) binding sites

10. Cr + aniline: Ring Bound B3LYP: N-bound (  E = 7.2 kJ/mol) MP1PW91: ring-bound (  E = 7.0 kJ/mol) Oomens, Moore, von Helden, Meijer & Dunbar, JACS 126, 724 (2004)

11. Cr + (acetophenone) 2 : Side-chain Bound

12. Spin State Cr + (Aniline) High spin Cr + (Aniline) 2 Low spin High-spin sextet in weak ligand field Low-spin doublet in strong ligand field Cr + is d 5

13. Cr + (aniline) 2 : High or Low Spin ? (S= 5 / 2 )

14. Exo Endo (Chelating) O-Binding modes Spectroscopically similar R Binding Characteristic spectrum M + /Acetophenone Binding Modes

15. Spectra of M + (Acet) 2 Cr and Ni very similar Co distinctly different Fe poorly resolved, not fully interpreted

16. Spectra of M + (Acet) 2 O bound O bound and R bound Ring umbrella Side-chain stretch C=O stretch

17. Fits to Calculations OO bound OR bound OO bound

18. Summary: Transition metals with Acetophenone Ni similar to Cr: All ligands O bound Co shows extra peaks: R bound and O bound ligands. Good fit to O/R complex (but mixtures possible) Special affinity of Co + for benzene ring Fe spectrum not as good: Clearly both O bound and R bound ligands Various possibilities

19. Characterizing a Rearrangement Co + active in bond activation chemistry Look at the product of the reaction Co + + Acet  Co + C 7 H 8 + CO

20. Spectrum of Co + C 7 H 8 Product Ion FELIX Spectrum Co + (Toluene) Calculation

21. Rearrangement Product The product ion spectrum fits the expected Co + (Toluene) spectrum Other possible product structures don’t fit Reflects Co + insertion into a C-C bond, rearrangement and coupling to form toluene, and expulsion of CO.

22. Conclusions FEL based IRMPD spectroscopy of trapped ionic species Application to transition metal complexes gives valuable new insights Structural characterization for ligands with competing binding sites ring versus side-chain binding gives clear IR fingerprint Spin state determination from vibrational spectrum is possible ring-bound Cr + bis-complexes are low spin Spectra can characterize products of complex rearrangement reactions

24. Possible Conformations

25. Spin state: mono vs bis complex Cr + Anisole mono and bis complexes are ring bound. Cr + (Anisole) 1 high spin Cr + (Anisole) 2 low spin