Computational and Experimental Structural Studies of Selected Chromium(0) Monocarbene Complexes

1. Computational and Experimental Structural Studies of Selected Chromium(0) Monocarbene Complexes MariléLandman University of Pretoria

2. Contents • Conformational analysis of heteroarenecarbene complexes • Comparison of experimental and theoretical data • Electronic and steric factors • Electrochemistry • Redox behaviour: Theoretical investigation

3. Synvs Anti conformation

4. List of complexes

5. Scan • Density Functional Theory calculations, using the GAUSSIAN09 • Dihedral scan of X-C-C-Y • Singlet spin state using the hybrid functional B3LYP; Stuttgart/Dresden (SDD) pseudo potential used to describe Cr electronic core while the valence electrons were described with the Karlsruhe split-valence basis set with polarization functions (def-SV(P)) • Scan performed in steps of 36°

6. Scan profiles of 1-3*

7. Optimization • No symmetry constraints applied for 0° and 180°;only default convergence criteria were used during the geometric optimizations • Vibrational frequencies were calculated at the optimized geometries and no imaginary frequencies were observed for the Eminconformers • TS 90° calculation froze dihedral angle at 90° • Donor−acceptor interactions have been computed using the natural bond orbital (NBO) method

8. Results after optimization

9. Comparing experimental and theoretical data

10. 1 2 3 1* 2* 3* Crystal structures of 1-3*

11. Structural comparison

12. NBO: Donor-acceptor interaction in 2 E = -4.9 kJ/mol Highest rotation barrier around the C(carbene)-C(aryl) bond for 2, with a value of 50.3 kJ/mol

13. Steric interaction in 2 • Delocalization of the lone pairs of electrons on the heteroatom of thiophene and furan, forms part of the aromatic system • Oxygen more electronegative heteroatom, furan shows less delocalization of electrons compared to thiophene • O6…O7 distance is 2.488 Å in 2(syn) Mullikencharges on these atoms are -0.410 and -0.395, respectively. • O6…S1 distance in thienylcomplex 1(syn) is 2.724 Å Mullikencharges -0.425 and +0.293.

14. Electrochemistry study • Redox behaviour of monomeric heteroarenecarbene complexes • Extend heteroarene substituent to dimericheteroarene • DFT study to understand redox behaviour

15. Crystal structures of 4 and 5

16. Electrochemistry • The Cr ethoxycarbene complexes of this study represent molecules with two redox active centres: the Cr metal and the carbene as “non-innocent” ligand • Three main redox processes observed: • one reduction process: reduction of the carbene carbon atom • two oxidation processes: the oxidation of the Cr(0) metal centre to Cr(I) and the oxidation of electrochemically generated Cr(I) species to either Cr(II) or (CO)5Cr(I)=C(OEt)R(+). • Comparing the LSV of the processes observed with that of ferrocene, it is concluded that each redox process represents a one electron process only

17. Electrochemistry

18. Reduction process • Reduction of a complex involves the addition of an electron to the LUMO of the complex. • The character of the LUMO of a complex should indicate where the reduction process will occur; the SOMO of the reduced complex will show where the first reduction took place. • Visualization of the (a) LUMOs of the neutral 1-5, (b) the SOMOs of the reduced (charge q = -1) 1-5 and (c) the spin density of the reduced radical anions of 1-5 provide the same information: the reduction involves the electrophilic carbene carbon and the added electron density is delocalized over the heteroarene five-membered rings.

19. Molecular orbitals of reduction process

20. Oxidation processes • Oxidation of a complex involves the removal of an electron from the HOMO of the complex. The character of the HOMO of the neutral complex will thus show where the oxidation will take place • First oxidation process: Cr(0)-Cr(I) oxidation

21. Oxidation processes • Second oxidation process: Cr(I)-Cr(II) or Cr(I)-(CO)5Cr(I)=C(OEt)R(+) oxidation? • Removal of an electron from the HOMO of the oxidized radical cation of 1-5 • 1-3: Second oxidation involves the removal of a dyz electron from the Cr(I)-metal centre • 4-5: Involves the removal of an electron from dimericheteroarene; leads to Cr(I)-(CO)5Cr(I)=C(OEt)R(+) radical species

22. Conclusion • The R group in [(CO)5Cr=C(OEt)R] plays a significant role in the energy, shape and distribution of the LUMO orbital, in other words, to the extent of electron delocalization, while the HOMO is Cr-based. Consequently the reduction of [(CO)5Cr=C(OEt)R] is sensitive to the electrophilic nature of the R substituent • The anodic peak potential of the first oxidation process of 1-5is Cr-based and is only sensitive to the electrophilic character of the heteroarene ring directly attached to the carbene carbon. • Second oxidation process different for monomeric and dimericheteroarene complexes

23. Acknowledgements • Students • Roan Fraser TamzynLevell • Stephen Thompson WynandLouw • René Pretorius • Prof J Conradie,R Lui, UFS • Prof PH van Rooyen, UP • NRF • University of Pretoria