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

Contents 1.Conformational analysis of heteroarene carbene complexes 2.Comparison of experimental and theoretical data 3.Electronic and steric factors 4.Electrochemistry 5.Redox behaviour: Theoretical investigation 2

Syn vs Anti conformation 3

List of complexes 4

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° 5

Scan profiles of 1-3* 6

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 E min conformers TS 90° calculation froze dihedral angle at 90° Donor−acceptor interactions have been computed using the natural bond orbital (NBO) method 7

Results after optimization Complex Dihedral Angle X-C-C-Y (°) ConformationEnergy (kJ/mol) Boltzmann Distribution Syn Anti TS % 1.3% Syn Anti TS a % 95.8% Syn Anti TS % 0.1% 1* Syn Anti TS % 3.0% 2* Syn Anti TS % 0.2% 3* Syn Anti TS % 37.9% 8

Comparing experimental and theoretical data 9

Crystal structures of 1-3* 10

Structural comparison 11

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 12

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 Å in 2 (syn) Mulliken charges on these atoms are and , respectively. O6…S1 distance in thienyl complex 1 (syn) is Å Mulliken charges and

Electrochemistry study Redox behaviour of monomeric heteroarene carbene complexes Extend heteroarene substituent to dimeric heteroarene DFT study to understand redox behaviour 14

Crystal structures of 4 and 5 15

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) 5 Cr(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 16

Electrochemistry 17

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. 18

Molecular orbitals of reduction process 19

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 20

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

Conclusion The R group in [(CO) 5 Cr=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) 5 Cr=C(OEt)R] is sensitive to the electrophilic nature of the R substituent The anodic peak potential of the first oxidation process of 1-5 is 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 dimeric heteroarene complexes 22

Acknowledgements Students – Roan FraserTamzyn Levell – Stephen ThompsonWynand Louw – René Pretorius Prof J Conradie, R Lui, UFS Prof PH van Rooyen, UP NRF University of Pretoria 23