Fischer and N-heterocyclic carbene complexes of tungsten(0)

Abstract

The synthesis of novel Fischer and N-heterocyclic tungsten(0) carbene complexes was endeavoured in this study and resulted in the synthesis, isolation and characterisation of eighteen new complexes. Sixteen novel Fischer carbene complexes were synthesised. In these complexes, both carbene ligand substituents were varied. Ethoxy as well as amino heteroatom substituents were used. Heteroaryl compounds thiophene and furan were employed as the second substituents on the carbene ligand. Complexes with combinations of these different substituents were synthesised and investigated to assess the influence the various substituents of the carbene ligand may have on the carbene complex itself. In addition, the metal ligand sphere was altered by substitution of one or two carbonyl ligands for either an amine or a phosphine ligand/s. These substitutions resulted in the formation of metal pentacarbonyl, metal tetracarbonyl as well as metal tricarbonyl systems. The complexes were successfully characterised by means of NMR and IR spectroscopy, and in selected cases X-ray diffraction and mass spectrometry. Synthesis of N-heterocyclic carbene complexes derived from isopropyl and mesityl imidazolium chlorides was attempted. The products were obtained in crude form, but could not be isolated due to decomposition during purification. Two novel decomposition products, which point towards a unique decomposition route, were isolated. Theoretical models of both the novel Fischer carbene complexes and the N-heterocyclic carbene complexes were calculated. This allowed for identification of infrared modes observed in experimental data. Furthermore, the HOMO and LUMO distributions and the HOMO-LUMO energy gaps were calculated, along with electrostatic potential maps. In all the Fischer carbene complexes the HOMOs were located on the metal centre and the LUMOs on the carbene ligand. In contrast, the HOMO and the LUMO were both located on the metal centre for the N-heterocyclic carbenes. The HOMO-LUMO energy gap decreased as follows: NHC complexes > Amino Fischer carbene complexes > Ethoxy Fischer carbene complexes Lastly, in all the complexes studied, the electrostatic potential maps indicated that the highest amount of electron density was found on the carbonyl ligands of these complexes. Both experimental and theoretical data indicated marked differences in the various classes of compounds, suggesting that these complexes would not only have different reactivities but also be suited to different applications. Experimental studies on reactivity and applications are thus future avenues of study which are made available from these results.