Abstract:
The synthesis and structure elucidation of fourteen novel Fischer ethoxy- and amino carbene
complexes of molybdenum and chromium metals of the type [(CO)3L2M=C(X)R] were performed.
Substitution of the parent pentacarbonyl complex [(CO)5M-C(OEt)(R)]; M = Mo, Cr; R = 2-thienyl,
2-furyl; with mono- and bidentate phosphine ligands yielded the corresponding tetracarbonyl
complexes [(CO)4(PR’3)M-C(OEt)R] with M = Mo, Cr, R = 2-thienyl, 2-furyl and R’ = Ph, Cy or
tricarbonyl complexes [(CO)3(DPPE)Mo-C(OEt)(2-furyl)], respectively. Aminolysis of these novel
complexes resulted in substitution of the ethoxy substituent with an amino group. Full
characterisation of these novel complexes included infrared spectroscopy, NMR (1H, 13C, 31P)
spectroscopy, x-ray crystallography and mass spectral analyses. From the characterisation data, it
was found that the cis isomer was dominant for the tetracarbonyl carbene complexes whereas the
mer isomer was favoured in most cases for the tricarbonyl carbene complexes.
Structural elucidation of nine of the novel complexes was confirmed with x-ray crystallography. The
amino carbene complexes showed preference for a syn conformation of the oxygen atom of the
furyl moiety and the nitrogen atom of the amino moiety in the crystal form. The ethoxy carbene
complexes showed a preference for the anti conformation in the crystal form. The NMR analysis
showed that resonance between the metal-carbene carbon-heteroatom stabilisation is more
prevalent in the case of the amino carbene complexes.
DFT calculations were performed for eleven of these complexes and their isomers. The infrared
stretching modes were calculated and compared to experimental data. The HOMO and LUMO
position and energy gaps were calculated. It was observed that the HOMO in all calculated
complexes lay on the metal centre, while the LUMO was centred on the carbene carbon. It was
found that the HOMO-LUMO gap was larger for the amino Fischer carbene complexes than for the
ethoxy Fischer carbene complexes. From frontier orbital considerations, it is concluded that ligand
substitution of carbonyls by phosphines as well as carbene substituent alterations does not seem to
improve the reactivity of the novel complexes of this study particularly for the potential application
as metathesis catalysts. The performed DFT calculations, in the future, can help determine which
type of catalytic reactions these fourteen complexes would be most effective for.
