Substituted Fischer carbene complexes of molybdenum(0)

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.