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

Abstract

The central focus of this study was the synthesis, structural investigation and characterisation of multiple chromium carbene complexes. Fifteen novel chromium(0) complexes were synthesised. The synthesis of the primary monocarbene starting material [Cr(CO)5{C(OEt)(heteroaryl)}], heteroaryl = thiophene, furan, 2,2’-bithiophene, was carried out utilising typical Fischer methodology. A wide variation of spacer ligands were reacted to obtain different carbene substituents. The ligand substitution reaction between carbonyl ligands and the bidentate ligands followed the techniques proposed in literature and produced distinctive chelated monocarbene complexes with the resulting structure mer- [Cr(CO)3(dppe){carbene)}]. An extensive collection of more sophisticated monocarbene complexes was synthesised via modification pathways (aminolysis). Conversion of original ethoxy-bearing monocarbenes through aminolysis provided the corresponding amine analogues, possessing both novel structure and unique chemical reactivity. The aminolysis reactions involved different sized amino reagents; both ammonia and cyclohexyl amine (bulky, cyclic chair amines) were employed to produce derivatives of the monocarbene starting complexes. Lastly, the synthesis of unique N-heterocyclic carbene (NHC) complexes was envisaged. Synthesis of both the pentacarbonyl-bearing and phosphine-bearing NHC complexes was attempted utilising an adapted version of a methodology proposed in literature. The synthesis of chelate NHC complexes, however, proved difficult and the resulting products were not obtained. All Fischer and N-heterocyclic carbene complexes were characterised using infrared spectroscopy, nuclear magnetic resonance spectroscopy and mass spectrometry. In cases where single crystals were obtained, X-ray crystallography was used to confirm molecular structures. X-ray crystallographic studies indicated that carbonyl substitution reactions performed on monocarbene starting material, will favour the formation of the meridonial isomer in molecules where the carbene substituents are less bulky. Due to steric considerations, the substitution of labile carbonyl ligands in the trans position to the carbene moiety will be favoured. Density functional theory (DFT) calculations were performed on the complexes synthesised in this study. The results obtained indicated the favoured isomeric form to be facial in some cases whereas crystallographic data signified the meridonial isomer as the more stable product, irrespective of the bulkiness of the carbene substituents. Bond lengths, geometry and bond angles were all comparable to those of the single-crystal X-ray data. Single-point energy calculations show clearly that modelling methods provide good estimations of energetically favourable geometries, and accurate DFT calculations also predict the HOMO and LUMO orientations around the metal or ligand spheres. The majority of the structures provided by the computed model, illustrated that the 3d atomic orbitals of the metal contributed significantly to the HOMO, whereas the LUMO was mostly orientated around the carbene carbon atom. Metathesis and polymerisation catalytic reactions were attempted on [Cr(CO)3(dppe){C(OEt)(thiophene)}], 2, whereas only metathesis studies were employed for [Cr(CO)3(dppe){C(NHCy)(thiophene)}], 6. Both complexes presented as inert to either reaction and no catalytic capability was witnessed. Gas chromatography was used to indicate the level of progression of the reaction and the chromatogram verified that neither pre-catalyst found application in metathesis or, in the case of 2, in polymerisation.