Synthesis and structural studies of N- and P-donor ligands in Chromium(III) complexes

Abstract

The fundamental knowledge of Cr(III) chemistry has been enhanced via detailed structural and spectroscopic studies of largely novel compounds that may potentially be active tri- and tetramerisation precursors. The compounds are based on various monodentate and bidentate nitrogen and phosphorus ligands which have been coordinated to [CrCl3 (thf) 3]. The few compounds that have been synthesised previously have in this study been made via novel synthetic routes and incorporate a combination of new and more detailed analysis than was carried out previously. The eight structures determined, in addition to offering novel crystallographic data, also provided insights into the synthetic pathways leading to compound formation. The isolation of monomeric structures suggests direct ligand substitution, while the cationic-anionic structures suggest the presence of dimeric intermediates which have been cleaved asymmetrically. Infrared and Raman spectra of these structures were able to add weight to these pathway proposals and, by means of vibrational comparisons, assisted in the general band assignments of the compounds' spectra where structures were not available. Vibrational shifts relative to the free ligands, as well as metal–ligand vibrations in the far infrared region, were also of significant value in terms of ligand coordination and geometry. Closely associated with the infrared and Raman spectra analysis was the generation of theoretical spectra using Density Functional Theory calculations. The excellent agreement between the calculated and experimental spectra confirmed the vibrational assignments. Also generated by computational means were the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of the compounds which indicated the sites of potential nucleophilic and electrophilic attack. 1H NMR spectroscopy is a technique normally avoided when studying paramagnetic materials. However, by employing a largely novel approach, information pertaining to both ligand coordination and reaction times was obtained. FAB-MS assisted in the confirmation that the single crystal determinations did indeed reflect the composition of the bulk precipitated samples. It also provided additional structural information through the identification of fragmentation patterns which could not be gained by techniques such as elemental analysis.