Structural and reactivity study of rhodium(i) carbonyl complexes as model nano assemblies

[摘要] This study focussed on the investigation of different β-diketonato ligands in coordination to rhodium(I). Square planar [Rh(O,O'-Bid)(CO)2] complexes have been shown to effectively facilitate metallophilic interactions between rhodium(I) centres in the construction of infinite one-dimensional metal chains in the solid-state. The classification of these systems as forming nano-wired assemblies has led to similar systems finding wide application in electronic and optical technologies. The study was focused on investigating the effect of an altered rhodium(I) environment on the metallophilic interactions and subsequent one-dimensional chains formed in the solid-state. These modifications were initiated by using a range of different coordinating β-diketonato ligands to induce either electronic or steric changes to the rhodium(I) centre. To this end, a range of rhodium(I) complexes were synthesized and characterized by IR, UV/Vis and NMR spectroscopy. Single crystal X-ray diffraction was used in the solid-state structure determinations of these complexes showing significant changes in the Rh···Rh distances in each rhodium(I) complex. It also provided valuable information with regards to how the molecules are arranged along these one-dimensional chains. Rh···Rh distances ranging from 3.134(3) Å to 3.617(3) Å were found in the solid-sate for the range of rhodium(I) complexes with the distances correlating to the UV/Vis absorption profile of each complex. A preliminary substitution reaction and equilibrium study was undertaken to further evaluate how changes at the rhodium(I) centre could affect the reactivity of the rhodium(I) complexes. An important equilibrium was shown to participate in the reaction using 31P NMR and UV/Vis spectroscopy. In this investigation it was seen that using second-order rate constants to describe the reactivity of the complexes correlated to the pKa values of the uncoordinated β-diketone ligands with an increased rate in substitution associated with a lower pKa of the free ligand. 103Rh NMR chemical shifts of the rhodium(I) complexes were also found to correlate to the pKa values of the free ligands as well as highlighting the electronic environment experienced by the metal centre. This provided an effective measure of how electronic changes to the rhodium(I) centre could affect the Rh···Rh interactions of the solid-state structures as well as the physical properties of the compounds. The study concluded with a comparison of all the parameters by which the rhodium(I) complexes were evaluated to assess how changes induced by using different coordinating β-diketonato ligands influence the one-dimensional chains constructed via metallophilic interactions as well as physical properties such as the colour exhibited by the bulk material. These parameters included pKa, UV/Vis absorbance properties, IR, 103Rh NMR, reactivity (k12 constants), Rh···Rh distances and the torsion angles of the assembled molecules.

[发布机构] University of the Free State