A mechanistic study of oxinato complexes of Rhodium (I)
Janse van Rensburg, Jacobus Marthinus
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The aim of this study was to functionalize 8-hydroxyquinoline bidentate ligand systems and introduce these bidentate ligands as well as tertiary phosphine ligands in a systematic way into Rh(I) complexes, in an attempt to determine the complex solid state geometrical parameters and manipulate the metal electron density. Functionalization of the oxine moiety was fairly easy and X-Ray crystallographic structure determinations are reported for a few ligand systems: 5-chloro-8-hydroxyquinoline (Orthorombic Fdd2, R = 3.19 %), 5-chloro-8-hydroxyquinolinium bromide (Monoclinic P21/n, R = 4.50 %), 5- chloro-8-hydroxyquinolinium hexafluorophosphate hydrate (Monoclinic P21/n, R = 4.07 %), 5-nitro-8- hydroxyquinoline (Orthorombic Fdd2, R = 2.49 %), 5-nitro-8-hydroxyquinolinium nitrate (Triclinic Pī, R = 2.44 %) and 5-chloromethyl-8-hydroxyquinoline hydrochloric acid solvate (Monoclinic P21/c, R = 4.76 %). From the solid state study it was established that for neutral ligands the cis-hydroxyl hydrogen configuration mostly leads to the formation of hydrogen bonded dimers and that the transhydroxyl hydrogen configuration will only occur in the presence of intermolecular soft contacts to neighbouring molecules. A theoretical investigation on functionalized 8-hydroxyquinoline compounds proved the cis-hydroxyl hydrogen configuration to be the energetically preferred isomer and that oxine functionalization influenced the molecular energy level. Various phosphine and phosphite ligands were introduced onto the rhodium(I) complexes, to note different intermolecular interactions in the solid state. The reported X-Ray crystallographic structure determinations include the following complexes: [Rh(ox)(CO)(P(O-2,4ditBuPh)3)] (Orthorombic Pccn, R = 4.26 %), [Rh(ox)(CO)(P(O-2MePh)3)] (Monoclinic P21/c, R = 4.63 %), [Rh(ox)(CO)(P(5FPh)3)] (Monoclinic C2/c, R = 6.39 %), [Rh(ox)(CO)(PCy3)] (Monoclinic C2/c, R = 4.73 %), [Rh(ox)(CO)(PCyPh2)] (Monoclinic P21/n, R = 4.25 %), [Rh(ox)(CO)(PPh3)] (Triclinic Pī, R = 7.52 %), [Rh(oxCl)(CO)P(O-4tBuPh)3] (Triclinic Pī, R = 3.97 %), [Rh(oxCl)(CO)(P(O-2,4ditBuPh)3)] (Monoclinic P21/n, R = 7.15 %), [Rh(oxCl)(CO)P(p-ClPh)3] (Monoclinic P21/n, R = 9.69 %), [Rh(oxCl)(CO)P(p-FPh)3] (Triclinic Pī, R = 4.14 %), [Rh(oxL)(CO)(PPh3)] (Monoclinic P21/c, R = 10.7 %), [Rh(oxL)(CO)(PCy3)] (Monoclinic P21/n, R = 6.74 %) and [Rh(oxL-Me3)(CO)(PPh3)] (Triclinic Pī, R = 4.70 %). From these data sets the phosphorous ligands steric effect, in the solid state, was determined by calculating the ligands effective cone angle. In general the molecules pack with a quinoline ligand to ligand’s π-stacking in a “head to tail” fashion, influenced by 8-hydroxyquinoline functionalization. A few phosphine substituted rhodium(I) complexes were selected to investigate the ligand’s effect on the electron density of the rhodium metal centre, by determining the complex catalytic activity towards a methyl iodide oxidative addition reaction. The PPh3, PCyPh2, PCy2Ph and PCy3 ligated rhodium(I) complexes were selected to hopefully obtain a stepwise change in the steric and electronic properties of the metal centre. The influence from functionalized 8-hydroxyquinoline ligands on the metal’s electron density was investigated in the same manner. It was found that by altering the electron withdrawing properties of substituents on the 8- hydroxyquinolinato backbone, the rate of Rh(III) alkyl formation during methyl iodide oxidative addition can be manipulated. The activity of functionalized 8-hydroxyquinolinato metal complexes towards oxidative addition decrease in the following order: [Rh(ox)(CO)(PPh3)] > [Rh(oxCl)(CO)(PPh3)] > [Rh(oxNO2)(CO)(PPh3)]. Both 8-hydroxyquinolinato and 5-chloro-8- hydroxyquinolinato phosphine ligated rhodium complexes displayed the following order of activity towards Rh(III) alkyl formation: [Rh(oxY)(CO)(PCyPh2)] > [Rh(oxY)(CO)(PPh3)] > [Rh(oxY)(CO)(PCy3)] > [Rh(oxY)(CO)(PCy2Ph)] where Y = H or Cl respectively, demonstrating that neither the phosphine ligand steric nor electronic properties of the ligands are dominating influences on the rate of Rh(III) alkyl formation. The proposed mechanism for methyl iodide oxidative addition to complexes of the type [Rh(oxY)(CO)(PR3)], where Y = H, Cl or NO2 and R = Ph3, CyPh2, Cy2Ph or Cy3, is depicted in Scheme I. The [Rh(ox)(CO)(PPh3)] complex was also exploited as a catalytic precursor for the hydroformylation of 1-octene, to investigate ligand influences on a Rh(I) metal centre under hydroformylation reaction conditions.