Sterically hindered chiral transition metal complexes

Abstract

This thesis describes the synthesis, characterization and study of a series of organometallic compounds which all contain the same new ligand, l-phenyl-3-methyl-4,5,6,7-tetrahydroindenyl. The ligand forms a chiral complex once coordinated, and is relatively bulky when compared with ligands such as cyclopentadienyl or 4,5,6,7-tetrahydroindenyl.Chapter one of this thesis introduces cyclopentadienyl ligand chirality, cyclopentadienyl metal complex chirality and sterically demanding cyclopentadienyl systems. The synthesis and chemistry of tetrahydroindenes and some applications of chiral cyclopentadienyl metal complexes and their bulky analogues are also reviewed. Chapter two describes modifications to a literature preparation of the tetrahydroindenone precursor of the new tetrahydroindenyl ligand which lead to higher yields. The synthesis of the ligand itself is described, as well as the synthesis of a benzylidene-substituted hexahydroindene, which demonstrates a limitation in the flexibility of the synthetic route chosen. The synthesis, characterization and various properties of the following iron(II) compounds are discussed in chapter two; bis-l-phenyl-3-methyl- 4,5,6,7-tetrahydroindenyl iron (II), 2.3, l-phenyl-3-methyl-4,5,6,7-tetrahydroindenyl iron(II) dicarbonyl dimer, 2.4, and l-phenyl-3-methyl-4,5,6,7-tetrahydroindaiyl methyl dicarbonyl iron(II), 2.5. For all these iron complexes, the solid state molecular structures and the absolute configuration of the chiral ligand were determined using single crystal X-ray d iffraction. For 23 and 2.4, three isomers are possible, two enantiomers that are collectively termed the rac-isomer and a third isomer, the meso- isomer. Cyclic voltammetric studies on 2.3 indicate that it has a reversible one electron oxidation at 0.187 V (with respect to a non-aqueous Ag/AgCl standard electrode). The difference between this and the reversible one electron oxidation for (η-C(_5)H(_5))(_2)Fe (with respect to the same standard) is -0.314 V, therefore 2.3 is shown to be much more easily oxidized than (η-C(_5)H(_5))(_2)Fe. The solution-state infi-a-red spectrum of 2.4 is explained, with reference to a literature analysis of the unsubstituted analogue [CpFe(CO)(_2)](_2). The steric forces present in the various molecular environments are discussed in connection with the degree of phenyl-ring tilt relative to the cyclopentadienyl mean plane and the deviation of the other cyclopentadimyl substituents away from the metal centre. Subsequent reactions of compounds 2.4 and 2.5 are described. Attempts to make linked analogues of the new ligand are summarized in chapter two. In chapter three, two Zr(rV) compounds are prepared, bis (l-phenyl-3-methyl-4,5,6,7-tetrahydroindenyi) zirconium(fV) dichloride, 3.1, and bis (l-phenyl-3-methyl-4,5,6,7-tetrahydroindenyl) dimethyl zirconium(TV), 3.2. Upon crystallization, rac-3.1 spontaneously resolves into crystals containing only one enantiomer. The similarities and differences in the spectroscopic data for the iron(n) compounds of chapter two and the zirconium(IV) compounds of chapter three are discussed and possible explanations offered . The solid state molecular structures of 3.1 and 3.2 were determined by single crystal X-ray diffraction. Experimental details are given in chapter four, whilst the characterizing data are presented in chapter five. Details of the X-ray structure determinations are given in Appendix A.