Pathways to more stable neutral and cationic alkane sigma complexes

Abstract

The work presented in this thesis details NMR spectroscopic studies undertaken at low temperatures aimed at generating, observing and characterising organometallic alkane complexes, which are alleged intermediates in C-H activation reactions of alkanes. The generation of alkane complexes was attempted primarily through two pathways, protonation of alkyl complexes and photolysis of complexes containing labile ligands. The use of weakly coordinating hydrofluorocarbon solvents to study cationic alkane complexes was developed. A new laser photolysis NMR apparatus was employed to assist in photolysis reactions. The protonation of rhenium alkyl complexes of the type (RCp)Re(CO)(NO)(alkyl) (R = i-Pr, alkyl= CH3; R = H, alkyl = cyclopentyl) resulted in the formation of trans alkyl hydrides at low temperature followed by evolution of free alkanes at higher temperature, whereas the protonation of both CpFe(CO)2CH3 and Re(CO)5CH3 resulted in the formation of methane at low temperature. CpRe(CO)2(cyclopentane) was the first alkane complex characterised using NMR spectroscopy and provides a benchmark for similar complexes. A series of complexes with substituted cyclopentadienyl ligands were photolysed in the presence of cyclopentane in order to generate the corresponding cyclopentane complexes. (C4F9Cp)Re(CO)2(cyclopentane) was found to have a half life surpassing that of the unsubstituted complex whereas the isopropyl-, acetyl- and pentamethylcyclopentadienyl complexes were found to have half lives less than or equal to that of the unsubstituted complex. The formyl complex proved difficult to photolyse and the formation of an alkane complex was not reproducible. An alkane bound to the CpRe(CO)2 fragment through a methine C-H bond is yet to be observed spectroscopically. The interaction of this fragment with cubane, which contains only tertiary carbons, resulted in the activation of a C-H bond upon complexation and a cubyl hydride complex was observed. The photolysis of a range of complexes, with the aim of observing alkane/solvent adducts, was also undertaken. Photolysis of (h6-hexaethylbenzene)W(CO)3 in the presence of methylcyclohexane resulted in the formation of alkane complexes in which the alkane was bound through the methyl and methylene groups. Photolysis of [(i-PrCp)Re(CO)2(NO)][Al(OC(CF3)3)4] was difficult and no alkane complex was observed. Several other combinations of precursor complexes and solvents yielded no definitive results.