The development of metal-based homogenous catalysts capable of oxidizing hydrocarbons under mild conditions continues to be a challenge for chemists around the world. There is a major need for the formation of alcohols (or oxygenates) by the hydroxylation of alkanes because the current commercial solid metal oxide catalysts are not sufficiently active for the functionalization of unsaturated C-H bonds and require high temperatures and pressures which often lead to low selectivity.19
Many transition metal complexes used as homogenous catalysts have shown high homogeneity, selectivity, reproducibility and activity, as well as the ability to catalyze reactions under mild conditions.151 However, the appropriate choice of ligand plays an important role, as this can fine tune the activity of the metal and alter the catalytic behavior.
In this study we have used aminodiphosphine ligands (PNP) in the oxidation of n-octane and styrene. This ligand system has been widely explored in ethylene oligomerisation with chromium as the active metal.152-158 These bi-dentate or multidentate ligands are part of a system that displays high activity, variability and stability.159-161 Modification of the ligand backbone, by using different donor substituents (such as soft (N) and hard (P) donors) or central anionic atoms, tailors the activity of the metals, allowing the reactions of the metal ions to be selective, due to the high demand the ligands place on the stereochemistry of the complex.162,163
In this study, the substituent on the nitrogen atom has been varied so as to vary the basicity of the ligand backbone (Fig 1.8). This was done with the intention of observing if these groups have an effect on the catalytic activity and selectivity to the products of oxidation. Alkyl substituents such cyclohexyl (a) (cyclic), iso-propyl (b) (branched) and n-pentyl (c) (straight chained) as well as phenyl (d) based substituents and substituted phenyl (chlorophenyl (e) and methoxyphenyl (f)) are used. In order to add flexibility to the ligand backbone, ethylene spacer groups between the N and P atoms are used (Fig 1.9). Again, the R group was varied by making use of different alkyl substituents on the nitrogen atom.
Figure 1.8. The ligands (1) used in this study containing the different substituents on the nitrogen atom.
Figure 1.9. Ligands (2) containing the ethylene spacer group between the N and P atoms, as well as different alkyl substituents on the nitrogen atom used in this study .
R
N
P P
Ph
Ph
Ph
Ph N
P P
Ph
Ph
Ph
Ph
N
P P
Ph
Ph
Ph
Ph
N
P P
Ph
Ph
Ph
Ph
N
P P
Ph
Ph
Ph
Ph
N
P P
Ph
Ph
Ph
Ph
N
P P
Ph
Ph
Ph
Ph O
Cl a
b
c
f
e d
R N
P P
Ph
Ph Ph
Ph
N
P P
Ph Ph Ph
Ph
N
P P
Ph Ph Ph
Ph
N
P P
Ph Ph Ph
a b Ph c
These ligands have been complexed to the transition metals Co, Rh, Ir and Ru and were used as catalyst in the oxidation of n-octane and styrene. Cobalt was chosen as one of the transition metals since it is relatively cheap, widespread and easily available. Furthermore, with the success of first row transition metals, such as iron and copper, in oxygenation reactions using biological enzymes, cobalt serves as a promising candidate in in the C-H activation of alkanes and alkenes.
The oxidation of both n-octane and styrene using iridium and rhodium complexes has not been thoroughly explored. Very few cases have been reported and most work was undertaken in the 1980’s. To gain scientific insight, it was most decided that complexes of these metals bearing the “PNP” ligand backbone, be investigated in the oxidation of these respective substrates.
Ruthenium oxidation catalysis has been thoroughly explored with a variety of ligand systems.
However, when using a phosphine-based ligand, the catalyst is highly susceptible to ligand degradation. This study presents new work using the aminodiphosphine ligand system, under mild conditions, in the oxidation of n-octane and styrene, in the hope of recovering the catalyst without ligand degradation.
In each of these studies, the solvent, oxidant and reaction temperature were varied so as to determine the optimum conditions for best selectivity and conversion to the desired products.
Below is a graphical summary of the content of each chapter.
octanols octanals octanoic acid
octanone n-octane
Catalysts 1 and 2 80 oC, TBHP, MeCN
R P N P
Co Ph Ph
Ph Ph
Cl Cl
R N
P P
Ph Co Ph
Ph Ph Cl Cl
1 2
R= cyclohexyl (a), pentyl (b) and iso-propyl (c)
Chapter 2 – Cobalt “PNP” aminodiphosphine complexes as catalysts in the oxidation of n-octane
Catalysts 1 and 2 80 oC, TBHP, DCE styrene
O + other products
O
+ other products
R P N P
Co Ph
Ph
Ph Ph
Cl Cl
R N
P P
Ph Co Ph
Ph Ph Cl Cl
1 2
R= cyclohexyl (a), iso-propyl (b) and pentyl (c)
Chapter 3 – Oxidation of styrene by TBHP using cobalt “PNP”
aminodiphosphine complexes as highly effective catalysts
R = cyclohexyl (a); pentyl (b); iso-propyl (c); phenyl (d); chlorophenyl (e);
methoxyphenyl (f).
M = Ir (1) and Rh (2)
Chapter 4 – Ir and Rh “PNP” aminodiphosphine complexes as catalysts in the oxidation of styrene
R P N P
M Ph
Ph
Ph Ph Cl
PF6 Catalysts 1 and 2
80 oC, TBHP, MeCN styrene
O + other products
O
+ other products
R = cyclohexyl (a); pentyl (b); iso-propyl (c); phenyl (d); chlorophenyl (e);
methoxyphenyl (f).
M = Ir (1) and Rh (2)
Chapter 5 – Ir and Rh “PNP” aminodiphosphine complexes as catalysts in the oxidation of n-octane
octanols octanals octanoic acid
octanone n-octane
Catalysts 1 and 2 80 oC, TBHP, DCE
R N
P P
M Ph Ph
Ph Ph Cl
PF6
R = cyclohexyl (a); pentyl (b); iso-propyl (c);
Chapter 6 – Ru “spider” complexes as catalysts in the oxidation of n-octane and styrene
O + other products
O
+ other products Ru"PNP" Catalysts
RT, TBHP, DCE styrene
octanols octanals octanoic acid
octanone n-octane 80 oC, TBHP, DCE
Ru"PNP" Catalysts
Ru P P P
P
N N R
Cl
Cl R
Ph Ph
Ph Ph Ph Ph Ph Ph