2.0 The characterisation of cobalt Til) and cobalt (III) complexes.
2.1. Aim.
The aim of this work was to synthesise a range of cobalt (II) and cobalt (III) complexes with a range of hydrophobicity. This was achieved by altering the axial and equatorial ligands. The effects of which will be discussed in this and other chapters.
2.2. Introduction
Effective catalytic chain transfer agents have been shown to be based on low spin cobalt (II) complexes M although cobalt (III) can also be used as a source of cobalt (II). The most effective being the BF2 bridged s e t5-6. It has been shown by various workers that altering catalyst structure can have a profound effect on molecular weight reduction 7. It has also been shown that cobalt (III) must reduce to cobalt (II) to become an effective CCTA *-9. The rate o f this reduction and hence effectiveness as a cobalt (II) species can be controlled by using different axial base ligands.
Characterisation of these CCTA’s is dependable on whether the species are low spin cobalt (II) or cobalt (III). Cobalt (II) CCTA’s are paramagnetic therefore analysis by NMR spectroscopy is not possible. An excellent technique to assure low spin confirmation for cobalt (II) is by calculation of its magnetic moment 10. Cobalt (III) species are diamagnetic and it is therefore possible to acquire useful interpretational NMR spectra IM3. Methods of characterisation which are useful for both cobalt (II) and (III) CCTA’s are infrared spectroscopy l214' 17, FAB MS, and CHN analysis, however the latter is not always interpretably useful owing to the boron and fluorine groups present. It is therefore the purpose o f this chapter to introduce the CCTA’s
which have been used in investigations throughout this thesis and discuss characterisation where necessary.
2.3. The structure of cobalt tilt complexes utilised in this work.
Table 2.1 shows which catalysts have been synthesised corresponding to the general structure outlined in Figure 2.1. Table 2.1 indicates the nature of the axial and equatorial ligands, as stated previously the ligands in cobalt (II) complexes are generally occupied by solvents which are used in the purification of the complex.
Figure 2.1. Structure of cobalt (II) complexes.
Fw F
Where L= CH3OH (I-IV), Ethyl acetate (V-VI), H20 (VII).
The aim of this work was to ascertain what role the equatorial groups (Ri + R2) played on the chain transfer activity of the complexes in bulk polymerisations using MMA and styrene. It was also interesting to see how the structure of the complex affected its partitioning properties when used with MMA and water. It would also be interesting to compare the cobalt (II) complexes with both the cobalt (III) analogues utilising pyridine and water as the axial ligands (L). It was also interesting to see what effect isomerisation had on complex III activity.
Table 2.1. Nature o f catalysts synthesised for cobalt (II) complexes.
Complex number Abbreviated name R, r2
I C0H4BF H H
II CoBF c h3 c h3
III CoEt2Me2BF CH3 c h2c h3
IV CoEt*BF CH2C H3 c h2c h3
V Co(Me2Prop2)BF c h3 c h2c h2c h3
VI Co(Me2But2)BF c h3 c h2c h2c h2c h3
VII Co(C5H5-CH3)BF (CsH5)CH3 (C5H5)CH3
2.4. The use of cobalt (111) complexes utilising pyridine as one of the axial ligands.
The following catalysts were synthesised corresponding to the general structure outlined in Figure 2.2, containing ethyl (R) and pyridine (B) axial ligands, table 2.2.
Figure 2.2. Structure of cobalt (III) complexes with pyridine as axial ligands.
The aim here was to see what effect using a strong base as one of the axial ligands had on the chain transfer activity of the complex. The results from these complexes in both MM A and styrene bulk polymerisations would be compared with both its cobalt
(II) and cobalt (III) - water analogues. The effect of the ligand would also be investigated in the partitioning experiments again using MMA and water. Again the effect of isomerisation on catalytic activity could also be important for complex IX.
Table 2.2. Nature of cobalt (III) complexes with pyridine as an axial ligand.
Complex number Abbreviated name R, r2
VIII Co(III)BF/PyEt CH3 CH3
IX Co(III)Et2Me2BF/PyEt CH3 CH2CH3
X Co(III)EuBF/PyEt CH2CH3 c h2c h3
2.5. The use of cobalt (HD complexes utilising water as an axial base ligand.
The following catalysts were synthesised corresponding to the general structure outlined in Figure 2.3, containing ethyl (R) and water (B) axial ligands, table 2.3.
Figure 2.3. Structure of cobalt (III) complexes with water as axial ligands. Fw F
The aim here was to compare the results obtained for the chain transfer activity of the complex in both MMA and styrene bulk polymerisations with those of the cobalt (II) and cobalt (III) pyridine analogues. The effect of the weak base ligand was also investigated in the partitioning experiments using MMA and water and again
compared with its cobalt (II) and cobalt (III) pyridine analogues. The effect of cis/trans isomers for complex XII could also play a role in catalytic activity.
Table 2.3. Nature of cobalt (III) complexes with water as one of the axial ligands.
Complex number Abbreviated name R i r2
XI Co(III)BF/H2OEt c h3 c h3
XII Co(III)Et2Me2BF/H2OEt c h3 c h2c h3
XIII Co(III)Et4BF/H2OEt c h2c h3 c h2c h3
2.6. The characterisation of the cobalt HI) and cobalt (HI) complexes. 2.6.1. Magnetic moment measurements.
Cobalt (II) complexes are d7 which exist in one of two electronic states either high or low spin (Figure 2.4).
d x 2 .y 2 d Z2