Catalytic Synthesis and Characterization of Biodegradable ...

Chapter 2
4 L 4 -Co III -dnp 25 70 81 3.1 1.4 1.2
a)
The reaction was performed in neat rac-PO ([rac-PO]:[Co]:[Bu4NBr] = 1000:1:1, molar ratio) in a 100
mL autoclave at 25 ºC with 2 MPa of CO2 for 2 h; b) Turnover frequency of rac-PO to products
(polycarbonate and cyclic carbonate); c) Selectivity for PPC over PC as determined by 1 H NMR
spectroscopy; d) Determined by 13 C NMR spectroscopy; e) Determined by 1 H NMR. f) Determined by GPC.
2.4 Electrochemistry of Cobalt(III) Complexes
The electrochemical data for the ligands and the L-Co III -dnp complexes were shown in
Table 2.4, and the representative voltammograms were presented in Figures 2.3 and 2.4. All
CV curves for the ligands were recorded in the potential range of -2.00 to 1.50 V. The
voltammograms showed an irreversible oxidation peak at potentials between 1.08 and 1.20 V.
These peaks were ascribed to the oxidation of the Schiff base ligands (Figure 2.3). For the
L-Co III -dnp complexes, the potential sweeps were made from -1.20 to 1.50 V. It is considered
that the nature of the cobalt(III) atom in the Schiff base complexes should dominate the
catalytic activity for the copolymerization of the epoxide and carbon dioxide, which prompted
us to pay close attention to the Co III + e -
Co II redox potential. The redox couple of Co III
+ e -
Co II was observed for L 1 -Co III -dnp, L 2 -Co III -dnp, L 3 -Co III -dnp, and L 4 -Co III -dnp
at E1/2 = 107, 134, 163, and 32 mV, respectively (Figure 2.4). The absence of any redox
responses near 0 to 0.5 V in the metal-free ligands (Figure 2.3) allowed one to ascribe these
quasi-reversible waves to the cobalt(II/III) redox couple.
Figure 2.3 Cyclic voltammograms of 1 mM solutions of H2L in DMF containing 0.1 M
NBu4ClO4 on a glassy carbon electrode (� 3.3 mm). Potential is shown in V vs. Ag/Ag + at the
scan rate of 100mV s -1 . I: H2L 1 ; II: H2L 2 ; III: H2L 3 ; IV: H2L 4 .
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