Catalytic Synthesis and Characterization of Biodegradable ...

Synthesis and Electrochemistry of Schiff Base Cobalt(III) Complexes and Their
Catalytic Activity for Copolymerization of Epoxide and Carbon Dioxide
were definitely assigned to the stretching of the C–O and C–N bonds, respectively. 31, 32 The
spectra of the complexes revealed that these vibrations shifted to lower wave numbers
compared to the corresponding bands observed for the free ligands, as a result of the decrease
in bond order upon complexation. Also, the characteristic bands for the out-of-plane
deforming, δOH, of the free ligands were absent in the case of the complexes, as a consequence
of the involvement of the oxygen anion in the bonds with the cobalt atom. The formation of
the cobalt–oxygen bonds led to major absorption bands around 571 cm -1 assigned to a vCo–O
vibration. 33 The IR spectra clearly showed the formation of the complexes by the coordination
33, 34
of cobalt(III) to the phenolic oxygens and the C=N nitrogens.
Table 2.2. Infrared spectroscopic data for L-Co III -dnp.
Complex vCo–O vC–N vC–O vC=N vAr-ring vN=O
L 1 -Co III -dnp
L 2 -Co III -dnp
571 1063 1252 1624 1610 1566
1525 1327
579 1065 1257 1626 1596 1331
1521 1569
L 3 -Co III -dnp 583 1056 1269 1625 1597 1555
1525 1310
L 4 -Co III -dnp 575 1061 1261 1625 1601 1568
1522 1318
The –OH proton signals in the 1 H NMR spectra of the free ligands (13.75, 13.58, 13.58,
and 13.82 ppm for H2L 1 , H2L 2 , H2L 3 and H2L 4 , respectively) disappeared in the complexes
indicating that the –OH group was deprotonated and bonded to the metal ion as an oxygen
anion. One 2, 4-dinitrophenoxide counter ion was involved in each cobalt(III) complex to
maintain electroneutrality. Comparing the 1 H NMR spectra of the free ligands with the
complexes, the imine proton resonances of the ligands (8.29, 8.44, 8.34 and 8.40 ppm in H2L 1 ,
H2L 2 , H2L 3 and H2L 4 , respectively) 15 shifted to lower fields upon the complexation (8.62,
9.01, 8.41 and 8.89 ppm, respectively), as a result of the deshielding effect. The data from the
elemental analysis and ESI MS also confirmed the structures of the complexes.
2.3 Copolymerization of CO2 and rac-PO
2.3.1 Mechanism for CO2/rac-PO Copolymerization
The molecular structure of L 1 -Co III -dnp clearly demonstrated that the cobalt(III) tends to
form a six coordination complex, which strongly suggested that all of the relevant
L-Co III -dnp complexes characterized by the axial 2,4-dinitrophenolate ligands were
‐ 63 ‐