Dinuclear Zinc Salen Catalysts for the Ring Opening Copolymerization of Epoxides and Carbon Dioxide or Anhydrides

A series of four dizinc complexes
coordinated by salen or salan ligands, derived from ortho-vanillin and bearing (±)-trans-1,2-diaminocyclohexane
(L1) or 2,2-dimethyl-1,3-propanediamine
(L2) backbones, is reported.
The complexes are characterized using a combination of X-ray crystallography,
multinuclear NMR, DOSY, and MALDI-TOF spectroscopies, and elemental
analysis. The stability of the dinuclear complexes depends on the
ligand structure, with the most stable complexes having imine substituents.
The complexes are tested as catalysts for the ring-opening copolymerization
(ROCOP) of CO2/cyclohexene oxide (CHO) and phthalic anhydride
(PA)/CHO. All complexes are active, and the structure/activity relationships
reveal that the complex having both L2 and imine substituents displays the highest activity. In the
ROCOP of CO2/CHO its activity is equivalent to other metal
salen catalysts (TOF = 44 h–1 at a catalyst loading
of 0.1 mol %, 30 bar of CO2, and 80 °C), while for
the ROCOP of PA/CHO, its activity is slightly higher than other metal
salen catalysts (TOF = 198 h–1 at a catalyst loading
of 1 mol % and 100 °C). Poly­(ester-block-carbonate)
polymers are also afforded using the most active catalyst by the one-pot
terpolymerization of PA/CHO/CO2.