Solid-State Structures of Zinc(II) Benzoate Complexes. Catalyst Precursors for the Coupling of Carbon Dioxide and Epoxides

Zinc complexes derived from benzoic acids containing electron-withdrawing substituents have been synthesized
from ZnII(bis-trimethylsilyl amide)2 and the corresponding carboxylic acid (2,6-X2C6H3COOH, where X = F, Cl, or
OMe) in THF and structurally characterized via X-ray crystallography. The 2,6-difluorobenzoate complex crystallizes
from THF or CH3CN as a seven membered zinc aggregate, where the metal atoms are interconnected by a
combination of 10 μ-benzoates and μ4-oxo ligands, that is, [(2,6-difluorobenzoate)10O2Zn7](solvent)2, solvent =
THF (1) and CH3CN (1a). On the other hand, the 2,6-dichlorobenzoate zinc derivative crystallizes from THF as a
dimer, [(2,6-dichlorobenzoate)4Zn2](THF)3 (2), where the two zinc centers are bridged by three benzoate ligand.
One of the zinc centers possesses a tetrahedral ligand environment where the fourth ligand is a unidentate benzoate,
and the other zinc center has an octahedral arrangement of ligands which is accomplished by the additional binding
of three THF molecules. Upon dissolution of complex 1 or 2 in the strongly binding pyridine solvent, disruption of
these zinc carboxylates occurs with concomitant formation of mononuclear zinc bis-benzoates with three pyridine
ligands in the metal coordination sphere. Complexes 1 and 2 were found to be effective catalysts for the
copolymerization of cyclohexene oxide and carbon dioxide to afford polycarbonates devoid of polyether linkages,
that is, completely alternating copolymers. Although these catalysts or catalyst precursors in the presence of CO2/propylene oxide afforded mostly propylene carbonate, they did serve as efficient catalysts for the terpolymerization
of carbon dioxide/cyclohexene oxide/propylene oxide. The reactivities of these zinc carboxylates were very similar
to those previously reported analogous complexes which have not been structurally characterized. Hence, it is
suggested here that all of these zinc carboxylates provide similar catalytic sites for CO2/epoxide coupling processes.